Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Report R15/60 ISBN: 978-0-478-15235-7 (print) 978-0-478-15236-4 (web) 978-0-478-15237-1 (cd)

Report prepared by Zeb Etheridge Environment Canterbury

August 2015

Report R15/60 ISBN: 978-0-478-15235-7 (print) 978-0-478-15236-4 (web) 978-0-478-15237-1 (cd)

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Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Summary Background Environment Canterbury is working with the Lower Waitaki – South Coastal Canterbury Zone Committee and the local community to set nutrient load limits for the lower Waitaki catchment. The nutrient limits are a way of managing diffuse sources of nitrogen loss in the catchment. In this report we look at different nutrient load scenarios to explore what might happen under a range of land management approaches, to help the community make decisions on appropriate limits. The problem Groundwater is an important source of drinking water in the lower Waitaki catchment, with around 180 actively used drinking water supply wells and springs listed on our database. Increasing the extent and/or intensity of land use generally increases nitrate and potentially microbial and phosphorus contamination in groundwater. Our monitoring wells in the catchment have recorded increasing nitrate concentrations. Groundwater is also an important flow component in some surface water bodies. Increases in groundwater nitrate and phosphorus concentrations can impact on surface water quality. What we did We used spatial modelling to assess nitrate concentrations in shallow groundwater under a range of nitrogen load scenarios, developed during a collaborative community process. We used our model to indicate the relative change in groundwater quality that could be expected for each scenario, and the impact of this on the number of wells in which the nitrate drinking water standard could be exceeded. What we found Most of the scenarios involved relatively small nitrogen load increases and associated water quality changes. The exceptions to this were the LUC and Flex Cap 15 scenarios in the Hakataramea freshwater management unit, the Green Rules scenario in the Valley and Tributaries unit and the Industrial Load scenario in Whitneys Creek. These scenarios all allowed for an increase in nitrogen leaching loads, which resulted in an increased risk to drinking water supply wells. Because irrigation practice in the Northern Fan freshwater management unit is currently changing from border dyke to spray irrigation, we expect groundwater quality to continue to deteriorate here. Average groundwater quality is not likely to deteriorate in the Hakataramea and Greater Waikākahi areas, may get slightly worse in the mid-Waitaki north bank and south bank tributaries areas, and is likely to deteriorate in the mid-Waitaki south bank, Northern Fan Riverside and Whitneys Creek areas. The ZIP Addendum for the lower Waitaki recommends that all farming is undertaken under the Good Management Practice (GMP) guidelines. If actual farm management at present results in higher nitrogen leaching than GMP, and if GMP is successfully implemented, the nutrient load increases predicted under the Solutions package relative to the Current scenario will be reduced. What does it mean? Deteriorating groundwater quality can increase the risk that water quality does not meet the drinking water standard. The long-term net effect of irrigation conversion in the Northern Fan freshwater management unit could be an increase in the number of wells that do not meet the drinking water standard for nitrate. There are two community supply wells in the Northern Fan, as well as over 40 private wells used for drinking water, all of which may be vulnerable to nitrate concentration increases in the future. A monitoring, trigger level and action plan is being developed for the lower Waitaki as part of a separate report, which could be used to help manage this risk.

Environment Canterbury Technical Report i Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

ii Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Table of contents

Summary ...... i

1 Introduction ...... 1 1.1 Purpose ...... 1 1.2 Scope ...... 1 1.3 Legal and planning framework ...... 1

2 Why we care about groundwater quality ...... 2

3 Assessment methodology ...... 3 3.1 Nitrogen load scenarios ...... 3 3.2 Methodology ...... 6 3.3 Freshwater management units ...... 6

4 Hydrogeology and current water quality ...... 8 4.1 Overview ...... 8 4.2 Hakataramea ...... 8 4.3 Valley and tributaries ...... 9 4.3.1 Mid-Waitaki north bank ...... 9 4.3.2 Mid-Waitaki south bank ...... 9 4.3.3 Mid - Waitaki south bank tributaries ...... 11 4.4 Northern Fan ...... 11 4.4.1 Whitneys Creek ...... 13 4.4.2 Greater Waikākahi ...... 14 4.4.3 Lower-Waitaki Riverside ...... 15 4.5 Current conditions summary ...... 15

5 Risk interpretation ...... 16

6 Nitrate load and soil drainage concentrations ...... 19 6.1 Methodology ...... 19

7 Scenarios assessment ...... 20 7.1 Assessment criteria...... 20 7.2 Assessment method ...... 20 7.3 Hakataramea ...... 21 7.4 Mid-Waitaki north bank ...... 21 7.5 Mid-Waitaki south bank ...... 21 7.6 Mid-Waitaki south bank tributaries ...... 22 7.7 Lower-Waitaki Riverside ...... 23 7.8 Greater Waikākahi ...... 24

Environment Canterbury Technical Report iii Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

7.9 Whitneys Creek ...... 25

8 Solutions Package assessment ...... 25

9 Phosphorus ...... 29

10 Contaminant microbiology ...... 29

11 Nitrate modelling results context ...... 30 11.1 The modelling process ...... 30 11.2 Underlying assumptions and the uncertainty of model predictions ...... 30

12 Discussion and conclusions ...... 34

13 Recommendations ...... 34

14 References ...... 35

15 Acknowledgements ...... 36

Appendix 1: Model validation ...... 37 Background ...... 37 Model and measured nitrate concentrations ...... 37

Appendix 2: Catchment maps ...... 39

Appendix 3: Elephant Hill nitrogen transport assessment ...... 48

Appendix 4: Lag-time assessment ...... 49 Background ...... 49 Lower-Waitaki lag-times ...... 49

Appendix 5: Groundwater recharge and quality trends in the lower Waitaki – Northern Fan area ...... 52

iv Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

List of Figures Figure 3-1: Lower Waitaki groundwater quality assessment areas ...... 7 Figure 4-1: Mid-Waitaki south bank drinking water supply wells and surface water influence areas ...... 10 Figure 4-2: Glenavy groundwater nitrate concentrations ...... 14 Figure 5-1: 2013 regional nitrate concentration histogram ...... 17 Figure 5-2: Regional average nitrate versus number of samples > MAV ...... 18 Figure 5-3 Regional average nitrate versus number of samples > ½ MAV ...... 18 Figure 7-1: Nitrate concentration in regional quality monitoring well ...... 24

List of Tables Table 3-1: Hakataramea nitrogen load scenarios ...... 3 Table 3-2: Valley and Tributaries Scenarios...... 4 Table 3-3: Northern Fan Scenarios ...... 5 Table 4-1: Soil drainage dilution factors – Current scenario ...... 12 Table 4-2: Soil drainage dilution factors – Irrigation Conversion scenario ...... 12 Table 4-3: Summary of drinking water use and nitrate data ...... 15 Table 6-1: Uncertainty factors ...... 19 Table 7-1: Model results for Hakataramea catchment ...... 21 Table 7-2: Model results for mid-Waitaki north bank ...... 21 Table 7-3: Model results for mid-Waitaki south bank – surface water influence area ...... 22 Table 7-4: Model results for mid-Waitaki south bank – LSR-dominated areas ...... 22 Table 7-5: Model results for south bank tributaries ...... 22 Table 7-6: Model results for lower Waitaki riverside ...... 23 Table 7-7: Model results for Greater Waikākahi ...... 24 Table 7-8: Model results for Whitneys Creek ...... 25 Table 8-1: Solution assessment results summary ...... 25 Table 8-2: Solution assessment results ...... 27 Table 8-3 Solution package results for CWMS target ...... 28 Table 11-1: Model assumptions and uncertainty ...... 31

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vi Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

1 Introduction Environment Canterbury and the Lower Waitaki – South Coastal Canterbury Zone Committee have been running a collaborative community process to help decide on water quality limits for the proposed Waitaki chapter of the Land and Water Regional Plan (LWRP). This report has been prepared as part of that process, and describes the work undertaken to assess the impact of various nutrient load scenarios on groundwater quality. The Waitaki catchment is the southernmost land in the Canterbury region. It drains a total area of about 12,120 km², which includes the full length of the Waitaki River and all its tributaries. The catchment straddles the boundary between the Canterbury and Otago regions. Most of the catchment land (c. 11,766 km²) falls within the Canterbury region. The remaining c. 354 km² of the catchment area is in the Otago region. On a hydrological basis, the Waitaki catchment is subdivided into upper and lower catchments, which cover 9,657 km² and 2,463 km², respectively. The two catchments connect hydrologically between Kurow and the Waitaki Dam through a bedrock gorge that narrows below Lake Waitaki, marking the division between them. There is no subsurface flow connection between the two catchments because the water-bearing sediments (aquifer systems) in both catchments are physically separate.

1.1 Purpose This report investigates how groundwater nitrate concentrations might change under a series of possible future nutrient load and management scenarios, to help inform the lower Waitaki catchment nutrient limit-setting process. The likely groundwater nitrate concentrations and associated risks to drinking water quality associated with the Solutions Package recommended by the Zone Committee are also evaluated. All nitrate concentrations in this report are presented in terms of nitrate-nitrogen.

1.2 Scope The most common groundwater contaminants associated with agricultural land use are nitrate and faecal bacteria. Phosphorous can also leach into groundwater. This assessment focuses on nitrate because it is the most prevalent groundwater contaminant. We also have models available to investigate the effects of different nitrogen load and management scenarios. Modelling phosphorous and faecal bacteria (which we refer to more broadly as contaminant microbiology in this report) at a sub-regional scale is more complicated, and it was not possible to do this within the time-frames of the upper Waitaki sub-regional planning process. We discuss these contaminants separately in Sections 9 and 10.

1.3 Legal and planning framework The Canterbury Water Management Strategy (CWMS) presents a vision to enable present and future generations to gain the greatest social, economic, recreational and cultural benefits from the water resources in the Canterbury region within an environmentally sustainable framework (Canterbury Mayoral Forum, 2009). The CWMS is the culmination of a process that started in 1999 as the Canterbury Strategic Water Study (Morgan et al., 2002). The proposed Land and Water Regional Plan (pLWRP) provides the regulatory framework to implement the community’s aspirations for water management under the Canterbury Water Management Strategy (Environment Canterbury, 2014). Zone and regional committees are the key delivery mechanism for the CWMS. The CWMS divides Canterbury into 10 zones, each of which has its special committee. In collaboration between the LW community, Lower Waitaki – South Coastal Canterbury (LWSCC) zone committee and the regional council, the LW catchment has been subdivided into freshwater management units as follows (see Figure 3-1):

₋ Hakataramea (899 km²) ₋ Waitaki valley and tributaries (1,007 km²) ₋ Northern fan (203 km²)

Environment Canterbury Technical Report 1 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

2 Why we care about groundwater quality Groundwater is an important source of drinking water in the lower Waitaki catchment, with around 180 drinking water supply wells and springs listed on our database. 15 of these wells feed public or community water supplies; the remainder are private water supplies. During our discussions with members of the community during the limit-setting process, we learned that there are many additional private water supply wells that are not included in our records. Around 75% of the recorded wells in the lower Waitaki catchment are shallow (<30 m deep) and are therefore more vulnerable to contamination than the deeper wells. Increasing the extent and/or intensity of land use generally increases nitrate concentrations in groundwater. It can also increase microbial contamination and phosphorus. Increasing nitrate concentrations are of concern because: • Nitrate can be toxic in drinking water supplies. The New Zealand drinking-water standards set a Maximum Acceptable Value (MAV) for nitrate nitrogen at 11.3 mg/L (equivalent to 50 mg/L of nitrate), based on a risk to bottle-fed babies. Community & Public Health recommends applying this value to bottle-fed babies less than six months old and to pregnant women. More frequent monitoring is required for community water supplies when nitrate concentrations exceed ½ MAV (5.6 mg/L). • Nitrate can also be toxic for aquatic life in groundwater and groundwater-fed streams/rivers, having chronic (not acute) effects on aquatic life at concentrations in the order of 1-10 mg/L depending on species. • Nitrogen (N) is a plant nutrient and contributes to nuisance periphyton and macrophyte growth in streams/rivers, increased algae (phytoplankton) growth in lakes, and associated deterioration of water quality (e.g. dissolved oxygen and pH fluctuations) that can stress ecological values. Nitrate concentrations above the MAV (>11.3 mg/L nitrate-N) makes groundwater unsuitable for drinking by pregnant women and bottle-fed babies unless nitrate treatment plant is installed. Nitrate treatment plant is very costly, both in terms of initial capital expenditure and operating costs. Phosphate is a plant nutrient which is applied as fertiliser to improve growth. It has a similar effect in surface waters, where increasing concentrations can result in excess growth and lead to eutrophication. There are no health-based limits set for phosphate and therefore the main concerns are environmental. Escherichia coli (E. coli) is a common gut bacterium of warm-blooded organisms which is used as an indicator organism for potential presence of pathogens (bacteria, viruses and protozoa). It is present in high numbers in faecal material and therefore indicates faecal contamination. Pathogens from human or animal waste can cause contamination of groundwater and make it unsuitable for drinking. Pathogens can enter groundwater from septic tank discharges, effluent disposal or grazing animals in areas where it can infiltrate from the surface into the groundwater, especially after rainfall or with excessive irrigation. Based on the observations of our groundwater monitoring field team across the region, we estimate that only 10% of private wells have some form of water treatment installed. Because of this, many of the households supplied by shallow private wells are vulnerable to microbial contamination.

2 Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

3 Assessment methodology

3.1 Nitrogen load scenarios A series of land management and nutrient load scenarios were developed by the communities of the Lower Waitaki and Lower Waitaki – South Coastal Canterbury zone committee to guide the load-limit setting process. I have summarised these scenarios in the following tables; full details are provided in Shaw and Palmer (2015). Table 3-1: Hakataramea nitrogen load scenarios

Scenario Scenario components and assumptions Current • Land use and irrigation in the Hakataramea as at December 2014. • All land users assumed to be operating at Good Management Practice Current + • Land use and irrigation and consented land use and irrigation in the Consented Hakataramea as at December 2014. • Assumes all unimplemented consents will be given effect to. • All land users assumed to be operating at Good Management Practice LUC • Assumes that all current and consented irrigation continues as per the Current + Consented scenario at Good Management Practice. • Non-irrigated land in the LUC 7+ class is assumed to continue as per current at Good Management Practice. • Constant leaching rates are assumed for non-irrigated land (excluding that in LUC 7+) in the catchment based on a Land Use Capability (LUC) assessment. 1. Non-irrigated land in LUC classes 1-3 have been assigned an assumed leaching rate of 20 kg/ha/yr 2. Non-irrigated land in LUC classes 4-6 have been assigned an assumed leaching rate of 5 kg/ha/yr Flex cap 15 • Assumes that all current and consented irrigation continues as per the Current + Consented scenario at Good Management Practice. • Non-irrigated land in the LUC 7+ class is assumed to continue as per current at Good Management Practice. • Constant leaching rates are assumed for non-irrigated land (excluding that in LUC 7+) in the catchment based on a Land Use Capability (LUC) assessment. 1. Non-irrigated land in LUC classes 1-3 have been assigned an assumed leaching rate of 15 kg/ha/yr 2. Non-irrigated land in LUC classes 4-6 have been assigned an assumed leaching rate of 8 kg/ha/yr Max Cap 20 • Assumes that all current and consented land use as at December 2014 that is modelled as leaching at a rate greater than 20 kg/ha/yr immediately reduces leaching to a rate of 20 kg/ha/yr. • Assumes that all current and consented land use as at December 2014 that is modelled as leaching at a rate less than 20 kg/ha/yr continues as per the Current + Consented scenario at Good Management Practice. Max Cap small • Assumes that all current and consented land use as at December 2014 flexibility that is modelled as leaching at a rate greater than 20 kg/ha/yr immediately reduces leaching to a rate of 20 kg/ha/yr.

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Scenario Scenario components and assumptions • Assumes that all current and consented land use as at December 2014 that is modelled as leaching at a rate less than 20 kg/ha/yr continues as per the Current + Consented scenario at Good Management Practice.

• In addition 1 kg/ha/yr of additional load is assumed to be applied to all current and consented land use as at December 2014 that is modelled as leaching at a rate less than 20 kg/ha/yr that is located within the Riverside or Flat/ Rolling bands.

Table 3-2: Valley and Tributaries Scenarios

Scenario Scenario components and assumptions Current • Land use and irrigation in the Valley and Tributaries as at December 2014. • All land users assumed to be operating at Good Management Practice Current + • Land use and irrigation and consented land use and irrigation in the Consented + WIC Valley and Tributaries as at December 2014. expansion • Assumes all unimplemented consents will be given effect to. • Assumes that an estimated additional area of 1,768 Ha for the Waitaki Irrigators Collective is implemented • All land users assumed to be operating at Good Management Practice Green Zone Rules • Assumes that all current and consented land use as at December 2014 that is modelled as leaching at a rate greater than 20 kg/ha/yr continues as per the Current + Consented scenario at Good Management Practice. • Assumes that all current and consented land use as at December 2014 that is modelled as leaching at a rate less than 20 kg/ha/yr immediately increases leaching to a rate of 20 kg/ha/yr.

WIC + Flats • Land use and irrigation and consented land use and irrigation in the Valley and Tributaries as at December 2014. • Assumes all unimplemented consents will be given effect to. • Assumes that an estimated additional area of 1,768 Ha for the Waitaki Irrigators Collective is implemented • Assumes that additional irrigation is implemented on all land with a slope less than 10 degrees (additional 2,500 ha irrigation) • All land users assumed to be operating at Good Management Practice

Border to Spray • Assumes that all current non irrigated land use as at December 2014 continues at Good Management Practice. (Northern Fan • Riverside only) Assumes that all current spray irrigation as at December 2014 continues at Good Management Practice. • Assumes that 100 percent of existing Border Dyke irrigation as at December 2014 is converted to spray irrigation.

LUC • Assumes that all current and consented irrigation continues as per the Current + Consented scenario at Good Management Practice. (Northern Fan • Non-irrigated land in the LUC 7+ class is assumed to continue as per Riverside only) current at Good Management Practice.

4 Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Scenario Scenario components and assumptions • Constant leaching rates are assumed for non-irrigated land (excluding that in LUC 7+) in the catchment based on a Land Use Capability (LUC) assessment. 1. Non-irrigated land in LUC classes 1-3 have been assigned an assumed leaching rate of 20 kg/ha/yr 2. Non-irrigated land in LUC classes 4-6 have been assigned an assumed leaching rate of 5 kg/ha/yr

Table 3-3: Northern Fan Scenarios

Scenario Scenario components and assumptions Current • Land use and irrigation in the Northern Fan as at December 2014. • All land users assumed to be operating at Good Management Practice LUC • Assumes that all current irrigation as at December 2014 continues as per the Current scenario at Good Management Practice. • Non-irrigated land in the LUC 7+ class is assumed to continue as per current at Good Management Practice. • Constant leaching rates are assumed for non-irrigated land (excluding that in LUC 7+) in the catchment based on a Land Use Capability (LUC) assessment. 1. Non-irrigated land in LUC classes 1-3 have been assigned an assumed leaching rate of 20 kg/ha/yr 2. Non-irrigated land in LUC classes 4-6 have been assigned an assumed leaching rate of 5 kg/ha/yr Border to spray • Assumes that all current non irrigated land use as at December 2014 continues at Good Management Practice.

• Assumes that all current spray irrigation as at December 2014 continues at Good Management Practice. • Assumes that 100 percent of existing Border Dyke irrigation as at December 2014 is converted to spray irrigation.

GMP Dairy + • Assumes that all current irrigated land as at December 2014 is converted Industrial to dairy land use with a stocking rate of 3 cows per hectare. • Assumes that all non-irrigated land use as at December 2014 is (Whitneys Creek converted to a spray irrigated dairy land use with a stocking rate of 3 only) cows per hectare. • An existing consented industrial discharge accounting for 9 tonnes has been accounted for. • Assumes that an additional ‘aspirational’ industrial load of 31 tonnes is to be discharged in the catchment

Environment Canterbury Technical Report 5 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

3.2 Methodology I have summarised the main components of the method I used to evaluate nitrate concentrations in groundwater as follows: • I divided some of the lower Waitaki freshwater management units into smaller assessment areas. These were generally areas with similar hydrological conditions, as I explain in Section 3.3. • I investigated the hydrogeology and current groundwater quality of each of area, and estimated how much the soil drainage water could be diluted by water from other groundwater recharge sources. I determined a dilution factor for each of the freshwater management units and assessment areas, which I have summarised in Section 4 • I investigated the statistical distribution of nitrate concentrations in our regional database in order to interpret the risk to drinking water quality associated with different groundwater nitrate concentrations (see Section 5). • I used nitrate leaching and drainage modelling to assess soil drainage nitrate concentrations for the various nitrogen load scenarios (Section 6.1). • I used the statistical relationships, modelled soil drainage nitrate concentrations and the dilution factors to estimate the risk that groundwater nitrate concentrations will exceed the MAV for each of the nutrient load scenarios (Section 7) and for the Solutions Package (Section 8). • Finally, I compared the model concentrations to measured concentration as part of a model validation assessment (Appendix 1).

3.3 Freshwater management units I have used the following areas for water quality assessment purposes: • Hakataramea River valley • Valley and Tributaries o Mid - Waitaki north bank and minor tributaries (excluding Penticotico Stream catchment) o Mid - Waitaki south bank and minor tributaries o Mid - Waitaki south bank tributaries o Northern Fan riverside sub-unit • Northern Fan o Greater Waikākahi sub-unit o Whitneys Creek sub-unit I have marked the locations of these catchments on Figure 3-1 below. I broke the Valley and Tributaries freshwater management unit into four separate sub-units based on hydrological characteristics and groundwater usage. I know from experience that groundwater in the north bank of the mid-Waitaki valley is closely connected to the river, so I assessed this area separately. I excluded the Penticotico Stream because I would not expect nutrient loads in this catchment to have a significant influence on the mid-Waitaki north bank area as a whole. Some areas of the mid-Waitaki south bank are also likely to be closely connected to the river. The south bank tributaries were grouped into a single assessment area. This is because, aside from the Maerewhenua River catchment, our records indicate that groundwater usage is very limited in these catchments.

6 Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Figure 3-1: Lower Waitaki groundwater quality assessment areas

Environment Canterbury Technical Report 7 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

4 Hydrogeology and current water quality

4.1 Overview Changes in groundwater nitrate concentrations are expected to be proportional to soil drainage nitrate concentration changes if land surface recharge is the dominant groundwater budget component, and if there is no denitrification in the aquifer. I have undertaken a high-level evaluation of the groundwater budget in different parts of the lower Waitaki catchment using simple calculations and information from previous studies. My evaluation suggests that in many areas land surface recharge (LSR – also referred to as soil drainage water in this report) is likely to be the dominant or only groundwater recharge component. Elsewhere, other components such as irrigation race losses and Waitaki River losses are likely to be important. These latter water sources typically have low nitrate concentrations and can therefore dilute nitrate in soil drainage water. In areas where river and race losses are significant, I used estimates of the ratio between LSR and the low nitrate water budget components to provide a dilution factor for soil drainage water from agricultural land. I have provided estimates of average groundwater nitrate concentrations for different areas of the lower Waitaki zone by applying the dilution factor to the soil drainage nitrate concentration. Scott (2015) reviewed groundwater quality data for the lower Waitaki valley and concluded that anoxic conditions are not widely encountered in shallow groundwater here. Because of this, large-scale denitrification is not expected throughout the catchment. There are localised areas where shallow groundwater could have suitable conditions for denitrification, however. In the following section of the report I look at the hydrogeological and water chemistry data for each of the freshwater management units, and consider the extent to which soil drainage concentrations could reduce within the aquifer due to dilution and denitrification.

4.2 Hakataramea There are currently four active domestic water supply wells in the Hakataramea catchment on the Environment Canterbury Wells database, all of which are shallow (<6 m deep) and located on the river flats (see Figure A2-1 in Appendix 2). Our records also list a spring used for a Small Community Water Supply in the valley. We understand from discussions in the community meetings that there are some additional domestic water supply wells within the catchment, which are not included in our database. Current groundwater nitrate concentrations appear to be fairly low, being generally less than 2 mg/L with a maximum concentration of 3.1 mg/L recorded. Seven wells have been sampled in total. Of the seven wells for which we have dissolved oxygen (DO) data in the Hakataramea valley, we recorded low concentrations (<2 mg/L) in two. Both of these samples showed elevated iron and were noted to have a “swampy” odour, suggesting that groundwater is anoxic here. The wells were located on Menzies Road, close to Cattle Creek and at the junction between Milne Road and Hakataramea Valley Road. The remaining five wells recorded higher DO concentrations, between 4 and 8 mg/L. The presence of anoxic conditions in two locations indicates that local denitrification could occur in some parts of the Hakataramea valley, but we do not have enough data to evaluate either the spatial extent of this or the mass of nitrate that could be reduced. I have therefore made the potentially conservative assumption that none of the nitrogen which leaches from soils in the Hakataramea valley is denitrified Nonetheless, I do not expect denitrification to be a significant factor for the catchment as a whole, and hence this assumption is unlikely to have a significant impact on the assessment results in this report. The local streams and the Hakataramea River itself are believed to gain flow from groundwater overall (Zemansky et. al., 2005). On this basis groundwater nitrate concentrations are generally not expected to be diluted by surface water discharges to groundwater. However, gaugings undertaken in the 1970’s, reported in Fancourt (1976) showed that in times of low flow, the Hakataramea River seeps into the floodplain adjacent to the main channel over a 7 km reach between Rocky Point and Foveran. Groundwater quality in these floodplain deposits here is therefore likely to be influenced by surface

8 Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

water quality. Because nitrate concentrations in the Hakataramea River (median = 0.028 mg/L at Cattle Creek confluence) are lower than groundwater concentrations, groundwater nitrate concentrations are likely to be diluted by river recharge in the floodplain deposits here. Irrigation water from the Waitaki River is brought into the catchment, but I have assumed that this does not provide any additional dilution of groundwater. This is probably a reasonable assumption because water is piped from the Waitaki River, and hence irrigation race losses are expected to be fairly low. The soil drainage rates provided in Mojsilovic (2015) account for the increase in LSR associated with irrigation. I have assumed that groundwater nitrate concentrations are likely to be similar to soil drainage concentrations in this catchment. This assumption is likely to be somewhat conservative for the Haktaramea River floodplain between Rocky Point and Foveran.

4.3 Valley and tributaries

4.3.1 Mid-Waitaki north bank This sub-unit covers the north bank of the Waitaki River between the Waitaki Dam and Stonewall (see Figure 3-1). I have excluded the Penticotico Stream catchment from this area because I do not expect there to be significant groundwater resources here, and there are no active wells on our database. There are 15 active drinking water supply wells in the mid-Waitaki north bank area, all of which are shallow (<30 m deep) and most are very shallow (6 m deep or less). We collected groundwater quality samples from six wells here in 2010, all of which recorded low nitrate concentrations (max 1.2 mg/L; see Figure A2-2 in Appendix 2). I consider that groundwater quality in this area is heavily influenced by the Waitaki River. From my previous (unpublished) investigations in the area I know that the transmissivity of the shallow Quaternary aquifer is very high, and that groundwater level fluctuations closely mirror Waitaki River stage changes. Natural tracer data also suggest that groundwater chemistry is probably dominated by the Waitaki River influence. There is no evidence of denitrifying groundwater conditions in our groundwater quality data from this area. The Current model soil drainage nitrate concentration of 6.5 mg/L (see Table 7-2) is eight times greater than the average groundwater concentration measured in our 2010 water quality survey (0.8 mg/L). Knowing that the aquifer here is very transmissive and thin, the depth to groundwater is low and that the low nitrate water quality samples were collected from shallow wells, I do not expect there to be a significant lag time between land use changes and the associated change in groundwater quality (see Table A1-1 in Appendix 1). I have therefore used the difference between the LUT soil drainage nitrate concentration and measured groundwater concentrations to provide an indication of the dilution ratio between soil drainage water and other (low nitrate) groundwater recharge sources – mainly the Waitaki River. Based on a Waitaki River nitrate concentration of 0.05 mg/L, groundwater in the Mid Waitaki North bank shallow aquifer is estimated to comprise approximately 90% Waitaki River water and 10% soil drainage water on average. Because of this I do not expect changes in nitrate concentrations in groundwater to be proportional to changes in soil drainage nitrogen loads. To be conservative I have assumed that groundwater concentrations are 50% less than soil drainage concentrations.

4.3.2 Mid-Waitaki south bank This sub-unit covers the south bank of the Waitaki River between the Waitaki Dam and Black Point (see Figure 3-1). Our wells database lists 48 drinking water water supply wells within this area, 44 of which are fairly shallow (<35 m deep). Most are very shallow, being between 1 m and 10 m deep. We collected groundwater quality samples from 20 wells here in 2010, 19 of which were from shallow wells. Nitrate concentrations were low, ranging from <0.1 mg/L to 2.3 mg/L, with an average of 0.7 mg/L (see Figure A2-3 in Appendix 2). Of the 17 wells in which we have measured dissolved oxygen in this area, concentrations were < 2 mg/L in two. There were no other indications of anoxic conditions (e.g. elevated iron or manganese) in these wells. I have assumed that denitrification is not a significant factor for groundwater quality in this area.

Environment Canterbury Technical Report 9 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Hydrological conditions are variable within this area, with LSR being the predominant groundwater recharge component in some areas and surface water recharge being a major influence elsewhere. This means that the effects of land use changes will be quite variable. I have split the area up into those parts in which I consider that groundwater quality is likely to be dominated by soil drainage water quality and those where I consider surface water influences to be important. Scott et al. (2012) identified several areas of losses to groundwater from surface water in the mid- Waitaki south bank area, based on evaluation of groundwater chemistry and environmental tracer data. We know from our stream gauging surveys that the Kurow, Otiake and Otekaieke Rivers lose the majority of their flow to groundwater in their lower reaches, so groundwater quality in these areas will be strongly influenced by surface water quality. I have delineated surface water influence areas on Figure 4-1 accordingly. I have also included the land adjacent to the Waitaki River, where I believe that groundwater is closely connected to the river.

Figure 4-1: Mid-Waitaki south bank drinking water supply wells and surface water influence areas Nitrate concentrations were low in the nine samples collected from surface water influence areas in 2010, all being <1 mg/L. The Current model mid soil drainage nitrate concentration in Table 7-3 later in this report is 7.6 mg/L. The lower and upper estimates are 4.7 mg/L and 12.6 mg/L respectively. The lower estimate is 10 times greater than the average groundwater measured in our 2010 water quality survey concentration (0.4 mg/L). As per the north bank area and the information in presented in Appendix 1, I do not expect long lag-times for the mid-Waitaki south bank. I have therefore assumed that the groundwater budget for the surface water influence areas is 50% LSR and 50% low nitrate river water. This may be conservative in some localities, but could overstate the dilution of nitrate in soil drainage water elsewhere. For the remaining areas I assumed that groundwater nitrate concentrations will be the same as soil drainage concentrations. Scott (2015) showed that dissolved oxygen concentrations are generally within the oxic range in this area (>2 mg/L), and iron and manganese concentrations are low. On this basis I have assumed that denitrification is not significant in the mid-Waitaki south bank area.

10 Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

4.3.3 Mid - Waitaki south bank tributaries We have only tested four groundwater samples for nitrate in this sub-unit: concentrations in three of the samples were low (<0.1 mg/L), with a very high concentration of 25.4 mg/L recorded in the fourth (see Figure A2-4 in Appendix 2). We have measured DO concentrations in five wells here, one of which recorded a low concentration (< 2 mg/L). High iron and manganese concentrations and a hydrogen sulphide odour were also recorded for this well (located close to Maerewhenua, downstream of a former gold mining area), all of which suggest that denitrification is likely to occur here. As per the Hakataramea we do not have enough data to evaluate either the spatial extent of this or the mass of nitrate that could be reduced. I have therefore again made the conservative assumption that none of the nitrogen which leaches from soils in the south bank tributaries is denitrified. Our Wells database lists 18 drinking water supply wells in this area, 15 of which are shallow (<36 m), one is deep (90 m) and the remainder are unspecified. There are two public water supply wells here, both of which feed the Duntroon town water supply and are very shallow (<2.5 m deep). Previous work (Scott, 2012) has indicated that groundwater in the area of these wells is influenced by surface water – from the Waitaki River and/or from the Maerewhenua River. This means that groundwater quality in these wells is unlikely to reflect soil drainage water quality entirely. However, we do not currently have enough information for this area to estimate the extent to which the effects of land management changes are likely to be buffered by the influence of surface water recharge. A specific study would be required for this. I have therefore made the conservative assumption that groundwater quality is dominated by soil drainage water quality here.

4.4 Northern Fan The Northern Fan covers an area of approximately 23,000 ha, of which 7,100 ha (roughly 30%) is currently irrigated with border dykes. 30% is spray irrigated and the remaining 40% (mainly in the upper Elephant Hill and Waihuna Stream catchments) is not irrigated. The Morven-Glenavy Irrigation (MGI) company operates two large irrigation schemes in the Northern Fan area: the MGI scheme and the Redcliffs scheme. The Redcliffs scheme is consented to take up to 6 m³/s of Waitaki River water from an intake close to Stonewall; the MGI consent allows for a maximum 14.3 m³/s take from the lower intake site, at Bells Pond. The maximum consented seasonal take rate is 330 M m³/year, which is equivalent to 10.5 m³/s as an annual average. The total annual take rate has only been measured for the last few years, after the annual volume limit was added to consent conditions in 2010. The total take for the 2014-2015 season up to 12/4/15 was ~300 M m³. The 2014-2015 season saw the highest demand in the last 45 years (Andy Guyton, MGI Ltd pers. comms.). The 2013-2014 season was wet and demand was lower than usual: the measured take was 210 M m³. The long-term average water take rate will be somewhere between these two figures, probably in the order of 250 M m³. This is equivalent to 12.6 m³/s over the irrigation season or 7.9 m³/s as an annual average. Water is supplied to irrigators via a series of water races, most of which are unlined and are expected to be losing water. The MGI races are therefore a key hydrological feature of the Northern Fan area. Because the MGI irrigation water comes from the Waitaki River, in which the nitrate concentration is low (roughly 0.05 mg/L), any race losses will dilute nitrate in soil drainage water when it mixes in the aquifer. I have assumed that dilution of nitrate in soil drainage water is only provided by irrigation race losses. Dilution associated with any losses from streams and rivers has been excluded. This assumption is not expected to affect the results of this assessment significantly, however, because the available groundwater contours do not indicate significant groundwater recharge from the Waitaki River to the northern fan (although water exchange in the Riverside area could be significant locally). The other main surface water bodies (Whitneys Creek and Waikakahi Stream) are groundwater-fed. The best available estimate of bulk leakage from the MGI races is 10% of the take (Andy Guyton and Robin Murphy, pers. comms.), i.e. 1.26 m³/s over the irrigation season or 0.79 m³/s as an annual average. Much of the loss is from the main races where they flow along the top of the Waitaki river terrace. Drainage ditches have been dug along the bottom of the terraces to prevent inundation of the adjacent land. MGI Ltd estimate that approximately ⅔ of the race losses are captured through these

Environment Canterbury Technical Report 11 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

drains, which discharge to the Waitaki River (Andy Guyton, pers. comms.). If this is the case, the total race network loss to groundwater would be around 260 L/s. Taking an upper estimate of 15% race losses with 30% captured by surface drainage, the groundwater recharge from the race network would be 830 L/s. A lower estimate of 8% total race losses and 70% capture through surface drainage would give a 190 L/s race loss to groundwater. 80% of the race losses are believed to occur in the network south of Pikes Point – i.e. within the Northern Fan area. North of here the near surface geology comprises Dog Kennel silts, which limit the leakage (Andy Guyton, pers comms). Race losses to groundwater in the northern fan area are therefore estimated to be between 150 and 660 L/s, with MGI’s estimate being 210 L/s (80% of 260 L/s). I used Aqualinc’s Irrigation Requirements Database (Irricalc) to estimate annual average soil drainage rates of 390, 280 and 470 mm/year for the Whitneys Creek, Greater Waikākahi and Riverside areas respectively. The rates were based on weighted average soil profile available water (PAW) data taken from the Landcare Research SMAP database and our irrigation database entries for irrigation methods within the catchment. Taking the information above together with the relative proportions of the MGI race network that falls within each of the water management units, I have estimated the soil drainage nitrate dilution factors in Table 4-1 below. Table 4-1: Soil drainage dilution factors – Current scenario Race losses based on MGI estimate Dilution factor based Proportion of (possible range) Drainage on MGI estimate Catchment total race length m³/s m³/s (possible range) Whitneys 33% 0.07 (0.05 – 0.21) 0.59 0.89 (0.73 - 0.92) Waikākahi 39% 0.08 (0.06 – 0.26) 0.94 0.92 (0.79 - 0.94) Riverside 28% 0.06 (0.04 – 0.19) 0.36 0.86 (0.66 - 0.90) Total 100% 0.21 (0.15 – 0.66) 1.88 0.90 (0.74 – 0.93) We know from our discussions with MGI Ltd personnel that work is underway to reduce losses from the most leaky race sections in the scheme. I have assumed that this will reduce race losses by 10% in the future. This does not change the dilution factors above significantly. For the irrigation conversion scenario we have assumed that 100% of the current border dyke area converts to 80% efficient spray irrigation. This would reduce the dilution factors slightly (i.e. increase the dilution ratio for soil drainage water), as shown on Table 4-2 below.

Table 4-2: Soil drainage dilution factors – Irrigation Conversion scenario Best estimate race losses future Best estimate Proportion of total (possible range) dilution factor Catchment race length m³/s Drainage m³/s (possible range) Whitneys 33% 0.06 (0.05 – 0.2) 0.48 0.89 (0.71 - 0.92) Waikākahi 39% 0.07 (0.05 – 0.23) 0.86 0.92 (0.79 - 0.94) Riverside 28% 0.05 (0.04 – 0.17) 0.28 0.84 (0.63 - 0.88) Total 100% 0.19 (0.14 – 0.6) 1.62 0.90 (0.73 – 0.92) My assessment does not account for Waitaki River losses to groundwater. Although the available groundwater contours do not indicate significant recharge to the Northern Fan aquifer from the river overall, river recharge is likely to be an important groundwater budget component for the land within close proximity of the river (i.e. parts of the Riverside area), particularly at times of high river stage and

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relatively low groundwater levels. This means that the groundwater assessment results could be conservative for parts of the Riverside areas. Further work would be required to investigate whether this is the case, and if so, to what extent. Scott (2015) showed that iron and manganese concentrations are generally low in the Northern Fan area. Although low dissolved oxygen concentrations have been recorded in some wells on a few occasions, we do not expect denitrification to be a significant factor for the Northern Fan as a whole. Lag-times could be quite long in the Northern Fan area, as I explain in Appendix 1. This means nitrate concentrations are generally expected to continue increasing for some time into the future, even if nitrogen loads in soil drainage water reduce. Further information on water quality trends in the Northern Fan area are provided in Etheridge (2015).

4.4.1 Whitneys Creek The Whitneys Creek sub-unit covers approximately 4,600 ha of irrigated land, with approximately 65% border dyke and 35% spray irrigation according to our records. Our Wells database records 13 domestic or stock water and domestic supply wells in this area (see Figure A2-7 in Appendix 2), most of which are shallow (<12 m deep). One well is deep (102 m) and three wells have no depth information. We collected groundwater samples from seven wells in this area in late 2014: the average nitrate concentration in these wells was 4.2 mg/L and the maximum was 6.7 mg/L. Some of these wells were again quite close to irrigation races, and hence the results may incorporate the effects of race leakage dilution. The maximum nitrate concentration recorded in our groundwater quality database for this area is 7.7 mg/L. The Glenavy township, which is located within the Whitneys Creek sub-unit, discharges septic tank effluent to ground. Septic tanks are a recognised source of nitrate in groundwater, and because of this I have reviewed the available groundwater quality data for this area. Environment Canterbury undertook a groundwater quality survey around Glenavy in 1996, which included sample collection from 13 shallow wells. I have plotted the nitrate concentrations recorded during this survey on Figure 4-2 below. The average nitrate concentration in 1996 was 1.9 mg/L. Only one of the wells has been monitored since 1996: well J41/0018, which is part of our long-term groundwater monitoring network. This well has recorded a trend of increasing nitrate concentrations since 1996, with the most recent concentration being 7.7 mg/L. The increasing nitrate trend in this well mirrors that seen in our other long-term monitoring wells in this area (see Scott, 2015), which are located away from Glenavy. It is therefore possible that the rising nitrate concentration relates to agricultural land use. Glenavy has grown in the last 10 years, however, with an extra 26 houses built on a subdivision about 8 years ago. Wastewater is treated locally and discharge to ground (Robin Murphy, pers. comms.). The current town population is estimated at 120-130 people. It is possible that the local increase in nitrate concentrations could be at least partly due to the additional domestic wastewater load. Although further work would be required to evaluate the main driver behind rising nitrate concentrations in well J41/0018, good management of the septic tanks, including regular emptying, is important to minimise impacts on groundwater quality.

Environment Canterbury Technical Report 13 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Figure 4-2: Glenavy groundwater nitrate concentrations

4.4.2 Greater Waikākahi The Greater Waikākahi sub-unit includes the Elephant Hill Stream, Waihuna Stream and Waikākahi Stream catchments, as shown on Figure 3-1 with a total area of 15,700 ha. Our records indicate that approximately 50% of the catchment is irrigated (20% border dyke, 30% spray); the remainder is predominantly dryland farms. There are 30 drinking water water supply wells in the Greater Waikākahi area on our Wells database (see Figure A2-6 in Appendix 2), all of which are listed as either domestic and stock water or domestic supply wells. 21 of these wells are shallow (<35 m deep), two are deep (46 and 95 m) and the remainder have no depth information. There are no community supply wells here. We collected groundwater samples from 11 shallow wells in this area in late 2014 (see Appendix 5). The average nitrate concentration was 4.0 mg/L and the range was 0.23 to 8.4 mg/L. The wells with low concentrations are again located close to the irrigation races. Looking at the wells away from the races, the average nitrate concentration is 5.6 mg/L. Environment Canterbury collects quarterly groundwater samples from three wells in this area, all of which show a trend of increasing nitrate concentrations. Peak annual nitrate concentrations in one of these wells have exceeded the drinking water limit on three occasions in the last 15 years. For further details please refer to Scott (2015). As per the Waitaki Riverside area, I have assumed that groundwater recharge is supplied through LSR and irrigation race losses. Because the Greater Waikākahi area includes the Elephant Hill and Waihuna catchments, in which groundwater is only recharged by LSR, the calculated dilution ratios (Table 4-1 and Table 4-2) are slightly lower than the Riverside area values. The LWSCCS Waitaki Zone Committee asked Environment Canterbury how much of the nitrogen load from the Elephant Hill catchment could end up in the Waikākahi Stream; this information was required to inform their decision-making during the nitrogen load limit assessment process. I reviewed all of the information we have available for the Elephant Hill stream catchment, and used this information to

14 Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

develop a groundwater model of part of the Northern Fan area. My assessment, which is included in Appendix 3, indicates that approximately 80% of the load could travel through the Northern Fan aquifer and into the springs at the headwaters of the Waikākahi Stream. I also used the model to evaluate whether nitrogen from the Waihuna Stream catchment is likely to be transported to the Waikākahi Stream. Model results indicate that this is unlikely to occur. This information was incorporated in our surface water quality assessment for the Northern Fan (see Greer et. al., 2015). I have recommended some additional investigative work in this area to improve our understanding of the local groundwater flow system, which would improve the certainty of our model results.

4.4.3 Lower-Waitaki Riverside This sub-unit covers the riverside strip of the Waitaki River north bank downstream of Stonewall. Although it has been included in the Valley and Tributaries freshwater management zone, the lower-Waitaki riverside area shares its hydrogeological characteristics with the other Northern Fan zones. In particular, it falls within the MGI irrigation area and groundwater quality is likely to be affected by race losses, as per the discussion above. Our records indicate that approximately 50% of the land in this area is currently irrigated using border dykes and 30% by spray irrigation. The remaining 20% is not irrigated. Our Wells database records 15 drinking water water supply wells in this area, two of which feed the Waimate District Council’s Lower Waihao community water supply (see Figure A2-8 in Appendix 2). The remainder are listed as individual domestic or stock water and domestic supply wells. Only one of these wells is more than 20 m deep according to our records. Our groundwater quality monitoring data for this area are limited to three samples collected in 2014 and six samples in 1996. Two of the 2014 samples were from wells located very close to irrigation races (see Appendix 5), and showed low nitrate concentrations (< 1 mg/L). The other sample was collected away from the race network, and recorded a high nitrate concentration of 8.4 mg/L. Two of the 1996 samples were collected from wells located within 200 m of the Waitaki River and recorded nitrate <1.2 mg/L; three were taken from Glenavy township wells and recorded a maximum nitrate concentration of 1.9 mg/L. The other sample was collected from a well in an irrigated area away from the river and races, and recorded 3.1 mg/L nitrate. The average measured concentration from all data is 2.1 mg/L; the average for the 2014 samples is 3.3 mg/L.

4.5 Current conditions summary The mean nitrate-N concentration for the lower Waitaki catchment is 3.1 mg/L. Ignoring the anomalous nitrate results from the south bank tributaries area, the highest concentrations have been recorded in the northern fav water management units (see Table 4-3), where a maximum nitrate-N concentration of 8.4 mg/L has been recorded. Table 4-3: Summary of drinking water use and nitrate data

No of active 1 1 Max nitrate-N Mean nitrate-N No of wells Area drinking water 1 (mg/L) (mg/L) sampled supply wells Hakataramea 4 3.1 0.8 7 Mid-Waitaki north bank 15 1.2 0.8 6 Mid-Waitaki south bank 48 2.3 0.7 20 Mid - Waitaki south bank 18 25.4 Insufficient data 4 tributaries Whitneys Creek 13 7.7 4.2 7 Greater Waikakahi 30 8.4 4.0 11 Riverside 15 8.4 3.3 6 Lower Waitaki total 143 25.4 3.1 61

1 Nitrate data are from a range of well use types, including drinking water supply wells and non-potable use wells (e.g. irrigation).

Environment Canterbury Technical Report 15 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

5 Risk interpretation The drinking water standard (DWS) MAV for nitrate (11.3 mg/L) is a short-term exposure limit – i.e. it should not be exceeded at any time. Because the outputs from the LUT are average nitrate concentrations, we need to consider the distribution of the “population” of nitrate concentration data around the average value. This helps us to understand how the risk of exceeding the drinking water limit changes in association with a changing average nitrate concentration. We do not have enough data for the upper Waitaki catchment to look at the nitrate population distribution. Environment Canterbury monitor nitrate concentrations in over 300 wells across the region each year, however, so we have investigated the distribution of this data set and assumed that this is valid for the study area. Although there is uncertainty around this assumption, we still consider that the regional data provides a useful indication of what a change in the mean nitrate concentration could signify for those people using groundwater for drinking water water supply. Figure 5-1 below is a histogram plot of the nitrate concentrations we measured in approximately 1,200 wells in 2013. The distribution is negatively skewed, with the mean concentration (5.9 mg/L) being greater than the median concentration (4.9 mg/L). Because the distribution is negatively skewed, we would expect the percentage of wells in which nitrate exceeds the MAV to be lower than the proportional difference between the mean nitrate concentration and the MAV. For instance, if the mean nitrate concentration was 5.65 mg/L (i.e. 50% of the MAV) we would expect less than half of the wells sampled to record nitrate concentrations at or above the MAV. The 2013 mean nitrate concentration was 5.9 mg/L, and nitrate concentrations in 15% of the samples were at or above the MAV. I have plotted the average nitrate concentrations recorded in our annual groundwater quality surveys for 1964, 1975, 1980, 1985, 1990, 1995, 2000, 2005, 2010 and 2013 against the proportion of samples which were at or above the MAV in each of those years on Figure 5-2 below. I have plotted a best-fit line through the data, and projected this forward to provide an indication of how the number of wells which exceed the MAV could increase as the average nitrate concentration increases. Because projecting trends well outside of the measured dataset is unreliable, I have added an uncertainty envelope around the best-fit line. The uncertainty envelope comprises Upper and Lower estimates of the relationship between mean nitrate concentrations and the number of wells which are likely to exceed the MAV. The Upper and Lower estimates were derived visually and on a judgement basis, in order to encapsulate the measured data and to make the uncertainty envelope to widen with increasing extrapolation beyond the measured data.

16 Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Figure 5-1: 2013 regional nitrate concentration histogram I have used the regional mean versus % of samples > MAV relationship shown on Figure 5-2 to estimate the percentage of wells in which a single sample could record a nitrate concentrations above the MAV for each of the nutrient load scenarios. This relationship is being used as a guide to inform plausible variability in nitrate concentrations associated with the scenarios modelled within this report. We also looked at the relationship between regional mean nitrate concentrations and the proportion of samples which exceeded 50% of the nitrate MAV (5.65 mg/L), as shown on Figure 5-3 below. We use this relationship later in this report to provide a basis for assessing the Solutions Package against the CWMS drinking water quality target (for all groundwater wells in Canterbury to be below 50% of the maximum allowable value for drinking water by 2040).

Environment Canterbury Technical Report 17 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Figure 5-2: Regional average nitrate versus number of samples > MAV

Figure 5-3 Regional average nitrate versus number of samples > ½ MAV

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6 Nitrate load and soil drainage concentrations

6.1 Methodology Whilst we can measure current groundwater nitrate-N concentrations, we use models to estimate what future concentrations could be. The method used to estimate nitrate-N concentrations in soil drainage water for the lower Waitaki zone is detailed in Mojsilovic (2015). In brief, the method uses data for land-use, soil types, irrigation, rainfall and nitrate leaching estimates in an emended version of the Canterbury Look-up-table [LUT], all expressed in Geographic Information System (GIS) layers that enable spatial analyses and calculations, to produce three modelled outputs: • Recharge (mm/year) • Nitrate nitrogen load (kg/ha/year) • Soil drainage nitrate nitrogen concentrations (mg/L)

The LUT essentially comprises a set of nitrate-N leaching rates for a range of land uses in Canterbury. Further explanation is provided in Mojsilovic (2015) and Lilburne et. al. (2010). The Current scenario assumes that all farming is undertaken to Good Management Practice standards. This is unlikely to be the case, and hence the Current scenario nitrate concentrations are expected to be lower than actual current soil drainage concentrations, for many areas. For the future scenarios a number of assumptions were made about possible land use changes; these assumptions are documented in Mojsilovic (2015). General model assumptions and uncertainty are discussed in Section 11. Because the drainage rates and nitrogen loads are model values with a fairly large uncertainty margin, and because the Current scenario assumes GMP rather than actual current land use practice, I have also taken upper and lower estimates for drainage rates and nitrogen loads to give upper and lower soil drainage nitrate concentrations. These upper and lower values were derived by multiplying the LUT soil drainage rates and nitrogen loads by the factors shown on Table 6-1 below. The factors are not based on quantitative uncertainty analysis because this work has not yet been done for the soil drainage nutrient model and its outputs. The factors are based on our judgement of the magnitude of potential error around the soil drainage nitrate concentrations. The lower estimate allows for greater dilution of nitrogen discharges by higher soil drainage rates; the upper estimate allows for nitrogen leaching loads to be higher than the LUT value, as discussed in Mojsilovic (2015). It is important to understand that the upper and lower estimates reflect uncertainty around the average concentration for the assessment area as a whole, and are based on my judgement of the model error margin. They do not show the full range of groundwater nitrate concentrations that could occur at specific locations or in individual wells. I would like to emphasise that for the reasons I have outlined above, decision-making should consider the full range of potential groundwater concentrations and the associated impact on drinking water and surface water quality, and not focus on the middle value. Furthermore, the reader should pay more attention to differences in concentration values for each of the scenarios than the absolute values resulting from the modelled scenarios. Table 6-1: Uncertainty factors

Irrigation Nitrate concentration outcome Drainage rate factor Nitrogen load factor Dry Lower estimate 1.5 1.0 Spray Lower estimate 1.5 1.0 Border Lower estimate 2.0 1.0

Irrigation Nitrate concentration outcome Drainage rate factor Nitrogen load factor Dry Upper estimate 1.0 1.2 Spray Upper estimate 1 1.2 Border Upper estimate 01 1.2

Environment Canterbury Technical Report 19 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

7 Scenarios assessment

7.1 Assessment criteria The Canterbury Water Management Strategy (CWMS) set the target for average annual nitrate levels in all groundwater wells in Canterbury to be below 50% of the maximum allowable value for drinking water by 2040 (Canterbury Water website – see references section for details). Nitrate levels in community drinking wells should be below the maximum allowable value of drinking water. This target has been modified in some of the CWMS zones to incorporate local aspirations and development objectives. The target outcomes developed by the lower Waitaki SCCS Zone Committee and community included: Safe and secure drinking water is available across the catchments. The Lower Waitaki South Coastal Canterbury Zone Implementation Programme notes that provision of safe and reliable drinking water is a priority consideration for the Zone Committee and the District Councils. The Committee believes that implementation of measures to improve nutrient management will play an important role in improving water quality. The scenarios and Solutions Package assessment for the lower Waitaki catchment is based on relative change – i.e. whether groundwater quality was expected to be better, the same or worse than current water quality, and the associated change in the number of wells in which nitrate could exceed the drinking water limit. Using a relative change approach is useful because it reduces the impact of uncertainty around exact groundwater nitrate concentrations, and allows decision-makers to focus on the relative merits of current versus the scenario and Solutions Package nutrient load limits and management regimes.

7.2 Assessment method I used the soil drainage nitrate concentrations from the model together with the dilution factors in Section 4 to evaluate groundwater nitrate concentrations for each of the nutrient load scenarios. I did this for each of the water management units. I then used the methodology described in Section 5 to estimate the risk of MAV exceedances in drinking water supply wells. I used the lower estimates function on Figure 5-2 with the lower groundwater nitrate concentrations on Table 7-1 to Table 7-8, the mid estimate function with the mid groundwater nitrate concentrations and the upper estimate function with the upper groundwater nitrate concentrations. Where the estimated percentage of wells expected to exceed the MAV is greater than 10%, I have rounded the value to the nearest 5% in recognition of the uncertainty associated with the method. The modelling method provides an estimate of the average groundwater nitrate concentration in each of the freshwater management units, under steady-state conditions. The term “steady state” is used to describe the conditions under which the groundwater system has reached equilibrium with catchment land use, in other words when nitrate concentrations have stopped increasing following land-use intensification. This is discussed further in Appendix 1. My comparison of Current scenario model concentrations to measured nitrate concentrations in Appendix 1 shows that model nitrate concentrations are generally higher than measured concentrations. The reasons for this are discussed in the appendix, and include lag times between land use intensification and the time at which the full groundwater quality effects are seen, potential overestimation of soil drainage nitrate concentrations in the nutrient modelling, the limitations of available groundwater quality data and in some areas the possible underestimation of the effects of surface water losses on dilution of soil drainage nitrate concentrations. It is also important to understand that the assessment considers the long-term cumulative effects of each scenario for the whole assessment area. The local effects of land use intensification will result in groundwater concentrations which are locally much higher than the mean groundwater basin in some places, and much lower in others. Local effects will need to be considered as part of the consent application process for those land use intensification activities which require consent.

20 Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

7.3 Hakataramea Table 7-1: Model results for Hakataramea catchment

Soil drainage nitrate-N Groundwater nitrate-N % of wells > N load Drainage MAV Scenario 2 2 concentration mg/L concentration mg/L T/year mm/year

Mid (lower- Lower Mid Upper Lower Mid Upper upper) Current 523 141 2.9 4.1 4.9 2.9 4.1 4.9 4% (0 - 15%) Current + 558 143 3.0 4.4 5.2 3.0 4.4 5.2 6% (0 - 15%) consented LUC (2aii) 978 183 4.1 5.9 7.1 4.1 5.9 7.1 10% (0 - 25%) Max Cap 20 501 141 2.7 4.0 4.9 2.7 4.0 4.9 4% (0 - 15%) Flex Cap 15 907 144 4.8 7.0 8.4 4.8 7.0 8.4 15% (1 - 35%) Max Cap, 559 143 3.1 4.5 6.7 3.1 4.5 5.4 6% (0 - 15%) small flexibility Based on the results above, I would not expect groundwater nitrate concentrations to change very much between the Current, Current + consented, Max Cap 20 and Max Cap, small flexibility scenarios. Implementation of the LUC (2aii) and Flex Cap 15 scenarios are likely to have an impact on groundwater quality, with the risk that individual wells will exceed the drinking water limit increasing from 4% to 10 - 15% for the mid values, or from 15% to 25 - 35% under the upper values.

7.4 Mid-Waitaki north bank Table 7-2: Model results for mid-Waitaki north bank

Soil drainage nitrate-N Groundwater nitrate-N % of wells > N load Drainage Scenario 2 2 concentration mg/L concentration mg/L MAV T/year mm/year Mid (lower- Lower Mid Upper Lower Mid Upper upper) Current 83 134 4.1 6.5 7.8 2.1 3.3 3.9 1% (0 - 10%) Current + 83 134 4.1 6.5 7.8 2.1 3.3 3.9 1% (0 - 10%) consented Green zone 188 159 8.3 12.4 14.9 4.2 6.2 7.4 15% (0 - 30%) rules (2ai) My results indicate that under the Green rules (2ai) scenario, the overall risk of a nitrate MAV exceedance would increase from negligible at present to 15% (mid results) or from 10% to 30% (upper estimates). Groundwater nitrate concentrations are unlikely to change significantly under the current + consented scenario.

7.5 Mid-Waitaki south bank As I explained in Section 4.3.2, hydrological conditions are variable within this area, with LSR being the predominant groundwater recharge component in some areas and surface water recharge (river losses to groundwater) being a major influence elsewhere. I have summarised model results for the surface water influence and LSR-dominated areas on Table 7-3 and Table 7-4 respectively.

2 Total load and average drainage for whole water management unit

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Table 7-3: Model results for mid-Waitaki south bank – surface water influence area3 Soil drainage nitrate- Groundwater nitrate- % of wells > MAV N load Drainage N concentration N concentration Scenario 2 2 T/year mm/year mg/L mg/L

Lower Mid Upper Lower Mid Upper Mid (lower-upper)

Current 230 119 4.7 7.6 9.1 2.3 3.8 4.6 3% (0 - 15%) Current + 248 129 4.5 7.6 9.1 2.3 3.8 4.5 3% (0 - 15%) consented Green rules (2ai) 363 128 7.5 11.2 13.4 3.8 5.6 6.7 10% (0 - 25%) WIC expansion 254 127 4.8 7.9 9.5 2.4 4.0 4.7 4% (0 - 15%) + flats

Table 7-4: Model results for mid-Waitaki south bank – LSR-dominated areas4 Soil drainage nitrate- Groundwater nitrate- % of wells > MAV N load Drainage N concentration N concentration Scenario 2 2 T/year mm/year mg/L mg/L

Lower Mid Upper Lower Mid Upper Mid (lower-upper)

Current 230 119 4.7 7.6 9.1 4.7 7.6 9.1 20% (0 - 40%) Current + 248 129 4.5 7.6 9.1 4.5 7.6 9.1 20% (0 - 40%) consented Green rules 363 128 7.5 11.2 13.4 7.5 11.2 13.4 35% (10 - 60%) (2ai) WIC expansion 254 127 4.8 7.9 9.5 4.8 7.9 9.5 25% (1 - 40%) + flats Based on the results above, I would not expect the catchment-average groundwater nitrate concentrations to change very much between the Current, Current + consented and WIC expansion scenarios. Implementation of the Green rules (2ai) scenario is likely to have an impact on groundwater quality, particularly in the LSR-dominated areas, which is the majority of this assessment area. The risk that individual wells will exceed the drinking water limit increases from 20% to 35% in these areas (mid values), or from 40 to 60% for the upper estimates.

7.6 Mid-Waitaki south bank tributaries Table 7-5: Model results for south bank tributaries Groundwater nitrate- % of wells > MAV N load Drainage Soil drainage nitrate-N Scenario 2 2 N concentration T/year mm/year concentration mg/L mg/L

Lower Mid Upper Lower Mid Upper Mid (lower-upper) Current 234 162 1.8 2.5 3.0 1.8 2.5 3.0 0% (0 - 5%) Current + 239 163 1.8 2.5 3.0 1.8 2.5 3.0 0% (0 - 5%) consented Green rules 927 176 6.5 9.1 10.9 6.5 9.1 10.9 25% (7 - 50%) (2ai) WIC expansion 239 163 1.8 2.5 3 1.8 2.5 3.0 0% (0 - 6%) WIC expansion 253 165 1.9 2.7 3.2 1.9 2.7 3.2 0% (0 - 8%) + flats

3 Assumes soil drainage nitrate concentrations diluted with 50% low nitrate water from river losses 4 Assumes no dilution of soil drainage water by surface water recharge

22 Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

The results above are similar to the mid-Waitaki south bank LSR-dominated areas: I would not expect groundwater nitrate concentrations to change very much between the Current, Current + consented and WIC expansion scenarios. Implementation of the Green rules (2ai) scenario is likely to have significant impact on the average groundwater quality. The risk that individual wells will exceed the drinking water limit increases from 0 to 25% (mid estimate), or from 5 to 50% (upper estimate).

7.7 Lower-Waitaki Riverside Table 7-6: Model results for lower Waitaki riverside Groundwater nitrate- Soil drainage nitrate-N N load Drainage N concentration % of wells > MAV Scenario 2 2 T/year mm/year concentration mg/L mg/L

Lower Mid Upper Lower Mid Upper Mid (lower-upper)

Current 103 283 6.8 12.6 15.2 4.5 10.9 13.6 35% (0 - 60%) Green rules 104 284 6.8 12.7 15.2 4.5 10.9 13.6 35% (0 - 60%) (2ai) Border to 84 144 13.6 20.3 24.4 8.5 17.1 21.5 60% (15 - >80%) spray The assessment results above indicate that the Current and, Green Rules scenarios are similar in terms of drinking water quality risk, with a mid estimate of 35% chance that nitrate will exceed the MAV in drinking water supply wells in the area. Our groundwater quality monitoring from elsewhere in the region has shown significant increases in nitrate concentrations following extensive border dyke to spray irrigation conversion. I have provided an example of this on Figure 7-1 below, which plots data from a well in the Amuri area. Conversion from border dyke to spray irrigation (around 2005) was followed by a 100% increase in nitrate concentrations over the next 10 years. The results above (and in Table 7-7 and Table 7-8) mirror what we have seen in our field measurements, with soil drainage concentrations increasing significantly between the Current and Border to Spray scenarios. The risk of MAV exceedances increases from 35% to 60% in the Riverside area. We therefore expect a general trend of increasing groundwater nitrate concentrations in the Waitaki Riverside area due to irrigation conversion, although the magnitude of this increase will be reduced if Good Management Practice reduces nitrogen leaching rates significantly. The Lower Waihao community water supply wells are located in an area of extensive border dyke irrigation, according to our database records. Groundwater contour data indicate that flow to these wells is likely to be from the north west, i.e. from beneath the irrigated land of the Northern Fan area. But because the wells are located only 200 m from the Waitaki River, a proportion of the abstraction could also be drawn indirectly from the river. Losses from the MGI race network could also provide dilution of soil drainage water from farmland upgradient of these wells. This situation makes it difficult to assess the effects of the nutrient management scenarios on water quality in the wells without investigating this area specifically. If groundwater drawn from the Lower Waihao wells is predominantly recharged from LSR, my assessment results indicate that border dyke conversion could cause nitrate concentrations to exceed the MAV here. If the Lower Waihao water supply wells do draw-in some Waitaki River water through the river bed/banks and intervening aquifer, the results on Table 7-6 will be somewhat conservative, and the risk of nitrate MAV exceedances will be lower than the results suggest.

Environment Canterbury Technical Report 23 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

WELL N33/0200 Nitrate Nitrogen mg/L

18

16

14

12

10 mg/L 8

6

4

2

0 1-Jan-95 6-Jun-11 9-Sep-08 2-Mar-14 23-Jun-00 14-Dec-05 26-Nov-16 27-Sep-97 20-Mar-03 Figure 7-1: Nitrate concentration in regional quality monitoring well

7.8 Greater Waikākahi Table 7-7: Model results for Greater Waikākahi

Soil drainage nitrate- Groundwater nitrate- % of wells > MAV N load Drainage N concentration N concentration Scenario 2 2 T/year mm/year mg/L mg/L

Lower Mid Upper Lower Mid Upper Mid (lower-upper)

Current 240 159 5.8 9.6 11.5 4.5 8.8 10.8 25% (0 - 50%)

LUC (2aii) 300 197 6.0 9.7 11.7 4.7 8.9 11.0 25% (0 - 50%) Border to 221 126 7.5 9.6 11.2 5.9 10.3 12.6 30% (5 - 55%) spray My results indicate that groundwater nitrate concentrations in the Greater Waikākahi area are not likely to change very much between the Current and LUC nutrient management scenarios. For the LUC (2aii) scenario we assumed that land eligible to move to 20 kg N/ha/year would be spray- irrigated. This assumption means that the additional soil drainage associated with new irrigation counterbalances the load increase, so the effect on water quality is minimal. The net effect of this assumption is that nitrate concentrations do not change much from the Current scenario, although total N load for the catchment would increase. Groundwater nitrate concentrations are expected to increase with irrigation conversion, but the effects of this are averaged-out in this water management unit because it includes large areas of land (mainly the Elephant Hill Stream and Waihuna Stream catchments), which do not have border dyke irrigation. The effects of irrigation conversion on groundwater quality in the lower parts of the Waikakahi Stream catchment will be greater than the results above indicate.

24 Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

7.9 Whitneys Creek Table 7-8: Model results for Whitneys Creek

Soil drainage nitrate-N Groundwater nitrate-N % of wells > N load Drainage Scenario 2 2 concentration mg/L concentration mg/L MAV T/year mm/year

Mid (lower- Lower Mid Upper Lower Mid Upper upper) Current 190 344 6.2 12 14.4 4.5 10.7 13.2 30% (0 - 60%) LUC (2aii) 196 358 6.2 11.9 14.3 4.5 10.6 13.1 30% (0 - 60%) Border to 135 147 12.1 20.0 23.2 8.6 17.7 21.2 60% (15 - >80%) spray GMP Dairy throughout, 242 344 7.9 15.3 17.8 5.8 13.6 16.4 45% (4 - 80%) plus industrial load The Current and LUC scenarios are likely to give similar average groundwater nitrate concentrations according to my assessment results, with both of these scenarios resulting in a risk of MAV exceedances in drinking water wells of somewhere between 6 and 60% when steady-state conditions are reached. The irrigation conversion scenario is expected to increase the risk of MAV exceedances from 30% to 60% and the industrial load would increase the risk from 30% to 45% based on the mid concentration values.

8 Solutions Package assessment The Solutions Package recommended by the Zone Committee allows for nitrogen leaching load increases between 0% (Greater Waikakahi) and 21% (mid-Waitaki south bank). The changing loads are summarised in Table 8-1. Details of the Solution loads are included in Table 8-2. Full details of the Solutions Package are provided in Shaw and Palmer (2015) and the Lower Waitaki ZIP Addendum (2015). I used two criteria for the Solutions assessment. Firstly, I looked at the difference in the risk of nitrate MAV exceedances between the Current scenario and the Solution to provide a qualitative assessment of the solution, as follows: • < 3% increase in risk of MAV exceedance = similar to current • 3-5% increase = slightly worse • 5-10 % = worse • >10% = much worse Table 8-1: Solution assessment results summary Solution N load Solution vs. current relative concentration Water management unit increase change Hakataramea 11% similar to current Mid-Waitaki north bank 15% similar to current or slightly worse Mid-Waitaki south bank 21% slightly worse or worse Mid-Waitaki south bank 19% similar to current or slightly worse tributaries Northern Fan riverside 10% slightly worse or worse Greater Waikākahi 0% similar to current or slightly worse Whitneys Creek 7% slightly worse or worse I have also provided more detailed solution assessment results in Table 8-2. The table generally has three rows of results: the first row, with black font, summarises the number of registered drinking water wells in the area, the Current Scenario and Solution nitrate concentrations (mid values) and the

Environment Canterbury Technical Report 25 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

associated number of wells exceeding the MAV. The second row, in blue font, gives my lower estimates of nitrate concentrations and number of wells exceeding the MAV for the Current Scenario and the Solution. The third row, in red font, gives my upper estimates of nitrate concentrations and number of wells exceeding the MAV. For the Northern Fan, where irrigation conversion is taking place, I have also added a row in green font to provide an indication of groundwater nitrate concentrations for Current scenario land use and for the Solution land use after irrigation conversion has taken place. Results represent stable groundwater nitrate concentrations, which could take many years to occur. Unlike the scenarios assessment, I have not rounded the MAV exceedance estimate to the nearest 5%: I used the qualitative criteria above to capture the uncertainty around the results. There is some risk that nitrate concentrations currently do or will exceed the drinking water MAV under current land use in all of the water management units. None of the scenarios or the final recommended solutions are expected to reduce this risk relative to the Current scenario. The Current scenario is based on the assumption that all farming is undertaken under the Good Management Practice (GMP) guidelines (see Shaw and Palmer [2015] for details). Because current farming practice is unlikely to align with GMP at present, soil drainage nitrate concentrations should decrease if GMP is successfully implemented. If this happens, the difference between present nitrate concentrations and the Solutions package nitrate concentrations will be less than my model results indicate. We have also assessed the solution against the CWMS drinking water target: for all groundwater wells in Canterbury to be below 50% of the maximum allowable value for drinking water by 2040. The assessment results are summarised in Table 8-3 below. Results indicate that a relatively high proportion of wells should meet the CWMS target in the Hakataramea. Mid-Waitaki north bank and mid-Waitaki south bank tributaries, and low proportions in the remaining water management units. The focus for future water quality management effort should focus on these latter management units. Regardless of this ongoing monitoring, education, and in some cases consideration of alternative supplies, may be needed. A monitoring, trigger level and action plan is under development for the lower Waitaki, which could be used for this purpose.

26 Environment Canterbury Technical Report

Environment CanterburyTechnical Report Table 8-2: Solution assessment results Predicting consequences of No. of Current N Current Water Current risk of Solution N Solution Solution risk of drinking source Model management DWS source load average DWS Assessment water load average NO3 unit exceedances (T/year) NO3 mg/L exceedances wells5 (T/year) mg/L Hakataramea 4 4.1 5 % 4.5 6 % Similar to current Best case 523 2.9 0 % 580 3.1 0 % Similar to current Conservative assessment 4.9 16 % 5.4 18 % Similar to current Mid-Waitaki 15 3.3 1 % 3.7 3 % Similar to current north bank 83 96

Best case 2.1 0 % 2.3 0 % Similar to current future Scenarios in Waitaki the Catchment: Lower Waitaki Conservative assessment 3.9 10 % 4.4 13 % Slightly worse Groundwater quality Mid-Waitaki 48 3.8 – 7.6 3 – 20 % 4.3 – 8.7 5 – 24 % Slightly worse

south bank Best case 230 2.3 - 4.7 0 % 279 2.6 – 5.3 0 – 2 % Similar to current

Slightly Conservative assessment 4.6 – 9.1 14 – 38 % 5.2 – 10.4 17 – 45 % worse/worse

Mid-Waitaki 18 2.5 0% 2.9 0 % Similar to current

south bank tribs 234 278 Best case 1.8 0 % 2.1 0 % Similar to current Conservative assessment 3.0 5 % 3.5 8 % Slightly worse

Greater 30 8.8 24 % 8.8 24 % Same as current Waikakahi Best case 240 4.5 0 % 240 4.5 0 % Same as current Conservative assessment 10.8 48 % 10.8 48 % Same as current With irrigation conversion 10.3 30 % 10.3 30 % Same as current

5 Numbers shown are for drinking water supply wells included in our Wells database. We believe that there are additional drinking water wells not in our database. 27

28

No. of Current N Current Water Current risk of Solution N Solution Solution risk of drinking source Model Predicting consequences of future Scenarios in Waitaki the Catchment: Lower Waitaki management DWS source load average DWS Assessment water load average NO3 unit exceedances (T/year) NO3 mg/L exceedances wells6 (T/year) mg/L Whitneys Creek 13 10.7 32 % 11.5 36 % Slightly worse Best case 4.5 0 % 4.9 1 % Similar to current 190 204 Conservative assessment 13.2 60% 14.2 66 % Worse With irrigation conversion 17.7 62% 19.0 67 % Worse Northern Fan 15 10.9 33 % 11.9 37 % Slightly worse riverside Best case 103 4.5 0 % 113 4.9 1 % Similar to current Conservative assessment 13.6 62 % 14.9 70 % Worse With irrigation conversion 17.1 59 % 18.8 66 % Worse

Groundwater quality Table 8-3 Solution package results for CWMS target

Water management unit Proportion of wells expected to meet <50% MAV CWMS target

Hakataramea 40 - 60% Mid-Waitaki north bank 40 - 70% Environment CanterburyTechnical Report Mid-Waitaki south bank 0 - 10%

Mid-Waitaki south bank 60 - 80% tributaries Northern Fan riverside 0 - 10% Greater Waikākahi 0 - 15% Whitneys Creek 0 - 10%

6 Numbers shown are for drinking water supply wells included in our Wells database. We believe that there are additional drinking water wells not in our database.

Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

9 Phosphorus Water quality modelling requires an understanding of, and the ability to quantify, the source, pathway and receptor for contaminants of concern. We need to understand contaminant loads discharged from point and diffuse sources, the pathway by which they are transported through the hydrological system and the processes that affect concentrations along that pathway, and finally the human and environmental receptors at the end of the pathway and their sensitivity to that contaminant. Although we can model phosphorus sources in most of the upper Waitaki catchment, we do not have sufficient understanding or the modelling tools available to allow us to evaluate the transport and attenuation of phosphorus in groundwater. We have therefore made some assumptions about the effects of the different nutrient load scenarios and the solution package on surface water quality. These are discussed in Greer et. al. (2015). Scott (2015) provided information on phosphorus sources and presence in the lower Waitaki catchment and noted that intensive land use may be a source of DRP in shallow groundwater in the lower Waitaki, which may contribute nutrients to surface waters. Wells with relatively high concentrations of DRP (> 0.06 mg/L) are mainly located in areas of very high or high phosphate leaching risk (see Scott, 2015, for details). These areas have shallow, very light soils which have low topsoil retention of phosphorus. Natural sources of phosphorus cannot be ruled out entirely, however; further investigations may be necessary in the future to exclude natural sources. Scott (2015) also provided a phosphate leaching risk map and concluded that controlling land use on the areas of very high leaching risk would give the greatest water quality benefits. Webb et. al. (2007) investigated the potential of soils to leach phosphorus to groundwater in Canterbury, and found that land with high leaching potential was confined to shallow to stony soils formed from alluvium on floodplains, stony Glasnevin soils and soils formed on sand dunes. Because the areas with high phosphorus leaching potentially described above are free-draining soils, they are also likely to have a high nitrate leaching potential. This means that controls on effluent spreading associated with nitrogen load limits will also serve to manage phosphorus discharges to groundwater from this source, to some degree. Phosphorus discharges to groundwater from phosphate fertiliser application will not be controlled by nitrogen load limits, however. Furthermore, land with medium phosphorus leaching potential will not necessarily coincide with areas of medium or high nitrogen leaching potential, so the nitrogen load limit is not expected to provide a comprehensive control on phosphorus discharges.

10 Contaminant microbiology The MAV for E. coli is set at less than one in 100 mL of sample (Scott, 2015). Environment Canterbury has tested water samples from 28 wells in the lower Waitaki for E. coli, 25 of which are in the Northern Fan area. The other three wells were located in the mid-Waitaki area. 27 of the wells were shallow (<25 m deep). Most of these wells have only been sampled once, with nine wells sampled on multiple occasions. I have included a map showing E. coli results from the Northern Fan area in Appendix 5 (Figure A2-9). E. coli counts exceeded the MAV in 16 wells (i.e. approximately 55% of those sampled). Most of the drinking water supply wells in the lower Waitaki zone are shallow. Based on our sampling results, I consider all shallow wells in the agricultural catchments of this zone to be at risk from microbial contamination. Although we have not been able to model the effects of different land management scenarios on groundwater microbiology, I consider that the risk of poor microbial water quality will increase with further land use intensification, especially in areas of light soils. Our groundwater monitoring field staff estimate that only 10% of well owners in the region have some form of treatment installed. This means that increased intensification equates to an increased health risk for shallow well water users. The increased health risks associated with any scenarios where loads increase significantly due to increased stocking rates should be considered carefully. Border dyke irrigation is likely to flush more pathogens into groundwater than spray irrigation, so there should be a reduction in pathogen concentrations in areas where conversion to spray irrigation occurs.

Environment Canterbury Technical Report 29 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

11 Nitrate modelling results context

11.1 The modelling process This report uses water quality modelling tools to provide an indication of the groundwater quality consequences of a range of possible future land-use and land-management scenarios. Anastasiadis et. al. (2013) provided some useful information on why models are used in water quality assessments, some of which I have summarised in the following paragraphs. The processes that determine nutrient loss, transport, and concentrations in groundwater and surface water are complex. This complexity arises from spatial variability of soil, topography and land use; and temporal variability of climate, land management practices and nutrient transport beneath the ground. In the face of such complexity, models can be very valuable. Although expert intuition and the use of proxies are sometimes suggested as alternatives to modelling, these are in fact simplistic informal models. Formal scientific modelling, with its processes of documentation, validation and review (all of which have been undertaken for the modelling included in this report) provides a consistent and transparent approach to understanding the factors that affect water quality and the limitations and uncertainty associated with the projected outcomes. Models can be valuable tools to help us extend our understanding beyond what measurements alone can tell us. Measurements provide values for a specific context or situation, but models enable us to generalize our results to other un-measured situations and predict future trends. Both measurements and models contain uncertainty. Good modelling practice includes accounting for this uncertainty and ensuring that the model results are consistent with observed measurements. The various uncertainties associated with the model used for this study are discussed throughout the report and are summarised and evaluated below. The model predictions of increases in groundwater nitrate-N concentrations in this report have been used by others in the limit-setting process to estimate nitrogen concentrations in rivers and streams (Greer, 2015), and the social and economic assessments (Taylor et. al., 2015) for the lower Waitaki.

11.2 Underlying assumptions and the uncertainty of model predictions The assumptions about the groundwater budget and associated dilution of nitrate in soil drainage water in this report are generally based on anecdotal information provided by members of the local community and my professional experience and judgement. I acknowledge the uncertainty of these estimates, but believe that it is important to account for surface water influences and consider that by doing this the results provide a more reasonable indication of the groundwater quality changes associated with the different nutrient management scenarios. The model used in this groundwater report simplifies reality, but is considered fit for purpose in estimating the potential effects of future land-use change scenarios at a catchment scale. However, it is important for decision makers to understand the limitations and uncertainty associated with model predictions. Knowing whether a model provides a worst-case estimate, and appreciating the extent to which real-life outcomes could be better or worse than model projections indicate, will enable decision- makers to develop robust solutions to the challenges of sustainable resource development. To this end, some of the main uncertainties around the model used in this study are summarised in Table 11-1 below.

30 Environment Canterbury Technical Report

Environment CanterburyTechnical Report

Table 11-1: Model assumptions and uncertainty Predicting consequences of future Scenarios in Waitaki the Catchment: Lower Waitaki Model Impact on model Basis Assumption Uncertainty rating component results Agribase map supplemented by Land Cover Database, Mix of land use is an accurate characterisation of High locally, Medium Land use. DOC boundary and irrigation current and potential future land uses in the lower General uncertainty on catchment scale layer and feedback from local Waitaki area stakeholders. S-map and supplemented by Soils classes are adequately characterised to produce High locally, Medium Soil type. the Fundamental Soil Layers an accurate estimate of drainage and leaching General uncertainty on catchment scale and Land Resource Inventory. potential. Mean annual precipitation classes are adequately Niwa Virtual Climate Station characterised to produce an accurate estimate of (VCS) mapped to same High locally, Medium Climate zone. drainage and leaching potential. VCS interpolation General uncertainty climate categories used in the on catchment scale Groundwater quality between rainfall measurement points does not LUT.

introduce significant error. Scenarios assessment: Soil nitrate Rates in lookup table provide a useful indication of High locally, Medium Look-up table for Canterbury General uncertainty leaching load. (adapted for Waitaki by likely current rates in the lower Waitaki area. on catchment scale Mojsilovic, 2015), developed through consensus by NZ research scientists using a

suite of nutrient simulation Very High locally, Soil drainage models, principally Overseer Rates in lookup table provide a useful indication of High at catchment General uncertainty rate. 6.1 (see Mojsilovic 2015 and likely future rates for the various scenarios considered. Lilburne et al. 2010 and scale 2013). Solution assessment: Overseer 6.1.3 modelling Nitrate Groundwater nitrate concentrations Dilution factors (where used) provide a reasonable High locally, low to concentrations = soil drainage in groundwater. indication of the influence of additional groundwater medium at catchment General uncertainty. water concentrations x recharge components (other than LSR). scale. dilution factor.

31

32

Model Impact on model Basis Assumption Uncertainty rating

component results Predicting consequences of future Scenarios in Waitaki the Catchment: Lower Waitaki Long term average model results are similar to short- term averages. This assumption will not be the case in LUT model results represent Underestimation of reality, with seasonal spikes and troughs seen in the long-term average soil High. peak nitrate measured data. This means that model results do not drainage water quality. concentrations. identify potential short term exceedances of the MAV for nitrate in drinking water. Low-Medium at Overestimation of Denitrification in groundwater Denitrification in groundwater is not significant in catchment scale, nitrate in areas excluded from model. overall results. potentially high where denitrification locally. is significant. Difference between assessment results

and measured Groundwater qua High for all sub-units Concentrations have been groundwater except Northern Fan assessed for the three water Hydrological conditions are consistent within each sub- concentrations in riverside, Whitneys management units, split into unit. individual wells likely Creek and mid-

eight sub-units. to be higher for sub- Waitaki north bank units with variable

Environment CanterburyTechnical Report hydrological

conditions. lity

Number of sampling points sufficient and locations None (used for Medium. Measured data Groundwater samples suitable to represent entire catchment. validation only). (model collected by Environment Low-Medium at validation). Canterbury. None (used for Local point-source nitrate does not bias overall results. catchment scale, High validation only). locally. Possible under- estimation of peak Climate Exclusion of short to medium-term climate variability nitrate if drought and variability/ Excluded from model. and long term climate change does not compromise Low-Medium. storm severity change. model’s usefulness. increases. Broader effects uncertain.

Environment CanterburyTechnical Report

Model Impact on model Basis Assumption Uncertainty rating component results Predicting consequences of future Scenarios in Waitaki the Catchment: Lower Waitaki Under- or over- estimation of nitrate. Additional improvements in Model assumes varying Irrigation Assumptions on irrigation type specified (Mojsilovic, efficiency would irrigation efficiencies based Medium. practices. 2015). reduce N loads but on type of irrigation. increase soil drainage nitrate concentrations, and vice-versa. Groundwater quality

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Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Despite uncertainty regarding the magnitude of change, scenarios involving intensification of land use (and/or a net reduction in recharge) are likely to increase average groundwater nutrient concentrations. Comparison of relative changes in groundwater quality between scenarios can provide a useful means to evaluate the relative merits of specific management options. Particular care should be taken applying results at local spatial scales because the results won’t be accurate at sub-catchment and property scales.

12 Discussion and conclusions Most of the scenarios that we evaluated on behalf of the LWSCCS Zone Committee involved relatively small increases in nitrogen loads compared to Current scenario, and hence the modelled groundwater quality changes associated with these scenarios are also small. The exceptions were the LUC and Flex Cap 15 scenarios in the Hakataramea freshwater management unit, the Green Rules scenario in the Valley and Tributaries unit and the Industrial Load scenario in Whitneys Creek. These scenarios all led to an increase in nitrogen leaching loads, which resulted in an increased risk that drinking water supply wells will not meet the drinking water standard for nitrate. The Solutions Package recommended by the LWSCCS Zone Committee results in no increase in the nitrogen leaching load in the Greater Waikakahi area, an increase of roughly 10% in Whitneys Creek, Hakataramea and Northern Fan Riverside, and a ~20% increase in nitrogen discharges in the mid- Waitaki south bank and tributaries areas. My Solutions Package assessment indicates that average nitrate concentrations in groundwater are not likely to increase in the Hakataramea and Greater Waikākahi areas, may get slightly worse in the mid-Waitaki north bank and south bank tributaries areas, and are likely to increase in the mid-Waitaki south bank, Northern Fan Riverside and Whitneys Creek areas. However, it should be noted that these changes are all relative to the Current scenario, which assumes all farming is undertaken under the Good Management Practice (GMP) guidelines. If actual farm management at present results in higher nitrogen leaching than GMP, and if GMP is successfully implemented, the nutrient load increases predicted under the Solutions package relative to the Current scenario will be reduced. Nitrate concentrations in groundwater in the Northern Fan area are currently increasing, probably because of conversion from border dyke to spray irrigation. As a result, the number of drinking water supply wells in which water quality does not meet the drinking water standards is likely to increase here.

13 Recommendations The recommended management approach for the cumulative effects of agricultural land use on groundwater quality in the lower Waitaki is through load limits and the associated management framework (e.g. Farm Environment Plans). Because the load limits are based on modelling, a monitoring, trigger level and action (MTA) plan is being developed for the lower Waitaki catchment. The Solutions Package does not allow for additional land intensification in the Elephant Hill and Waihuna Stream catchments. This is because nitrogen leaching from these catchments could potentially end up in the spring system which feeds the Waikakahi Stream. I recommend that further work should be undertaken to investigate this linkage. I have provided an outline scope for this investigation in Appendix 3. Drilling deeper wells is often suggested as a remedy for nitrate contamination, but this is often only a short-term solution. Deeper wells often have lower nitrate only because the groundwater at depth is older and has not yet been affected by recent land use changes. Over the long-term, deep groundwater quality can become similar to shallow groundwater quality, since deeper groundwater is often recharged from the shallower aquifer. It is only in those areas where the deep aquifer is recharged by a low nitrate source, such as alpine rivers and rainfall in low-intensity hill country

34 Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

catchments, or where denitrification occurs at depth, that sustainable high quality deep groundwater is likely to be available. Because of this, I do not recommend that drilling deeper wells should be assumed as a viable long-term management option to achieve nitrate quality objectives for the lower Waitaki catchment. The risk of microbial contamination in properly constructed deep wells is much lower than in shallow wells, and hence drilling deeper is expected to improve microbial water quality. But water treatment may be a more cost-effective approach for management of this risk. Nitrate concentrations in our monitoring well in Glenavy are trending upwards, and it is not clear whether this is due to agriculture or local septic tank discharges. A groundwater quality investigation could be undertaken in this area to try to confirm the cause of the nitrate increases.

14 References Canterbury Water website: http://ecan.govt.nz/get-involved/canterburywater/targets/Pages/drinking- water.aspx Canterbury Land and Water Regional Plan, Schedule 1 Chapelle F. H., Campbell B. G., Widdowson M. A. and Landon M. K. 2013. Modelling the long-term fate of agricultural nitrate in groundwater in the San Joaquin Valley, California. Intech Open Science Close M. 2013. Critical review of contaminant transport through the vadose zone. Environment Canterbury Report No. R10/113 Environment Canterbury, 2014, ‘Canterbury Land and Water Regional Plan’, 14 January 2014. Fancourt, T, (1976) The resources and usage of water in the Hakataramea River catchment, Waitaki Catchment Commission and Regional Water Board Greer, M.; Clarke, G.; Gray, D. 2015: Waitaki limit setting process. Predicting consequences of future scenarios: Surface water quality and associated values. Environment Canterbury Technical Report. Hesketh, N and Brookes, P (2000) The leaching of soil phosphorus: a hundred years of getting it wrong, Proceedings of BSSS Conference, Eurosoil 2000 4-6 September 2000, University of Reading Heller T. (2015) External peer review comments for this report. Landcare Research (2014), S-map, GIS layer, Accessed February 2014. Lilburne, L., Webb, T., Ford, R., Bidwell, V., (2010 + updated in 2013): Estimating nitrate-nitrogen leaching rates under rural land uses in Canterbury. R10/127 and R14/19, Environment Canterbury. McDowell R., Cox N., Daughney C., Wheeler D., and Moreau M. 2014. A national assessment of the potential linkage between soil and surface and groundwater concentrations of phosphorus. Journal of American Water Resources Association Ministry of Health (2008) Drinking-Water Standards for New Zealand 2005 (revised 2008). Published by the New Zealand Ministry of Health, Wellington. Mojsilovic, O. 2015: Generation of nitrogen loss estimates in the Waitaki catchment. Environment Canterbury Technical Report. Morgan, M., Bidwell, V., Bright, J., McIndoe, I., Robb, C. 2002: Canterbury strategic water study. Lincoln Environmental Report No 4557/1, Lincoln University, New Zealand. Opus 2012. Ashburton District Council Water Investigation Project Redding, M, Ghani, A, Kear, M, O’Connor, M and Catto, W (2006) Phosphorus leaching from pastures can be an environmental risk and even a significant fertiliser expense, AgResearch and Ballance Agri-nutrients, Proceedings of the New Zealand Grassland Association 68: 293–296.

Environment Canterbury Technical Report 35 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Rosen M.R., 2001. Hydrochemsitry of Groundwater In Groundwaters of New Zealand. Edited by Rosen, M.R. and White, P. For the New Zealand Hydrological Society. ISBN 0-473-07816- 3. Scott L. 2013. Lag times in the Hinds catchment. Environment Canterbury memo dated 31/7/2013 to Don Vattala referenced WATE/INGW/QUAL/INVE/5 Scott L., Hanson C., and Cressy R. 2012. Groundwater Quality Investigation of the mid-Waitaki valley. Environment Canterbury Report No R12/71 Scott M. 2015. The current state of groundwater quality in the lower Waitaki. Environment Canterbury report No R15/41 Selwyn Waihora Zone Implementation Programme 2011. Shaw, H.; Palmer, K. 2015: Waitaki limit setting process. Predicting consequences of future scenarios: Overview report. Environment Canterbury Technical Report. Taylor, N., S. Harris, W. McClintock, and M. MacKay. 2015 Lower Waitaki Limit Setting Process: Social-Economic Assessment. Environment Canterbury Technical Report. URS 2003. Project Aqua – hydrogeological assessment of effects. Prepared for Meridian Energy Ltd. Volume 1 of 2 - main report Weissmann G. S., Zhang Y., LaBolle E.M., and Fogg G.E. 2002. Dispersion of groundwater age in an alluvial aquifer system. Water Resourxes Research Vol 38 No. 10. Zemansky G., White P., Barrell D., McDowell R., Norton N., Rouse H., and Miller M. 2005. Water quality impacts from irrigation development in the Hakataramea River catchment. Environment Canterbury Report No E05/13.

15 Acknowledgements I would like to thank Robin Murphy and Andy Guyton of Morven Glenavy Irrigation for their invaluable knowledge of the irrigation scheme and local groundwater system. Ognjen Mojsilovic provided technical inputs to this report and Carl Hanson, Lisa Scott, Hisham Zarour, Helen Shaw and Kelly Palmer provided useful comments. I am also grateful to Tom Heller for carrying out an independent external science review.

36 Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Appendix 1: Model validation

Background Although we have confidence in the reliability of our assessment when used to assess the relative differences in nitrate results between the Current scenario and the other nutrient load scenarios and solution package, our confidence over the absolute nitrate concentrations presented in this report is lower. Model validation, or “truthing” against real/measured data, is often undertaken to assess whether model results align with what is actually seen in the environment. I have compared Current scenario model results (see Section 7) against measured groundwater nitrate concentrations in this section to provide an indication of the accuracy of the model results. Before comparing model results to measured data, however, it is important to think about what the model results and the measured data actually represent. The modelling method used for this study provides an estimate of the average groundwater nitrate concentration in each of the water management units, under steady-state conditions. We know from our monitoring data that nitrate concentrations are still increasing in some parts of the lower Waitaki zone – i.e. they have not reached steady-state. In other areas we do not have enough data to assess this. I have made a rough estimate the potential lag-time between nitrate leaching from the root zone and arrival at monitoring wells in different parts of the catchment in Appendix 4. I used simple calculations to do this because we do not have groundwater age data for most of the lower Waitaki area. Results (Table A1-1 below) indicate that lag-times could be up to 260 years and 85 years for the Hakataramea catchment and Northern Fan area respectively. Recent land use change has occurred in both of these areas, and hence the recently measured nitrate concentrations are unlikely to represent steady-state conditions. Lower lags are expected in the mid-Waitaki north and south bank areas, where the aquifer transmissivity is generally high and Waitaki River water exchange with groundwater is likely to increase the aquifer through-flow rates significantly. If land use and management practices in these areas have been reasonably consistent for the last few years, recently measured nitrate concentrations could be somewhere close to steady-state conditions. I have not considered lag times in the mid-Waitaki tributaries because they will be highly variable here. Table A1-1: Lag-time estimates (update with new Overseer version LSR-based values)

Lag time (years) Catchment Lower estimate Upper estimate Hakataramea (well I40/0033) 15 260 Mid-Waitaki north bank (well I40/0145) <1 7 Mid-Waitaki south bank (well I40/0675) <1 7 Northern Fan (well J41/0003) 5 85

Model and measured nitrate concentrations I have compared Current Scenario model nitrate concentrations to measured values on Table A1-2 below. Model concentrations are generally higher than measured values, for reasons I discuss below.

Environment Canterbury Technical Report 37 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Table A1-2: Measured and Current Scenario model nitrate concentrations Current scenario model average Measured nitrate (mg/L) Area nitrate (mg/L) Average Max Lower Mid Upper 1 1 Hakataramea 0.7 3.1 2.9 4.1 6.2 Mid-Waitaki north bank 0.8 1.2 2.1 3.3 5.3 Mid-Waitaki south bank – 0.5 0.7 2.4 3.8 6.3 surface water influenced Mid-Waitaki south bank – 0.8 2.3 4.7 7.6 12.6 LSR-dominated Mid-Waitaki south bank 6.41 25.41 1.8 2.5 3.7 tributaries 1 1 Northern Fan Riverside 3.1 8.4 4.5 10.8 19.9 Greater Waikakahi 4.0 8.4 4.6 8.8 15.1 Whitneys Creek 4.2 7.7 4.5 10.7 20.7 Notes: 1. These results are based on very few samples and hence average and max may not be representative. As noted above, measured groundwater nitrate concentrations are unlikely to be in equilibrium with current land use in many of the water management units. We know from our regular monitoring that groundwater nitrate concentrations are still increasing in the Northern Fan area. Most of the land use intensification happened many years ago, but large areas of land have converted to from border dyke irrigation in recent years, so the increasing nitrate concentrations we see in our monitoring data probably relate to this. Aside from lag times, there are other reasons why model results would not be more similar to the measured values: • The model could be under-estimating land surface recharge rates or over-estimating nitrogen leaching rates. Soil drainage rates are generally higher in Version 6.2 of Overseer, but nitrate leaching rates are correspondingly higher (i.e. soil drainage nitrate concentrations are similar), so this is unlikely to be the only reason for differences. • The measured concentrations are based on very few wells in some of the water management units; a well with very high or very low concentration can significantly impact on the measured average (as per the mid-Waitaki south bank tributaries area). • We do not have recent groundwater quality measurements for much of the lower Waitaki zone, and current concentrations may be higher than the measured values shown on Table A1-2. • Our modelling does not allow for attenuation in the system (i.e. denitrification). Although the water quality data indicate that this is unlikely to be a significant factor overall, it could explain part of the difference in some areas. • I have accounted for the influence of other recharge sources (e.g. river losses to groundwater), but these are poorly quantified and some of the difference between model and measured nitrate concentrations can be attributed to this.

38 Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Appendix 2: Catchment maps

Figure A2-1: Hakataramea catchment

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l Reportl Figure A2-2: Mid-Waitaki north bank catchment

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chment: Lower Waitaki

Figure A2-3: Mid-Waitaki south bank catchment 41

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l Reportl Figure A2-4: Mid-Waitaki south bank hill country catchment

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chment: Lower Waitaki

Figure A2-5: Maerewhenua River catchment 43

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l Reportl Figure A2-6: Greater Waikakahi catchment

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chment: Lower Waitaki

Figure A2-7: Whitneys Creek catchment 45

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l Reportl Figure A2-8: Lower Waitaki north bank riverside area

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chment: Lower Waitaki

Figure A2-9: E coli data for Northern Fan area 47

Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Appendix 3: Elephant Hill nitrogen transport assessment

48 Environment Canterbury Technical Report Memo

Date 26/5/2015 To Kelly Palmer, Helen Shaw CC Michael Greer From Zeb Etheridge

Subject: Elephant Hill nitrogen load fate and transport assessment

1. Introduction

The purpose of this document is to evaluate where nitrogen discharges to the Elephant Hill Stream catchment are likely to end up. I have reviewed hydrological, geological and hydrogeological information and used this information to develop a conceptual model of the Elephant Hill/north bank Waitaki River surface water and groundwater system. I have used my conceptual model as a basis for construction of a groundwater model of the Waitaki River north bank in the area of the Elephant Hill Stream. The groundwater model provides insights into the transport and fate of nitrogen discharged in the Elephant Hill Stream catchment.

2. Background

Environment Canterbury and the Lower Waitaki Zone Committee have been running a collaborative community process to help decide on water quality and quantity limits for the proposed Lower Waitaki chapter of the Land and Water Regional Plan. Setting nutrient load budgets is one of the tools being used to help protect water quality, and one such nutrient budget is currently being determined for the Elephant Hill stream catchment.

The Elephant Hill stream discharges to the north bank of the Waitaki River, and a number of springs are located immediately down gradient of the discharge zone. These springs (which I refer to as the Waikakahi springs in this report) feed into the Waikakahi Stream.

Environment Canterbury collected monthly water quality samples from four of the springs on the north bank Waitaki River flats between September and December 2014. We recorded mean nitrate-N concentrations ranging from 3 to 6 mg/L, and a maximum concentration of 6.7 mg/L.

The amended National Policy Statement (NPS) for Freshwater Management was released by central government in July 2014. The NPS provides a National Objectives Framework to assist regional councils and communities to more consistently and transparently plan for freshwater objectives. The NPS sets National Bottom Lines for two compulsory values: ecosystem health and human health for recreation, and minimum acceptable states for other national values. The National Bottom Line median and 95th percentile nitrate-N

Page 1 of 27 concentrations are 6.9 mg/L and 9.8 mg/L respectively (National Policy Statement for Freshwater Management 2014).

Because nitrate concentrations in some of the Waikakahi springs are close to the National Bottom Line median concentration, the community have asked whether the elevated concentrations relate to recent land use intensification in the Elephant Hill stream catchment. This information is required to help understand the likely effects of future land use intensification in the catchment, and to inform the nutrient load limit-setting programme.

Greer (2015) used surface water quality data from the Waikakahi Stream springs to assess the importance of the Elephant Hill Stream catchment nitrogen load contribution on Waikakahi Stream water quality. His results suggested that groundwater from the Elephant Hill area discharges into the springs on the flat land immediately north of the Waitaki River. Although instream nutrient loads were not quantified, Greer concluded that the discharge of nitrogen and phosphorus from the springs is likely to be a significant contributor to observed nitrogen enrichment in Waikakahi Stream. Because the Greer study was a qualitative assessment, further work was required to provide a more comprehensive assessment of the source of nitrate in the stream.

3. Study area

The area of interest for my study covers the upper Elephant Hill Stream catchment and the north bank Waitaki River flats between Fairway and the Whitneys Creek headwaters. The Waitaki River flats in this area are bordered to the north by a series of small hill country catchments, including the Redcliff Stream catchment, Ryde Stream catchment and upper Waikakahi Stream catchment. I have marked the study area and some of the hydrological features on Figure 1 below.

4. Location and climate

The upper Elephant Hill Stream drains east from the southern end of the Hunter Hills onto the Lower Waitaki Plains. The catchment area comprises roughly 5,600 ha of hill country to the north of the Waitaki River (see Figure 1). The ground elevation falls from 500 – 600 m above sea level (asl) at the top of the catchment to 100 m at the lower end, where the stream discharges to the north bank Waitaki River flats.

Zarour (2015) analysed rainfall data for the Lower Waitaki zone and classified the upper Elephant Hill Stream catchment as moist sub-humid. The lower catchment was classified dry sub-humid. The isoheyatal map presented in Zarour (2015) indicates mean annual rainfall of 625 mm and 575 mm in the upper and lower Elephant Hill Stream catchment respectively.

Meteorological data from URS (2003) summarised in Table 1 below indicates that mean annual evapotranspiration is significantly greater than annual rainfall, and hence the estimated recharge rate for non-irrigated land is very low (approximately 50 mm/year).

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Figure 1 Study area showing main drainage features

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Table 1 Average monthly rainfall, potential evaporation and soil drainage (URS, 2003)

Annual Station Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total

Duntroon rainfall (mm) 59 47 52 41 39 36 41 37 38 45 49 64 548

Tara Hills potential 140 130 97 75 45 30 29 44 69 98 117 145 1,019 evaporation (mm)

Calculated soil drainage with 1 2 − 6 − 3 1 12 14 9 2 − 5 54 no irrigation (mm)

5. Nutrient loads and soil drainage

Nutrient loads and soil drainage rates in the Lower Waitaki zone were estimated by Mojsilovic (2015) through GIS modelling. I summarise the GIS modelling results as follows: • Current Elephant Hill Stream catchment nitrogen load: 47,650 kg/year, equivalent to 8.5 kg/ha/year. • Elephant Hill catchment weighted mean soil drainage (irrigated and dry land): 0.13 m/year, equivalent to 20,000 m³/d or 230 L/s over 5,600 ha catchment. • The Elephant Hill soil drainage and nutrient load estimates equate to a mean nitrate- N groundwater concentration of 6.5 mg/L • Waitaki River flats mean soil drainage 0.25 m/year, equivalent to 35,000 m³/d or 400 L/s over 5,100 ha catchment.

Woodward et al (2013) note that groundwater is the dominant flow path for transport of dissolved contaminants to surface waters draining a catchment. Their study of a downland and alluvial plain stream catchment in the Waikato indicated that approximately 5% of the total surface water nitrogen load was derived from near surface flow. The remainder of the nitrogen load in the stream was found to come from groundwater flow. I will discuss the relevance of this information later in this report.

Scott (2014) concluded that most groundwater nitrate in the lower Waitaki zone is likely to remain in shallow groundwater before it discharges to surface water. She considered that anoxic conditions are not widely encountered in shallow groundwater in the lower Waitaki. I have assumed for this study that no denitrification occurs in groundwater.

1 Assumes average soil moisture holding capacity of 90 mm and an evaporation/evapotranspiration ratio of 1 (URS, 2003).

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6. Geology and hydrogeology

Elephant Hill Stream and hill country catchments

The published geological map for the Lower Waitaki area (Forsyth GNS QMAP Sheet 19, 2001) indicates that the Elephant Hill catchment is underlain by Rakaia Terrane in the upper 4,000 ha of the catchment and by Elephant Hill Gravels, Otakou Group (Mount Harris Formation) deposits and Quaternary alluvium and fan deposits in the lower catchment (Figure 2).

The geological map shows that the small hill country catchments to the east of Elephant Hill are underlain by Elephant Hill Gravels. The Rakaia Terrane rocks are typically quanzofeldspathic sandstone and mudstone or argillite with sandstone generally dominant (Forsyth 2001). The Elephant Hill Gravels consist of alluvial coarse sands and well-bedded gravels that are composed chiefly of weathered greywacke approximately 30 m thick. The matrix of the gravels consists of rusty coarse sands (Riddolls, 1966). Forsyth (2001) describes the underlying Otakou Group (Mount Harris Formation) as blue-grey siltstone, sandstone and carbonaceous mudstone in shallowing-up sequences, up to 200 m thick.

The Environment Canterbury Wells database has strata logs from five wells within the upper Elephant Hill Stream catchment. I have summarised these records in Table 2 below. The locations of these wells are shown on Figure 2.

Wells J40/0463, J40/0692 and J40/1010 are located within 2 km of one another and within 500 m of Elephant Hill Stream. Both J40/0692 and J40/1010 were drilled through the Quaternary alluvium and were screened in the Elephant Hill Gravels. These wells recorded groundwater at 25 – 40 m depth at the time of drilling. There are no peat deposits recorded in any of these well logs. Table 2 Elephant Hill catchment well records

Well No Geology Notes

J40/1079 Silt (Colluvium) to 3.5m, sandstone Initial water level 7.4 m depth, 508 m asl Sep (Rakaia Terrane) to 100 m 2009. Screen 18 - 25 m and 90 to 100 m depth.

J40/0463 Sandstone to 32 m, shingle and Initial water level 25 m depth, 137 m asl May sand to 40 m (Otakou Group) 1956

Purpose = Monitoring well. Well record notes presence of spring at NZTM 1430081, 5035176 (approximately 145 - 150 m asl) with flow of several thousand gallons per hour (e.g. 3-6 L/s).

J40/0692 Clay to 7 m, gravel to 58 m, silt to 82 Initial water level: 37 m depth, 98 m asl Sep m, gravels and sands to 150 m. 2002

QMap: Quaternary alluvium, Yield 33 L/s for approximately 50 m drawdown, Elephant Hill Gravels. Wells screen in gravel at 116 to 142 m depth. Specific Database lists aquifer as Kowai capacity = 0.52 L/s/m. Empirical BalT Formation (Elephant Hill Gravels) transmissivity = 200 m²/d

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Well No Geology Notes

J40/1010 Gravel to 8 m, clay & clay-bound Initial WL: 35 m depth, 113 m asl July 2007 gravel to 103 m, gravel to 107 m, clay-bound gravel and mudstone to Yield 50 L/s for 60 m drawdown. Specific 132 m, gravel and sand to 144 m capacity = 3 L/s/m. Empirical BalT transmissivity = 900 m²/d QMap: Quaternary alluvium, Elephant Hill Gravels. Wells Screen in gravel at 129 – 137 m depth Database lists aquifer as Kowai Formation (Elephant Hill Gravels) Water levels Oct 09 and May 2010 45 m and 106 m depth respectively

J40/0502 Clay to 1.8 m, boulders to 2.8 m No water level data (dry?) Jan 1923

North bank Waitaki River flats

The geological map shows that the near surface material of the north bank Waitaki River flats is undifferentiated Late Quaternary river deposits (Q4a). I have reviewed stratigraphic data from seven wells in this area and summarised the data into the general stratigraphic sequence in Table 3 below. Table 3 Waitaki River flats stratigraphy

Depth m Strata Description

0 to 15 Quaternary river gravel Gravel, sand and silt of low river terraces with patchy loess cover in places (thickness range: 6-32)

15 to 40 Elephant Hills Gravel Alluvial coarse sands and well-bedded gravels (thickness range: 11-47)

40 to >130 Otakou Group Blue-grey siltstone, sandstone and carbonaceous mudstone Specific capacity data are available for five wells within the study area. I used these data to provide a rough estimate of transmissivity (see Table 4.) using the following empirical equation presented in Bal (1996):

Where T = transmissivity and SPC = specific capacity. Because the more permeable sand and gravel deposits in this area are frequently interlayered with silt and clay, confined groundwater conditions are likely to be found at fairly shallow depths. The same is true in much of Canterbury, and because of this we usually assume that aquifers below 10 m depth are confined or leaky-confined, and shallow aquifers less than 10 m deep are unconfined.

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Figure 2 Geology of the Elephant Hill Stream catchment

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Table 4 Well log data for north bank Waitaki River flats

Well No Screen Strata Specific Transmissivity K (m/d)2 interval (m capacity (m²/d) bgl) (L/s/m)

J40/0620 85-89 Otakou Group – 1.13 333 83 White Rock Coal Measures?

J40/0631 51-53 Otakou Group – 0.49 150 75 White Rock Coal Measures?

J40/0965 35-39 Kowai Formation - 4 1129 282 Elephant Hill Gravels

J40/0989 25-27 Kowai Formation - 0.04 13 7 Elephant Hill Gravels

J40/1031 41-45 Kowai Formation - 0.06 19 5 Elephant Hill Gravels

Groundwater flow

URS (2003) interpolated piezometric contours from water levels recorded in 96 Waitaki valley wells and pits in June 2001. I have reproduced these contours on Figure 3.

Information provided in the URS report indicates that the vast majority of the 96 wells and pits were located to the north west of the study area. Only three were within the current area of interest, and these were located close to the Waitaki River. These three wells would have been insufficient to interpolate the contours presented in the report. It appears that the groundwater contours for the Waitaki River north bank below Fairway (the western limit of the study area) were based on contour drawings in MacFarlane (1988), which were interpolated from groundwater levels recorded in 1980 and 1985 – 1986. These earlier contours seem to have been based on five or six monitoring locations within the study area. This provides a fairly sparse coverage of the study area, and because of this I consider that there is some uncertainty around the interpolated groundwater levels. Nonetheless, the contours do provide a useful indication of the overall groundwater flow pattern.

The groundwater contours indicate a general south easterly flow direction, parallel to the Waitaki River. The contours indicate that groundwater discharging from the upper Elephant Hill Stream catchment is likely to flow towards both the Waikakahi Springs and Waitaki River. But the resolution of the contours is insufficient to draw any firm conclusions on the transport and fate of nitrogen discharges from the Elephant Hill Stream catchment.

2 Assumes horizontal flow into well screen – no vertical flow

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7. Groundwater takes

The only active groundwater take within the Elephant Hill catchment on the Environment Canterbury consents database is Consent CRC074131. This consent permits a combined abstraction of up to 6,307 cubic metres per day and 946,080 cubic metres between 1st July and the following 30th June each year, from wells J40/0692 and J40/1010. This equates to a combined annual average abstraction of 30 L/s. Water usage records for 2012-2013 show that approximately 500,000 m³ of water was abstracted (approximately 50% of the allocation); records for 2013-2014 show 903,847 m³ of abstraction (approximately 95% of the allocation). If 70% of the allocation limit is abstracted on average, and applied with a 70% irrigation efficiency, this equates to removal of 15 L/s flow on average from the Elephant Hill Stream catchment, ignoring the increase in rainfall recharge associated with irrigated land.

There are 12 active groundwater takes on the north bank Waitaki River flats within the study area, with a combined maximum consented abstraction rate of 500 L/s. The estimated annual allocation for these takes is 4,429,082 m³ in total, equivalent to 140 L/s. Water use records are available for five of these takes, and show usage of between 3% and 70% of the estimated annual allocation. Based on actual use records where available and an assumed usage of 75% of the allocated rate where no water use records are available, I have estimated the total groundwater take rate from the north bank Waitaki River flats part of the study area at 50 L/s (averaged over a calendar year). Of this, 30 L/s is from consent CRC000945, located in the vicinity of the Thorntons/Tawai Ikawai Road intersection. This is 2.5 km away from the headwaters of the Waikakahi Stream, which is the key area of interest for the study. Because groundwater abstraction is negligible in the main area of interest for this study I did not include it in my model.

8. Hydrology

Elephant Hill Stream

The Elephant Hill Stream discharge was monitored between June 2003 and May 2004, approximately 3 km upstream of the Elephant Hill Road Bridge, by ECS Ltd. The monitoring was undertaken to support a proposal to take water from the Elephant Hill Stream for irrigation, which was ultimately withdrawn by the applicant. A discussion of the streamflow monitoring in ECS (2007) notes that the entire stream flows over exposed greywacke bedrock at the recorder site location (referred to as Gorge). Downstream of the recorder site the stream flows over an increasing thickness of alluvial gravel. Anecdotal information and evidence from others indicate that the stream ceases to flow at and downstream of the Elephant Hill Road Bridge (ECS, 2007). I have summarised the flow monitoring and rainfall data provided in the ECS report on Table 5 below. I use the term ’effective rainfall‘ to describe that part of rainfall which is not taken-up by evapotranspiration. Because the Gorge site gauging station was installed on very low permeability bedrock, the volume of groundwater flow which bypassed the gauge is likely to have been negligible. I have therefore assumed that the flow record for this site measured the effective rainfall in the upstream catchment.

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Figure 3 North bank Waitaki River flats drainage features

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Table 5 Flow monitoring and rainfall data summary

Month Rainfall (mm) Mean flow L/s Effective rainfall3

Jun 2003 20 22.4 5.2%

Jul 2003 6.6 20.4 22.2%

Aug 2003 25.2 57.0 16.3%

Sep 2003 63.9 306.5 33.4%

Oct 2003 26.7 199.3 53.7%

Nov 2003 28.5 42.8 10.5%

Dec 2003 6.8 6.1 6.5%

Jan 2004 46.8 2.1 0.3%

Feb 2004 54.8 27.6 3.4%

Mar 2004 19.1 12.3 4.6%

Apr 2004 7.7 0.6 0.5%

May 2004 5.5 2.4 1.1%

Total 311.6 Average 15.6% Assuming a mean annual rainfall of 600 mm/year for the Elephant Hill catchment and a catchment area of 4,350 ha at the Elephant Hill Road Bridge, a total effective rainfall estimate of 16% (94 mm/year) equates to a long term average flow of 130 L/s.

Gabites and Horrell (2005) also note that Elephant Hill is generally an ephemeral stream downstream of the Elephant Hill Road Bridge. Based on flow regression analysis using gauging data from the Maerewhenua or Hakataramea Rivers, the authors estimated a mean flow of 200 L/s at Elephant Hill Road Bridge site.

The mean soil drainage estimate of 130 mm/year provided by Mojsilovic (2015) equates to 180 L/s of flow at Elephant Hill Road Bridge.

The three estimates above are based on different data sources and methods, but are of the same order. This gives me some confidence that the likely value is somewhere between 130 and 200 L/s. I believe the 130 L/s estimate to be the most reliable because it is based on measured data from the Elephant Hill catchment itself, but have assumed a mean flow of 150 L/s for this study.

3 Effective rainfall = measured stream flow as a proportion of rainfall (assumes no flow loss to greywacke basement). Gauging station catchment area ≈ 3,700 ha

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An assessment of environmental effects was submitted in support of a consent application to take and use groundwater from well J40/1010, located approximately 2 km downstream of the Elephant Hill Road Bridge. The assessment noted that the Elephant Hill Stream seldom flows at this location, and then only for short periods during heavy rainfall. An inspection of the stream recorded the presence of well-established non-aquatic vegetation in the stream bed, implying dry periods of significant duration. During sustained heavy rainfall the estimated stream dimensions are 12 - 20 m wide and 0.5 - 1.0 m deep. Flow in the stream typically terminates soon after rain has passed (Goldie 2007).

The 2003-2004 flow record includes 18 days when flows at the Gorge site exceeded 200 L/s and eight days when flows exceeded 500 L/s. If we assume that the maximum infiltration capacity of the stream channel and floodplain is between 200 and 500 L/s, there would have been between eight and 18 days within that one year period when flows occurred downstream of the Elephant Hill Road Bridge. This flow frequency would align with anecdotal observations that the stream “seldom flows” at this location.

Summing all flows in excess of 200 - 500 L/s gives a total annual runoff of 400,000 to 700,000 m³/year, which represents 20 - 35% of the total annual flow recorded at the Gorge site.

Based on the hydrological information above, I have made the following assumptions: • Mean annual effective rainfall = 15% of total mean annual rainfall (600 mm/year). • The mean annual Elephant Hill Stream flow at the Elephant Hill Road Bridge is 150 L/s • 20 - 35% of the mean annual effective rainfall discharges from the Elephant Hill Stream catchment to the Waitaki River as surface water flow. • The remaining 65 - 80% of mean annual effective rainfall discharges from the Elephant Hill Stream catchment to the north bank Waitaki river valley floor via groundwater flow through the Quaternary river gravel deposits and Elephant Hill Gravels. • The mean annual groundwater discharge from the Elephant Hill catchment at the Elephant Hill Road Bridge is therefore 100 - 120 L/s. • The Elephant Hill Stream catchment area at Redcliff Back Road, where the stream discharges to the north bank Waitaki River flats is approximately 5,200 ha; i.e. approximately 20% greater than the Elephant Hill Road Bridge catchment. The estimated mean annual groundwater discharge to the north bank Waitaki River flats is therefore 120 - 140 L/s, minus 15 L/s used for irrigation: 105 – 125 L/s.

Groundwater is the dominant flow path carrying dissolved contaminants to surface waters. As I explained previously the Elephant Hill Stream only flows after heavy rainfall, when it is fed by overland and near-surface flow rather than groundwater. The water that discharges to the Waitaki River via the stream will therefore be small. I have assumed a value of 10% for this study, but it could be evel less than this. The remaining 90% (42,900 kg/year) is expected to seep into groundwater and discharges to the north bank Waitaki river groundwater system. This assumption is comparable with the findings of Woodward et al (2013), which I discussed earlier in this report.

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North bank Waitaki River flats

The main drainage features of the north bank Waitaki River flats are the Elephant Hill Stream, the Waikakahi Stream (including its tributary Redcliff Stream and the Whakaihi springs) and the Waihuna Stream.

Waikakahi Stream

Environment Canterbury carried out spot gaugings in the Waikakahi Stream at Cock and Hen Road on 24 occasions between December 1996 and June 1999. The gaugings recorded a mean flow of 280 L/s and the median flow of 255 L/s. The lowest flows were recorded during the winter months, with the mean winter month flows being 160 L/s. Flows increased significantly in the spring of 1997 and 1998, and the highest flows were recorded in the spring and summer months. This is likely to relate to border dyke irrigation in the area, which provides significant additional groundwater recharge and surface runoff.

I reviewed rainfall and evapotranspiration data for the gauging period from one of NIWA’s Virtual Climate Station sites for the study area. The data show that rainfall was generally less than average (80% of the long term mean) over the measurement period, whilst evapotranspiration was close to average (95% of the mean). The measured flows are therefore likely to be somewhat lower than the long term mean flow.

The Waikakahi Stream catchment at Cock and Hen Road comprises approximately 215 ha of hill country to the north of the Waitaki River flats and 135 ha of river flats catchment. Because the hill country catchment is underlain by Elephant Hill Gravels, I expect runoff rates from this part of the catchment to be low, similar to Elephant Hill Stream. Assuming 94 mm/year of effective rainfall with 25% discharging as surface flows and the remaining 75% as groundwater seepage, the discharge from the Waikakahi Stream hill country catchment to the Northern Fan aquifer would be small: 6 L/s. I also believe that runoff rates on the north bank Waitaki River flats are likely to be low. The majority of streamflow is probably sourced from groundwater discharge, principally from the Waikakahi springs. Anecdotal infomormation indicates that most of the Waikakahi Stream flow derives from a spring close to Corner Farm (see Figure 3), which flows at a reasonably consistent rate year-round (i.e. not reducing significantly outside of the irrigation seasn), visualy estimated to be in the order of 200 L/s (Andy Guyton, MGI, pers. comms). This means that the spring provides a major proportion of the 255 L/s median flow recorded during our gaugings further downstream, at Cock and Hen Road. I have assumed that the Waikakahi Stream acts as a drain, removing 250 L/s of groundwater from my study area, with 200 L/s coming from the Corner Farm spring.

The Redcliffe Stream catchment, located immediately east of the Elephant Hill Stream catchment, covers approximately 800 ha. Assuming flows from the two adjacent catchments are similar, the estimated groundwater discharge rate for Redcliffe Stream is 20 L/s.

Five other small hillside catchments discharge to the river flats between Elephant Hill Stream and the eastern limit of the study area, with a combined catchment area of approximately 2,500 ha. The split of this flow between groundwater seepage and surface runoff is not known for these catchments, but again I have assumed that it is similar to the Elephant Hill

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Stream, i.e. 30% surface runoff and 70% groundwater recharge. I have used this method to estimate groundwater seepage rates for these remaining catchments. Values are summarised in Table 6.

Waihuna Stream

Only one stream gauging is available for the Waihuna Stream catchment, so I have assumed that the groundwater discharge rate from the upper (hill country) part of the catchment to the Waitaki River flats is similar to the Elephant Hill Stream. The catchment area of the Waihuna Stream is approximately 1,650 ha; roughly 30% of the Elephant Hill Stream. The estimated groundwater discharge rate is therefore 40 L/s (equivalent to 76 mm/year).

The Waihuna Stream dissects the north bank Waitaki River flats in the lower part of its catchment, and is likely to act as a drain for groundwater here. Because the Waihuna Stream is much smaller than the Waikakahi Stream, I have assumed that the average groundwater loss to this stream is in the order of 10 – 50 L/s.

9. Morven Glenavy Irrigation Scheme

The Morven-Glenavy Irrigation (MGI) company operates two large irrigation schemes in the Northern Fan area: the MGI scheme and the Redcliffs scheme. The Redcliffs scheme is consented to take up to 6 m³/s of Waitaki River water from an intake close to Stonewall; the MGI scheme consent permits a maximum 14.3 m³/s take from the lower intake site, at Bells Pond. The maximum consented seasonal take rate is 330 M m³/year, which is equivalent to 10.5 m³/s as an annual average. The total annual take rate has only been measured for the last few years, after the annual volume limit was added to consent conditions in 2010. The total take for the 2014-2015 season up to 12/4/15 was approximately 300 M m³. The 2014- 2015 season saw the highest demand in the last 45 years (Andy Guyton, MGI Ltd pers. comms.). The 2013-2014 season was wet and demand was lower than usual: the measured take was approximately 210 M m³. The long term average water take rate will be somewhere between these two figures, probably in the order of 250 M m³. This is equivalent to 12.6 m³/s over the irrigation season or 7.9 m³/s as an annual average.

Water is supplied to irrigators via a series of water races, much of which are unlined and are expected to be losing water. Studies in other parts of the region have found race losses of 80-90% of the total water take (see Opus 2012, which references various other studies of race losses). The MGI schemes are therefore a key hydrological feature of the Northern Fan area.

Because the MGI irrigation water comes from the Waitaki River, in which the nitrate concentration is low (0.05 mg/L), race losses will dilute nitrate in soil drainage water when it mixes with race loss water in the aquifer

The best available estimate of bulk leakage from the MGI races is 10% of the take (Andy Guyton and Robin Murphy, pers. comms.), i.e. 1.26 m³/s over the irrigation season or 0.79 m³/s as an annual average. Much of the losses are from the main races sections running along the top of the Waitaki river terrace. Drainage ditches have been excavated along the

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bottom of the terraces to prevent inundation of the adjacent land with race loss water. MGI Ltd estimate that approximately ⅔ of the race losses are captured through these drains, which discharge to the Waitaki River (Andy Guyton, pers. comms.). If this is the case, the total race network loss to groundwater would be around 260 L/s.

Taking an upper estimate of 15% race losses with 30% captured by surface drainage, the groundwater recharge from the race network would be 830 L/s. A lower estimate of 8% total race losses and 70% capture through surface drainage would give a 190 L/s race loss to groundwater.

80% of the race losses are believed to occur in the network south of Pikes Point – i.e. within the northern fan area. North of here the near surface geology comprises Dog Kennel silts, which limit the leakage (Andy Guyton, pers. comms.). Race losses to groundwater in the northern fan area are therefore estimated to be between 150 and 660 L/s.

Of the total approximately 105 km of irrigation races within the Northern Fan area, approximately 47 km of races are located within the study area – i.e. around 45% of the total. Assuming that 45% of the Northern Fan race losses occur in the study area, I estimate that race losses provide somehwere between 70 and 300 L/s of recharge to groundwater in my area of interest. Much of this loss is probably from the main race which runs along the top of the river terrace and carries ⅔ of the 6 m³/s Stonewall intake. I have assumed tat 70% of race losses to groundwater in my study arewa (i.e. 50 – 210 L/s) occur along the main race, with the remaining 20 – 90 L/s being evenly distributed along the remainder of the race network.

10. Groundwater model

Model summary

I developed a MODFLOW model of the river flats part of the study area in order to evaluate the fate of the estimated approximately 43,000 kg/year of nitrogen which discharges to the north bank Waitaki River aquifer system from the Elephant Hill catchment. The model was constructed using the GMS interface and incorporated river-type boundary cells along the southern border, constant head cells along the eastern border and a combination of constant flux and now flow boundary cells on the northern and western model borders (Figure 4). A steady-state model was considered to be sufficient for the questions addressed in this study; transient modelling was not required. Summary information on the main elements of the model is provided in Table 6. I have listed the broader set of assumptions made for the model in Appendix 1.

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Figure 4 Model boundary conditions

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Model calibration criteria

Because the fate of nitrogen in groundwater discharges from the Elephant hill Stream catchment will mainly be determined by the groundwater flow direction, groundwater contours interpolated from measured groundwater levels provide a good basis for calibrating the model.

I discussed the source and uncertainty of the currently available groundwater contours for the study area earlier in this report. In summary, the contours are based on data from a sparse monitoring network, and the water level readings underlying the contours may not be representative of long-term average conditions. Because of this it would not be appropriate to constrain the model too closely to these contours: I have used the contours as a general guide only.

Table 6 Model summary

Model element Parameters Basis Fine resolution for transport simulation Model grid 100 x 100 m cells between Elephant Hill Stream discharge and Waikakahi springs Single layer with constant top (100 m Simplifying assumption, unlikely to affect Model structure asl) and base model results significantly elevation (0 m asl) 100 m asl in west South boundary: Waitaki River URS (2003) groundwater contours declining to 60 m asl in

east East boundary: constant head 60 m asl URS (2003) groundwater contours North boundary Zero flux Low permeability of greywacke bedrock West boundary Constant flux Estimated stream discharge – see below Elephant Hill Stream discharge Flow estimate from gauging data minus to Waitaki river flats: constant Mean flow 115 L/s irrigation water take (assumed 70% flux efficient) Waihuna Stream catchment discharge to Waitaki river flats: Mean flow 40 L/s Factored from Elephant Hill Stream constant flux Redcliffe Stream catchment discharge to Waitaki river flats: Mean flow 20 L/s Factored from Elephant Hill Stream constant flux Hill country stream catchments immediately east of Redcliffe Mean flow 5 L/s Factored from Elephant Hill Stream Stream: constant flux Ryde Stream + Mount Harris Stream + upper Waikakahi Mean flow 45 L/s Factored from Elephant Hill Stream Stream catchments: constant flux MGI race losses to aquifer 70 – 300 L/s Anecdotal information from MGI Ltd

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Model element Parameters Basis Irricalc database using SMAP database 4 Mean 0.00073 m/y Land Surface Recharge PAW and Environment Canterbury (270 mm/year) irrigation type database Assumes that 10% of N load transported Model Elephant Hill N load 42,900 kg/year by surface runoff to Waitaki River Groundwater takes: 30 L/s constant Rate of take for all wells small, except for CRC000945 included as well abstraction consent CRC000945 boundary ~250 L/s, implemented Groundwater loss to as drain cells in Stream gauging data see Hydrology Waikakahi Stream above Cock springs area and Section and Hen Road mapped stream channels Elephant Hill Stream discharge (source = river flats 10 L/s Estimate groundwater) Waihuna stream discharge (source = river flats 10 L/s Estimate groundwater) Empirical transmissivity estimates for Transmissivity of north bank 100 – 1,000 m²/d screened well horizons + literature river flats aquifer (K4 = 1 – 10 m/d) values for soil types described in unscreened horizons Hydraulic conductivity is usually higher parallel to the flow direction due to Horizontal anisotropy 0.8 particle alignment in the aquifer. Value used was refined during model calibration

11. Model results

I ran the groundwater model with three different river flats aquifer hydraulic conductivity values. I took this approach because I do not consider that the available data are sufficient to constrain the model to a single parameter set. The model results show the nutrient transport outcomes are not very sensitive to hydraulic conductivity changes within the range I looked at.

Model parameters and results are summarised in Table 7 below. I have plotted model and URS (2003) groundwater contours for each of the model runs in Appendix 2.

I used a horizontal anisotropy ratio of 0.8, as explained in Table 6 above, to account for the alignment of alluvial sediment particles parallel to the flow direction. This improved the alignment of the model and measurement-based groundwater contours.

I was unable to replicate the estimated 200 L/s Corner Farm spring flow in my model if I applied a consistent hydraulic conductivity value across the whole model. A zone of higher transmissivity in the location of the spring would have been required to replicate a discharge of this magnitude. But we do not have sufficient information to define such a zone, and I

4 Land Surface Recharge is aquifer recharge from rainfall and irrigated land drainage. K = hydraulic conductivity

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wanted to keep the model simple. I therefore implemented the spring discharge as a series of drains which remove water from the broader spring area, and accepted that model groundwater levels would not replicate the contours interpolated from measured data very well in this area.

The Run 1 contours match the URS (2003) contours most closely, but the Waikakahi Stream baseflow is lower than our records show. The reduction in hydraulic conductivity in Runs 2 and 3 causes the groundwater level in the area of the springs to increase. This results in higher model stream flows which are closer to the rate we have measured.

The three model runs provide very similar estimates of the proportion of the Elephant Hill nitrogen load discharging to the streams, being 75-80%. All three runs indicate that nutrient discharges from the Waihuna Stream catchment are unlikely to be transported into the Waikakahi Stream. I have plotted model flow paths for groundwater outflows from the Waihuna and Elephant Hill Stream valleys on Figure 5 to illustrate this. Table 7 Model results

Run Hydraulic Waikakahi stream Waikakahi N Proportion With No conductivity base flow (L/s) load5 (kg/year) Elephant Waihuna (m/d) Hill N load6

1 10 150 33,000 75% 33,000

2 5 240 34,000 80% 34,000

3 3 290 34,000 80% 34,000

12. Conclusions

The Waikakaki Stream is fed by a series of springs which issue from the north bank Waitaki River flats. Most of the flow comes from a single spring in the vicinity of Corner Farm. Recent groundwater quality monitoring undertaken by Environment Canterbury has recorded fairly high nitrate concentrations in the springs, with mean concentrations between 3 and 6 mg/L for the four springs sampled. The mean concentrations in two of the springs are close to the mandatory National Bottom Line value of 6.7 mg/L nitrate-N defined in the NPS for Freshwater. Future increases in the nutrient loads within the springs’ catchment area could push the median concentration over the National Bottom Line.

I have reviewed available information on the hydrology, geology, hydrogeology and groundwater chemistry for the study area to develop a conceptual understanding of the hydrological system. I translated my conceptual understanding into a numerical groundwater model, and used this model to investigate the fate and transport of nitrogen discharges in the Elephant Hill Stream catchment. I also assessed whether nitrogen discharges from the Waihuna Stream catchment are likely to be transported to the Waikakahi Stream.

5 N load from Elephant Hill catchment only – not total N load in stream

6 Proportion of Elephant Hill nitrogen load that discharges to Waikakahi Stream

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Model results suggest that around 80% of the Elephant Hill nitrogen discharge could end up in the Waikakahi Stream, principally via the spring system. Results indicate that nitrogen from the Waihuna Stream catchment is unlikely to be transported to Waikakahi Stream.

13. Recommendations

The following work would improve our understanding of nitrogen fate and transport in the study area: • Combined piezometric survey and groundwater quality sampling in the study area, focusing on the area between the Elephant Hill Stream and Waikakahi springs. At least two surveys should be undertaken, inside and outside of the irrigation season. • Flow gauging of springs and streams (Waikakahi springs, lower Elephant Hill Stream, Waihuna Stream, Redcliffe Stream). • Flow and water quality monitoring of Elephant Hill Stream under low and high flow conditions. • Evaluation of denitrification potential in the north bank Waitaki River aquifer upgradient of the Waikakahi springs. • The groundwater model developed for this study should be updated with this additional information, and re-run to provide a better understanding of nitrogen fate and transport.

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Figure 5 Model flow lines for Elephant Hill and Waihuna Stream catchment groundwater discharge

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14. References

Bal A. A. 1996. Valley fills and coastal cliffs buried beneath an alluvial plain: evidence from variation of permeabilities in gravel aquifers, Canterbury Plains, New Zealand. Journal if Hydrology NZ 35(1): 1-27, 1996

Forsyth, P.J. 2001. Geology of the Waitaki area : scale 1:250,000. Lower Hutt: Institute of Geological & Nuclear Sciences Limited. Institute of Geological & Nuclear Sciences 1:250,000 geological map 19.

Gabites S. and Horrell G. 2005. Seven day mean annual low flow mapping of the tributaries of the Waitaki River. ECan Report No. R05/16

Greer, M. 2015. Memorandum regarding the influence of groundwater inputs from Elephant Hill on the Water Quality of Waikakahi Stream. Environment Canterbury memo to the Lower Waitaki Zone Committee

Goldie R. 2007. Application for Resource Consent by Mr Lindsay White to Take and Use Grounwater. Consent application and assessment of environmental effects prepared by R J Hall Civil and Environmental Consulting Ltd

Macfarlane D. F. 1988. Lower Waitaki Power Investigations. Preliminary assessment of the likely effects of river diversion on the groundwater regime. Department of Scientific and Industrial Research

Mojsilovic, O. 2015: Generation of nitrogen loss estimates in the Waitaki catchment. Environment Canterbury Technical Report.

National Policy Statement for Freshwater Management 2014. Issued by notice in gazette on 4 July 2014

Riddolls, B.W. 1966. The Geology of the Waihao District, South Canterbury, with Particular Refernece to the Pre-Quaternary Rocks. A thesis presented for the degree of Master of Science in Geology at the University of Canterbury, Christchurch; New Zealand

Scott M. 2014. The current state of groundwater quality in the lower Waitaki. Environment Canterbury Report, currently in draft format

Stenger R., Clague J., Woodward S., Moorhead B., Burbery L. and Canard H. 2012. Groundwater Assimilative Capacity – an Untapped Opportunity For Catchment-Scale Nitrogen Management?

URS 2003. Project Aqua – hydrogeological assessment of effects. Prepared for Meridian Energy Ltd. Volume 1 of 2 - main report

Wheeler D. M. and Rutherford J. C. 2014. OVERSEER® Technical manual Technical manual for the description of the OVERSEER® model - Hydrology ISSN: 2253-461X

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Woodward S., Stenger R. and Bidewell V. 2013. Using Stream Flow and Chemistry Data to Estimate Catchment Scale Groundwater and Nitrate Fluxes

Zarour, H. 2015. Lower Waitaki hydrogeology. Environment Canterbury report, currently in draft form

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Appendices

Appendix 1 - Model assumptions

No. Assumption Rationale

1 Homogeneous aquifer Simplifying assumption; data available do not support interpretation of spatial variability

2 Waitaki River: stage = groundwater Stage based on URS (2003) contours, which appear contours, depth = 4 m, bed to be based on a series of river stage conductance = 1,000 m²/d/m measurements. River depth not critical to model. High conductance considered appropriate for braided river

3 Aquifer thickness 60-100 m Wells screened up to 90 m depth.

4 Transport through full saturated Reasonable assumption under steady state thickness conditions, but steady state would probably take a very long time to occur (100’s of years)

5 Streams implemented as constant flux Considered reasonable for a steady state model boundaries

6 10% of N load discharged direct to See discussion in report body: nitrogen loads Waitaki transport in surface runoff are only a small part of the total load.

7 Nitrate transport conservative – no Groundwater quality data show no evidence of denitrification in the aquifer anoxic groundwater.

8 Elephant Hill groundwater nitrate-N Based on N load and estimated groundwater concentration = 13 mg/L recharge. Concentration not relevant to model results because results are discussed in terms of N load only

9 Northern + western border = no flow Greywacke bedrock hydraulic conductivity likely to be very low

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Appendix 2 - Model results and calibration plots

Memo, 1 September 2015 Page 25 of 27

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Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Appendix 4: Lag-time assessment

Background Understanding how closely current monitoring data reflect current land use, and how long it is likely to take for groundwater quality to respond to changes in land management practices, is important for model validation and for long-term natural resource planning and management. I use the term lag-time to describe the time it takes for a particle in the soil root zone at the top of the catchment to travel through the unsaturated zone and groundwater system and arrive at a monitoring well at the bottom of the catchment. It is only when contamination from the furthest part of the catchment (for that well) reaches the monitoring well that water quality will reach steady-state. Close (2010) reports on a travel time estimates for a meat processing site with high rates of land surface recharge associated with effluent disposal. The data showed that monitoring wells down- gradient of the site responded to land use change within less than a year. Data from a monitoring well down-gradient of a freezing works site showed that nitrate concentrations decreased approximately seven years after effluent discharge ceased. Scott (2013) reviewed lag-times between water and nutrients entering the groundwater system via recharge and exiting the system via springs, drains and pumping wells near the coast in the Hinds zone. She noted that lag times for the Selwyn-Waihaora were estimated to be 10 to 30 years. In the Hinds catchment the lag time appears to be slightly faster, probably in the order of 0 to 20 years. Both of the above examples relate to the shallow part of the aquifer system. Lag-times for the deeper aquifer are much longer. Chapelle et. al. (2013) investigated the long-term fate of nitrate in a river valley alluvial fan system using a groundwater model. Model results (shown on Figure A4-1 below) indicated that ten years after land use intensification with an associated soil drainage nitrate concentration of 20 mg/L, nitrate at concentrations of about 10 mg/L had moved into the shallow portions of the aquifer, which was consistent with observed nitrate contamination in this system. After 50 years the shallowest portion of the aquifer showed nitrate concentrations ranging from 15 to 20 mg/L, and nitrate had reached the discharge area. After 100 years of simulation, nitrate concentrations in portions of the aquifer near the discharge area had risen above 10 mg/L. The results of these simulations indicate that nitrate will penetrate deeper into the aquifer for the foreseeable future. Similar patterns have been observed throughout Canterbury, with deep wells showing lower nitrate concentrations than shallow wells. This is generally because it takes a very long time for recharge from the land surface to penetrate deeper into the aquifer. The results show that if the deeper aquifer is predominantly recharged from the overlying shallower aquifer system (which is normally the case), drilling deeper wells may not be an effective long term strategy for management of agricultural impacts on drinking water supplies.

Lower-Waitaki lag-times The lag-time in different parts of the lower Waitaki catchment will vary substantially, depending on land surface recharge rates, vadose zone thickness and properties, aquifer hydraulic properties and the catchment length. Heterogeneity, in particular the interlayering of fine, low permeability material with course permeable deposits, can result in significant dispersion of groundwater residence times. Steadily declining water quality can occur for many decades into the future can occur in such areas, even if the sources are reduced today (see discussion in Weissmann et. al. 2002). I have selected four wells that we have tested for nitrate in the Hakataramea, mid-Waitaki north bank, mid-Waitaki south bank and Northern fan catchments. I carried out some basic aquifer lag-time calculations for these wells using two methods: For the mid-Waitaki north bank and south bank wells, where groundwater contour and aquifer property data were available (or could reasonably be inferred) I used Darcy velocity calculations to estimate travel times within the aquifer.

Environment Canterbury Technical Report 49 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

For the Hakataramea catchment and Northern Fan area, where no reliable groundwater contour data are available and/or where aquifer properties are unknown, I used a simple piston-flow model, taking the weighted average soil drainage rate for each area from Overseer V. 6.2.1 (plus estimates of race losses for the Northern Fan area) together with an assumed flow horizon thickness and aquifer effective porosity. It should be noted that neither of these estimates account for dispersion within the aquifer and should be regarded as indicative only.

Figure A4-1: Model nitrate concentrations in groundwater through time

Lag-times in the vadose zone are more difficult to estimate because we do not have the requisite data for much of the lower Waitaki zone – e.g. the depth to groundwater is not known for most of the Hakataramea catchment and mid-Waitaki south bank tributaries. Close (2010) used depth to water data from 21 wells in the Ikawai area together with profile available water data and annual dryland recharge values to estimate average travel times through vadose zone sandy gravel material. Results showed an average travel time of 2.4 years with a 0.5 to 5.6 year range. In reality travel times will be faster than this because the vast majority of the land in the Ikawai and broader Northern Fan area is irrigated. Estimated vadose zone travel times for various other parts of Canterbury reported in Close (2010) ranged from 1.7 to 7.6 years. I have assumed vadose zone travel times of 2 – 10 years for the Hakataramea catchment, 1 – 3 years for the Northern Fan area and 0.25 – 5 years for the mid-Waitaki area. I have summarised calculation inputs and results for the piston flow and Darcy flow models on Table A4-1 and Table A4-2 below.

50 Environment Canterbury Technical Report Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Table A4-1: Lag-time calculations – piston flow model

Piston flow model Est Catchment Well L (m) D (m) LSR (m/y) P V (m/yr) A t (y) V t (y) T t (y)

High Hakataramea I40/0033 7000 100 0.1 0.25 28 250 10 260

Low Hakataramea I40/0033 7000 50 0.2 0.05 560 13 2 15 High Northern Fan J41/0003 8000 30 0.3 0.25 96 83 3 86 Low Northern Fan J41/0003 8000 20 0.6 0.05 1920 4 1 5 Notes: L = flow path length, D = effective aquifer depth; LSR = weighted average land surface recharge; P = effective porosity; V = velocity; A t = aquifer travel time; Vt – Vadose zone travel time; T t = total travel time

Table A4-2: Lag-time calculations – Darcy flow model

Darcy flow model D T K w Q V A t V t T t Est Catchment Well L (m) (m) i (m²/d) (m/d) (m) (m³/d) P (m/y) (y) (y) (y) Mid- Waitaki Low north I40/0145 8000 8 0.003 14,000 1,750 1500 70,000 0.05 42,583 0.2 0.25 0.4 Mid- Waitaki High north I40/0145 8000 8 0.003 7,000 875 1500 35,000 0.3 3,549 2.3 5 7 Mid- Waitaki Low south I40/0675 6000 8 0.003 14000 1,750 1500 70,000 0.05 42,583 0.1 0.25 0.4 Mid- Waitaki 0.00 High south I40/0675 6000 8 3 7000 875 1500 35,000 0.3 3,549 1.7 5 7 Notes: i = hydraulic gradient; T = aquifer transmissivity; K = hydraulic conductivity; w = aquifer width; Q = aquifer specific discharge

These lag-time estimates are based on limited data and many assumptions and experience-based estimates. The piston-flow estimates do not account for gains and losses from/to surface water. Whilst they are useful for current purposes, they should be updated using an alternative approach when additional information becomes available. Notes on data sources: 1. An effective aquifer depth of 20-30 m has been assumed for the Hakataramea and Northern Fan areas. This is based on groundwater quality data from other parts of the region, which frequently show that so far nitrate, has generally not penetrated much beyond this depth. 2. Effective porosity values of 0.3, 0.25 and 0.05 have been assumed: these are literature values for gravel, silt and sand/gravel 3. Hydraulic gradient data are based on contours provided in URS (2003) 4. T and aquifer thickness based on unpublished (confidential) reports for the mid-Waitaki north bank, and previous experience in the south bank area

Environment Canterbury Technical Report 51 Predicting consequences of future Scenarios in the Waitaki Catchment: Lower Waitaki Groundwater quality

Appendix 5: Groundwater recharge and quality trends in the lower Waitaki – Northern Fan area

52 Environment Canterbury Technical Report

16 February 2015

MEMORANDUM

File Reference: WATE/INGW/PLAD

FROM : ZEB ETHERIDGE

TO : Carl Hanson cc Lisa Scott, Helen Shaw, Kelly Palmer, Mike Greer

SUBJECT : GROUNDWATER RECHARGE AND QUALITY TRENDS IN THE LOWER WAITAKI – NORTHERN FAN AREA

SUMMARY Environment Canterbury collected groundwater samples from 26 shallow wells in the Northern Fan area of the lower Waitaki zone in late 2014. We undertook this sampling to investigate nitrate concentration changes since our last water quality survey in 1996. Natural tracer data were also collected to investigate groundwater recharge sources. We found that the mean nitrate concentration recorded in the February and May 1996 surveys was 2.2 mg/L; in 2014 was 3.9 mg/L. This represents a shallow groundwater nitrate load increase of approximately 80%. I have looked at the ratio between mean and peak nitrate concentrations in this area, and used this ratio to estimate the number of wells where peak annual nitrate concentrations could exceed the drinking water limit. My assessment indicates that exceedances could occur in approximately 10% of wells, which equates to five of the total 50 wells used for drinking water in the study area. My interpretation of the natural tracer test results aligns with our conceptual understanding of groundwater recharge in the zone: land close to the Waitaki River shows a predominance of recharge from this source (either directly or via irrigation), and recharge to land close to the northern margin appears to be dominated by rainfall recharge – either directly onto the fan or as runoff from the hill country to the north. The remainder of the fan shows mixed recharge influences. The natural tracer data collected for this survey has not improved our understanding of the influence of Elephant Hill Stream recharge on groundwater quality in the upper Waikakahi Stream catchment. A targeted investigation would be required to address this question.

INTRODUCTION During our review of groundwater quality in the Lower Waitaki for the sub-regional planning process in mid-2014, we found that water quality data in some areas is quite limited. In particular we wanted to know how much nitrate concentrations have increased, and what the predominant groundwater recharge sources are. We therefore scheduled a water quality investigation to fill-in some of the gaps in our dataset. This memorandum discusses the area we investigated and what we have learned from the sampling results. The investigation area covers part of the north bank of the lower Waitaki River known as the Northern Fan. The area extends from Fairway in the west to the coast in the east. The Waitaki River forms the southern boundary and the hills form the northern boundary of the sampling area, except along the coastal plain where the area extends north as far as Whitneys Creek surface water catchment. We also collected samples from the lower part of the South Canterbury Coastal Stream (SCCS) zone, on the coastal plains north of Whitneys Creek (see Figure 1). We last investigated groundwater quality in this area in 1996: during that investigation most of the wells had fairly low nitrate-N concentrations. Farming land use has intensified and conversions from border dyke to spray irrigation have occurred in this area since 1996. The shallow wells which we have continued to sample as part of our long-term monitoring programme all now have nitrate-N concentrations above ½ of the drinking water limit. We think that this is a direct result of land use and irrigation practice changes.

SAMPLING LOCATIONS Environment Canterbury undertook two sampling rounds in 1996, in February and May. We collected samples from 26 wells in total, 21 of which are within the Lower Waitaki zone and the remaining five within the South Canterbury Coastal Streams zone (see Figure 1). We collected groundwater samples from the same wells in 2014, between September and December. We only sampled shallow wells with depths between 4 m and 17 m, the median depth being 8.5 m. We chose shallow wells because these wells most closely reflect the effects of recent land use change. Groundwater in deeper wells is typically older and is therefore not equilibrated with current land use practice in most instances. Shallow groundwater quality is also the main area of interest for resource management purposes because 75% of the circa 50 wells used for domestic and stock water supply in the Lower Waitaki zone are shallow (< 20 m deep). Shallow groundwater is also an important source of recharge for many streams and rivers.

HYDROLOGICAL CONDITIONS DURING SAMPLE COLLECTION Shallow groundwater quality often varies in response to seasonal climatic conditions. As a general rule, we expect nitrate concentrations in groundwater to be highest in winter or spring. This is because plant activity is lower in winter, allowing nitrate to build-up in the soil. Rainfall is higher than evapotranspiration, so more water infiltrates through the ground and carries nitrate down to the water table. Nitrate concentrations then decline over the summer when there is little available soil moisture, and they are generally lowest in the autumn (Hanson 2002). We would therefore normally expect samples collected in February and to a lesser extent May (the 1996 sampling dates) to show nitrate concentrations toward the lower end of the seasonal range. Samples collected in September (when half of the 2014 samples were collected) would typically show nitrate concentrations somewhere close to the seasonal highs.

2 I have reviewed rainfall data from NIWA’s Virtual Climate Station network and groundwater level data from one of our long-term monitoring wells (J41/0018) in order to understand hydrological conditions during and immediately prior to our sampling rounds. Summary data are plotted on Figure 2. I have also reviewed data from Environment Canterbury’s three long-term quality monitoring wells in the area, to see how nitrate concentrations at the time of the surveys compare to longer term seasonal averages. From Figure 2 we can see that rainfall during February 1996, when the first sampling round of that year was undertaken, was much higher than normal. The groundwater level was slightly below the long-term monthly average. The high rainfall could have flushed some nitrate from the soil into groundwater, and caused concentrations to be above normal for that time of year. Data from the long-term quality monitoring wells (see Figure 3) show that nitrate concentrations in February 1996 were probably close to the annual average in the two wells that Environment Canterbury was monitoring regularly at that time. Rainfall and groundwater levels in May 1996 were much lower than normal, but rainfall in April was significantly above average. Nitrate concentrations were approximately 20% lower in the two long- term monitoring wells on the second sampling round compared to the first. The average nitrate-N concentrations in the 26 wells sampled in 1996 were roughly 20% higher in May than in June, however: 2.4 mg/l compared to 2.0 mg/L. I believe that by taking the average nitrate concentrations from the two 1996 sampling rounds we end up with a value that is reasonably representative of the average for that year. Rainfall between September and December 2014 was significantly less than long-term monthly average. Winter recharge was also well below the long-term average, and this was reflected by lower than normal groundwater levels. Nitrate concentrations in the quarterly wells were well below their maximum recorded values, being closer to the annual average. My review of hydrological and water quarterly water quality monitoring data therefore indicates that nitrate concentrations recorded during the 1996 and 2014 sampling rounds were probably close to the annual average for those years. This means that the two sets of samples provide a good basis for assessing the long-term increase in average nitrate concentrations.

SURVEY RESULTS: NITRATE Survey results show that nitrate concentrations in 19 of the 26 wells sampled were higher in 2014 than in 1996; concentrations were lower in the remaining seven wells. Five of the wells sampled in 2014 had concentrations above half the drinking-water standard (the Maximum Acceptable Value, or MAV) of 11.3 mg/L, whereas none of the wells sampled in 1996 had concentrations above half the MAV. None of the wells in either survey had a concentration above the MAV. The highest concentration recorded in 2014 was 8.4 mg/L. The mean nitrate concentration recorded in the February and May 1996 surveys was 2.2 mg/L. The mean concentration in 2014 was 3.9 mg/L. This represents a shallow groundwater nitrate load increase of approximately 80%. Concentrations in our three local long-term monitoring wells increased by 75 – 135% on average over this period. The long-term wells do therefore provide a reasonably representative picture of nitrate trends in the broader area. To consider what the survey results mean for drinking water quality in the sampling area, we need to consider the relationship between average nitrate concentrations and peak concentrations. If peak concentrations were typically only slightly above average concentrations, an average concentration of 3.9 mg/L would be unlikely to signify widespread exceedances of the nitrate-N drinking water limit of 11.3 mg/L. Conversely, if peak concentrations are several times higher than average

3 concentrations, the mean of 3.9 mg/L could mean that nitrate periodically exceeds the limit in many wells. Peak concentrations in the three long-term monitoring concentrations vary between 1 x and 5 x the annual average, with a mean ratio of 1.5. If we assume that the 2014 sample results represent an average concentration for that year, and take a peak/mean nitrate ratio of 1.5, all wells with a nitrate concentration above 7.5 mg/L could see peak concentrations above the 11.3 mg/L limit. This occurred in 2 of the 26 wells sampled, with a third well coming close (7.0 mg/L nitrate-N). This represents approximately 10% of the wells sampled. If we extrapolate this to the circa 50 wells used for drinking water in the area, it indicates that the peak annual nitrate concentrations in approximately five wells may currently exceed the drinking water limit (aka MAV – maximum acceptable value). Carl Hanson investigated whether a mean concentration of half the MAV (5.6 mg/L N) could maintain nitrate concentrations below the MAV in at least 90% of a random set of groundwater samples from the Ashburton-Rakaia, Rangitata-Orari and Morven-Glenavy zones. Statistical analysis of groundwater quality data from these zones indicated that this was likely to be the case. The mean nitrate-N concentration in all South Canterbury wells was 5.2 mg/L, and 14% of the samples exceeded 11.3 mg/L (Hanson 2012). This broadly aligns with the 3.9 mg/L mean vs. 10% exceedance estimate above.

NATURAL TRACER DATA Concentrations of natural isotopes and ions can provide useful insights into the origin of groundwater recharge. Ratios of oxygen isotopes in natural waters reflect the temperature at which precipitation (rain or snowfall) occurred, which provides an indication of whether the water is sourced from a high altitude or low altitude area. Scott (2014) reviewed several years of δ18O data from central Canterbury and found that that groundwater recharge from: • rivers with high catchments to the east of the main divide (e.g. Orari River and Ashburton River/Hakatere) is likely to have δ18O more negative than −10‰ • rivers with high alpine catchments (or irrigation water from these rivers) is likely to have δ18O more negative than −9‰ • rainfall on the upper plains (above about 200 m elevation) is likely to have δ18O more negative than −8‰ • rainfall near the coast is likely to have δ18O less negative than −8‰.

The Northern Fan area predominantly comprises low-lying land less than 100 m above mean sea level. The area is bordered by hill country to the north and the Waitaki River to the south. The largest hill country catchment discharging to the northern flats is Elephant Hill stream, which predominantly lies between 200 and 600 m above sea level. In the upper part of the catchment, west of Waikakahi, various hill country streams discharge onto the fan.

Much of the Northern Fan area is irrigated with Waitaki River water, and over half of the irrigated area comprises border dyking at present. Border dyke irrigation often provides a significant source of groundwater recharge, with land surface recharge (LSR) estimates for border dyke areas typically being three times greater than spray-irrigated areas.

Groundwater in the Northern Fan area is therefore likely to be recharged by a combination of rainfall and irrigation recharge directly onto the low-laying land, recharge from the hill country streams as they cross the fan, and recharge from the Waitaki River.

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Scott (2014) estimated that the mean δ18O in the Waitaki River would be approximately −9.7‰. Recharge from irrigation and directly from the river is therefore likely to have δ18O in the order of −9.7‰. Rainfall on the fan area is likely to have δ18O less negative than −8‰ and the hill country streams are likely to have δ18O more negative than −8‰.

Overall, the analysis I have undertaken for this study suggests that the δ18O data do not provide a very clear picture of recharge sources in the Northern Fan area. This is because we cannot distinguish between areas where groundwater has been recharged by the Waitaki River, and areas of irrigation recharge (using Waitaki River water). Furthermore, where groundwater is recharged by a combination of mechanisms (e.g. lowland rainfall recharge + Waitaki River irrigation recharge), it is not possible to determine the proportion of each component. I looked at various other environmental tracers such as silica and the calcium/magnesium ratio, but found that these do not clarify matters.

I have assumed that groundwater recharge in areas where the δ18O value is significantly more negative than -9‰ is dominated either by border dyke irrigation or by Waitaki River recharge. A plot of δ18O and nitrate data on Figure 4 shows that δ18O values significantly below -9‰ are generally found in the wells closest to the Waitaki River and in the SCCS area to the north (where border dyke irrigation with Waitaki River water is predominant). Nitrate concentrations are generally low in these locations.

δ18O values are around -8‰ along the northern edge of the fan. I anticipate that groundwater recharge in these areas is likely to comprise local land surface recharge and recharge from the hill country streams. The less negative δ18O values are likely to reflect a lack of Waitaki River recharge.

The remaining areas have δ18O values between -8.5 and -9.0‰, and I believe this reflects a mixture of rainfall and Waitaki River-sourced irrigation recharge, with a hill country stream recharge component in some locations.

The influence of runoff from the Elephant Hill stream catchment on nitrate concentrations in the upper Waikakahi Stream catchment has been under question during the current nitrogen load limit setting process. We collected δ18O samples from three locations in this vicinity (i.e. close to Ikawai) in 2014. The sample collected on the flats upgradient of the Elephant Hill Stream recorded -9.5‰ δ18O; the two downstream samples recorded less negative values, of -8.5‰ and -8.8‰. Whilst this suggests less Waitaki River water influence in the downstream samples than upstream, I cannot determine whether this is the result of land surface recharge on the fan itself, or recharge from the Elephant Hill Stream. The expected δ18O values of the latter two components are not sufficiently different to distinguish between them. A targeted investigation in this area would be required to gain insights into the Elephant Hill Stream / Waikakahi Stream interaction.

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Figure 1 Groundwater nitrate concentrations in 1996 and 2014 100 0

Mean rainfall 1996 rainfall

90 2014 rainfall Mean depth to water -1 1996 depth to water 2014 depth to water 80

Mean NO -N 2.0 mg/L -2 70 3

60 -3 Mean NO3-N 3.9 mg/L

Mean NO3-N 2.4 mg/L 2014 sampling 50 February 1996sampling

-4

Mean monthly rainfall (mm) rainfall monthly Mean 40 Mean depth to groundwater (m) to groundwater depth Mean

30 -5

20

-6

10 May 1996 sampling 1996 May

0 -7 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 2 Hydrological data summary

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J41/0004 J40/0163 J41/0018 14

12

10

8 Nitrate mg/L Nitrate Dec Dec 2014 sampling - Sept

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4 Feb & MayFeb & 1996sampling 2

0 1-Jan-96 31-Dec-97 31-Dec-99 30-Dec-01 30-Dec-03 29-Dec-05 29-Dec-07 28-Dec-09 28-Dec-11 27-Dec-13

Figure 3 Nitrate data from long-term groundwater monitoring wells

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18 Figure 4 Northern Fan zone δ O and 2014 nitrate data

REFERENCES Hanson C. R. 2002: Nitrate concentrations in Canterbury groundwater – a review of existing data. Environment Canterbury Report No. R02/17 ISBN 1-86937-457-6 Hanson C. 2012: LUWQ - Target Nitrate Concentrations in Groundwater Zones. Environment Canterbury memorandum Ref: WATE/INGW/QUAL/INVE/2 Scott L. 2014: Review of Environment Canterbury’s groundwater oxygen-18 data. Environment Canterbury Report (currently in draft format). Hanson C. & Abraham P. 2009: Depth and spatial variation in groundwater chemistry – Central Canterbury Plains. Environment Canterbury Report No. R09/39 Scott L., Hanson C. & Cressy R. 2012. Groundwater quality investigation of the mid-Waitaki valley. Environment Canterbury Report No. R12/71