The current state of groundwater quality in the upper Waitaki

Report No. R15/42 ISBN 978-0-478-15176-3 (print) 978-0-478-15177-0 (web) 978-0-478-15178-7 (cd)

Report prepared by Marta Scott Environment Canterbury

August 2015

Report No. R15/42 ISBN 978-0-478-15176-3 (print) 978-0-478-15177-0 (web) 978-0-478-15178-7 (cd)

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The current state of groundwater quality in the upper Waitaki

Summary

Environment Canterbury is working with the zone committee and the local community to set nutrient load and flow limits for the lower Waitaki catchment. The nutrient limits are a way of managing diffuse sources of nitrogen loss in the catchment. This report describes the current state of groundwater quality in the lower Waitaki catchment to inform that process. The technical team has used test scenarios to explore groundwater quality under different management actions and nitrogen loads. This is discussed in a separate report (see Etheridge and Scott 2015). The upper Waitaki catchment has two major depressions, the Mackenzie and Omarama basins. The area may also be further divided into smaller sub-basins, based on geological features and glacial movements. The surrounding hills have provided the deposits which form the aquifer material, but various glacial and interglacial periods have resulted in substantial heterogeneity. Quaternary deposits host the main aquifers in the area. There are also deeper, semi-confined aquifers which are likely to be located within old fans deposited by rivers, but may not be laterally continuous. The groundwater recharge sources include: rainfall, rivers, lakes and canals. The interaction between groundwater and surface water is evident throughout the area. Distinguishing between the different recharge sources using water chemistry can be difficult, however, due to low concentrations of dissolved ions. There are three large natural lakes (Lake Tekapo, Lake Pukaki and Lake Ohau), which are lined with thick silt deposits and therefore unlikely to recharge the local groundwater. Other major Lakes (Lake Benmore, Lake Aviemore and Lake Waitaki) are man-made and therefore lack the thick silt deposits of the natural lakes. Lake Benmore provides the only exit point for groundwater in the catchment, with groundwater flow generally following surface topography. In general groundwater from the Mackenzie basin will flow towards the Haldon arm of Lake Benmore whereas groundwater in the Omarama basin will mainly drain to the Ahuriri arm of Lake Benmore. The groundwater divide is positioned somewhere near Wairepo Creek. In general, the electrical conductivity and average concentrations of major ions in upper Waitaki groundwater are lower than average concentrations in the whole of Canterbury. This is likely to be due to the large component of alpine water in the groundwater recharge, greater distance from the coast and a lower intensity of land use. Most of the wells have high dissolved oxygen concentrations and therefore denitrification in groundwater is not likely to be significant. Concentrations of iron and manganese are generally below their Guideline Values, which further supports the inference that denitrification is not prevalent. The average nitrate-N concentrations in upper Waitaki are significantly lower than the average for the whole of the Canterbury region. Concentrations are elevated in areas where land use is more intensive and some wells show an increasing trend. There may be a time lag between land use changes and increasing nitrate concentrations. Dissolved reactive phosphorus concentrations are the highest either in deeper wells, where they are likely to be natural, or in shallow wells, where they may be due to land use intensification. E. coli have been detected in those areas of more intense land use where significant numbers of animals are present. E. coli are normally detected in shallow wells but deeper wells with poor wellhead security or soils prone to bypass flow can provide pathways for E. coli to travel deeper into groundwater. The main uses of wells are for domestic and/or stock supply but significant numbers of wells are also used for irrigation water. Groundwater is used as a source of drinking-water supply in most of the major population centres (Twizel, Tekapo, Omarama, Otematata, Mt Cook and Parsons Rock). Other public supplies rely on surface waters. I recommend that community drinking water supply wells should have their protection zones reviewed, and protection zones should be delineated for wells with no protections zones currently set.

Environment Canterbury Technical Report i The current state of groundwater quality in the upper Waitaki

ii Environment Canterbury Technical Report The current state of groundwater quality in the upper Waitaki

Table of contents

Summary ...... i

1 Project area ...... 1 1.1 Upper Waitaki ...... 1 1.1.1 General topography and geology...... 1 1.1.2 Groundwater occurrence ...... 2 1.1.3 Groundwater recharge ...... 3 1.1.4 Groundwater flow direction ...... 3 1.1.5 Groundwater age ...... 4 1.1.6 Groundwater chemistry ...... 4 1.1.7 Groundwater use...... 7

2 Contaminant sources ...... 9 2.1 Nutrients in groundwater ...... 10 2.1.1 Nitrate ...... 10 2.1.2 Phosphate ...... 16 2.2 Bacterial contamination in groundwater...... 19 2.3 Drinking water wells ...... 20

3 Summary of current state of groundwater quality ...... 22

4 Acknowledgements ...... 22

5 References ...... 23

Environment Canterbury Technical Report iii The current state of groundwater quality in the upper Waitaki

List of Figures

Figure 1-1: Location of the project area showing the upper Waitaki and Lower Waitaki sub- catchments ...... 1 Figure 1-2: Location of upper Waitaki sub-catchment and major lakes and towns ...... 2 Figure 1-3: Comparison of screen depth within the well and groundwater age ...... 4 Figure 1-4: Minimum conductivity recorded in wells from Environment Canterbury sampling or from consent monitoring data ...... 6 Figure 1-5: Minimum DO recorded in wells from Environment Canterbury sampling or from consent monitoring data ...... 7 Figure 1-6: Locations of wells used for the abstraction of water ...... 8 Figure 2-1: Locations of discharge sources which contribute nutrients to groundwater ...... 9 Figure 2-2: Maximum nitrate-N concentrations ever recorded in wells ...... 11 Figure 2-3: Locations of long-term monitoring wells...... 12 Figure 2-4: Trends in Environment Canterbury long-term monitoring wells ...... 13 Figure 2-5: Trends in Environment Canterbury long-term monitoring wells ...... 14 Figure 2-6: Locations of long-term monitoring wells H39/0229 and CA15/5007 and aerial view indicating areas of more intense land use where irrigation occurs ...... 15 Figure 2-7: Nitrate leaching risk map ...... 16 Figure 2-8: Maximum DRP concentrations ever recorded in wells...... 18 Figure 2-9: Phosphate leaching risk map ...... 19 Figure 2-10: Locations of E. coli detections in wells ...... 20 Figure 2-11: Individual household and public supply wells ...... 21

List of Tables

Table 1-1: Comparison of the median values of water parameters for upper Waitaki with whole Canterbury region ...... 5 Table 1-2: Well use breakdown for active and proposed wells ...... 8 Table 2-1: Comparison of nitrate-N concentrations in groundwater in upper Waitaki with whole Canterbury region during the last annual survey ...... 10 Table 2-2: Average DRP concentrations for wells of various depths ...... 17

iv Environment Canterbury Technical Report The current state of groundwater quality in the upper Waitaki

1 Project area The project area covers the Waitaki River catchment as shown in Figure 1-1. The area is divided into two sub-catchments; upper Waitaki and lower Waitaki, which are discussed in separate reports. This report discusses the upper Waitaki sub-catchment.

Figure 1-1: Location of the project area showing the upper Waitaki and Lower Waitaki sub- catchments

1.1 Upper Waitaki

1.1.1 General topography and geology The upper Waitaki sub-catchment has two major depressions, the Mackenzie and Omarama basins, with the underlying basement being composed of greywacke-argillite rock. The area can be divided into smaller sub-basins based on geological features, and glacial movements have created other troughs within the area. Surrounding hills have provided most of the deposits that have in-filled these

Environment Canterbury Technical Report 1 The current state of groundwater quality in the upper Waitaki

basins and which hold most of the groundwater in the catchment. Reworking of deposits during various glacial and inter-glacial periods has resulted in substantial heterogeneity. There are three large, natural lakes in the upper Waitaki sub-catchment (Lake Tekapo, Lake Pukaki and Lake Ohau), which are lined with thick silt deposits of glacial origin. Figure 1-2 shows the locations of these lakes. Other major Lakes (Lake Benmore, Lake Aviemore and Lake Waitaki) are man-made and were created to form part of the hydroelectricity generation network. These lakes lack the thick silt deposits of the natural lakes.

Figure 1-2: Location of upper Waitaki sub-catchment and major lakes and towns Land surrounding the basins and lakes is fairly steep and the northern part of the catchment includes New Zealand’s highest mountains. Most of the upper Waitaki soils are well drained. The soils are usually shallow or very shallow, with some parts of the basins having moderately deep or deep soils (Landcare Research, 2014).

1.1.2 Groundwater occurrence Quaternary deposits within the major basins are up to 500 m thick and host the main aquifers in the area (GHD, 2009). Shallow, unconfined, alluvial aquifers are located close to the active river beds, and these respond to rainfall and river level fluctuations. Cooksey (2008) suggested that the shallow

2 Environment Canterbury Technical Report The current state of groundwater quality in the upper Waitaki

groundwater is perched on a less permeable layer and only some water percolates down to recharge the deeper aquifers. The deeper, semi-confined aquifer does not seem to respond to river flows, rainfall events or barometric pressure changes. This aquifer is likely to be located within old fans deposited by rivers, but may not be laterally continuous. Large alluvial fans near the foothills also host groundwater which responds to rainfall events. In some areas this groundwater re-emerges as springs at the bottom of the fan, where a change in permeability is encountered. Groundwater is also known to occur within bedrock fractures and can re-emerge in springs even at high elevations. Flowing artesian wells have also been reported in moraines east of Lake Pukaki (Cooksey, 2008). There are limited numbers of drillers logs available within the area and many of the wells tend to be clustered around the townships or along the canals. There are very few deep wells to provide information about groundwater at depth.

1.1.3 Groundwater recharge The recharge sources in the upper Waitaki include: rainfall, rivers, lakes and canals. Distinguishing between the different recharge sources using water chemistry can be difficult, however, due to low concentrations of dissolved ions. Oxygen-18 testing, which has proven to be useful in differentiating river and rainfall recharge in Canterbury Plains, has limited applicability to the upper Waitaki where the differences in river and rainfall signatures are much smaller (Cooksey, 2008). The Mackenzie and Omarama basins are in the rain shadow of the Southern Alps. There is a sharp gradient in annual rainfall across the basins, varying from about 800 mm per year in the northwest to approximately 400 mm per year in the southeast. Even in the west the rainfall in the basin is significantly less than rainfall closer to the main divide. The monthly rainfall is reasonably constant throughout the year, with summers receiving slightly higher rainfall due to the north westerly winds. Glacial silts have created thick deposits at the bottom of the three major natural lakes (Tekapo, Pukaki and Ohau). The lakes are perched on these silt deposits and may not provide significant amounts of recharge to groundwater. When Lake Pukaki levels were increased as part of the hydroelectricity development, the surrounding groundwater levels did not rise, indicating minimal interaction between the lake and groundwater. The man-made Lake Ruataniwha lacks the thick silt deposits of the natural lakes and the local groundwater levels rose following the filling of this lake. Lake Ruataniwha may therefore contribute significant recharge to groundwater. Due to large daily water level variations in Lake Ruataniwha, there may also be a reversal of flows at different times of the day (Cooksey, 2008). Groundwater gains from rivers in some areas and loses to rivers in others. For example, Irishman Creek flows below ground for a substantial part of its length. Cooksey reported that in the Tekapo sub- basin, at high flow, surface water receives about 10% contribution from groundwater but at low flow the groundwater contribution is 80%. In the Twizel sub-basin, Cooksey estimated that surface waters lose about 35% to the groundwater system. The groundwater must however discharge into the lower reaches of the rivers near Lake Benmore as this lake provides the only exit point for water within the catchment. The canal networks used in hydroelectricity generation may also provide some recharge to the shallow groundwater, as they have been noted to lose water. Recently, works have been undertaken to install liners in the Tekapo canal where leakage threatened the canal wall stability (Genesis Energy, 2014).

1.1.4 Groundwater flow direction Lake Benmore provides the exit point for groundwater in the catchment, with groundwater flow generally following surface topography. There are however some barriers to groundwater flow in the catchment, which cause groundwater to discharge to surface water bodies. These may again lose to groundwater further downstream (GHD, 2009). For example, the Grays Hills are a barrier to flow, and force groundwater to move to the surface. This is evident in the form of the large swampy areas and from Grays River gaining flow. The Pukaki and Tekapo Rivers are likely to gain from groundwater in their lower reaches, although electrical conductivity data indicate that the lower Tekapo River reach does lose some flow to groundwater (see Figure 1-4 below). In general groundwater from the Mackenzie basin will flow towards the Haldon arm of Lake Benmore, whereas groundwater in the Omarama basin will mainly flow towards the Ahuriri arm of Lake Benmore. The groundwater divide is positioned somewhere near Wairepo Creek, but the exact location is unknown. Local scale groundwater flow directions are more difficult to establish because the small numbers of wells means there is not enough data available.

Environment Canterbury Technical Report 3 The current state of groundwater quality in the upper Waitaki

1.1.5 Groundwater age Cooksey (2008) summarised results from age analysis carried out by GNS (van der Raaij, 2008). The mean groundwater ages for wells sampled in the Mackenzie ranged from 11 to 115 years. Figure 1-3 below, adapted from Cooksey (2008), provides a summary of screen depth versus mean groundwater age. It indicates two distinct groupings of groundwater ages. Deeper wells generally had older groundwater than shallow wells. Shallow wells generally had mean groundwater ages of a decade or two. Deeper wells had groundwater ages between 80 and 100 years.

Figure 1-3: Comparison of screen depth within the well and groundwater age1

1.1.6 Groundwater chemistry Major ions present in groundwater either originate from the surrounding aquifer material or enter with water through the soil after rainfall. Measuring concentrations of major ions and other chemical parameters can help to define aquifer characteristics and flow paths, and to measure land use effects on groundwater quality. Environment Canterbury samples a number of wells on a regular basis as part of our regional monitoring programme. Data is also collected when wells are sampled during particular investigations. Table 1-1 shows the comparison of chemistry results in the upper Waitaki and the whole Canterbury region from the last Environment Canterbury annual survey. In general, the conductivity and average concentrations of major ions in upper Waitaki groundwater are lower than the concentrations in the broader region. This is likely to be due to the large component of alpine water in the groundwater recharge, greater distance from the coast and lower intensity of land use. Areas further away from the coast and at higher altitudes are likely to receive less sea spray and therefore have lower conductivity. On average the wells are also not as deep as those sampled throughout the whole Canterbury region, and this may also result in lower average concentrations of dissolved ions due to shorter residence times.

1 As age ranges have been suggested for most samples, the lower age has been used for plotting purposes. The two samples with no recommended age have been plotted using the suggested age from the CFC-12 analysis.

4 Environment Canterbury Technical Report The current state of groundwater quality in the upper Waitaki

Table 1-1: Comparison of the median values of water parameters for upper Waitaki with whole Canterbury region

Canterbury Upper Waitaki Bicarbonate (mg/L at 25°C) 61 36 Chloride (mg/L) 8.3 1.1 Calcium (mg/L) 17.9 7.2 Iron (mg/L) <0.02 0.03 Magnesium (mg/L) 4.5 1.6 Manganese (mg/L) 0.001 0.002 Potassium (mg/L) 1.3 0.7 Sodium (mg/L) 10.9 4.7 Nitrate Nitrogen (mg/L) 3.6 0.45 Reactive Silica (mg/L as SiO2) 16.5 14.1 Sulphate (mg/L) 9.1 2.0 Total Alkalinity (mg/L as CaCO3) 50 29 Total Ammoniacal-N (mg/L) < 0.01 < 0.01 Total Hardness (g/m3 as CaCO3) 66 25 Conductivity (mS/m) 19.4 6.9 Dissolved Oxygen (mg/L) 6.7 7.6 pH 7.1 6.9 Depth to Water (m) -5.3 -9.3 Water Temperature (°C) 12.4 11.3 Escherichia coli (MPN / 100mL) <1 <1 Total Coliforms (MPN / 100mL) <1 <1

Electrical Conductivity

Electrical Conductivity (referred to as conductivity in this report) is a measure of the ability of water to carry an electrical charge, and is directly related to concentrations of dissolved substances. Figure 1-4 shows the minimum conductivity measured in wells in the upper Waitaki. The wells with higher conductivity are either close to bedrock or in areas of more intensive land use. Cooksey (2008) suggested that surface waters recharge the shallow and perched groundwater near Twizel, and this appears to be reflected in the lower conductivities measured in that area. Similarly, lower conductivities near the Tekapo River suggest that the river is providing groundwater recharge. Concentrations of other ions generally follow the same pattern as conductivity.

Environment Canterbury Technical Report 5 The current state of groundwater quality in the upper Waitaki

Figure 1-4: Minimum conductivity recorded in wells from Environment Canterbury sampling or from consent monitoring data

Dissolved oxygen Oxygen is supplied to groundwater by recharge with oxygenated water, or by movement of air through the unsaturated material above the groundwater table. Water temperature affects the amount of oxygen which can dissolve in it: cold water can hold more oxygen than warmer water. Dissolved oxygen (DO) is important because it can impact on concentrations of other compounds present in groundwater. For example, in some areas low DO can result in high concentrations of dissolved iron and manganese and low concentrations of nitrate. Cooksey (2008) noted that DO is highest in active river beds and close to alluvial fans and lowest close to bedrock, within moraine deposits or in low permeability formations. Figure 1-5 shows the minimum DO ever recorded in each well and this aligns with Cooksey’s description. The DO results indicate that most of the sampled wells have high DO concentrations. Therefore, denitrification in groundwater is not likely to be significant in the catchment. Concentrations of iron and manganese are generally below their Guideline Values (set for aesthetic reasons) in drinking water, and no well has concentrations exceeding the Maximum Acceptable Value

6 Environment Canterbury Technical Report The current state of groundwater quality in the upper Waitaki

(MAV, set for health reasons) for either of these parameters. This further supports that denitrification is not prevalent in upper Waitaki groundwater.

Figure 1-5: Minimum DO recorded in wells from Environment Canterbury sampling or from consent monitoring data

1.1.7 Groundwater use The breakdown of water use for active and proposed wells in the catchment is shown in Table 1-2. Figure 1-6 shows the locations of wells for each water use. The main uses of wells are for domestic and/or stock supply. Significant numbers of wells are also used for irrigation water. Each of the major population centres relies on groundwater for at least part of its public water supply.

Environment Canterbury Technical Report 7 The current state of groundwater quality in the upper Waitaki

Table 1-2: Well use breakdown for active and proposed wells

Well use Number of wells Commercial / Industrial 3 Dairy Use 3 Domestic and Stock water 31 Domestic Supply 58 Irrigation 43 Public Water Supply 15 Small Community Supply 1 Stock Supply 10 Total 164

Figure 1-6: Locations of wells used for the abstraction of water

8 Environment Canterbury Technical Report The current state of groundwater quality in the upper Waitaki

2 Contaminant sources Discharges to land may introduce contaminants into groundwater. Figure 2-1 shows the locations of the main point-source discharge sources which are likely to contribute nutrients to groundwater in the upper Waitaki. There are also other diffuse sources that arise from agricultural activities. There are few domestic onsite wastewater treatment systems which discharge to land, due to the low population density. There are six community wastewater treatment plants which discharge to land. Twizel is the largest population centre, and is likely to contribute the most contaminants. There are four discharge consents for diary effluent.

Figure 2-1: Locations of discharge sources which contribute nutrients to groundwater

Environment Canterbury Technical Report 9 The current state of groundwater quality in the upper Waitaki

2.1 Nutrients in groundwater

2.1.1 Nitrate Nitrate is a plant nutrient which is applied as a fertilizer on land to improve plant growth. Nitrate also comes from grazing animals and effluent discharges. The nitrate molecule contains nitrogen, and typically we report nitrate concentrations in terms of nitrate nitrogen (nitrate-N). This is useful as nitrate can be converted to other nitrogen compounds in the environment by naturally occurring bacteria, but the amount of nitrogen remains the same unless it is removed from the system. The main forms of nitrogen in water are: nitrate, nitrite, ammonia and organic nitrogen. Dissolved inorganic nitrogen (DIN) refers to nitrate-N plus nitrite-N plus ammonia-N. Nitrate is the dominant form of DIN in natural oxygenated environments. That means that in groundwater, nitrate-N concentrations are roughly equivalent to concentrations of DIN. However, close to some discharge sources or where the water is low in dissolved oxygen, other forms of nitrogen may predominate. Nitrate-N concentrations in groundwater Nitrates occur naturally in groundwater, but generally at concentrations less than about 1 to 3 mg/L nitrate-N (Close and Smith, 2001; Chapelle, 1993; Madison and Brunett 1985 referenced in Hanson 2002). More recent analysis carried out by Morgenstern and Daughney (2012) shows that natural concentrations of nitrate-N in New Zealand groundwater are likely to be below 0.25 mg/L.

Table 2-1 summarises data from our last annual water quality survey, which show that the median and average concentrations in the upper Waitaki are much lower than the average for the whole of Canterbury, due to less intense land use. Table 2-1: Comparison of nitrate-N concentrations in groundwater in upper Waitaki with whole Canterbury region during the last annual survey

Nitrate Nitrogen Canterbury Upper Waitaki Median 3.60 0.45 Average* 4.74 0.91 Min <0.05 <0.05 Max 24.00 6.7 * Results below the detection limit are divided by two to calculate the average

Figure 2-1 shows the highest nitrate-N concentrations ever recorded in monitored wells in the upper Waitaki. This includes both Environment Canterbury data and data from consent monitoring. The reason for showing maximum concentrations is to emphasise areas where concentrations are always low, indicating that the groundwater is derived primarily from surface water recharge (such as near Twizel and near the Tekapo River), or from land with low nitrogen leaching rates. In areas dominated by land surface recharge, rainfall or irrigation carries nitrate down into the groundwater, resulting in higher concentrations. Nitrate travels with the groundwater until it discharges in spring-fed streams or into Lake Benmore. The concentration in the discharged water is the same as that in the groundwater. From Figure 2-2 it is evident that concentrations within the areas with more intense land use are elevated above natural concentrations. Nitrate concentrations in groundwater only decrease if they are diluted with water of lower nitrate concentrations, or if denitrification occurs under anoxic conditions. Anoxic conditions are not widely encountered in groundwater within the upper Waitaki. It is only in deeper wells, or wells in areas where groundwater flow is likely to be slow, that low DO levels encountered. Denitrification within groundwater is not likely to be significant in this catchment, and most of the nitrate is likely to remain within the shallow groundwater.

10 Environment Canterbury Technical Report The current state of groundwater quality in the upper Waitaki

Figure 2-2: Maximum nitrate-N concentrations ever recorded in wells

Why do we care about nitrate-N concentrations in groundwater? Increasing the extent and/or intensity of agricultural land use generally increases nitrate-N concentrations in groundwater. Increasing nitrate-N concentrations are of concern because: • Nitrate-N 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 when nitrate concentrations in public supplies exceed ½ MAV (5.6 mg/L).

• Nitrate-N can also be toxic for aquatic life in groundwater and groundwater-fed streams/rivers, having chronic (not acute) effects on aquatic life.

• Nitrogen (N) is a plant nutrient and contributes to nuisance periphyton and macrophyte growth in streams/rivers, increased algae (phytoplankton) growth in Lake Benmore, and associated deterioration of water quality (e.g. dissolved oxygen and pH) that can stress ecological values.

Environment Canterbury Technical Report 11 The current state of groundwater quality in the upper Waitaki

Environment Canterbury data Environment Canterbury samples groundwater on a regular basis to monitor the nitrate-N trends over time in our annual and quarterly monitoring netwrok. Other wells have only been sampled once or twice for a particular investigation and some data comes from consent monitoring results. There are 370 existing wells in the study area. Environment Canterbury has nitrate-N concentration data for 76 of these wells. Of the 76 wells sampled, 22 wells are part of Environment Canterbury’s regular, long-term monitoring programme (Figure 2-3). These wells are used to observe trends in nitrate-N over time. However, some of those wells have only recently been added to the programme and we do not have enough data to analyse their trends.

Quarterly

Figure 2-3: Locations of long-term monitoring wells. Areas of groundwater occurrence are shown in yellow There are eight wells which have been part of Environment Canterbury’s regular monitoring programme since 2007 or 2009; nitrate-N trends in these wells are shown in Figure 2-4. Seven of the wells do not have clear trends and their concentrations are generally low. However, well H38/0229 has elevated and increasing nitrate-N concentrations. When Environment Canterbury started sampling this well in 2009 the concentrations were close to natural levels, but they have increased over time and are now regularly over 2 mg/L. This well is close to a dairy farm which was developed in 2004 and shows clear effects of increasing nitrate-N concentrations, most likely resulting from land use intensification.

12 Environment Canterbury Technical Report The current state of groundwater quality in the upper Waitaki

Figure 2-4: Trends in Environment Canterbury long-term monitoring wells

There are 14 wells which were added to Environment Canterbury’s regular monitoring programme in 2012 or 2013 and their nitrate-N trends are shown in Figure 2-5. The sampling record is quite short, but it appears that 13 of these wells have fairly stable trends, even though some have concentration elevated above natural values.

Well CA15/5007 is located in an area with dairy farming and has very high and increasing trends of nitrate-N concentrations. This well shows clear effects of increasing nitrate-N concentrations resulting from land use intensification. The nitrate-N concentration in this well is approaching the MAV but there are no recorded drinking water wells in the vicinity of this well. The high concentrations could potentially have an impact on drinking water in wells down-gradient, however, and may impact on groundwater-fed surface waters.

Evaluation of the data from well CA15/5007 (Heller, 2015) indicated an inverse relationship between nitrate-N and groundwater level. There was some concern when the well was drilled that it was of insufficient depth to adequately describe groundwater conditions. This is because it only just penetrates groundwater and on some occasions may be more representative of drainage water than aquifer water quality. Heller considered that at low groundwater levels the measured nitrate-N concentrations may reflect local land surface recharge concentrations (of around 10 mg/L) more closely than groundwater nitrate-N. The highest SWL gives the lowest nitrate-N result of 2 mg/L. The situation is likely to become clearer as more data are gathered.

Environment Canterbury Technical Report 13 The current state of groundwater quality in the upper Waitaki

Figure 2-5: Trends in Environment Canterbury long-term monitoring wells

Figure 2-6 indicates the location of wells H39/0229 and CA15/5007 and also shows an aerial view of the area. It is likely that that the intensive agricultural lands use within this area is responsible for the increasing nitrate trend.

14 Environment Canterbury Technical Report The current state of groundwater quality in the upper Waitaki

Figure 2-6: Locations of long-term monitoring wells H39/0229 and CA15/5007 (indicated by yellow arrows) and aerial view indicating areas of more intense land use where irrigation occurs Groundwater which discharges to Lake Benmore, either directly or by discharging to rivers and streams, will show the effects of various upgradient land uses. We do not know if concentrations in the upper Waitaki are likely to increase to similarly high concentrations as those measured on the Canterbury Plains. We do however know that concentrations in some areas are elevated and increasing following recent intensification. Further development within the catchment is likely to cause further increases in nitrate-N concentrations. There may also be a time lag between land use changes and their effects, due to the time it takes for contaminants to travel through the unsaturated zone and through the aquifer. In areas where the water table is deeper it may take some years for the recharge to enter groundwater. Nutrients are likely to stay within the shallow groundwater system, but even then, it may take from years to a few decades for nutrients to travel to Lake Benmore. Nitrate leaching risk Figure 2-7 shows the nitrate leaching risk determined by Webb et al., 2010. This risk is based on the types of soils and their likelihood to provide conditions suitable for denitrification. Soils with a very high risk will leach more nitrate than soils with a very low risk.

Environment Canterbury Technical Report 15 The current state of groundwater quality in the upper Waitaki

The map shows that farms within the Willowburn, Wairepo Creek and Ben Omar areas have been developed mainly on very light soils with a high leaching risk. Restricting development on areas of very high leaching risk would give the greatest benefits in terms of reducing nitrate-N leaching to groundwater. Further work is underway to improve our understanding of nitrate leaching risk in some of these areas.

Figure 2-7: Nitrate leaching risk map

2.1.2 Phosphate 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. Phosphorus (P) is part of the phosphate molecule. Phosphate concentrations are normally reported in terms of P. Total phosphorus includes dissolved reactive phosphorus (DRP), soluble unreactive phosphorus and particulate (undissolved) bound forms. For groundwater, we are particularly interested in DRP which is readily available for plant growth.

16 Environment Canterbury Technical Report The current state of groundwater quality in the upper Waitaki

In New Zealand, the most common source of phosphorus is pollution from fertiliser use in agricultural areas (Rosen, 2001). Effluent is also very high in phosphorus, therefore effluent discharges to land will increase phosphorus in the soil. Phosphorus can also occur naturally in groundwater as it is present in some rocks and minerals. Although the phosphate ion, like the nitrate ion, is negatively charged, it interacts quite differently with soil. It adsorbs onto the soil particles and becomes immobile. High levels of calcium, iron, or aluminium will also reduce phosphorus solubility. Phosphorus loss therefore is mainly associated with runoff, where the phosphorus is carried as part of sediment into surface waters. Phosphorus can desorb from the sediment and result in nuisance growth in rivers and streams. While the main loss of phosphorus is via runoff, recent research has shown that phosphorus can leach through the soil if it is applied in excess of the soil’s capacity to retain it (Redding et al., 2006). This will depend on the type of soil and factors which affect soil sorption. Examples in Europe have shown that a build-up of excess phosphorus in soil over time will eventually lead to leaching into groundwater (Hesketh and Brookes, 2000). McDowell et al. (2014) recently published a paper which determined a linkage between soil and surface and groundwater enrichment with phosphorus. They demonstrated that soil was especially enriched under dairying with high cow numbers and high P-fertiliser use. They concluded that groundwater could contribute significant phosphorus to connected surface waters on soils prone to leaching and with intensive land use. DRP concentrations in groundwater Figure 2-8 shows the highest DRP concentrations ever recorded in each well in upper Waitaki. This includes both Environment Canterbury data and data from individual consent monitoring. It appears that DRP concentrations are the highest either in deeper wells, where they are likely to be natural, or in shallow wells (Table 2-2) where they are likely to be due to land use intensification. The wells in the Willowburn, Wairepo Creek and Ben Omar areas follow the expected pattern based on phosphate leaching risk (refer to the following section). This means that within the areas of intensive land use, land with a low leaching risk has lower groundwater DRP concentrations than areas with a moderate or a high leaching risk. Table 2-2: Average DRP concentrations for wells of various depths

Well depth (m) DRP (mg/L) 0-20 0.030 20-40 0.006 40-60 0.019 60-90 0.007

The observation of elevated DRP concentrations in shallow groundwater is at odds with two previous investigations (Hanson and Abraham, 2009; Hanson and Abraham, 2013) where the authors noted higher DRP concentrations in deep groundwater, likely due to reactions with the aquifer materials. In their investigations, shallow groundwater recharged from land surface had the lowest DRP concentrations and the authors concluded that leaching from the ground surface is not a major source of phosphorus in groundwater in those areas. In contrast to this, it appears that in upper Waitaki intensive land use may be a source of DRP in shallow groundwater which may contribute nutrients to surface waters. Further investigations would be necessary to exclude natural sources.

Environment Canterbury Technical Report 17 The current state of groundwater quality in the upper Waitaki

Figure 2-8: Maximum DRP concentrations ever recorded in wells

Phosphate leaching risk Figure 2-9 shows the phosphate leaching risk determined by Webb et al., 2010. This risk is based on P-retention in various soil types and the soil thickness. Soils with a very high risk will leach more phosphate than soils with a very low risk. This figure shows that farms within the upper Willow Burn and Wairepo Creek areas have been developed mainly on soils with a low risk of phosphate leaching. Areas close to active river beds have the highest leaching risk. Restricting development on areas of very high leaching risk would give the greatest benefits in terms of reducing phosphate leaching to groundwater. There are a number of tests available to monitor the phosphorus content in soils such as Olsen P, Resin P, total phosphorus and anion storage capacity tests. The most appropriate test will depend on the particular farming practices. Testing would ensure that phosphate is not applied above the soil holding capacity. Fertiliser applications would be adjusted depending on the results from testing to minimise the risk of leaching.

18 Environment Canterbury Technical Report The current state of groundwater quality in the upper Waitaki

Figure 2-9: Phosphate leaching risk map

2.2 Bacterial contamination in groundwater Escherichia coli (E. coli) is a common gut bacterium of warm-blooded organisms which is used as an indicator organism for the 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. E. coli can enter groundwater from septic tank discharges, effluent disposal or grazing animals, in areas where these discharges infiltrate to groundwater, especially after rainfall or with excessive irrigation. Because the unsaturated zone provides some pathogen removal, areas with thin soils and shallow groundwater are more prone to E. coli contamination. The MAV for E. coli is set at less than one per 100 mL of sample. I have plotted E. coli concentration data for all the wells we have tested on Figure 2-10. Many of these wells are used for stock supply, dairy or irrigation rather than for drinking water. The plot shows that in the upper Waitaki, E. coli have been detected in the areas of more intense land use where significant numbers of animals are present. E. coli are normally detected in shallow wells but deeper wells with

Environment Canterbury Technical Report 19 The current state of groundwater quality in the upper Waitaki

poor wellhead security or soils prone to bypass flow can provide pathways for E. coli to travel deeper into groundwater. There are 50 wells which have been sampled for E. coli and 14 (~30%) of those have had detections.

Figure 2-10: Locations of E. coli detections in wells

2.3 Drinking water wells Groundwater is used as a source of drinking-water supply in most of the major population centres within the upper Waitaki. Figure 2-11 shows the locations of active wells which supply drinking water to individual households and public supplies. Environment Canterbury’s database has active public supply wells listed in most of the major population centres in Twizel, Tekapo, Omarama, Otematata, Mt Cook and Parsons Rock. Other public supplies rely on surface waters. We do not know how many of the 105 drinking water wells (active and proposed) could potentially be contaminated with E. coli because we have not sampled all of these wells. If we assume that 30% of those wells could potentially be contaminated (in line with the proportion of all wells sampled which recorded positive E. coli detections) then that equals approximately 30 wells. However, the areas with

20 Environment Canterbury Technical Report The current state of groundwater quality in the upper Waitaki

E. coli detections currently do not have many drinking water wells and therefore the real number of drinking wells which could be contaminated with E. coli is likely to be lower.

Figure 2-11: Individual household and public supply wells

Existing community drinking water supply wells will have provisional protection zones as specified in the Canterbury Land and Water Regional Plan.

Environment Canterbury Technical Report 21 The current state of groundwater quality in the upper Waitaki

3 Summary of current state of groundwater quality The following general statements can be made about groundwater quality in the upper Waitaki: • The groundwater recharge sources include: rainfall, rivers, lakes and canals but distinguishing between the different recharge sources using water chemistry can be difficult. • The three large natural lakes are not likely to provide recharge to groundwater. • Lake Benmore provides the only exit point for groundwater in the catchment, with groundwater flow generally following surface topography. • There are areas which are intensively farmed and these are likely to contribute to a significant proportion of nutrients or E.coli that enter groundwater. These areas represent a small proportion of the total catchment. • The average nitrate-N concentrations in upper Waitaki are much lower than the average for the whole of Canterbury region, but in areas where land use is intensive the groundwater quality is degraded. • Large-scale denitrification is not expected throughout the catchment due to dissolved oxygen concentrations being generally high. • DRP concentrations in some shallow wells are elevated, and this may be due to land use activities. • Shallow wells in areas of effluent disposal or animal grazing are at a risk of pathogenic contamination. • The main uses of wells are for domestic and/or stock supply but significant numbers of wells are also used for irrigation water. • Each of the major population centres relies on groundwater for at least part of its public water supply. Protection zones should be established or reviewed for all of the public supply wells.

4 Acknowledgements

I would like to thank Carl Hanson, Helen Shaw and Kelly Palmer for providing useful comments on this report and Tom Heller for carrying out an independent external science review.

22 Environment Canterbury Technical Report The current state of groundwater quality in the upper Waitaki

5 References

Cooksey, K. (2008) Hydrogeology of the Mackenzie Basin. Master of Science in Engineering Geology thesis. University of Canterbury. Etheridge Z. & Scott M. Upper Waitaki limit setting process. Predicting consequences of future scenarios: Groundwater quality. Environment Canterbury Report No R15/61. Genesis Energy, Tekapo Canal Remediation Project, Accessed online 19 March 2014: https://www.genesisenergy.co.nz/c/document_library/get_file?uuid=ff815f4f-df80-47ce-8d89- 940c68d75614&groupId=10314 GHD, (2009) Cumulative Water Quality Effects of Nutrients from Agricultural Intensification in the Upper Waitaki Catchment, Groundwater Report, Prepared for Russell McVeagh on behalf of Mackenzie Water Research Limited. Hanson C., (2002) Nitrate concentrations in Canterbury groundwater – a review of existing data. Environment Canterbury Report No R02/17.

Heller T. (2015) External peer review comments for this 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.

Landcare Research (2014), S-map, GIS layer, Accessed February 2014. Ministry of Health (2008) Drinking-Water Standards for New Zealand 2005 (revised 2008). Published by the New Zealand Ministry of Health, Wellington. Morgenstern, U and Daughney C (2012) Groundwater age for identification of baseline groundwater quality and impacts of land-use intensification – The National Groundwater Monitoring Programme of New Zealand. Journal of Hydrology, Geological and Nuclear Sciences. 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. van der Raaij, R. (2008). Age dating of groundwaters from the Mackenzie Basin. Lower Hutt: Geological and Nuclear Sciences. Webb, T, Hewitt, A, Lilburne, L, McLeod, M and Murray Close (2010) Mapping of vulnerability of nitrate and phosphorus leaching, microbial bypass flow, and soil runoff potential for two areas of Canterbury, Environment Canterbury Technical Report, R10/125.

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