Summary of Existing Environmental Data for Foxton Wastewater Treatment Plant

Summary of Existing Environmental Data For Foxton Wastewater Treatment Plant

(LEI, 2014:B1)

Prepared for

Horowhenua District Council

Prepared by

August 2014

Summary of Existing Environmental Data For Foxton Wastewater Treatment Plant (LEI, 2014:B1)

Horowhenua District Council

This report has been prepared for the Horowhenua District Council by Lowe Environmental Impact (LEI). No liability is accepted by this company or any employee or sub-consultant of this company with respect to its use by any other parties.

Quality Assurance Statement

Task Responsibility Signature

Project Manager: Hamish Lowe

Prepared by: Philip Lake

Reviewed by: Hamish Lowe

Approved for Issue by: Hamish Lowe

Status: Final

Prepared by :

Lowe Environmental Impact Ref: Foxton_WWTP_B1 - P O Box 4467 Existing_environmental_data-FINAL Palmerston North 4462 Job No.: 10 172 | T | [+64] 6 359 3099 | E | [email protected] Date: August 2014 | W| www.lei.co.nz

TABLE OF CONTENTS

1 EXECUTIVE SUMMARY ...... 1

2 INTRODUCTION ...... 4

2.1 Purpose ...... 4

2.2 Background ...... 4

2.3 Scope ...... 4

3 ENVIRONMENTAL DATA SOURCES AND DESCRIPTIONS ...... 6

3.1 Environmental Data Sources ...... 6

3.2 Environmental Data Descriptions ...... 6

3.3 Completeness of Datasets...... 7

4 ENVIRONMENTAL DATA SUMMARIES ...... 8

4.1 Merging and Correlation of Data ...... 8

4.2 Manawatu River Flow and Level Data ...... 8

4.3 Rainfall and Evapotranspiration Data for Moutoa and Levin ...... 10

4.4 Water Quality Changes and River Flow Effects at Whirokino ...... 13

4.5 Water Quality Changes and River Flow Effects in Foxton Loop Near Foxton WWTP ...... 21

4.6 Comparison of Whirokino and Foxton Loop Water Quality Data ...... 26

4.7 Cawthron Institute Assimilative Capacity Report ...... 27

5 CONCLUSIONS ...... 30

5.1 Manawatu River Flow and Level Data ...... 30

5.2 Water Quality Changes and River Flow Effects at Whirokino ...... 30

5.3 Water Quality Changes and River Flow Effects in Foxton Loop ...... 31

5.4 Comparison of Whirokino and Foxton Loop Water Quality Data ...... 31

5.5 Cawthron Institute Assimilative Capacity Report ...... 32

6 RECOMMENDATIONS ...... 33

7 REFERENCES ...... 34

1 EXECUTIVE SUMMARY

Horowhenua District Council (HDC) operate the Foxton wastewater treatment plant (WWTP) which is currently located on land known as Matakarapa Island. This land is located south-west of the Foxton township and surrounded by the Foxton Loop, which is a former meander of the main Manawatu River channel that was bypassed with a new channel directly to the final reach into the Tasman Sea in 1943.

The WWTP consists of three oxidation ponds in series, with the effluent discharging into an open drain that leads to the western arm of the Foxton Loop about 1 km upstream of its confluence with the Manawatu River and its estuary.

The discharges of treated wastewater from the Foxton WWTP into the Foxton Loop require re- consenting by Horizons Regional Council (HRC) upon expiry of the current resource consents on 1 December 2014. HDC has committed to investigate changing the treated effluent discharge method prior to the expiry of the resource consents. An upgrade of the treatment system may also be required. The consenting and upgrade requirements will be influenced by understanding the existing environment among other factors such as the Manawatu River Accord which HDC and HRC are signatories to. This Accord requires all parties to avoid discharges of wastewater into the Manawatu River during periods of low flows (less than half median flow rate).

The purpose of this report is to identify and summarise the existing available information about the environment surrounding the Foxton WWTP and in particular the Manawatu River and Foxton Loop. The specific types of data sought and summarised are river levels, river flow rates, tidal impacts on river levels and flow rates, river and estuary water quality, and climate data (ie. rainfall and evapotranspiration).

The WWTP treatment performance and resource consent compliance data were not assessed in this report, as these aspects are assessed in other reports.

The following environmental data was obtained by LEI:

• River level data on an hourly basis at Teachers’ College in Palmerston North, Moutoa, and Foxton from 1 January 2008 to 31 December 2013 was obtained from HRC. • River flow rate data on an hourly average basis at Teachers’ College in Palmerston North and calculated on an hourly basis at Moutoa due to tidal influences on flows at Moutoa from 1 January 2008 to 31 December 2013 was obtained from HRC. • Daily total rainfall data and soil moisture deficit at Moutoa covering 1 January 1980 to 1 May 2012 was obtained from NIWA, and daily total rainfall data at Moutoa covering 2 January 2009 to 31 December 2013 was obtained from HRC. • Daily total rainfall data, soil moisture deficit, and various evapotranspiration data at Levin covering 1 January 1993 to 31 December 2013 was obtained from NIWA. • Water quality data at Whirokino about 3 km upstream of the Foxton WWTP discharge covering January 1990 to December 2013 and data for Foxton Loop Wharf covering July 2000 to June 2008 and for Foxton Wharf (adjacent to Manawatu Marine Boating Club on Hartley Street, Foxton Beach) for January 2010 to June 2014 was obtained from Aquanet and HRC. • Water quality data for the Foxton WWTP effluent and within Foxton Loop 200 m upstream and downstream of the Foxton WWTP discharge covering November 2009 to December 2013 was obtained from Beca on behalf of HDC. The monitoring frequency was weekly during summer months (November to March) and monthly during the remainder of each year.

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This environmental data was supplemented by a report by Cawthron Institute that assessed the assimilative capacity of the Manawatu Estuary for contaminant loads.

The river level data was able to be used by LEI to assess the nature of the tidal cycle occurring during most of the sampling times at Whirokino and within the Foxton Loop between 1 January 2008 and July 2012. The river flow data was able to be used to assess whether the Manawatu River was in flood or at very low flows during each of the sampling times at Whirokino and within the Foxton Loop between 1 January 2008 and July 2012.

A very small number of samples were collected on the same day since 2009 by HRC at Whirokino and by HDC in the Foxton Loop which allowed an assessment of the degree of connection between the water quality of the main Manawatu River, its estuary, and Foxton Loop. A larger number of samples were collected on the same day by HRC at Whirokino and at Foxton Loop Wharf on Harbour Street, Foxton, during 2000-08. A comparison of these datasets showed no relationship between the water quality of the two sites.

A number of Foxton WWTP effluent samples were collected at times similar to the Foxton Loop samples, and these were analysed for a range of parameters ranging from only E. coli to a total of eight parameters. Comparisons of the effluent results with the Foxton Loop results were possible on these occasions to determine whether the Foxton WWTP effluent was the likely cause of the water quality changes in the Foxton Loop and the potential maximum extent of any changes that could be attributed to the effluent discharge.

Overall, the data indicated that the Manawatu River water quality within the estuary and Foxton Loop is primarily influenced by upstream sources, flooding, and tidal restrictions on river flow rates through the estuary into the Tasman Sea. The water quality within Foxton Loop appeared to be independent of the water quality measured at Whirokino, and this indicated that the Foxton Loop is likely to be subjected to other sources of contaminants, and not just influenced by the discharge of the Foxton WWTP discharge.

A long term trend of gradually decreasing dissolved reactive phosphorus (DRP) was apparent at Whirokino, but all other parameters remained stable within broad ranges.

LEI’s assessment of HRC’s Whirokino monitoring data found that E. coli, nitrate-nitrogen, TSS and turbidity at Whirokino clearly increased with increasing river flow rates. Ammoniacal nitrogen concentrations were generally lowest when river flow rates were below HMF (34.6 m3/s), and gradually increased with increasing river flow rates. DRP concentrations were lowest when river flow rates were less than HMF, but otherwise had no relationship with river flow rates. DO had a weak relationship of increasing with increasing river flow rates. Conductivity and pH clearly decreased with increasing river flow rates, particularly when flows were below HMF.

E. coli, TSS and turbidity at Whirokino were generally highest during low tides. Nitrate-nitrogen concentrations had a weak tendency to be highest during high tides. Ammoniacal nitrogen, DRP, pH concentrations had no relationship with tidal phases. Conductivity strongly tended to be highest during high tides and outgoing tides (which reflects the presence of seawater at Whirokino during these tidal phases). DO strongly tended to be lowest (worst quality) during incoming tides.

The ammoniacal nitrogen concentrations in the Foxton Loop near the Foxton WWTP discharge had a strong tendency to be highest both upstream and downstream of the discharge during low tides in Foxton Loop and/or when river flow rates were between HMF and median flow (ie. 34.6

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– 69.2 m 3/s). The ammoniacal nitrogen concentration changes in the Foxton Loop were generally minor (less than 1.0 g/m 3) compared with the effluent concentrations.

The E. coli concentrations in Foxton Loop near the Foxton WWTP discharge had a strong tendency to be lowest both upstream and downstream of the discharge during outgoing tides and clearly increased with increasing river flow rates.

The Foxton WWTP effluent was not capable of causing the upper range (>1,000 cfu/100mL) of changes in E. coli concentrations observed in the Foxton Loop; however the effluent was potentially responsible for smaller E. coli increases of 10’s to 100’s cfu/100 mL. These results indicate that other sources of E. coli are significant contributors to the bacterial contamination of the Foxton Loop.

The scBOD 5 concentrations in the Foxton Loop were consistently very low and any changes were within or below detection limits.

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2 INTRODUCTION

2.1 Purpose

This report summarises the nature of the available environmental data, the timeframes of each dataset, the extent of missing data, and the relationships between the data. This report also aims to build understanding of the existing environment around the Foxton WWTP and into which the treated effluent is currently discharged. The data and this report can then be used in future for informing decisions on any upgrades to the Foxton WWTP, any changes to the location and nature of the treated effluent discharges, and for applying to HRC (and potentially also HDC) for the appropriate resource consents.

2.2 Background

HDC operate the Foxton WWTP on Matakarapa Island which discharges the treated wastewater into the Foxton Loop. The discharges require re-consenting by HRC upon expiry of the current resource consents on 1 December 2014. HDC has committed to investigate changing the treated effluent discharge method prior to the expiry of the resource consents. An upgrade of the treatment system may also be required. The consenting and upgrade requirements will be influenced by understanding the existing environment among other factors such as the Manawatu River Accord which HDC and HRC are signatories to. This Accord requires all parties to avoid discharges of wastewater into the Manawatu River during periods of low flows (less than half median flow rate).

In December 2013 Cawthron Institute completed a study that assessed the assimilative capacity of the Manawatu Estuary for contaminant loads including those of the Foxton WWTP. They concluded that the lower Manawatu River’s assimilative capacity was limited by tidal influences, and that effluent should only be discharged during outgoing tides. Lower flow rates in the river resulted in lower assimilative capacities. The average treated effluent flow rates from the Foxton WWTP were 0.01% of the average Manawatu River flow rates, and the combined upstream sources of contaminant loads limited or already exceeded the river’s assimilative capacity in the estuary.

LEI have been providing advice to HDC regarding treated effluent discharge to land as an alternative or supplement to the existing discharges into the Foxton Loop. LEI have also been assisting HDC with identifying potential sites for land application of treated effluent and with assessing the suitability of these sites for land application.

2.3 Scope

Relevant environmental data relating to the Manawatu Estuary and Foxton Loop have been sourced from relevant public agencies such as HRC and NIWA. The data includes hydrological and water quality data for the Manawatu Estuary, and meteorological data (primarily rainfall and evapotranspiration) that relates to river flow rates and soil moisture and/or groundwater levels. This data has been summarised to allow an overview assessment of the impact of the WWTP discharge on the Foxton Loop to be undertaken. It should be noted that this report is merely a

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presentation of existing data and no interpretation is provided. Further, there are other and more detailed trend analyses that could be applied.

The WWTP treatment performance and resource consent compliance were not required to be assessed in this report, as these aspects are assessed in other reports.

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3 ENVIRONMENTAL DATA SOURCES AND DESCRIPTIONS

3.1 Environmental Data Sources

The sources of environmental data obtained by LEI were from HRC, NIWA, and Beca on behalf of HDC. The Manawatu Estuary assimilative capacity report by Cawthron Institute was also a useful reference for additional data and interpretation.

The following environmental data was found to be available:

• Manawatu River flow and level data at various locations from 1880 to present day with data currently recorded at 5 minute intervals (the reliability of this data is reduced prior to January 1924); • Total daily rainfall data at Moutoa from 1 October 1941 to present day (monitoring by NIWA was transferred to HRC on 1 May 2012); • Total daily rainfall and potential evapotranspiration data at Levin from 1 November 1990 to present day; • Water quality data for Manawatu River at Whirokino from January 1990 to present day; • Water quality data for Foxton Loop at Foxton Wharf on Harbour Street, Foxton for July 2000 to June 2008; • Water quality data for Manawatu River at Foxton Wharf (adjacent to Manawatu Marine Boating Club on Hartley Street, Foxton Beach) for January 2010 to June 2014; and • Water quality data for Foxton Loop and Foxton WWTP effluent from November 2009 to present day.

3.2 Environmental Data Descriptions

The following environmental data was obtained by LEI:

• River level data on an hourly basis at Teachers’ College in Palmerston North, Moutoa, and Foxton Beach (at the Foxton Beach boat club wharf on Hartley Street); while this data is available at 5 minute intervals hourly data from 1 January 2008 to 31 December 2013 was obtained from HRC for this report. • River flow rate data on an hourly average basis at Teachers’ College in Palmerston North and calculated on an hourly basis at Moutoa due to tidal influences on flows at Moutoa. Flow rate data from 1 January 2008 to 31 December 2013 was obtained from HRC for this report. • Daily total rainfall data and soil moisture deficit at Moutoa covering 1 January 1980 to 1 May 2012 was available from NIWA, and daily total rainfall data at Moutoa covering 2 January 2009 to 31 December 2013 was obtained from HRC. • Daily total rainfall, soil moisture deficit, and evapotranspiration data at Levin covering 1 January 1993 to 31 December 2013 was obtained from NIWA. • Water quality data at Whirokino about 3 km upstream of the Foxton WWTP discharge from January 1990 to December 2013. The frequency of data is variable, but generally monthly. A wide range of analytical parameters was monitored, but some parameters were only monitored for portions of the timeframe that this dataset covered. • Water quality data for Foxton Loop at Foxton Wharf on Harbour Street, Foxton from July 2000 to June 2008. The frequency of data is monthly during July to June of 2000/01, 2003/04, and 2007/08. A wide range of analytical parameters was monitored, but E. coli were not monitored during 2000/01. • Water quality data for Manawatu River at Foxton Wharf on Hartley Street, Foxton Beach (adjacent to Manawatu Marine Boating Club) from January 2010 to June 2014. The

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frequency of data is variable, but generally monthly during winter months, and weekly during summer months. A wide range of analytical parameters was monitored monthly, while E. coli was monitored weekly. • Water quality data within Foxton Loop 200 m upstream and downstream of the Foxton WWTP discharge covering November 2009 to December 2013. The only analytical parameters monitored were ammoniacal nitrogen, E. coli, and scBOD 5. The monitoring frequency was weekly during summer months (November to March) and monthly during the remainder of each year.

3.3 Completeness of Datasets

The HRC river level data for Foxton Beach was complete for every hour of every day between 1 January 2008 and 31 December 2013, but the level sensor was noted as erroneously indicating a continuous level of 1168 mm from 6 pm on 22 July 2011 to 10 am on 25 July 2011, and the readings did not make sense for actual river flow rates and tidal influences until 3pm on 25 July 2011. The Manawatu River was entering high flood conditions when the sensor became static, and was still dropping from high flood conditions when it began reading correctly again. The river flow rate peaked at about 349 m3/s at the Teachers’ College monitoring station at about 7 pm on 23 July 2011.

The daily rainfall and soil moisture deficit data series for Moutoa was missing 454 days of data between 1 January 1980 and 1 May 2012, and out of these it was missing 74 days of data after 1 January 2002, and 51 days after 1 January 2009. The largest continuous data gap occurred from 2 April 1987 to 1 August 1987. Most other data gaps were only 1 – 4 days in duration, but some were 10 – 16 days long. Only one day of daily rainfall data was missing from the HRC rainfall data set for Moutoa between 1 January 2009 and 31 December 2013.

The daily rainfall and soil moisture deficit data series for Levin had no missing days, but the three evapotranspiration data series for Levin were missing between 100 and 204 days of data between 1 January 1993 and 31 December 2013. All three of these evapotranspiration data series were missing 11 days after 1 January 2009. The duration of any continuous data gap was generally only 1 – 4 days at a time.

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4 ENVIRONMENTAL DATA SUMMARIES

4.1 Merging and Correlation of Data

The HRC river level data for Foxton Beach was able to be used to assess the phase of the tidal cycle occurring during each of the HRC sampling times at Whirokino after 1 January 2008 and within the Foxton Loop between November 2009 and June 2012. This only applied to when sampling times were recorded in the monitoring data provided to LEI. The HRC river flow data was able to be used to assess whether the Manawatu River was in flood or at very low flows during each of the sampling times at Whirokino and within the Foxton Loop after 1 January 2008.

There was an overlap between the rainfall data obtained from NIWA and that obtained from HRC. Both organisations were recording rainfall at Moutoa between 2 January 2009 and 1 May 2012. This overlap enabled a comparison of the consistency of the rainfall measurements to be undertaken.

HRC monitored water quality of the Manawatu River at Whirokino and Foxton Loop at Foxton Wharf on Harbour Street, Foxton on the same day on 35 occasions between August 2000 and July 2008. This enabled comparisons of the water quality to determine whether the Foxton Loop water quality was related to or different from the Manawatu River water quality at Whirokino.

The Manawatu River water quality was monitored at Whirokino by HRC and in the Foxton Loop by HDC on the same day on five occasions after 2 November 2009. This has enabled a very limited comparison of the HRC and HDC data for assessing whether the Foxton Loop water quality was related to or different from the Manawatu River water quality at Whirokino during more recent years than the previous HRC monitoring of Foxton Loop.

4.2 Manawatu River Flow and Level Data

Figure 4.2.1: Hourly flow rate and level data for Manawatu River

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Figure 4.2.2: Daily Statistics for Manawatu River levels at Foxton Beach

The hourly river flow data at Teachers’ College is presented in Figure 4.2.1 with the water level data as measured at Foxton Beach. HRC advise that the very gentle gradient and minimal change of height of the Manawatu River between Foxton and Foxton Beach enables the Foxton Beach water level data to be viewed as representative of Foxton Loop water levels as well.

Figure 4.2.1 shows that the river levels at Foxton Beach are primarily responsive to tidal cycles but there is clearly also an underlying relationship between water levels and major rainfall events causing high flow rates (floods) and general seasonal changes (wetter months versus drier months). The daily minimum, mean, and maximum Foxton Beach water levels presented in Figure 4.2.2 more clearly separate these features of the river level trends, and can be readily compared with the maximum hourly flow rates at Teachers’ College.

The lowest water level at Foxton Beach during 1 January 2008 – 31 December 2013 was 103 mm above mean sea level (AMSL) (Wellington datum) and this occurred on 29 March 2010. The highest water level at Foxton Beach during this period was 3,069 mm AMSL and this occurred on 23 July 2009 during an exceptionally high tide and moderate flood flows of about 100 m 3/s at Teachers’ College and 128 m 3/s at Moutoa.

HRC advised that a flood return period of 1:100 is expected to generate a maximum water level of 4,050 mm AMSL, and a flood return period of 1:200 is expected to generate a maximum water level of 4,150 mm AMSL at Foxton Beach. The river flow rates at Teachers’ College that match these flood return periods are 4,000 m 3/s and 4,600 m 3/s respectively. For comparison, the February 2004 flood event had a peak flow rate of 3,515 m 3/s at the Teachers’ College flow gauge. HRC advise that the Moutoa and Whirokino river flow rate generally corresponds with about 120% of the Teachers’ College flow rate.

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The overall statistics for HRC’s Moutoa and Teachers’ College river flow rate data during 2008- 2013 were as follows:

Table 4.4.4.24. 222.1.1.1.1:::: Statistics for Manawatu River Flow Rates (m(m(m 333/s)/s)/s) –––source:–source: HRC Statistical Measure Teachers’ College Moutoa Estimate Minimum 9.1 22 Mean 104 130 Maximum 2,479 1,694

The reasons for the much lower maximum flow rate at Moutoa compared with the Teachers’ College measurements are not clear, particularly when the HRC advise that Moutoa flow rates should be about 120% of the Teachers’ College flow rates. Because the Moutoa flow rates are calculated while the Teachers’ College flow rates are actual measurements, the Teachers’ College flow rate data has been used throughout this report for consistency and improved reliability of data. This ignores the delay in flow pulses traversing the 80 km to Foxton Beach and the tidal influences on flow rates between Moutoa/Opiki and Foxton Beach.

Cawthron (2013) relied upon HRC’s advice that daily river flow rates at Whirokino are assumed to be 120% of the Teachers’ College daily flow rates. Table 4.2.2 presents the flow statistics for the Manawatu River at Teachers’ College and calculated on this basis at Whirokino as quoted by Cawthron (2013:A3).

Table 4.2.2: Manawatu River Flow Statistics Monitoring Mean Annual Half Median Median 20FEP Location Low Flow Teachers’ College 16.0 m3/s 34.6 m3/s 69.2 m3/s 152.8 m3/s Whirokino 19.2 m3/s 41.6 m3/s 83.1 m3/s 183.4 m3/s

4.3 Rainfall and Evapotranspiration Data for Moutoa and Levin

The daily rainfall data as obtained from NIWA for Moutoa and Levin is presented in Figures 4.3.1 and 4.3.2 below, and a correlation between the two sites for 1993-2012 is presented in Figure 4.3.3.

Figure 4.3.1: NIWA Daily Total Rainfall Data for Moutoa 1980-2012

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Figure 4.3.2: NIWA Daily Total Rainfall Data for Levin 1993-2013

Figure 4.3.3: Correlation of NIWA Daily Total Rainfall Data for Moutoa and Levin for 1 January 1993 – 1 May 2012

A comparison of the NIWA daily total rainfall data for Levin and Moutoa demonstrated that the overall average daily rainfall and total rainfall for the 10.5 year period were almost identical (less than 1% difference between the two sites) despite the obvious day to day differences shown in Figure 4.3.3 above.

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Figure 4.3.4: Correlation of NIWA and HRC Rainfall Data for Moutoa for 2 January 2009 – 1 May 2012

A comparison of the overlapping 2009-2012 rainfall data for Moutoa obtained from NIWA and HRC is presented in Figure 4.3.4 above. Analysis of the data demonstrated that the HRC measurements were overall lower than the NIWA measurements by about 10-15%. Individual days of data also generally did not match, with one data set often indicating no rainfall while the other indicated measured rainfall; this was not able to be explained by different daily recording times (NIWA used 8am to 8am NZST while HRC used midnight to midnight). This is a much worse correlation than that found for NIWA’s data for Levin and Moutoa.

Figure 4.3.5: Daily Total Evapotranspiration at Levin 1993-2013

Figure 4.3.5 above clearly shows the seasonal variations in the daily total Priestley-Taylor evapotranspiration measured for Levin. Significant rainfall during summer months also clearly reduces the evapotranspiration for short periods of time. The trend is quite consistent across the 20 years of data, so the gaps in data could conceivably be filled by referring to similar months and rain events in other years where the data is complete.

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4.4 Water Quality Changes and River Flow Effects at Whirokino

The Whirokino sampling site used by HRC for water quality monitoring of the Manawatu River is located inland of the estuary about 2 km upstream of the confluence with the western arm of Foxton Loop and about 3 km from the Foxton WWTP effluent discharge into the western arm of Foxton Loop. HRC’s hourly river level data for Foxton Beach was able to be used to assess whether the tide was rising or falling at most of the times of sampling at Whirokino after 1 January 2008. The HRC monitoring results are also graphed against the river flow rates as measured at the Teachers’ College gauge in Palmerston North in order to assess the influence of river flow rates on contaminant concentrations.

Figure 4.4.1: Ammoniacal Nitrogen Concentrations at Whirokino

Figure 4.4.2: Ammoniacal Nitrogen at Whirokino Against River Flow Rate

These graphs show that the ammoniacal nitrogen concentrations at Whirokino have been stable and mostly under 0.15 g/m 3. The lowest ammoniacal nitrogen concentrations (<0.04 g/m 3) most commonly occurred when river flow rates were below the half median flow (HMF) rate of 34.6 m3/s, the highest ammoniacal nitrogen concentrations occurred across a wide range of river flow rates, and overall there appeared to be a gradual small increase in ammoniacal nitrogen

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concentrations with increasing river flow rates. A tidal cycle assessment of the 2008—13 data showed little or no relationship with tidal phases.

Figure 4.4.3: Nitrate Nitrogen Concentrations at Whirokino

Figure 4.4.4: Nitrate Nitrogen at Whirokino Against River Flow Rate

These graphs show that the nitrate nitrogen concentrations at Whirokino have been more variable than ammoniacal nitrogen (the peaks and troughs appear to reflect annual cycles) but generally remained under 1.5 g/m 3 throughout the series. There is a clear relationship of increasing nitrate nitrogen concentrations with increasing river flow rates. The nitrate nitrogen results were mostly less than 0.5 g/m 3 when river flow rates were below the HMF of 34.6 m3/s, and the highest nitrate nitrogen results (above 1.0 g/m 3) generally occurred when river flow rates were above the median flow rate of 69.2 m3/s. A tidal cycle assessment of the 2008—13 data showed that the highest concentrations also tended to occur during high tides, but this relationship was not particularly strong.

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Figure 4.4.5: E. coli Concentrations at Whirokino

Figure 4.4.6: E. coli at Whirokino Against River Flow Rate

These graphs show that the E. coli concentrations at Whirokino have been reasonably stable and generally less than 10,000 MPN. There is a clear relationship of increasing E. coli with increasing river flow rates. The lowest E. coli results (<80 MPN/100 mL) all occurred when river flow rates were less than the median flow rate of 69.2 m3/s, and the highest E. coli results (>1,000 MPN/100 mL) most commonly occurred when river flow rates were above the median flow rate of 69.2 m3/s. A tidal cycle assessment of the 2008—13 data showed that the highest concentrations tended to occur during low tides, and this relationship was quite strong.

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Figure 4.4.7: Dissolved Reactive Phosphorus Concentrations at Whirokino

Figure 4.4.8: Dissolved Reactive Phosphorus at Whirokino Against River Flow Rate

These graphs show that the dissolved reactive phosphorus (DRP) concentrations at Whirokino have gradually decreased by about 50% throughout the 22.5 year duration of this series. There appears to be little or no overall relationship between DRP and river flow rates, but almost all of the lowest results (<0.01 g/m 3) occurred when river flow rates were below the HMF of 34.6 m3/s. A tidal cycle assessment of the 2008—13 data showed no relationship with tidal phases.

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Figure 4.4.9: pH at Whirokino

Figure 4.4.10: pH at Whirokino Against River Flow Rate

The pH was generally consistently around 7.5 until 2005 when it became more variable both downwards and upwards. There is a clear relationship of decreasing pH with increasing river flow rates, particularly when river flow rates were less than the 20 th flow exceedance percentile rate (20FEP) of 152.8 m3/s. The highest pH results (above 8.0) almost all occurred when river flow rates were below the HMF of 34.6 m 3/s. A tidal cycle assessment of the 2008—13 data showed no relationship with tidal phases.

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Figure 4.4.11: Dissolved Oxygen at Whirokino

Figure 4.4.12: Dissolved Oxygen at Whirokino Against River Flow Rate

The dissolved oxygen (DO) concentration was consistently around 7-12 g/m3 until 2007 when it became more variable both downwards and upwards. There appears to be a weak relationship of increasing dissolved oxygen concentrations with increasing river flow rates. The lowest (worst) dissolved oxygen concentrations (<6.5 g/m3) all occurred when river flow rates were between HMF and 20FEP (ie. 34.6 – 152.8 m3/s). A tidal cycle assessment of the 2008—13 data showed that the lowest (worst) DO concentrations tended to occur during incoming tides (when slack water would be most common), and the highest (best) DO concentrations tended to occur during low tides (when river flow and aeration would be expected to be at their optimum).

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Figure 4.4.13: Conductivity at Whirokino

Figure 4.4.14: Conductivity at Whirokino Against River Flow Rate

The original conductivity data showed a 10-fold increase after 2006 which is suspected by LEI to be a change of measurement units from µS/cm to mS/m in July 2006. The graphs above show the conductivity data after adjusting the more recent data to a tenth of their original results.

Apart from a number of notable peaks, the conductivity results were generally stable and low. There is a strong relationship of decreasing conductivity with increasing river flow rates when river flow rates were less than HMF of 34.6 m3/s. All of the highest conductivity results occurred when river flow rates were less than HMF of 34.6 m3/s.

A tidal cycle assessment of the 2008-13 data showed that the lowest conductivities tended to occur during low and incoming tides, and the highest conductivities tended to occur during high and outgoing tides; these results reflect the seawater content of the river due to tidal phases.

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Figure 4.4.15: Total Suspended Solids Concentrations at Whirokino

Figure 4.4.16: Total Suspended Solids at Whirokino Against River Flow Rate

These graphs show that the peak total suspended solids concentrations at Whirokino have been variable throughout the duration of this series but the more usual concentrations have been consistently under 100 g/m 3. There is an overall relationship of increasing total suspended solids with increasing river flow rates, particularly for river flow rates above the median flow rate of 69.2 m3/s. A tidal cycle assessment of the 2008—13 data showed that the highest concentrations tended to occur during low tides, while the lowest concentrations tended to occur during high tides, and these relationships were strong.

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Figure 4.4.17: Turbidity at Whirokino

Figure 4.4.18: Turbidity at Whirokino Against River Flow Rate

These graphs show that the peak turbidity results at Whirokino have been variable throughout the duration of this series but the more usual concentrations have been consistently under 100 NTU. There is a clear relationship of increasing turbidity with increasing river flow rates, particularly for river flow rates above HMF of 34.6 m3/s. A tidal cycle assessment of the 2008— 13 data showed that the highest turbidity results tended to occur during low tides, the lowest turbidity results tended to occur during high tides, and these relationships were strong. As expected, the turbidity results mirrored the total suspended solids results.

4.5 Water Quality Changes and River Flow Effects in Foxton Loop Near Foxton WWTP

When the exact sampling times were recorded in the HDC monitoring dataset for Foxton Loop near the Foxton WWTP discharge, HRC’s hourly river level data for Foxton Beach was able to be used to assess whether the tide was rising or falling at the times of sampling. LEI noted that the sampling site location for the higher contaminant results usually reflected the actual direction of flow within the Foxton Loop caused by tidal fluctuations ie. during incoming tides, the Foxton

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Loop flows towards the “upstream” sampling location, and this “upstream” location usually gave higher results than the “downstream” sampling location during incoming tides. Unfortunately, the time of sampling was not always recorded in the HDC dataset, so LEI assumed that the maximum result for each sampling event was always the true downstream result, and that the other (minimum) result was always the true upstream or background result for each sampling event.

The graphs below show the minimum, maximum, and changes in contaminant concentrations in Foxton Loop for each sampling event. This data is also graphed against the river flow rates as measured at the Teachers’ College gauge in Palmerston North in order to assess the influence of river flow rates on the minimum contaminant concentrations and on the changes in contaminant concentrations.

Figure 4.5.1: Minimum (Upstream) Ammoniacal Nitrogen in Foxton Loop Against River Flow Rate

Figure 4.5.1 indicates that the highest upstream or background ammoniacal nitrogen results most commonly occurred when river flow rates are between HMF and 20FEP (ie. 34.6 – 152.8 m3/s), but otherwise there is little or no relationship between upstream ammoniacal nitrogen concentrations and river flow rates. Two of these results were higher than the highest results for Whirokino. A tidal cycle assessment of the data showed no clear relationship of upstream ammoniacal nitrogen concentrations with tidal phases.

Figure 4.5.2: Maximum (Downstream) Ammoniacal Nitrogen in Foxton Loop Against River Flow Rate

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The highest downstream ammoniacal nitrogen results were notably higher than the highest results for Whirokino and the highest results for the upstream Foxton Loop results. The highest downstream ammoniacal nitrogen concentrations most commonly occurred when river flow rates are between HMF and 20FEP (ie. 34.6 – 152.8 m 3/s), but otherwise there is little or no relationship between downstream ammoniacal nitrogen concentrations and river flow rates. A tidal cycle assessment of the data showed that the highest downstream ammoniacal nitrogen concentrations generally occurred during low tides, and this relationship was strong.

Figure 4.5.3: Ammoniacal Nitrogen Concentration Changes in Foxton Loop

Figure 4.5.4: Ammoniacal Nitrogen Changes in Foxton Loop Against River Flow Rate

Figures 4.5.3 and 4.5.4 show that the changes in ammoniacal nitrogen concentrations in Foxton Loop have been variable but still small at less than 1.0 g/m 3 compared with the effluent concentrations of up to 39 g/m 3 and an average of 13.6 g/m 3. Most of the results have remained low and unchanged during the four years of monitoring, but the highest results (above 0.15 g/m 3) have all occurred between May 2011 and August 2013. The greatest changes in ammoniacal nitrogen concentrations almost always occurred when river flow rates were between HMF and median flow rates (ie. 34.6 – 69.2 m 3/s), but otherwise there was no relationship between changes in ammoniacal nitrogen concentrations and river flow rates.

A tidal cycle assessment of the data showed that the greatest changes in ammoniacal nitrogen concentrations generally occurred during low tides, and this relationship was strong.

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Figure 4.5.5: Minimum (Upstream) E. coli Concentrations in Foxton Loop Against River Flow Rate

Figure 4.5.5 shows a clear trend of increasing upstream or background E. coli concentrations with increasing river flow rates. Low upstream E. coli results (<100 cfu/100 mL) only occurred when river flow rates were less than the median flow rate of 69.2 m 3/s.

A tidal cycle assessment of the data showed that the highest upstream E. coli concentrations tended to occur during low and incoming tides, and the lowest results tended to occur during outgoing tides. These tidal phase relationships were quite strong.

Figure 4.5.6: Maximum (Downstream) E. coli Concentrations in Foxton Loop Against River Flow Rate

Figure 4.5.6 shows a trend of more gradually increasing downstream E. coli concentrations with increasing river flow rates. Low downstream E. coli results (<100 cfu/100 mL) only occurred when river flow rates were less than the median flow rate of 69.2 m 3/s, and there are no longer any results below 26 cfu/100 mL. At very low river flow rates (10 – 20 m 3/s), the highest downstream E. coli results tend to be higher than at river flow rates between 20 m 3/s and HMF (34.6 m 3/s).

A tidal cycle assessment of the data showed that the lowest downstream E. coli concentrations tended to occur during outgoing tides, but there was no clear relationship with the other tidal phases.

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Figure 4.5.7: E. coli Concentration Changes in Foxton Loop

Figure 4.5.8: E. coli Concentration Changes in Foxton Loop Against River Flow Rate

Figures 4.5.7 and 4.5.8 show that the changes in E. coli in Foxton Loop have generally been less than 1,000 cfu/100 mL across a wide range of river flow rates, and the usual ranges of results each year have not changed during the four years of monitoring. There appears to be a relationship of increasing E. coli concentration changes with increasing river flow rates for river flows above the median flow rate of 69.2 m 3/s, but this is much weaker than the relationship of the minimum (upstream) E. coli concentrations increasing with increasing river flow rates. At very low river flow rates (10 – 20 m 3/s), the changes in E. coli in Foxton Loop tend to be higher than at river flow rates between 20 m 3/s and HMF (34.6 m 3/s).

A tidal cycle assessment of the data showed that the lowest changes in E. coli concentrations tended to occur during outgoing tides, but there was no clear relationship with the other tidal phases.

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Figure 4.5.9: E. coli Concentration Changes in Foxton Loop Against Potential Change Attributable to Foxton WWTP Effluent

The blue line in Figure 4.5.7 indicates the highest possible change in E. coli in Foxton Loop if the Foxton WWTP effluent was undiluted (i.e. the Loop contained only the effluent). This is of course impossible because the daily effluent volume discharged is minor compared with the actual volume and flow rates of Foxton Loop, and this does not take into account the dilution by Foxton Loop water. Any results that fall below this line have exceeded the highest possible change attributable to the Foxton WWTP effluent and therefore these E. coli increases must have alternative sources.

It is notable on this graph that a significant number of results fall into this group, including all of the increases in Foxton Loop E. coli concentrations that were higher than 1,000 cfu/100 mL. It is clear that the Foxton WWTP discharge can only be responsible for E. coli increases of 10’s to 100’s cfu/100 mL.

A review of HRC’s water quality data for Foxton Loop Wharf sampling location during 2000-08 indicated that the E. coli and ammoniacal nitrogen results were similar to the Foxton Loop results near the Foxton WWTP discharge during 2009-13, but the lack of overlapping sampling dates did not allow for any assessments of relationship of the Foxton Loop Wharf water quality with the WWTP effluent quality or Foxton Loop water quality near the WWTP discharge.

A review of HRC’s water quality data for Foxton Beach Wharf and HDC’s water quality data for Foxton Loop near FWWTP also did not have a direct relationship with each other or with the FWWTP wastewater discharge quality.

4.6 Comparison of Whirokino and Foxton Loop Water Quality Data

On five occasions both HRC and HDC sampled the Manawatu River at Whirokino and Foxton Loop near the Foxton WWTP discharge respectively at similar times on the same days since 2009. The only parameters common to both sets of monitoring were E. coli and ammoniacal nitrogen.

This enabled a comparison of these water quality monitoring results to be undertaken by LEI as an indication of whether the Foxton Loop water quality was linked to that of the main river flow entering the estuary from Whirokino or whether its water quality was independent of changes in the Manawatu River water quality. The Foxton Loop sampling sites were located 200 m upstream and downstream of the Foxton WWTP discharge point.

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Table 4.6.1 below provides the results of these comparisons (note that the original order of upstream and downstream results for Foxton Loop is shown here and does not necessarily reflect the actual tidal flow direction). Table 4.64.64.6.14.6 .1.1.1:::: Comparisons of Manawatu River and Foxton Loop WaterWater Quality Data

Sampling Date Ammoniacal Nitrogen (g/m 3) Escherichia coli (cfu/100 mL) and River Flow Manawatu Foxton Foxton Manawatu Foxton Foxton Conditions River at Loop Loop River at Loop Loop Whirokino Upstream Downstream Whirokino Upstream Downstream 7/04/2011 52 m 3/s 0.127 0.07 0.07 300 540 400 Incoming tide 14/06/2012 56 m 3/s 0.11 0.09 0.46 100 47 49 Outgoing tide? 11/10/2012 57 m 3/s 0.053 0.10 0.06 16 59 39 Incoming tide? 15/08/2013 110 m 3/s 0.004 0.04 0.04 434 660 540 Incoming tide? 10/10/2013 97 m 3/s 0.036 0.01 0.01 102 87 93 Incoming tide?

Although this is a very small number of results over a period of about 2.5 years, it would seem apparent that the water quality of Foxton Loop is independent of the water quality of the Manawatu River at Whirokino. The Whirokino sampling site is located inland of the estuary about 2 km upstream of the confluence with the western arm of Foxton Loop and about 3 km from the Foxton WWTP effluent discharge into the western arm of Foxton Loop.

The Manawatu River at Whirokino had higher ammoniacal nitrogen concentrations than either of the Foxton Loop samples on the first and last occasions, and similar to the lower results for Foxton Loop on two other occasions. The Foxton Loop contained about 10 times as much ammoniacal nitrogen as Whirokino on 15 August 2013 due to a very low (but not unusually low) Whirokino result.

The Manawatu River at Whirokino had higher E. coli concentrations than either of the Foxton Loop samples on the second and last occasions, and lower than either of the Loop results on the other three occasions.

A review of HRC’s water quality data for Foxton Loop Wharf and Manawatu River at Whirokino sampling locations indicated that both sites were sampled on the same day 35 times between August 2000 and July 2008. Comparisons of the matched results for ammoniacal nitrogen, nitrate nitrogen, E. coli, and DRP confirmed that Foxton Loop Wharf and Whirokino concentrations did not have any relationship with each other. Foxton Loop generally had much higher DRP and much lower E. coli and nitrate-N concentrations than the Manawatu at Whirokino. This confirms that the Foxton Loop water quality both near the WWTP and near Foxton township is influenced mostly by sources other than the Manawatu River upstream of Whirokino.

4.7 Cawthron Institute Assimilative Capacity Report

A number of key points raised by Cawthron Institute’s assimilative capacity report were of relevance to the environmental data summary addressed in this report, and these are summarised below.

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Cawthron observed that the tidal influences on water levels and flow rates are clearly evident at the Moutoa gauge even when the Manawatu River is flowing above the 20FEP rates of 152.8 m3/s when measured at the Teachers’ College gauge. The Foxton Loop and Manawatu Estuary in the vicinity of Whirokino was therefore expected to be affected by tides to at least the same extent throughout at least the same range of river flow rates.

Cawthron determined that the tidal influences prevented the Manawatu River estuary having any available assimilative capacity, as determined by HRC’s OnePlan water quality targets, during incoming tidal phases. Therefore any potential assimilative capacity was only available during outgoing tides. However, the existing upstream water quality as measured at Whirokino indicated that some of the One Plan water quality targets were already exceeded as a result of upstream contaminant sources. The assimilative capacity of the estuary was therefore calculated on the basis of discharge rates that would not increase the Foxton Loop water contaminant concentrations beyond the limit of detection for each water quality parameter.

The key parameters found to be potentially limiting the discharge of Foxton WWTP effluent into Foxton Loop and the Manawatu Estuary were identified as DRP, soluble inorganic nitrogen (SIN), ammoniacal nitrogen, and E. coli. Table 6 of Cawthron’s report presented the averages of these parameters in the Manawatu River at Whirokino for several different key flow rate brackets, and this table is shown below:

Note: 20 th percentile flow = 152.8 m 3/s, median flow = 69.2 m 3/s, half median flow = 34.6 m 3/s at HRC’s Teachers’ College flow gauge.

The data in this table clearly shows increases in mean SIN and median E. coli with increasing river flow rates. Mean DRP rises to a peak during mid to low flows but overall decreases with increasing river flow rates. Mean ammoniacal nitrogen rises to a peak during mid to low flows but otherwise has no distinct relationship with river flow rates.

The capacity of the Manawatu River for assimilating Foxton WWTP effluent was found to be limited by ammoniacal nitrogen for river flows exceeding the 20FEP, and limited by SIN for all other river flows that were less than the 20FEP. As would be expected, the effluent SIN loads able to be assimilated by the Manawatu River reduced substantially as the river flow rates decreased. The actual mean SIN load of the Foxton WWTP effluent exceeded the assimilative

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limit for SIN when river flows were at or below the 20FEP flow rate i.e. based on HRC One Plan water quality targets there is no assimilative capacity during non-flood flows.

The hydrology of Foxton Loop was noted to be different from the Manawatu River due to its limited through flow and its inability to fully empty through its downstream leg and flush out contaminants at each low tide. Contaminants were therefore expected to accumulate within Foxton Loop, even if discharging effluent from the WWTP only occurred during the outgoing tidal phases.

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5 CONCLUSIONS

5.1 Manawatu River Flow and Level Data

River flow and level data is held by HRC dating back as far as 1880, but the reliability of this data is reduced as it becomes more historic. HRC currently collect river flow and level data at 5 minute intervals, but LEI obtained hourly data for 2008-13 from HRC for this assessment.

The river flow and level data has no gaps since 1 January 2008, but the river level monitor at Foxton Beach boat club wharf was faulty for almost three days during flooding conditions in July 2011. HRC advise that the very gentle gradient and minimal change of height of the Manawatu River between Foxton and Foxton Beach enables the Foxton Beach water level data to be viewed as representative of Foxton Loop water levels as well.

The river levels at Foxton Beach are primarily responsive to tidal cycles but there is clearly also an underlying relationship between water levels and major rainfall events causing high flow rates (floods) and general seasonal changes (wetter months versus drier months).

The calculated river flow rate data for Moutoa did not match the expected 120% of Teachers’ College flow rates, particularly for the highest flow rates, so for the sake of consistency and known accuracy the Teachers’ College flow rate data was used throughout this report as the basis for flow rate assessments at Foxton. This ignored the delay in flow pulses traversing the 30 km to Foxton and the tidal influences on flow rates between Moutoa/Opiki and Foxton.

5.2 Water Quality Changes and River Flow Effects at Whirokino

The water quality data at Whirokino is comprehensive and generally monthly since January 1990 for a wide range of analytical parameters, but some parameters were only monitored for portions of the timeframe that this dataset covered. LEI correlated the water quality data for Whirokino and Foxton Loop with tidal phases and river flow rates between 1 January 2008 and June 2012 when sampling times were identified and able to be matched with the hourly river flow rate and water level data.

Overall, the combined water quality, flow rate, and water level data for Whirokino indicated that the Manawatu River water quality upstream of Foxton Loop is primarily influenced by existing upstream contaminant loads, river flow rates (ie. low flows through to flooding), and tidal influences on seawater content and river flow rates through the estuary into the Tasman Sea.

Cawthron’s assessment of Whirokino environmental data indicated that mean SIN and median E. coli increased with increasing river flow rates.

LEI’s assessment found that E. coli, TSS and turbidity at Whirokino were generally highest during low tides and clearly increased with increasing river flow rates. Ammoniacal nitrogen concentrations were generally lowest when river flow rates were below HMF (34.6 m 3/s), and gradually increased with increasing river flow rates, but had no relationship with tidal phases. Nitrate-nitrogen concentrations had a weak tendency to be highest during high tides and clearly increased with increasing river flow rates.

DRP concentrations gradually fell by about 50% during the 22 year period of monitoring data. DRP concentrations were lowest when river flow rates were less than HMF, but otherwise had no relationship with river flow rates, and also had no relationship with tidal phases.

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DO strongly tended to be lowest (worst quality) during incoming tides but had only a weak relationship of increasing with increasing river flow rates. River pH clearly decreased with increasing river flow rates but had no relationship with tidal phases. Conductivity clearly decreased with increasing river flow rates, particularly when flows were below HMF, and strongly tended to be highest during high tides and outgoing tides (which reflects the presence of seawater at Whirokino during these tidal phases).

5.3 Water Quality Changes and River Flow Effects in Foxton Loop

The water quality within the Foxton Loop 200 m upstream and downstream of the Foxton WWTP discharge has been monitored weekly during summer months (December to March) and monthly during the remainder of each year. The dataset obtained by LEI covered November 2009 to December 2013 and monitored ammoniacal nitrogen, E. coli, and SC BOD 5 as required by the resource consent conditions for the Foxton WWTP effluent discharge.

LEI found that ammoniacal nitrogen concentrations had a strong tendency to be highest both upstream and downstream of the discharge during low tides in Foxton Loop and/or when river flow rates were between HMF and median flow (ie. 34.6 – 69.2 m3/s). The E. coli concentrations in Foxton Loop had a quite strong tendency to be lowest both upstream and downstream of the discharge during outgoing tides. Upstream E. coli concentrations clearly increased with increasing river flow rates, while this trend was less apparent for the downstream E. coli concentrations in Foxton Loop.

The Foxton WWTP effluent was not capable of causing the upper range (>1,000 cfu/100mL) of changes in E. coli concentrations observed in the Foxton Loop. The effluent was potentially responsible for smaller E. coli increases of 10’s to 100’s cfu/100 mL. The observed changes in ammoniacal nitrogen concentrations in the Foxton Loop have been variable but still small at less than 1.0 g/m 3 compared with the effluent concentrations of up to 39 g/m 3 and an average of 3 13.6 g/m . The SC BOD 5 concentrations in the Foxton Loop were consistently very low and any changes were barely detectable.

HRC’s water quality data for Foxton Loop at the Foxton Wharf sampling location generally mirrored the Foxton Loop results near the Foxton WWTP discharge, but did not have any relationship with the WWTP effluent quality. This confirms that the Foxton Loop water quality both near the WWTP and near Foxton township is influenced mostly by sources other than the Foxton WWTP discharge. The data also showed a tendency for reduced DRP and ammoniacal nitrogen since 2011, an increased upper range of nitrate nitrogen since 2011, and no change of range over the years for E. coli.

5.4 Comparison of Whirokino and Foxton Loop Water Quality Data

The water quality of the Foxton Loop appeared to be independent of the water quality of the Manawatu River at Whirokino. Only a very small number of concurrent water quality results for E. coli and nitrate-nitrogen were available for the HDC sampling sites near the Foxton WWTP discharge since 2009 for making this assessment. However, a larger number of HRC’s water quality results for Foxton Loop Wharf and Manawatu River at Whirokino sampling locations indicated that the water quality of these two sites did not have any relationship with each other over a range of parameters during 2000/01, 2003/04, and 2007/08. These comparisons indicated that Foxton Loop water quality both near the WWTP and near Foxton township is likely to be subjected to different sources of contaminants from the sources of contaminants for the Manawatu River at Whirokino.

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5.5 Cawthron Institute Assimilative Capacity Report

The Cawthron report identified tidal and river flow rate effects on contaminant concentrations at Whirokino. They determined that the Manawatu River and estuary only have potential assimilative capacity for additional contaminant loads from Foxton WWTP during outgoing tidal phases. This assimilative capacity is limited by its capacity for ammoniacal nitrogen when river flow rates are above 20FEP, and limited by its capacity for SIN at all other flow rates. The One Plan target concentrations for E. coli and SIN were found to be exceeded at Whirokino when river flow rates are higher than the median flow rate, while the One Plan target for DRP is exceeded at all river flow rates.

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6 RECOMMENDATIONS

The Teachers’ College flow rate data should be used as the basis for calculating daily minimum, average, and maximum flow rates at Foxton.

The total daily rainfall data recorded for Levin should be used to fill gaps in the NIWA data for Moutoa prior to 1 May 2012, and to substitute for Moutoa data since that date. The Levin evapotranspiration data should be used as the de facto evapotranspiration data for Moutoa.

The water quality of the Foxton Loop should be recognised as being independent of the water quality of the main Manawatu River at Whirokino. The water quality of the Foxton Loop should also be recognised as being subjected to significant contaminant loads from sources other than the Foxton WWTP effluent. The observed changes in E. coli concentrations in Foxton Loop often exceeded the highest possible changes in E. coli concentrations that could be caused by the Foxton WWTP effluent discharge.

Future monitoring records for Whirokino and Foxton Loop should record the time of day and the observed tidal phase if at all possible, and the upstream versus downstream monitoring results for Foxton Loop should reflect the true flow direction occurring in Foxton Loop at the times of sampling.

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7 REFERENCES

Allen C, Young RA, Gillespie P 2013. Foxton wastewater — estuary assimilative capacity assessment. Prepared for Lowe Environmental Impact. Cawthron Report No. 2440.

Horizons Regional Council 2011. Proposed One Plan Schedule D as amended by Environment Court Decisions as at 9 May 2013.

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