Lowe Environmental Impact Limited - Horizons · 2019. 11. 19. · Stream, 2013-2016 (Aquanet, 2016:B6b) Additional technical information referenced in the above reports were not reviewed

Lowe Environmental Impact Limited The Pot Levin WWTP

Groundwater Assessment

June 2018

GHD, 2018:E2

GHD | Report for Lowe Environmental Impact Limited - The Pot Levin WWTP, 51/37501/ | i

Executive summary Horowhenua District Council (HDC) propose to upgrade and re-consent the Levin wastewater land treatment system (LWWLTS) This report is a technical assessment of the groundwater system and the potential effects on groundwater quality associated with the existing and proposed wastewater land disposal system. This report is subject to, and must be read in conjunction with, the limitations set out in Section 1 and the assumptions and qualifications contained throughout the Report.

Wastewater is stored in the unlined Pot pond and discharged to land via spray irrigation into the forested area surrounding the pond. HDC proposes to make changes to the irrigation regime, including an increase in the total area of irrigation and decrease in irrigation depth per event. No changes are proposed to the management of the Pot pond and seepage rates through the base of the Pot pond are expected to remain the same.

A summary of the environmental setting is included, which relies on the previous reports published by Lowe Environmental Impact (LEI) and Aquanet. This information was used to develop a conceptual model of the groundwater and surface water systems, which provided the basis for identifying the likely sources, pathways, and receptors associated with the losses of wastewater from storage and irrigation. A 2-D numerical groundwater model was developed to better understand groundwater flow paths, pore velocity, and hydraulic gradients, all of which influence contaminant migration through the groundwater system.

Key findings of the assessment were:

The application of wastewater to land and storage of wastewater at the Pot pond have resulted in localised mounding of the groundwater table.

Surface drains provide a good mechanism for intercepting shallow groundwater impacted by wastewater. This water ultimately flows into Waiwiri Stream and then to the coast.

Seepage from the Pot pond appears to be the main influence on the local groundwater system. Therefore, proposed changes to the irrigation regime are unlikely to have a significant impact on the local groundwater system.

Modelling shows that variations in pond level will affect groundwater fluxes and contaminant travel times.

Groundwater travel times are relatively long, with an estimate of five years for a particle of groundwater to travel a distance of 200 m from the Pot pond to the nearest drain (Drain 3). Groundwater travel times beneath the Bare Sands irrigation area and deep groundwater flow paths will be longer due to the low hydraulic gradient.

Groundwater quality has been influenced by seepage from the Pot pond and beneath wastewater irrigation areas. The surface water quality in drains and Waiwiri Stream has also been affected by the discharge of nutrient rich groundwater.

There is no available information that supports significant attenuation via denitrification in the groundwater system. Therefore, it is a reasonable conclusion to conserve the nitrogen mass in groundwater, with the concentration of nitrate-nitrogen likely to be reduced by dilution and dispersion.

The proposed changes to the irrigation regime will result in an initial decrease in the mass nitrogen flux to groundwater. However, as wastewater inflows increase the estimated mass nitrogen flux will increase by approximately 6% (Y35) compared to the current scenario.

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At the western site boundary, the model indicates that approximately 80% of the groundwater is captured by Drain 4, with 20% flowing to the coast. It is difficult to quantify the amount of groundwater flow past the site boundaries in other directions, however it is considered likely that a high proportion of the shallow groundwater will be captured by the surface water system (drains and ultimately into Waiwiri Stream).

The effect on groundwater and groundwater users outside of the property boundaries was not directly assessed. However, given the action of the surface drains to intercept groundwater, it is unlikely that the discharge will affect existing groundwater users in the area.

It is considered that the hydraulic properties of the groundwater system are likely to support a range of potential mitigation methods to reduce nitrate-nitrogen concentrations in Waiwiri Stream, such a permeable reactive barrier and/or wetlands. The potential mitigation options are addressed in the mitigation package report. The mitigation toolbox is intended to reduce the mass of contaminants discharging to the receiving environments, and overall enhance the immediate environment.

GHD | Report for Lowe Environmental Impact Limited - The Pot Levin WWTP, 51/37501/ | iii

Table of contents 1. Introduction ............................................................................................................................... 1

1.1 Background ..................................................................................................................... 1 1.2 Purpose of this report ...................................................................................................... 1 1.3 Limitations ....................................................................................................................... 1 1.4 Assumptions.................................................................................................................... 2

2. Description of proposal .............................................................................................................. 1

2.1 The Pot Water Balance ................................................................................................... 1 2.2 Wastewater nitrogen concentrations ................................................................................ 1 2.3 Wastewater Discharge .................................................................................................... 1

3. Environmental Setting ............................................................................................................... 5 3.1 Introduction ..................................................................................................................... 5 3.2 Geology .......................................................................................................................... 5 3.3 Hydrogeology .................................................................................................................. 5 3.4 Hydrology ...................................................................................................................... 10

4. Assessment ............................................................................................................................ 12 4.1 Introduction ................................................................................................................... 12 4.2 Conceptual Groundwater Model .................................................................................... 12 4.3 Groundwater Flow ......................................................................................................... 13 4.4 Groundwater Quality...................................................................................................... 17

5. Findings .................................................................................................................................. 22

5.1 Discussion ..................................................................................................................... 22 5.2 Conclusion .................................................................................................................... 22

6. References ............................................................................................................................. 24

Table index Table 2-1 Wastewater Quality (2012-2017) ......................................................................................... 1

Table 2-2 Summary of Discharge Parameters (annual) ....................................................................... 3

Table 2-3 Drainage losses................................................................................................................... 4

Table 3-1 Hydraulic Gradients ............................................................................................................. 6

Table 3-2 Hydraulic Conductivity Values ............................................................................................. 7

Table 3-3 Bores located within 2.5 km of The Pot (from LEI, 2016:B3) ............................................... 10

Table 3-4 Stream Gauging Results (Aquanet, 2016:B6b) ................................................................... 10

Table 3-5 Surface Water Chemistry (median values from Aquanet, 2016:B6b) .................................. 10

Table 4-1 Average Particle Velocities from Seep/W Model ................................................................ 15

Table 4-2 Seepage Flux from Pond - Medium Conductivity Value ...................................................... 16

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Table 4-3 Seepage Flux from Pond - High Conductivity Value ........................................................... 16

Table 4-4 Seepage Flux into Drain 3 ................................................................................................. 17

Table 4-5 Seepage Flux into Drain 4 ................................................................................................. 17

Table 4-6 Indicator Parameters for denitrification (Rivas et al 2015) ................................................... 19

Table 4-7 Mass flux ........................................................................................................................... 20

Figure index Figure 2-1 Current Irrigation Layout, Bare Sand (green) and Ring Main (red) Irrigation Lines ............... 2

Figure 3-1 Daily Pot Pond Levels January 2012 to June 2015 (from LEI 2016:B3) ............................... 6

Figure 3-2 Local Bores within 2.5 km of The Pot and Tucker Block (from LEI, 2016:B3) ....................... 9

Figure 4-1 Conceptual Model (W-E transect) ..................................................................................... 13

Figure 4-2 Conceptual Model (SW-NE transect) ................................................................................ 13

Figure 4-3 Groundwater Flow Paths (W-E transect) ........................................................................... 14

Figure 4-4 Groundwater Flow Paths (SW-NE transect) ...................................................................... 15

Figure 4-5 Pond to Drain 3 Shortest Flow Path .................................................................................. 16

Figure 4-6 Processes affecting nitrogen speciation in aquatic systems (Bohlke et al, 2006) ............... 18

Appendices Appendix A – Geological Logs

Appendix B – Groundwater Contours

Appendix C – Groundwater Chemistry

Appendix D – Seep/W

GHD | Report for Lowe Environmental Impact Limited - The Pot Levin WWTP, 51/37501/ | 1

1. Introduction GHD Limited (GHD) was engaged by Horowhenua District Council (HDC) to prepare an assessment of groundwater effects from the proposed re-consenting and upgrade of the Levin wastewater land treatment system (LWWLTS).

1.1 Background

The HDC engaged Lowe Environmental Impact (LEI) to manage the re-consenting and upgrade to the LWWLTS. Over the past five years HDC and LEI have undertaken a range of environmental investigations and monitoring programmes. GHD has relied upon those reports and others to construct the conceptual site model and undertake the assessment of potential effects on the groundwater system. Specifically, the following reports have been relied upon to inform our conceptual understanding of groundwater system and overall wastewater discharge proposal:

Levin Wastewater Discharge, The Pot Discharge Description and Assessment of Effects to Land (LEI, 2017:D1,E1)

Levin Wastewater Land-Application Re-consenting: The Pot Groundwater Monitoring (LEI, 2016:B3b)

Groundwater Monitoring 2017 (Memorandum) (LEI, 2017:B3c)

Levin WWTP land application at “The Pot”: Water quality and ecology of the Waiwiri Stream, 2013-2016 (Aquanet, 2016:B6b)

Additional technical information referenced in the above reports were not reviewed in detail as part of this phase of works. However, a review of groundwater data and interpretation presented in LEI (2016:B3b) and LEI (2017:B3c) memorandum was undertaken.

1.2 Purpose of this report

The purpose of the report is to provide a technical assessment of the groundwater system and the potential effects on groundwater quality associated with the existing and proposed wastewater land disposal system.

1.3 Limitations

This report: has been prepared by GHD for Horowhenua District Council and may only be used and relied on by Horowhenua District Council for the purpose agreed between GHD and Horowhenua District Council as set out in section 1 of this report.

GHD otherwise disclaims responsibility to any person other than Horowhenua District Council arising in connection with this report. GHD also excludes implied warranties and conditions, to the extent legally permissible.

The services undertaken by GHD in connection with preparing this report were limited to those specifically detailed in the report and are subject to the scope limitations set out in the report.

The opinions, conclusions and any recommendations in this report are based on conditions encountered and information reviewed at the date of preparation of the report. GHD has no

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responsibility or obligation to update this report to account for events or changes occurring subsequent to the date that the report was prepared.

The opinions, conclusions and any recommendations in this report are based on assumptions made by GHD described in this report. GHD disclaims liability arising from any of the assumptions being incorrect.

GHD has prepared this report on the basis of information provided by Lowe Environmental Impact Limited and others who provided information to GHD (including Government authorities), which GHD has not independently verified or checked beyond the agreed scope of work. GHD does not accept liability in connection with such unverified information, including errors and omissions in the report which were caused by errors or omissions in that information.

1.4 Assumptions

GHD has made the following assumptions, in addition to those addressed in the limitation statement, in preparing the report:

The monitoring data contained in the LEI reports is representative of the current state of the environment.

The changes to the irrigation approach and the associated leaching of nutrients to groundwater is adequately represented by nutrient modelling undertaken by LEI.

Hydraulic performance of the aquifer is represented by the slug testing of shallow piezometers.

Bore logs collected during piezometer installation are representative of the unconfined groundwater system.

The drainage network and surface water system is represented by the information presented in Aquanet (2016:B6b).


2. Description of proposal The following provides a brief description of the LWWLTS, including the proposed changes to the land application system. The aspects of the proposal to renew and amend the storage and discharge of wastewater at the Pot which influence groundwater are addressed herein.

2.1 The Pot Pond Water Balance

There are no changes proposed to the management of The Pot pond level and storage volume. All of the changes relate to the irrigation practice, which is discussed below.

Following the replacement of the rising main between the wastewater treatment plant (WWTP) and the Pot, conveyance losses were considered to be negligible. Therefore, LEI (2017:D1,E1) estimated pond seepage using the measured flow data between the influent volume entering the Pot and the effluent volume applied to land. The water balance calculated that approximately 448,801 m3 of water is lost to ground via seepage per year, which is comprised mostly of wastewater (~423,961 m3) with rainfall (minus evaporation) equating to 6% of the wastewater volume (i.e. 24,840 m3).

The seepage rate estimated through the base of the pond is therefore 17.5 mm/d based on the water balance and an approximate pond wetted area of seven hectares. The seepage rate is equivalent to a flux of 0.018 m/d.

2.2 Wastewater nitrogen concentrations

Wastewater characteristics are detailed in Table 6.1 of LEI (2017:D1,E1). Summary statistics of nitrogen concentrations obtained from historical wastewater samples, described in LEI (2017:D1, E1), are provided in Table 2-1. The data presents the wastewater nitrogen concentrations stored in The Pot pond prior to irrigation, or lost to ground via seepage. The data indicates that the composition of nitrogen in wastewater is primarily in its ammonium form. Phosphorus concentrations in the wastewater were measured as Total-P and in its dissolved form as DRP (Dissolved Reactive Phosphorus). DRP is more mobile in waters and susceptible to leaching.

Table 2-1 Wastewater Quality (2012-2017)

Parameter (g/m3)

n range mean median 95 %ile

Total N 261 21-53 39 40 47

NH4 - N 238 15.3-53.8 34.5 34.8 42

Nitrate - N 61 0.01-2.61 0.44 0.24 1.53

DRP 63 2.8-8.1 5.2 5.2 7.6

2.3 Wastewater Discharge

Treated wastewater is discharged to land at The Pot land via two mechanisms; seepage from the unlined inter-dune basin which is used as a buffer storage pond for wastewater (referred to as ‘The Pot pond’), and spray irrigation.


The Pot pond level is drawn down (through increased irrigation) during the months of January to April and allowed to refill from May to December due to higher flow volumes from the LWWTP and lower irrigation rates. The proposed monthly irrigation schedule is described in LEI (2017:D1,E1), with winter application rates of 25 mm/event increasing to up to 45 mm/event in summer months. The ability to irrigated up to 50 mm/event will provide some contingency for HDC to managing extreme events.

Spray irrigation occurs in two areas of the broader Site; Ringmain and Bare Sands (Figure 2-1). The application rates and schedules of historical practices are described in LEI (2017:D1,E1) and summarised in Table 2-2. The changes to the irrigation practice include:

A decrease in the application depth (per event) from 100 mm to 25 mm.

An increase in the area irrigated per day from 8 ha to 20 ha.

A decrease in the return time between applications from once weekly to twice per week.

An increase to the total irrigated area from 40 ha to more than 60 ha.

An increase to the total annual volume of wastewater discharge to land via spray irrigation.

A decrease in the annual application depth of wastewater from 4,667 mm to less than 3,698 mm.

Figure 2-1 Current Irrigation Layout, Bare Sand (green) and Ring Main (red) Irrigation Lines


Table 2-2 Summary of Discharge Parameters (annual)

Parameter Current Future

Wastewater to The Pot (m3/yr)

2,314,469 2,661,530

Irrigation

Irrigation area (ha) 40 60

Irrigation area per discharge per day (ha)

8 20

Irrigation event application (mm/event)

100 25-45 (max 50)

Average annual application volume (m3/yr)

1,890,509 2,237,569

Average annual application depth (mm)

4,667 3,698

Total nitrogen load (kg N/y) 73,730 87,265

Nitrogen load (kg N/ha/y) 1,820 1,442

Total phosphorus load (kg P/y)

10,587 12,530

Phosphorous load (kg P/ha/y)

261 207

Pond

Average annual seepage volume (m3/yr)1

448,801 448,801

Average annual seepage depth (mm)

6,400 6,400

Total nitrogen load (kg N/y) 14,813 14,813

Nitrogen load (kg N/ha/y) 2,116 2,116

1 Includes (rainfall-evaporation) received to the pond surface

2.4 Wastewater Irrigation Water Balance

Estimates of the annual drainage losses to ground for current and future scenarios are included in Table 2-3 (LEI 2017: D1, E1). The drainage loss comprises the applied irrigation and rainfall, less evapotranspiration.


Table 2-3 Drainage losses

Current Y1 future Y35 future

Irrigation applied (mm/yr) 4,667 3,151 3,924

Rainfall received (mm/yr) 1,021 1,021 1,021

Annual Drainage (mm/yr) 5,153 3,382 3,593

Notes:

Assumes no change to rainfall

Y1 future: proposed irrigation regime for current wastewater inflows

Y35 future: proposed irrigation regime for predicted future wastewater inflows


3. Environmental Setting 3.1 Introduction

The following provides a summary of the key environmental factors that are likely to influence the assessment of contaminant fate and transport within the groundwater system. It focuses on the seepage from The Pot pond directly to the groundwater system, and the application of wastewater to land via spray irrigation.

3.2 Geology

The regional and local geological setting was described in LEI (2017: B3c), which has been adopted for this report. The Pot pond and irrigation area is situated in an area of paleo-dune deposits, which have developed over the past ~5,000 years as described in Cowie (1963). Clement et al (2009) noted that the oldest dune sequence (referred to as the Foxton Phase) are large parabolic dunes which can be up to 30 m in elevation. These dunes were considered to have migrated over forests, possibly creating peaty layers (Clement et al, 2009). Wetlands/swamps can form in the inter-dune areas the base of dunes due to organic matter and silt deposits. There is evidence of these features in the general area, where land drainage has been installed to lower water tables and divert water from low-lying areas to streams.

A series of investigation wells were installed by HDC at The Pot land as described in LEI (2016:B3b). Geological logs are included in Appendix A. The logs show grey fine to medium sand. The drilling investigation did not encounter any organic or peat layers (i.e. inter-dune swamp deposits). However, these may be present given the dune setting but not logged. Logs of older site boreholes (BHA-E) are not available. In addition, the description of the sands from the recent piezometers were consistent, with no descriptions of iron-pans or iron staining of the sands or groundwater.

3.3 Hydrogeology

Section 3.3 of LEI (2016:B3b) provides an overview of the regional groundwater setting. Regional groundwater flow is generally towards the west (i.e. towards the coast). The regional piezometric surface shown in LEI (2016:B3b) indicates a distinctive change in the groundwater flow direction in the vicinity of the Pot. The contours show the influence of the Pot on groundwater flow direction, particularly to the south. It is considered that the interpretation of the groundwater flow direction is affected by the limited number of monitoring sites and the effects of groundwater mounding around the Pot. It is unlikely that the flow direction to the south of Waiwiri Stream is a significantly altered as indicated in Figure 3.5 of LEI (2016:B3b).

The groundwater monitoring has been undertaken in monitoring wells installed around the Pot as described by LEI (2016:B3b). The monitoring shows mounding of the shallow groundwater table in the vicinity of The Pot pond (LEI 2016:B3b). There is no evidence of a perched water table with similar water levels in twin (shallow and deep) piezometers.

3.3.1 Hydraulic Gradient

As discussed in Section 2.3, the level of The Pot pond is managed to provide additional wastewater storage during the winter months. The Pot pond water level is drawn down to its minimum level (~13 metres above sea level (m asl)) over the summer months, with the lowest level occurring in March. The Pot pond is generally highest between October and December at a level of approximately 15 m asl. Figure 3-1 reproduced from LEI (2016:B3b) shows the seasonal variation in pond water levels.


Figure 3-1 Daily Pot Pond Levels January 2012 to June 2015 (from LEI 2016:B3)

The Pot pond is unlined and is considered to be hydraulically connected to the shallow groundwater system. However, it is believed that a layer of sludge is present which may limit leakage through the base of the pond (LEI, 2016:B3b).

LEI (2016: B3b) presented groundwater contours showing the radial flow outwards from the Pot and towards drainage features and the Waiwiri Stream. GHD reproduced the groundwater contours for the Pot (Appendix B). Groundwater contours were constructed for two monitoring events:

April 2015 - representing low pond water levels.

December 2015 - representing high pond water levels.

The April 2015 contours were constructed using water level measurements from piezometers (deep piezometer for P3, P4 and P5), standpipes and surface water points. However, only piezometer water level measurements were taken in December 2015. Therefore, the groundwater contours do not extend as far to the north and west compared to the April 2015 plot. Both contour plots show groundwater mounding around the Pot pond, with a steep groundwater gradient within 200 m of the pond. The gradients are influenced by the surface water features around the Pot, including farm drains and Waiwiri Stream.

Surface water level measurements were not collected in December 2015. Therefore, an artificial SW7 point (based on the relative difference between SW7 and P7 in April 2015) was created to constrain the groundwater contours near Waiwiri Stream. However, there is low confidence in the groundwater contours near the stream due to the limited number of surface water monitoring points.

Table 3-1 below provides the range of hydraulic gradients that were adopted for this investigation.

Table 3-1 Hydraulic Gradients

Low Pond Level High Pond Level

Section A (W-E transect)

Pond to Drain 3 0.025 0.031


Section B (SW-NE transect)

Pond to Waiwiri Stream

0.026 0.030


3.3.2 Hydraulic Conductivity

The hydraulic conductivity of the shallow groundwater system was determined from a series of single well hydraulic tests (slug tests) undertaken on two piezometers (P2 and P6). Both piezometers have a standard construction with a 50 mm slotted PVC screen and filter cloth, Walton Park gravel filter pack and bentonite seal (LEI, 2016:B3b). Data processing and analysis was undertaken by Terry Hughes (Groundwater Services Ltd). The hydraulic conductivity of the shallow groundwater system was interpreted to be between 1.9 m/d and 2.4 m/d (2.1 x 10-5 and 2.8 x 10-5 m/s) (LEI, 2016:B3b).

GHD undertook a review of the slug test data and analysis to confirm the derivation of the hydraulic conductivity values. GHD noted that the initial analysis using the Bower and Rice (1976) solution was fitted to the early time water level recovery data. These measurements may be influenced by the Walton Park filter pack, which is coarser than the surrounding formation (fine to medium sands). GHD reanalysed that response data, utilising the mid-time data points. The results of the analysis indicated a slightly lower hydraulic conductivity of between 0.6 m/d and 1.8 m/d (i.e. 6.9 x 10-6 and 2.1 x10-5 m/s).

With this in mind, for the purposes of this assessment, a range of hydraulic conductivity values were adopted (Table 3-2). These were applied to the conceptual model, treating the unconfined sand aquifer as a single hydrostatic unit.

Table 3-2 Hydraulic Conductivity Values

Scenario Hydraulic Conductivity

Low 0.5 m/day

Med 1.8 m/day

High 3.1 m/day

3.3.3 Groundwater Chemistry

A review of the long term groundwater monitoring undertaken at the site indicated that the site boreholes (BHA-BHE) were screened too deep to intercept the wastewater signature in the shallow groundwater (LEI, 2016:B3b). In 2015, LEI supervised the installation of seven piezometers (P1-P7). A figure showing the location of these piezometers is provided in Appendix B.

As discussed above, water levels in the shallow and deep piezometers are similar with no evidence of a perched aquifer. Therefore, it is considered that the twin piezometers are intercepting different levels of the same aquifer. Groundwater sampled from the shallow piezometers is likely to represent a combination of drainage from irrigation and pond leakage. Groundwater sampled from the deeper piezometers surrounding the Pot are likely to be more influenced by pond leakage and regional groundwater flow.

The following is a summary of water quality data from the piezometers and the Pot pond (2015-2017):

Ammoniacal nitrogen (NH4-N) is elevated in samples from the Pot pond, P3 and P2.

Nitrate nitrogen (NO3-N) is elevated in groundwater samples from P1, P4, P5 and P6.

Dissolved reactive phosphorus (DRP) is elevated in samples from the Pot pond, P2 and P3.


The concentration of nitrogen and phosphorous is generally similar (same order of magnitude) in samples collected from twin piezometers (shallow and deep) with the following exceptions:

o NH4-N is higher in P4d relative to P4s, with one exception (Dec 2015).

o DRP is higher in P4s relative to P4d.

o DRP is one to two orders of magnitude higher in P5s compared to P5d.

o NH4-N was elevated in P5s in samples collected in 2015, however recent results show a similar NH4-N concentration in both the shallow and deep piezometers.

No clear seasonal trends were observed in water quality data.

The concentration of nitrogen and phosphorous in P7 was low relative to the other piezometers.

Escherischia coli (E.coli) was counted at high levels in samples collected in 2015. In 2016-2017, only the Pot pond, P2 and P3 have recorded E.coli on a regular basis.

Few dissolved oxygen and redox measurements (late 2016 to 2017) indicate reducing conditions and low dissolved oxygen in P2. Dissolved oxygen levels are highest in P1 and P7.

The average analyte concentrations are shown spatially in Appendix C (from LEI, 2016:B3b).

3.3.4 Other Groundwater Users

Nearby groundwater wells are shown in Figure 3-2 and listed in Table 3-3 (from LEI, 2016:B3b).


Figure 3-2 Local Bores within 2.5 km of The Pot and Tucker Block (from LEI, 2016:B3b)


Table 3-3 Bores located within 2.5 km of The Pot (from LEI, 2016:B3b)

3.4 Hydrology

The surface water environment is described in detail in Aquanet (2016:B6b) and the Surface Water Assessment of Environmental Effects (Aquanet, 2017:E4). Waiwiri Stream flows immediately to the south of the Pot land. Two surface water drains (Drain 3 and Drain 4) traverse the irrigation areas and drain into Waiwiri Stream in the western part of the Pot land. Flow gauging was completed as part of Aquanet’s study (2016:B6b), a summary is reproduced below.

Table 3-4 Stream Gauging Results (Aquanet, 2016:B6b)

Location Average flow (m3/s) Median flow (m3/s)

Waiwiri Stream 2

(up gradient of pond)

0.092 0.085

Waiwiri Stream 3

(down gradient of pond)

0.130 0.125

Drain 3 0.013 0.012

Drain 4 0.008 0.008

3.4.1 Surface Water Chemistry

A summary of surface water chemistry data collected by Aquanet (2016:B6b) is reproduced in Table 3-5.

Table 3-5 Surface Water Chemistry (median values from Aquanet, 2016:B6b)

NO3-N Total N DRP

Waiwiri Stream 2

(up gradient of pond)

0.025 1.96 0.041

Waiwiri Stream 3

(down gradient of pond)

1.39 2.88 0.044

Drain 3 11.50 11.80 0.117

Drain 4 2.89 3.94 0.063

Identification Map_Ref x NZMG y NZMG Depth (m) Distance (km) Owner361043 S25:957-625 2695700 6062500 0.77 AP&SV Tucker361021 S25:943-608 2694396 6060883 24 1.31 JA& Shaw362414 S25:970-616 2697055 6061660 19.8 1.92 P Cockrell361060 S25:948-597 2694800 6059700 25 2.28 Easton362196 S25:972-607 2697200 6060700 2.40 Law BrosNo1362123 S25:976-623 2697600 6062300 8 2.47 Roslene EnterpNo2362110 S25:976-623 2697600 6062300 8 2.47 Fuller_Est?


Waiwiri Stream 4

(down gradient of pond and Drain 3 and 4)

2.17 3.74 0.054

The surface water monitoring has shown that the total nitrogen and NO3-N increases downstream of the Pot pond. Elevated levels of nitrogen in Drain 3 and 4 contribute to the higher concentrations seen at the Stream 4 monitoring point. However, the stream will also receive nitrogen from other sources, including groundwater seepage, which has been impacted by the Pot pond leakage, and groundwater and surface water from surrounding farmland.

A longitudinal study of Drain 3 (Aquanet 2016:B6b) showed an overall increase of nitrogen and phosphorous within the irrigation area. However, it was noted that NO3-N increased significantly between the northern property boundary (site Drain 3d) and the sampling point immediately before the irrigation area (Drain 3c), suggesting a non-point source input such as groundwater. In contrast, DRP did not increase before entering the irrigation area but increased within it, indicating direct inputs from irrigation.


4. Assessment 4.1 Introduction

An initial conceptual model of the groundwater system was developed based on the information present within the referenced LEI reports. The conceptual model was used to identify the likely sources, pathways, and receptors associated with the losses of wastewater from storage and irrigation.

A 2-dimensional (2-D) numerical groundwater model was constructed to calculate groundwater flow paths, pore velocity, and hydraulic gradients to better understand contaminant migration through the groundwater system.

4.2 Conceptual Groundwater Model

The conceptual model for the shallow groundwater system is shown in Figure 4-1 and Figure 4-2. The topography shown is approximated from Google Earth and surveyed well levels.

The conceptual model shows groundwater mounding with flow in all directions away from the Pot pond. To the west of the Pot pond (Figure 4-1), the groundwater gradient changes from steep to a gentle gradient at a distance of approximately 200 m from the pond. This gradient change is due to the presence of the drain (Drain 3) intercepting groundwater. Drain 3 flows into Waiwiri Stream near the western end of the Pot land. To the south of the Pot, groundwater discharges to Waiwiri Stream. It is possible that groundwater flow may transport contaminants to the south of the Stream. However, there is no groundwater monitoring data immediately to the south of the stream to confirm the extent of the contaminant plume. The fate of groundwater flowing to the north and east of the Pot is also less certain due to the limited groundwater data, although it is likely that shallow groundwater will be intercepted by drains that ultimately flow into Waiwiri Stream.

The conceptual model shows that the primary source of contaminants is leakage from the Pot pond. In groundwater close to the pond, nitrogen is primarily in the form of ammoniacal nitrogen (wells P2 and P3), converting to oxidised forms of nitrogen within 200 m (P4). The nitrogen mass entering the groundwater system from the pond has been estimated by LEI (2017, D1:E1) to be 14,813 kg nitrogen/year.

No seasonal trends in water quality were observed in the groundwater data. The impact of spray irrigation on groundwater quality could not be determined as it could not be differentiated from the pond leakage. However, the proposed changes to the irrigation regime involve a reduction in event application from 100 mm to 25-45 mm. This change is expected to reduce runoff to surface drains and drainage to the groundwater table.


Figure 4-1 Conceptual Model (W-E transect)

Figure 4-2 Conceptual Model (SW-NE transect)

4.3 Groundwater Flow

An estimate of groundwater flux and linear velocity was undertaken to determine the likely residence time of contaminants within the groundwater system beneath the Pot pond and irrigation areas. The calculations are based on Darcy flow principles through porous media.

Darcy flow is given by the following equation:

Where Q is the discharge (m3/d), K is the hydraulic conductivity, i is the hydraulic gradient, and A is the area (given by aquifer thickness per unit aquifer width).

The pore/linear velocity that groundwater moves through the matrix is important to consider as this is a measure of how long a contaminant will take to travel in groundwater, ignoring attenuation effects.

Linear velocity is given by:

=

Where v is the linear or pore velocity (m/d), and ne is the effective porosity.

A numerical groundwater model was created using Seep/W to simulate the interaction between the Pot pond, the groundwater system, and the surface water (i.e. drains and Waiwiri Stream).


Seep/W (by Geostudio®) is a two-dimensional groundwater modelling software, which allows the user to model groundwater flow through a cross section (“slices”) of the area of interest.

Two slices were created to represent the groundwater system in the transects shown in Figure 4-1 and Figure 4-2. A steady-state and transient model were created for both transects. The transient simulation included changes in the Pot water level. The water levels used were based on a daily water level elevation recorded from January 2009 to April 2012. The water level data sequence was replicated (from April each year) to provide water levels for a full ten-year transient simulation. Input variables used in the model simulation are included in Appendix D.

The Seep/W model was used to inform the groundwater flow paths and pore velocities of the two transects. The model replicated groundwater mounding which occurs due to the seepage losses from the Pot, with the surface water drains and Waiwiri Stream, which intercept groundwater also replicated. The drains act as collection systems for shallow groundwater, draining the water table and conveying water to the coast.

4.3.1 Seep/W model

For the W-E transect (Figure 4-3), Drain 3 acts as important collection system, intercepting groundwater from the Pot seepage and irrigation areas. Drain 4 collects groundwater flowing from the western half of the irrigation area. The eastern boundary conditions influences the groundwater flow in that direction.

Figure 4-3 Groundwater Flow Paths (W-E transect)

In the SW-NE transect, pond leakage enters groundwater and flows towards Waiwiri Stream and surface drains. The conceptual model considered that groundwater to the south of Waiwiri Stream was likely to be draining towards it. This differs from the piezometric surface shown in Figure 3-5 of LEI (2016:B3b), where groundwater flow is depicted to toward the SW, with groundwater flowing beneath the Waiwiri Stream. Until groundwater levels and water quality is investigated to the south of Waiwiri Stream, this remains an area of uncertainty.


Figure 4-4 Groundwater Flow Paths (SW-NE transect)

4.3.2 Pond leakage

The model was used to inform pore velocities through the groundwater system and likely groundwater flux entering surface water. The model was calibrated to the measured hydraulic gradient between the pond and the drain/surface water environment, with a single hydraulic conductivity of the sand matrix applied based on Table 3-2.

Table 4-1 presents the average pore velocities for the section of the model between the Pot pond and Drain 3 (Figure 4-5). This is considered to be the shortest flow path for pond leakage, with Drain 3 showing the greatest effects on water quality. The table includes output for the medium and high hydraulic conductivity scenarios. Three scenarios are shown representing a low, medium and high water level. Transient model outputs for the three water level scenarios are include in Appendix D. The model indicates that it would take approximately 5 years (high K) for a particle of water to travel from the pond to Drain 3. Using a lower conductivity value the travel times are increased to approximately 9 years. However, both scenarios show that the deeper flow path will result in contaminants entering the drains over a much longer period (i.e. decades).

By comparison, the pore velocity that was calculated using first principles yielded a value of approximately 0.1 m/d. This is more consistent with the outputs using the high hydraulic conductivity scenario.

Table 4-1 Average Particle Velocities from Seep/W Model

Pond Level Water Level (m) Average Pore Velocity (m/d)

Estimated Travel Time*

(Distance 200 m Pond –Drain 3)

Low: K = 1.8 m/d

K=3.1 m/d 13.1

0.05 4,000 days 11.0 years

0.08 2,500 days 6.8 years

Med: K = 1.8 m/d

K=3.1 m/d 14.1

0.06 3,300 days 9.1 years

0.11 1,800 days 5.0 years

High K = 1.8 m/d

K=3.1 m/d 15.3

0.08 2,500 days 6.8 years

0.13 1,500 days 4.2 years

*the travel time represents the shortest flow path between the Pot pond and the Drain.

SW NE


Figure 4-5 Pond to Drain 3 Shortest Flow Path

Based on the hydraulic properties applied in the model, the seepage rate through the base of the pond ranged between 285 m3/d to 680 m3/d (Table 4-2). Increasing the hydraulic conductivity of the sand matrix resulted in an increase in seepage through the base of the pond (Table 4-3), which were closer to the estimate of seepage losses provided in LEI (2017 D1:E1)

Table 4-2 Seepage Flux from Pond - Medium Conductivity Value

Pond Scenario Pond Water Level (m RL)

Seepage Rate (mm/d)

Seepage Flux from Pond (m3/d)

Low 13.1 2.7 185

Med 14.1 8.0 560

High 15.1 9.7 680

Table 4-3 Seepage Flux from Pond - High Conductivity Value

Pond Scenario Pond Water Level (m RL)

Seepage Rate (mm/d)

Seepage Flux from Pond (m3/d)

Low 13.1 6.9 485

Med 14.1 14.5 1,015

High 15.1 16 1,130

4.3.3 Seepage into Drains

The model indicates that the seepage flux in the drains is very slow. Model flux into Drain 3 and Drain 4 was compared against average stream gauging measurements collected by Aquanet (2016:B6b). This comparison is presented in Table 4-4 and Table 4-5. The rate of flux into the drains will vary depending on the topography and orientation of the drains relative to the hydraulic gradient. It is assumed that the majority of the water enters the drains when they traverse across the hydraulic gradient (from north to south). Therefore, for the purposes of assessment, an approximate length has been used to calculate the total flux based on the length of the drain segment that crosses the hydraulic gradient.


Table 4-4 Seepage Flux into Drain 3

Drain 3 Flux / m (m3/day) Total Flux (m3/s)* Aquanet (m3/s)

Medium K High K Medium K High K

0.012 Low Pond 2.59 3.77 0.015 0.022

Medium Pond 2.62 3.97 0.015 0.023

High Pond 2.59 4.74 0.017 0.027

*Based on 500 m length of drain

Table 4-5 Seepage Flux into Drain 4

Drain 4 Flux / m (m3/day) Total Flux (m3/s)* Aquanet (m3/s)

Medium K High K Medium K High K

0.008 Low Pond 0.91 1.02 0.004 0.004

Medium Pond 0.91 1.02 0.004 0.004

High Pond 0.7791 1.02 0.004 0.004

*Based on 350 m length of drain

At the western site boundary, the model indicates that approximately 80% of the shallow groundwater flux was captured by Drain 4, with 20% flowing to the coast. Due to the constraints of the 2-D model and available information it is difficult to quantify the proportion of groundwater flux in other directions. However, based on the conceptual model it is likely that a high proportion of the shallow groundwater will be captured by the surface water system (drains and ultimately into Waiwiri Stream), with limited groundwater flow past the site boundaries to the south and north.

4.3.4 Irrigation Areas

The irrigation of wastewater on the dunes has been shown to result in localised mounding of the water table (LEI, 2016:B3b). The mounding will affect local groundwater flow paths, with the surface drainage intercepting groundwater around the disposal areas. The irrigation losses were modelled in Seep/W by assuming a constant flux of 10 mm/d, which equates to approximately 3,650 mm/yr. The recharge depth was equivalent to the Y35 drainage depth modelled by LEI (2017: D1:E1), and shown in Table 2-3.

The hydraulic gradient beneath the dune irrigation area is significantly less than the pond mound, which means that travel times to the drains are expected to be very long. The hydraulic gradient beneath the Bare Sands area is approximately 0.006. Under the high hydraulic conductivity scenario (i.e. 3.1 m/d) the pore velocity is approximately 0.02 m/d (7.3 m/yr).

4.4 Groundwater Quality

As discussed in Section 4.3, the Seep/W model illustrates the influence that the drains and Waiwiri Stream have on the groundwater gradients and flow paths. The drains act as collection systems for shallow groundwater, draining the water table and conveying water to the coast.


The groundwater and surface water monitoring data which has been collected to date was used to inform the conceptual understanding of the transport and fate of nutrients (predominately nitrogen) through the groundwater system and into surface water. LEI (2016:B3b) provides a summary of the groundwater monitoring data, whilst Aquanet (2016) summarise the surface water quality.

4.4.1 Nutrient Attenuation

The physio-chemical processes that affect nitrogen speciation and concentrations within the environment are well established, and is commonly referred to as the nitrogen cycle (Figure 4-6). The attenuation of nitrogen concentrations in groundwater includes the following processes; dilution, sorption/retardation, advection and dispersion, and volatilisation (via denitrification).

Figure 4-6 Processes affecting nitrogen speciation in aquatic systems (Bohlke et al, 2006)

In general terms, ammoniacal-nitrogen, which is the dominant nitrogen species in wastewater, is fully oxidised to nitrate-nitrogen in the presence of certain bacteria (i.e. Nitrosomonas and Nitrobacter) and where soil and groundwater conditions are aerobic. Whilst ammoniacal-nitrogen is readily available to plants for up-take, it is readily oxidised in the soil, vadose, and shallow groundwater.

Nitrate-nitrogen is a very stable form of dissolved nitrogen and is generally persistent in the environment unless redox conditions change. Where the groundwater or surface water system becomes oxygen limited (i.e. anaerobic / anoxic) then nitrate-nitrogen can be consumed by bacteria, in the presence of electron donor species (e.g. Iron, Manganese, Dissolved Inorganic Carbon), and reduced to N2 (gas). Environments which are typically associated with denitrifying conditions are paleo-peat, swamp, wetland deposits or deep mineralised groundwater.

Rivas et al. (2015) undertook a study on the denitrification potential of nitrate-nitrogen in groundwater in the Manawatu catchment. The investigation noted that for denitrification to


occur then there needed to be a low dissolved oxygen concentration and the presence of electron donors such as iron in the groundwater system to facilitate the process (Table 4-4).

Table 4-6 Indicator Parameters for denitrification (Rivas et al 2015)

The water chemistry data collected to date suggests that denitrification of nitrate-nitrogen is not a significant component of attenuation in groundwater. It is likely that some attenuation occurs by dilution and dispersion of the plume as it radiates outwards from the Pot Pond and irrigation area. In addition, LEI (2016:B3b) state that denitrification may be occurring in the zone at the base of the pond, where anoxic conditions are likely to exist. This may explain the difference in nitrogen concentrations measured in the wastewater in the Pot and those in adjacent piezometers.

It is also possible that denitrification is occurring in the seepage zones in the bed substrate of the drains and Waiwiri Stream, where sediment and organic matter create an anoxic zone with appropriate conditions for denitrification. In addition, the presence of macrophytes in the drains is expected to reduce nutrient concentrations through plant uptake, as described in Aquanet (2016). These physiochemical processes in the drains may explain the differences in the groundwater nitrate concentrations compared to the concentrations in Drain 3. However, there is no specific groundwater chemistry data that would enable a reduction factor to be applied.

4.4.2 Pond leakage

There are no substantial changes proposed for the operation of the wastewater stored in the Pot pond. Therefore, it is expected that the groundwater mound will remain and the movement of nutrients through the groundwater system, and into surface water, will continue.

LEI (2017: D1,E1) calculated a nutrient flux from the pond based on the estimated seepage volume and nutrient concentrations. Based on a basic water balance, using flow measurements to establish the likely differential between influent and effluent flows at the Pot pond, a seepage volume of approximately 448,000 m3/yr and a nitrogen mass of 14,813 kg/yr were calculated. The pond area was estimated to be approximately seven hectares.

The Seep/W modelling indicated that LEI value is likely to be conservative, with seepage losses less during low pond levels and greater during higher pond levels. A cumulative seepage loss was not modelled. However, the modelling indicated that using an average pond level of 14.1 m RL, the annual seepage losses were approximately 370,000 m3/yr (high hydraulic conductivity scenario). The mass lost from the pond was estimated using the average concentration of nitrogen observed in pond wastewater and the modelled annual seepage of 370,000 m3/yr. This resulted in a nitrogen input of 12,210 kg/yr (or 33.5 kg/d).


The monitoring data indicates that there is transformation of ammoniacal-nitrogen (NH4-N) to nitrate-nitrogen (NO3-N) within the groundwater system (LEI, 2016:B3b). The oxidisation of NH4-N to NO3-N occurs over a very short distance (~200 m), with monitoring wells P1 and P6 indicating that almost all of the nitrogen is present in its fully oxidised form. The nitrogen concentrations measured in groundwater between the pond and the drains do not show any significant evidence of attenuation. Therefore, continuation of high nitrogen concentrations in groundwater beneath the Pot pond are expected under the future use scenario.

Most of the groundwater is expected to discharge into the surface drains and Waiwiri Stream. However, the concentration of nutrients measured in surface waters samples is substantially less than measured in nearby groundwater wells (eg P4 and P6). This is likely due to the uptake of nutrients by macrophytes and potentially some denitrification in the bed material/seepage zone. This is discussed in more detail in Aquanet (2016:B6b). The Aquanet report noted a 30% reduction in the concentration of soluble inorganic nitrogen in samples upstream and downstream of macrophyte beds.

4.4.3 Irrigation of Wastewater

HDC are proposing to increase the area of land that receives wastewater, from 40 ha to in excess of 60 ha. The irrigation depths and return periods are also proposed to change to more effectively manage the discharge of wastewater to the vegetated dunes. LEI (2016:B3b) have calculated that the changes proposed are expected to result in a decrease in the drainage losses from the soils. In addition, by applying the wastewater over a larger area with lower application rates, nutrient recycling is expected to be increased (i.e. nitrogen losses are decreased) (LEI, 2017: D1:E1).

An estimate of nitrogen mass flux entering the groundwater system was undertaken using the estimated drainage losses presented in Table 2-3. The mass flux estimate uses a mean dissolved inorganic nitrogen (DIN) concentration based on a weighted average of nitrogen in wastewater irrigation (i.e. 35 mg/L), rainfall (i.e. 0.1 mg/L), and forestry land which is not currently irrigated (i.e. 2 mg/L for 19.5 ha). The calculated mass flux is summarised in Table 4-7.

Table 4-7 Mass flux

Current Future (Y1) Future (Y35)

Weighted nitrogen concentration (g/m3)

29 26 27

Drainage loss (mm) 5,153 3,382 3,924

Area (ha) 40.5 (irrigated)

19.5 (unirrigated)

60 60

Total drainage (m3/yr) 2,286,060 2,029,200 2,354,400

Total nitrogen load (kg N/yr)

60,368 53,691 64,221

Nitrogen load (kg N/ha/yr)

1,006 895 1,070


The proposed changes to the irrigation regime will result in an initial decrease in the mass nitrogen flux to groundwater. However, as wastewater inflows increase the estimated mass nitrogen flux will increase by approximately 6% (Y35) compared to the current scenario.

The Seep/W model indicates that most groundwater from the irrigated area will flow towards the drains and Waiwiri Stream. Groundwater flow to the west is not entirely captured by the drains, with some nutrients expected to flow slowly towards the coast. However, it is considered that the drains intercept the majority of shallow groundwater where the highest nutrient concentration are recorded. The model also indicates some groundwater flow to the east. However, this is expected to be captured by drains and directed to Waiwiri Stream.

From the mass balance, it is considered that the future scenario at Y35 is likely to result in a very similar loss of nutrients to the groundwater system, albeit over a larger area. Therefore, a similar nutrient concentration in groundwater is expected under the Y35 future scenario, unless denitrification and/or phytoremediation methods are utilised to reduce concentrations in groundwater.

It is considered that some of the proposed mitigation options identified in LEI (2017: D1,E1) are likely to be successful. In particular, the creation of wetlands along surface drains and/or a permeable reactive barrier (PRB) along Waiwiri Stream are likely to be solutions that could reduce nutrient concentrations before leaving the site. These systems have been shown to reduce the nitrogen concentrations in water through denitrification processes elsewhere. The relative success depends on the environment in which they are placed (i.e. low flux areas with high residence time tend to achieve better outcomes). It would be prudent to trial some of the mitigation approaches to demonstrate the effectiveness of each prior to investing in large scale mitigation plans. However, it is considered that the mitigation tools proposed are likely to enhance the environmental outcomes.


5. Findings 5.1 Discussion

Seepage from the Pot pond into groundwater has been confirmed by the monitoring data. The seepage has caused localised mounding of the water table, with perimeter drains acting to intercept the groundwater. Without the drains it is likely that groundwater seepage at the toe of the dunes would occur, giving rise to the creation of open bodies of water (i.e. swamp/wetland conditions). The seepage losses may not be as significant as estimated by LEI (2016:B3b) i.e. LEI have assumed a high seepage (conservative) case scenario.

The hydraulic conductivity of the dune sands was determined from in-situ testing of monitoring wells. The hydraulic conductivity together with the hydraulic gradient and porosity were used to calculate particle velocities through the water table. The modelling showed that the travel times are long, with more than five years for a particle from the pond to travel to the nearest drain. Travel times beneath the irrigated area will be longer, given the flatter hydraulic gradients.

The groundwater quality is clearly influenced by the seepage from the Pot pond and beneath the irrigated areas. The surface water quality in the drains has also been affected by the discharge of nutrient rich groundwater. However, it is noted that the concentration of nutrients in surface water is less than measured in nearby groundwater wells.

There is no available information that supports significant attenuation via denitrification in the groundwater system. Therefore, it is a reasonable conclusion to conserve the nitrogen mass in groundwater, with the concentration of nitrate-nitrogen likely to be reduced by dilution and dispersion. This is particularly relevant for the deeper groundwater flow paths, and is likely why deeper groundwater is shown not to be significant affected by the past discharges.

The modelling undertaken in this assessment indicates that the drains provide a very good mechanism for collecting groundwater containing irrigated wastewater seepage. Therefore, it is unlikely that the discharge will affect existing groundwater users in the area.

LEI (2017: D1:E1) identified a range of potential mitigation options for reducing the effects of elevated nitrate-nitrogen and DRP in groundwater and surface water. It is considered that the hydraulic properties of the sand aquifer are appropriate for the use of a permeable reactive barrier or for wetlands.

5.2 Conclusion

The following conclusions are made about the groundwater system and the effect of the wastewater application and pond leakage on groundwater:

The application of wastewater to land and storage of wastewater at the Pot have resulted in localised mounding of the groundwater table.

Surface drains provide a good mechanism for intercepting shallow groundwater impacted by wastewater. This water ultimately flows into Waiwiri Stream.

Seepage from the Pot pond appears to be the main influence on the groundwater system. Therefore, proposed changes to the irrigation regime are unlikely to have a significant impact on the groundwater system.

Modelling shows that variations in pond level impact groundwater fluxes and travel times.

Groundwater travel times are long, with an estimate of five years for a particle of groundwater to travel a distance of 200 m from the Pot pond to the nearest drain (Drain


3). Groundwater travel times beneath the Bare Sands irrigation area and deep groundwater flow paths will be longer.

Groundwater quality has been influenced by seepage from the Pot pond and beneath wastewater irrigation areas. The surface water quality in drains and Waiwiri Stream has also been affected by the discharge of nutrient rich groundwater.

There is no available information that supports significant attenuation via denitrification in the groundwater system. Therefore, it is a reasonable conclusion to conserve the nitrogen mass in groundwater, with the concentration of nitrate-nitrogen likely to be reduced by dilution and dispersion.

The proposed changes to the irrigation regime will result in an initial decrease in the mass nitrogen flux to groundwater. However, as wastewater inflows increase the estimated mass nitrogen flux will increase by approximately 6% (Y35) compared to the current scenario based on preliminary modelling.

At the western site boundary, the model indicates that approximately 80% of the groundwater is captured by Drain 4, with 20% flowing to the coast. It is difficult to quantify the amount of groundwater flow past the site boundaries in other directions, however it is considered likely that a high proportion of the shallow groundwater will be captured by the surface water system (drains and ultimately into Waiwiri Stream).

The effect on groundwater and groundwater users outside of the property boundaries was not assessed. However, given the action of the surface drains to intercept groundwater, it is unlikely that the future discharge will affect existing groundwater users in the area based on current monitoring data.

It is considered that the hydraulic properties of the groundwater system are appropriate for the use of a permeable reactive barrier and/or wetlands. A field trial of mitigation options would be a prudent step to ensure the receiving water environment is enhanced and the most effective mitigation tool(s) are adopted to enhance the environmental outcomes.


6. References

Aquanet Consulting Limited (2016:B6b) Levin Wastewater Land Application: Water quality and ecology of the Waiwiri Stream, 2013-2016

Aquanet Consulting Limited (2017:E4) Levin Wastewater Land Application: Surface Water Assessment

Clement. A.J.. et al. 2009. Late Quaternary geomorphology of the Manawatu Coastal Plain, North Island, New Zealand. Quaternary International, doi:10.1016/j.quaint.209.07.005

Cowie, J. D. 1963. Dune building phases in the Manawatu district, New Zealand. New Zealand Journal of Geology & Geophysics 6, 268-280

Lowe Environmental Impact (2016:B3b) Levin Wastewater Land Application: The Pot Groundwater Monitoring

Lowe Environmental Impact (2017: B3c) Groundwater Monitoring 2017 (Memorandum)

Lowe Environmental Impact (2017:D1, E1) Levin Wastewater Discharge, The Pot Discharge Description and Assessment of Effects to Land

Rivas, A., Singh, R., Horne, D., Roygard, J., Matthews, A., and Hedley, M. (2015): An assessment of the denitrification potential in shallow groundwaters of the Manawatu River catchment. http://www.massey.ac.nz/~flrc/workshops/15/Manuscripts/Paper_Rivas_2015.pdf

http://www.massey.ac.nz/~flrc/workshops/15/Manuscripts/Paper_Rivas_2015.pdf

GHD | Report for Lowe Environmental Impact Limited - The Pot Levin WWTP, 51/37501/

Appendices


Appendix A – Geological Logs


Appendix B – Groundwater Contours

6

6

7 9 10 11

11

11

12

12

12

13

13

1784000 1784500 1785000 1785500 17860005499500

5500000

5500500

5501000

Pond

SP4SP5

SP11

SP12

SP15

SP20

SP24

SW7

P2

P3dP4d

P7

P1

P5d

P6

A

A'

B

B'

The Pot Groundwater Contours April 2015

Waiwiri Stream

Cross Section A-A'

0 200 400 600 800 1000 1200 1400Distance (m)

5

10

15

Gro

undw

ater

Ele

vatio

n (m

asl)

Cross Section B-B'

0 100 200 300 400 500 600 700 800 900 1000Distance (m)

5

10

15

Gro

undw

ater

Ele

vatio

n (m

asl)

Drain 4

Drain 3

5 6 7 8 9 10 1112

12 13

1314

15

15 P2

P3dP4d

P7

P1

P5d

P6 Pond

SW7 (Artificial)

1784000 1784500 1785000 1785500 17860005499500

5500000

5500500

5501000

A

A'

B

B'

The Pot Groundwater Contours December 2015

Waiwiri Stream

Cross section A-A'

0 200 400 600 800 1000 1200 1400Distance (m)

5

10

15

Gro

undw

ater

Ele

vatio

n (m

asl)

Cross Section B-B'

0 100 200 300 400 500 600 700 800 900 1000Distance (m)

5

10

15

Gro

undw

ater

Ele

vatio

n (m

asl)

Drain 4

Drain 3


Appendix C – Groundwater Chemistry


Average Analyte Concentrations: The Pot March 2015 to March 2016 (geomean used for E.coli) (from LEI, 2016:B3b)


Appendix D – Seep/W


Table A1 Seep/W input variables (all water level based on April 2015 measurements)

Value Comment

Hydraulic Conductivity

(fine-medium sand)

1.8 m /day Based on slug test

(0.5-5 m/day for sensitivity analysis)

Pond level (steady state) 13.44 m April 2015 measurement

Pond level (transient) variable Based on pond water levels from Jan 2009 to April 2012

Irrigation 8 mm/day Assumes ~80% loss to groundwater

W-E transect Water level

Western Boundary 5 m Estimated from P5 water level

Eastern Boundary 9.5 m Estimated from P1 water level and groundwater gradient

Drain 3 8.4 m Estimated based on groundwater elevation in P4

Drain 4 5.6 Estimated based on groundwater elevation in P5

SW-NE transect Water level

Northern Boundary 9.5 m Estimated based on SP5 water level

Southern Boundary 9.5 m Estimated based on SP5 water level and regional groundwater flow direction (towards the west)

Waiwiri Stream 5.5 m Based on measurement at SW7

Drain 10 m Estimate based on SP4/SP5 water level


W-E transect Seep/W model set up

SW-NE transect Seep/W model set up

A check of groundwater levels and gradients calculated by the model was undertaken by comparing to gradients presented in Section 3.3 measured from groundwater contours. This comparison is presented in the following table:

Location Seep/W Model Gradient (Steady State)

Contour Gradient (April 2015)

W-E transect Pond to Drain 3 0.028 0.025


SW-NE transect Pond to Waiwiri Stream

0.024 0.026


W-E transect –Transient Model – Medium Conductivity

High, Medium and Low Pond Level. Note colours and contour labels indicate total head in metres (groundwater level) over different parts of the model domain


SW-NE transect –Transient Model

High, Medium and Low Pond Level. Note colours and contour labels indicate total head in metres (groundwater level) over different parts of the model domain


Sensitivity Assessment – Hydraulic Conductivity

An assessment was undertaken to determine the models sensitivity to variation in the hydraulic conductivity of the sand aquifer. A low (0.5 m/day) and high (3.1 m/day) hydraulic conductivity was compared to the medium case (1.8 m/day) as per Table 3-2.

Under the low conductivity scenario, the model did not calibrate with erroneous outputs particularly in irrigation areas. On that basis, it is considered that a hydraulic conductivity of 0.5 m/day is not representative of the medium-fine sand aquifer.

The calibration was good under the high conductivity scenario. The model show increased fluxes (pond leakage and discharge into drains) and reduced groundwater mounding under the Bare Sands Irrigation area.

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Lowe Environmental Impact Limited - Horizons · 2019. 11. 19. · Stream, 2013-2016 (Aquanet, 2016:B6b) Additional technical information referenced in the above reports were not reviewed