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12 June 2018

To: Gallo Saidy

Group Manager Infrastructure Services

Horowhenua District Council

From Olivier Ausseil

Principal Scientist

Aquanet Consulting Ltd

Technical Memo

Levin WWTP Land Application at The Pot

Assessment of Effects on water quality and ecology

(Aquanet, E4:2018)

1. Introduction

1.1. Context

The Horowhenua District Council (HDC) operates the Levin Wastewater Treatment Plant (WWTP). At present secondary treated wastewater is piped from the WWTP to an unlined storage pond, before being irrigated to surrounding pine forest. The site is referred to as “the Pot”; the storage pond is referred to as the “Pot pond”.

The Waiwiri Stream flows immediately to the south of the Pot. It takes its source from Lake Papaitonga and flows through pastoral farmland, before reaching the Tasman Sea to the southwest of the Pot site.

To provide an assessment of the current state of the ecological, recreational and cultural values in the vicinity of the Pot, and an assessment of the effects of the activities undertaken at the site, HDC initiated a comprehensive monitoring programme, covering surface and groundwater flows and quality, soil and sediment sampling, ecological monitoring and a range of other investigations.

Aquanet Consulting Ltd (Aquanet) have undertaken surface water monitoring and investigations at, and in the vicinity of, the Pot site since 2013. We previously reported on the results of this programme in

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technical reports dated January 20151 (2013-2014 results) and June 20162 results (2016:B6b) and an update technical memo dated November 2017 (2017:B6c).

1.2. Proposed future activities

HDC are preparing a series of resource consent applications to enable to continued operation of the Levin wastewater treatment and discharge to land at the Pot site. A number of changes and modifications to the activities are proposed, including extended irrigation areas, different irrigation rates, and mitigation options, as described in a technical report provided (LEI, 2018:D1,E1). Our assessment is made on the basis of the information provided in that report, and a map of proposed irrigation areas, provided by LEI on 7th November 2017.

1.3. Scope

This memo provides:

• a summary of the surface water investigations programme, which started in May 2013

• a summary of the effects on water quality and freshwater ecology under the current wastewater

treatment and discharge regime, including an assessment against key planning provisions relating

to water quality and/or ecological health, i.e. RMA S107, NPSFM (2017) and One Plan Schedule E

Water quality targets and Schedule B values;

• an assessment of the potential effects of proposed activities, including mitigation options, based

on the description provided in LEI (LEI, 2018:D1,E1), on water quality and freshwater ecology.

This memo is focused on effects on water quality and ecology of the Waiwiri Stream itself, as the key receiving environment for activities undertaken at the Pot site. Significant amount of water quality monitoring data were also collected in the surface drains present in the vicinity of, (drains 1 and 2) and within (drains 3 and 4), the wastewater irrigation area, primarily as a means of identifying sources of contaminants from the site to the Waiwiri Stream. These monitoring results are detailed in the Aquanet technical reports (2015, 2016 (Aquanet 2016:B6b)) and update memo (Aquanet 2017:B6c).

2. Surface water monitoring programme

Surface water instigations started in May 2013 and involved three distinct phases, as outlined below. Monitoring site locations are shown on Figure 1. Photographs of the monitoring sites are provided in the 2016 technical report (Aquanet, 2016:B6b)

• Phase 1 aimed at providing an assessment of the current state of the ecological values of the

Waiwiri Stream in the vicinity of the “Pot” site. Phase 1 included flow gaugings, water quality

monitoring and visual assessment of macrophyte cover monthly between May 2013 and July 2014

in the Waiwiri Stream and in a number of tributary drains upstream of, within, and downstream of,

the “Pot” and associated land irrigation area. Macroinvertebrate and fish communities were also

1Aquanet (2015). Levin WWTP land application at “The Pot”: Water quality and ecology of the Waiwiri Stream, 2013 – 2014. Report Prepared for Horowhenua District Council by Aquanet Consulting Ltd, January 2015. 2 Aquanet (2016). Levin WWTP land application at “The Pot”: Water quality and ecology of the Waiwiri Stream, 2013 – 2016. Report Prepared for Horowhenua District Council by Aquanet Consulting Ltd, June 2016.

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sampled in March 2014. Phase 1 was followed by a review of the results in late 2013, leading to

Phase 2 starting in October 2014.

• Phase 2 saw a shift in focus of the monitoring programme in October 2014 whereby sampling was

undertaken in order to:

▪ determine and quantify the sources of nitrogen and phosphorus from the “Pot”, the land

irrigation area and the surrounding land use;

▪ better understand the key drivers of macrophyte growth in the lower Waiwiri Stream;

▪ include coastal water quality and a record of beach usage in the vicinity of the Waiwiri

Stream Mouth; and

▪ continue regular water quality monitoring at key monitoring sites to provide a continuous

water quality record, and rationalise the monitoring sites and methods to meet the above

objectives.

▪ In September 2015, Phase 2 of monitoring wrapped up after eleven months of sampling

at sites within the Waiwiri Stream and drains of the “Pot”.

• Phase 3 of monitoring began In December 2015. Phase 3 involved sampling at the same water

quality sites as for Phase 2, but at a reduced sampling frequency. Phase 3 also included a one-off

metal sampling in stream water and sediments at selected sites.

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Figure 1: Sites monitored on the Waiwiri Stream and drains during Phase 1, 2 and 3 of monitoring between May 2013 and March 2016 are shown in white. Sites shown in orange were sampled in Phase 1 of monitoring and then discontinued. Additional sites monitored during Phase 2 are shown in yellow. Sampling at site Stream 5 is ongoing in Phase 3 of monitoring.

Beach North

Beach South

Beach Reference

Drain #3 (d)

Drain #3 (b)

Drain #3 (c)

Stream #1

Drain #1 Stream #2

Drain #2

Drain #3 (a) Stream #3

Stream #4 Drain #4

Stream #5

Beach North

Beach South

Beach Reference

Drain #3 (d)

Drain #3 (b)

Drain #3 (c)

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3. Current effects

3.1. Summary of findings

With regards to the effects of the current wastewater treatment and discharge regime at the Pot on surface quality and ecology of the Waiwiri Stream, we draw the following conclusions from the information and data collected since May 2013:

1. There were no statistically significant increases in the Waiwiri Stream downstream of the “Pot” in

total ammoniacal-nitrogen, E. coli, Chlorophyll a, suspended solids or turbidity;

2. There was a statistically significant increase in nitrate-nitrogen, soluble inorganic nitrogen (SIN)

and total nitrogen (TN) concentrations in the Waiwiri Stream between upstream and downstream

of the “Pot”. There are also indications of an increase in DRP concentrations downstream of the

“Pot” although the increase was not statistically significant;

3. Nitrate-nitrogen concentrations at the Drain 3 site (but not any of the other sites) exceeded the

NPSFM (2014) national bottom line for ecosystem health. The risk of actual toxic effects

associated with nitrate-nitrogen is however considered to be low once toxicity thresholds were

corrected for site-specific water hardness;

4. Sources of nitrogen and phosphorus into Drain 3 appear to predominantly originate from the

“Pot” and/or wastewater irrigation as opposed to farmland located at the upstream end of the

drain;

5. Concentrations of nutrients present in coastal waters North and South of the Waiwiri Stream

mouth were not significantly different to that of the reference site;

6. Metals measured in the water column of the main drain running through the irrigation area were

all below the ANZECC 95% species protection levels, indicating a low risk of toxic effects from

metals;

7. Metals measured in sediment in the Waiwiri Stream bed upstream, adjacent to, and downstream

of, the “Pot” irrigation area were all below the ANZECC ISQG-Low trigger values, indicating that

the risk of toxic effect from these metals is low;

8. There was an increase in the abundance of water celery (Apium nodiflorum), an invasive exotic

emergent macrophyte in the Waiwiri Stream at the site located downstream of the “Pot”, which

generally presented 100% macrophyte cover between December and May. Upstream of the

“Pot”, macrophyte cover was greatest between November and January and varied between 15%

and 100% cover;

9. When present, the macrophyte beds in the lower Waiwiri Stream (i.e. downstream of the land

irrigation area) appeared to provide significant reduction in the concentrations of nitrogen (58 to

69 % reduction in nitrate-nitrogen, SIN and total nitrogen) and phosphorus (63% reduction in DRP)

present in the Waiwiri Stream before its discharge into the coastal area. When macrophyte beds

were absent or significantly reduced (i.e. following the removal by stream freshes/floods) little

nutrient removal was measured in the lower Waiwiri Stream;

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10. With regards to macroinvertebrate communities, biotic indices in the Waiwiri Stream are

indicative of fair to poor water quality. No overall differences were identified in MCI between sites

upstream and located immediately adjacent to the land irrigation area, and QMCI increased at

the site located immediately adjacent to the land irrigation area, indicating an overall

improvement in macroinvertebrate community health. This improvement is thought to be

associated with better riparian cover at the downstream site, and occurred in spite of the

increases in nutrient (DRP and SIN) concentrations measured between the two sites;

11. Fish communities observed in the Waiwiri Stream included eels, inanga, Cran’s bullies and banded

kokopu. Higher numbers of species and individuals were observed in the stream reach

immediately adjacent to the land irrigation area compared with upstream of the “Pot”;

12. Recreational use of the beach and dune area near the Waiwiri Stream appears to be low, with no

recreational use recorded on any of the eleven site visits except for the collection of drift wood

approximately 300m south of the stream mouth on one occasion.

In summary, monitoring to date suggests that activities at the “Pot” site are having an effect on some, but

not all, water quality determinands in the Waiwiri Stream.

• Concentrations of E. coli, total ammoniacal nitrogen, dissolved reactive phosphorus (DRP), total

suspended solids (TSS) and turbidity in the Waiwiri Stream do not seem to be significantly affected

by operations at the “Pot”;

• Nitrate-nitrogen, Soluble Inorganic Nitrogen (SIN) and Total Nitrogen (TN) do increase significantly

in the Waiwiri Stream between upstream and downstream of the site. Nitrate-nitrogen

concentrations exceed national bottom lines in one of the drains within the irrigation area, although

not in the Waiwiri Stream itself. The risk of actual toxic effects associated with nitrate toxicity is,

however, considered to be low at all sites once toxicity thresholds are corrected for site-specific

water hardness.

• There was no detectable effect on coastal water quality near the Waiwiri Stream mouth compared

the reference site.

With regards to ecological effects:

• No significant effects were noted on macroinvertebrate communities, or on the presence/absence

of fish species. On the contrary a larger number of fish species and slightly “healthier”

macroinvertebrate communities were found at the site immediately adjacent to the land irrigation

area, indicating that better physical habitat (riparian cover and no stock access) at that site seem

to outweigh the potential effects of increased nutrients;

• Thick beds of the tall exotic emergent macrophyte water celery (Apium nodiflorum) were present

in the lower reaches of the Waiwiri Stream (i.e. downstream of the effluent irrigation area) on most

monitoring occasions.

• Rooted macrophytes such as water celery are able to meet their nutrient requirements from the

stream bed sediments in which they are anchored, and their growth tends to be controlled by the

availability of soft substrate, and sunlight and favourable (slow) flow conditions rather than the

availability of dissolved nutrients;

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• Although it may contribute to it, it is unlikely that the increase in dissolved nutrient concentrations

caused in the Waiwiri Stream by activities at the “Pot” is a key driver of the macrophyte growths in

the lower Waiwiri Stream. The combination of sunlight, broader channel and lower water velocity

present in the lower reaches of the Waiwiri Stream downstream of site Stream 3 provide favourable

conditions for macrophyte growth and are the likely key drivers of the increased macrophyte

abundance at the site compared with the upstream reaches of the stream;

• The macrophyte growths present in the lower Waiwiri Stream can be considered excessive at times,

and likely impact negatively on fish and macroinvertebrate habitat quality, by reducing open stream

channel habitat and causing lower oxygen levels at night;

• On the other hand, the macrophyte beds appear to provide significant reductions in the

concentrations of nutrients delivered to the coast by the Waiwiri Stream. Potential mechanisms of

nutrient attenuation/removal may include bacterial uptake (e.g. de-nitrification under low oxygen

conditions), algal uptake (from “epiphytic” algae growing on the macrophytes)

• From a management point of view, if a reduction in macrophyte growth was sought for the lower

Waiwiri Stream, a reduction in the amount of nutrients present in the water column (as opposed

to contained in sediment) is not likely to be effective for reducing macrophyte growth. A reduction

of light availability is the most effective option for reducing macrophyte growth and this is best

achieved through increased shading of the stream bed through increased riparian planting. This is

likely to provide benefits in terms of in-stream habitat quality, which outweigh the potential effects

of increased nutrients, as is the case immediately upstream.

3.2. Assesssment against RMA S107(1)

This section examines whether S107(1) standards, specifically clauses 107(1) (c), (d), (f) and (g) are met in the Waiwiri Stream and the surface drains. Clause 107(1)(e) is not covered in this report as it relates to odour, which is not a water quality issue.

• S107(1)(c) (the production of conspicuous oil or grease films, scums or foams, or floatable or

suspended materials). Conspicuous oil or grease films, scums or foams, or floatable or suspended

materials were not observed on any of the site visits or monitoring runs undertaken by Aquanet

Staff since 2013 in either the drains or the Waiwiri Stream itself;

• S107(1)(d) (conspicuous change in water clarity). There is no evidence of effects of the current

activities Pot on Total Suspended Solids (TSS) or turbidity in the Waiwiri Stream. Although visual

clarity was not regularly monitored, this provides a strong indication that the activities do not result

in a breach of S107(1)(d);

• 107(1)(f) (the rendering of fresh water unsuitable for consumption by farm animals). A number of

water quality determinands are relevant to the suitability of water for farm animal consumption,

including microbiological water quality, toxicants such as metals etc. Metal concentrations in both

the water column and the sediment of the Waiwiri Stream were found to be low and of no material

concern from a livestock drinking water point of view. There is no evidence of an impact of activities

at the Pot on microbiological water quality (E. coli) of the Waiwiri Stream.

• 107(1)(g) (significant adverse effects on aquatic life). As explained above there is no evidence of

significant adverse effects on macroinvertebrate or fish communities in the Waiwiri Stream. The

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excessive macrophyte growths observed at times downstream of the Pot are likely to result in some

habitat degradation, but are unlikely to be directly caused by activities at the Pot.

3.3. Assesssment against One Plan Water Quality targets

Table 1 below provide a summary of whether the water quality targets are met in the Waiwiri Stream and whether the activities at the Pot have a significant or measurable impact on each water quality or ecological determinand. The 2015 and 2016 Aquanet technical reports and the 2017 update memo compare water quality results to relevant water quality targets from Schedule E of the One Plan for both drain and stream sites. This assessment is not repeated here in detail.

Table 1: Summary of key water quality determinants measured in the Waiwiri Stream upstream and downstream of the Pot site, and assessment against One Plan water quality targets.

One Plan target Waiwiri Stream Significant impact of

activities at the Pot? Upstream Downstream

Ammoniacal nitrogen

(average 0.4 g/m3) √ √ No

Ammoniacal nitrogen

(Maximum 2.1 g/m3) √ √ No

SIN

(average, 0.167 g/m3) × × Yes

DRP

(average, 0.010 g/m3) × × Possible/localised

E. coli

(95th percentile √ √ No

Chlorophyll a

(4 mg/m3, estuaries only) × × No

Water clarity change

(33% reduction) √ No

QMCI change

(20% reduction) √ No

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3.4. Assesssment against NPSFM (2017) Numeric Attribute States

Table 2 below provide a summary of the water quality data available for the Waiwiri Stream at sites upstream (Stream Site 2) and downstream (Stream Site 4) of the Pot Site. The 2015 and 2016 Aquanet technical reports and the 2017 provide further detail, including an assessment for drain sites as well as other Waiwiri Stream sites.

The NPSFM (2014, edited September 2017) describes five “Attribute States” defined by the percentage of exceedances over 540 cfu/100ml, the percentage of exceedances over 260 cfu/100ml, the median concentration and the 95th percentile of E. coli/100ml based on a minimum of 60 samples over a maximum of 5 years. Band A represents the lowest risk of effects (i.e. the highest quality “band”), whilst band E represents the highest risk of effects (i.e. the lowest quality “band”)

All sites on the Waiwiri Stream fell within band A for each E. coli “attribute state”.

One of the water quality “attributes” relating to ecosystem health is the nitrate-nitrogen concentration

for toxicity risk. The NPSFM 2014 describes four “Attribute States” defined by nitrate concentration

ranges, or “bands”. Band A represents the lowest risk of effects (i.e. the highest quality “band”), whilst

“band” D is below the national “bottom line” for ecosystem health. Band D is the lowest quality “band”.

We note that the One Plan Schedule D water quality targets seek a 95% species protection level for “Other

toxicants” in the Water Management Zone containing the Waiwiri Stream (West_8), which is generally

consistent with the NPSFM B “band”.

The Waiwiri Stream upstream of the Pot was in Band A in all years of monitoring. Of the three monitoring

sites on the Waiwiri Stream adjacent to, or downstream of the Pot:

• Stream Site 4 essentially sits on the threshold between Band B and Band C for median nitrate-

nitrogen concentrations (2.4mg/L), and has exceeded the Band B threshold for 95th %ile

concentration (3.5 mg/L) in one of the four years of monitoring (in 2015);

• Stream sites 3 (adjacent to land irrigation area) and 5 (stream mouth) were in Band B in all years.

Some relatively modest reductions in in-stream nitrate-nitrogen reductions would be required to bring

Site 4 into Band B.

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Table 2: Assessment against NPSFM (2017) Attribute States for nitrate-nitrogen, ammoniacal nitrogen and E.coli upstream (Stream Site 2) and downstream (Stream Site 4) of the Pot site. Note that ammoniacal nitrogen data is not corrected for pH.

One Plan target

Waiwiri Stream

Upstream Downstream

Nitrate nitrogen

(median concentration) A B/C

Nitrate nitrogen

(95th %ile concentration) A B/C

Ammoniacal nitrogen

(Median Concentration) B C

Ammoniacal nitrogen

(Max concentration) C C

E. coli

(%exceed 540/100mL) A A

E. coli

(%exceed 260/100mL) A A

E. coli

(%exceed 540/100mL) A A

E. coli

(Median concentration) A A

E. coli

(95th %ile concentration) A A

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4. Potential effects of proposed future activities

4.1. Wastewater storage, treatment and discharge

LEI (LEI, 2018:D1,E1) provides a description of proposed changes to the wastewater storage, treatment and discharge to land at the Pot site. It is our understanding that, overall:

• The wastewater storage and treatment, including contaminant losses from the storage pond to

groundwater will remain similar to what they are currently;

• The wastewater irrigation areas will be expanded, albeit within the same general spatial footprint,

meaning that the same surface water bodies, i.e. the same surface drains and the same reach of

the Waiwiri Stream will be potentially affected by the discharge to land;

• Changes in irrigation management, including irrigation rates, are proposed to try and reduce

contaminant (including nutrients) losses to groundwater. However, growth in Levin will

counterbalance these reductions, and nutrient losses from the irrigation area will remain similar to

current losses.

As a result, and without the adoption of specific mitigation measures, the effects of the proposed activities at the Pot can be expected to remain similar in nature, intensity and spatial extent as those described in Section 3 of this memo. The implementation of additional mitigation measures have the potential to enhance water quality and ecological health, as detailed below.

4.2. Potential Mitigation options

LEI (2016:C3) describes a number of measures potentially able to be implemented at the Pot site in order to reduce the losses of contaminants from the wastewater storage and irrigation areas to groundwater (i.e. reduced irrigation rates, management of pond levels), or to intercept and treat these contaminants before they reach surface water bodies (e.g. wetlands, de-nitrification trenches). Subsequent work has identified the location, design and likely effectiveness of the various mitigation measures; as well as whether they will be implemented for certain or are possible options. This is summarised in Table 8.1 and Appendix A, Figure A11-P of the Application (HDC, 2018:E8). It is our understanding that the various mitigation measures aiming at reducing contaminant losses to and within groundwater will result in a material reduction in the amount of nutrients, in particular nitrogen, reaching surface water from groundwater recharge.

These measures are:

• Revised loading rate and irrigation scheduling;

• Increased irrigation area within the Pot; and

• Biodiversity trial.

The expect nett result is a material reduction in the in-stream loads and concentrations of nutrients in the Waiwiri Stream downstream of the “Pot”, compared with the current situation. We note that the exact degree of enhancement cannot be quantified at this stage on the basis of information available, and will need to be confirmed via on-going monitoring. In addition to the above measures, it is our understanding that the proposal also includes riparian planting of the northern bank (true right) of the lower Waiwiri Stream within the Pot site’s boundary (refer to Figure 2), in order to provide shading to the stream reach where excessive macrophyte growth has been observed. It is well established that shading is often the most efficient means by which the growth of aquatic macrophytes may be controlled. It is also generally well accepted that shading from riparian vegetation is most efficient at controlling plant growth in small streams, up to 10m in width (with some variability depending on site characteristics).

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The Waiwiri Stream channel downstream of the Pot Site is less than 10m in width. It runs on a mostly East-West direction, meaning that the northern bank (true right) should be given priority for riparian planting. This northern bank is a steep sand dune, which will aid in the shading efficiency of vegetation on that bank. Given the reach’s physical setup, it is likely that tall riparian vegetation will provide significant shading of the Stream bed and should, once sufficiently established, provide a reduction in macrophyte growth, as well as wider in-stream habitat benefits.

In conclusion, a range of mitigation options have been proposed, and it is expected they will have a positive impact on in-stream water quality and ecology compared with the current situation. The extent of the positive impact is difficult to quantify at this stage but can be assessed by monitoring over time.

Figure 2: Location of proposed riparian planting (map provided by LEI, May 2018).