Otara Creek Catchment

Otara Lake Water Quality Technical Report - Report Prepared by Worley Consultants Ltd

Summary | Introduction | Water Quality | Discussion | Conclusions

Executive Summary

A water quality assessment of Otar Lake and surrounding catchments was undertaken between October 1999 and January 2000. The purpose of the study was to estimate the baseline water quality of the Otara Lake and to determine the primary pollution impacts on the Lake. A further study aim was to estimate the impact of rain events on the Lake water quality.

Tamaki Estuary Water Quality
The water quality in the Tamamki Estuary represents the basic level of achievable water quality in the Otara Lake. Results indicate that Tamaki Estuary water quality does not support primary contact (e.g. full immersion bathing. It is, however, of sufficient quality to support secondary recreational use (e.g. boating).

Otara Boat Ramp Water Quality
The curve in the Otara Lake to the east of the Boat ramp means that flow velocity decreases at the boat ramp point. This promotes the deposition of solids (and their associated contaminants) and means the area may require dredging if regular access to the channel is required.

Overall the water quality at the boat ramp was similar to that found at Gayes Creek (mid-Lake) or the weir (lower Lake).; the only significant difference being high levels of nitrogen compounds and heavy metals at the boat ramp.

BOD₅ concentrations were high at this site and indicative of a polluted water body; on half the sampling occassiosn BOD₅ levels were over 5mg/L.

During wet weather the suspended solids concentrations at this site were some of the highest recorded and heavy metal concentrations rose by a factor of ten. This suggests that low level, long term pollutants are accumulating at the site.

Basic levels were also high at this site, especially during dry weather, suggesting that bathing and shellfish gathering should not be undertaken.

Inputs into Otara Lake

Industrial Stormwater
The industrial stormwater sampling site ws onteh northern arm of the Otara Lake. This site had consistently poor water quality during both dry weather and during rainfall. During wet weather there was a consistent and unexplained increase in pH suggesting that there was some alkaline input into this site. This site was also subject to what appeared to be sporadic elevated pollution events leading to high nitrate and BOD₅levels.

Rural Stormwater
The site at Flatbush represented the storwater quality from rural runoff. This site consistently had the best water quality of all the sampling sites (during both dry and wet weather) with low suspended solids, Bod_5, COD, total phosphorus, amonia, nitrite, nitrate. This site also had the lowest bacterial levels during wet weather.

Subdivision Storwater
The most demonstrable trend from the subdivision sites were the high levels of suspended solids which entered the stormwater systems during wet weather.

Residential Stormwater
Sampling was done at a number of stormwater sites in residential areas:

• East Tamki Road
• Bairds Road

The most significant results from the residential stormwater analysis was that during storm events the bacterial levels were indicative of stormwater contaminated with sewerage.

Bacterial levels at East Tamaki Road during dry weather were also very high suggesting that the problem at this site was due to failed infrastructure together with wet weather overflows. Nitrite concentrations at East Tamaki Avenue were the highest recorded during dry weather.

Mixed Stormwater Inputs
One of the storwater sampling sites was representative of storwater from rural, industrial and residential inputs. This site had:

• The highest total phosphorous during wet weather
• The highest nitrate during dry weather
• Lower bacterial levels than those usually found in stormwater

What were the major sources of contamination?
The most prominent pollution sources were:

• the industrial site at Kerwyn Ave in the northern arm of the tributary entering Otara Lake
• the residential discharge from Bairds Road with bacterial loadings indicative of sewer overflows

Risk Associated with Contact Recreation in Otara Lake
In wet weather the bacterial levels are very high. It is also fair to say that the Tamaki Estuary falls short of the contact recreation standard. Under dry weather conditions Otara Lake water quality is determined by Tamaki Estuary water quality. This water quality is suitable for non-contact recreation (e.g. boating) subject to some improvements (e.g. dredging, gross pollutant removal).

Introduction

Background
Otara Lake is approximately 50 hectares in size and receives run off from a catchment area of 3487 hectares. Land usage in the catchment and around the area is mainly residential and rural, although in recent years there has been an increase in industry and development.

Project Aims and Objectives
The monitoring programme was split into two distinct sections:

1. A baseline study to determine "normal" water quality in the Otara Lake and its associated creeks and streams during dry weather.
2. A stormwater study to investigate the effect of rainfall on various contaminants entering the Otara Lake.

Hypotheses
The baseline study was designed to determine whether:

1. rural land uses contributes significant nutrient loads into the Otara Lake (e.g. ready lawy, dairy farming etc)
2. land development earthworks contribute the majority of the sediment loads to Otara Lake
3. Tamaki Estuary has only a minor adverse effect on water quality in the Otara Lake
4. the majority of heavy metals and hydrocarbon contaminants entering the Otara Lake orginate from industrial areas and arterial roads.

Methodology and Programme Design

Description of Sampling Programmes
The study was separated into two main parts. The first was a programme to establish the baseline water quality of Otara Lake and its surrounds. This study involved collecting samples during periods of mostly fine weather. The second study was designed to observe changes in the water quality during periods of rainfall. To this end three rain events were chosen and samples collected from the same sites as the baseline study.

Sampling Locations
(See Table 2).

Baseline Study Methodology

Sample Collection
Water samples were collected from all baseline sampling sites after a period of dry weather greater than two days.

Stormwater Study Methodology

Sample Collection
Sampling was undertaken in response to rainfall events. Different intensity storms were targeted with an aim of ensuring at least 10mm of rain had fallen prior to sample collection.

Results

These results represent only those collected during the course of this sampling programme.

Physical Water Quality Characteristics

Conductivity
Conductivity is a measure of the presence of ions in water which allow the passage of an electric current. In this instance conductivity is primarly an indicator of the salinity of water.

Baseline:Highest conductivity were at sites 1,2,3,6, and 11. Lowest conductivity was found at sites 4,5,6,7,8,9 and 10.

Wet Weather: As a storm event occurs the influx of freshwater into a saline environment will lead to a decrease in salinity and, from that, to a decrease in conductivity. This phenomenon was observed at all sampling sites. The greatest change in conductivity occurred at site 4 where it appears that a large influx of freshwater caused a significant decreaase in conductivity.

Chloride
Chloride may be used as an indicator of the water salinity as it quantifies the presence of the chloride component of sodium chloride.

Baseline: These results therefore reflect those found in the conductivity section with high levels of chlorides at sites 1,2,3,6 and 11.

Wet Weather: The trends in chloride concentrations included significant decreases during rain events. The smallest changes in salinity occurred at the most saline sites - 11,1 and 2.

Suspended Solids
Suspended solids provide an indication of the amount of sediment entering the aquatic environment. High suspended solids levels can have an adverse impact ona aquatic fauna and can lead to clogging.

Baseline: The highest suspended solids concentration was found at site 2.

Wet Weather: In general, suspended solids concentrations increase during storm events as silt and sediments are sluiced off non-permeable surfaces. This phenomenon was observed at all the sampling sites except sites 2 and 10. The highest suspended solids concentrations were found at site 11. Significant increases in suspended solids concentrations during rain events were also found at sites 3,4,5 and 6.

Turbidity
Turbidity is another measurement of the presence of solids in a water sample.

Baseline: Dry weather sampling for turbidity reflected those results found in the suspended solids analysis with high levels at sites 2,3 and 11.

Wet Weather: As with suspended solids, turbidity levels are expected to increase during wet events. The increases observed in this data set are not as marked as those seen in the suspended solids section.

pH

Baseline: pH in saline environments. The pH in saline waters should generally fall between 7.8 and 8.3. On this basis site 2 has slightly lower pH on three of the four sampling occassions. Site 11 also has lower than normal pH. The highest pH reading was recorded at site 1.

pH in freshwater environments: the pH of freshwater should lie between 6 and 9, but on average around 7. In this study all freshwater samples had pH levels within this range. The lowest pH reading of 7 was recorded at site 10.

Wet Weather: The pH observed during wet weather was generally more acidic than levels found during dry weather. This is because rain water reacts with carbon dioxide in the atmosphere forming weak carbolic acid. However, this increase in acidity was not observed at site 4 where during rainfall PH rose consistently. This may be due to some industrial discharge.

 

Chemical Water Quality Characteristics

BOD₅
Biochemical Oxygen demand (BOD₅) indicates the amount of dissolved oxygen required to break down organic material in water over 5 days at 20^oC. High BOD₅indicates a nutrient rich envrinoment which may be come oxygen deprived. Waters with greater than 5mg/L BOD₅ are considered to be polluted.

Baseline: On this basis the dry weather water quality at sites 4 and 5 are above recommended levels for aquatic environments.

Wet Weather: BOD₅ levels during rainfall tended to increase as nutrients were washed into the sampling areas. BOD₅ decreased during rainfall at sites 2,4 and 5. Sites 4 and 5 had elevated BOD₅ concentrations which were attributed to single outliers while their rainfall data were consistently high.

COD
Chemical Oxygen demand (COD) is a quantification of the amount of oxygen required to completely oxidise the organic matter in a water sample. High COD can indicated a nutrient rich environment which may be oxygen deprived.

Baseline: COD concentrations were only quantified in the freshwater environments. The highest COD was from the site 4 while lower levels were found at less developed stormwater discharges (e.g. sites 8 and 10).

Wet Weather: There are no comprehensive trends associated with COD concentrations before or during rain.

Total Phosphorus
Phosphates are commonly found in detergents and fertilisers but may also be sourced from organic decay. High concentrations of total phosphorous, in conjunction with high nitrate levels, are responsible for causing algal blooms and eutrophication.

Baseline: The highest phosphorous concentrations were found at site 4.

Wet Weather: Total phosphorous concentrations increased at all sites during rainfall.

Nitrogen Compounds
Amonia
Amonia is the initial product of the decay of nitrogenous wastes and its presence frequently indicates the presence of wastes including wastewater and fertilisers. Amonia concentrations vary according to the pH, temperature and salinity of the aquatic system.

Baseline:
Ammonia levels in saline water: Chronic toxicity in saline waters occurs at levels fo more than 2.3mg/L. The highest recorded levels in saline water in this study was at site 2 with levels of less than 0.35mg/L.

Ammonia levels in freshwater: The long term exposure limit for ammonia in freshwater is 0.77mg/L. All ammonia readings, except one (see below), were below 0.36mg/L.

However, the ANZECC states that for the protection of freshwater aquatic systems ammonia levels should be below 0.02 to 0.03 mg/L. None of the sites recoreded mean ammonia levels of below 0.02mg/L.

The highest ammonia levels in freshwater in this study were recorded at site 7 with 3mg/l.

Wet Weather: During rainfall ammonia concentrations increased with distance from the Otara Weir. This may be attributed to decreasing dilution and the increasing presence of pollution sources. The highest level was recorded at site 9.

Nitrite
Nitrite (NO₂) occurs in water as an intermediate oxidation state of nitrogen in teh conversion of ammonia to nitrate.

Baseline: On this basis the results suggested that sites 4 and 6 may be influenced by point sources of pollution.

Wet Weather: At sites 7 and 9 concentrations were higher during wet weather than dry weather.

Nitrate
Nitrate (NO₃) is the end product of the breakdown of ammonia through the intermediate step of nitrite by micorbial decomposition.

Baseline: As with both ammonia and nitrite the concentrations of nitrate increased with distance from the weir (sites 1,2 and 3) during both dry and wet weather. As with the nitrite results, the lowest concentrations were found at site 10, sites 8 and 9 and at site 1. High nitrite concentrations were consistently found at site 7 and site 5.

Wet Weather: Published literature suggests that nitrate levels in stormwater ranges between 0.4mg/L (weak) and 1.5mg/L (strong). On this basis the leels of ntirate found in dry weather water samples are very high at sites 5 and 7. Levels of nitrates in sites 3 and 6 were also high.

Nitrate/nitrite
The combined Nitrate/nitrite concentrations give an indication of the overall presence of nitrogen compounds.

Baseline: Nitrate/nitrite presence during baseline monitoring was elevated in the stormwater catchments at sites 5 and 7. The lowest levels found in stormwater were both upstream at site 9 and downstream at site 8 and in the rural stormwater discharge at site 10.

Wet Weather: Samping during wet weather determined that nitrate/nitrite concentrations decreased at most sites with the exception of sites 8 and 9.

Sulphide
Sulphide levels in water samples during this study were all consistely below the level of detection (<0.10 mg/L).

Heavy Metals
Heavy metals in water are commonly sourced from road runoff and industrial wastewater. The most commonly found heavy metals are zinc and copper and lead.

Copper
Copper is a commonly found heavy metal sourced both from industrial catchments and from roads where vehicle wear leads to deposits.

Baseline: Copper levels increased from site 1 to site 3. Site 4 had the highest concentrations of all the water samples.

Wet weather: Copper concentrations increased during wet weather and generally mimicked those concentrations found during dry weather. This suggests that the sources of heavy metal pollution are predominantly in the upper regions of the lake.

Zinc
Zinc is also sourced from industry and vehicle wear which deposits in roadside dust.

Baseline: As with copper, the zinc concentrations found in the Otara Lake increase further up the Lake (from sites 1 to 3) again suggesting that site 3 is the most heavily impacted by heavy metals. Elevated zinc levels were consistently found at site 4.

Wet weather: Zinc concentrations rose during wet weather. Highest concentrations were at sites 4 and 5. Site 3 had the hightest zinc concentrations of any of the saline water bodies.

Lead
Lead used to be commonly found as a result of leaded petrol but has become more scarce with the introduction of unleaded petrol. High levels are generally only found in sediment studies. Lead levels were all below the level of detection with the exception of one reading in the Tamaki Estuary.

Microbial Water Quality Characteristics

Indicator bacteria are used to give an indication of the microbial contamination of water bodies.

Enterococci
Baseline: Enterococci levels were consistently higher at site 4. Site 7 also had consistently elevated enterococci concentrations. The lowest levels were found at site 1.

Wet weather: Particularly high concentrations of enterococci were found throughout the lake (sites 1,2 and 3).

Faecel coliforms
Baseline: Levels mirrored those trends suggested by the enterococci results.

Wet Weather: All faecal coliforms levels were elevated during wet weather by at least an order of magnitude.

Total Petroleum Hydrocarbons (TPH)

Although hydrocarbon type sheens were sometimes observed on the Lake at site 4, TPH's were not found at any of the sites during dry weather and were only found at one site (site 4) during wet weather.

 

Discussion

Tamaki Estuary Water Quality
The water quality in the Tamaki Estuary ultimately determines the water quality in the Otara Lake. Therefore the quality of this water can be viewed as the deciding factor in establishing whether adequate water quality is achievable in the Lake. The water quality parameters found in the Tamaki Estuary in this study are summarised in table 8.

Otara Lake Water Quality
Otara Lake water quality can be summarised from the data collected at sites 1,2 and 3.

Site 1 - Otara Weir
Site was directy adjacent to the Otara Weir which floods the Otara Lake with water from the Tamaki. During dry weather this site was characterised by:

• the lowest enerococci and faecal coliforms concentrations
• the highest chloride and conductivity levels
• the second highest pH
• low nitrate/nitrite concentrations

The ARC had continued monthly water quality sampling at this site since 1985. Date for 1977/1998 was compiled with the following results:

• Mean salinity of 18.9%
• Mean dissolved oxygen levels of 8.0 mg/L
• Median enerococci concetrations of 210/100ml
• Mean pH of 7.8

Site 2 - Gayes Creek
Gayes Creek sampling point was a muddy water body which fed into the Otara Creek from alongside Ngati Otara Park. Table 9 represents a comparison between this study and one undertaken by MCL (1995).

Site 3 - Boat Ramp
Overall the water quality at the boat ramp was similar to that found at Gayes Creek (mid-Lake) or the Weir (lower Lake); the only significant difference being in high levels of nitrogen compounds and heavy metals at the boat ramp.

Otara Creek
Site 6 - Otara Footbridge
The Otara Footbridge overlooks the main body of esuarine water from the mouth of Otara Creek which feeds into the lake. Table 10 represents the results of this study for comparison with those of Challis (1985) and MCL (1995).

Industrial Stormwater
The only fully industrial stormwater sampling site was at Kerwyn Ave. This site had consistently poor water quality during both dry weather and during rainfall.

Rural Stormwater
Site 10 at Flat Bush represented the stormwater quality from rural runoff. The site consistently had the best water quality of all the sampling sites. Table 11 compares a downstream ARC historical sampling site with site 10.

Subdivision Stormwater
The two sites which were based in an area undergoing subdivision were Site 12 and Site 9. Subdivision stormwater was good at site 9 but poor at site 12. Sediment control methods at site 12 do not appear to have been effective.

Residential Stormwater
Sampling ws done at a number of stormwater sites in residential areas:

• East Tamaki Road - site 7 (full analysis)
• Bairds Road - sites 13 and 14 (bacterial counts only)

Bacterial levels at East Tamaki Road were very high during both dry and wet weather. Nitrite concentrations at East Tamaki Ave were the highest of any site recorded during dry weather. Table 12 compares data collected in other studies with those from site 7. It would appear from these data that this site has a poor history of water quality.

Comparing the bacterial levels found here with those of other studeis shows that the range of bacterial concentrations found at sites 7,13 and 14 are comparable to sewage contaminated water concentrations found by Williamson and are significantly higher than concentrations found in previous Auckland stormwater studies. This suggests that the sites are impacted by domestic sewage.

The most significant result from the residential storwater analysis ws that during storm events the bacterial levels discharged from all three sites were indicate of storwater contaminated with sewage.

Mixed Storwater Inputs
Site 5 was representative of stormwater from mixed sources. Table 14 presents previous recorded data from Site 5 and cmpares it with this study.

Rongomai Stormwater Treatment Pond
The Rongomai stormwater treatment pond is approximately eight years old and was constructed to detain stormwater run off from the Flatbush area. During dry weather the majority of the water quality parameters were the same upstream and downstream of the wetland. The data suggests that Rongomai wetland was effective in removing some heavy metals, COD, suspended solids, some bacterial indicators and some nutrients. Its poorest removal efficiency was in zinc removal.

How Contaminated Is The Stormwater Entering Otara Lake?
Table 16 presents a comparison between storwater run off commonly found both in New Zealand and overseas and those found discharging into the Otara Lake in this study. The results presented in table 16 suggest that during rain events the most significantly contaminated discharges are from the industrial site 4 and residential site 7.

How contaminated is Otara Lake?
On the basis of a comparison between The Australia and New Zealand Environment and Conservation Council guidelines for Fresh and Marine waters and levels found in the receiving water systems in the Otara Catchment during dry weather it has been found that the water quality in not sufficient to protect the aquatic ecosystem or promote health within the system.

Sediment Studies
Sediment studies have shown that contaminant levels increase towards the head of the lake.

Ecological Studies
A number of ecological studies have been performed in the Otara Lake area:

• Bethnic Sampling from Otara Lake and upper Tamaki Estuary (Kingett Mitchell 1992).
• Bethnic Sampling from Otara Lake (Kingett Mitchell 1995).
• ethnic Sampling from Tamaki Estuary (Kingett Mitchell 1996).
• Shellfish resources (Bioresearches 1990).
• Bethnic invertebrates and fisheries were surveyed in the power station holding pond and in the Tamaki Estuary (Kingett Mitchell and Associates, 1999).

The 1999 study had the following findings:

Sediment Characteristics: Both the subtidal and intertidal sampling locations were dominated by sand sized particles. Sediment further from the diffusers was muddier.

Bethnic Communities: The dominant taxa were crustaceans, polychaeates adn bivalve and gastropod molluscs.

Baseline fisheries survey: The most commonly caught fish were yellow eyed mullet, shortfinned eel and Koheru.

Conclusions

The following conclusions can be drawn from this study"

• The water quality in Otara Lake is very poor especially following rain. Bacteria levels are very high as are heavy metal loadings and some nutrients.
• Water quality throughout the Lake and its catchment deteriorated significantly during rainfall.
• The enerococci levels are sufficiently high to suggest there is a public health risk associated with the water body and as such should not be used for primary contact (e.g. swimming). The faecal coliform concentrations found during both dry and wet weather are higher than 14 faeceal coliforms/100ml. This is above the level recommended for recreational shellfish gathering.
• Water quality throughout the Otara Lake is not sufficient to protect the aquatic ecosystem or promote health within the system.
• The Rongomai stormwater treatment wetland was effective in removing suspended solids, some heavy metals and some nutrients. The wetlands were not effective in removing faecal coliforms, nitrate or zinc.
• Under dry weather conditions, the water quality of Otara Lake is dependent on the quality of water entering it frm the Tamaki Estuary. The water quality of Tamaki Estuary supports non-contact recreational use such as boating. Under wet weather conditions it is discharge from within the Otara Catchment itself that determines water quality in the Lake. After rain it is recommended that both contact and non-contact recreational activities do not occur for at least 72 hours.
• The water quality in Otara Lake is too poor for primary contact (e.g. swimming).

Recommendations

The following recommendations are made solely on the results of the water quality study and will be further extrapolated and prioritised as part of the strategic review and recommended actions.

General Water Quality Improvement Strategies:

1. Immediate improvement of amenity value could be achieved with the removal of debris from the upper lake area.
2. Some dredging at the boat ramp will improve the velocity of the water, allow boat access to the water and remove some of the most contaminated sediment in the Lake area.
3. Lake water quality during dry weather would need to be improved to acheive secondary contact (non-contact) status such as boating.
4. Secondary (no contact) recreational use (e.g. boating) should not be promoted after rainfall unless pollution levels are significantly reduced.
5. The lake should not be used for contact recreation such as swimming.
6. Manukau City Council should assist Auckland regional council in site by site investigation of the Kerwyn Ave Industrial Area.
7. Consideration should be given to the further use of stormwater ponds.

Infrastructure Investigations:

1. Some investigations of sewerage integrity should be performed at East Tamaki Road and Bairds Road.
2. Sporadic pollution from the industrial site at Kerwyn Ave should be investigated and recommendations made as to the feasibility of stormwater quality improvement strategies.
3. The ARC should be informed regarding the high suspended solid levels at Site 12 and action taken to investigate the function of sediment ponds in this area.
4. Manukau water should investigate the occurrence of illegal connections and possible sewerage leaks in its system.

Further Monitoring Studies

1. The Kerwyn Bridge Catchment and East Tamki Drive sites should be more extensively studied in order to determine the most significant sources of pollution.
2. Baseline monitoring should continue at the industrial stormwater site, subdivision stormwater site, boat ramp and catchment focal point for a large area of discharge.
3. A comprehensive ecological survey should be undertaken in the area to determine the diversity of species in Otara Lake. This would estabilsh the baseline poplutions of flora and fauna currently present in the Lake area adn would act as a benchmark for future investigations.

Education

1. The expectations of the community should be addressed in terms of realistic,