5. Water Quality

• Introduction
• Water quality parameters
• Natural variations
• Human impacts
• Lake Illawarra water quality
• Monitoring
• Overview

Introduction

Previous information sheets have examined the flow of water into and out of Lake Illawarra due to tides, rainfall, catchment runoff, evaporation, etc. This information sheet looks at the quality of the water and some of the factors which affect water quality.

Water quality parameters

When investigating water quality it is common practice to look at certain physical, chemical and biological indicators as a guide to the health or otherwise of an estuary. Some of the more common indicators are:

· colour,

· turbidity and suspended solids, which indicate the concentration of particles (sediment or microscopic aquatic life) in the water,

· temperature, which influences the productivity of the ecosystem,

· pH, which measures acidity/alkalinity. A level of 7 is neutral. The pH of sea water is slightly alkaline at around 8.2,

· salinity (or conductivity), which measures the amount of salts,

· dissolved oxygen, which indicates the amount of oxygen available for aquatic animals,

· nutrients such as phosphorus and nitrogen, which are essential for plant growth and indicate the potential for nuisance algae blooms,

· inorganic chemicals, mainly heavy metals, which can be toxic to aquatic life,

· organic chemicals, such as pesticides, petroleum products, etc., which can also be toxic to aquatic life,

· chlorophyll-a, which is a measure of the amount of plant matter, including microscopic algae and seaweeds,

· algal levels, or the number of microscopic algae,

· faecal coliform numbers, or how many bacteria such as E. Coli which come from animal wastes.

Australian Water Quality Guidelines have been developed using the above indicators and aim to protect the whole aquatic ecosystem, i.e. all the plants and animals which rely on freshwater streams or sea water. The Guidelines also cover other uses of the waterway such as recreational activities.

Natural variations

Natural variations in water quality are related to factors such as the lake's water balance (see Information Sheet No.4), mixing and flushing characteristics and physical shape.

Parameters also change gradually over the seasons of the year. For example, the quantity of microscopic plants and animals in the water increases during the warmer months raising nutrient levels and turbidity.

Shallow lagoons that are open only intermittently with irregular tidal flows (such as Lake Illawarra) have naturally high turbidity and salinity variations.

Human impacts

Human activity in the lake can result in major water quality changes. The construction of entrance training walls can increase tidal flows. Conversely, causeways and similar structures can restrict flows. These changes affect the water balance and hence water quality. Catchment development can also change the physical, chemical and biological composition of the runoff water with potentially adverse effects on the lake.

Lake Illawarra Water Quality

European settlement, and particularly urban development over the past 30 years, has resulted in increased inputs of pollutants and a subsequent change in the rates of processes occurring on the lake, The water frequently fails to meet national water quality guidelines for aquatic ecosystems i.e. < 25% of the time, but meets primary contact and fish consumption guidelines > 75% of the time.

Of 8 water quality parameters tested for the period between 1996 and 2001, only 3 complied with the ANZECC (2000) guidelines for the majority of the time. These being dissolved oxygen, pH and oxidised nitrogen. Of the remaining constituents ammonia, total nitrogen, orthophosphorus rarely complied with the guidelines, while chlorophyll-a complied at some sites but not others.

The quality of the water in Lake Illawarra is influenced by rainfall, wind, temperature and entrance conditions. Although the lake is shallow, wind induced circulation patterns result in the water being well mixed in the main body of the lake.

In the absence of significant stormwater runoff the lake is saline and if the entrance is closed lake waters can become more saline than ocean water due to evaporation. Following heavy rainfall the water is less saline particularly near creek mouths. Rainfall runoff also contains pollutants such as sediments and nutrients. Sewage overflows into the lake can add to these pollutants.

Sediments which enter the lake may be transported directly to the ocean or be carried by tidal flows. The remainder either settles to the lake’s bottom or remains suspended in the lake. Strong winds and storms can readily disturb these sediments increasing turbidity especially in the shallower areas of the lake.

Sediments which flow into the lake carry with them nutrients, particularly phosphorous and nitrogen, which are present in animal faeces, detergents, fertilisers and sewerage, which are all part of urban and rural runoff.

Nutrient levels have substantially increased, with phosphate and nitrate concentrations exceeding 200 ug/L (with concentrations of 5 -15 ug/L of phosphorous and 10 - 100 ug/L of nitrate likely to cause nuisance algal growths). Nuisance algal growths occur regularly in Lake Illawarra. The first reports of excessive algal growth were made in the early 1970s.

Monitoring

Monitoring of the lake’s chemical water quality parameters has been undertaken at various times since 1972. The parameters tend to fluctuate considerably, due mainly to the periodic opening and closing of the lake’s entrance and the prevailing weather conditions.

Based on extensive water quality sampling carried out in the lake between 1996 and 2001 the following water quality parameters are derived.

Lake Illawarra Water Quality

Parameter	Range
Salinity	5 to 38 (seawater being 35.3)  parts per thousand consistent throughout the lake.
Temperature( Cel)	7 to 35, average of 20 C
Dissolved Oxygen	10% to 165% (% saturation) 0.8 to 13 mg/L
pH	7.0 to 8.8,  average of 7.8
Turbidity	NTU - 3.5 Secchi depth - 0.1m to 2.5m, average of 1.0m NFR - 0.5 mg/L
Total Nitrogen	0.15 to 0.65 mg/L, average of 0.5 mg/L
Total Phosphate	10 to 175 ug/L, average of 50 ug/L
Oxidised Nitrogen	<0.01 to 0.10 mg/l average of 0.015 mg/l
Chlorophyll-a	0 to 20 ug/L,  average of 4 ug/L
Orthophosphorus	<0.01 to 0.20 mg/L, average 0.05 mg/L

Source: Lake Illawarra Estuary Processes Study

Growths of algae can rapidly deplete oxygen levels and decrease the suitability of the lake for other aquatic life forms. Measurements over the past 16 years have indicated that oxygen concentrations frequently fall below a recommended minimum of 6 mg/L. The large volume of sediment which is easily stirred up in Lake Illawarra contributes to poor conditions for aquatic life by reducing the level of light penetration into the water, thus reducing photosynthetic activity, especially in the deeper parts of the lake.

Salinity in Lake Illawarra is generally lower than the ocean, reflecting physical factors such as the intermittently closed nature of the entrance, the volume of stormwater, rainfall and runoff occurring. The level of salinity tends to drop most when minimal tidal effects are occurring.

Faecal coliforms show a similar trend to nutrients in Lake Illawarra, by rising after rain, but are satisfactory during drier weather.

Overview

Overall, Lake Illawarra water quality is poor and varies with climatic and tidal conditions. This could happen naturally even if the catchment was not developed at all, but is more likely caused by polluted urban stormwater runoff from creeks and drains after rainfall.

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