W3: Water quality

Introduction

For information on this indicator see Background: Water.

This indicator assesses the water quality of rivers and streams in the ACT. A comprehensive assessment of the ACT’s urban waters, including lakes and wetlands, can be found in the 2022 investigation into the State of the Lakes and Waterways in the ACT.

Climate trends are an important consideration for aquatic ecosystem assessments. The period from 2015 and 2022 saw extremes in weather conditions across the ACT, from the hot and dry conditions between 2017 to 2019, to the very wet conditions from mid-2020 to 2022 (see Climate Change). These extremes had a significant impact on river flows in the ACT (see Indicator W2: River flows) and have influenced the water quality assessments presented in this report.

Overall water quality

Water quality — Catchment Health Indicator Program (CHIP)

CHIP assessments of the overall aquatic ecosystem health condition for ACT waterways, and the macroinvertebrate and riparian components of the CHIP are presented in Indicator W1: Aquatic ecosystem health.

The CHIP assesses water quality for each reach using the results of monthly surveys from all sites (where possible). Water quality parameters monitored are pH (a measure of acidity or alkalinity of water), electrical conductivity (salinity), turbidity, dissolved oxygen, phosphorus and nitrate. These parameters have been widely established as the best indicators of water quality while being relatively easy to measure. For information on water quality see Background: Water. 

Water quality was assessed for 71 reaches in the Ginninderra, Molonglo and Southern ACT catchments. Water quality was found to be excellent for 41% of reaches and good for 55%, with only three reaches assessed as fair condition (Figure 16 and Figure 17). All catchments had the majority of their reaches in good to excellent condition. The Southern ACT catchment had 20 (67%) of its reaches in excellent condition. It should be noted that the results of monthly monitoring are often biased toward more favourable water quality assessments due to the likelihood of missing high pollution events. Consequently, the CHIP water quality results may underestimate the actual condition of water quality and catchment pressures.

The 2019 to 2022 period saw improved water quality scores compared to the 2015 to 2018 period with a higher percentage of reaches in excellent condition (41% compared to 35%). The difference between the two periods may reflect the very wet conditions and increased waterway flows over the 2020 to 2022 period, with high flows diluting concentrations of nutrients in urban waters.

Although the ACT’s water quality was generally good, nitrogen concentrations are higher in the Murrumbidgee River downstream of the Lower Molonglo Water Quality Control Centre (LMWQCC), with concentrations second only to the Molonglo River immediately downstream of the LMWQCC.^[3]Murrumbidgee River downstream of the LMWQCC refers to those reaches downstream of the Molonglo River confluence. The Molonglo River receives discharges from the LMWQCC immediately upstream of the confluence with the Murrumbidgee River.

Nitrogen (measured as nitrate) for the Murrumbidgee River downstream of the LMWQCC had a maximum concentration of 40 milligrams per litre (mg/l) in 2019-20, compared to a range of 0 to 3 mg/l upstream of the ACT and 0 to 5 mg/l for the Murrumbidgee River reach upstream of the LMWQCC discharge. A nitrate concentration of greater than 2.6 mg/l is considered to be degraded under Waterwatch condition thresholds.

The high amount of rainfall over the 2020 to 2022 period saw an improvement in nitrate concentrations for the Murrumbidgee River downstream of the LMWQCC, from a median concentration of 12.5 mg/L in 2019 to a median of 1.5 mg/L in 2022.

The high nitrate concentration in the Murrumbidgee River continues downstream of the ACT, particularly during dry periods when the LMWQCC discharge contributes a higher proportion of the total river flows. More information is required on the impacts of the LMWQCC on aquatic ecosystems in the Murrumbidgee River.

As with all other CHIP parameters, water quality condition is linked to land use with the majority of excellent condition reaches on conservation and protected land. However, despite the added pressures imposed by urban and rural land uses, water quality was still good in these areas, with some reaches attaining excellent condition ratings. These assessments demonstrate the effectiveness of water quality management in some urban areas, particularly as a result of constructed wetlands and other water-sensitive design approaches.

Urban waterways

In 2022, the Office of the Commissioner for Sustainability and the Environment undertook an investigation into the State of the Lakes and Waterways. This investigation assessed the environmental health and management of Canberra’s urban lakes, ponds and wetlands, and waterways. This urban water quality section should be read in conjunction with this investigation to obtain a more detailed understanding of urban waters and their management. The investigation found that of the water quality variables monitored, increased turbidity and nutrients were the ACT’s main urban water quality issues impacting on aquatic health and recreational closures. Some of the main water quality findings for each urban lake and waterway are discussed below. Recreational water quality is discussed in Indicator W4: Recreational water quality.

Lake Tuggeranong

Lake Tuggeranong has a reputation for poor water quality and regularly suffers from cyanobacterial blooms and high levels of faecal contamination. These are often accompanied by odours and highly turbid water, reducing the recreational and aesthetic value of the lake and surrounds. A significant issue affecting the aquatic health of Lake Tuggeranong is its small size in comparison to its catchment area.

The main pressure on the lake’s water quality is the significant inflow of pollutants from urban areas which occupy 50% of the catchment. Urban development is continuing in the catchment, which will likely further degrade water quality in the absence of effective pollution controls.

Nutrient concentrations in Lake Tuggeranong were regularly above the acceptable levels for urban lakes. Nitrogen levels are particularly high with over 80% of water samples above the acceptable range for most years (Figure 18).

Lake Tuggeranong does not appear to be consistently mitigating the effects of urbanisation for the downstream waters with periods of higher phosphorus and nitrogen concentrations within and downstream of the lake compared to upstream of the lake.

Lake Ginninderra

Current water quality issues in Lake Ginninderra are confined to rare cyanobacterial blooms and regular high enterococci concentrations, with all other water quality parameters typically within the acceptable range. However, nitrogen concentrations are an issue in some years with around 70% of samples taken in 2021 and 2022 above guideline levels.

The main pressure on the lake’s water quality is the significant inflow of pollutants from urban areas. With urban development continuing in the catchment, it is likely that water quality impacts will increase in the absence of effective pollution controls.

Lake Ginninderra does not appear to be effectively mitigating the effects of urbanisation for the downstream waters with increased nutrient levels found downstream of the lake in some years.

Lake Burley Griffin

The main pressures on the Lake Burley Griffin’s water quality and aquatic health are pollutants from the Molonglo River, the Queanbeyan Sewage Treatment Plant discharges and pollutants from the ACT’s urban runoff, including from the construction of new urban and commercial developments in the lake’s catchment.

There are regular occurrences of very high nutrient concentrations in the lake. The main sources of nutrients are rural and urban runoff and the Queanbeyan Sewage Treatment Plant. The lake’s sediments are also a source of nutrients and are thought to be causing some of the cyanobacterial blooms in the lake. Phosphorus concentrations in the lake are consistently above the algal growth limiting levels and are sufficient to support algal blooms (Figure 19).

With climate change and increasing urbanisation, cyanobacterial (blue-green algae) blooms are likely to occur with increasing frequency unless interventions are undertaken to improve the quality of water entering the lake.

Turbidity levels generally improve as water moves through the lake toward Scrivener Dam. However, during high rainfall runoff events, urban inputs from Sullivans Creek and other sources can significantly increase turbidity in the lake.

Lake Burley Griffin was found to be effectively mitigating pollutants, especially nitrogen, and protecting the waters downstream of the lake in most years.

Molonglo River

Discharges from the Queanbeyan Sewage Treatment Plant can be a substantial source of flow for the Molonglo River during drought periods. In 2019, it contributed nearly 40 per cent of additional flows. Whilst this is an important source of water, it introduces higher nutrient concentrations to the river in dry periods, increasing the risk of cyanobacterial blooms in Lake Burley Griffin.

Whilst concentrations of phosphorus recorded in the upper and lower reaches of the Molonglo River were commonly within the acceptable range, nitrogen concentrations were regularly outside of the acceptable range with the high concentrations likely to have adverse effects on the instream aquatic ecosystems. Nitrogen concentrations were found to be higher in the upper Molonglo River in comparison to the lower reaches showing that Lake Burley Griffin is trapping some of the nitrogen and preventing its movement downstream.

High turbidity levels and associated sediment inputs are of particular concern for aquatic health in the lower Molonglo River.

Turbidity levels were regularly outside of the acceptable range for both the upper and lower Molonglo River, particularly in wetter years (Figure 20). Since 2019, the turbidity in the lower Molonglo River has increased notably with more than 75% of the turbidity samples above the acceptable range with some very high readings. The increase in turbidity is highly likely to be from sediment runoff caused by urban development in the Molonglo Valley (see Land).

Turbidity issues in the lower Molonglo River suggest that sediment contributions from urban development are negating the water quality benefits provided by Lake Burley Griffin, and are impacting on the water quality of the Murrumbidgee River (see Sedimentation in Deep Creek). This highlights the need to better manage the runoff from the urban development occurring in the Molonglo Valley (see 5. Canberra’s urban boundary).

Tuggeranong Creek

The concrete lined open drains that characterise much of the Tuggeranong Creek upstream of Lake Tuggeranong provide no physical or biological value for the improvement of water quality. The concrete channels cause high levels of nitrogen, turbidity and electrical conductivity (a measure of salt concentration).

Concentrations of nitrogen in Tuggeranong Creek upstream and downstream of Lake Tuggeranong were consistently outside the acceptable range and were significantly higher downstream of Lake Tuggeranong (Figure 21). For some years, none of the recorded levels were within the acceptable range. Turbidity levels were also significantly higher downstream of Lake Tuggeranong.

The high nitrogen and turbidity are a concern for the ecosystem health of Tuggeranong Creek and the downstream Murrumbidgee River. Turbidity is also the main cause of the poor recreation and aesthetic values associated with the creek.

The continuing urban development in the Tuggeranong District, and especially the construction phase of new suburbs, is increasing water pollutant contributions to the creek.

Ginninderra Creek

Whist concentrations of phosphorus recorded in Ginninderra Creek upstream and downstream of Lake Ginninderra were nearly always within the acceptable range, nitrogen (as nitrate) was consistently outside the acceptable range.

Turbidity levels in Ginninderra Creek were also consistently outside the acceptable range, with higher levels of turbidity upstream of Lake Ginninderra. The higher levels are possibly caused by runoff from the urban development that has been occurring in the upper catchment over the past ten years. The decreased turbidity downstream of Lake Ginninderra is likely to be in part caused by the mitigation of turbidity by the lake.

Periods of low concentrations of dissolved oxygen were found downstream of Lake Ginninderra and may be caused by a high organic load in the creek, possibly from the leaf fall of deciduous trees that line the creek in sections.

Sullivans Creek

Concrete lined drains make up a significant length of the Sullivans Creek channel within the Canberra urban area. These provide no physical or biological value for the improvement of water quality. The concrete channels also cause elevated electrical conductivity (a measure of salt concentration) in the creek.

Concentrations of total phosphorus, nitrogen, turbidity and dissolved oxygen were consistently outside the acceptable range in the creek with some very high concentrations recorded. The high phosphorus concentrations are likely because of the greatly modified catchment and aquatic ecosystems in the creek.

Sources of high turbidity are most likely caused by sediment transported by rainfall runoff from urban areas and developments. Another driver of high turbidity is the amount of fine organic content in the creek from the large loads of organic matter from urban areas.

Periods of high levels of nutrients and turbidity in Sullivans Creek impact on the water quality and amenity of Lake Burley Griffin, particularly during times of high rainfall.

ACT impacts on water quality

It is important that the quality of water leaving the ACT via the Murrumbidgee River should be comparable to that entering the region. However, a range of pressures, especially those related to land use, can degrade water quality both within and downstream of the ACT. The impact of the ACT on water quality is determined by comparing monitoring results for the Murrumbidgee River at sites upstream and downstream of the ACT. In situ water quality probes measure pH (a measure of acidity or alkalinity of water), electrical conductivity (salinity), dissolved oxygen and turbidity. Phosphorus and nitrogen are not assessed by these probes.

Water quality results for the period 2015 to 2022 show that the ACT can impact on the downstream levels of turbidity and electrical conductivity in the Murrumbidgee River. Turbidity exceeded guidelines in all years from 2015 to 2022, both upstream and downstream of the ACT, with some years having exceedances of around 50% or higher of the days monitored (Figure 22). In 2020, turbidity levels downstream of the ACT experienced guideline exceedances for 85% of the days monitored, this increased to 100% in 2021. The number of turbidity exceedances was highly influenced by increased rainfall runoff and river flows with higher turbidity levels observed in response to significant rain events. In addition, extreme levels of ash and sediment from the 2020 Orroral Valley bushfire contributed to high turbidity levels that persisted for long periods in 2020 and 2021 (see 4. Bushfires in the ACT).

In the high rainfall years of 2016 and 2020 to 2022, the number of turbidity exceedances significantly increased and were higher downstream of the ACT. This shows that during periods of increased rainfall runoff, the ACT contributes additional sediment and other particles that cause turbidity into the Murrumbidgee River via Canberra’s urban waterways. Sources of the increased turbidity is likely from urban areas — especially development sites — and lands with little vegetation including rural areas. It should be noted that Canberra’s urban lakes mitigate the downstream levels of turbidity in most years, therefore reducing downstream impacts in the Murrumbidgee River. However, new land developments in the ACT’s western edge have no large lake to reduce potential downstream impacts. This may mean that the ACT will have a greater impact on the Murrumbidgee River in the future. Turbidity issues for urban waterways are discussed in the 2022 Investigation into the State of the Lakes and Waterways in the ACT and Water quality in the ACT’s urban waterways.

During the dry years with low flow conditions from 2017 to 2019, the number of turbidity exceedances were lower and more common upstream of the ACT. This shows that in dry years turbidity slightly improves as the river moves though the region. This is likely due to the limited inputs of sediments in rainfall runoff and the reduced movement of instream channel sediments due to low river flows. In addition, during low flow periods when the Lower Molonglo Water Quality Control Centre (LMWQCC) contributes a significant proportion of the Murrumbidgee River flows, the discharged water is very low in suspended particles levels which further reduces overall turbidity in the river.

Results also show that turbidity is the main water quality issue for the Murrumbidgee River for the variables assessed. Turbidity was also reported as one of the most serious water quality issues for urban waters in the 2022 investigation into the State of the Lakes and Waterways in the ACT.

Electrical conductivity results for the period 2015 to 2022 show that there are no exceedances of guidelines upstream of the ACT. However, in dry periods, electrical conductivity exceedances occurred for around 30% to 40% of the days monitored downstream of the ACT in 2018, 2019 and 2020 (before the drought ended). This increase in downstream guideline exceedances is largely driven by low flow conditions which can increase electrical conductivity levels, especially in the inflowing waters from urban waterways. Electrical conductivity is also increased downstream of the ACT from the contribution of increased salts from the discharge waters of the LMWQCC during extreme low flow periods.

It should also be noted that results for water quality monitoring undertaken for CHIP show that the ACT is increasing nitrogen levels downstream of the LMWQCC (see the Water quality — Catchment Health Indicator Program).

Data gaps

• Current monitoring programs are not sufficient to accurately assess water quality in the ACT. To understand the sources and volume of pollutants, more frequent monitoring needs to occur, as well as events-based monitoring that assesses periods of high pollutant loads.
• More information is required on the impacts of the Lower Molonglo Water Quality Control Centre on aquatic ecosystems in the Murrumbidgee River, particularly with regard to nitrogen concentrations.
• Information is poor for groundwater resources and condition, including contamination.