5: Past Water Quality Trends
- 5.1 Overview of Trends
- 5.2 General
- 5.3 Turbidity and Suspended Material
- 5.4 Phosphorus
- 5.5 Nitrogen
- 5.6 Algae and Chlorophyll-a
- 5.7 Conductivity and pH
- 5.8 Bacteria
- 5.9 Metals
- 5.10 Dissolved Oxygen
5.1 Overview of Trends
The results of the monitoring program from 1981 to 2004 indicate that:
- The overall environmental health of the Lake is generally good, and gradually improving (although its continued health is likely to be linked to management practices in its catchment).
- Turbidity and suspended material have remained in the same general range.
- Phosphorus concentrations have gradually decreased in the Lake, probably due to the phosphorus reduction measures implemented at the Queanbeyan Wastewater Treatment Plant during the mid 1980s, and other catchment management and Lake management practices.
- Nitrogen concentrations have also decreased gradually, but not as markedly as phosphorus. The decrease in both nutrients has probably been due to the same factors.
- Algal and chlorophyll-a values have generally remained in the same range, despite the above mentioned reductions in phosphorus and nitrogen concentrations. However, the intensity of late summer algal blooms has generally decreased.
- Conductivity values have remained in the same general range, except in the last two years, when they increased slightly, probably due to drought conditions. Even with this increase, values were generally low, and unlikely to result in public health or environmental concerns.
- pH values have remained in the same general range, and close to neutral.
- Bacterial counts remain generally within the guideline values with minor exceedences, but appear to have increased during the late summer months.
- Metal concentrations have gradually decreased, probably as a result of remedial works at abandoned mines in the Captains Flat area in the mid 1970s. Metal concentrations in biota (such as fish that may be caught in the Lake) are generally low or below detection levels, and thus appear not to be a significant environmental or public health concern.
- Dissolved oxygen concentrations were at their maximum during the late winter months, and at their minimum during the late summer months.
Generally, observed values for the above mentioned water quality characteristics were:
- Turbidity, usually below 60 NTU in East Basin and below 30 NTU in West Lake.
- Suspended solids, usually below 50 mg/L in East Basin and below 25 mg/L in West Lake.
- Total phosphorus concentrations, usually below 0.08 mg/L in East Basin and below 0.06 mg/L in West Lake (although concentrations in the past were generally 2-3 times higher).
- Total nitrogen concentrations, usually below 1.6 mg/L in East Basin and below 1.2 mg/L in West Lake (although concentrations in the past were generally 20-30% higher).
- Ammonia concentrations, usually below 0.1 mg/L.
- Algal numbers, usually below 5,000 cells/mL.
- Chlorophyll-a, usually below 30 μg/L.
- Conductivity, generally below 500 μS/cm.
- pH, usually in the range of 6.5-9.0.
- Bacterial counts, usually below 150 CFU/100mL
(except in some late
summer months). - Metal concentrations, generally low or below detection levels in fish.
- Dissolved oxygen concentrations were generally over 5mg/L.
5.2 General
Routine water quality monitoring in Lake Burley Griffin started in December 1981 and with some minor modifications has been continued to the present day by ECOWISE Environmental (ActewAGL Corporation). Public health monitoring is undertaken by ACT Health Protection Service. Intensive monitoring of specific events or at specific locations is undertaken by a wide range of organisations (including government agencies, research organisations and community groups).
The results of various monitoring programs and research projects are summarised in a number of previous reports (ACT Electricity and Water, 1988; ACT Parks and Conservation Service, 1988a and b; Burgess and Olive, 1975; Cullen, Rosich and Bek, 1978; Department of Housing and Construction, 1982 and 1984; EcoChemistry Laboratory, 2003; Ecowise Scientific and Aquatech, 1995; Gutteridge, Haskins and Davey, 1982; Hillman, 1980; Maher et al. 1992; Nagy and Butters, 1987 and 1988; Nazer, 1986; Norris, 1983; Office of ACT Administration, 1987; Scientific Services and Aquatech, 1994). Collectively, these research and monitoring programs contain a considerable amount of valuable information on Lake Burley Griffin water quality.
Most of the monitoring effort has used the following four main sites, and generally a monthly sampling frequency:
- East Basin (site 529).
- Central Basin (site 530).
- West Lake (site 504).
- Scrivener Dam (site 507).
Refer to Appendixes A and B for location of sites.
At each site, both surface and tube samples have been collected. Surface samples (taken at 0.3 m below the surface) were generally used for bacteriological analysis. Tube samples, which consisted of a composite of the top 5 m of the water column, were used for chemical and algal analyses. The only exception to this was East Basin (site 529), where only surface samples were taken, because the site is relatively shallow.
The remainder of this section provides an overall assessment of the results from the past 23 years of monitoring focusing on:
- Turbidity and suspended material.
- Phosphorus.
- Nitrogen.
- Algae and chlorophyll-a.
- Conductivity and pH.
- Bacteria.
- Metals.
- Dissolved oxygen.
5.3 Turbidity and Suspended Material
5.3.1 Background
Turbidity measures the amount of suspended material in the water, and is important for environmental, public health and aesthetic reasons.
Turbidity is important environmentally, as it reduces the clarity of the water and thus the amount of sunlight available for algae and aquatic plants. Also, nutrients such as phosphorus and nitrogen are often attached to the surface of suspended material.
Turbidity is also important in terms of public health, as bacteria and pollutants (such as heavy metals) are often attached to the surface of suspended material.
Furthermore, turbidity is an important aesthetic measure, particularly for recreational areas, as it is an indication of water clarity.
Suspended material and turbidity can come from the surrounding catchment (as part of rainfall runoff during storm events), or it can be resuspended Lake sediment (due to wind mixing in shallow areas).
Like most lakes in south-east Australia, Lake Burley Griffin has always been a relatively turbid lake. The Lake's catchment contains:
- Large areas of agricultural land (used primarily for grazing).
- Large areas of urban land (involving residential and light commercial uses).
Such relatively high levels of turbidity can introduce significant amounts of nutrients into a lake, but can also reduce the ability of algae to use such nutrients (due to reduced light penetration into the water column). The relatively high turbidity can also serve to reduce the aesthetic value of the water for recreational activities (such as swimming or boating).
The Lake also experiences a considerable amount of wind induced sediment resuspension. This is especially the case in East Basin, which is relatively shallow (less than 3 metres) over large parts of its area, and relatively exposed to strong westerly winds (Scientific Services and Aquatech, 1994).
5.3.2 General Trends
Turbidity is measured in nephelometric turbidity units (NTU), and its monitoring in Lake Burley Griffin commenced in 1981. Information is generally available from four sites in the Lake and usually at a monthly frequency.
From the collected information (EcoChemistry Laboratory, 2003; Ecowise Scientific and Aquatech, 1995; Office of ACT Administration, 1987), it is possible to indicate the following general trends:
- Turbidity is higher in the shallower eastern parts of the Lake than in the deeper western parts. This is to be expected, as suspended solids settle out on entering most lakes, and indeed one of the intended functions of the Lake was to serve as a settling basin for catchment runoff.
- Turbidity is higher just after stormwater inflow into the Lake. Once again, this is to be expected, as stormwater runoff contains higher levels of suspended material, resulting in higher turbidity values.
- Turbidity is higher just after strong windy periods, especially in the shallower eastern parts of the Lake. This is largely the result of wind resuspension of bottom sediments (Scientific Services and Aquatech, 1994).
- Turbidity values in East Basin are generally below 60 NTU, whereas in West Lake values are generally below 30 NTU.
- Suspended solids show very similar patterns to turbidity, with values in East Basin generally below 50 mg/L, whereas in West Lake values are generally below 25 mg/L.
Turbidity values for East Basin (site 529 surface) and West Lake (site 504 tube) are shown in figures 1A and 1B respectively for the period from 1981 to 2004.
The figures show that turbidity in both basins has remained in the same general range over the 23 years of available monitoring data.
The water quality standards in other parts of the ACT for water based recreational swimming areas state that turbidity should not be objectionable. The standard for water based recreational boating areas states the same condition (ACT Government, 2003).
It is possible to suggest that an appropriate water quality benchmark value for turbidity in East Basin is 60 NTU, and for West Lake 30 NTU. Corresponding values for suspended solids are 50 mg/L and 25 mg/L. These will be discussed in more detail in Chapter 6.
Figure 1: Turbidity (NTU) values in Lake Burley Griffin for 1981-2004
- A. East Basin (site 529 surface)
- B. West Lake (site 504 tube)
5.4 Phosphorus
5.4.1 Background
Phosphorus is an essential nutrient in aquatic ecosystems, particularly to photosynthetic organisms such as algae and macrophytes. However, high concentrations of phosphorus increase the amount of biological activity. Consequently, phosphorus promotes algal activity and this in turn can result in serious water quality problems. During algal blooms, the water becomes unsuitable for a number of recreational activities, whereas after a large algal bloom, the decaying material can deplete the oxygen levels in the water column (and thus result in fish deaths).
Phosphorus originates from a number of sources including:
- soils in the catchment;
- wastewater discharges;
- fertilizers applied to agricultural land or suburban gardens;
- waterbirds defecating into or near the water body; and
- releases from lake sediments if oxygen becomes depleted in the overlying waters.
In environmental waters, both total phosphorus and filterable phosphorus concentrations are measured. Filterable phosphorus is generally regarded as the fraction that is biologically available in the short term. In Australian lakes and rivers, filterable phosphorus usually represents 30-60% of the total phosphorus in the water column.
5.4.2 General Trends
Total phosphorus and filterable phosphorus concentrations are both measured in terms of mg/L. Generally, both have been monitored in Lake Burley Griffin since 1981, at four sites and usually at a monthly frequency.
From the collected information (EcoChemistry Laboratory, 2003; Ecowise Scientific and Aquatech, 1995; Office of ACT Administration, 1987), it is possible to indicate the following general trends:
- Both total phosphorus and filterable phosphorus concentrations have been gradually decreasing since the mid 1980s. The main reason for this decrease has been a reduction in the amount of phosphorus entering the Lake from the Queanbeyan Wastewater Treatment Plant. This plant was upgraded in the mid 1980s and its phosphorus discharges were reduced by over 90%. Other contributing factors have included a range of catchment management and lake management practices (such as the harvesting and removal of aquatic plants from some areas).
- Total phosphorus and filterable phosphorus concentrations are higher in the shallower eastern parts of the Lake, than in the deeper western parts. This is to be expected, as higher suspended solids concentrations (caused by stormwater runoff and wind resuspension of sediment) generally correlate with higher phosphorus and nitrogen values.
- Total phosphorus concentrations are higher just after stormwater inflows into the Lake.
- Total phosphorus concentrations are higher just after strong windy periods, especially in the shallower eastern parts of the Lake.
- Total phosphorus concentrations in East Basin are generally below 0.08 mg/L, whereas in West Basin values are generally below 0.06 mg/L.
- Filterable phosphorus concentrations in East Basin and West Basin are generally below 0.02 mg/L.
Total phosphorus concentrations for East Basin (site 529 surface) and West Lake (site 504 tube) are shown in figures 2A and 2B respectively for the period from 1981 to 2004.
The figures show that total phosphorus concentrations in both basins have gradually decreased since the mid 1980s. As indicated previously, the main reason for this decrease has been the upgrading of the Queanbeyan Wastewater Treatment Plant (during the mid 1980s). Ongoing reductions since then have probably been due to a range of other catchment management practices.
The water quality standards in other parts of the ACT for water based recreational activities and for aquatic habitat indicate a value of 0.1 mg/L total phosphorus (ACT Government, 2003). The values observed for Lake Burley Griffin are generally below this level. It is possible to suggest that an appropriate water quality benchmark for total phosphorus in Lake Burley Griffin is 0.1 mg/L. This will be discussed in more detail in Chapter 6.
Figure 2: Total phosphorus (mg/L) in Lake Burley Griffin for 1981-2004
- A. East Basin (site 529 surface)
- B. West Lake (site 504 tube)
5.5 Nitrogen
5.5.1 Background
Nitrogen is also an essential nutrient in aquatic ecosystems. High nitrogen concentrations can promote nuisance growths of algae and macrophytes, which in turn can result in eutrophication.
Nitrogen originates from the same general sources as phosphorus, including:
- soils in the catchment;
- wastewater discharges;
- fertilizers applied to agricultural land, or suburban gardens;
- waterbirds defecating into or near the water body; and
- releases from lake sediments as ammonia, if oxygen becomes depleted in the overlying waters.
In environmental waters, nitrogen is measured in a number of forms, including ammonia, nitrate, nitrite, total kjeldahl nitrogen, and total nitrogen. Of these, total nitrogen and ammonia are particularly important (especially for algal and fish activity). High concentrations of total nitrogen and ammonia can promote algal blooms, whereas high ammonia concentrations can interfere with the ability of fish gills to absorb oxygen (and can thus result in fish deaths).
5.5.2 General Trends
Total nitrogen and ammonia concentrations are both measured in terms of mg/L. Generally, both have been monitored in Lake Burley Griffin since 1981, at four sites and usually at a monthly frequency.
From the collected information (EcoChemistry Laboratory, 2003; Ecowise Scientific and Aquatech, 1995; Office of ACT Administration, 1987), it is possible to indicate the following general trends:
- Both total nitrogen and ammonia concentrations have decreased slightly over the past 23 years of monitoring (but not as dramatically as total phosphorus and filterable phosphorus). This decrease has probably been the result of a range of catchment management practices.
- Nitrogen concentrations are higher just after stormwater flows into the Lake.
- Nitrogen concentrations are higher just after strong windy periods, especially in the shallower eastern parts of the Lake.
- Total nitrogen concentrations in East Basin are generally below 1.6 mg/L, whereas in West Basin values are generally below 1.2 mg/L.
- Ammonia concentrations are generally below 0.1 mg/L.
Total nitrogen concentrations for East Basin (site 529 surface) and West Lake (site 504 tube) are shown in figures 3A and 3B respectively for the period from 1981 to 2004. The figures show that total nitrogen in both basins has decreased slightly over the two decades of available monitoring data.
There are no total nitrogen water quality standards for other parts of the ACT for recreational waters or for aquatic habitat. Nitrogen is not regarded as a limiting factor for algal growth in regional waters and is non-toxic to other organisms (ACT Government, 2003). Furthermore, the availability of nitrogen will generally favour the growth of green algae as opposed to blue-green algae (which are less desirable from the recreational, ecological or aesthetic perspective). Consequently, the water quality benchmark proposed for total nitrogen concentrations is 1.6 mg/L in East Basin, and 1.2 mg/L in West Lake. This is discussed in more detail in Chapter 6.
Ammonia concentrations for East Basin (site 529 surface) and West Lake (site 504 tube) are shown in figures 4A and 4B respectively for the period from 1981 to 2004. The figures show that ammonia concentrations in both basins have decreased slightly over the 23 years of available monitoring data.
The water quality standard for ammonia in other parts of the ACT is calculated from ANZECC/ARMCANZ (2000), and is dependent on water pH and temperature at the time of sample collection. However, a level of 0.1 mg/L generally provides a suitable benchmark value. This will be discussed in more detail in Chapter 6.
Figure 3: Total nitrogen (mg/L) in Lake Burley Griffin for 1981-2004
- A. East Basin (site 529 surface)
- B. West Lake (site 504 tube)
Figure 4: Ammonia (mg/L) in Lake Burley Griffin for 1981-2004
- A. East Basin (site 529 surface)
- B. West Lake (site 504 tube)
5.6 Algae and Chlorophyll-a
5.6.1 Background
Algae are aquatic plants that can range in size from microscopic to several metres long. Water quality problems can occur when some algae rapidly increase in numbers (during a bloom) and make the water unsuitable for a wide range of recreational uses (such as swimming, boating, or even passive recreation, such as sightseeing). Even more significant problems can occur when such blooms die and decompose. The decomposing algae use up the oxygen in the water column, which in turn often results in fish deaths.
5.6.2 General Trends
Algal numbers are measured in terms of cells per mL. In most cases the cells are also further classified into algal types (such as green, blue-green etc.). Blue-green algae tend to be more important as they clump together and make the water unattractive for a wide range of recreational uses. In some cases, blue-green algae can also produce toxins which can be harmful to animals and humans.
Often algal levels are measured not in terms of cells per mL, but rather by the amount of chlorophyll-a in the water (which is produced by algal cells to help with photosynthesis). Chlorophyll-a is measured in μg/L.
Both algal numbers (in cells per mL) and chlorophyll-a concentrations (in μg/L) have been monitored in Lake Burley Griffin since 1981, at four sites, and usually at a monthly frequency. As the monitoring program is time based, as opposed to event based, it is possible that the true peaks in algal activity have been under-reported.
From the collected information (EcoChemistry Laboratory, 2003; Ecowise Scientific and Aquatech, 1995; Office of ACT Administration, 1987), it is possible to indicate the following general trends:
- Both algal numbers and chlorophyll-a concentrations have remained in the same general range. This is despite the previously mentioned reductions in the amount of phosphorus entering the Lake, and better flow management through the Lake during the key summer months (Nagy and Butters, 1988). However, the intensity of late summer algal blooms have generally decreased.
Total algal numbers for East Basin (site 529 surface) and West Lake (site 504 tube) are shown in figures 5A and 5B respectively, for the period from 1981 to 2004.
Water quality standards for total algal numbers in other parts of the ACT provide a value of 5,000 cells per mL for water based recreation and for aquatic habitat (ACT Government, 2003). As shown in figure 5, algal numbers were usually below this level over the last 10 years. A value of 5,000 cells/mL may be an appropriate algal water quality benchmark for Lake Burley Griffin. This will be discussed in more detail in Chapter 6.
Chlorophyll-a concentrations for East Basin (site 529 surface) and West Lake (site 504 tube) are shown in figures 6A and 6B respectively for the period from 1981 to 2004.
The water quality standards in other parts of the ACT for water based recreation and for aquatic habitat indicate a chlorophyll-a concentration of 10 μg/L (ACT Government, 2003). As shown in figure 6, chlorophyll-a concentrations in West Lake were often above this level over the past 10 years. Consequently, a concentration of 30 μg/L chlorophyll-a may be an appropriate water quality benchmark for Lake Burley Griffin. This will be discussed in more detail in Chapter 6.
Figure 5: Total algae (cells/mL) in Lake Burley Griffin for 1981-2004
- A. East Basin (site 529 surface)
- B. West Lake (site 504 tube)
Figure 6: Chlorophyll-a (μg/L) in Lake Burley Griffin for 1981-2004
- A. East Basin (site 529 surface)
- B. West Lake (site 504 tube)
5.7 Conductivity and pH
5.7.1 Background
Conductivity measures the amount of inorganic ions (salts) in the water, and thus provides a measure of water salinity (in μS/cm). Freshwaters in south-east Australia generally have values under 1,000 μS/cm. Salinity in aquatic environments is highly dependent on catchment geology. It is further influenced by land use and weather fluctuations (particularly rainfall). Salinity can influence a wide range of ecological processes and high levels of salts can significantly degrade freshwater ecosystems.
Acidity and alkalinity (expressed in pH) is a measure of the amount of hydrogen ions in the water. Neutral pH is 7, with waters becoming increasingly acidic as the scale decreases from 7 to 1, and increasingly alkaline as the scale increases from 7 to 14. Like conductivity, pH influences a wide range of ecological processes.
5.7.2 General Trends
Conductivity values (in μS/cm) have been monitored in Lake Burley Griffin since 1981, at four sites, and usually at a monthly frequency.
From the collected information (EcoChemistry Laboratory, 2003; Ecowise Scientific and Aquatech, 1995; Office of ACT Administration, 1987), it is possible to indicate the following general trends:
- Conductivity values have remained relatively unchanged in the Lake over the past two decades except in the last two years, possibly as a result of drought, when values increased (probably as a result of increased base flow and evaporation).
Conductivity values for East Basin (site 529 surface) and West Lake (site 504 tube) are shown in figures 7A and 7B respectively for the period from 1981 to 2004. The figures show that conductivity in both basins has generally remained in the same range, except in the last two years, when values increased slightly (probably as a result of the drought).
Values of pH have been measured in Lake Burley Griffin since 1981, at four sites, and usually at a monthly frequency.
From the collected information (EcoChemistry Laboratory, 2003; Ecowise Scientific and Aquatech, 1995; Office of ACT Administration, 1987), it is possible to indicate the following general trends:
- pH values have remained in relatively the same range over the past 23 years of monitoring.
- pH values have generally been higher in winter and lower in summer, probably as a result of greater algal decomposition in the summer months (lowering the pH).
Values for pH for East Basin (site 529 surface) and West Lake (site 504 tube) are shown in figures 8A and 8B respectively for the period from 1981 to 2004. These were in the 6.5-9.0 range, which is the water quality standard for pH in other parts of the ACT for recreational waters (ACT Government, 2003). This same range may be appropriate as a water quality benchmark for Lake Burley Griffin. This will be discussed in more detail in Chapter 6.
Figure 7: Conductivity (μS/cm) in Lake Burley Griffin for 1981-2004
- A. East Basin (site 529 surface)
- B. West Lake (site 504 tube)
Figure 8: pH in Lake Burley Griffin for 1981-2004
- A. East Basin (site 529 surface)
- B. West Lake (site 504 tube)
5.8 Bacteria
5.8.1 Background
Bacteria are microorganisms which are present in most environments, including aquatic environments such as Lake Burley Griffin. Some bacteria are pathogenic or harmful to humans, and their presence in lake water can make it unsuitable for some recreational uses (especially swimming).
Because it is impossible to regularly monitor for the many different potential pathogens, scientists have developed an indicator system (called faecal coliforms) which can be monitored and used as a warning tool.
5.8.2 General Trends
Faecal coliforms are measured in colony forming units per 100 mL (CFU/100mL) of water. Faecal coliforms have been monitored in Lake Burley Griffin since the late 1980s, at a number of swimming locations, particularly in the summer months.
From the collected information (EcoChemistry Laboratory, 2003; Ecowise Scientific and Aquatech, 1995; Lawrence, 2001; Office of ACT Administration, 1987) it is possible to indicate the following general trends:
- Faecal coliforms have remained in the same general range except in late summer. This may be due to a number of reasons, including greater bacterial inputs from the catchment, as well as in-lake regrowth.
Faecal coliform numbers are indicated for East Basin (site 529 surface) and West Lake (site 504 surface) in figures 9A and 9B respectively.
In other parts of the ACT, water quality standards for faecal coliforms consist of:
- 150 CFU/100mL for water based recreational activity such as swimming.
- 1,000 CFU/100mL for water based recreational activity such as boating.
Values in Lake Burley Griffin are sometimes above these values (particularly during the late summer/early autumn period). However, these may be appropriate water quality benchmarks for Lake Burley Griffin. This will be discussed in more detail in Chapter 6.
Although a number of water quality issues in the Lake have improved over the past 17 years of monitoring, faecal coliform numbers may have increased. There are several possible reasons for this, including:
- Increased urbanisation of the catchments, with urban runoff resulting in greater bacterial levels in the runoff (from such things as food wastes, and animal and pet droppings).
- The possibility of sewage overflow during heavy rain periods (although this is relatively rare in Canberra, and does not explain the observed relationship between faecal coliform numbers and high water temperatures).
- In-lake regrowth of coliforms (Lawrence, 2001).
- Better and more accurate faecal coliform monitoring techniques. The accuracy and sensitivity of faecal coliform enumeration techniques has generally increased over the past 20 years, and this may give the appearance of an increase in faecal coliform levels.
- Some changes to the monitoring methodology for faecal coliforms.
- Increased abundance of waterbirds in and around the Lake. Waterbird faeces will contain large numbers of bacteria, including some faecal coliforms.
It is not possible to indicate at this stage which of the above mentioned factors may be the most important in terms of managing bacterial pollution of the Lake. Indeed, it is not even possible to state conclusively if bacterial levels have generally increased in the Lake, even though the issue has been receiving research attention since the early 1970s (Burgess and Olive, 1975).
Figure 9: Faecal coliforms (CFU/100mL) in Lake Burley Griffin for 1987-2004
- A. East Basin (site 529 surface)
- B. West Lake (site 504 surface)
5.9 Metals
5.9.1 Background
Trace metal concentrations in Lake Burley Griffin used to be relatively high, due to runoff from abandoned zinc mining areas in the catchment, primarily in the Captains Flat area (Joint Government Technical Committee, 1974 and 1978). However, as a result of major remediation works in the 1970s, the abandoned mining areas were stabilised and metal concentrations in the subsequent runoff were significantly reduced (Hillman, 1980; National Capital Development Commission, 1981). This in turn has resulted in lower concentrations of metals entering Lake Burley Griffin.
5.9.2 General Trends
Trace metal concentrations in Lake Burley Griffin waters, sediments and biota have been measured by several research projects (EcoChemistry Laboratory, 2003; Maher et al, 1992; Norris 1983), as well as by some ongoing monitoring programs (Ecowise Scientific and Aquatech, 1995). These research and monitoring programs indicate that generally:
- Trace metal concentrations in the Lake are still elevated, especially in Lake sediments, but metal concentrations are slowly decreasing in most parts of the Lake.
- Trace metal concentrations in biota (such as fish that may be caught in the Lake) are generally low or below detection levels specified by the Australian National Food Authority, and thus appear not to be a significant environmental or public health concern.
5.10 Dissolved Oxygen
5.10.1 Background
Dissolved oxygen is an important aspect of Lake biodiversity, as most fish species require concentrations above 5mg/L. Furthermore, low dissolved oxygen concentrations at the sediment surface can result in the anoxic release of nutrients such as phosphorus and nitrogen into the overlying water column. Such releases of nutrients can then encourage the growth of blue-green algae in late summer.
5.10.2 General Trends
Dissolved oxygen values (in mg/L) have been monitored in the Lake since 1981, at four sites, and usually at a monthly frequency.
From the collected information (Eco Chemistry Laboratory, 2003; Ecowise Scientific and Aquatech 1995; Office of ACT Administration, 1987) it is possible to indicate the following general trends:
- Dissolved oxygen values in East Basin are generally above 5mg/L, most likely due to wind mixing and its shallow profile.
- Dissolved oxygen values in Central Basin, West Lake, and Scrivener Dam are at their maximum during the late winter months and at their minimum during the late summer months. This is because during the winter months of the year, water temperatures are relatively homogeneous through the depth profile. However, during the warmer months of the year, water temperatures become stratified (with warmer water at the surface and colder water at the bottom). This temperature stratification together with greater biological activity in the sediments during the warmer months of the year can deplete dissolved oxygen in the deeper parts of the lake.
Dissolved oxygen concentrations for Central Basin (site 530), West Basin (site 504), and Scrivener Dam (site 507) are shown in figures 10A, 10B, and 10C respectively for the period from 1981 to 2004.
The figures show dissolved oxygen concentrations through the water column averaged for each quarter of the year (i.e. 1st quarter - Jan to March, 2nd quarter - April to June, 3rd quarter - July to Sept, and 4th quarter - Oct to Dec).
As shown in the figures, dissolved oxygen concentrations at Central Basin range from about 8-12 mg/L at the water surface, and about 4-10 mg/L at the bottom (near the sediment).
At West Lake and Scrivener Dam, dissolved oxygen concentrations range from about 8-12 mg/L at the water surface and from about 0-10 mg/L at the bottom (near the sediment). This is because West Lake and Scrivener Dam are deeper sites than Central Basin and consequently experience greater temperature and dissolved oxygen stratifications during the summer months.
Figure 10: Dissolved Oxygen in Lake Burley Griffin for 1981-2004
- A. Central Basin (site 530)
- B. West Lake (site 504)
- C. Scrivener Dam (site 507)
No particular benchmark value is proposed at this stage for dissolved oxygen concentrations through the water profile at the various Lake Burley Griffin sites. However, as with other water quality characteristics, continued monitoring will be an important component of the WQMP.