Steve Appleyard from the Department of Environment has summarised the first year of monitoring results from the 24 shallow bores installed between Torbay Inlet and lake Powell in March 2004. The bores have been regularly sampled and analysed for nitrogen and phosphorus. This study, like the Geoscience Australia team's study on the interaction of nutrients and sediments in groundwater, is another piece in the jigsaw about sources of nutrients in the Torbay catchment.

The report indicates that control of groundwater through drainage management significantly affects the watertable behaviour in the lower drainage district. There is a large area of seasonal groundwater mounding beneath Torbay Inlet when the sandbar is closed and water sits in the waterbody. This recharge is important for maintaining the height of the watertable within this area and without it the watertable near Lake Powell would be depressed below sea-level over a much larger area and for a much longer period of the year than is currently the case.

It is unlikely that the high nitrogen concentrations measured in groundwater near Lake Powell come from fertilisers used on seed potato crops in the area as the kind of nitrogen found in bore samples has the signature of the chemical reaction between soils and oxygen. The lower part of the Torbay catchment has soil with large amounts of organic carbon. Chemical reactions between nitrates in the soil and the organic matter driven by soil bacteria causes conversion of nitrates to nitrous oxide and nitrogen gases, carbon to carbon dioxide or methane and these are lost from the soil.

The highest nitrogen concentrations in the groundwater samples generally occurred in areas where and when the watertable is at its lowest level and then rapidly drops when the watertable begins to rise. This is consistent with soils being a significant source of nitrogen in groundwater through the oxidation of organic matter in soils.

When the watertable levels drop, oxygen can enter soils causing the chemical process which leads to oxidation of soils contributing to nitrogen and phosphorus levels. The presence of acid sulphate soils across much of the lower Torbay catchment has also been highlighted in the report as a potential threat.

The report recommended that further work be undertaken to find out the distribution and severity of acid sulphate soils across the lower Torbay catchment; that any drainage management options adopted minimise mobilisation of acid and nutrients from the soil; and that training be provided for landholders on management of acid sulphate soils.

These recommendations have now been included in the Restoration Plan for implementation.

The Watershed Torbay project has funded research to work out how important different sources of nutrients are to driving algal blooms. One source are the sediments in Torbay waterbodies -Lake Powell, Lake Manarup, Torbay Inlet and Marbellup Brook. Work on sediments was done in April 2005 by the Geoscience Australia team from Canberra. They spent two weeks using benthic chambers in the Torbay waterbodies as well as Oyster Harbour and Princess Royal Harbour. The team set out to find:

• If sediments are an important source of nutrients compared with other catchment sources such as farm runoff or groundwater sources.
• The total amount and type of nutrients in sediments.
• The rate at which sediments are released into surrounding water.
• What processes control the storage and release of nutrients.

Torbay Inlet: There are very high levels of nitrogen and phosphorous release and biological activity at the research locations in the channel where there are deep deposits of black sludge. In the more shallow sandy areas of the Inlet there is a much lower level of sediment deposition, less biological activity and release of nutrients.

Lake Powell: Of the five sites where research was undertaken, the site on the south west corner of the Lake had a very significant deposition of sludge, significant levels of nutrient release and biological activity.

Torbay sediments compared nationally: The research group showed comparisons with other waterbodies across Australia, including the Swan-Canning in Perth and Wilson Inlet in Denmark. The levels of nutrients and biological activity in the Torbay waterbodies was much higher than any of the other examples provided. This is not good news! The sediments are a result of nutrients and organic matter coming from the catchment or via groundwater. These nutrients feed algal growth, and when the algae die, they settle to the bottom to form the black sludge with other incoming sediments and organic matter. This then releases nutrients back to the water column, setting up a cyclic pattern of algal growth, collapse, and nutrient release.

It was suggested that there is no gain in trying to remove the sediments if sources of nutrients from the catchment are not turned off. If the level of input from catchment nutrients is significantly reduced, then there would be a case for dealing with the sediments in the waterbodies. The increasing use of urea in agriculture was also raised as an issue.

Further results will be provided in the Geoscience Australia's report due at the end of June 2005.