Assessment of the effectiveness of the Lake Goollelal floating islands for nutrient control.
Mine Water and Environment Research Final Report No. 2020-01
Mine Water and Environment Research Centre
School of Science
The Friends of Yellagonga Inc. have designed and are trialling a treatment system for nutrient and metal concentrations in Lake Goollelal. The design uses previously constructed small dams situated on the edge of the lake. Approximately 5 % of the dam is covered by floating islands of rushes. The dam had a hydraulic residence time of 86 days in December 2019 which offers plenty of time for treatment to occur. A solar panel is used to pump water from a smaller dam connected to the lake into the treatment area. The floating island trial was sampled in June/July and in December in 2019. Samples were collected at the inlet (before the islands), amongst the islands (middle) and after the islands at the outlet and in the lake opposite the outlet. Additionally, three piezometers were installed in the dam walls opposite the inlet and outlet and on the land side. Samples were analysed for a range of nutrients and metals. Sediment traps were installed in the inlet, middle and outlet areas. No significant difference (P > 0.05) was found in nutrient and metal concentrations at the lake, inlet, outlet and middle sites but there was a significant difference to the groundwater. The lack of difference between sites suggests that the floating islands were not influencing water quality in a measurable way. The dam walls were effective at preventing lake water entering the dam and the high water levels in the dam were believed to be sufficient to prevent groundwater entering the dam from the landside. Despite the floating islands not proving to be effective at nutrient or metal removal, the reasons for this may be due to the relatively small area of plants, the depth of the dam and low concentrations of metals and nutrients. It is not recommended to increase the plant area/number of islands to increase efficiency at Lake Goollelal as water quality within the lake is sufficiently good that there is little to be gained from the system. Although the design proved to be ineffective, there is still potential that could be explored at other sites with substantial nutrient or metal pollution problems.