This project had dual aims of building the community’s capacity to understand local water quality issues while at the same time investigating the elevated dissolved inorganic nitrogen (DIN) levels in Lake Cootharaba, which negatively impacted on the estuarine / marine report card grade for the Noosa system in 2006.

Concerns were raised as to possible causes as Noosa River has a reputation for having excellent water quality compared with other catchments in South East Qld. Possible reasons for DIN fluctuations were:

a) Septic contamination of groundwater around Boreen Point

b) Catchment run-off from disturbed catchments to the west of the Noosa River system

c) Resuspension of organic nitrogen deposited in sediments of Lake Cootharaba during periods of high wave energy being processed and made biologically available in the form of inorganic nitrogen.

d) Disruptions to the benthic nitrogen cycle following rainfall leading to the release of inorganic nitrogen

A multi-disciplinary team was assembled to determine the relative contributions of groundwater, surface flows and lake sediment processes to the whole system nitrogen budget. Scientists from Queensland EPA and University of Queensland were commissioned to investigate to the relative contribution of within lake sediment nutrient processes and analyse monthly monitoring data collected under the EHMP. Scientists from University of the Sunshine Coast and SEQ Catchments were commissioned to monitor surface water inflows to the Lake from Boreen Point and surrounding areas and conduct analyses of nutrients, thermotolerant coliforms and faecol sterols in groundwater samples. The project was coordinated by Noosa and District Landcare, who worked closely with Noosa Waterwatch volunteers, resident bore owners from Boreen Point, Qld National Parks and Wildlife rangers at Elanda Point, the Lake Cootharaba Community Association, the Lake Cootharaba Sailing Club and the Boreen Point Bushland Care Group to ensure that local knowledge was incorporated into the scientific investigation process.

Results revealed that benthic sediments contained a huge pool of nitrogen and that high rates of sediment resuspension caused by wind over the shallow lake increased DIN release into the water column. Groundwater analyses from Boreen Point bores indicated that these waters were generally lower in nitrogen than the lake and could not be considered significant sources. Inputs from surrounding disturbed catchments by comparison carried higher nitrogen, although mostly organic in nature and had higher sediment loads than were present in Noosa River. During high flow events, this organic nitrogen may accumulate in the Lake sediment, leading to sediment enrichment which under certain conditions may be released into the Lake.

Overall, the community was greatly relieved to find out that groundwater inputs from Lake Cootharaba surrounds including septic outflow from Boreen Point, a small township of approx 280 people, could be discounted as the source of the problem.

A series of community consultation workshops and information sessions were held to discuss the project aims, its progress and final outcomes. A concept diagram and informative newsletter were generated and disseminated together with a series of press releases to inform everyone in the greater Sunshine Coast region.

For more information, view or download

> In lake Processes (2009) by Dr Dave Rissik and Dr Alistair Grinham,

> Contributions from groundwater and catchment runoff (2009) by Dr Peter Brooks, Darryle Sullivan and Prof Neil Tindale.

> Concept Diagram_Lake Cootharaba Nitrogen.