Section 1   Introduction

Wilson Inlet is a shallow, seasonally closed estuary located in Denmark, on the south coast of Western Australia (Figure 1). The inlet has a surface area of about 48 km2, about 14 km long from east to west and 4 km wide. The average depth of the inlet is less than 2 m. Huizinga (2003) estimated that Wilson Inlet has a volume of about 90GL at 0 m AHD and 130GL at 1 m AHD.

Department of Water
Wilson Inlet Groundwater Monitoring
Location Map
Drawn: WY Approved: WY Date: 15/12/07 Figure: 1 Rev. A.
Job No. 42906500 File No. A4

Wilson Inlet receives discharge from rivers, constructed drains, surface runoff, and groundwater from a catchment of 2280 km2 (Lukatelich, et al., 1987). The major rivers are the Denmark River and the Hay River, and the minor ones are the Sleeman River, Cuppup Creek and the Little River. Deep drainage systems have been constructed in the Sleeman and Cuppup catchment by the Water Corporation to reduce groundwater recharge and waterlogging. The distribution of surface runoff follows the rainfall pattern. The runoff coefficient (proportion of runoff to rainfall) is in the order of 0.2-0.3 near the coast and decreases markedly inland. Groundwater interacts with the rivers and the inlet.

A sand bar blocks the Inlet from late January to August. The width of the opening at the mouth is between 100 to 150 m. The Inlet is separated into two basins: western and eastern (Hodgkin and Clark 1988). During the summer months, Wilson Inlet becomes stratified and a large influx of nutrients causes eutrophication. It was estimated that the rivers bring over 200 tonnes of Nitrogen and 10 tones of phosphorus into the inlet every year. The ruppia which grows in the Inlet also has a foul smell which becomes offensive to tourists and local residents near the Inlet. In recent years, the bar was artificially opened every September to increase the circulation and outflow and therefore flush out the nutrient-rich water. The bar open can generate over 1 m drawdown in groundwater level based on an experience by the Water and Rivers Commission (1998).

Land use, especially potential urban development along the inlet foreshore causes environmental concern. Yu (1998) has identified some hot spots where high nutrient concentrations were detected in groundwater. The study also estimated that the annual nutrient load discharged from groundwater into the inlet is 498 kg of nitrogen and 35 kg of phosphorous, contributed as diffusive source from the sand dune and weathered granite areas.

The diffuse sources were related to applications of fertilisers on the agricultural land in the upper catchment, while the unsewered commercial activities along the inlet foreshore produced point sources and posed potential threats to the health of the inlet.

The project included a site inspection, drilling and installing monitoring bores, groundwater sampling, and data analysis. Before joining URS, Wen Yu was a Senior Hydrogeologist in the DoW and was involved in providing technical advice to the project. A limited fund was provided to URS for wrapping up the work.

Wilson Inlet Groundwater Nutrient Monitoring
URS Australia Pty Ltd

Draft Report

Prepared for Department of Water, 18 December 2007