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
|Job No. 42906500
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.