Section 2   Hydrogeological Setting

2.1 Climate

The climate in the study area is typically Mediterranean with mild, wet winters and warm to hot dry summers. Mean annual rainfall at Denmark is around 1200 mm. Highest rainfall generally occur in June and the lowest rainfall in January. About 75 per cent of rain falls during May to October when average monthly rainfall exceeds average monthly pan evaporation. Rainfall isohyets representing average annual rainfall range from 1200 in the west to 600 mm in the east.

2.2 Major Land Use Activities

Residential and commercial activities in Denmark have been carried out mainly on the northern shore of Wilson Inlet, predominantly at the Denmark town site, and to the west of the town. At present over 80 per cent of householders and 51 per cent of commercial premises use septic systems for waste water disposal in the urban areas (Denmark Environmental Centre Inc, 1995).

Areas to the east and south east of the township are predominantly agricultural land where sheep and cattle farming are often incorporated with some oats and barley cropping. The remaining area is covered by native forest. At the south eastern part of the inlet where soils are loamy red earths or sandy peat associated with granite outcrop, potato and fruit and vegetable growing are the main land use activities.

The peninsula on the southern shore of the inlet is largely covered with native forest with limited agricultural and horticultural activities.

2.3 Geology

The study area is situated within the Bremer Basin and is underlain by sediments which are Tertiary to Quaternary in age. Sediments of Quaternary age are found at the coast and on the peninsula to the south of the inlet. These consist of calcareous sand of eolian origin up to 20 m thick, and silty and clayey estuarine sediments. In the valleys of the Hay and Sleeman Rivers, alluvial deposits occur which comprise sand, silty sand, clay and peat. The alluvial sediments overly a thick layer of weathered granite rocks (Figure 2).

Department of Water
Wilson Inlet Groundwater Monitoring
Geology Map
Drawn: DoW Approved: WY Date: 15/12/07 Figure: 2 Rev. A.
Job No. 42906500 File No. A4

The eolian sediments are underlain by sediments of the Werillup Formation of Tertiary age. The formation is comprised of limestone, siltstone, sandstone, peat and basal conglomerates. The distribution of this formation is limited in Rudgyard area. The formation is moderately weathered. It outcrops as isolated limestone caps or as laterite sediments.

The basement rock consists of granitoid rock, granitoid geniss, and / or quartzite. The top portion of the rocks is generally weathered to clayey sand, and clay.

2.4 Groundwater Occurrence

The occurrence of groundwater is determined by geological conditions. There are three main aquifer types around Wilson Inlet: a porous medium aquifer in sand dunes of Quaternary age; localised aquifers within weathered granite of basement rock; and aquifers formed by structural fractures in basement rock. Groundwater flows generally towards Wilson Inlet except in the southern part of the peninsula where a groundwater divide separates flow to the inlet and the ocean. Groundwater levels respond rapidly to the opening of the sand bar at the mouth of the inlet, and it is likely that this feature exerts a strong control on the rate of groundwater flow to Wilson Inlet. Groundwater levels vary significantly over the season, responding rapidly to rainfall.

2.5 Groundwater Chemistry

Yu (1998) has carried out a hydrogeological investigation around the inlet. In the sand dune area situated on the southern shore, groundwater is fresh to slightly brackish with TDS (Total Dissolved Salts) ranging from 500 to 1500 mg/L. Water types are either Na-Ca-Cl-HCO3 or Ca-Na-HCO3-Cl. The high calcium content is due to the dissolution of calcareous materials in the coastal sediments. Groundwater in the weathered granite area is highly saline with TDS ranging from 10,000 - 50,000 mg/L. The water type is either Na-Cl or Na-Mg-Cl-SO4. Groundwater in the fractured rock area is fresh with TDS ranging from 130 to 600 mg/L. The water type is Na-Cl or Na-Cl-HCO3, similar to the rainwater.

Wilson Inlet Groundwater Nutrient Monitoring
URS Australia Pty Ltd

Draft Report

Prepared for Department of Water, 18 December 2007