Studies have shown that for the Lake Clifton thrombolites to thrive they require a flow of fresh, or at least no more than brackish, groundwater. This groundwater enters the lake via springs though the lake bed near the shoreline and it’s in these locations that thrombolites form. To help monitor the flow of this water PHCC has recently acquired a “spear probe” … but what is that exactly?
A “spear probe” is essentially a portable piezometer – a tube placed in the ground from which groundwater samples can be extracted. The tube in this case is a stainless steel “spear” comprising a head, which is a short section of slotted pipe screened with stainless steel mesh to exclude silt and sand, fitted with a sharp point (hence the “spear” analogy), and extension pipes. When all the extension pipes are added the hollow tube can be driven to a total depth of about 3 m below the ground surface.
To take samples the spear is driven to the required depth and then a weighted tube is inserted down the spear tube into the head. This tube is connected to a peristaltic pump which is then used to pump the water that has seeped into the spear head to the surface. The water is collected in a bottle and analysed using a handheld water quality meter to measure its salinity with some selected samples sent to a laboratory for further analysis. Typically, the water collecting in the spear head is pumped out a number of times until stable readings are achieved. The spear can then be withdrawn, or if required, driven deeper to take more readings.
The salinity of water is often expressed as the quantity of Total Dissolved Solids (TDS) found in a sample. Basically, this is a measure of the weight of “salt” in a litre of sample water (expressed as grams per litre, or g/L). During the recent trial the surface water in Lake Clifton returned a reading of 120 g/L TDS while groundwater collected from about 2m below the surface of the lake bed gave a reading of 20.5 g/L. The salinity of seawater is typically 35 g/L TDS so the contrast between the super-salty (“hypersaline”) lake water, about 3 to 4 times that of seawater, and the underlying groundwater, about half the salinity of seawater, is obvious.
The trial of the equipment was a success and so a comprehensive program of assessments is now being planned as part of PHCC’s National Landcare Program / Regional Land Partnerships-funded Wetlands and People project. In this program we will relate the groundwater salinity measurements to the surface water level of the lake to create a contour map of the groundwater salinity in the vicinity of the thrombolites. From this mapping we will be able to observe how the flow of groundwater into the lake changes with the lake level and rainfall events throughout the year to better understand and manage the threat of decreasing fresh groundwater flows to the survival of the Lake Clifton thrombolites.
This project is supported by Peel-Harvey Catchment Council, through funding from the Australian Government’s National Landcare Program.