Influences on water quality in a groundwater dependent wetland system

Abstract:

Ewens Ponds is a unique series of connected wetlands situated within the Gambier Limestone formation in the Gambier Basin on the southeastern coast in South Australia. The system is composed of three consecutive ponds, each with a total depth of 9 to 13 m deep. Groundwater is the sole water source for the ponds, and the clear water, lush flora, and rare indigenous fish that characterize these wetlands typically lure thousands of divers to the Ponds each year. Over the past century, agricultural practices in the area have changed the hydrology of this system in many ways; first with an extensive system of drains on both sides of the Ponds to make the surrounding area viable for agricultural use, subsequently with the dredging of the outflow of the Ponds and addition of synthetic fertilizers, and most recently with heavy aquifer pumping for widespread use of center pivot irrigation systems. Beginning in the 1970s, diebacks in the Ponds’ flora were documented, concurrent with spikes in nutrient concentrations. In nearby waters, reductions in key wetland species have been observed during periods of high alkalinity (pH>10). Following these concerns, the current study aims to quantify the water budget within the ponds, identify sources of nutrients, and estimate the age of groundwaters entering the wetlands for correlation with longterm agricultural trends. Groundwater ages were sampled in May 2014 and analysed for Carbon 14 and SF6, in addition to the installation of salinity and water level sensors and flow gaging. Preliminary results show that approximately 70 percent of the water enters the system through groundwater inflow in the first pond, with the remaining water entering within the third pond. A slight increase in the electrical conductivity of the ponds (average 750 μS in the first pond, up to 800 μS in the third pond) also differentiates the water.