Recharge to Blue Lake and Strategies for Water Security Planning, Mount Gambier, South Australia

Abstract

Blue Lake, a volcanic crater provides municipal water supply to the city of Mount Gambier, population of 26,000. Current average annual pumping from the lake is 3.6 × 10 6 m 3. The lake is fed by karstic unconfined Gambier Limestone aquifer. Storm water of the city discharges to the aquifer via about 400 drainage wells and three large sinkholes. Average annual storm water discharge is estimated at approximately 6.6 × 10 6 m 3 through drainage wells and sinkholes within 16.8 km 2 of the central part of the city. Chemical mass balance for calcium was used to estimate groundwater inflow to the lake at 6.3 × 10 6 m 3 , almost equal to the volume of storm water discharge and slightly higher than the previous estimates using environmental isotopes (4.8-6.0 × 10 6 m 3). Considering the lake outflow volume of 2.7 × 10 6 m 3 , the net inflow to the lake equates to the current annual pumping and therefore it is considered that the current pumping rate is at the upper limit. For meeting the short-term future demand, confined aquifer water may be used and in the longerterm, an additional well field is required outside the Blue Lake capture zone, preferably to the northeast of the city. For water supply security, inflow to the lake along with water quality has to be maintained within the city. Current annual private abstraction within the capture zone is about 4.4 × 10 6 m 3 and in order to maintain aquifer water levels, no additional allocation should be allowed.

Characteristics of Point Recharge in Karst Aquifers

“…In Poocher Swamp, the majority of recharge is from a large volume of creek water discharge into two sinkholes, resulting in formation of a fresh water lens [29]. Groundwater basins Uley South [30,31], Robinson lens [31], and Blue Lake capture zone in Mount Gambier [28] are previously described and therefore essential hydrogeogical features of the basins are provided below.…”

“…The non-fractured sections of the subunit are composed of fine dolomitic rhombs and have transmissivities probably similar to or lower than bryozoal sections of limestone. Greenways Member is typically described as grey basal marl with resultant low transmissivities and hydraulic conductivity [28].…”

“…In the Robinson lens and Uley South basin, point recharge is through solution features and diffuses recharge through granular porosity of pockets of soil and minor fissures and cracks. In the city of Mount Gambier within the Blue Lake capture zone, point recharge is mainly through about 400 storm water drainage wells and three sinkholes [28]. In Poocher Swamp, the majority of recharge is from a large volume of creek water discharge into two sinkholes, resulting in formation of a fresh water lens [29].…”

“…Low-flow purging is considered [46] superior to bailing and high-rate pumping and results in a more representative sample than the typical well purge methodology. The assumption in the grab sampling is that the hydrostratigraphy in the well is in hydraulic equilibrium prior to sampling [28]. To collect the sample by this method, an electronic depth sampler connected to a geophysical logging line is advanced to the target sampling depth and the unit is electronically opened, allowing groundwater to enter the sampler.…”

“…Selected monitoring wells are away from brackish water upward leakage areas and salinity stratified wells [30], the swamp and coastal monitoring wells to avoid chloride contamination from other sources. For the Blue Lake capture zone, existing groundwater chloride data were supplemented with samples taken from unconfined aquifer monitoring wells within and outside the city [28]. Selected sampling wells are away from historically known contaminated sites.…”