Using elemental profiles in the sediment of a lake used to supply drinking water to understand the impacts of urban stormwater recharge

摘要

The regional city of Mount Gambier, South Australia, recharges stormwater directly into the underlying unconfined, karstic Gambier Limestone aquifer. This aquifer provides the majority of recharge to Blue Lake, a groundwater-fed volcanic crater lake, used for Mount Gambier's drinking water supply. However, concern remains regarding the risk posed by contaminants within stormwater, in particular when stormwater recharge may ultimately contribute to a source of drinking-water supply. The present research examined the role of the annual calcite precipitation in the lake, in protecting the quality of its water supply, by examining the composition of particulate matter in the lake and on the lake bottom. The sediment did not reveal negative impacts of stormwater recharge, but did highlight the increase in erosion as a result of settlement and extensive land clearing for agriculture at the time of settlement. Analysis of lake-floor sediment revealed increased accumulation of the lithogenic elements within the lake-floor sediment during this interval, owing to the cleansing capacity of the calcite precipitation cycle. Extraction of water from Blue Lake for water supply has resulted in a reduced water residence time in the lake and a three-fold increase in the accumulation of calcium carbonate on the lake floor.