Tracing the outputs from drained acid sulphate flood plains to minimize threats to coastal lakes.

Abstract

AbstractDrainage of acid sulfate floodplains for agriculture and urban development has led to the acidification of coastal waterbodies, major fish kills and other environmental effects in the subtropical areas of eastern Australia. These have produced problems for local governments and conflicts in communities. Here, we trace the effects of drainage from an acidified, subtropical floodplain on water quality and sediments in an estuarine lake, Cudgen Lake, in northern New South Wales. This shallow, brackish lake was once a renowned fish and prawn nursery. The local government has monitored water quality in drains and the lake since 1990. This has revealed episodic discharge events with pH as low as 2 and concentrations of dissolved iron and aluminium that are toxic to gilled organisms. Highly acidic waters were found to accumulate in drainage channels after rains following long dry periods. These were then discharged into the lake, causing major fish kills, low benthic organism biodiversity and infestations of acid-tolerant reeds and disease-bearing mosquitoes. High concentrations of arsenic, mercury and lead were found to have accumulated in iron monosulfides in the lake sediments, together with massive amounts of aluminium and iron. Ion ratios show that these metals were mobilized by drainage from the acidified soils within the floodplain. Although the lake sediments at present represent a sink for these metals, disturbance and oxidation of the monosulfides in the sediments could release major contaminants into the lake. The heavy metals sequestered in the lake sediments could partly explain the low benthic biodiversity. These results provide pointers for enhancing the conservation and fishery values of the lake.