The acid flux dynamics of two artificial drains in acid sulfate soil backswamps on the Clarence River floodplain, Australia

The export of acidity, iron, aluminium, and sulfate to an estuary from 2 drains in acid sulfate soil backswamps was monitored over 18 months. The backswamps had similar geomorphology, stratigraphy, and drainage density, and comparable soil and groundwater acidity. However, the flux rates, temporal dynamics, and export pathways of acid and other sulfide oxidation products varied greatly and were controlled to first order by (i) the saturated hydraulic conductivity (K) of sulfuric horizons and (ii) the tidally influenced groundwater gradients. The site with very high K and large tidally influenced groundwater gradients had high acid flux rates (5300 mol H+/ha.year), chronic acid discharge, high drain water acid and metal concentrations, and the primary flux pathway was direct groundwater seepage (interflow/bypass flow) to the drain. The site with lower K and smaller groundwater gradients displayed low acid flux rates (50 mol H+/ha.year), infrequent, highly episodic discharge, and the primary flux pathway was dilute surface runoff following dissolution of sulfide oxidation products accumulated on the soil surface. Importantly, the majority of acid export at both sites occurred while the backswamp groundwater level was within a very narrow elevation range.