Hydrochemistry of episodic drainage waters discharged from an acid sulfate soil affected catchment R. Green a,b , B.C.T. Macdonald c , M.D. Melville b , T.D. Waite a, * a School of Civil and Environmental Engineering, University of New South Wales, Sydney, NSW 2052, Australia b School of Biological, Earth and Environmental Sciences, University of New South Wales, NSW 2052, Australia c Centre for Resource and Environmental Studies, Australian National University, Canberra 0200, Australia Received 18 September 2003; revised 24 October 2005; accepted 28 October 2005 Abstract The water quality of drainage discharged via pumping from an acid sulfate soil (ASS) affected catchment used for sugar cane farming is temporally very variable and is influenced by the various rain event magnitudes, their antecedents, and the particular phase of the discharge in any rain event. Rainfall episodes can cause substantial changes in acidity and dissolved metal concentrations in ASS drainage waters over very short time scales with minimum pH often reached within a few hours of initiation of the rainfall event. The initial increase in acidity and dissolved metals concentrations often observed can be attributed mainly to ‘first flush’ effects resulting from mobilization of salts present in the upper soil profile. During the middle of a large rainfall event dilution effects may result in a decrease in concentrations of dissolved species, but increases in acidity and dissolved metals (particularly aluminium) concentrations in the recession portion of the hydrograph often occur as small field drains discharge into main channels. These observations assist both in understanding of the hydrogeochemical processes leading to acid and metals release from acid sulfate soils affected catchments, and in developing appropriate strategies to treat contaminated discharge waters from such catchments. q 2005 Published by Elsevier B.V. Keywords: Acidification; Acid sulfate soils; Episodic events; Water quality; Contaminant transport 1. Introduction Acidity and dissolved metals such as iron and aluminium (products of pyrite oxidation and resultant clay breakdown) are discharged from acid sulfate soil (ASS) floodplains during and after rainfall events. Pyrite oxidation is a complicated process that involves a number of sequential chemical reactions, some of which are bacterially-mediated (Stumm and Morgan, 1996). Acidity is produced both in the initial pyrite dissolution steps and on hydrolysis of Fe(III) produced from oxidation of ferrous iron. The overall reaction describing these sequential processes is shown in Eq. (1) (Nordstrom, 1982a). A variety of secondary minerals, such as jarosite [KFe 3 (SO4) 2 (OH) 6 ], may be formed as a result of the interaction of different species. The acidic conditions that often result can also Journal of Hydrology 325 (2006) 356–375 www.elsevier.com/locate/jhydrol 0022-1694/$ - see front matter q 2005 Published by Elsevier B.V. doi:10.1016/j.jhydrol.2005.10.022 * Corresponding author. Tel.: C61 2 9385 5060; fax: C61 2 9385 6139. E-mail address: d.waite@unsw.edu.au (T.D. Waite).