title: Acidification of floodplains due to river level decline during drought.
abstract: A severe drought from 2007 to 2010 resulted in the lowest river levels (1.75 m decline from average) in over 90 years of records at the end of the Murray-Darling Basin in South Australia. Due to the low river level and inability to apply irrigation, the groundwater depth on the adjacent agricultural flood plain also declined substantially (1-1.5 m) and the alluvial clay subsoils dried and cracked. Sulfidic material (pH>4, predominantly in the form of pyrite, FeS2) in these subsoils oxidised to form sulfuric material (pH<4) over an estimated 3300 ha on 13 floodplains. Much of the acidity in the deeply cracked contaminated soil layers was in available form (in pore water and on cation exchange sites), with some layers having retained acidity (iron oxyhydroxysulfate mineral jarosite). Post drought, the rapid raising of surface and ground water levels mobilised acidity in acid sulfate soil profiles to the floodplain drainage channels and this was transported back to the river via pumping. The drainage water exhibited low pH (2-5) with high soluble metal (Al, Co, Mn, Fe, Mn, Ni, and Zn) concentrations, in exceedance of guidelines for ecosystem protection. Irrigation increased the short-term transport of acidity, however loads were generally greater in the non-irrigation (winter) season when rainfall is highest (0.0026 tonnes acidity/ha/day) than in the irrigation (spring-summer) season (0.0013 tonnes acidity/ha/day). Measured reductions in groundwater acidity and increases in pH have been observed over time but severe acidification persisted in floodplain sediments and waters for over two years post-drought. Results from 2-dimensional modelling of the river-floodplain hydrological processes were consistent with field measurements during the drying phase and illustrated how the declining river levels led to floodplain acidification. A modelled management scenario demonstrated how river level stabilisation and limited irrigation could have prevented, or greatly lessened the severity of the acidification.
authors.affiliation: Environment Protection Authority (South Australia), GPO Box 2607, Adelaide, SA 5001, Australia; CSIRO Land and Water, Private Bag No. 2, Glen Osmond, SA 5064, Australia; Acid Sulfate Soils Centre, The University of Adelaide, SA 5005, Australia. Electronic address: luke.mosley@epa.sa.gov.au.
authors.forename: Luke M
authors.initials: LM
authors.lastname: Mosley
authors.affiliation: Environment Protection Authority (South Australia), GPO Box 2607, Adelaide, SA 5001, Australia.
authors.forename: David
authors.initials: D
authors.lastname: Palmer
authors.affiliation: Environment Protection Authority (South Australia), GPO Box 2607, Adelaide, SA 5001, Australia.
authors.forename: Emily
authors.initials: E
authors.lastname: Leyden
authors.affiliation: Freeman Cook and Associates Ltd, PO Box 97, Glasshouse Mountains Q4518, Queensland, Australia.
authors.forename: Freeman
authors.initials: F
authors.lastname: Cook
authors.affiliation: Environment Protection Authority (South Australia), GPO Box 2607, Adelaide, SA 5001, Australia.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Zammit
authors.affiliation: CSIRO Land and Water, Private Bag No. 2, Glen Osmond, SA 5064, Australia; Acid Sulfate Soils Centre, The University of Adelaide, SA 5005, Australia; School of the Environment, Flinders University, PO Box 2100, Adelaide, SA 5001, Australia.
authors.forename: Paul
authors.initials: P
authors.lastname: Shand
authors.affiliation: CSIRO Land and Water, Private Bag No. 2, Glen Osmond, SA 5064, Australia.
authors.forename: Andrew
authors.initials: A
authors.lastname: Baker
authors.affiliation: CSIRO Land and Water, Private Bag No. 2, Glen Osmond, SA 5064, Australia; Acid Sulfate Soils Centre, The University of Adelaide, SA 5005, Australia.
authors.forename: Rob
authors.initials: R
authors.lastname: W Fitzpatrick
mesh_terms: D055864:Droughts; D004784:Environmental Monitoring; D006863:Hydrogen-Ion Concentration; D013058:Mass Spectrometry; D045483:Rivers / Q000737:chemistry; D012621:Seasons; D012989:Soil Pollutants / Q000032:analysis; D013021:South Australia; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D012989:Soil Pollutants; D014874:Water Pollutants, Chemical
keywords: Acid drainage; Acid sulfate soils; Climate change; Pyrite oxidation; Surface–groundwater interactions