Acidification of floodplains due to river level decline during drought

• Published in: Journal of contaminant hydrology Vol. 161; pp. 10 - 23
• Main Authors: Mosley, Luke M., Palmer, David, Leyden, Emily, Cook, Freeman, Zammit, Benjamin, Shand, Paul, Baker, Andrew, W. Fitzpatrick, Rob
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier B.V 01.06.2014; Elsevier
• Subjects: Acid drainage; Acid sulfate soils; Climate change; Droughts; Earth sciences; Earth, ocean, space; Environmental Monitoring; Exact sciences and technology; Hydrogen-Ion Concentration; Hydrogeology; Hydrology. Hydrogeology; Mass Spectrometry; Pyrite oxidation; Rivers - chemistry; Seasons; Soil Pollutants - analysis; South Australia; Surface–groundwater interactions; Water Movements; Water Pollutants, Chemical - analysis; Australia; South Australia; Surface–groundwater interactions; Climate change; Acid drainage; Pyrite oxidation; Acid sulfate soils; fissures; floods; Australasia; ground water; exhibits; sulfates; acidification; pyrite; clay; irrigation; aquifers; pore water; pH; soils; clastic rocks; sulfides; rivers; cations; iron sulfides; jarosite; transport; pumping; floodplains; sedimentary rocks; acidity; depth; acid sulfate soils; channels; materials; drought; Surface-groundwater interactions; iron; drainage
• ISSN: 0169-7722; 1873-6009
• DOI: 10.1016/j.jconhyd.2014.03.003

A severe drought from 2007 to 2010 resulted in the lowest river levels (1.75m decline from average) in over 90years 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 (1–1.5m) 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 3300ha 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 (0.0026tonnesacidity/ha/day) than in the irrigation (spring–summer) season (0.0013tonnesacidity/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. [Display omitted] •River level decline during drought caused acidification on floodplains.•Rising river levels and irrigation mobilised acidity to drainage channels.•A 2D saturated–unsaturated zone model could predict the acidification zone.•Acidification has persisted for several years in the soil and groundwater.