Metal speciation and potential bioavailability changes during discharge and neutralisation of acidic drainage water Stuart L. Simpson a,⇑ , Christopher R. Vardanega a,b , Chad Jarolimek a , Dianne F. Jolley b , Brad M. Angel a , Luke M. Mosley c a Centre for Environmental Contaminants Research, CSIRO Land and Water, Locked Bag 2007, Kirrawee, NSW 2232, Australia b School of Chemistry, University of Wollongong, New South Wales, Australia c Environment Protection Authority South Australia, GPO Box 2607, Adelaide, SA 5001, Australia highlights  Discharge of acid drainage from farm irrigation areas represents a risk to ecosystem health.  Rapid precipitation of Al and Fe increases removal of other metals from dissolved phase.  Many dissolved metals in labile and potential bioavailability forms.  Similar factors controlling the dissolved concentrations (pH, dilution and mixing time).  Water quality guideline exceedance is unlikely for drainage waters dilution to 1%. article info Article history: Received 12 August 2013 Received in revised form 21 November 2013 Accepted 22 November 2013 Available online 18 December 2013 Keywords: Acid sulfate soils Water quality guidelines Ecotoxicology Risk assessment Dairy farm Murray River abstract The discharge of acid drainage from the farm irrigation areas to the Murray River in South Australia represents a potential risk to water quality. The drainage waters have low pH (2.9–5.7), high acidity (up to 1190 mg L 1 CaCO 3 ), high dissolved organic carbon (10–40 mg L 1 ), and high dissolved Al, Co, Ni and Zn (up to 55, 1.25, 1.30 and 1.10 mg L 1 , respectively) that represent the greatest concern relative to water quality guidelines (WQGs). To provide information on bioavailability, changes in metal specia- tion were assessed during mixing experiments using filtration (colloidal metals) and Chelex-lability (free metal ions and weak inorganic metal complexes) methods. Following mixing of drainage and river water, much of the dissolved aluminium and iron precipitated. The concentrations of other metals generally decreased conservatively in proportion to the dilution initially, but longer mixing periods caused increased precipitation or adsorption to particulate phases. Dissolved Co, Mn and Zn were typically 95–100% present in Chelex-labile forms, whereas 40–70% of the dissolved nickel was Chelex-labile and the remaining non-labile fraction of dissolved nickel was associated with fine colloids or complexed by organic ligands that increased with time. Despite the different kinetics of precipitation, adsorption and complexation reactions, the dissolved metal concentrations were generally highly correlated for the pooled data sets, indicating that the major factors controlling the concentrations were similar for each metal (pH, dilution, and time following mixing). For dilutions of the drainage waters of less than 1% with Murray River water, none of the metals should exceed the WQGs. However, the high concentrations of metals associated with fine precipitates within the receiving waters may represent a risk to some aquatic organisms. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. 1. Introduction Submerged soils and sediments are frequently observed to accumulate pyritic (FeS 2 ) phases as a result of the sulfate reduction that occurs naturally through microbial respiration of organic car- bon (Dent and Pons, 1995). When undisturbed and covered with water, the pyrite poses little or no threat of acidification, however considerable oxidation and generation of acidity can occur when pyrite is exposed to the air (Bronswijk et al., 1993; Dent and Pons, 1995). The rewetting of oxidised acid sulfate soils may release sig- nificant quantities of metals to associated water (Cook et al., 2000; Simpson et al., 2010; Nystrand et al., 2012). Drought conditions and long-term low inflows in the Murray- Darling Basin system from 2006 to 2010 led to unprecedented low water levels in the lower river reaches in South Australia 0045-6535/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.11.059 ⇑ Corresponding author. Tel.: +61 2 9710 6807. E-mail address: stuart.simpson@csiro.au (S.L. Simpson). Chemosphere 103 (2014) 172–180 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere