The characterisation of metal and metalloid contaminants in re-flooded Acid Sulfate Soils of Lake Alexandrina, South Australia

[摘要] Increased flow from the River Murray since 2010caused a return to pre-drought water levels in the Lower Lakes that helped to mitigate hazards associated with acid sulfate soils along the previously exposed shores of the Lakes. The high rate of inflow helped to minimise impacts to surface water from acid and metal mobilisation.This was largely due to dilution and the establishment of high surface water head which induced a downward flux of acidity and contaminants.Soil monitoring by CSIRO to date has shown that soil recovery, in terms of acid neutralisation, is taking a considerable time to return to pre-drought conditions. However, the hazards posed by trace metal and metalloid contaminants and their fate were poorly known. This study presents a summary of the chemistry of subaqueous soil porewater, and ‘solid phase’ speciation of metals and metalloids using sequential extractions, of four selected soil profiles from Lake Alexandrina. The soil cores were sampled from Point Sturt (two profiles), Dog Lake and Boggy Lake, all sites which oxidised during the drought to produce sulfuric soil materials.The porewater chemistry data show that the soils are very variable within soil profiles as well as between different sites. In general, metal and metalloid contaminant concentrations were lowest in the upper parts of the profiles, due to infiltration from fresh surface lake water. This has resulted in a downward flux of solutes and acidity. For all profiles, the concentrations of a number of contaminants breached ANZECC Guideline values. The worst affected sites were Dog Lake and Boggy Lake, with one soil layer in Dog Lake having extremely high concentrations of some contaminants. A number of contaminants were identified as being well above ANZECC Guideline values including aluminium (up to 875 mg l-1), arsenic (up to 186 µg l-1), beryllium (up to 91 µg l-1), cadmium (up to 5 µg l-1), chromium (up to 288 µg l-1), cobalt (up to 1744 µg l-1), copper (up to 82 µg l-1), iron (3206 mg l-1), manganese (896 mg l-1), lead (up to 10 µg l-1), nickel (up to 3225 µg l-1), uranium (up to 35 µg l-1)and zinc (up to 2072 µg l-1).A comparison of different pore size filters showed that in most instances the metal and metalloids were present as dissolved species (3 kDa), although some deeper soil samples contained colloidal material (between 0.45 µm and 1kDa), and two samples contained colloidal manganese at shallow depth.The sequential extraction data showed that a large number of trace metals and metalloids were present at high concentrations in different ‘soil fractions’. Although the exchangeable fraction formed a small part of the total, for some metals, they were still high enough to be of concern in relation to water quality limits. Most metals were present in a range of fractions, but were often highest in the weak hydrochloric acid (potentially representing poorly crystalline materials) and nitric acid extractions (potentially representing pyrite fraction). The pyrophosphate fraction (potentially representing the organic fraction) was also important for a number of metals as well as the metalloid arsenic. In contrast, zinc concentrations were generally highest in the dithionite fraction (potentially representing crystalline iron and manganese oxide phases) and uranium concentrations were typically highest in the weak hydrochloric acid extraction. The data provide a good basis for making future predictions of metal mobilisation under different geochemical scenarios, and as a basis from which to ascertain in more detail the mineralogical phases present in the soils and their metal/metalloid associations.The implications for ecological recovery at the study sites are significant. The loss of water, as well as acid generation and contaminant release, had a severe impact on the lakes marginal subaqueous soil biodiversity, through loss of much of the soil food-web biodiversity. The presence of soil porewaters with high contaminant concen

[发布日期] 2012-11-01 [发布机构] CSIRO

[学科分类] 地球科学(综合)