Factors influencing arsenic release into Canterbury groundwaters
Thesis DisciplineWater Resource Management
Degree GrantorUniversity of Canterbury
Degree NameMaster of Water Resource Management
Arsenic is a toxic metalloid element which is naturally occurring in sediments, rocks, water and air. In some areas of the world, particularly Bangladesh, India, Pakistan, and Eastern Europe, elevated arsenic in groundwater causes health issues such as skin lesions, gastrointestinal disease, and cancer. Arsenic has been previously detected at concentrations exceeding the drinking water limit, 10 μg/L, in some groundwater wells in Canterbury. An investigation into the mechanism of arsenic enrichment in groundwater was undertaken. Four possible mechanisms of arsenic release were investigated: the reductive dissolution of iron and magnesium species with adsorbed arsenic, competitive sorption with phosphate, pH-related release in arid conditions, and oxidation of arsenic-bearing sulfide minerals. The microbial transformation of arsenic to more mobile species was not investigated in detail as no microbial studies were undertaken. Initially an assessment of the existing groundwater monitoring data held by Environment Canterbury was carried out. In addition to this, nine wells with previously detected arsenic were selected and paired with nearby wells of a similar depth and with similar characteristics but lower historical arsenic. Samples were collected twice from each well in 2015; once in March or April (autumn), and once in July (winter). Each well was tested for conductivity, pH, temperature, and dissolved oxygen concentration and a full suite of chemicals including arsenic, manganese, iron, nitrate, phosphate, sulfate, cadmium, chromium, antimony and lead. Significant positive correlations were observed between arsenic and iron, manganese, and dissolved reactive phosphorus (DRP) in both the analysis of Environment Canterbury data and the analysis of sampling results. Mutually exclusive relationships were observed between arsenic, sulfate and dissolved oxygen. Elevated arsenic concentrations were found in lower-lying wells with a flood risk, less permeable sediments, and silt-sized soil particles. The Environment Canterbury data showed that wells within 4.0 km of the coast had significantly elevated arsenic concentrations relative to those further inland, but this was not confirmed by the sampling analysis. The Environment Canterbury data also showed that arsenic concentration was significantly higher in winter and autumn than summer and spring. Due to the relationship between arsenic and chemicals that exist in, or are elevated in, reducing redox waters (i.e. anoxic); and the absence of arsenic in fully oxygenated groundwaters, it is suggested that elevated arsenic occurs to a greater extent in reducing conditions. A positive relationship between arsenic and organic carbon suggests that reducing conditions are contributed to by carbon-rich horizons in alluvial rock hosting the aquifer. This mechanism has previously been proposed for other arsenic-rich groundwaters in Marlborough and in other deltaic sediment hosted groundwater systems. The findings show that elevated arsenic concentrations occur in wells with reducing conditions. Given this, and the significant relationships discussed previously, it is suggested that the reductive dissolution of iron oxide and manganese oxide minerals is the dominant mechanism of arsenic release into Canterbury groundwater.