Dr. Dave Bull and Waikato Regional Council team leader Michelle Begbie were awarded best podium presentation on the second day of the Australasian Land and Groundwater Association's annual Ecoforum conference recently held in Auckland.
Michelle and Dave argued that arsenic is the most common contaminant of concern in New Zealand, especially in the Waikato Region. Arsenic contamination is often a result of human activities such as sheep dipping, codling moth control in apples, or timber treatment and use. But elevated arsenic can also be geogenic – naturally occurring – and in these cases the conceptual site model and risk mitigation are always much harder to resolve. They introduced a natural source of arsenic in the Waikato Region; not a new topic in itself, but our approach to identifying the risk it poses and how it should be managed are new.
The Waikato Region is blessed with rich volcanic minerals and fluids. It has long been known that the Waikato River itself has elevated arsenic; mostly derived from naturally occurring geothermal inputs, but enhanced by human factors such as geothermal power stations and water extraction and use. Natural groundwater in the region is also elevated with arsenic thanks to our unique combination of geology and enhanced rates of weathering from geothermal heating. Multiple contaminated land investigations in the region have identified naturally occurring arsenic, which has posed significant challenges to land redevelopment. In particular, certain soils of the Hamilton Basin feature an iron pan heavy with naturally enriched arsenic, often exceeding Soil Contaminant Standards (SCS) and at times well into the hundreds of mg/kg.
Michelle selected a number of sampling sites in an area currently under heavy development pressure, where arsenic is often reported elevated, in the absence of any obvious man-made source of contamination. Samples were analysed for arsenic bioaccessibility, and for a variety of chemical and mineralogical determinands so as to provide multiple lines of evidence. Dave presented information collated to date. Elevated arsenic in the study area seems strongly associated with Bruntwood type soils, apparently formed on riverbanks deposited after the Oruanui eruption 26,000 years ago, and associated breakout floods from the Lake Taupo area. "Iron pans" that have formed in Hamilton Basin soils also seem to be generally enriched in arsenic. These iron pans can sometimes be broken up and brought to surface, increasing topsoil arsenic concentrations, by ploughing - or as an unintended consequence of soil mixing intended as a remedial action!
Initial results suggest that arsenic bioaccessibility and bioavailability in these impacted soils is considerably less than the 100 % assumed in guidance, indicating that the risk these soils pose is lower than a simple comparison with the SCS would suggest. Supporting analysis is still ongoing. Michelle hopes that the project will enable some broader discussions about sustainable land management and how policy might be able to respond in a way that enables practical and risk-based land management before development.