Characterisation of a mining-related arsenic-contaminated site, Cornwall, UK

Arsenic contamination in Cornwall is widespread due to the historic mining of polymetallic ores and the calcination of ores. In areas such as Camborne/Redruth and Hayle in West Cornwall, arsenic values typically exceed 100 mg kg −1 in sampled topsoils, with high values also being present in the subsoil. To investigate the dispersion of arsenic from a calciner stack and from fugitive dusts, the New Mill site at Roseworthy, near Camborne, Cornwall was chosen for detailed investigation. The site contains a calciner which was fed with handpicked ore and a refinery where arsenic was refined to a saleable product. In total, an area of 700×600 m on a grid with a spacing of 100 m was sampled. Seventy-eight soil samples were obtained from pits up to 30 cm deep and analysed by ICP-MS, while seven water samples were also analysed for arsenic. Two soil samples were analysed in more detail. Measured properties were pH, particle size distribution, the arsenic distribution in the different size fractions, and the mineralogy of arsenic in the soil using XRD, SEM and QemSCAN. Twenty samples crossing the area were subjected to water extraction and speciation analysis by HPLC-HG-AFS. High values of total arsenic, above 4000 mg kg −1, were recorded in the soils in the valley floor adjacent to the calciner. Lower values, close to 500 mg kg −1 arsenic, were measured near the stack, with a background value of up to 125 mg kg −1. Agricultural disturbance by ploughing and down-slope leaching has probably dispersed the arsenic contamination in the soil. The majority of the arsenic contamination was found to occur in the silt and sand fractions. Speciation of the water-soluble component revealed high values of As 5+ plus As 3+, between 0.3% and 1.7% of the total arsenic value. QemSCAN results indicate that coal/coke/fly ash particulates are a significant carrier of arsenic contamination. Furthermore, locked and liberated particles associated with silicates and oxides often contain arsenic-iron oxide as rims and infillings. Water sample concentrations are below 50 μg l −1, indicating relatively low mobility of arsenic contained in soils. This could be due to the oxidation state of arsenic and redox conditions. However, a large reservoir of arsenic could be remobilised if there is a change in ambient conditions, allowing for potential retoxification of the site.