ABSTR ACTUnderstanding the biogeochemical behaviour of arsenic in the weathering and shallow subsurface environment depends critically upon determining the nature and distribution of the chemical species present in natural waters. To this end, coupled ion chromatography-inductively coupled plasma mass spectrometry (IC-ICP-MS) is widely used, though species fractionation during ultrasonic nebulization, and matrix-dependent ionization in the plasma are analytical issues that need to be addressed. Hexapole collision cell technology is shown to be effective in suppressing chloride-based polyatomic interferences. Irrespective of the analytical technique used, As(III)/As(V) ratios of natural waters may change substantially during storage due to (1) differential adsorption of arsenic species on hydrated ferric oxides (HFOs); and (2) microbial activity. A wide range of apparently contradictory speciation changes observed by various workers can be rationalized in terms of the differences of microbial consortia present in different water samples. Arsenic speciation in certain water types can be stabilized for days or even weeks by combined filtration, acidification and refrigeration whilst the addition of EDTA and the use of 0.1 mm filters is indicated for iron-rich waters and waters with high activities of redox-active bacteria, respectively. Although the use of hydrochloric acid has been reported elsewhere as resulting in the apparent oxidation of As(III), we show that for certain water types it acts as an extremely effective preservative of arsenic speciation.

The physical and chemical effects of bacterial biofilm formation upon hydraulic conductivity, mineral-solution interactions and the formation of biogenic mineral precipitates have been studied over a wide range of scales, from microscopic to macroscopic. Several novel pieces of equipment have been designed, constructed and commissioned in order to measure the physical effects of biofilms upon fluid flow through fractures and porous media, the overall effects of biofilm formation upon mineral surface reactivity, and the imaging and identification of mineral precipitates formed due to the presence of biofilm and bacterial cell surface polymers on a quartz surface. This paper presents an overview of key experimental methods and selected results; further experimental information is being published elsewhere.Biofilm formation within quartz sand in artificial groundwater resulted in a two orders of magnitude reduction in hydraulic conductivity under bench-scale constant head conditions. However, under quasi-environmental conditions within macroscopic centrifuge experiments, a reduction of 21% was measured, revealing differences in measurements and, hence, the value of the macroscopic experimental work in scaling from micro to macro.

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The dispersion and sediment-water partitioning of Pb and Zn have been studied in the Conwy River, North Wales. Analysis included major and trace element water chemistry and concentrations of sediment-hosted Pb and Zn. In situ solution pH, Eh, temperature and conductivity were also measured. Sediments were characterized via SEM, XRD, nitric acid leaching and sequential chemical extraction to quantify metal distribution and sediment phase associations. Dissolved and sediment-bound Pb and Zn within river and estuary waters and sediments have been used to calculate whole sediment-and phase-specific apparent partition coefficients.Weathering of galena and sphalerite, associated with the Llanrwst Mining Field, provide point sources of elevated dissolved and sedimentary Pb and Zn in the upper catchment. Dissolved Pb is actively adsorbed onto sedimentary surface coatings of Fe-Mn (hydr)oxides and organics whilst Fe-Mn (hydr)oxides, organics and carbonates were the main hosts for Zn. Systematic changes in metal sorption are evident between the estuary and the upper catchment, with organic matter becoming progressively more important upstream. This change is ascribed to the sorptive properties of sedimentary organic material. Solution pH, [Cl] and aqueous metal speciation are the dominant solution controls on metal partitioning. Laboratory sorption experiments parallel Pb sorption behaviour in the natural system.