This paper describes the Late Pleistocene to Holocene stratigraphy of the River Lambourn; a minor headwater of the River Thames in the Berkshire Downs. The Quaternary valley-fill comprises around 5-8. m of Late Pleistocene gravels overlain by Holocene peats and chalky clays. Quaternary deposits overlie an irregular rockhead erosion surface with deep scouring particularly evident on prominent bends in the valley. The gravels subdivide into a lower unit of chalky gravels overlain by coarse flint gravels. Ground penetrating radar suggests that gravels at depth are relatively structureless, but at the top show well-developed point-bar accretion surfaces which occur in association with peat-filled sinuous channels. These probably date from around the Pleistocene-Holocene boundary and may have formed in response to climate change and increased groundwater outflow as stream hydrology changed from the short-duration, high-magnitude flows of the Lower Dryas to the uniform, low-magnitude flows of the Holocene. Holocene peats initially infilled abandoned floodplain channels at around 10. kyr BP but later encroached over much of the Lambourn floodplain. A progressive upward decrease in organic material and an increase in the proportion of chalky clays from around 4. kyr BP probably occurred in response to floodplain accretion coupled with increased erosion of the chalk catchment related to agricultural clearance and a wetter climate.

Millions of people globally, and particularly in South and Southeast Asia, face chronic exposure to arsenic from reducing groundwaters in which. Arsenic release to is widely attributed largely to reductive dissolution of arsenic-bearing iron minerals, driven by metal reducing bacteria using bioavailable organic matter as an electron donor. However, the nature of the organic matter implicated in arsenic mobilization, and the location within the subsurface where these processes occur, remains debated. In a high resolution study of a largely pristine, shallow aquifer in Kandal Province, Cambodia, we have used a complementary suite of geochemical tracers (including ¹⁴C, ³H, ³He, ⁴He, Ne, δ¹⁸O, δD, CFCs and SF₆) to study the evolution in arsenic-prone shallow reducing groundwaters along dominant flow paths. The observation of widespread apparent ³H-³He ages of <55years fundamentally challenges some previous models which concluded that groundwater residence times were on the order of hundreds of years. Surface-derived organic matter is transported to depths of >30m, and the relationships between age-related tracers and arsenic suggest that this surface-derived organic matter is likely to contribute to in-aquifer arsenic mobilization. A strong relationship between ³H-³He age and depth suggests the dominance of a vertical hydrological control with an overall vertical flow velocity of ~0.4±0.1m·yr^-1 across the field area. A calculated overall groundwater arsenic accumulation rate of ~0.08±0.03μM·yr^-1 is broadly comparable to previous estimates from other researchers for similar reducing aquifers in Bangladesh. Although apparent arsenic groundwater accumulation rates varied significantly with site (e.g. between sand versus clay dominated sequences), rates are generally highest near the surface, perhaps reflecting the proximity to the redox cline and/or depth-dependent characteristics of the OM pool, and confounded by localized processes such as continued in-aquifer mobilization, sorption/desorption, and methanogenesis.