Hyporheic zone controls on arsenic fluxes from shallow groundwater to streams in agricultural catchments

Potentially toxic elements (PTEs), including arsenic (As), pose a risk to groundwater and surface water quality, particularly in agricultural catchments where organic matter loading can enhance PTE mobility from bedrock sources. The hyporheic zone (HZ) comprising the saturated interstices of streambed sediments forms a dynamic groundwater-surface water interface characterised by variable hydraulic gradients, geological heterogeneity and intense biogeochemical activity. Steep redox gradients, driven by hydrological exchange and organic matter inputs may exert strong control on the fate of redox-sensitive contaminants such as arsenic (As). This study is undertaken as part of the HYPOFLUX project which investigates the fate of geogenic PTEs in headwater agricultural catchments in extreme vulnerability fractured bedrock environments in Ireland. A multi-scale monitoring network comprising shallow riparian groundwater monitoring wells, clustered piezometers, field drains and nested stream sites were used to characterise spatial and temporal variability in pH, DO, ORP, DOC, DIC, alkalinity, PTEs, Fe, Mn, Si and specific ultraviolet absorbance at 254 nm (SUVA). Riparian and streambed core were collected for extraction and analysis of PTEs using pXRF. Results demonstrate that deep field drainage and permeable bedrock pathways control the interaction between groundwater and surface water at stream reach scale. Solid phase As was found to be generally very low in heavily weathered mudstone bedrock and alluvial sediments and not strongly related to Fe or organic matter content. Dissolved As was spatially variable but elevated with an average of 43.9 ± 31.3 µg L^–1 in DOC-rich shallow riparian groundwater (15.1 ± 5.2 mg L^–1). Streambed piezometer profiles showed pronounced declines in dissolved As from 150 cm to 20 cm depth below bed level with concomitant increase in DO and solid phase As (measured from streambed cores). These findings provide field evidence of in-situ attenuation of groundwater As in the HZ and underscore the need to explicitly incorporate HZ processes into groundwater-surface water frameworks and models in agricultural catchments.