Understanding groundwater, surface water, and hyporheic zone biogeochemical processes in a Chalk catchment using fluorescence properties of dissolved and colloidal organic matter

Abstract

Understanding groundwater–surface water (GW–SW) interaction in Chalk catchments is complicated by the degree of geological heterogeneity. At this study site, in southern United Kingdom, alluvial deposits in the riparian zone can be considered as a patchwork of varying grades and types with an equally varied lateral connectivity. Some display good connection with the river system and others good connection with the groundwater system and, by definition, poorer connectivity with the surface water. By coupling tangential flow fractionation (TFF) with fluorescence analysis we were able to characterize the organic matter in the river and hyporheic zone. There is a significant proportion of particulate and colloidal fluorescent organic matter (FOM) within the river system and at depth within the gravels beneath the river channel. At depth in the hyporheic zone, the surface water inputs are dampened by mixing with deeper groundwater FOM. The shallow (0–0.5 m below river bed) hyporheic zone is highly dynamic as a result of changing surface water inputs from upstream processes. Labile C in the form of protein‐like FOM appears to be attenuated preferentially compared to fulvic‐like fluorescence in the hyporheic zone compared to the adjacent gravel and sand deposits. These preliminary findings have important implications for understanding nutrient and trace element mobility and attenuation within the groundwater, surface water, and hyporheic zone of permeable Chalk catchments. Fluorescence analysis of dissolved organic matter has been shown to be a useful environmental tracer that can be used in conjunction with other methods to understand GW–SW processes within a permeable Chalk catchment.