Jonathan Paul, Doris Bolaji-Dada, Aneena Ajmi, Samuel Bordessoule

Groundwater behavior in superficial gravel aquifers is globallypoorly understood, especially across urban regions where drinking water is sourced from elsewhere. We focus on one such region around Staines, SE UK, where local River Terrace Gravels form a thin (<10 m) superficial aquifer. Our objective was to explain the unusually broad and long-lived distribution of flooding by investigating local groundwater level fluctuations and flow. Over a period in January 2024, we instigated a targeted citizen science program to leverage local knowledge of floodwater, which was determined to match groundwater chemistry. We designed geophysical surveys (ground-penetrating radar and seismic refraction) to produce high-resolution water table maps, validated against well measurements. Flow rates and hydraulic conductivity, K, of the gravels were determined both in the field (via pumping and tracer tests) and laboratory, to obviate any scale effects. K depended non-linearly on hydraulic gradient, with Darcyan behaviour breaking down at low (<0.03) gradients, in conditions approaching turbulent flow. Dramatic, localized fluctuations in groundwater level, combined with the existence of several fast-flow pathways, are explained by the strong heterogeneity of the gravels, as well as their sensitivity to the imposition of sub-surface obstacles such as clay-lined backfilled gravel pits, or deep basements. These manifestations of urbanization drive observed patterns of groundwater emergence, together with aquifer thickness, rather than changes in river stage or surface elevation alone. Our experience motivates us to suggest that groundwater flooding be considered as significant as fluvial flooding in the production of risk maps by environmental regulatory bodies.