An assessment of methods for catchment-scale identification of goundwater-suface water interractions in lowland permeable catchments
PublisherUniversity of Exeter
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A better understanding f groundwater-surface water interactions is urgently required o underpin the implementation of the Water Framework Directive and to increase our knowledge of environmental processes in relation to base flow, chemical and nutrient fluxes, contaminated groundwater-surface water transfers, the selection of spawning sites by Atlantic salmon( Salmos alar) and the abstraction of groundwater. This project assessed a range of methods, other than the time-consuming technique of flow gauging, as fast and effective indicators of groundwater-surface water interactions over a catchment-wide scale.The study was aided by the results of two high resolution catchment-wide flow-accretion surveys. Physical and chemical tracers were shown to be ineffective when compared against the results of the flow-accretion surveys. A number of factors, such as long reach lengths, anthropogenic discharges, other in-stream processes and similar groundwater and surface water composition, appeared to diminish the signals produced by groundwater inputs to below detectable levels. A detailed reach-scale investigation assessed a range of methods over different temporal and spatial scales. The direct measurement of groundwater and surface water levels were 100% accurate in the identification of groundwater-surface water interactions, but this method would be impractical for catchment-scale assessment. Three methods for predicting groundwater-surface water interaction sites were assessed based on hydrogeological theory, such as groundwater flow systems, dry valley inputs and localised aspects of the solid geology. The effectiveness of the predictions were compared against the results of the Frome and Piddle flow-accretion surveys. The method using the localised aspects of the solid geology showed relatively high accuracy (60-70%)and was quite fast for catchment-scale assessment. The final investigation showed a statistically insignificant correlation between groundwater inputs and the location of Atlantic salmon( Salmos alar) redds. However, lower densities of Atlantic salmon redds were observed a long certain survey reaches that were subjected to surface water losses.