The influence of karst on hydrogeology and water quality in the Chalk

In southern England, groundwater from the Chalk aquifer accounts for over 60% of the potable supply provided by Southern Water and Affinity Water, and up to 40% for Thames Water. Karst development in the Chalk aquifer is ubiquitous and certain areas of chalk outcrop are also impacted by prevalent karst landform development, rendering the aquifer highly vulnerable to pollution via rapid, low attenuation pathways from the surface to public water supply springs and abstraction boreholes. This thesis is a study of tracer testing and hydrogeology in chalk aquifers that are affected by karst landform development. The results of 11 tracer tests conducted in south Hertfordshire are reported. The study quantitatively characterises the impact of high-magnitude short-term natural fluorescence events after rainfall on the detectability of fluorescent dye tracers using in-situ fluorometers. Excitation-emission matrix spectroscopy (EEMS) is shown to be a vital technique for the detection of fluorescent dye tracers in the Chalk. A comparative tracer test demonstrates that MS2 bacteriophage is a suitable tracer to estimate groundwater transit times from stream sinks to abstraction boreholes, despite losses generally exceeding 99%. Fluorescent dissolved organic matter (FDOM) is also successfully applied as a ‘natural’ tracer of rapid transport from stream sinks to abstraction boreholes. Tracer testing from stream sinks to abstraction boreholes provides a novel opportunity to study the hydraulic relationship between conduit networks and the bulk groundwater flow system in the Chalk. This study demonstrated that some abstraction boreholes were extremely well-connected to conduit transport networks, whereas others were not. The large vertical component of transport required for tracer to be recovered at abstraction boreholes is also used to infer extensive vertical karst permeability development in the Chalk aquifer.