The River Derwent in North Yorkshire has been identified by the Environment Agency as a river with a high priority for flow alleviation due to low flows during summer months. The river flows across the Corallian Limestone aquifer and loses flow to this aquifer, via a set of solution hollows in the stream bed. This loss in flow has been gauged, as of the order of 15 to 35 Ml/day, and in dry years the entire river flow can be lost to the aquifer. Groundwater abstraction from the Corallian Limestone aquifer varies seasonally, and is currently between 25 and 35 Ml/day, with the higher rates of pumping occurring during the summer months. One of the main groundwater abstractions is for public water supply, and this source is located about 1.5 km from the river. Abstraction from this source is largely sustained by the leakage from the river.

The Corallian Limestone aquifer and its hydraulic relationship with the River Derwent has been represented using the USGS modelling package MODFLOW, to improve the understanding of the flow regime and to provide a tool to assist in the evaluation of different water resource management options. This model has provided a good representation of the aquifer system, and in particular has shown that leakage from the River Derwent is a key factor in controlling the groundwater regime and in maintaining a water balance for the aquifer. The model has been used to examine different management options for the aquifer system, including the influence of reducing the volume of water available to leak through the river bed on the groundwater abstraction sources.

This paper will describe how the model has been applied to this aquifer system, the benefits obtained and the problems encountered.

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