Preliminary numerical modelling of groundwater flow in the Dumfries basin

The Dumfries Basin occupies the lower part of the River Nith catchment in south-west Scotland. The basin is a deep, partly fault bounded, outlier of Permian sandstone and breccia, with a partial superficial cover comprising a variety of lithologies which range from gravel to sandy clay. There is a complex interaction between the River Nith and the aquifer as the river and its various tributaries cross the basin. Piecemeal investigation of the aquifer had taken place since the first public supply borehole was commissioned in the late 1970s. The main driver of these studies was groundwater development and latterly also pollution protection. The purpose of the current study was to bring these findings together, to identify gaps in data and to develop and test a conceptual flow model for the basin. The key objective of the study was to determine the total renewable resource available in the Dumfries Basin Aquifer as a part of the overall Nith catchment, by: o Defining the groundwater flow system, its principal recharge and discharge zones. o Developing a catchment scale water balance. o Identifying data gaps. The conceptual flow model of the aquifer was developed and this work drew on new drilling, monitoring, and analytical activities, which together allowed the new conceptualisation to be developed. The conceptual model has now been tested with the development of a distributed recharge model for the basin, which depends partly on surface water accretion data, an overall basin-wide water balance and a steady state groundwater flow model. This report describes the modelling and water balance studies. Annual precipitation totals vary from approximately 1000 mm in coastal areas to more than 2000 mm over the high ground near the western watershed. Average potential evaporation is typically in the range 450 to 550 mm a-1. The bedrock aquifer sequence of the Dumfries Basin comprises the Doweel Breccia and Locharbriggs Sandstone formations that are Permian in age. The Doweel Breccia comprises predominantly sedimentary breccia interbedded with sandstone and underlies the western part of the basin. The formation extends eastward toward the centre of the basin where it interfingers with the Locharbriggs Sandstone that underlies the eastern and northern parts of the basin. The superficial geology of the Dumfries Basin is dominated by an extensive development of glacigenic deposits, including lodgement tills and sand and gravel deposits, with marine clays towards and at the coast. Whereas the Locharbriggs Sandstone has high storage and low permeability, fractures control the hydraulics of the Doweel Breccia which has the opposite characteristics. The effective depth of the aquifer is variable, but evidence from boreholes in the Terregles area and elsewhere indicates that active fracture flow occurs to depths in excess of 100 m. The piezometry of the aquifer, for which data are concentrated in the central and western parts of the basin, suggests that the main rivers are the principal discharge areas for groundwater in both the Permian and superficial aquifers. It was also accepted that the low permeability fluvio-marine silts and clays in the south of the basin, both onshore and offshore, allowed little groundwater flow directly to the sea, whilst also acting as a barrier to sea water intrusion.