The Permian aquifer of Dumfries : groundwater chemistry and age

The Dumfries aquifer has been studied for over 20 years. However, investigations have been piecemeal and little attempt has been made to bring all the different approaches together. This study, although entirely hydrochemical, aims to help understanding of the groundwater flow within the basin in order to validate (or otherwise) existing hydrogeological conceptual models. The age and hydrochemistry of groundwater in the Permian sediments of the Dumfries Basin was assessed from 21 groundwater samples and 14 age determinations using CFCs and SF6. A summary of the important features of the groundwater chemistry is given below. 1. Groundwater is of Ca-Mg-HCO3 type and is moderately mineralised; pH is near neutral. The chemistry is dominated by maritime recharge and the dissolution of carbonate minerals in the sandstone, breccia and drift. 2. Groundwater within the Dumfries basin ranges from older (perhaps greater than 100 years) to modern. Groundwater in the west of the basin within the Doweel Breccia Formation has a large component of modern (less than 10 years) water. In the east of the basin, within the Locharbriggs Formation the groundwater is older, with generally less than 10% modern water (apart from the far south-east edge of the basin where modern groundwater is present). 3. Nitrate concentrations are high (median value of 27 mg/l NO3 (6.1 mg/l NO3-N)). The concentration of nitrate is directly related to the age of the groundwater; with older groundwater having low concentrations of nitrate and younger groundwater higher concentrations. The relationship between nitrate and age suggests that recharge currently contains 40-50 mg/l NO3. Lowest (<10 mg/l NO3) nitrate concentrations are found in older water in the Locharbriggs formation to the east of the basin. 4. Limited depth sampling suggests that groundwater within the Doweel Breccia Formation in the east of the Basin is stratified. Deeper water tends to be less mineralised than shallow water, and contains lower concentrations of NO3, Ca, Mg and HCO3. Although the lower NO3 concentration can be accounted for by the increased age of the deeper groundwater, the reduced concentration of the other minerals suggests different sources for the deep and shallow groundwater. The deep groundwater may have originated from recharge in another part of the basin, where the drift and lithology produce subtly different chemistry from the shallow, more localised groundwater. 5. Many of hydrochemical variations reflect the geology of the basin. Concentrations of Ca, Mg and HCO3 are higher in the Doweel Breccia Formation, where the sediments are mainly fluviatile. Concentrations are generally lower within the aeolian sandstones of the Locharbriggs Formation apart from in the south-east where the drift or geology may be sufficiently different from the rest of the Locharbriggs formation to alter groundwater chemistry. Sulphate concentrations increase to the south in both formations, possibly reflecting occurrence of mudstone within the sediment or leakage from the overlying raised beach deposits. Groundwater flow in the basin is complex. Flow within the Dowell Breccia Formation is relatively rapid and groundwater storage is limited; groundwater is stratified and deeper water may flow considerable distances (>3 km) to abstraction boreholes. Groundwater in the Locharbrigggs Formation is older, suggesting groundwater flow is slower and storage high. A change in water chemistry in the southern outcrop of the Locharbriggs Formation, however, suggests that the flow characteristics may become more like the Doweel Breccia Formation in this area.