GROUNDWATER CONTRIBUTION TO AN ACID UPLAND LAKE (LOCH FLEET, SCOTLAND) AND THE POSSIBILITIES FOR AMELIORATION

The Loch Fleet catchment lies in an upland region in the centre of the outcrop of the Cairnsmore of Fleet granite. It is a recently acidified lake (pH = 4.4) which has been the subject of a liming experiment to restore fisheries. In the present study, hydrogeological and geochemical techniques were used to determine the contribution of ground water to the loch and its role in buffering the lake water chemistry. Diffuse groundwater seepage was detected by infrared linescan survey, and overflowing ground water (2 m above the level of the loch) was encountered in a shallow borehole. This ground water has an alkaline geochemistry (pH = 7.2, HCO3- = 142 mg l-1) determined by secondary vein calcite and hydrolysis of silicate minerals. The net gains or losses of various constituents in the ground water and in the loch outflow have been determined relative to rainfall inputs. Na, K, Ca, Mg, HCO3, SO4, Cl, Si, Sr, Fe, Mn, Li and F all show net gain in the ground water; NO3, Al, Zn and B show a net loss. In the acidic loch outflow, Ca, Mg, Si, Sr, Ba, Fe, Mn, Al, Zn and Li show a net gain over rainfall inputs; most of these elements derive from ground water, enhanced by evapotranspiration by a factor of 1.8. The chemical results have been used to determine that ground water contributes around 3.5 l s-1 to the loch, compared with an estimated 3-4 l s-1 derived from hydrograph analysis. This constitutes 5% of the mean loch outflow, which was sufficient to buffer the loch at around pH = 6.0 until the late 1960s. Titrations of ground water with loch water show that as little as 0.06 l s-1 (1656 m3 year-1) of additional ground water would be required to restore the loch to conditions suitable for a self-sustaining fish population. Twice this flux (3310 m3 year-1) would restore the loch to the conditions pertaining in the pre-industrial era. These targets could be achieved at an economic cost, it is suggested, by induced abstraction of ground water in the upper reaches of the catchment without any harmful ecological effect.