Sustainable groundwater resources in a hard-rock island aquifer — the Channel Island of Guernsey

Abstract

AbstractThe annual volume of water in the Guernsey public supply, which derives largely from surface storage, is approximately 5 Mm3. Additional abstraction from private surface and groundwater sources amounts to a further 1.5 Mm3. A shallow weathered zone in ancient crystalline metamorphic rocks forms the main aquifer, and this has a significant resource potential in maintaining baseflow to streams. The average annual water budget for the island is 831 mm rainfall, which supports 613 mm potential evapotranspiration, 90 mm surface runoff and 128 mm groundwater recharge. These figures contrast with a poor rainfall year in which infiltration may be zero; the annual variation in rainfall from the long-term mean is often considerable. Annual rainfall has also been declining on the island since the 1940s, and although Guernsey has survived droughts in the past it may be less able to do so in the future. Groundwater on the island is moderately mineralized, but over half of the 21 samples collected recently contained nitrate at concentrations greater than the EU maximum admissible concentration 11.3 mg-N l−1. Some of the nitrate may derive from leaking cesspits, but past application of nitrogenous fertilizer to cultivated land accounts for the major component. Attempts at groundwater dating by analysis of chlorofluorocarbon species at a small number of sites was hindered by contamination, although much of the water sampled is apparently young, and recently recharged. The long-term sustainability of the shallow island aquifer and its associated surface waters requires careful husbanding to protect it from conflicting land use interests and water demands.