Reconnaissance hydrogeological survey of Guernsey

Abstract

The key findings from this study are: • There is no evidence of any old waters or of any mineralisation indicative of long or deep flow paths from an off-island source; • The unpredictable nature of the so called “deep aquifer” (25 m plus beneath the water table) does not warrant its further exploitation; • The uppermost “shallow aquifer” can best be exploited by dispersed small volume sources • The groundwater body and the surface water body are part of a single water system; • Reduction in aquifer yield with depth provides a self protection mechanism to the aquifer; • Based on the survey conducted there is a consensus on Guernsey that the renewable water resource is finite and that it needs safeguarding. The annual volume of water supplied by the States of Guernsey Water Board is approximately 5 Mm3. Additional abstraction from private sources may amount to a further 1.5 Mm3, particularly in a relatively dry year. The public water supply derives from surface water courses and is stored in surface water reservoirs. The private abstractions draw on groundwater (wells, boreholes and springs), roof top collection and surface water abstraction. The groundwater body and the surface water body are part of a single water system. Groundwater discharge (or baseflow) from springs and seepages into streams maintains the surface water low-flows during prolonged dry weather. The groundwater body is itself divisible into three contiguous levels. Where present, there is an upper granular aquifer within superficial deposits of alluvium and raised beach material. Beneath this is the main aquifer which is contained within the shallow weathered zone of the bedrock, which is underlain by a deeper aquifer with groundwater flow restricted to occasional dilated fractures. Bedrock mainly consists of ancient crystalline metamorphic rocks. Generally, the water table lies within 3 to 8 metres of the ground surface, and the main aquifer, in which the majority of groundwater flow takes place, is situated in a 25 m zone immediately below the water table. Beneath this depth there is some groundwater flow in deeper fractures, but borehole yields from the greater depths are commonly less than those from the shallow weathered zone. This reduction in aquifer yield with depth provides an element of self protection, whereby baseflow discharge from the aquifer and abstraction from boreholes is automatically reduced as the water table falls. Groundwater recharge occurs in the High Parishes and flows south to the coast and north-west to the coast. There is little potential for groundwater flow beneath the low-lying land towards the northern part of the island. Using meteorological and catchment recharge calculations from Jersey as an analogue, the normal average annual water budget for the island can be assigned: Rainfall = 831 mm = Potential evapotranspiration = 613 mm + Streamflow = 226 mm (of which nearly 60% derive from groundwater recharge as baseflow) + Groundwater recharge = 128 mm iv These figures do not reflect a poor rainfall year in which resource renewal may be small and infiltration may be zero. Annual variation in rainfall from the long-term mean is significant, and annual rainfall has been declining since the 1940s over Guernsey. The groundwater contains an element of salinity derived from rainfall and sea spray, although there is little evidence of actual physical marine invasion of the aquifer. The groundwater samples were otherwise moderately mineralised with specific electrical conductance in the range 427 to 1578 μS cm-1. Over half the 21 samples collected contained NO3-N (nitrate) at concentrations greater than the EC maximum admissible concentration. Some of the NO3-N may be derived from leaking cess pits, but past application of nitrogen fertilizer to cultivated land probably accounts for the majority. The average pH of the samples was 6.4. This acidity reflects the lack of carbonate material in the aquifer, other than shelly debris present in the superficial deposits. Attempts at groundwater dating by analysis of CFC species at a small number of sites was hindered by local contamination, particularly in the vicinity of the airport. It is likely that the pumped groundwaters are mixtures of young and older water. If this is the case, then at three of the four sites sampled, between 75 and 100% of the water can be identified as young, recently recharged water, in keeping with shallow, short flow path groundwater circulation in the main weathered aquifer. At the fourth, only between 40 and 50% of the sample consisted of the young component. However, there was no evidence of any old waters or of any mineralisation indicative of long or deep flow paths from an off-island source. There is an urgent need to create a small groundwater level and groundwater quality monitoring network. This would provide early warning of any change that may be occurring to the groundwater body. A catalogue of point source pollution risk activities is also recommended. Given consumption of about one third of the overall water resource it is also recommended that a comprehensive evaluation of the groundwater system would assist the overall management of the resource