WATER RESOURCES ON VINALHAVEN ISLAND, MAINE

Water resources on Vinalhaven Island have been an issue of concern for many decades. Vinalhaven represents a fractured-granite island aquifer. The main sources of water are private wells and the Vinalhaven Water District that obtains its water from a pond. Water supply is particularly problematic during the drier summer months when the demand is highest due to the increased summer population (year-round pop. = 1500, peak summer pop. > 4000). Very low yields, saltwater intrusion, and increasing consumption (estimated to increase by 100% by 2025) all pose challenges to the future supply of water. We measured water-levels in 9 wells over various time periods on Vinalhaven. One-time measurements were made on numerous other wells. Ocean tides affected 7 of the 9 wells monitored. Water levels in the wells varied from 0.15 m to over 1.3 m compared to the local ocean tidal range of ~3 m. There was no relationship between the elevation of the water table or the distance from the shore of the wells, suggesting that other factors including variations in bedrock fractures determines the tidal influence on wells. Water chemistry was determined on samples from wells, springs and surface water, as well as vertical interval samples from 7 wells. Three wells showed salt water intrusion, demonstrating the existence of salt water below the freshwater lens. In at least one case a major fault zone trended directly toward a ‘salt' well. Limited storage capacity of the fractured granite bedrock is illustrated by the response of the wells to precipitation. In one case the groundwater table rose by 1 m in response to 0.4 m of rain. Taking into account trans-evaporation, run-off and near-surface flow, this is consistent with the very low porosities (> 0.1%) of fractured granite. This information is useful in determining the overall carrying capacity of the Vinalhaven aquifer and combined with the existing GIS database, predicts areas susceptible to saltwater intrusion and low yields.