GIS AND GROUNDWATER RESOURCES ON VINALHAVEN

Vinalhaven ME is an example of a fractured granite island aquifer with a limited groundwater resource. We have calculated the groundwater resource on Vinalhaven using 3-D modeling in GIS. Previous observations have confirmed that the Ghyben-Herzberg relationship, which estimates a ratio of 40:1 of the freshwater lens above and below the mean sea level, applies to Vinalhaven. Based on limited well data we modeled the groundwater table as approximating one third of the topography and then calculated the lower surface of the fresh water lens based on the Ghyben-Herzberg relationship. This model produced a maximum freshwater lens depth of 850 m and a total rock volume of 13.9 km3.

Using typical values for secondary porosity in granites of between 0.1% and 0.01% and assuming uniform porosity with depth, the volume of freshwater is between 370 and 3,700 million gallons. This value assumes uniform sub-surface geology and fracture characteristics. Further modeling will take into account variations in these factors.

While this volume of freshwater is substantial, it is important to relate this to the recharge of the aquifer and the consumption of freshwater on the island. Based on an average precipitation of 42 inches per yr and an infiltration rate of 10%, the recharge rate is 1,700 million gallons per yr. This translates to a very short residence time of between 0.22 and 2.2 yrs and suggests that the freshwater resource is vulnerable to periodic variations in the recharge rate. The year round population on Vinalhaven is approximately 1200 and the summer population swells to over 4000. Assuming an average per capita consumption rate of 66 gallons of water per day, we estimate the total annual consumption on the island at 44 million gallons. Annual water consumption is between 1.2% and 12% of the total resource. Since the freshwater lens thins towards the shore and most residents live close to the ocean, changes in the recharge rate could threaten many groundwater wells.