LONG-TERM MONITORING OF GROUNDWATER RECHARGE USING ELECTRICAL RESISTIVITY TOMOGRAPHY

Water resources are under increasing stress in New England due to a rapidly growing population. The Lamprey River Watershed (LRW) is a dominant source of water for Rockingham County, New Hampshire. From 1970 to 2000, the population of Rockingham County doubled, and it is projected to grow by an additional 30% by 2025. In addition to population growth, climate change models suggest that the region will experience changes in the amount, timing, and form of precipitation over the next several decades. The inevitable increase in the region's water demand in a potentially changing climate necessitates a greater understanding of the processes that govern the quality and quantity of its water resources.

The LRW is comprised primarily igneous and metamorphic bedrock covered by thin sandy till. Valley-fill glacial outwash and glaciomarine deltaic deposits are present in the watershed and provide a limited amount of groundwater storage. Groundwater is a primary contributor to streamflow and usable water resources in the Seacoast Region, especially during low flow periods.

This research uses a combination of geophysical and hydrological methods to investigate the geological, hydrological, and meteorological factors that control groundwater recharge in a glaciomarine deltaic deposit in the Lamprey River Watershed. Recharge to the stratified drift is of particular interest because of their ability to store rainfall and snowmelt. The primary method for monitoring the timing and spatial pattern of recharge is Electrical Resistivity Tomography (ERT) using a small fixed surface arrays. ERT is particularly appealing because it is non-invasive, allowing for long-term quasi-continuous and spatially extensive monitoring in the saturated and unsaturated zones. Long-term monitoring of precipitation, soil moisture, soil temperature, snowpack, and runoff complement the ERT surveys. Preliminary results suggest that the presence of a significant snowpack positively affects recharge in that it prevents ground frost and it provides temporary storage for spring rains. Long-term monitoring of recharge at a fixed location under different meteorological conditions will enable an improved understanding of groundwater recharge mechanisms for this critical component of the watershed system.