USGS Grant Number:
Year Established: 2009 Start Date: 2009-09-01 End Date: 2012-08-31
Total Federal Funds: $174,490 Total Non-Federal Funds: $181,817
Principal Investigators: David Boutt, Stephen Mabee
Abstract: The use of bedrock wells to meet residential and commercial water supply needs continues to increase in the northeastern United States. Over 50% of the rural New England population relies on fractured bedrock aquifers to meet its water supply needs. Although it is well established that overburden deposits are often hydraulically linked to fractured bedrock aquifers, quantitative measures of recharge rates and potential leakage across the overburden-bedrock boundary remains one of the greatest sources of uncertainty in delineating zones of contribution or evaluating sustainability of public water supply wells. In addition, water level data from bedrock monitoring wells in New Hampshire and Massachusetts show an alarming downward trend over the last 10 years while water levels in surface aquifers have remained relatively constant. This suggests that the groundwater flow system in the bedrock operates at a different temporal scale and is a complicated, dynamic system. We propose to make direct measurements of temperature and hydraulic head at discrete intervals across the overburden-bedrock interface in three hydrogeologic settings typical of glaciated terrain. The temperatures and heads will be monitored for one year and used to estimate formation transmissivities using the established temperature gradient method. Three wells will be constructed and instrumented using Solinst multiport PVC pipe. Wells will be located in the Nashoba formation, a late Proterozoic migmatitic gneiss and schist that is strongly foliated. This unit was chosen because a wealth of data has already been collected from this formation and it is located in the Nashoba terrane, an area that has undergone comprehensive study by both the University of Massachusetts and the USGS Water Science Center in Northborough, MA. Three hydrogeologic settings will be evaluated. One well will be located in an area with very thin to non-existent overburden, one with thick, permeable sand and gravel deposits and one located in an area with thick till. The objectives of the study are to: 1) construct multiport monitoring wells in three hydrogeologic settings and install temperature sensors and pressure transducers; 2) monitor the temperature and hydraulic heads for a period of one year to evaluate aquifer responses to seasonal changes and significant hydrologic events; and, 3) model the transmissivity of the aquifer at discrete intervals using the temperature gradient method and estimate the flux across the boundary for the three hydrogeologic settings. The expected outcomes are a clearer understanding of the groundwater flux across the overburden-bedrock boundary and how the system responds to seasonal changes, individual recharge events and stresses due to pumping. This knowledge is transferable to any area of the world containing crystalline igneous and metamorphic rock that has been subjected to glaciation. The proposed work makes use of existing USGS data and resources. It is also part of a larger, ongoing and integrated study being conducted by the Northborough office the USGS and University of Massachusetts, with financial support from the Massachusetts Department of Environmental Protection, on the geohydrology of the Nashoba terrane, Massachusetts. This larger study has involved acquisition of borehole geophysical data, collection of outcrop-scale fracture characterization data, discrete fracture flow modeling, a statistical assessment of the factors affecting well yield as well as supporting quadrangle-scale geologic mapping by the Office of the Massachusetts State Geologist and Greg Walsh from the USGS. The funds requested are primarily for student support for three years and some equipment and drilling costs.