GROUNDWATER-SURFACE WATER INTERACTIONS – CHALLENGES IN FRACTURED BEDROCK SETTINGS

Instream flows in rivers and streams of northern New England are becoming increasingly important as water resources in the region become further developed. One of the challenges in predicting and managing instream flows is developing appropriate modeling tools to quantify groundwater-surface water interactions. Much of the success to date has been in watersheds that are comprised primarily of sediments and/or sedimentary rocks.

Analysis of stream flow in the Lamprey River, NH, suggests that baseflow is derived from gravity drainage of two distinct groundwater reservoirs. The glacial deposits in the valley floors appear to drain first over a period of weeks and the more extensive fractured bedrock appears to drain more slowly. Extrapolation of the bedrock drainage rate suggests that the bedrock aquifer would drain more than 75% of its storage in one year. This relatively fast drainage rate of the natural system, along with rapid population growth, is one reason that water availability has become a primary concern to the communities in the Seacoast region of New Hampshire. Successful modeling in this type of hydrogeologic setting, common for northern New England, will depend on the ability to adequately represent the groundwater flow patterns and gravity drainage in both the sedimentary valley fill and the more extensive fractured rock system.

The bedrock geology and geochemistry of the Lamprey River Watershed provide an excellent opportunity to 1) study the groundwater flowpaths and 2) assess the adequacy of hydrologic models to quantify groundwater-surface water interactions. While most of the watershed is underlain by Silurian-Devonian metamorphic and igneous rocks, an igneous intrusion of the Cretaceous White Mountain Plutonic series, covering 6km2, exists near the geographic center of the watershed, providing a distinct strontium isotopic signature (Smith et al., this volume). Pathlines from a simple steady-state two-dimensional ground-water model suggest that the flowpaths emanating from the Mt Pawtuckaway area generally follow a radial pattern, but exhibit significant topographic control. Comparison of the modeled flowpaths with those observed will provide insights into the dominant factors controlling groundwater flow in fractured bedrock over scales of 10s of kilometers.