ESTIMATING REGIONAL-SCALE GROUNDWATER FLOW AND TRANSPORT PROPERTIES IN THE FRACTURED ROCK TERRAIN OF THE VALLEY AND RIDGE AND BLUE RIDGE PROVINCES, USA

Mountainous fractured-rock terrains in the Mid-Atlantic region of the USA underlie more than half of the Chesapeake Bay watershed, where land use practices affect the types and amount of pollutants that ultimately enter the Bay. To better understand the nature and timing of groundwater and nitrogen movement to streams in the Bay watershed, we have developed a numerical simulation model of the Upper Potomac River Basin (~24,000 sq. km) that includes Proterozoic metamorphic rocks of the Blue Ridge Mountains and Paleozoic siliciclastic and carbonate rocks of the Valley and Ridge Province. Inverse modeling was undertaken to estimate regional values and distributions of: (a) recharge using 12 base-flow measurements in watersheds, (b) hydraulic conductivity (K) using 200 water-level measurements in wells, (c) specific yield using 12 stream regressions in watersheds and 120 water-level regressions in wells, and (d) effective porosity using >100 environmental tracer (CFC-113, SF₆, ³H, ³He) measurements from wells, springs and the Potomac River at the basin outlet.

Results indicate K is strongly depth-dependent, declining by 4-6 orders of magnitude within 100 m of land surface, so that most of the transmissivity is in the upper 10 m. Specific yield values reflect fracture porosity (<2%) in the upper ~20 meters but drainage of matrix porosity (>5%) below ~20 meters. The strong depth-dependence of K has major implications for the transport of solutes in these terrains, as the bulk of the transport occurs close to the water table with travel times of a few years or less, yet at greater depths flow paths are characterized by very long residence times (centuries to millennia or more). A tritium record from 1961-1991 in the Potomac River at the outflow of the basin can be reproduced in the model using advective transport and a single regional porosity value of 2-3%. The fit to the tritium data can be improved at early and late times by assuming fracture porosity (1-2%) dominates transport at shallow depths (<10 m), and matrix porosity (5-15%) dominates at deeper depths (>10 m). Tracers collected from production wells reflect local heterogeneity and downward vertical transport. Tracers collected from springs were limited to the carbonate-dominated Shenandoah Valley and represent a broad mixture of ages.