Influence of geology on the water resources of the Upper Roanoke River Basin, Virginia

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Virginia Polytechnic Institute and State University


The Upper Roanoke River Basin contains Precambrian Blue Ridge Complex rocks and 15,000 feet of Paleozoic sedimentary rocks in a complexly folded and faulted structural framework. Three geohydrologic units were identified--Precambrian and Ordovician metamorphics and elastics, Cambrian and Ordovician carbonates, and Ordovician to Mississippian elastics. Geohydrologic analyses of climate, surface-water, ground-water, and water-quality data identified the geohydrologic characteristics of each unit.

Precipitation on 15 subbasins during the study year (April 1969 to March 1970) ranged from 30.77 to 39.68 inches; potential evapotranspiration ranged from 24.52 to 27.53 inches; runoff ranged from 7.00 to 16.68 inches; runoff per square mile ranged from 0.52 to 1.23 CFSM; and ratios of runoff to rainfall ranged from 0.218 to 0.421. Precipitation, runoff, and the ratio of runoff to rainfall increased and evapotranspiration decreased with increased elevation.

Hydrograph separations showed that the metamorphics and elastics unit had 8 to 9 inches of ground-water runoff and 4 to 8 inches of surface runoff, the carbonates unit had 5 to 7 inches of ground-water runoff and 2 to 3 inches of surface runoff, and the elastics unit had 2 inches of ground-water runoff and 5 inches of surface runoff. The high ground-water runoff from the metamorphics and elastics unit may be due in part to ground-water diversion into the basin. Flow duration analysis showed that each geohydrologic unit had different low-flows and similar high-flows relative to mean flows.

The metamorphics and elastics unit is a fair, the carbonates unit a good, and the elastics unit a fair to poor ground-water producing unit. The ground-water resource potential of these units and the unconsolidated deposits has not been fully utilized. The most productive wells are likely to occur where carbonate rocks with significant secondary porosity and permeability occur near perennial streams and thrust faults and are overlain by unconsolidated deposits.

Each geohydrologic unit has distinct water quality characteristics with a general trend from ground-water to low-flow surface-water to high-flow surface-water to rainwater quality.

The pressing water resources issues in the basin and the role of utility of the geohydrologic units in evaluating these issues were discussed. Four water resources management units were identified and the water resources planning, development, and management potentials of each unit for ground-water supply, storage reservoirs, ground-water quality degradation, hydrologic changes due to urbanization, flooding, stormwater management, and solid and liquid waste management were established. Guidelines were established for sound management of three critical water-related activities which impact on urban growth and development in the Upper Roanoke River Basin--water supply, waste management, and hydrologic changes due to urbanization.

Specific problems appropriate for future investigations in the basin were identified. Guidelines for the application of the results in the Appalachian Valley and Ridge Province and west slope of the Blue Ridge were presented and the type and scope of basic information needed for long-range water resources planning in those areas were identified.