ANALYSIS OF WATER WELL YIELDS IN THE FRACTURED-ROCK AQUIFER OF THE BLUE RIDGE GEOLOGIC PROVINCE, LOUDOUN COUNTY, VIRGINIA

Loudoun County, Virginia, the fourth-fastest growing county in the U.S., includes part of the Blue Ridge Geologic Province, in which the primary source of ground water is fractures in Mesoproterozoic gneisses and Neoproterozoic-Lower Cambrian metasedimentary cover rocks. A digital database of yields, depths, and locations for 3,561 wells in the Blue Ridge was used in combination with a digital geologic map for a statistical study of crystalline-rock hydrogeology.

Mean yield of wells (MYOW) in Mesoproterozoic gneisses was 26% higher than MYOW in the metasedimentary cover rocks. Comparison of well yields in different geologic map units shows that MYOW in Cambrian Harpers Formation phyllite is significantly lower than in all other units; MYOW in Neoproterozoic Catoctin Formation metabasalt is significantly lower than in two Mesoproterozoic metagranites; and MYOW in a coarse-grained metagranite is significantly higher than in a medium-grained biotite metagranite. Grouping geologic map units by rock types reveals that MYOW in all granitic rocks is significantly higher than in charnockite plus metanorite combined, and higher than in metavolcanic rocks. MYOW in charnockite plus metanorite and in carbonate rocks is substantially lower than in all other rock types. MYOW in generally well-foliated Mesoproterozoic rocks, such as paragneiss and granitic gneisses, is higher than MYOW in poorly-foliated Mesoproterozoic charnockite and metanorite together.

Time-trend analysis of the 2,641 wells with reported well-completion dates was conducted by partitioning the well data into 15 temporal increments spanning 1953 to 1996, each containing an equal number of wells. Mean yield per increment has generally increased over time, but mean well depth has increased at a greater rate, resulting in a decrease over time in mean yield per foot drilled. The number of very-low-yielding wells (<0.5 gpm) has increased over time, while low yielding (0.5-<2.5 gpm) and moderate yielding (2.5-40 gpm) wells have stayed the same and declined in number, respectively, while high-yielding wells (>40 gpm) have increased.