WATER PRODUCTION FROM FRACTURED PRECAMBRIAN CRYSTALLINE AQUIFERS, CENTRAL BLACK HILLS, SOUTH DAKOTA

Water wells for home and commercial use in the central Black Hills (specifically the Hill City, Mt. Rushmore, Pactola Dam and Silver City Quadrangles) are chiefly drilled in Precambrian phyllite, schist, metagraywacke, metabasalt and granite with lesser production from alluvial aquifers. Fracture porosity (faults and joints) in differing quadrangles in the crystalline rocks yield flows averaging 17 to 32 gallons per minute (gpm) in varying quadrangles at the time of initial well completion, but vary from one-half to 350 gpm. Granite and metagraywacke averages vary from 19 to 27 gpm whereas schist and phyllite averages vary from 1 to 23 gpm. Wells deeper than 300 feet rarely produce more than one to two gpm, but in exceptional cases yield greater than 100 gpm: average well depths vary from 141 to 215 feet and average static water levels from 35 to 45 ft.

Drill logs compiled by commercial drillers indicate production from fractured quartz bodies averaging 16 gpm: Vertical quartz “veins” (1-2 feet wide by 3-10 high) are noted in surface exposures as are iron-stained fractures (indicative of groundwater flow) in the metamorphic country rock. Trends of subvertical fracture sets in the Pactola Dam Quadrangle vary from location to location, but NE and/or NW trends are most common. Such orientations likely lend anisotropy to the aquifer and have an effect on ground water flow paths, as might several major NNW-trending faults.