Daily precipitation data were collected from April to October, 2011, in an area underlain by Precambrian metamorphic rocks near Hill City, South Dakota. Ground water discharged from a nearby abandoned mine adit from May 20 to June 9 following two days of intense rain. The maximum discharge was 0.437 cfs (0.0124 m(3)/s), and the total volume of water that discharged during the 21-day interval was 425,000 ft(3) (12,040 m(3)). The onset of discharge lagged about 12 hours following 2.15 inches (5.46 cm) of rain on May 20. The recharge area serving the mine adit is approximately 26.4 acres (10.7 ha). The precipitation during the 21-day interval affecting the mine discharge was 5.07 inches (12.9 cm). This is equivalent to a volume of water falling on the recharge area of 486,000 ft(3) (13,760 m(3)), slightly more than the volume of water discharged from the mine adit during this period. Because nearly all the rain recharged the ground water, and because precipitation infiltrated the metamorphic rocks and recharged the water table within 12 hours, the metamorphic rocks demonstrate considerable permeability at shallow depths. This conclusion is supported by hydro geologic studies of metamorphic rocks at other places in the Black Hills, such as a shallow ground-water contaminant plume at Nemo. The abundant base flow in streams draining metamorphic rocks also indicates shallow metamorphic rocks can store meteoric water for subsequent release to streams. Data from this study site, supplemented by published permeability data, indicate the hydraulic conductivity of metamorphic rocks varies from approximately 1 m/d near the surface to 10(-4) m/d at 1 km depth. A general formula relating this exponential decrease in hydraulic conductivity with depth was determined. The permeable nature of the near-surface metamorphic rocks has practical ramifications. For instance, a water well that is open over the upper 100 ft (30.5 m) of saturated near-surface metamorphic rocks would probably have a greater specific capacity than if the well were drilled much deeper. The permeable nature of near-surface metamorphic rocks also helps explain the rapid transport of bacteria from onsite wastewater systems.
