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tichwartrwalJer Mine Hydrologic Evaluation of Mine Closure and Reclamation 30 <br />Installation of drainage/dewatering boreholes. During the late 1970's and early 1980's drainage <br />from dewatering boreholes completed on the 19 Level increased mine intlow from about 300 gpm <br />to 600 gpm before declining to near 300 gpm again in 1981. <br />Plugging of productive boreholes that intersect permeable pegmatites or fractures. In 1994, <br />boreholes that intersected pegmatite veins were grouted and mine intlow decreased from about ;00 <br />gpm to I i0 gpm. <br />goo <br />soo <br />soo <br />t9 a00 <br />$~ <br />a <br />~ a6o <br />zoo <br />~~ <br />4 ~ <br />_ ~ <br />~~ ~'~ • <br />~ ~ '~ ~ <br />,_ ~ <br />1 ~ <br />g : ~r~ <br />1 <br />I ~ ~ t I & i <br />y <br />s? I ~ s- I <br />u ~ u t <br />N S ~ ~ g ~ O <br />"' A a g € ° c <br />2 ~ O i S ,'n ~ C <br />ta'0 1915 t?FC 1gB5 IV90 1595 2060 2005 <br />YEAR <br />Figure 18. Groundwater Inflow to the Schwartzwalder Mine, 1973-1999 <br />Much of the groundwater intlow to the mine from about 1988 through 1994 was derived from core holes <br />drilled into pegmatite dikes in the hornblende-biotite-gneiss (H[3GN) on the 1,800-?A00 levels of the mine. <br />The water-producing boreholes were plugged and pressure-grouted in May 199-1. This reduced intlow <br />from about ;10 gpm to about 140 gpm. Continued mining from 1994-1999 brought the average intlow rate <br />up to about 190 gpm. The increased volume of the mine excavation after 1994 was partially responsible for <br />the observed increase in intlow, but it is also likely that increased storage of water in the rock mass caused <br />by the grouting of boreholes which previously drained the area contributed to the observed gradual increase <br />in intlow. In other words. once the storage deficit had been met, intlow to the mine gradually increased. <br />~fhe increase in inflow, however, was lower than the pre-groutim~ flow rates. Mine pumping data are <br />contained in Appendix E. <br />Under dewatered conditions. the intlow to the mine at its maximum extent (1991 - 1999) wa_s 189 gpm. <br />Given that the mine workings extend more than '_',000 feet below the pre-mining potentiometric surface, <br />this rate of inflow is small and reflects the overall low permeability of the rock mass. <br />Mine pumping records hehveen 1991 and 1999 indicate that intlow rates to the mine vary seasonally, with <br />the lowest average intlow occurring in February and March (180 gpm) and the highest average intlow <br />41 u911.071 116 Whetstone Associates • <br />