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Appendix E - Mine Pumping Data 2 <br />is Much of the groundwater inflow to the mine from about 1988-1994 was derived from core holes drilled <br />into pegmatite dikes in the hornblende-biotite-gneiss (HBGN) on the 1,800-2,000 levels of the mine. The <br />water-producing boreholes were plugged and pressure-grouted in 1994. This reduced inflow from about <br />310 gpm to about 140 gpm. Continued mining from 1994-1999 brought the inflow rate up to about 190- <br />200 gpm. The increased volume of the mine excavation after 1994 was partially responsible for the <br />observed increase in inflow, but it is also likely that increased storage of water in the rock mass caused by <br />the grouting of boreholes which previously drained the area contributed to the observed gradual increase in <br />inflow. In other words, once the storage deficit had been met, inflow to the mine gradually increased. The <br />increase in inflow, however, was lower than the pre-grouting flow rates. Mine pumping data are contained <br />in Table 1. <br />• <br />Under dewatered conditions, the inflow to the mine at its maximum extent (1995 - 1999) was about 200 <br />gpm. Given that the mine workings extend more than 2,000 feet below the pre-mining potentiometric <br />surface, this rate of inflow is small and reflects the overall low permeability of the rock mass. <br />Mine pumping records between 1995 and 1999 indicate that inflow rates to the mine vary seasonally, with <br />the lowest average inflow occurring in February and March (180 gpm) and the highest average inflow <br />occurring in June (209 gpm) (Figure 2). This pattern is consistent with the annual distribution of <br />precipitation, but comparison of peak precipitation and infiltration indicates a lag time of approximately 1 <br />month for precipitation to infiltrate into the mine. If it is assumed that February and March represent the <br />infiltration to the mine from January and February when the ground is frozen and infiltration is essentially <br />negligible, the average inflow to the mine by infiltration of precipitation to the upper levels can be <br />calculated on a monthly basis for the referenced time period (Table 2). <br /> <br />240 <br /> <br />230 <br /> <br />220 <br /> <br />210 <br /> <br />200 <br />E <br />a <br /> <br />? 190 <br />O <br />J <br />LL <br />? 180 <br />W <br />Z_ <br />170 <br /> <br />160 <br /> <br />150 <br /> <br />140 <br /> <br />130 <br />Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec <br />MONTH <br />Figure 2. Average Groundwater Inflow by Month, 1990-1999 <br />CI <br />4109B.071113 Whetstone Associates •