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B. Probable Hydrologic Consequences of the Hayden Gulch Mine <br />1. Ground Water <br />The probable hydrologic consequences to ground water from Hayden Gulch t4ine <br />can be classified into two categories: impacts to the coal aquifers and <br />development of spoils aquifers. It was originally projected that the coal <br />aquifer would form a cone of depression with a radius of 420 feet within the <br />coal seam aquifers and that the flow of two springs would be interrupted. <br />Water quality of the coal aquifer would be degraded by the inflow from the <br />spoils aquifers (both from the backfilled areas and from the excess overburden <br />pile). Spoils aquifer development would change the physical characteristics <br />of the recharge zone to the coal aquifers, resulting in a diminution of water <br />quality. Water from spoils aquifers would discharge to surface water at a far <br />greater rate than previously, as will be described in the Section 2) Probable <br />Hydrologic Consequences to Surface Water. <br />Wells for monitoring de-watering were constructed two years following mine <br />establishment, following a five-year drought period. The updip well (UC-3) <br />and the well immediately downdip of the mine (UC-1) showed minimal water level <br />changes over the last five years. Well UC-2 is the furthest well north of the <br />pit area and is recharged from the west. It showed a decrease in water level <br />of over 100 feet in two readings taken in October, 1982, and October, 1984, <br />with a return to baseline levels in 1985. This does not appear to be a <br />mine-related impact and may have been an operator error. Data collected to <br />date shows the upper coal aquifer was not significantly influenced by mining, <br />unless water levels at the time of well development were already depressed. <br />Spring 4 issued from the upper coal and stopped flowing fora period between <br />1977 and 1984. The spring was observed during Division field inspections <br />during April and June, 1984, original predictions called for re-establishment <br />of this spring thirty years following backfilling of the pit. Those <br />predictions appeared to err due to unreasonably low expectations of the <br />porosity of the spoils (6%). <br />There were four wells monitoring the lower coal. Well LC-4 is the furthest <br />updip well north of the mine. Water levels started rising in 1982 and seem to <br />have plateaued in 1984 and 1985. The excess overburden pile is 450' feet east <br />of this well and construction was completed in 1982. This may have initiated <br />the rise in water levels or levels may have risen in response to increased <br />recharge during higher water years following the summer of 1981. 'Hell LC1 is <br />directly downdip from LC-4 and water levels did not start to increase until <br />1985, when they rose 45 feet. <br />LC-3 and LC-2 are a half-mile north of the furthest northern extension of the <br />pit. LC-3 has always been dry. LC-2 is further downdip and was dry until <br />inid-1982 when it developed 50' of water. Future years of monitoring should <br />better delineate whether this aquifer was affected by the mine or was <br />influenced by drought. <br />-18- <br />~ .. <br />