Ground-water changes near Montana coal mines
Conference
·
OSTI ID:61218
Hydrologic conditions near surface coal mines in southeastern Montana are being monitored because the coal beds are the most accessible and widely used aquifers of the region. Mining has thus far affected water levels in alluvium only locally, if at all. Water levels in coal-bed aquifers are being lowered within about 2 miles of active mines near Colstrip. As mine pits are backfilled with spoil, hydrostatic pressures in both the spoil and adjacent aquifers are recovering toward pre-mining levels. At Decker, a potentiometric depression has formed over an area more than 5 miles wide and 15 miles long, caused by converging of drawdowns created by 3 mines. At all mines, water-level declines clearly illustrate the influences of directional permeabilities and hydrologic boundaries. As pits are being backfilled, spoils are rapidly becoming resaturated by inflow from undisturbed aquifers, laterally at all mines, and vertically where conditions are appropriate. It is hypothesized that pre-mining non-plugged drill holes allow vertical flow. In the resaturation process, strongly increased concentrations of dissolved solids (primarily calcium, magnesium, sodium, and sulfate) are evolving. The initial chemical quality of spoils water is highly diverse because of the variable distribution of soluble salts in the spoils. As groundwater flow in the spoils continues, trends are becoming apparent. Some sites are currently showing increasing concentrations where surface recharge introduces dissolved solids faster than they are flushed from the system. Other sites show decreasing trends where new salts are not introduced by recharge. In these cases, chemical quality of spoils water may approach pre-mining quality within the predictable future.
- OSTI ID:
- 61218
- Report Number(s):
- CONF-9003298--
- Country of Publication:
- United States
- Language:
- English
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