Methods for the Estimation of Mine Infiltration

1. Methods for the Estimation of Mine Infiltration Bruce Leavitt PE PG, Consulting Hydrogeologist Washington, Pennsylvania

2. Mine Infiltration • has been used to: • Estimate inflow during mining; • Calculate the rate of mine flooding; • Estimate post closure mine discharge; and • Inform permitting decisions.

3. New Applications of Mine Infiltration Project mine discharge from underground mines for mine water utilization under varying amounts of annual precipitation. Project groundwater discharge from surface mine backfills in support of stream mitigation.

4. Mine Infiltration Methods • Rule of Thumb • 0.50 gpm / acre • Possibly based on diPretoro 1986 • 56 surface mines principally on the Waynesburg (14) and the Upper Freeport (31) coals. Pittsburgh seam excluded. • Flow, one time site visit; <10 gpm bucket and stop watch; >10 gpm visual estimate ± 10 gpm. • Data range, 0.02 to 4.35 gpm / acre

5. Mine Infiltration Methods • Rule of Thumb • Leavitt (1997) • Pittsburgh seam underground mines • gpm/acre = 1.117e-0.0045H where H = average of the Maximum and minimum overburden thickness. • Based on pump curves and hour meter data from active mines. • Forced to 0.50 gpm / acre at 200 ft overburden • Includes longwall and full extraction room and pillar.

6. Mine Infiltration Methods • Rule of Thumb • Leavitt (1997) • McCoy (2002) • Primarily flooded Pittsburgh Seam underground mines. • Removed gaining and loosing barrier pillar leakage. • Based on the percent of the mine deeper than 150 meters (492 ft). • Places a lower limit on infiltration of about 0.003 meters per year which equals 0.006 gpm / acre.

7. McCoy 2002 mines gaining water from barriers "average" inflow rates "shallow mines" mines losing water to barriers

8. Mine Infiltration Methods

9. New Applications of Mine InfiltrationMine Water Utilization - Underground mines Large quantities of water are discharged annually from flooded mines in the Pittsburgh Basin. The bulk of this water is untreated. Water quality has improved over time. The flooded mines of the Pittsburgh Basin represent the second largest aquifer in the region. Utilization of this water could benefit industrial water users, and reduce the environmental mining legacy.

10. Power Plant Cooling • Power plant cooling requires large amounts of water and cool water improves the efficiency of plant operation. • The power plants need for water must be satisfied in both wet and dry years, or a second water source will be required. • Existing mine infiltration methods do not provide for variations in precipitation.

11. Project WV 232

12. Hydrograph JB-1

13. Recharge Based on Precipitation Recharge inches = ( Precipitation (October – May) – 6.68 inches) * 0.27 Based on: Average Killing Frost – October 1 End of Recharge – May 31

14. New Applications of Mine InfiltrationMine Water Utilization - Surface mines Surface mining operations are being required to provide mitigation for stream values that are lost due to the mining operation. One mitigation option is to create a constructed stream segment to replace the lost stream. Discharge from mine spoils, either back stack or valley fill, can be used as a water source for the mitigation. An estimate of discharge from mine spoils is needed to project the flow and duration of flow from these designs. The Rule of Thumb method is not appropriate in this application.

15. USGS Studies • Ballard Fork and two of its tributaries were monitored for two years November 1999 to November 2001. • Spring Branch, unmined, 339.2 acres • Unnamed tributary, mined, 121.6 acres • Ballard fork, mixed, 1,401.6 acres • Four rain gauges, two mountain top, and two at valley locations. • Based on this work two reports have been published - Messinger, 2003; and Messinger and Paybins, 2003 – focused on storm response, and precipitation effect on mean flows.

16. USGS Studies continued Precipitation data obtained from Messinger and Paybins, 2003 Daily stream flow data downloaded from USGS Web site.

17. Stream Flow Hydrographs

18. Base Flow Averaging Minimum 0.5 inches of rain in a 2 day period. Base flow proportionally increased from pre rain level to the post rain level either 3, 4, or 6 days following the rain event.

19. Comparison of Discharge

20. Monthly Precipitation Vs. Mine Discharge

21. Estimation of Mine Discharge Three Outliers: two were months of normal precipitation preceded by a month of abnormally high precipitation. The outliers skew the regression to a higher flow rate. Equation with outliers removed: Q = 0.405e0.130P Where: Q is gpm per acre and P is monthly precipitation.

22. Summary The Rule of Thumb is quick and easy, but is only appropriate for surface mines and even then may be significantly in error. The Leavitt 1996 method is based on Pittsburgh seam unflooded underground mines and is not appropriate at less than 200 feet of overburden. The McCoy method is appropriate to flooded Pittsburgh seam mines, but places a lower limit on the infiltration rate despite overburden thickness.

23. Summary Continued Surface mine hydrographs show significant response to precipitation, while the hydrographs from underground mines do not. Surface mine recharge can occur any time there is sufficient precipitation. Recharge to underground mines is negligible during the growing season and can be delayed until the soil has been resaturated. Estimation of the amount of groundwater available from surface and underground mines based on precipitation is possible.

24. Questions ?