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 lt10 gpm bucket and
  stop watch gt10 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
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 ?