Projects > Hydrologic Suitability of Mine Spoil as a Medium for Septic-Tank Absorption Fields, Warrick County, Indiana

EXECUTIVE SUMMARY

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The project was conducted over a three-year period from January 1998 through December 2000.

The project had three main parts: (1) Compilation of a postmining land-use map for Warrick County, showing four principal reclamation categories, and creation of a geographic information system (GIS) for the county. (2) Establishment of intensive monitoring sites at three new residences having on-site septic disposal systems. The residences are located on three of the four reclamation categories. (3) Installation of instruments and samplers for intensive monitoring at the three sites.

The four reclamation categories that were mapped are (1) Category 1A: no soil, ungraded, (2) Category 1B: no soil, graded, (3) Category 2: graded, having less than 30 inches of restored topsoil, and (4) Category 3: graded, having more than 30 inches of restored topsoil.

Hydrologic monitoring indicates that Category 1A (unreclaimed spoil) allows the greatest amount of vertical water movement and the most rapid ground-water recharge. In Categories 2 and 3, the layer of restored topsoil effectively traps infiltrated precipitation and thereby inhibits vertical percolation through the unsaturated zone.

Because the residence at Site 3 was never occupied during the period of monitoring, the chemistry values from that location could be used only to indicate natural background conditions for Category 3. There was no indication that leachate from the active septic disposal systems at Sites 1 and 2 was reaching the saturated zone. The deepest penetration of septic leachate occurred at Site 1 (Category 1A), where elevated values (14 mg/L) of nitrate-nitrite nitrogen were observed at a depth of about 1 meter below a septic system finger. At Site 2 (Category 2) the concentration of nitrate-nitrite nitrogen was reduced to less than 3 mg/L by the time leachate reached a depth of 1 meter.

On the basis of the data gathered during this investigation, there is no indication that leachate from on-site septic disposal systems poses a threat to ground water in the spoil-covered areas of Warrick County. Where layers of restored topsoil are present, movement of leachate is retarded and rendered harmless by the low permeability of the compacted topsoil layer. Where no such retarding layer is present, the great thickness of the unsaturated zone, consisting largely of clayey shale, absorbs and dilutes the downward percolating leachate. Therefore, any water that reaches the saturated zone is of relatively good quality, and concentrations of potential contaminants, such as nitrate, are below the maximum limits recommended by the EPA for drinking water.

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