Based upon detailed mapping and evaluation of boring logs, the native material downslope and underlying the northern portion of the spoil consists of poorly indurated volcaniclastic sandstone and lahars of andesitic composition.  Inspection of the only sedimentary outcrop in the area revealed calcite filling fractures within the sandstone.  The presence of calcite in these sedimentary units was further confirmed by x-ray diffraction (XRD) analysis of core samples.  Calcite was found in nine of 13 samples collected from the sedimentary units immediately downslope or underlying the spoil.      

These analytical results and corresponding observations are indicative of buffering of the acidic groundwater by calcite in the sedimentary units.  This is further supported by an observed groundwater pH of 7.1 from a well near the northern limits of the spoil piles.  The well was completed in the sedimentary unit immediately underlying the saturated acidic spoil and calcite was found in sedimentary unit core samples collected from the well boring.
This paper demonstrates the potential for passive natural chemical buffering and associated treatment at sites with acid mine drainage (AMD).  Such potential buffering should always be considered and investigated.  The investigation should include mapping and evaluation of formation mineralogy coupled with evaluation of the groundwater geochemistry.