UNSATURATED HYDROGEOLOGIC PROPERTIES OF RECLAIMED COAL STRIP MINES

Two nuclear methods were used to quantify hydrogeologic parameters in the unsaturated zone of a reclaimed and revegetated coal strip mine in Clarion, Pennsylvania. Am-Be (neutron) and Cs-137 (gamma-gamma) geoph ysical logging tools were used to quantify volumetric moisture content , bulk density, total porosity, and unsaturated hydraulic conductivity . An additional study at the same site used postsampling neutron activ ation analysis to determine concentrations of a bromide tracer in unsa turated zone water samples. Those additional data were used to indepen dently calculate unsaturated hydraulic conductivity. Geophysical loggi ng of six boreholes at the site was conducted on seven different dates . Temporal variations in volumetric moisture content versus depth were observed to be short-lived, with the general shape of the volumetric moisture content profiles remaining spatially stable over the eight mo nth period of investigation. Bulk density values ranged from less than 1.14 to 1.86 g/cm3, corresponding to total porosities of greater than 57% to 30.1%. Large void spaces were encountered during past and pres ent drilling and observed at a measurement point. Unsaturated hydrauli c conductivities were calculated using draining profile volumetric moi sture content data as input to an explicit numerical solution of the u nsaturated flow equation. Calculated values ranged from 2.4 X 10(-7) t o 1.5 X 10(-3) cm/s. Examination of all of the geophysical log data to gether showed sharp increases in volumetric moisture content spatially coincident with zones where bulk density increases (and porosity decr eases). The bulk density contrast appears to be of more influence than the magnitude of the material property. Increased unsaturated hydraul ic conductivity associated with increased volumetric moisture content was seen in several boreholes. Bromide tracer-labeled waters were coll ected from pressure-suction lysimeters installed at depths of up to 18 .1 m for a period of 16 months. Unsaturated hydraulic conductivities, calculated by interpreting concentration peaks as average arrival time s of steadily infiltrating water through a uniformly porous media, ran ged from 2.8 X 10(-6) to 7.2 X 10(-5) cm/s. However, a dual-permeabili ty mechanism is suggested by the observed behavior of the tracer. Anal ysis of the data suggests that fluid flow in this hydrogeologic settin g is dominantly transient. Ground-water recharge occurs in short-lived pulses. The periodicity of acid mine drainage formation and flushing to the water table is expected to correspond to infiltration and recha rge events.