Groundwater Discharge in the Illinois Basin as Suggested by Temperature Anomalies

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<jats:p>Areas where ground water in the Illinois basin is moving vertically should be reflected by temperature anomalies. To test this hypothesis, an isothermal map for a depth of 500 feet was constructed, based on bottom hole temperatures of deep wells. This map was compared with a theoretical isothermal map for a depth of 500 feet that was made by projecting temperature gradients (calculated from the thermal conductivities of the rock) downward from the surface where mean annual air temperatures were known. The residual temperature map that was made on the basis of this comparison (calculated minus observed temperature) shows several warm and cool anomalies that are postulated to be discharge and recharge areas, respectively. Analysis of the curvature of the earth's thermal profile calculated for the basin suggests that approximately 59,000 acre‐feet of ground water per year are discharged upward from the deep rocks in the Illinois basin. This figure is of the same magnitude as an estimated excessive groundwater discharge into the region's streams of 43,000 acre‐feet, based on comparison of unit‐area discharge of streams in the basin with streams in similar areas outside the basin. From a simplified model of the basin, a groundwater inflow of about 36,500 acre‐feet from the surrounding highlands is estimated. A significant portion of this discharge must be through fracture zones associated with faults and anticlines in the basin, inasmuch as this amount of flow would require that the shales have a vertical permeability from one to three orders of magnitude higher than is generally observed. One such discharge area in southwestern Illinois is suggested by field data.</jats:p>

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