Interaction of ground water with the Rock River near Byron, Illinois

Ground-water discharge to the Rock River in the study area, estimated by three independent methods, ranged from 16,300 to 30,900 cubic feet per day; the low value, determined by the use of the modified Darcy equation, is an estimate only of ground-water discharge from the southern side of the Rock River. The vertical distribution of trichloroethene (TCE) in ground water was determined at a test hole along the estimated centerline of the contaminant plume and as close to the river as property access would allow. The maximum concentrations of TCE of 3 micro- grams per liter were found at depths of 59 and 64 feet. The contaminant was dispersed across a verti- cal interval of about 75 feet at depths of 19 and 94 feet. All of the TCE in ground water discharges to the Rock River because no TCE was detected below a depth of 109 feet, and increasing vertical head gradients with depth indicate ground-water flow from a depth of 119 feet is to the river. The maximum possible discharge of TCE is estimated to be about 1.7 grams per day. A finite-difference numerical model was used to simulate ground-water flow along a vertical section through the ground-water system from the Byron Superfund site to the Rock River. Results of the ground-water flow simulation indicate that, if underflow in the St. Peter aquifer occurs beneath the Rock River, it would be water that was present at depth in the flow system at the Byron Superfund site rather than contaminated water that had recharged the system in the vicinity of the Byron Superfund site. (USGS)