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This report presents results of a study, in cooperation with the City of Columbus, Ohio, to investigate transport of Scioto River, ground-water flow and quality, and surface-water/groundwater interaction near the City of Columbus South Well Field during low-flow periods. Water quantity and quality of Scioto River in the study area are significantly affected by effluent from the Jackson Pike Wastewater Treatment Facility. During lowflow periods, this effluent can comprise as much as 90 percent of the total flow in Scioto River. Data-collection activities included water-quality sample collection and analysis; water-level measurement in wells completed in the carbonate bedrock and glacial drift aquifers near the well field; and water-quality-sample collection, discharge measurement, traveltime measurement, and dye-dispersion measurement of Scioto River from the Jackson Pike Wastewater Treatment Facility to the Southerly Wastewater Treatment Facility. This reach encompasses the area from which the South Well Field is designed to induce infiltration of water across the riverbed. During low flow, waters in Scioto River above the Jackson Pike Wastewater Treatment Facility and in its tributaries within the study area are dominated by the cations calcium and magnesium and by the anions bicarbonate and sulfate. This composition, which is similar to that of ground waters in the area, indicates that ground water is a principal component of streamflow. The river below the wastewater-treatment facility is dominated by effluent, as evidenced by increased concentrations of dissolved solids, chloride, sodium, nitrate, phosphorus, and several trace elements. Ground waters from the carbonate bedrock and glacial drift aquifers are hard, calcium bicarbonate-type waters. Concentrations of iron and manganese routinely exceed State drinking-water standards. Concentrations of trace metals generally are below the detection limit, and the combined concentration of all nitrogen species generally is less than 1 milligram per liter. Ground waters from the carbonate bedrock aquifer can be distinguished from ground waters from the glacial drift aquifer by their greater dissolved-solids, sulfate, and calcium concentrations. The surface-water-transport system was simulated by use of a one-dimensional Lagrangian transport model. Traveltimes for various flow conditions were simulated. For a discharge of 93 cubic feet per second from the Jackson Pike Wastewater Treatment Facility, simulated traveltime from Frank Road to the South Well Field during a 7-day, 2-year low-flow period is approximately 35 hours; for an effluent discharge of zero, traveltime would increase to 94 hours. No lateral nonpoint sources of discharge to Scioto River were identified. A lateral sink of 0.00078 cubic feet per second per foot was identified in the stream reach from collector well 101 to collector well 103. This represents a loss of approximately 6.5 million gallons per day (10 cubic feet per second) over a 2.1-mile segment at a time (August 1988) when pumpage from three collector wells was approximately 18.4 million gallons per day. A finite-difference ground-water-flow model was used to simulate leakage from Scioto River, lateral flow within the glacial drift aquifer, and upflow from the carbonate bedrock aquifer into the glacial drift aquifer. The model was calibrated to steady state by use of water levels measured in March 1986 and before 1967. Simulations show that, for pumping rates of 7.5 and 18.4 million gallons per day, 7 to 9 percent of the pumpage from three collector wells originates from bedrock, 51 to 75 percent originates from glacial drift, and 16 to 42 percent originates from Scioto River. Sensitivity analyses of model input parameters showed that, at increased pumping rates, the proportion of collector-well pumpage derived from the river is increased. An increase in vertical riverbed hydraulic conductivity at values greater than 0.5 foot per day does not result in a significant change in contribution from the river. A mixing analysis of collector-well pumpage generally corroborates the percentage contributions from the glacial drift aquifer and Scioto River derived from the ground-water-flow model. A two-step mixing analysis shows that approximately 50 to 60 percent of pumpage was derived from the glacial drift aquifer and 5 to 25 percent was derived from Scioto River. At the time that water-quality samples were collected in August 1987, the pumping rate from the collector wells was approximately 15.8 million gallons per day. The contribution from the carbonate bedrock aquifer, 23 to 32 percent, is greater than that calculated by the ground-water-flow model. This difference could be due to a combination of factors, including variable data on chemical quality of the carbonate bedrock aquifer and uncertainties associated with estimates of vertical hydraulic conductivity of the glacial drift and carbonate bedrock aquifers. INTRODUCTION At present (1990), about 15 percent of the total water supply for the City of Columbus, Ohio, is provided by ground water withdrawn from the South Well Field in southern Franklin County. Four radial collector wells, completed in the glacial drift aquifer along Scioto River (three wells) and Big Walnut Creek (one well), are designed to induce stream infiltration. Much of the flow of Scioto River at the South Well Field originates from the Jackson Pike Wastewater Treatment Facility (WTF), one of two municipal wastewater-treatment plants operated by the City of Columbus. The Jackson Pike WTF discharges effluent into Scioto River about 6 mi upstream from the South Well Field (fig. 1). During periods of dry weather, effluent from the Jackson Pike WTF has comprised as much as 90 percent of the flow in the river. When the City of Columbus recently implemented a new system, the rate at which effluent discharges from the Jackson Pike WTF was reduced by as much as 150 Mgal/d since 1970 to a relatively constant rate of about 60 Mgal/d, or about 93 ft 3 /s. This reduction in treated effluent will result in reduced streamflow during future low-flow periods compared to past low-flow periods. Traveltime of the river will be affected, and, depending on upstream water quality compared to treated effluent quality, this reduction could have a positive effect (reduction of effluent loading) or a negative effect (reduction of dilution) on Scioto River quality in southern Franklin County. In addition, potential changes to low flows of Scioto River have led to questions about the proportion of water pumped at the South Well Field that originates from ground-water and surface-water sources. To address these anticipated issues, the U.S. Geological Survey (USGS), in cooperation with the City of Columbus, undertook a study to evaluate the hydrologic consequences of changes in effluent discharge to Scioto River and to clarify the interaction of Scioto River and carbonate bedrock and glacial drift aquifers at the South Well Field. Purpose and Scope This report describes the hydrology and water quality of the South Well Field area by presenting the results of (1) surfacewater-transport simulations of a conservative constituent during low flow of Scioto River from Frank Road to the South Well Field and (2) ground-water-flow simulations of the glacial drift and carbonate bedrock aquifers at the South Well Field. These simulations are examined along with surface-water and groundwater quality to describe the relation between the quantity and quality of flow in the Scioto River; the nature of the groundwater-flow system; and the quantity, quality, and sources of water pumped from the South Well Field. Data collection in the study area in August and September 1987 and 1988 included

water-resources investigations report

Hydrology and water quality near the South Well Field, southern Franklin County, Ohio, with emphasis on the simulation of ground-water flow and transport of Scioto River