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The hydrology and quality of ground water were evaluated for the surficial sand and carbonate aquifers in northwestern Ohio. A locally important surfrcial sand aquifer in western Lucas County was evaluated on the basis of data from 10 wells completed in undeveloped and developed areas. The carbonate aquifer in Silurian and Devonian bedrock at its northernmost extent on the Ohio mainland was evaluated on the basis of data from previous studies and data from 466 wells and 11 springs. Most data are for the period 1985-88. The unconfined surficial sand aquifer is less than 50 ft (feet) thick. Clay-rich drift, which restricts vertical movement of water, underlies the aquifer. Recharge is from precipitation, and discharge is by evapotranspiration and by flow to local streams and drainage ditches. Water levels are generally 2 to 8 ft below land surface and fluctuate a total of about 3.5 ft seasonally in a forested area. Concentrations of iron and manganese in ground water are excessive in some areas. Waters from shallow drive-point wells in residential areas contained larger concentrations of dissolved solids, hardness, sodium, and chloride than did waters from identical wells in undeveloped areas. The presence of nitrate nitrogen and other selected constituents in ground water in residential areas, and the absence of these constituents in ground water in undeveloped areas, indicate that the surficial sand aquifer has been affected by development. In the carbonate aquifer, fractures, bedding-plane joints, and other secondary openings are the principal water-bearing zones. These zones can be areally and stratigraphically separated by low-permeability rock. Leaky artesian or semiconfined conditions predominate beneath most of the 1,400-mi 2 study area. The aquifer is confined by relatively impermeable underlying shale of Silurian age and overlying clay-rich drift of Quaternary age. Unproductive strata, including evaporites, within the sequence of carbonate rocks also confine some water-bearing zones. The carbonate aquifer is part of a regional ground-waterflow system; however, subsystems such as the eastern karst and central outcrops are locally important. The potentiometric surface indicates that recharge from areas south and west of the study area flows toward discharge areas along major rivers (Maumee, Portage, and Sandusky), to a buried bedrock valley in central Sandusky County, and to springs and flowing wells. The potentiometric surface flattens markedly near the southern shore of Lake Erie, where ground-water levels approximate those of the lake, indicating a hydraulic connection between the lake and the aquifer. Hydrogeologic characteristics and water-quality data indicate that Lake Erie is not a major source of recharge to the aquifer. Ground-water ages inferred from tritium concentrations and potentiometric-surface maps indicate that recharge from precipitation enters the aquifer by subsurface drainage in karstified strata in eastern Sandusky County and by infiltration in shallow bedrock areas where drift is less than 20 ft thick. The quality of water in the carbonate aquifer is described with reference to 52 properties and constituents that characterize chemical, radiochemical, bacteriologic, and physical conditions. Ground-water samples from 135 wells and 11 springs are used in the characterization. On the basis of these data, water from the aquifer is generally suitable for drinking and for most domestic purposes. The most areally widespread aesthetic factors limiting the use of ground water are hardness, concentrations of dissolved solids, sulfate, and iron, and the presence of hydrogen sulfide. Selected bacteria are commonly present and may compromise the potability of water from the aquifer. Coliform bacteria from surface sources were found in 47 of 143 water samples. Analyses for total coliform bacteria indicate that 36 of the 125 samples from wells maintained for potable supply have bacteria counts of 4 or more colonies per 100 mL counts that are bacteriologically unsafe. Concentrations of alphaand beta-particle radioactivity equaled or exceeded 5 pCi/L in many areas. The largest concentrations of beta-particle radioactivity are in waters with large potassium concentrations. The trace elements selenium, silver, lead, antimony, cadmium, and copper are rarely detected in the samples analyzed. Concentrations detected are generally less than 5 pg/L and never exceeded the SMCL or MCL's for these elements. Arsenic, chromium, lithium, mercury, barium, nickel, aluminum, and zinc are commonly detected in the samples analyzed. Concentrations of 10 yg/L or greater are commonly reported for zinc, lithium, and barium. Few aluminum and nickel concentrations exceed 10 pg/L. Few arsenic and mercury concentrations exceed 2 yg/L. Chromium concentrations greater than 50 yg/L are reported in 3 of 54 wells sampled. Volatile-organic compounds in concentrations greater than 3 yg/L were detected in only 1 of the 45 wells sampled. Cyanide in concentrations greater than 10 yg/L was not detected in any of the 48 wells sampled. Variations in water quality are related to the geochemical makeup of rock units, the thickness of drift overlying the aquifer, and past and current uses of land in areas of shallow bedrock. The presence or absence of calcium sulfate minerals in the rock causes a bimodal distribution of concentrations of dissolved solids, hardness, and sulfate in waters from the Bass Islands Group. Dissolution of calcium sulfate minerals contributes to excessive concentrations of sulfate that approach 2,000 mg/L. Sulfate reduction probably contributes to excessive hydrogen sulfide concentrations in some sulfate-rich waters. Waters derived solely from the Lockport Dolomite are relatively dilute and are a calcium magnesium bicarbonate type. Strontium concentrations as large as 50 mg/L characterize sulfate-poor waters and are the result of dissolution of strontium-bearing minerals in the aquifer matrix. Shale mineralogies naturally soften water and increase sodium concentrations, most notably in western Lucas County. BrrCl ratios were useful in tracing the source of large chloride concentrations. Sources were the dissolution of salt in evaporite-bearing strata and brine produced by oil and gas development. Ground water in shallow bedrock areas is most likely to indicate effects of past and current uses of the land. Concentrations of nitrogen, phosphorus, chloride, and dissolved-organic carbon generally are elevated only in areas of shallow bedrock.

Geohydrology and quality of water in aquifers in Lucas, Sandusky, and Wood counties, northwestern Ohio