Selected geochemical characteristics of ground water from the Marshall aquifer in the central Lower Peninsula of Michigan

Chemistry and stable-isotope data for water from wells completed in the Marshall aquifer within the Michigan Basin were used to prepare maps that show area! variations of δ¹⁸O; distribution of dissolved solids, dissolved chloride, dissolved iron, and dissolved sulfate; and distribution of hydrochemical facies. Delta oxygen-18 values indicate the presence of modern meteoric water (δ¹⁸O approximately -10 parts per mil) as well as isotopically light meteoric water (δ¹⁸O less than -15 parts per mil). Isotopically light ground water is present in the Michigan Lowland. Dissolved-solids concentrations range from 273 to 412,000 milligrams per liter, and dissolved-chloride concentrations range from less than 10 to greater than 250,000 milligrams per liter. Dissolved-solids and dissolved-chloride concentrations increase toward the center of the study area. Dissolved-iron concentrations which range from less than 0.002 to 100 milligrams per liter, are low in the subcrop areas, and increase toward the center of the study area. Dissolved-sulfate concentrations range from 4.5 to 3,500 milligrams per liter and generally increase then decrease from the subcrop area toward the center of the study area. Most ground water in the Marshall aquifer is classified as either a calcium bicarbonate or a sodium chloride type.