USGS1998
In the spring of 1996, the Upper Mississippi River Basin Study Unit of the National Water-Quality Assessment Program drilled 30 shallow monitoring wells in a study area characterized by urban residential and commercial land uses. The monitoring wells were installed in sandy river-terrace deposits adjacent to the Mississippi River in Anoka and Hennepin Counties, Minnesota, in areas where urban development primarily occurred during the past 30 years. Analyses of sediments collected during well drilling indicated that at most well sites the aquifer materials had relatively high hydraulic conductivities (ranging from 0.01 to 238 feet per day), and relatively low organic carbon contents (0.10 to 41 grams per kilogram), indicating a high susceptibility to leaching of fertilizers and organic substances used on the land surface. Sediment pH values, which can affect leaching of pesticides, were generally alkaline, ranging from 5.1 to 9.6. Ground-water levels ranged from 2.39 to 23.14 feet below land surface, and indicated that shallow ground water flows primarily toward the Mississippi River. Dissolved-oxygen concentrations in water samples from the wells were generally less than 3 milligrams per liter (mg/L) and specific conductances were generally greater than 600 microsiemens per centimeter. Calcium, magnesium, sodium, bicarbonate, chloride, and sulfate were the primary dissolved constituents in the water samples. Sodium and chloride concentrations were generally greater than commonly reported in the region, probably due to leaching of sodium chloride applied to roads during the winter. Most tracemetal concentrations in ground-water samples were less than 10 micrograms per liter (μg/L), but concentrations of iron and manganese commonly exceeded Secondary Maximum Contaminant Levels set by the U.S. Environmental Protection Agency of 300 and 50 μg/L, respectively. Mineral saturation indices indicated that calcite, dolomite, and gypsum were slightly undersaturated in most water samples, and that quartz, and many oxides and hydroxides of iron and manganese were oversaturated in all of the water samples. Concentrations of nitrate nitrogen, the primary nutrient of concern in ground water, ranged from less than 0.05 to 16 mg/L, with a median concentration of 1.4 mg/L. Dissolved phosphorus concentrations ranged from less than 0.01 to 1.5 mg/L in the water samples, with a median concentration of 0.02 mg/L. Water from one well was oversaturated with respect to hydroxyapatite, a phosphatic mineral used as a fertilizer. Pesticide compounds were detected in water samples from 16 wells, but concentrations of those compounds were less than 1.0 μg/L. Prometon, an herbicide commonly used for right-of-way weed control, was detected in water samples from 10 of the wells. Atrazine, and its metabolite deethylatrazine, were detectable in water samples from 6 and 8 wells, respectively. Atrazine is commonly applied to land planted with corn and is detectable in rainfall and air samples in concentrations of less than 1 ng/L in the urban land use study area. Other detected pesticide compounds, which are used in agriculture, right-of-way weed control, or lawn care included: tebuthiuron, EPTC, p , p '-DDE, metolachlor, simazine, bentazon, and bromacil. Volatile organic compounds were detected in water samples from 26 wells, but the concentrations of most of those compounds were less than 1 μg/L. Carbon disulfide, which may be produced by bacteria in soils, was the most commonly detected volatile organic compound in water samples from the wells. Other detected volatile organic compounds included: methyl chloride, acetone, dichlorofluoromethane, tetrahydrofuran, trichlorofluoromethane, methyl iodide, 1,1-dichloroethane, chloroform, toluene, trichloroethane, trichloroethene, cis -1,2-dichloroethene, methylene chloride, bromodichloromethane, benzene, methylisobutylketone, ethyl ether, and tetrachloroethene. Tritium concentrations, analyzed in water samples from 15 of the 30 wells, indicated that shallow ground water has been recharged since the mid-1950's, and that shallow ground water should be affected by urban development that has taken place over the past 40 years. Land uses in the urban land use study area affected the concentrations of some water-quality constituents. Concentrations of nitrate and chloride, and frequencies of detection of pesticides and of volatile organic compounds, were greater in water samples from the surficial sand and gravel aquifer underlying the urban land use study area than in water samples from similar aquifers from part of the Upper Mississippi River Basin National Water-Quality Assessment study unit. Land uses within 500-meter radii of each well were quantified by digitizing overlays of aerial photographs that were verified and updated in the field. Concentrations of magnesium and sulfate were greater in ground water beneath areas of denser residential development, which may be a natural artifact of better drainage and a deeper water table in those areas. Frequencies of detection of some pesticides and volatile organic compounds were greater in water from wells with greater proportions of industrial and transportation land uses. Ground water in areas with less dense residential development, mostly the more recently-developed areas, tended to have greater concentrations of agricultural herbicides and some nutrients probably a relict of previous agricultural land use.