North Branch Milwaukee River Watershed, Sheboygan County, Wisconsin (Till Land-Use Study Area)
The focus of this Study was to understand variations in ground-water quality (mainly nutrients and pesticides) along shallow ground-water flowpaths in an agricultural area representative of the till land-use study area (area 1). In 1994 and 1995, water-quality and geohydrologic data were collected from 19 monitor wells and a stream in an agricultural area in southeastern Wisconsin (click thumbnail to the right).
These sites were located along a 2,700-ft transect from a local ground-water high to the stream (see figure 3, Saad and Thorstensen, 1998). The transect is approximately parallel to the horizontal direction of ground-water flow at the water table. Most of the wells were installed in unconsolidated deposits at five locations along the transect and include an upgradient well nest, a midgradient well nest, a downgradient well nest, wells in the lowland area near the stream, and wells installed in the stream bottom.
The data collected from this study site were used to describe the water quality and geohydrology of the area and to explain and model the variations in water chemistry along selected ground-water flowpaths.
Water samples from most wells and the stream were analyzed for major ions, nutrients, pesticides, dissolved organic carbon, aluminum, tritium, CFCs, 15N, 18O, and dissolved gases. Measurements of temperature, pH, specific conductance, and dissolved oxygen were made in the field. Concentrations of all dissolved constituents were below Wisconsin ground-water quality enforcement standards. The concentrations of both nitrate and ammonium in precipitation concentrated by evapotranspiration are roughly equal to the concentrations of either in the shallow ground waters. The nitrogen and oxygen isotope data, however, indicate that soil ammonium, ammonium fertilizer, and animal waste are possible nitrate sources. Concentrated precipitation can also supply dissolved sulfate to the shallow ground waters and may be a principal source of pesticides to the ground water. However, some input of dissolved chloride to the ground water from mineral or anthropogenic sources is necessary.
X-ray diffraction analyses of samples from 2 cores show the most abundant mineral to be dolomite, with subordinate quartz, microcline, and plagioclase, and minor amounts of mica, hornblende, and chlorite. Hydraulic conductivities determined from slug tests at selected wells range from 0.006 to 55 feet per day, with most values between 0.4 and 12 feet per day.
A cross-sectional ground-water flow model, representing the water-table flow system, was developed for the site and was used to identify possible ground-water flowpaths for geochemical modeling. The model was calibrated against measured water levels and was most sensitive to variation in recharge and hydraulic conductivity. The calibrated model shows that downward flow from shallow to deeper wells within a nest may occur at the upgradient and midgradient well nests, but that flow from each well nest travels beneath downgradient nests to the stream. Pathline and travel-time analysis performed on the calibrated flow-model output yielded travel times that range from 5.8 to 59 years with a recharge of 4 inches per yr. Recharge dates based on tritium and CFC concentrations range from pre-1955 to 1986 and are consistent with flowpaths and travel times in the calibrated flow model.
Changes in water quality along ground-water flowpaths were evaluated using the geochemical model PHREEQC. Geochemical mole balance models of shallow ground-water formation show that the principal reaction, by an order of magnitude, is dissolution of dolomite with CO2. Concentration factors in the mole-balance models range from 1 to 11, with most values between 5 and 10, which provides independent support for the concentration factor of 8 based on recharge estimates used in the flow model.
Ground water recharging at mid- and downgradient wells is oxic and contains dissolved nitrate, whereas the ground water discharging to the stream is anoxic and contains dissolved ammonium. Redox environments were defined at each well on the basis of relative concentrations of various dissolved redox-active species. Chemically permissible flowpaths inferred from the observed sequence of redox environments at well sites are consistent with flowpaths in the ground-water flow model. The transition from nitrate in recharging ground water to ammonium in ground water discharging to the stream suggests the possibility of nitrate reduction along the flowpath. None of the techniques employed in this study, however, were able to prove the occurrence of this reaction.
Water-quality data for this flowpath study can be found on the Data page. The data is also included in Saad (1998).
Tomorrow River Watershed, Portage County, Wisconsin (Sand & Gravel Land-Use Study Area)
In Cycle 2, a second flowpath study was conducted in the WMIC. The specific objective of the study was to address the question: "How have concentrations of nitrate and selected pesticides in ground water changed over the past 50 years at selected NAWQA study sites?" (Larry Puckett, USGS, written comm., March 14, 2003). The answer to this question, relative to this flowpath study area, is described in detail by Saad (2008). The paper describes trends in ground-water quality based on data collected from the Area 2 land-use and flowpath studies. A brief description of the study area and selected results are included below.
The combination of agricultural land use and permeable surficial deposits in land-use area 2, made it a suitable location for this type of study. The Tomorrow River Watershed was chosen for study because it was nested within and fairly representative of land-use study area 2 (click thumbnail to right or see figures 1 and 2 in Saad, 2008). The study site is located on the west side of the Tomorrow River about 3 km upstream from the confluence with Pancho Creek and includes 4 nests of monitor wells installed along a ground-water flowpath (as inferred from ground-water flow simulations). The well nests are located upgradient from the Tomorrow River at distances of 65 m (well nest 4), 220 m (well nest 3), 600 m (well nest 2), and 1500 m (well nest 1). These well nests represent the downgradient part of the entire flowpath length that discharges to the stream at this location. The full length of the flowpath is about 3.5 to 4.5 km (based on ground-water simulations). Water samples were analyzed for major ions, nutrients, organic carbon, pesticides, dissolved gases, and chlorofluorocarbons (CFCs). CFCs were used for ground-water age dating. Field measurements of temperature, pH, specific conductance, and dissolved oxygen, were collected.
The dominant ions in ground-water samples collected from the study site (from the 17 monitor wells and 2 mini-piezometers) were calcium, magnesium, and bicarbonate. Dissolved oxygen concentrations ranged from 3.1 to 9.7 mg/L, indicating that the entire aquifer system is oxygenated at this location. Nitrate concentrations ranged from 1.4 to 11.9 mg/L, while ammonia and nitrite concentrations were generally less that the laboratory reporting limits. Nitrate concentrations generally decrease with age and are correlated with historical fertilizer use.
Six pesticides or metabolites were detected in ground-water samples. These included acetachlor ESA, alachlor ESA, atrazine, deethyl atrazine, metolachlor ESA, and simazine. Atrazine and deethyl atrazine were detected in nearly all samples. Alachlor ESA and Metolachlor ESA were detected in slightly less than half of the samples. The highest concentrations were generally in the shallow and upgradient parts of the study area. Concentrations of all pesticides were below drinking-water standards.
Ground-water recharge dates (based on CFCs) ranged from 1964 to 1996. Ground-water age generally increases with depth and with distance along the flowpath and generally correlates well with expected relative residence times based on flow directions determined from measured heads. Recharge dates also correlate well with pesticide tracers like atrazine and metolachlor. Atrazine concentrations by recharge date are correlated to historical atrazine use. Additionally, metolachlor detections generally match recharge dates that correspond to the period after metolchlor was introduced (1977).
Water-quality data for this flowpath study can be found under Data. The data is also included in Saad (2008).
Saad, David A., 2008, Agriculture-Related Trends in Groundwater Quality of the Glacial Deposits Aquifer, Central Wisconsin: Journal of Environmental Quality 37:S-209-S-225, doi:10.2134/jeq2007.0053
Saad, D.A., and Thorstenson, D.C., 1998, Ground-water flow and geochemistry along shallow ground-water flowpaths in an agricultural area in southeastern, Wisconsin: U.S. Geological Survey Water-Resources Investigations Report 98-4179 62 p.