Central Sand Plain Research Articles

Measuring sand content using sedimentation, spectroscopy, and laser diffraction

Sand (2,000–50 µm), silt (50–2 µm) and clay (<2 µm) have been determined by sedimentation methods (i.e., hydrometer and pipette), but increasingly laser diffraction and visible and near-infrared (vis-NIR) or mid-infrared (MIR) spectroscopy are being used. It is necessary to understand the comparability and limitations of these methods. We compared these different methods using 121 soil samples from the Central Sand Plains of Wisconsin, for total sand content as well as five sand fractions (very coarse, coarse, medium, fine, and very fine). Spectroscopic methods are not hindered by assumptions of sphericity or constant particle density; yet these model-based methods require calibration by laboratory methods that do rely on these assumptions. Vis-NIR and MIR spectroscopic model predictions of total sand content differed when calibration laboratory data were obtained from the hydrometer and pipette method. The laser method was not able to accurately measure sand fractions and the fractions were poorly to moderately predicted by vis-NIR (R2 = 0.01 to 0.61) but slightly better using MIR (R2 = 0.23 to 0.71). The hydrometer method overestimated the total sand content compared to the pipette; the laser method was only moderately correlated to both pipette and hydrometer. Spectroscopic modeling of total sand content was very good (R2 = 0.91 to 0.94), but results were affected by the laboratory calibration method and models calibrated with reference hydrometer data overestimated total sand content. We conclude that MIR predictions of sand content provide the best results, but models must be calibrated with the pipette method for soils with high sand contents.

Predicting the color of sandy soils from Wisconsin, USA

Color is an important soil property and often used to infer soil properties and delineate soil horizons. We investigated the effect of soil texture, carbon, total elements, and particle-size fractions on the color of sandy soils using five color models. In total, 915 soil samples were collected to a maximum depth of 220 cm at 400 locations in the Wisconsin Central Sand Plains, and the soils were sandy throughout. The samples were scanned using a visible-near infrared spectrometer, with soil color models (HSV, RGB, CIE L*a*b*, CIE L*u*v*, and redness index (RI)) extracted from the reflectance spectra. Cubist models were used to predict each of the soil color coordinates from the soil properties. The models showed high prediction accuracy for V, R, SRGB, L*, a*, b*, u*, and v* color coordinates for both calibration and validation. The CIE L*a*b* color model was generally better than other color models. Silt and Fe were used in all of the Cubist models, while sand, clay, carbon, Al, Si, Mn, and Zn were used in most models. The 45–125 µm, 125–250 µm, and 1000–2000 µm fractions affected the soil color as opposed to the 250–500 µm and 500–1000 µm fractions. A clustering analysis on the CIE L*a*b* color model showed that soil lightness was higher with higher sand content, but lower with an increase in silt and clay content, carbon, Fe, Al, Zn, and Mn. The LUCAS dataset has approximately 20,000 soil samples and was used to explore the relationships between soil color and soil properties in sandy soils and to test the robustness of the model established with the Central Sands dataset. The Cubist models developed from either dataset (Central Sands, LUCAS) were not useful to predict CIE L*a*b* color using the other dataset.

Spatial-temporal analysis of soil water storage and deep drainage under irrigated potatoes in the Central Sands of Wisconsin, USA

Knowledge of soil water storage and deep drainage is important for improving irrigation efficiency, maximizing crop water use, and understanding groundwater table fluctuation. This is particularly important in sandy soils that depend on irrigation to produce high crop yields. In sandy soils under potato in the Wisconsin Central Sand Plains, soil water storage using tension probes was measured in three irrigation zones over the 2014 and 2015 growing seasons. Soil water storage was estimated across a 78 ha field using apparent electrical conductivity maps. Deep drainage was estimated using the Richards’ equation and Hydrus-1D software. It was found that the average soil water storage ranged from 74 to 110 mm in the top 0.45 m in three irrigation zones in 2014, and from 70 to 95 mm in 2015. Rainfall and irrigation was 387 and 269 mm in 2014, and 328 and 281 mm in 2015. Estimated deep drainage was uniform in three irrigation zones, and ranged from 222 to 244 mm in 2014, and from 167 to 180 mm in 2015. A negative correlation was found between soil water storage and potato yields possibly due to over-irrigation. The methods used in this study can be applied to improve irrigation and water use efficiency.

Predicting Nitrate Retention at the Groundwater‐Surface Water Interface in Sandplain Streams

AbstractGroundwater‐surface water ecotones present opportunities for nitrate retention because changes in organic carbon availability, redox potential, and nitrate demand often occur at these locations. Although there have been many measurements of nitrate retention and denitrification at the groundwater‐surface water interface, few investigators have quantitatively predicted where nitrogen transformation occurs in this zone. Our main objective was to describe and predict spatial variation in nitrate retention and removal in shallow groundwater among and within streams in the central sand plains of Wisconsin. We predicted that nitrate transformation rates would be positively related to nitrate and particulate organic carbon availability, a negative nonlinear function of dissolved oxygen availability, and would peak at intermediate rates of groundwater discharge. Five sites on four streams were chosen that spanned an order of magnitude in groundwater nitrate concentration. Nitrate retention and N2 production were quantified by determining how nitrate and N2 fluxes changed along nominal 55‐cm groundwater flow paths. Nitrogen transformation was widespread but highly variable within the study sites. Partial least squares regression models explained a much larger amount of variation in nitrate retention and N2 production at the local scale than at the regional scale. Dissolved oxygen availability and groundwater discharge were the most consistent predictors of nitrate retention at both scales. We conclude that nitrate retention at the groundwater‐surface water interface can be predicted but that more research is necessary to determine if nitrate transformation at this ecotone can be modeled at broader scales.

Fire History at the Confluence of the Driftless Area and Central Sand Plains of Wisconsin: A Case Study from Castle Mound Pine Forest State Natural Area

Castle Mound Pine Forest State Natural Area (CMPF) is a 48-ha reserve at the confluence of the Driftless Area and Central Sand Plains of Wisconsin. Here, we report the first tree-ring—based fire history study for central Wisconsin and examine the relationships among fire, forest structure and composition, and historical land use at the site. We crossdated 12 fire-scar samples from Pinus resinosa stumps and inventoried and cored 83 trees along four transects to quantify the fire history, forest composition, and forest age structure at the site. The fire history data span the years 1788–2006 and include 15 years when trees were scarred by a fire on the site. Most fire scars were recorded in the earlywood of the recording growth ring, suggesting spring or early summer fires. The mean fire return interval for the site was six years when calculated between the first fire in 1841 and the last fire in 1923. The time-since-fire at the time of our study was 91 years. The canopy of the site was dominated by P. resinosa, individuals of which represent the oldest trees on the site. The subcanopy and recent age structure was dominated by mesic species, indicating that the forest at CMPF is transitioning away from the historical dominance by P. resinosa to a more closed forest following the cessation of fire at the site. Our data highlight an opportunity to establish ecological baseline data for this area to inform fire management and restoration activities.

Long‐term shifts in the patterns and underlying processes of plant associations in Wisconsin forests

AbstractAimPlant species co‐occur within communities in response to variation in environmental conditions, limited species dispersal and biotic interactions. We used surveys and resurveys of the same sites of three temperate forest plant communities to study patterns of association between co‐occurring species pairs and to infer how these mechanisms contribute to community assembly. Our goal was to compare these forces among communities occupying more and less disturbed landscapes and examine how these have changed over the last 50 years.LocationWisconsin, USA.MethodsWe resurveyed 266 sites first surveyed in the 1950s to assess the patterns and dynamics of co‐occurrence among understorey plant species in three community types. We then used checkerboard scores, null models and a newly developed framework to infer the mechanisms likely to have driven community assembly. Finally, we compared these across the three communities and two time periods.ResultsSpecies co‐occur less often than expected in all three vegetation types and both periods. We detected high fractions of aggregated and segregated species pairs (up to 14% and 17%, respectively). In the fragmented southern upland forests and central sand plains, both biotic interactions and dispersal limitation may play important roles in community assembly with inferred dispersal limitation becoming more important since the 1950s. In the more continuous and intact northern upland forests, environmental filtering and biotic interactions appear to dominate community dynamics with little change over time. Aggregated and segregated species pairs made a similar contribution to our ability to infer these mechanisms.Main conclusionsEnvironmental filtering, biotic interactions and dispersal limitation all appear to affect plant community structure in both time periods. However, the influence of dispersal limitation seems to be increasing in more fragmented forest landscapes, portending shifts in community composition and dynamics. Because aggregated and segregated species pairs may be shaped by similar processes both can be used to infer processes of community assembly.

Groundwater Quantity and Quality Issues in a Water-Rich Region: Examples from Wisconsin, USA

The State of Wisconsin is located in an unusually water-rich portion of the world in the western part of the Great Lakes region of North America. This article presents an overview of the major groundwater quantity and quality concerns for this region in a geologic context. The water quantity concerns are most prominent in the central sand plain region and portions of a Paleozoic confined sandstone aquifer in eastern Wisconsin. Water quality concerns are more varied, with significant impacts from both naturally occurring inorganic contaminants and anthropogenic sources. Naturally occurring contaminants include radium, arsenic and associated heavy metals, fluoride, strontium, and others. Anthropogenic contaminants include nitrate, bacteria, viruses, as well as endocrine disrupting compounds. Groundwater quality in the region is highly dependent upon local geology and land use, but water bearing geologic units of all ages, Precambrian through Quaternary, are impacted by at least one kind of contaminant.

Groundwater recharge under compacted agriculturalsoils, pineand prairie in Central Wisconsin, USA

The water table in Wisconsin Central Sand Plain (CSP) dropped 1.8 m between 2002 and 2010, and several lakes in the area suffer from low water levels. The lower groundwater level has been attributed to agricultural cropping practices, specifically irrigation, and a reduction in ground-water recharge. Dominant soils in this area are sands (Udipsamments). The objective of our research was to quantify groundwater recharge under (i) irrigated agricultural crops, (ii) prairie, and (iii) a 50-year old pine tree plantation in the CSP. Equipment was installed at five sites to monitor water table elevation, soil water content, and precipitation at 15-minute intervals. It was found that, when the soil was at field moisture capacity, precipitation during the growing season resulted in 1.4 cm more water table rise under a prairie than under irrigated agricultural fields. Agricultural crops used groundwater through irrigation, but natural vegetation relied on soil available water, and capillary rise of water from the shallow groundwater table (1 to 2 m to water table), for daily transpiration. After snowmelt, prairie vegetation yielded greater rise, up to 16 cm, in the water table than agricultural fields. Lack of crop residue on the soil surface of agricultural fields resulted in a continuous layer of frost in the soil profile that extended to about a meter depth. This thick, frozen layer was enhanced by greater soil compaction under irrigated crops compared to limited or no compaction in prairie areas. The key finding was that this deep frost in the soil profile inhibited snowmelt water from infiltrating and recharging the groundwater. Thus, compacted-irrigated agricultural soils in the CSP alter groundwater recharge characteristics during frozen and non-frozen ground periods. Increased crop residues on the surface of agricultural fields might enhance groundwater recharge during winter snow melt periods.

Groundwater recharge under compacted agriculturalsoils, pineand prairie in Central Wisconsin, USA

The water table in Wisconsin Central Sand Plain (CSP) dropped 1.8 m between 2002 and 2010, and several lakes in the area suffer from low water levels. The lower groundwater level has been attributed to agricultural cropping practices, specifically irrigation, and a reduction in ground-water recharge. Dominant soils in this area are sands (Udipsamments). The objective of our research was to quantify groundwater recharge under (i) irrigated agricultural crops, (ii) prairie, and (iii) a 50-year old pine tree plantation in the CSP. Equipment was installed at five sites to monitor water table elevation, soil water content, and precipitation at 15-minute intervals. It was found that, when the soil was at field moisture capacity, precipitation during the growing season resulted in 1.4 cm more water table rise under a prairie than under irrigated agricultural fields. Agricultural crops used groundwater through irrigation, but natural vegetation relied on soil available water, and capillary rise of water from the shallow groundwater table (1 to 2 m to water table), for daily transpiration. After snowmelt, prairie vegetation yielded greater rise, up to 16 cm, in the water table than agricultural fields. Lack of crop residue on the soil surface of agricultural fields resulted in a continuous layer of frost in the soil profile that extended to about a meter depth. This thick, frozen layer was enhanced by greater soil compaction under irrigated crops compared to limited or no compaction in prairie areas. The key finding was that this deep frost in the soil profile inhibited snowmelt water from infiltrating and recharging the groundwater. Thus, compacted-irrigated agricultural soils in the CSP alter groundwater recharge characteristics during frozen and non-frozen ground periods. Increased crop residues on the surface of agricultural fields might enhance groundwater recharge during winter snow melt periods.

Macrophyte beds contribute disproportionately to benthic invertebrate abundance and biomass in a sand plains stream

Macrophyte beds have been shown to influence organic matter retention and nutrient processing in streams. Less is known about the extent to which plant beds contribute to abundance, biomass, and diversity of macroinvertebrate assemblages in low-order streams. We measured aquatic invertebrate abundance, biomass, and diversity associated with plant beds and sand/gravel patches in a low-gradient second-order stream in the Central Sand Plains of Wisconsin, USA from March to October. Invertebrate abundance and biomass were higher on average in plant beds (2,552 m−2 and 1,575 mg m−2) than in sand/gravel patches (893 m−2 and 486 mg m−2). Although sand/gravel habitat was over three times more abundant than plant beds in the study reach, plant beds and sand/gravel patches contributed similarly to invertebrate abundance and biomass at the whole-reach scale. The abundance and biomass of invertebrates associated with plant beds decreased from spring to autumn. Non-insect invertebrates in the plant beds increased in relative abundance as the year progressed. Shannon–Weiner diversity and taxa richness of invertebrates were higher in the plant beds than in the sand/gravel habitat. Our results suggest that plant beds can represent hot spots for invertebrate abundance and production in low-gradient streams, and have implications for stream management and restoration in these types of ecosystems.

Groundwater nitrate contamination costs: A survey of private well owners

Groundwater is an important source of drinking water in Minnesota and nation-wide. In Minnesota, 5% to 10% of drinking water wells have nitrate (NO₃) concentrations that exceed health standards. Well owners incur direct costs associated with the presence of NO₃, including costs related to treatment systems, well replacement, and purchasing of bottled water. The objective of this study was to quantify actual amounts spent by private well owners when NO₃ levels are elevated, regardless of whether the owners are aware of the contamination. Survey questionnaires asking about well characteristics, NO₃ testing, and costs of actions taken in response to elevated NO₃ were mailed to 800 private well owners in the central sand plains of Minnesota. Sixty percent of recipients returned surveys and then were sent water sampling bottles, of which 77% were returned. Nitrate was determined in the returned water samples. About 6% of wells tested greater than the US Environmental Protection Agency health standard maximum of 10 mg L^-1 (10 ppm) nitrate-nitrogen. Less than one-third of respondents had tested their water for NO₃ within the past three years. Average remediation costs were $190 y^-1 to buy bottled water, $800 to buy a NO₃ removal system plus $100 y^-1 for maintenance, and $7,200 to install a new well. Of well owners with nitrate-nitrogen over 10 mg L^-1, 24% bought bottled water, 21% installed treatment systems, 24% installed new wells, and 31% were unaware of the contamination and took no actions. Water resource planners can compare the costs described in this study to the costs of preventing aquifer contamination through education and technical and financial support. This study also demonstrates a method for representative sampling of private wells without on-site visits, and the continued need for educational programs related to routine testing.

Groundwater nitrate contamination costs: A survey of private well owners

Groundwater is an important source of drinking water in Minnesota and nation-wide. In Minnesota, 5% to 10% of drinking water wells have nitrate (NO₃) concentrations that exceed health standards. Well owners incur direct costs associated with the presence of NO₃, including costs related to treatment systems, well replacement, and purchasing of bottled water. The objective of this study was to quantify actual amounts spent by private well owners when NO₃ levels are elevated, regardless of whether the owners are aware of the contamination. Survey questionnaires asking about well characteristics, NO₃ testing, and costs of actions taken in response to elevated NO₃ were mailed to 800 private well owners in the central sand plains of Minnesota. Sixty percent of recipients returned surveys and then were sent water sampling bottles, of which 77% were returned. Nitrate was determined in the returned water samples. About 6% of wells tested greater than the US Environmental Protection Agency health standard maximum of 10 mg L^-1 (10 ppm) nitrate-nitrogen. Less than one-third of respondents had tested their water for NO₃ within the past three years. Average remediation costs were $190 y^-1 to buy bottled water, $800 to buy a NO₃ removal system plus $100 y^-1 for maintenance, and $7,200 to install a new well. Of well owners with nitrate-nitrogen over 10 mg L^-1, 24% bought bottled water, 21% installed treatment systems, 24% installed new wells, and 31% were unaware of the contamination and took no actions. Water resource planners can compare the costs described in this study to the costs of preventing aquifer contamination through education and technical and financial support. This study also demonstrates a method for representative sampling of private wells without on-site visits, and the continued need for educational programs related to routine testing.

Late Pleistocene dune construction in the Central Sand Plain of Wisconsin, USA

Wisconsin's Central Sand Plain east of the Wisconsin River is composed of eolian sand forming high-relief dunes surrounded by sand sheets and scattered low-relief dunes. To establish a maximum age for dune formation, three samples for optical dating were taken from glacial Lake Wisconsin lacustrine sediment that underlies eolian sand. These age estimates range from 19.3 to 13.6ka. Age estimates taken from within or at the base of the dunes range from 14.0 to 10.6ka. Samples taken from < 2m of the ground surface were slightly younger, indicating dunes were stabilized between 11.8 and 5.5ka. The younger ages near the surface of some dunes were most likely the result of pedoturbation or localized problems with applying the optical dating method. The majority of the optical age estimates from dunes (18 of 21) indicated that most of the dunes were active between 14 and 10ka and that most dune activity ended by 10ka. These ages suggest that localized activity on dune crests may have occurred in the Holocene but would have been limited to < 1m of sand accumulation. The timing of dune activity and the lack of any significant Holocene reactivation suggest that dune activation in this setting cannot be attributed solely to changes in aridity. Instead, we attribute dune formation to changes in sediment availability from either sand inputs from the Wisconsin River or the melting of permafrost.

Nitrate impacts on groundwater from irrigated-vegetable systems in a humid north-central US sand plain

Groundwater is frequently susceptible to nitrate pollution in irrigated regions possessing a humid climate, coarse soil, and shallow water table. Such pollution degrades local drinking water resources and increases watershed nitrate export. The irrigated-vegetable production agroecosystem of the Wisconsin Central Sand Plain (WCSP), north-central USA, exemplifies the problem. This study’s research goal was to assess groundwater nitrate loading in this agroecosystem, with a view to manage groundwater quality and nitrate export in the WCSP and similar regions. Nitrate loading was measured beneath a 44 ha field for 4 years using a novel “water-year” method, during three crops of sweet corn ( Zea mays L.) and one of potato ( Solanum tuberosum L.). Nitrate-N concentrations averaged 20 mg l −1 in shallow (upper 3 m) groundwater beneath the study field. Nitrate-N loading from the sweet corn ranged from 126 to 169 kg ha −1 per year; loading from potato was 228 kg ha −1 per year. Groundwater loading amounted to 61% of total available N and 77% of fertilizer N. Measured loadings compared well with those calculated using a budget approach, supporting the validity of both methods. N budgets were calculated from average regional harvests and two fertilizer rates: a typical grower rate, and the more conservative University Extension rate. Budget-derived nitrate-N loadings from sweet corn are 151 kg ha −1 per year (typical) and 119 kg ha −1 per year (University Extension). For potato, typical and recommended fertilizer rates are equal, and the budget-derived nitrate-N loading is 203 kg ha −1 per year. Budget-derived loadings imply that limiting the basin-averaged nitrate-N concentration in groundwater to 10 mg l −1 (the US drinking water standard) would require each irrigated-vegetable hectare to be offset by 4.5–6.5 ha of land supplying nitrate-free groundwater recharge. Further, a WCSP watershed with half irrigated-vegetable cover, even with no other nitrate sources, would export 50–74 kg ha −1 per year of nitrate-N. This export is greater on a per-hectare basis than the large maize- and soybean-producing basins of Iowa, USA. Most orthodox nitrate control strategies (e.g., decreasing and splitting fertilizer, irrigation scheduling) have already been implemented in the WCSP, and have little further potential to substantially reduce loading if profitability must be maximized. Better control will require new and unconventional approaches for controlling nitrate leaching from mineralized crop residue as well as additional curbing of direct fertilizer nitrogen leaching.

A digital procedure for ground water recharge and discharge pattern recognition and rate estimation.

A digital procedure to estimate recharge/discharge rates that requires relatively short preparation time and uses readily available data was applied to a setting in central Wisconsin. The method requires only measurements of the water table, fluxes such as stream baseflows, bottom of the system, and hydraulic conductivity to delineate approximate recharge/discharge zones and to estimate rates. The method uses interpolation of the water table surface, recharge/discharge mapping, pattern recognition, and a parameter estimation model. The surface interpolator used is based on the theory of radial basis functions with thin-plate splines. The recharge/discharge mapping is based on a mass-balance calculation performed using MODFLOW. The results of the recharge/discharge mapping are critically dependent on the accuracy of the water table interpolation and the accuracy and number of water table measurements. The recharge pattern recognition is performed with the help of a graphical user interface (GUI) program based on several algorithms used in image processing. Pattern recognition is needed to identify the recharge/discharge zonations and zone the results of the mapping method. The parameter estimation program UCODE calculates the parameter values that provide a best fit between simulated heads and flows and calibration head-and-flow targets. A model of the Buena Vista Ground Water Basin in the Central Sand Plains of Wisconsin is used to demonstrate the procedure.

Nitrate and chloride loading to groundwater from an irrigated north-central U.S. sand-plain vegatable field.

Groundwater pollution and associated effects on drinking water have increased with the expansion of irrigated agriculture in north-central U.S. sand plains. Controlling this pollution requires an ability to measure and predict pollutant loading by specific agricultural systems. We measured NO3 and Cl loading to groundwater beneath a Wisconsin central sand plain irrigated vegetable field using both a budget method and a new monitoring-based method. By relying on frequent monitoring of shallow groundwater, the new method overcomes some limitations of other methods. Monitoring-based and budget methods agreed well, and indicated that loading to groundwater was 165 kg ha(-1) NO3-N and 111 kg ha(-1) Cl for sweet corn (Zea mays L.) in 1992, and 228 kg ha(-1) NO3-N and 366 kg ha(-1) Cl for potato (Solanum tuberosum L.) in 1993. Nitrate N loading was 56 to 60% of available N, or 66 to 70% of fertilizer N. Sweet corn NO3 loading was about typical for this region, but potato NO3 loading was probably 50% greater than typical because heavy rains provoked extra fertilizer application. Our results imply that typical NO3-N loading would be 119 kg ha(-1) for sweet corn and 203 kg ha(-1) for potato, even with strict adherence to University Extension fertilizer recommendations. To keep average groundwater NO3-N within the 10 mg L(-1) U.S. drinking water standard, each irrigated vegetable field would need to be offset by five to eight times as much land supplying NO3-free groundwater recharge.

Groundwater Quality Beneath Irrigated Vegetable Fields in a North‐Central U.S. Sand Plain

AbstractThe dramatic expansion of irrigated agriculture since about 1970 in the north‐central USA has been accompanied by NO3 and pesticide pollution of groundwater. The expansion has been concentrated in areas with sandy soils and shallow water tables, such as the Wisconsin central sand plain. In some parts of this sand plain, most wells contain detectable pesticide residues and NO3‐N concentrations that exceed the 10 mg L−1 U.S. drinking water maximum contaminant level (MCL). To evaluate the effects on groundwater quality of this agricultural system, we monitored solutes 23 times during a 2‐yr period in the upper 3 m of the aquifer beneath and immediately upgradient of four irrigated vegetable fields. Groundwater beneath fields had significantly greater concentrations of most solutes and lower pH than upgradient groundwater. Especially pronounced were Ca, Cl, K, Mg, and NO3 differences, with concentrations 5 to 26 times greater under fields. Nitrate N concentrations averaged 21 mg L−1 under fields, compared with 1 mg L−1 upgradient. Pesticide residues were ubiquitous beneath fields, and generally persisted for many months after application. Pesticide concentrations often exceeded Wisconsin preventive‐action limits (PALs), but seldom exceeded federal MCLs. Even when agricultural management approximated best management practice (BMP) recommendations, the NO3 concentration beneath these fields approached double the MCL, indicating a need for new approaches to control agricultural groundwater pollution.

Impacts of Irrigated Vegetable Agriculture on a Humid North‐Central U.S. Sand Plain Aquifer

AbstractWe determined the impacts of irrigated vegetable agriculture on ground water quality in a 29 km2 portion of the Wisconsin central sand plain (WCSP). Vegetable fields cover 22% of the study area and are used for growing potatoes, sweet corn, snap beans, peas, field corn, and soybeans. We found that contaminant plumes from fields underlay about 54% of the area. Plumes were 3 to &gt;16 m thick and in places occupied the entire aquifer thickness. Impacted ground water retained the chemical signature observed under fields, and compared with unimpacted ground water, had elevated NO3‐N (median 13.7 mg L−1 impacted vs. 0.5 for unimpacted) and Cl (median 23 vs. 2.0 mg L−1) and 1.4 to 2.5 times the Ca, Mg, K, Na, and SO4. NO3‐N detections exceeded 10 mg L−1 in 58% of impacted monitoring wells compared with none of the unimpacted wells. Evidence of denitrification was observed, but rarely. Residues of the pesticides alachlor, atrazine, carbofuran, metolachlor, and metribuzin were detected, but usually at summed concentrations of &lt;1 μg L−1 outside of fields. The maximum observed sum of pesticide residues was 8 μg L−1. Not all potential pesticide residues were detectable by the analytical method.Agricultural impacts limit the aquifer's value as a source of potable water, because wells are unable to tap unimpacted ground water through much of the area. Results of this study are generally applicable to other humid sandy areas in the north‐central United States and elsewhere. Impacts might be more severe elsewhere because the agricultural land use is frequently denser and ground water flowpaths are longer.

Tracer Test Evaluation of a Drainage Ditch Capture Zone

AbstractRecent analytical and numerical modeling studies (Zheng et al., 1988a, 1988b) suggest that drainage ditches within the central sand plain of Wisconsin may serve to limit the spread of agricultural chemicals. In this study a natural‐gradient tracer test was conducted to determine flow paths around a sand plain drainage ditch. The flow paths define a capture zone for the ditch. These results were used to evaluate the predictive capabilities of an analytical model (Zheng et al., 1988a).The natural‐gradient tracer test began with injections of iodide at two depths and bromide at an intermediate depth. Movement of these tracers reveals that average ground‐water velocity ranged from 0.4 ft/day at a depth of 20 ft to greater than 1 ft/day near the water table. The delineated capture zone was deeper than 12 ft below the water table. These results are encouraging because they indicate that the capture depth of the ditch should allow for the effective removal of shallow agricultural contaminants. The observed capture depth falls within the range predicted by the analytical model of Zheng et al. (1988a). However, a sensitivity analysis of the analytical method reveals that the predicted range of capture depths is large. This range results from the variability and uncertainty of the model parameters. The analytical solution appears best suited for estimating the minimum capture depth, which is a conservative estimate of contaminant capture.

Dependence of Aldicarb Residue Degradation Rates on Groundwater Chemistry in the Wisconsin Central Sands

AbstractAldicarb sulfoxide (ASO) (2‐methyl‐2‐(methylsulfinyl)propanal O‐[(methylamino)carbonyl]oxime) and aldicarb sulfone (ASO2) (2‐methyl‐2‐(methylsulfonyl)propanal O‐[(methylamino)carbonyl]oxime) degradation rates were measured in incubations simulating three depths (0.5, 5, and 10 m) of Wisconsin central sand plain (WCSP) aquifer. The incubations differed mainly in pH (5.8, 6.5, and 7.2, respectively, for the three depths) and pO2 (20 kPa [0.2 atm] for the 0.5‐ and 5‐m depths, 1.5 kPa [0.015 atm] for the 10‐m depth). Degradation rates corresponded to half‐lives ranging from years to less than a month. Aldicarb sulfone (ASO2) degraded faster than ASO at the same depth. No ASO degradation was measured under aerobic, pH 5.8 conditions; no ASO oxidation was observed in any incubation. The shortest half‐lives, 19 and 32 d for ASO and ASO2, respectively, were obtained under the low pO2 conditions. The results of this and a previous experiment provide degradation rates for a cross‐section of WCSP groundwater chemistries. Thus, with knowledge of the groundwater chemistry at a given WCSP location, degradation rates can be estimated. Degradation rates of common groundwater contaminants as functions of chemical and physical controlling factors could prove valuable to groundwater managers for endeavors such as wellhead protection.