Geology topics

R. J. Hunt

Publications and source records attributed to R. J. Hunt.

30 records · Page 2Linked to original sources

USGS2005

Improving a complex finite-difference ground water flow model through the use of an analytic element screening model

This paper demonstrates that analytic element models have potential as powerful screening tools that can facilitate or improve calibration of more complicated finite-difference and finite-element models. We demonstrate how a two-dimensional analytic element model was used to identify errors in a complex three-dimensional finite-difference model caused by incorrect specification of boundary conditions. An improved finite-difference model was developed using boundary conditions developed from a far-field analytic element model. Calibration of a revised finite-difference model was achieved using fewer zones of hydraulic conductivity and lake bed conductance than the original finite-difference model. Calibration statistics were also improved in that simulated base-flows were much closer to measured values. The improved calibration is due mainly to improved specification of the boundary conditions made possible by first solving the far-field problem with an analytic element model.

Groundwater↗

USGS2005

Speciation and transport of newly deposited mercury in a boreal forest wetland: A stable mercury isotope approach

As part of the Mercury Experiment to Assess Atmospheric Loadings in Canada and the United States (METAALICUS) the fate and transport of contemporary mercury (Hg) deposition in a boreal wetland was investigated using an experimentally applied stable mercury isotope. We applied high purity (99.2% ± 0.1) 202 Hg(II) to a wetland plot to determine if (1) the 202 Hg was detectable above the pool of native Hg, (2) the 202 Hg migrated vertically and/or horizontally in peat and pore waters, and (3) the 202 Hg was converted to methylmercury (MeHg) in situ. The 202 Hg was easily detected by ICP/MS in both solid peat and pore waters. Over 3 months, the 202 Hg migrated vertically downward in excess of 15 cm below the water table and traveled several meters horizontally beyond the experimental plot to the lake margin along the dominant vector of groundwater flow. Importantly, at one location, 6% of aqueous 202 Hg was detected as Me 202 Hg after only 1 day. These results indicate that new inorganic Hg in atmospheric deposition can be readily methylated and transported lakeward by shallow groundwater flow, confirming the important role of wetlands as contributors of Hg to aquatic ecosystems.

Water Resources Research↗

USGS2005

Investigating surface water-well interaction using stable isotope ratios of water

Because surface water can be a source of undesirable water quality in a drinking water well , an understanding of the amount of surface water and its travel time to the well is needed to assess a well's vulnerability. Stable isotope ratios of oxygen in river water at the City of La Crosse, Wisconsin, show peak-to-peak seasonal variation greater than 4‰ in 2001 and 2002. This seasonal signal was identified in 7 of 13 city municipal wells, indicating that these 7 wells have appreciable surface water contributions and are potentially vulnerable to contaminants in the surface water. When looking at wells with more than 6 sampling events, a larger variation in δ 18 O compositions correlated with a larger fraction of surface water, suggesting that samples collected for oxygen isotopic composition over time may be useful for identifying the vulnerability to surface water influence even if a local meteoric water line is not available. A time series of δ 18 O from one of the municipal wells and from a piezometerlocated between the river and the municipal well showed that the travel time of flood water to the municipal well was approximately 2 months; non-flood arrival times were on the order of 9 months. Four independent methods were also used to assess time of travel. Three methods (groundwater temperature arrival times at the intermediate piezometer, virus-culture results, and particle tracking using a numerical groundwater-flow model) yielded flood and non-flood travel times of less than 1 year for this site. Age dating of one groundwater sample using 3 H– 3 He methods estimated an age longer than 1 year, but was likely confounded by deviations from piston flow as noted by others. Chlorofluorocarbons and SF 6 analyses were not useful at this site due to degradation and contamination, respectively. This work illustrates the utility of stable hydrogen and oxygen isotope ratios of water to determine the contribution and travel time of surface water in groundwater, and demonstrates the importance of using multiple methods to improve estimates for time of travel of 1 year or less.

Journal of Hydrology↗

USGS2004

Vulnerability of drinking-water wells in La Crosse, Wisconsin, to enteric-virus contamination from surface water contributions

Human enteric viruses can contaminate municipal drinking-water wells, but few studies have examined the routes by which viruses enter these wells. In the present study, the objective was to monitor the municipal wells of La Crosse, Wisconsin, for enteric viruses and determine whether the amount of Mississippi River water infiltrating the wells was related to the frequency of virus detection. From March 2001 to February 2002, one river water site and four wells predicted by hydrogeological modeling to have variable degrees of surface water contributions were sampled monthly for enteric viruses, microbial indicators of sanitary quality, and oxygen and hydrogen isotopes. 18O/ 16O and 2H/1H ratios were used to determine the level of surface water contributions. All samples were collected prior to chlorination at the wellhead. By reverse transcription-PCR (RT-PCR), 24 of 48 municipal well water samples (50%) were positive for enteric viruses, including enteroviruses, rotavirus, hepatitis A virus (HAV), and noroviruses. Of 12 river water samples, 10 (83%) were virus positive by RT-PCR. Viable enteroviruses were not detected by cell culture in the well samples, although three well samples were positive for culturable HAV. Enteroviruses detected in the wells by RT-PCR were identified as several serotypes of echoviruses and group A and group B coxsackieviruses. None of the well water samples was positive for indicators of sanitary quality, namely male-specific and somatic coliphages, total coliform bacteria, Escherichia coli, and fecal enterococci. Contrary to expectations, viruses were found in all wells regardless of the level of surface water contributions. This result suggests that there were other unidentified sources, in addition to surface water, responsible for the contamination.

Applied and Environmental Microbiology↗

USGS2004

Simulation of ground-water flow, surface-water flow, and a deep sewer tunnel system in the Menomonee Valley, Milwaukee, Wisconsin

Numerical models were constructed for simulation of ground-water flow in the Menomonee Valley Brownfield, in Milwaukee, Wisconsin. An understanding of ground-water flow is necessary to develop an efficient program to sample ground water for contaminants. Models were constructed in a stepwise fashion, beginning with a regional, single-layer, analytic-element model (GFLOW code) that provided boundary conditions for a local, eight layer, finite-difference model (MODFLOW code) centered on the Menomonee Valley Brownfield. The primary source of ground water to the models is recharge over the model domains; primary sinks for ground water within the models are surface-water features and the Milwaukee Metropolitan Sewerage District Inline Storage System (ISS). Calibration targets were hydraulic heads, surface-water fluxes, vertical gradients, and ground-water infiltration to the ISS. Simulation of ground-water flow by use of the MODFLOW model indicates that about 73 percent of recharge within the MODFLOW domain circulates to the ISS and 27 percent discharges to gaining surface-water bodies. In addition, infiltration to the ISS comes from the following sources: 36 percent from recharge within the model domain, 45 percent from lateral flow into the domain, 15 percent from Lake Michigan, and 4 percent from other surface-water bodies. Particle tracking reveals that the median traveltime from the recharge point to surface-water features is 8 years; the median time to the ISS is 255 years. The traveltimes to the ISS are least over the northern part of the valley, where dolomite is near the land surface. The distribution of traveltimes in the MODFLOW simulation is greatly influenced by the effective porosity values assigned to the various lithologies.

Wisconsin↗

USGS2001

Evaluating the effects of urbanization and land-use planning using ground-water and surface-water models

Why are the effects of urbanization a concern? As the city of Middleton, Wisconsin, and its surroundings continue to develop, the Pheasant Branch watershed (fig.l) is expected to undergo urbanization. For the downstream city of Middleton, urbanization in the watershed can mean increased flood peaks, water volume and pollutant loads. More subtly, it may also reduce water that sustains the ground-water system (called "recharge") and adversely affect downstream ecosystems that depend on ground water such as the Pheasant Branch Springs (hereafter referred to as the Springs). The relation of stormwater runoff and reduced ground-water recharge is complex because the surface-water system is coupled to the underlying ground-water system. In many cases there is movement of water from one system to the other that varies seasonally or daily depending on changing conditions. Therefore, it is difficult to reliably determine the effects of urbanization on stream baseflow and spring flows without rigorous investigation. Moreover, mitigating adverse effects after development has occurred can be expensive and administratively difficult. Overlying these concerns are issues such as stewardship of the resource, the rights of the public, and land owners' rights both of those developing their land and those whose land is affected by this development. With the often- contradictory goals, a scientific basis for assessing effects of urbanization and effectiveness of mitigation measures helps ensure fair and constructive decision-making. The U.S. Geological Survey, in cooperation with the City of Middleton and Wisconsin Department of Natural Resources, completed a study that helps address these issues through modeling of the hydrologic system. This Fact Sheet discusses the results of this work.

Wisconsin↗

USGS2001

Use of a watershed-modeling approach to assess hydrologic effects of urbanization, North Fork Pheasant Branch basin near Middleton, Wisconsin

The North Fork Pheasant Branch Basin in Dane County, Wisconsin is expected to undergo development. There are concerns that development will adversely affect water resources with increased flood peaks, increased runoff volumes, and increased pollutant loads. To provide a scientific basis for evaluating the hydrologic system response to development the Precipitation Runoff Modeling System (PRMS) was used to model the upper Pheasant Branch Creek watershed with an emphasis on the North Fork Basin. The upper Pheasant Branch Creek (18.3 mi2; 11,700 acres) Basin was represented with 21 Hydrologic Response Units (daily time step) and 50 flow planes (5-minute time steps). Precipitation data from the basin outlet streamflow-gaging station located at Highway 12 and temperature data from a nearby airport were used to drive the model. Continuous discharge records at three gaging stations were used for model calibration. To qualitatively assess model representation of small subbasins, periodic reconnaissance, often including a depth measurement, was made after precipitation to determine the occurrence of flow in ditches and channels from small subbasins. As a further effort to verify the model on a small subbasin scale, continuous-stage sensors (15-minute intervals) measured depth at the outlets of three small subbasins (500 to 1,200 acres). Average annual precipitation for the simulation period from 1993 to 1998 was 35.2 inches. The model simulations showed that, on average, 23.9 inches were intercepted by vegetation, or lost to evapotranspiration, 6.0 inches were infiltrated and moved to the regional ground-water system, and 4.8 inches contributed to the upper Pheasant Branch streamflow. The largest runoff event during the calibration interval was in July 1993 (746 ft3/sec; with a recurrence interval of approximately 25 years). Resulting recharge rates from the calibrated model were subsequently used as input into a ground-water-flow model. Average annual recharge varied spatially from 2.3 inches per year in the highly impervious commercial/industrial area to 9.7 inches per year in the undeveloped North Fork Basin with an average overall recharge rate of 8.1 inches per year. Two development scenarios were examined to assess changes in water-budget fluxes. In scenario A, when development was predominantly low-density residential with 5 to 10 percent commercial development along principal roadways, mean annual streamflow increased by 53 percent, overland flow increased by 84 percent, base flow decreased by 15 percent and annual recharge to the regional ground-water system was reduced by 10 percent. In development scenario B, the entire North Fork and intervening area basins contained 50 percent commercial and 50 percent medium density residential land use. Annual storm runoff increased by over 450 percent. The ground-water model for the Pheasant Branch that used the scenario B recharge rates simulated a lowered water table with zero base flow and that flow from Frederick Springs would be reduced 26 percent from present-day (1993?98) conditions.An additional example application of the model evaluated locations of flood detention ponds and potential recharge areas that may mitigate the changes in flood peaks and ground-water recharge resulting from urbanization. From February 1998 through July 1998, water-quality samples were collected by use of stage-activated automated samplers. Median suspended- sediment concentrations were similar between the North and South Fork Basins (194 and 242 mg/L, respectively); however, for other constituents, North Fork values were considerably higher: median phosphorus concentrations by 4 times (1.5 and 0.35 mg/L), median ammonia concentrations by 13 times (1.9 and 0.14 mg/L), and the phosphorus-to-sediment ratio by more than 6 times (21 and 3.1 mg/g).

Wisconsin↗

USGS2000

Development of Maximum Considered Earthquake Ground Motion Maps

The 1997 NEHRP Recommended Provisions for Seismic Regulations for New Buildings use a design procedure that is based on spectral response acceleration rather than the traditional peak ground acceleration, peak ground velocity, or zone factors. The spectral response accelerations are obtained from maps prepared following the recommendations of the Building Seismic Safety Council's (BSSC) Seismic Design Procedures Group (SDPG). The SDPG-recommended maps, the Maximum Considered Earthquake (MCE) Ground Motion Maps, are based on the U.S. Geological Survey (USGS) probabilistic hazard maps with additional modifications incorporating deterministic ground motions in selected areas and the application of engineering judgement. The MCE ground motion maps included with the 1997 NEHRP Provisions also serve as the basis for the ground motion maps used in the seismic design portions of the 2000 International Building Code and the 2000 International Residential Code . Additionally the design maps prepared for the 1997 NEHRP Provisions , combined with selected USGS probabilistic maps, are used with the 1997 NEHRP Guidelines for the Seismic Rehabilitation of Buildings .

Earthquake Spectra↗

USGS2000

The importance of ground water in the Great Lakes Region

Ground water is a major natural resource in the Great Lakes Region because it indirectly contributes more than 50 percent of the stream discharge to the Great Lakes. In addition, ground water is the source of drinking water for millions of people in the region, is an important source of supply for agriculture and many industries, and provides a relatively uniform supply of water in some ecologically sensitive areas to sustain plant and animal species. Therefore, to improve our understanding of water-resources issues in the Great Lakes Region, it is important to have a better understanding of the role that ground water plays in the overall hydrologic system of the lakes. The main ground-water resources issues in the Great Lakes Region are related to the amount of ground water, the interaction of ground water and surface water, changes in ground-water quality as development expands, and ecosystem health related to quantity and quality of water. Issues related to the amount of ground water Although the amount of water in the Great Lakes Region is vast, issues related to relatively small quantities of water are being raised more and more often. For example, even though the amount of ground water pumped in the region is small compared to the total amount of water present, ground water is an important source of public-water supply as well as an important source of supply for industrial, agricultural, and domestic needs. Less clearly understood, however, is the relation between the amount of streamflow discharging to the Great Lakes and the large portion of that flow that originates as ground water. The implications of this understanding for water- and land-use practices and, in turn, their effects on water quantity and quality, have not been fully incorporated into a policy framework. To help include information about the implications of the role that ground water plays in addressing regional water issues, a comprehensive analysis of indirect ground-water discharge to the Great Lakes is needed. Direct ground-water discharge to the Great Lakes is not a large factor in water-budget analyses for the Great Lakes. Locally, however, direct ground-water discharge to the Great Lakes may be important, even though the rates and places of discharge are not well known. A long-term evaluation of direct ground-water discharge to the Great Lakes would help place this hydrologic process in proper perspective. Near-shore areas with high rates of direct ground-water discharge may provide valuable habitat for aquatic organisms. Issues related to the interaction of ground water and surface water Withdrawal of ground water removes that water from the watershed when it is consumptively used or when the return flow is discharged to another drainage basin. Under these circumstances, pumping ground water constitutes a diversion of Great Lakes water. Alternatively, ground-water withdrawal could have the opposite effect of diverting ground-water flow into the watershed by altering the ground-water divides. In particular, as withdrawals associated with urban expansion increase, more accurate data on the amount and effects of ground-water use need to be collected. Data on the amounts of ground water pumped both within the watershed and outside, but near the watershed boundaries needs to be collected and evaluated for potential diversion of water to or from the Great Lakes. It is currently thought that both irrigation and ground-water withdrawals near the watershed boundaries constitute relatively small amounts of water; however, both rapidly changing farming practices and rapidly expanding urban communities could alter these amounts in a relatively short timeframe, especially during drought periods. At present, the effects of ground-water withdrawals have been quantified in detail at only a few urban locations. In addition to quantifying the amount of water pumped out of aquifers, it is also important to improve our knowledge of the amount of water that is recharging them. Ground-water recharge rates estimated in earlier studies cover only a small part of the Great Lakes Region. A comprehensive study of ground-water recharge rates for the entire watershed is needed to more completely determine the role of ground water in the hydrologic budget of the Great Lakes. Issues related to changes in ground-water quality as development expands Ground-water quality is as important as quantity for most water uses. As ground-water development proceeds, the possibility of altering the quality of ground water increases. The quality of ground water can be altered when water levels are drawn below the layer that confines the aquifer or by inducing water of lesser quality into an aquifer. Many local studies of these problems have been conducted, but few regional-scale analyses of changes in ground-water quality as a result of ground-water development have been done. Issues related to ecosystem health and quantity and quality of ground water Ground water is essential to maintain wetlands and to provide healthy habitat for other aquatic systems. Wetland hydrology is widely recognized as the primary influence on wetland ecology, development, and persistence, and information about hydrology is essential to understanding and quantifying wetland functions and processes. Studies of the role of ground water in selected wetlands in a range of physiographic settings throughout the Great Lakes watershed are needed to more fully understand the role of wetlands in the Great Lakes Region.

Lake Erie, Lake Huron, Lake Michigan, Lake Ontario↗

USGS2000

Simulation of the shallow hydrologic system in the vicinity of Middle Genesee Lake, Wisconsin, using analytic elements and parameter estimation

Middle Genesee Lake is a ground-water flow-through lake located in a developing area in southeastern Wisconsin. Because the lake is in good connection with the shallow ground-water system, hydrologic stresses to the shallow ground-water system could adversely affect the lake system. In order to assess the effects of potential stresses on the lake, a study was completed by the U.S. Geological Survey, in cooperation with the Middle Genesee Lake Management District. The objective of the study was to identify areas that contribute ground water to the lake and estimate the hydrologic budget of the lake and hydraulic parameters affecting ground-water flow. A two-dimensional, steady-state analytic element model of the lake and surrounding area was developed using the computer code GFLOW. A parameter estimation model, UCODE, was used to optimize the calibration to measured water levels and streamflow. The calibrated model was used to evaluate the effect of three hypothetical stress scenarios on the stage of Middle Genesee Lake; the simulations were linked to UCODE, which formally incorporated parameter uncertainty into 95-percent confidence intervals around the simulated value. The scenarios included: (1) pumping from upgradient irrigation wells, (2) pumping from Lower Genesee Lake to lower lake levels, and (3) reduction in recharge resulting from development. The results of the simulations demonstrated that lake levels could be affected by hydrologic stresses in the shallow hydrologic system, with effects ranging from a 2.7 feet decline in lake stage resulting from pumping in Lower Genesee Lake to a 0.1 feet decline in lake stage from development in part of the upgradient recharge area. The range of lake stage decline increased when parameter uncertainty was included, from a decline of 3.1 feet for pumping from Lower Genesee Lake to no reduction in lake stage for the development in the recharge area. Whereas these simulated effects are within the natural variation in lake stage, they represent a systematic reduction of ground-water flow to the lake. Therefore, these hypothetical stresses are expected to establish a new, lower, baseline lake stage over which the natural variation due to climatic effects are added and subtracted.

Wisconsin↗

USGS2000

Simulation of the recharge area for Frederick Springs, Dane County, Wisconsin

The Pheasant Branch watershed in Dane County is expected to undergo development. There are concerns that this development will adversely affect water resources, including Frederick Springs, a large spring complex in the watershed. The spring's recharge area was delineated using a telescopic mesh refinement (TMR) model constructed from an existing regional-scale ground-water flow model, and further refined by adding nearby surface-water features, a refined recharge array based on a surface-water model, and increasing the vertical leakage between the deep aquifers. This TMR model was formally optimized using the parameter estimation code UCODE. The results of optimization demonstrated that the best fit to measured heads and fluxes was obtained by using a horizontal hydraulic conductivity two times that of the original regional model for layer 2 and 80 percent smaller for layer 3. This range of parameter values was formally considered using a stochastic Monte Carlo approach. Two-hundred model runs used uniformly distributed, randomly sampled, horizontal hydraulic conductivity values within the range given by the TMR optimized values and the previously constructed regional model. A probability distribution of particles captured by the spring, or a probabilistic capture zone' was calculated from the realistic Monte Carlo results (136 runs of 200). In addition to portions of the local surface watershed, the capture zone encompassed distant areas in the North Fork of the Pheasant Branch watershed and areas entirely outside of the Pheasant Branch - demonstrating that the ground-watershed and surface watershed do not coincide. Analysis of samples from the springs and a nearby municipal well identified large contrasts in chemistry, even for springs within 50 feet of one another. The differences were stable over time, were present in both ion and isotope analyses, and showed a distinct gradation from high nitrate, high calcium, Ordovician-carbonate dominated water in western spring vents to low nitrate, lower calcium, Cambrian-sandstone influenced water in eastern spring vents. The difference in chemistry was attributed to distinctive bedrock geology as demonstrated by overlaying the 50 percent probability capture zone over a bedrock geology map for the area. This finding gives additional confidence to the capture zone calculated by the ground-water flow model.

Wisconsin↗

USGS1997

Assessing hydrogeochemical heterogeneity in natural and constructed wetlands

While 'water quality function' is cited as an important wetland function to design for and preserve, we demonstrate that the scale at which hydrochemical samples are collected can significantly influence interpretations of biogeochemical processes in wetlands. Subsurface, chemical profiles for both nutrients and major ions were determined at a site in southwestern Wisconsin that contained areas of both natural and constructed wetlands. Sampling was conducted on three different scales: (1) a large scale (3 m between sampling points), (2) an intermediate scale (0.15 m between sampling points), and (3) a small scale (1.5 cm between sampling points). In most cases, significant vertical heterogeneity was observed at the 0.15 m scale, which was much larger than previously reported for freshwater wetlands and not detected by sampling water table wells screened over the same interval. However, profiles of ammonia and total phosphorus showed tenfold changes in the upper 0.2 meters of the saturated zone when sampled at the small (1.5 cm) scale, that was not depicted by sampling at the intermediate scale. At the intermediate scale of observation, one constructed wetland site differed geochemically from the natural wetlands and the other constructed wetland site due to application of off-site salvaged marsh surface and downward infiltration of rain. While important differences in dissolved inorganic phosphorus and dissolved inorganic carbon concentrations existed between the constructed wetland and the natural wetlands, we also observed substantial differences between the natural wetland sites for these constituents. A median-polishing analysis of our data showed that temporal variations in constituent concentrations within profiles, although extensively recognized in the literature, were not as important as spatial variability.

Biogeochemistry↗