Simulation of Contributing Areas and Surface-Water Leakage to Potential Replacement Wells Near the Community of New Post, Sawyer County, Wisconsin, by Means of a Two-Dimensional Ground-Water-Flow Model (original) (raw)
Scientific Investigations Report, 2007
Cabot WaterWorks, located in Lonoke County, Arkansas, plans to increase ground-water withdrawals from the Mississippi River Valley alluvial aquifer from a 2004 rate of approximately 2.24 million gallons per day to between 4.8 and 8 million gallons per day by the end of 2049. The effects of increased pumping from several wells were simulated using a digital model of ground-water flow. The proposed additional withdrawals by Cabot WaterWorks were specified in three 1square-mile model cells with increased pumping beginning in 2007. Increased pumping was specified at various combined rates for a period of 44 years. In addition, augmented pumping from wells owned by Grand Prairie Water Users Association, located about 2 miles from the nearest Cabot WaterWorks wells, was added to the model beginning in 2007 and continuing through to the end of 2049 in 10 of the 16 scenarios analyzed. Eight of the scenarios included reductions in pumping rates in model cells corresponding to either the Grand Prairie Water Users Association wells or to wells contained within the Grand Prairie Area Demonstration Project. Drawdown at the end of 44 years of pumping at 4.8 million gallons per day from the Cabot WaterWorks wells ranged from 15 to 25 feet in the three model cells; pumping at 8 million gallons per day resulted in water-level drawdown ranging from about 15 to 40 feet. Water levels in those cells showed no indication of leveling out at the end of the simulation period, indicating non-steady-state conditions after 44 years of pumping. From one to four new dry cells occurred in each of the scenarios by the end of 2049 when compared to a baseline scenario in which pumping was maintained at 2004 rates, even in scenarios with reduced pumping in the Grand Prairie Area Demonstration Project; however, reduced pumping produced cells that were no longer dry when compared to the baseline scenario at the end of 2049. Saturated thickness at the end of 2049 in the three Cabot WaterWorks wells ranged from about 52 to 68.5 feet for pumping rates of 4.8 million gallons per day, and from about 38 to 64 feet for pumping rates of 8 million gallons per day, the latter causing water levels to fall below half the aquifer thickness in the most heavily pumped of the three cells. analysis of the effects of withdrawals to be performed prior to the issuance of a water-withdrawal permit. To address this need and to improve the understanding of the effect of municipal users on ground-water flow in this type of aquifer, the USGS in cooperation with CWW used the north alluvial model of to simulate ground-water flow and water-level changes for the period 1918-2049 for various ground-water pumping scenarios. The purpose of this report is to compare simulated water levels derived from the north alluvial model with and without several additional ground-water withdrawal rates from several 1-square-mile model cells pumped for a period of 44 years simulated to begin in 2007 (table ). These cells include pumping from the Grand Prairie Water Users Association (GPWUA), the Grand Prairie Area Demonstration project (GPADP), as well as CWW. GPWUA plans to increase pumping and has proposed the construction of an additional well near its current well field. The GPADP is a project, under the management of the ANRC and U.S. Army Corps of Engineers, designed to route water from the White River to water users in Arkansas, Monroe, Prairie, and eastern Lonoke counties to supplement ground-water demand. By supplying this supplemental water, it is anticipated that wateruse pressure on the alluvial aquifer will decrease. Drawdown and resulting saturated thickness of the alluvial aquifer after 44 years of pumping are presented for a radial distance of about 20 miles from the pumped model cells located in eastern Lonoke County. The study area includes Lonoke, Prairie, Arkansas, White, and Jefferson Counties, all of which have been designated as Critical Ground-Water Areas by the ANRC (fig. ). Information in this report can be used by water managers to evaluate simulated effects of additional ground-water withdrawals on the ground-water resource. Because alluvial aquifers commonly provide sources of water for municipal, industrial, and agricultural use, it is important to improve the understanding of ground-water flow in this type of aquifer. Additionally, the types of scenarios analyzed using the ground-water flow model in this report demonstrate the utility of the approach for assessing complex ground-water flow systems and pumping distributions.
Sources of water captured by municipal supply wells in a highly conductive aquifer western Montana
2005
The sole-source unconfined M issoula Aquifer provides drinking water to 60,000 residents. The Clark Fork River reportedly provides 50-90% o f the aquifer recharge. If this recharge source becomes contaminated, water from municipal wells may be at risk. The goal o f this study was to examine the river-groundwater recharge process and to quantify the sources o f water pumped for water supply. This goal was accomplished by investigating the geology, the groundwater occurrence and flow, aquifer properties and streambed properties. Results o f these investigations coupled with components from past studies generated a conceptual model and ultimately a transient three-dimensional groundwater flow model. Finally, water sources were assessed by particle tracking. Geologic investigations revealed a course grained aquifer with discontinuous packages o f fine grained material. Observations at 29 monitoring wells from May 2004-June 2005 enabled construction o f potentiometric maps revealing an east to west flow direction divided by the Clark Fork throughout most o f the study region. Aquifer investigations by pum ping tests, slug tests and peak delay analysis yielded hydraulic conductivities ranging from 2,000 to 48,000 ft/day in the eastern portion o f the aquifer. Streambed investigations by vertical gradient measurements, temperature monitoring and modeling, stream discharge measurements and tracer tests revealed the river to be perched above the aquifer and losing water. In Hellgate Canyon the river is perched approximately 5 ft above the aquifer and leaking 1.9-4 ft^/day per o f river bed. In the M adison Area the river is perched approximately 17ft above the aquifer and leaking 7-14 ft^/day per ft^ o f riverbed. A three-dimensional transient model was calibrated to March 17, 2005 water level data and water level changes over the study period. The ground water budget from the model suggested 82% o f the groundwater in the study region is from Clark Fork River leakage and 12% is from up gradient underflow. Particle tracking to delineate capture zones revealed dominant horizontal flow with wells adjacent to the river on the south side receiving 50-80% o f their water from the river, while distal wells on the south side are dominated by underflow and wells on the north side receive recharge from a losing portion o f Rattlesnake Creek. 10 CHAPTER II METHODS This chapter describes the field and data analysis methods for all investigations conducted during this study. This includes: geologic, groundwater occurrence and flow, aquifer property, streambed behavior, geochemical, groundwater budget, and modeling investigations. Geologic Investigation The geologic and hydrogeologic setting was characterized by performing a detailed review o f the literature, interpreting well driller's logs, examining drill cuttings and drilling four new monitoring wells. Two wells were completed in Hellgate Canyon (HGS and HGD) (Figure 4) and two wells were drilled on either side o f Madison St. Bridge (D H l and DH2) (see inset Figure 4). These wells were constructed with an air rotary drilling rig equipped with an eccentric (off-centered) bit. Environmental West Drilling Co. performed drilling and set these wells. Well cuttings were reviewed and logged during the drilling process. The two wells completed adjacent to Madison Street Bridge included the collection o f split spoon samples reviewed every five feet. The Hellgate Canyon wells were sand-packed with 10+20 silica sand and screened with 20 ft filter packed 0.12 slot PVC screen. Well HGD was drilled to bedrock (247 ft) and then cased to a depth o f 172 ft. Well HGS was drilled and finished to 51 ft. The M adison Area wells were natural packed and screened with 0.040 slot screen. Well D H l was drilled and finished to 50 ft with 10 ft o f 0.040 slot screen. Well DH2 was drilled and finished to 70 ft with 20 ft o f 0.040 slot screen (see well logs Appendix B). These well 1070' 70 ' I = 0.0024 K = 21,500
Groundwater models and wellfield management: a case study
Environmental Engineering and Policy, 1998
The highly permeable sand and gravel of the Little Miami River Valley Aquifer System near Milford, Ohio provide the community with a high-yielding source of drinking water. While this hydrogeologic setting is ideal from a water quantity standpoint, it is greatly vulnerable to contamination. The future viability of the wellfield came into question when it was discovered that the wellfield was contaminated with volatile organic contaminants, leaving the City with the costly cleanup. Milford's perseverance is also challenged by having to deal with a deteriorating treatment plant. Furthermore, larger water suppliers in the area have threatened the community's independence. The question facing this community, and many other smaller communities, was whether to surrender its independence or invest in their future. The city has decided to keep their wellfield and to conduct a groundwater study. The objectives of the study were to: (a) collect and evaluate hydrogeological data; (b) develop a conceptual model of the groundwater system; (c) construct groundwater flow and geochemical models; (d) delineate wellhead protection area; and (e) develop a comprehensive management program. Collected hydrogeologic data served as a basis for the conceptual model. The U.S. Geological Survey (USGS) three-dimensional groundwater flow model MODFLOW was used in conjunction with MODPATH, a particle-tracking program, to identify travel times and paths of contaminants. This approach ultimately lead to the delineation of the wellhead protection area (WHPA). Geochemical mixing models were constructed using the USGS PHREEQC to verify the flow model results. The use of both flow and geochemical models to delineate the WHPA and to manage groundwater resources is a unique approach. The modeling results provide the City of Milford a management tool in making difficult policy decisions regarding future land use, siting for new monitoring and production wells, and identification of potential pollution sources.
The North Platte Natural Resources District (NPNRD) has been actively collecting data and studying groundwater resources because of concerns about the future availability of the highly inter-connected surface-water and groundwater resources. This report, prepared by the U.S. Geological Survey in cooperation with the North Platte Natural Resources Dis¬trict, describes a groundwater-flow model of the North Platte River valley from Bridgeport, Nebraska, extending west to 6 miles into Wyoming. The model was built to improve the understanding of the interaction of surface-water and ground¬water resources, and as an optimization tool, the model is able to analyze the effects of water-management options on the simulated stream base flow of the North Platte River. The groundwater system and related sources and sinks of water were simulated using a newton formulation of the U.S. Geo¬logical Survey modular three-dimensional groundwater model, referred to as MODFLOW–NWT, which provided an improved ability to solve nonlinear unconfined aquifer simulations with wetting and drying of cells. Using previously published aquifer-base-altitude contours in conjunction with newer test-hole and geophysical data, a new base-of-aquifer altitude map was generated because of the strong effect of the aquifer-base topography on groundwater-flow direction and magnitude. The largest inflow to groundwater is recharge originating from water leaking from canals, which is much larger than recharge originating from infiltration of precipita¬tion. The largest component of groundwater discharge from the study area is to the North Platte River and its tributar¬ies, with smaller amounts of discharge to evapotranspiration and groundwater withdrawals for irrigation. Recharge from infiltration of precipitation was estimated with a daily soil-water-balance model. Annual recharge from canal seepage was estimated using available records from the Bureau of Reclamation and then modified with canal-seepage potentials estimated using geophysical data. Groundwater withdraw¬als were estimated using land-cover data, precipitation data, and published crop water-use data. For fields irrigated with surface water and groundwater, surface-water deliveries were subtracted from the estimated net irrigation requirement, and groundwater withdrawal was assumed to be equal to any demand unmet by surface water. The groundwater-flow model was calibrated to measured groundwater levels and stream base flows estimated using the base-flow index method. The model was calibrated through automated adjustments using statistical techniques through parameter estimation using the parameter estimation suite of software (PEST). PEST was used to adjust 273 parameters, grouped as hydraulic conductivity of the aquifer, spatial multipliers to recharge, temporal multipliers to recharge, and two specific recharge parameters. Base flow of the North Platte River at Bridgeport, Nebraska, streamgage near the eastern, downstream end of the model was one of the primary calibration targets. Simulated base flow reasonably matched estimated base flow for this streamgage during 1950–2008, with an average difference of 15 percent. Overall, 1950–2008 simulated base flow followed the trend of the estimated base flow reasonably well, in cases with generally increasing or decreasing base flow from the start of the simulation to the end. Simulated base flow also matched estimated base flow reasonably well for most of the North Platte River tributar¬ies with estimated base flow. Average simulated groundwater budgets during 1989–2008 were nearly three times larger for irrigation seasons than for non-irrigation seasons. The calibrated groundwater-flow model was used with the Groundwater-Management Process for the 2005 version of the U.S. Geological Survey modular three-dimensional groundwater model, MODFLOW–2005, to provide a tool for the NPNRD to better understand how water-management deci¬sions could affect stream base flows of the North Platte River at Bridgeport, Nebr., streamgage in a future period from 2008 to 2019 under varying climatic conditions. The simulation-optimization model was constructed to analyze the maximum increase in simulated stream base flow that could be obtained with the minimum amount of reductions in groundwater withdrawals for irrigation. A second analysis extended the first to analyze the simulated base-flow benefit of groundwater withdrawals along with application of intentional recharge, that is, water from canals being released into rangeland areas with sandy soils. With optimized groundwater withdrawals and intentional recharge, the maximum simulated stream base flow was 15–23 cubic feet per second (ft3/s) greater than with no management at all, or 10–15 ft3/s larger than with managed groundwater withdrawals only. These results indicate not only the amount that simulated stream base flow can be increased by these management options, but also the locations where the management options provide the most or least benefit to the simulated stream base flow. For the analyses in this report, simulated base flow was best optimized by reductions in groundwater withdrawals north of the North Platte River and in the western half of the area. Intentional recharge sites selected by the optimization had a complex distribution but were more likely to be closer to the North Platte River or its tributaries. Future users of the simulation-optimization model will be able to modify the input files as to type, location, and timing of constraints, decision variables of groundwater withdrawals by zone, and other variables to explore other feasible management scenarios that may yield different increases in simulated future base flow of the North Platte River.