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At least 649 records · Page 36Linked to original sources
USGS2008

Ground-Water Storage Change and Land Subsidence in Tucson Basin and Avra Valley, Southeastern Arizona, 1998-2002

Gravity and land subsidence were measured annually at wells and benchmarks within two networks in Tucson Basin and Avra Valley from 1998 to 2002. Both networks are within the Tucson Active Management Area. Annual estimates of ground-water storage change, ground-water budgets, and land subsidence were made based on the data. Additionally, estimates of specific yield were made at wells within the monitored region. Increases in gravity and water-level rises followed above-average natural recharge during winter 1998 in Tucson Basin. Overall declining gravity and water-level trends from 1999 to 2002 in Tucson Basin reflected general declining ground-water storage conditions and redistribution of the recent recharge throughout a larger region of the aquifer. The volume of stored ground-water in the monitored portion of Tucson Basin increased 200,000 acre-feet from December 1997 to February 1999; however, thereafter an imbalance in ground-water pumpage in excess of recharge led to a net storage loss for the monitoring period by February 2002. Ground-water storage in Avra Valley increased 70,000 acre-feet during the monitoring period, largely as a result of artificial and incidental recharge in the monitored region. The water-budget for the combined monitored regions of Tucson Basin and Avra Valley was dominated by about 460,000 acre-feet of recharge during 1998 followed by an average-annual recharge rate of about 80,000 acre-feet per year from 1999 to 2002. Above-average recharge during winter 1998, followed by average-annual deficit conditions, resulted in an overall balanced water budget for the monitored period. Monitored variations in storage compared well with simulated average-annual conditions, except for above-average recharge from 1998 to 1999. The difference in observed and simulated conditions indicate that ground-water flow models can be improved by including climate-related variations in recharge rates rather than invariable rates of average-annual recharge. Observed land-subsidence during the monitoring period was less than 1 inch except in the central part of Tucson Basin where land subsidence was about 2-3 inches. Correlations of gravity-based storage and water-level change at 37 wells were variable and illustrate the complex nature of the aquifer system. Storage and water-level variations were insufficient to estimate specific yield at many wells. Correlations at several wells were poor, inverse, or resulted in unreasonably large values of specific yield. Causes of anomalously correlated gravity and water levels include significant storage change in thick unsaturated zones, especially near major ephemeral channels, and multiple aquifers that are poorly connected hydraulically. Good correlation of storage and water-level change at 10 wells that were not near major streams where significant changes in unsaturated zone storage occur resulted in an average specific-yield value of 0.27.

Scientific Investigations Report↗
USGS1997

Ground-water flow in the Saginaw aquifer in the vicinity of the north Lansing well field, Lansing Michigan — Part 1, simulations with a regional model

Vinyl chloride has been detected in water from the Saginaw aquifer near Lansing Board of Water and Light wells in the north Lansing well field. These public-supply wells have the potential to withdraw contaminated ground water. Groundwater-flow simulations and particle-tracking analyses with a regional model were used to investigate local ground-water movement. The effectiveness of hypothetical purge wells to remove ground water containing vinyl chloride was also evaluated. Five pumping scenarios were developed to assess effects of existing groundwater pumping conditions and alternative groundwater management options on the movement of the vinyl chloride plume in the Saginaw aquifer. Results indicate that under 1995 average pumping conditions, four public-supply wells in the north Lansing well field will remove water that originates in a portion of the Saginaw aquifer known to be contaminated with vinyl chloride. When pumping rates by wells in the north Lansing well field are reduced to simulate winter withdrawals, four public-supply wells to the west and south of the north Lansing well field remove water that originates in a portion of the Saginaw aquifer known to be contaminated with vinyl chloride. Simulation results indicate that purge wells can be used to capture most contaminated water and prevent interception of contaminated water by supply wells. However, further analysis is needed to determine the full extent of the vinyl chloride plume and the potential impact on Lansing Board of Water and Light public-supply wells.

Michigan↗
USGS1999

Evaluation of surface-water/ground-water interactions in the Santa Clara River Valley, Ventura County, California

The interactions of surface water and ground water along the Santa Clara River in Ventura County, California, were evaluated by analyzing river-discharge and water-quality data and geohydrologic information collected by the U.S. Geological Survey between 1993 and 1995 for the Piru, Fillmore, and Santa Paula subbasins. Measurements of discharge and water quality were made at multiple locations along the Santa Clara River and its tributaries at eight different time periods during different releases from Lake Piru. Geologic, hydraulic, and water-quality data were collected from three new multiple-completion ground-water monitoring wells. These data, together with data collected as part of the U.S. Geological Survey Southern California Regional Aquifer-System Analysis (RASA) study, were analyzed in order to quantify rates and locations of ground-water recharge and discharge within the river, characterize the correlation of recharge and discharge rates with ground-water conditions and reservoir releases, and better characterize the three-dimensional ground-water flow system. Analysis of the data indicates that the largest amount of ground-water recharge from the river consistently occurs in the Piru subbasin. Some ground-water recharge from the river may occur in the upper part of the Fillmore subbasin. Increases in sulfate concentrations indicate that increases in flow at the lower ends of the Piru and Fillmore subbasins result from high-sulfate ground-water discharge. Increases in flow in the lower part of the Santa Paula subbasin are not accompanied by significant sulfate increases. Several sets of regressions indicate possible correlation between net flow changes in the river and depths to ground water and release rates from Lake Piru. These statistical relations may be of use for evaluating alternative Lake Piru release strategies. Data on the stable isotopes of hydrogen and oxygen from the ground-water monitoring wells that were installed as part of this investigation were used to distinguish between zones affected by recharge from the Santa Clara River and zones affected by recharge from local precipitation. Tritium data from a new multiple-completion monitoring site indicate that near the river in the upper Santa Paula subbasin, recent (post-1950) recharge water is not present at depths greater than about 350 feet below land surface. Water-level and lithologic data from the monitoring site indicate that the river and the Shallow aquifer have only limited hydraulic connection to the underlying aquifers at this location. Water-level data from the Shallow aquifer and from an in-stream drive point were used in an analytic model to estimate hydraulic properties governing stream?aquifer interactions in the upper Santa Paula subbasin. Hydraulic conductivities in all the USGS monitoring wells were estimated on the basis of slug tests.

Water-Resources Investigations Report↗
USGS2000

Effects of ground-water withdrawals on the Rock River and associated valley aquifer, eastern Rock County, Minnesota

A better understanding of the ground-water and surface-water resources of the Rock River Valley in southwestern Minnesota was needed due to concerns surrounding future reliable sources of water for public supply. The Rock River Valley aquifer consists of a surficial sand and gravel unit that underlies the entire Rock River Valley and a buried sand and gravel unit that is present only in the vicinity of the Luverne Municipal and Airport well fields. The surficial and buried units of the aquifer are separated by a clay and till layer ranging in thickness from 1 to 38 feet. The combined maximum saturated thickness of the aquifer is 52 feet, with a median of 22 feet. The thickness of the buried unit ranges from 3 to 17 feet. Recharge to the Rock River Valley aquifer occurs primarily by infiltration of precipitation to the saturated zone (areal recharge) and by induced infiltration from the Rock River due to withdrawals from supply wells near the river. Discharge from the aquifer occurs primarily as leakage to streams and ground-water evapotranspiration. The water budget for the calibrated steady-state simulation indicated that areal recharge accounts for 38 percent of the sources of water to the Rock River Valley aquifer and leakage from streams contributes 58.7 percent. The largest discharge from the aquifer is leakage to streams, (71.1 percent). The net stream-aquifer leakage is approximately 5 cubic feet per second from the aquifer to the streams. The simulated contributing areas for the wells in the three well fields generally extend to the aquifer boundaries on the west and are generally truncated at the Rock River. The simulated transient water budget for 1996 indicated that the principal sources of water to the aquifer were as follows: (1) winter, spring, and late summer stress periods— leakage from streams and water released from storage and (2) early summer and fall stress periods—areal recharge and leakage from streams. The principal discharges from the aquifer were leakage to streams for all stress periods, ground-water evapotranspiration for the early and late summer stress periods, and addition to storage for the early summer and fall stress periods. The herbicides atrazine, alachlor, metolachlor, acetachlor, and cyanazine, and metabolites of these herbicides, occurred in concentrations of 0.05 to 11.5 micrograms per liter in the Rock River at Luverne during major runoff events following application of herbicides in the spring. Atrazine and metabolites, alachlor ESA (a metabolite of alachlor), metolachlor and metabolites, metolachlor ESA and metolachlor OA, and acetochlor metabolites acetochlor ESA and acetochlor OA, were detected at concentrations of 0.05 to 2.8 micrograms per liter in municipal supply wells less than 500 feet from the river during November 1995 through August 1997. The Rock River is the major source of the herbicides and metabolites. However, concentrations of atrazine and metabolites, alachlor ESA, metolachlor ESA, and metolachlor OA in supply wells may also reflect sources of these herbicides and metabolites in the ground-water contributing areas to the supply wells. Nitrite plus nitrate nitrogen concentrations in supply wells and in the ground-water contributing area to the Luverne Municipal well field were generally less than 1.5 milligrams per liter. Nitrite plus nitrate nitrogen concentrations of 2.4 to 8.5 milligrams per liter in the Rock River in the Rock County Rural Water well field and 14 to 18 milligrams per liter in the ground-water contributing area to the Rock County Rural Water supply wells are not having a substantial affect on nitrite plus nitrate nitrogen concentrations in most supply wells. Isotopic mixing calculations indicate that proportions of river water withdrawn from supply wells less than 500 feet from the river range from 5 to 60 percent of total withdrawals. The Rock River is a gaining stream in most reaches, but is losing water to the aquifer in the vicinity of the Luverne Municipal and Rock County Rural Water well fields, located 150 to 1,500 feet from the river. Simulated streamflow losses due to ground-water withdrawals in the well fields were approximately 2.1 cubic feet per second. Because an average of about 1.5 cubic feet per second of the water pumped by Luverne is returned to the Rock River as wastewater discharge, the net steady-state simulated streamflow loss for the study area is 0.6 cubic feet per second. The streamflow losses as a result of ground-water withdrawals are insignificant in comparison to typical streamflow, and are likely to have a measurable effect on streamflow only during low-flow conditions of less than approximately 10 cubic feet per second. Model results indicate that the additional water withdrawn by wells due to anticipated increased ground-water withdrawals was derived from a decrease in net leakage of ground water from the aquifer to the streams. The simulations indicated that the increased ground-water withdrawals and normal precipitation resulted in an increase in induced infiltration from the Rock River of 0.1 cubic feet per second for the Luverne Municipal well field and 0.3 cubic feet per second for the Rock County Rural Water well field. Maximum drawdowns ranged from 0.5 to 1.4 feet near the three well fields. For drought conditions, the simulated streamflow losses constituted approximately 30 percent and nearly 65 percent of the flows in the Rock River for the Luverne Municipal and Rock County Rural Water well fields, respectively. Maximum drawdowns ranged from 3.8 to 7.0 feet near the three well fields. Transient simulations with anticipated increased ground-water withdrawals and drought conditions indicated declines in hydraulic heads ranging from 0.2 to 0.4 feet per year in the vicinity of the three well fields, except for near the Rock River.

Minnesota↗
USGS1971

Hydrologic data for Horseshoe Lake, Arkansas and vicinity

During the summer and fall, seepage and evaporation losses from Horseshoe Lake, an oxbow or an 'old river' lake adjacent to the Mississippi River, exceed inflow to the lake, and seasonal declines of 2.5-3.0 feet in the lake level are common. In exceptionally dry years, the minimum lake level has been as much as 4 feet below the normal seasonal low. These low levels severely affect the recreational uses of the lake. Seepage and evaporation rates at Horseshoe Lake were determined from hydrologic and meteorologic data. Analysis of these data indicates that the direction of seepage is out of the lake except for a period of about 2 months in the spring, when the stage of the Mississippi River is high. The lake can be maintained at a constant level by supplementing the inflow to the lake with surface or ground water. Contributions to the lake from local drainage can be increased, but this water contains undesirable amounts of pesticides, herbicides, and plant nutrients, and the flow is insufficient to eliminate seasonal declines in the lake level. Water from r the Mississippi River can be used to maintain a given lake level, but the bacteriological quality of water from the river makes this an undesirable source of supplemental water. Water from the Quaternary alluvium contains troublesome amounts of iron, but it probably is free of pesticides, herbicides, and coliform bacteria which are commonly found in surface water. An electric-analog model was used to determine the rate at which inflow to the lake must be supplemented to maintain various lake levels. During this investigation, the lake could have been maintained very near the normal spring level by supplementing the inflow at a maximum rate of 10,600 gallons per minute. The analog model was also used to determine the effects of pumping wells on seepage. With the exception of wells near the southeast end of the lake, wells located within one-half mile of the lake would obtain more than 50 percent of their yield from the lake after pumping for 90 days.

Open-File Report↗
USGS1999

Conceptual Model and Numerical Simulation of the Ground-Water-Flow System in the Unconsolidated Sediments of Thurston County, Washington

The demand for water in Thurston County has increased steadily in recent years because of a rapid growth in population. Surface-water resources in the county have been fully appropriated for many years and Thurston County now relies entirely on ground water for new supplies of water. Thurston County is underlain by up to 2,000 feet of unconsolidated glacial and non-glacial Quaternary sediments which overlie consolidated rocks of Tertiary age. Six geohydrologic units have been identified within the unconsolidated sediments. Between 1988 and 1990, median water levels rose 0.6 to 1.9 feet in all geohydrologic units except bedrock, in which they declined 1.4 feet. Greater wet-season precipitation in 1990 (43 inches) than in 1988 (26 inches) was the probable cause of the higher 1990 water levels. Ground-water flow in the unconsolidated sediments underlying Thurston County was simulated with a computerized numerical model (MODFLOW). The model was constructed to simulate 1988 ground-water conditions as steady state. Simulated inflow to the model area from precipitation and secondary recharge was 620,000 acre-feet per year (93 percent), leakage from streams and lakes was 38,000 acre-ft/yr (6 percent), and ground water entering the model along the Chehalis River valley was 5,800 acre-ft/yr (1 percent). Simulated outflow from the model was primarily leakage to streams, springs, lakes, and seepage faces (500,000 acre-ft/yr or 75 percent of the total outflow). Submarine seepage to Puget Sound was simulated to be 88,000 acre-ft/yr (13 percent). Simulated ground-water discharge along the Chehalis River valley was simulated to be 12,000 acreft/yr (2 percent). Simulated withdrawals by wells for all purposes was 62,000 acre-ft/yr (9 percent). The numerical model was used to simulate the possible effects of increasing ground-water withdrawals by 23,000 acre-ft/yr above the 1988 rate of withdrawal. The model indicated that the increased withdrawals would come from reduced discharge to springs, seepage faces, and offshore (total of 51 percent of increased pumping) and decreased flow to rivers (46 percent). About 3 percent would come from increased leakage from rivers. Water levels would decline more than 1 foot over most of the model area, more than 10 feet over some areas, and would be at a maximum of about 35 feet. Contributing areas for water discharging at McAllister and Abbott Springs and to pumping centers near Tumwater and Lacey were estimated using a particle-tracking post-processing computer code (MODPATH) and a MODFLOW model calibrated to steady-state (1988) conditions. Water discharging at McAllister and Abbot Springs was determined to come from water entering the ground-water system at the water table in an area of about 20 square miles (mi2) to the west and south of the springs. This water is estimated to come from recharge (both precipitation and secondary) and from leakage from Lake St. Clair and several other surface-water bodies. Southeast of Lacey, about 3,800 acre-ft of ground water were pumped from five municipal wells during 1988. The source of the pumped water was determined to be an area that covers about 1.1 mi2. The water was estimated to come from recharge (both precipitation and secondary) and leakage from surface-water bodies. Along the lower Deschutes River nearly 3,900 acre-ft/yr of ground water were pumped during 1988 from 15 wells for municipal and industrial use. The calculated source of this water was an area that covers about 1.3 mi2. Within the calculated contributing area the pumped ground water comes from recharge (both precipitation and secondary) and leakage from the Deschutes River and several other surface-water bodies.

Water-Resources Investigations Report↗
USGS1983

Hydrogeology of parts of the Central Platte and Lower Loup Natural Resources Districts, Nebraska

Water-level declines of at least 15 feet have occurred in this heavily irrigated area of central Nebraska since the 1930's, and potential for additonal declines is high. To test the effects of additional irrigation development on water levels and streamflow , computer programs were developed that represent the surface-water system, soil zone, and saturated zone. A two-dimensional, finite-difference ground-water flow model of the 3,374 square-mile study area was developed and calibrated using steady-state and transient conditions. Three management alternatives were examined. First, 125,000 acre-feet of water would be diverted annually from the Platte River. During a water year in which flows are similar to those in 1957, months of zero streamflow at Grand Island increased from the historical 2, to 7. After 5 years of such low flows, in 36 nodes (997.4 acres per node) water levels declined more than 5 feet, with a maximum decline of 10.7 feet. A second alternative would allow no new ground-water development after 1980. The third alternative would allow irrigable but unirrigated land to be developed at an annual rate of 2, 5, and 8 percent and to apply irrigation water at 80, 100, and 120 percent of consumptive irrigation requirements. The maximum projected declines by 2020 are 119 and 139 feet, respectively, for the second and third alternatives. (USGS)

Water-Resources Investigations Report↗
USGS2001

Optimization of ground-water withdrawal at the old O-Field area, Aberdeen Proving Ground, Maryland

The U.S. Army disposed of chemical agents, laboratory materials, and unexploded ordnance at the Old O-Field landfill at Aberdeen Proving Ground, Maryland, beginning prior to World War II and continuing until at least the 1950?s. Soil, ground water, surface water, and wetland sediments in the Old O-Field area were contaminated by the disposal of these materials. The site is in the Atlantic Coastal Plain, and is characterized by a complex series of Pleistocene and Holocene sediments formed in various fluvial, estuarine, and marine-marginal hydrogeologic environments. A previously constructed transient finite-difference ground-water-flow model was used to simulate ground-water flow and the effects of a pump-and-treat remediation system designed to prevent contaminated ground water from flowing into Watson Creek (a tidal estuary and a tributary to the Gunpowder River). The remediation system consists of 14 extraction wells located between the Old O-Field landfill and Watson Creek.Linear programming techniques were applied to the results of the flow-model simulations to identify optimal pumping strategies for the remediation system. The optimal management objective is to minimize total withdrawal from the water-table aquifer, while adhering to the following constraints: (1) ground-water flow from the landfill should be prevented from reaching Watson Creek, (2) no extraction pump should be operated at a rate that exceeds its capacity, and (3) no extraction pump should be operated at a rate below its minimum capacity, the minimum rate at which an Old O-Field pump can function. Water withdrawal is minimized by varying the rate and frequency of pumping at each of the 14 extraction wells over time. This minimizes the costs of both pumping and water treatment, thus providing the least-cost remediation alternative while simultaneously meeting all operating constraints.The optimal strategy identified using this objective and constraint set involved operating 13 of the 14 extraction wells at rates ranging from 0.4 to 4.9 gallons per minute.

Water-Resources Investigations Report↗
USGS1988

A digital simulation of the glacial-aquifer system in Sanborn and parts of Beadle, Miner, Hanson, Davison, and Jerauld counties, South Dakota

The drought in South Dakota from 1974-76 and the near drought conditions in 1980-81 have resulted in increased demands on the groundwater resources within many of the irrigated areas of the James River basin in eastern South Dakota. These increases in demand for irrigation water from the glacial aquifer system, and continued requests to the State for additional irrigation well permits, have created a need for a systematic water management program to avoid over-development of this system in the James River basin. An equally spaced grid containing 56 rows and 52 columns used to simulate the glacial aquifer system, was calibrated using water level data collected before significant groundwater development (before 1973). The aquifer was also simulated in 11 annual transient stress periods from 1973 through 1983 and in 12 monthly transient stress periods for 1976. The simulated pre-development potentiometric heads were compared to average water levels from 32 observation wells to check the accuracy of the simulate potentiometric surface. The average arithmetic difference between the simulated and observed water levels was 1.68 ft and the average absolute difference was 4.38 ft. The non-pumping steady-state simulated water budget indicates that recharge from precipitation accounts for 97.1% of the water entering the aquifer and evapotranspiration accounts for 98.2% of the water leaving the aquifer. The sensitivity analysis of the steady-state model indicates that the model is most sensitive to reductions in recharge and least to changes in hydraulic conductivity. The maximum annual recharge varied from 0.10 inch in 1976 to 8.14 inches in 1977. The potential annual evapotranspiration varied from 29.9 inches in 1982 to 48.9 inches in 1976. Withdrawals from the glacial aquifer system increased 2.6 times between 1975 and 1976. The average annual arithmetic difference between the simulated and observed water levels ranged from 3.88 ft in 1974 to 2.23 ft in 1982; the average absolute difference ranged from 4.70 ft in 1973 to 11.70 ft in 1982. In the 1976 monthly transient simulation, the maximum annual recharge rate 0.10 inch was distributed over the months of March, April, and September. The potential monthly evapotranspiration rate ranged from 12.50 inches in August to 0.00 inch during the winter when the ground was frozen. (Author 's abstract)

Water-Resources Investigations Report↗
USGS1981

Simulative models for the analysis of ground-water flow in Vekol Valley, the Waterman Wash area, and the Bosque area, Maricopa and Pina Counties, Arizona

Simulative ground-water flow models for Vekol Valley, the Waterman Wash area, and the Bosque area were developed for use in evaluating alternatives for developing a ground-water supply for the Ak-Chin Indian Community. The hydraulic properties of the basin-fill deposits used in the models were estimated primarily from aquifer tests made by the U.S. Geological Survey. Annual recharge to Vekol Valley and the Waterman Wash area is negligible in comparison to the quantity of water in storage and the quantity proposed to be pumped. The models are based on a three-dimensional, block-centered, finite-difference scheme. The Vekol Valley model was calibrated for steady-state onditions, and the Waterman Wash area model was calibrated for steady-state and transient conditions. The sensitivity of calibrated heads to changes in transmissivity was also investigated. An uncalibrated storage-depletion model was developed for the Bosque area. Simulated water levels for steady-state conditions average within 5 feet of measured values for Vekoi Valley and within 6 feet for the Waterman Wash area. Simulated water levels for transient conditions in the Waterman Wash area average within 8 feet of measured values for 15 years of analysis and within 15 feet for 24 years. Water-level declines simulated by the Waterman Wash area model average within 17 feet of those measured during the 24-year period, 1951-75.

Arizona↗
USGS1998

Steady-state simulation of ground-water flow in the Rush Springs Aquifer, western Oklahoma

A simplified steady-state ground-water flow model was prepared for the Rush Springs aquifer in western Oklahoma. A 3-kilometer square grid was established over the area containing two layers with 674 active nodes simulated in the model. The steady-state model simulation used a mean recharge rate of 3.05 x 10-4 feet per day and a hydraulic conductivity range from 0.8 to 10 feet per day. The error at each node in the model is defined as the difference between the measured and simulated water levels.The arithmetic mean error for 170 of the 674 active nodes was -0.11 feet, the absolute value mean error was 7.55 feet, and the standard deviation of the error was 10.21 feet. A net simulated recharge of 231 cubic feet per second is balanced by a discharge to drains and seeps of 190.6 cubic feet per second about 82 percent of the total recharge. Discharge to the main stem of the Washita River is about 41 cubic feet per second about 18 percent of the recharge.

Water-Resources Investigations Report↗
USGS2001

Simulation of a valley-fill aquifer system to delineate flow paths, contributing areas, and traveltime to wellfields in southwestern Broome County, New York

A valley-fill aquifer system that extends along a 14-mile reach of the Susquehanna River valley in southwestern Broome County, N.Y., is a major source of water supply to local municipalities and industries, but is highly susceptible to contamination from human activities. Protection of ground-water supplies requires accurate delineation of the areas that are the sources of water pumped by wells. A previously developed two-layer steady-state ground-water flow model of the aquifer system was upgraded with an improved method of simulating stream-aquifer interactions, then recalibrated and coupled to a particle-tracking program. Three-dimensional, ground-water flow modeling coupled with particle tracking is the most reliable method of simulating groundwater flow paths in multiaquifer systems such as this; it also allows delineation of contributing areas to well.elds. A primary advantage of three-dimensional particle-tracking analysis is that it shows the complexities of the flow paths in each aquifer. Model and particle tracking analyses indicate that groundwater frequently follows convoluted three-dimensional flow paths. The contributing areas of individual supply wells in this aquifer system each has a unique flow pattern and shape. Results of the model simulation indicate that recharge from precipitation, rivers, and tributaries contribute 35 percent, 29 percent, and 25 percent, respectively to the aquifer system and that pumpage from supply wells accounts for 67 percent of the discharge from the aquifer system. Particle-tracking results indicate that the simulated contributing areas to the 24 supply wells includes most of the valley floor.

New York↗
USGS2006

Hydrogeology and simulation of ground-water flow in the Silurian-Devonian aquifer system, Johnson County, Iowa

Bedrock of Silurian and Devonian age (termed the “Silurian-Devonian aquifer system”) is the primary source of ground water for Johnson County in east-central Iowa. Population growth within municipal and suburban areas of the county has resulted in increased amounts of water withdrawn from this aquifer and water-level declines in some areas. A 3-year study of the hydrogeology of the Silurian-Devonian aquifer system in Johnson County was undertaken to provide a quantitative assessment of ground water resources and to construct a ground-water flow model that can be used by local governmental agencies as a management tool. Johnson County is underlain by unconsolidated deposits of Quaternary age and Paleozoic-age bedrock units. The bulk of the Quaternary deposits consists of weathered and unweathered glacial till; however, shallow alluvium and buried sand and gravel deposits also are present. Six bedrock hydrogeologic units are present in Johnson County (oldest to youngest): Maquoketa confining unit, Silurian aquifer, Wapsipinicon Group (aquifer and confining unit), Cedar Valley aquifer, Upper Devonian shale confining unit, and Cherokee confining unit. Although separate aquifers and confining units are described, the Silurian- and Devonian-age units are considered as a single aquifer system. The top of the Silurian-Devonian aquifer system is considered as the top of the Cedar Valley aquifer, where present, and the base of the aquifer system is considered as the top of the Maquoketa confining unit. The hydraulic properties of the rocks that comprise the Silurian-Devonian aquifer system are highly variable as a result of the variable composition of the rocks and the presence of solution features in some of the carbonate-rock units. For the combined Silurian-Devonian aquifer system, specific capacity averages 2.1 gallons per minute per foot of drawdown, transmissivity averages about 580 feet squared per day, and hydraulic conductivity averages 8.3 feet per day. Recharge to the Silurian-Devonian aquifer system in Johnson County is predominantly from infiltration of precipitation to the bedrock. Discharge from the aquifer is primarily to municipal, industrial, and private-development wells. Reliable measurements of the amount of recharge to or discharge from the ground-water system in Johnson County, however, are not available. Altitude of the 1996 potentiometric surface ranged from more than 750 feet above the North American Vertical Datum of 1988 (NAVD88) in northern Johnson County to less than 575 feet above NAVD88 in the central part of the county. A large cone of depression within the potentiometric surface is present in the central part of the county, between Coralville and Iowa City. A large limestone quarry is located near the center of this cone of depression. Ground water generally flows from the northern and western parts of Johnson County either toward the cone of depression in the center of the county or south out of the county. Ground water also flows toward the Cedar River in the northeastern part of the county. A ground-water divide in the northeastern part of the county roughly approximates the surface-water divide between the Iowa River and Cedar River drainages. A numerical ground-water-flow model of the Silurian-Devonian aquifer system in Johnson County was used to test concepts of ground-water flow, to assess the need for additional data, and to evaluate the potential effects of anticipated increased ground-water development and drought. The 1-layer model was calibrated to average 1996 ground-water conditions, which were assumed to approximate steady-state flow conditions. The model also was used to simulate steady-state conditions for 2004, steady-state conditions using anticipated pumping rates for 2025, and potential future drought conditions. The simulated potentiometric surface generally replicated the potentiometric surface for 1996 and 2004 conditions. The calculated root mean squared error values for the 1996 and 2004 simulations were 13.6 and 18.6 feet, respectively. The mean absolute differences between measured and simulated water levels for the 1996 and 2004 simulations were about 11 and 14 feet, respectively. Total model-calculated inflow to the ground-water system for the 1996 simulation was 19.6 million gallons per day (Mgal/d), and the largest model-calculated inflow component was areal recharge (15.1 Mgal/d). Total model-calculated outflow from the ground-water system was 19.7 Mgal/d, and the largest outflow component was discharge to wells (10.5 Mgal/d). Model-calculated water-budget components for the 2004 simulation were similar to the 1996 components. Potential future steady-state conditions were simulated using anticipated 2025 pumping rates. Pumpage both for existing wells and for assumed new wells, based on anticipated population growth in the northern part of the county and for the nearby municipalities, was included in the model. Simulated 2025 pumpage was about 1.5 Mgal/d greater than simulated 2004 pumpage. Simulated steady-state ground-water levels, using anticipated 2025 pumping rates, were lower than 2004 simulated levels throughout the county, and simulated water-level declines ranged from less than 1 foot near the county boundaries to about 11 feet. Potential future drought conditions were simulated by assuming that recharge to the Silurian-Devonian aquifer system is reduced by a factor of 0.75 and that water-supply pumpage is increased by a factor of 1.25 over the anticipated 2025 pumping rates. Overall, simulated water levels for future drought conditions were greater than 5 feet lower than simulated 2004 conditions and were a maximum of about 30 feet lower in the northeastern part of the county. The greatest limitation to the model is the lack of measured or estimated water-budget components for comparison to simulated water-budget components. Because the model is only calibrated to measured water levels, and not to water-budget components, the model results are nonunique. Other model limitations include the relatively coarse grid scale, lack of detailed information on pumpage from the quarry and from private developments and domestic wells, and the lack of separate water-level data for the Silurian- and Devonian-age rocks.

Iowa↗
USGS2008

Conceptual Model of Hydrologic and Thermal Conditions of the Eastbank Aquifer System near Rocky Reach Dam, Douglas County, Washington

The Lower and Combined Aquifers of the Eastbank Aquifer system, located in a river-terrace deposit along the Columbia River near Rocky Reach Dam, Washington, are primarily recharged by the Columbia River and provide water to the Eastbank Hatchery and the regional water system servicing the cities of Wenatchee, East Wenatchee, and parts of unincorporated Chelan and Douglas Counties. In 2006, mean annual pumpage from the aquifers by the hatchery and regional water system was about 43 and 16 cubic feet per second, respectively. Reportedly, temperatures of ground water pumped by the hatchery have been increasing, thereby making water potentially too warm for salmonid fish production. An evaluation of hourly ground-water and river temperatures from January 1991 through August 2007 indicates increasing interannual trends in temperatures in most of the Lower and Combined Aquifers from 1999 through 2006 that correspond to increasing trends in the annual mean and annual maximum river temperatures during the same period of 0.07 and 0.17?C per year, respectively. There were no trends in the annual minimum river temperatures from 1999 through 2006, and there were no trends in the annual minimum, mean, and maximum river temperatures from 1991 through 1998 and from 1991 through 2007. Increases in river temperatures from 1999 through 2006 are within the natural variability of the river temperatures. Most of the Lower and Combined Aquifers reached thermal equilibrium?defined by constant time lags between changes in river temperatures and subsequent changes in ground-water temperatures?during 1991?98. The only exceptions are the Combined Aquifer north of the well field of the regional water system, which had not reached thermal equilibrium by 2006, and the Lower Aquifer west of the well fields of the hatchery and the regional water system, which reached thermal equilibrium prior to 1991. Because most of the Lower and Combined Aquifers were in thermal equilibrium from 1999 through 2006 and seasonal pumpage patterns were relatively stable, reported trends of increasing temperatures of water pumped by the hatchery well field are most likely explained by increasing trends in river temperatures. Most of the water pumped by the hatchery well field recharges in an area west to southwest of the well field about 2 months prior to the time it is pumped from the aquifer. The northern extent of the hatchery well field may pump some colder water from a bedrock depression to the north and west of the well field. The conceptual model of hydrologic and thermal conditions is supported by analyses of historical water temperatures, water-level data collected on July 18, 2007, and dissolved-constituent and bacterial concentrations in samples collected on August 20?22, 2007.

Scientific Investigations Report↗
USGS2006

The Cerrillos Uplift, the La Bajada Constriction, and Hydrogeologic Framework of the Santo Domingo Basin, Rio Grande Rift, New Mexico

The geologic, geophysical, and hydrogeologic properties of the La Bajada constriction and Santo Domingo Basin, northern New Mexico, result from tectonic and volcanic processes of the late Tertiary and Quaternary Rio Grande rift. An integrated geologic and geophysical assessment in the La Bajada constriction allows development of a geologic framework that can provide input for regional ground-water flow models. These models then can provide better estimates of future water supplies in a region that largely subsists on aquifers in Rio Grande rift basins. The combination of surface geologic investigations (stratigraphic and structural studies; chapters A, B, C, and E), airborne geophysics (aeromagnetic and time-domain electromagnetic surveys; chapters D and F), ground geophysical measurements (gravity and magnetotelluric surveys; chapters D and F), and data from the few wells in the area (chapter G) provides new constraints on the hydrogeologic framework of this area. Summary results of our investigations are synthesized in chapter G. Through-going aquifers consisting of ancestral Rio Grande axial-river sand and gravel and of coarse western-piedmont gravel form the predominant ground-water pathways through the partly buried structural trough defining the La Bajada constriction between Española and Santo Domingo Basins. Thick, clay-rich Cretaceous marine shales of low hydraulic conductivity form a pervasive regional confining unit within the Cerrillos uplift on the southeast flank of the constriction. Numerous, dominantly north-northwest-striking, intrabasin faults that project part way across the La Bajada constriction create a matrix of laterally and vertically variable hydrogeologic compartments that locally partition and deflect ground-water flow parallel to faults.

New Mexico↗
USGS2009

Reconnaissance Assessment of the Potential for Roadside Dry Wells to Affect Water Quality on the Island of Hawai'i

The County of Hawai'i Department of Public Works (DPW) uses dry wells to dispose of stormwater runoff from roads. Recently, concern has been raised that water entering the dry wells may transport contaminants to groundwater and affect the quality of receiving waters. The DPW operates 2,052 dry wells. Compiling an inventory of these dry wells and sorting it on the basis of presence or absence of urbanization in the drainage area, distance between the bottom of the dry well and the water table, and proximity to receiving waters helps identify the dry wells having greatest potential to affect the quality of receiving waters so that future studies or mitigation efforts can focus on a smaller number of dry wells. The drainage areas of some DPW dry wells encompass urbanized areas, which could be a source of contaminants. Some dry wells penetrate close to or through the water table, eliminating or substantially reducing opportunities for contaminant attenuation between the ground surface and water table. Dry wells that have drainage areas that encompass urbanization, penetrate to near the water table, and are near the coast have the highest potential to affect the quality of coastal waters (this study did not consider specific sections of coastline that may be of greater concern than others). Some DPW dry wells, including a few that have drainage areas that encompass urbanization, lie within the areas contributing recharge (ACR) to drinking-water wells. Numerical groundwater modeling studies by previous investigators indicate that water infiltrating those dry wells could eventually be pumped at drinking-water wells. Dry wells that have a high potential for affecting coastal receiving waters or drinking-water wells can be the focus of studies to further understand the effect of the dry wells on the quality of receiving waters. Possible study approaches include sampling for contaminants at the dry well and receiving water, injecting and monitoring the movement of tracers, and numerical modeling. To fully assess whether dry wells actually pose a significant contamination threat to receiving waters, results from modeling or monitoring must be compared to limits for contaminant concentration at receiving waters. These limits are usually established by the agencies tasked with protecting those waters.

Scientific Investigations Report↗
USGS2003

Simulation of Ground-Water Flow in the Irwin Basin Aquifer System, Fort Irwin National Training Center, California

Ground-water pumping in the Irwin Basin at Fort Irwin National Training Center, California resulted in water-level declines of about 30 feet from 1941 to 1996. Since 1992, artificial recharge from wastewater-effluent infiltration and irrigation-return flow has stabilized water levels, but there is concern that future water demands associated with expansion of the base may cause a resumption of water-level declines. To address these concerns, a ground-water flow model of the Irwin Basin was developed to help better understand the aquifer system, assess the long-term availability and quality of ground water, and evaluate ground-water conditions owing to current pumping and to plan for future water needs at the base. Historical data show that ground-water-level declines in the Irwin Basin between 1941 and 1996, caused the formation of a pumping depression near the pumped wells, and that recharge from the wastewater-treatment facility and disposal area caused the formation of a recharge mound. There have been two periods of water-level recovery in the Irwin Basin since the development of ground water in this basin; these periods coincide with a period of decreased pumpage from the basin and a period of increased recharge of water imported from the Bicycle Basin beginning in 1967 and from the Langford Basin beginning in 1992. Since 1992, artificial recharge has exceeded pumpage in the Irwin Basin and has stabilized water-level declines. A two-layer ground-water flow model was developed to help better understand the aquifer system, assess the long-term availability and quality of ground water, and evaluate ground-water conditions owing to current pumping and to plan for future water needs at the base. Boundary conditions, hydraulic conductivity, altitude of the bottom of the layers, vertical conductance, storage coefficient, recharge, and discharge were determined using existing geohydrologic data. Rates and distribution of recharge and discharge were determined from existing data and estimated when unavailable. Results of predictive simulations indicate that in 50 years, if artificial recharge continues to exceed pumpage in Irwin Basin, water levels could rise as much as 65 feet beneath the pumping depression, and as much as 10 feet in the wastewater-treatment facility and disposal area. Particle-tracking simulations were used to determine the pathlines and the traveltimes of water high in dissolved solids into the main pumping area. The pathlines of particles from two areas with high dissolved-solids concentrations show that in 50 years water from these areas almost reaches the nearest pumped well.

Water-Resources Investigations Report↗
USGS2002

Spatial and temporal variations in streamflow, dissolved solids, nutrients, and suspended sediment in the Rio Grande Valley study unit, Colorado, New Mexico, and Texas, 1993–95

Streamflow and water quality vary spatially and temporally in the Rio Grande from Del Norte, Colorado, to El Paso, Texas. The variations in streamflow and in concentrations of selected waterquality constituents—dissolved solids, dissolved nitrite plus nitrate as nitrogen, total phosphorus, and suspended sediment—are described in this report. A multivariate linear regression model, ESTIMATOR2000, was used to estimate loads for selected constituents. Streamflow decreases in the downstream direction throughout most of the basin because outflows (due to agricultural use, leakage to ground water, and evapotranspiration) are greater than inflows. Streamflow increases between Rio Grande above the mouth of Trinchera Creek, near Lasauses, Colorado, to Rio Grande at Otowi Bridge, near San Ildefonso, New Mexico, because ground-water and tributary inflow are greater than outflow. Concentrations of dissolved solids, dissolved nitrite plus nitrate, total phosphorus, and suspended sediment generally increase in the downstream direction. Concentrations of dissolved solids, dissolved nitrite plus nitrate, and total phosphorus decrease between Rio Grande above the mouth of Trinchera Creek, near Lasauses, Colorado, and Rio Grande at Otowi Bridge, near San Ildefonso, New Mexico, because of dilution by tributary inflow. Concentrations of dissolved nitrite plus nitrate, total phosphorus, and suspended sediment decrease between Rio Grande Floodway at San Marcial, New Mexico, and Rio Grande below Leasburg Dam, near Leasburg, New Mexico, because of reservoir effects (nutrient uptake and settling of sediment). Several instances of decreasing streamflow and increasing loads indicate the presence of inflows with large constituent concentrations (relative to those of the Rio Grande immediately upstream from that inflow); this occurs (1) between Rio Grande near Del Norte, Colorado, and Rio Grande above the mouth of Trinchera Creek, near Lasauses, Colorado, for dissolved solids, (2) between Rio Grande at Otowi Bridge, near San Ildefonso, New Mexico, and Rio Grande Floodway at San Marcial, NewMexico, for all constituents, and (3) between Rio Grande below Leasburg Dam, near Leasburg, New Mexico, and Rio Grande at El Paso, Texas, for all constituents. Streamflow increases along every reach of the Rio Grande between the streamflow-gaging station Rio Grande above the mouth of Trinchera Creek, near Lasauses, Colorado, and the station Rio Grande at Otowi Bridge, near San Ildefonso, NewMexico. These increases in streamflow result in increases in the loads of dissolved solids, total phosphorus, and suspended sediment regardless of changes in concentrations.

Water-Resources Investigations Report↗