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128 records · Page 8Linked to original sources
USGS2015

Conceptual and numerical models of groundwater flow in the Ogallala and Arikaree aquifers, Pine Ridge Indian Reservation area, South Dakota, water years 1980-2009

The Ogallala and Arikaree aquifers are the largest sources of groundwater on the Pine Ridge Indian Reservation and are used extensively for irrigation and public and domestic water supplies. To assess the potential for decreased water levels and discharge to streams in the Pine Ridge Indian Reservation, conceptual and numerical models of groundwater flow in the Ogallala and Arikaree aquifers in southwestern South Dakota were developed by the U.S. Geological Survey in cooperation with the Oglala Sioux Tribe. The study area includes most of the Pine Ridge Reservation in Jackson and Shannon Counties and Indian trust lands in Bennett County in southwestern South Dakota. The High Plains aquifer, which includes the Ogallala and Arikaree aquifers, generally is less developed in South Dakota compared with other areas underlain by this aquifer; therefore, water levels in the High Plains aquifer in South Dakota generally fluctuated by less than 5 feet (ft) from 1980 to 1999. Despite minimal water-level changes in the High Plains aquifer in South Dakota, extensive withdrawals of groundwater for irrigation have caused water-level declines in many areas and increased concerns about the long-term sustainability of the aquifer; therefore, continued or increased withdrawals from the aquifer or prolonged drought may have the potential to affect water levels within the aquifer and discharge to important streams in the area. The Ogallala and Arikaree aquifers generally consist of poorly consolidated claystones, siltstones, sandstones, and shale deposited in fluvial and lacustrine environments. Saturated thicknesses ranged from 10 to 314 ft for the Ogllala aquifer and from 10 to 862 ft for the Arikaree aquifer. Previous hydraulic conductivity estimates ranged from less than 1 to 180 feet per day (ft/d) for the Ogallala aquifer and from less than 1 to 13 ft/d for the Arikaree aquifer. Recharge to the Ogallala and Arikaree aquifers is from precipitation on the outcrop areas, and discharge occurs through evapotranspiration, discharge to streams, and well withdrawals. Evapotranspiration generally occurs in topographically low areas along streams, and maximum evapotranspiration occurs when the water level is at the land surface. The generalized groundwater-flow direction is to the northeast with local flow towards streams. Precipitation for water years 1980–2009 ranged from about 11 to 39 inches per year (in/yr) and averaged about 19 in/yr. Estimated mean recharge for water years 1980–2009 was about 17.3 percent of precipitation for the Ogallala aquifer and 7.9 percent of precipitation for the Arikaree aquifer. The estimated mean maximum evapotranspiration for water years 1980–2009 was about 35 in/yr. Estimated mean base flow for gaged streams was about 0.06 cubic foot per second (ft 3 /s) per square mile of drainage area. Estimated mean total water use for water years 1980–2009 was 5.4 ft 3 /s from the Ogallala aquifer and 7.1 ft 3 /s from the Arikaree aquifer. A two-layer numerical groundwater-flow model was constructed using MODFLOW–NWT with a uniformly spaced grid consisting of 166 rows and 288 columns with cells 1,640 ft on a side. The numerical model of the Ogallala and Arikaree aquifers was used to simulate steady-state and transient conditions for water years 1980–2009. Model calibration was accomplished using the Parameter ESTimation (PEST) program that adjusted individual model input parameters and assessed the difference between estimated and model-simulated values of hydraulic head and base flow. Aquifer boundaries were no-flow on the northern and western sides and constant-head on the southern and eastern sides. The mean arithmetic difference was 1.4 ft between the 731 simulated and observed hydraulic heads in the Ogallala aquifer and 9.8 ft between the 2,754 simulated and observed hydraulic heads in the Arikaree aquifer. Simulated mean discharge from the Ogallala and Arikaree aquifers to selected stream reaches was 92.1 ft 3 /s compared to estimated discharge of 88.7 ft 3 /s. Calibrated recharge for the transient simulation averaged 3.3 in/yr for the Ogallala aquifer and 1.1 in/yr for the Arikaree aquifer. The mean maximum potential evapotranspiration rate was 35.4 in/yr. Streambed conductance for perennial stream reaches averaged 530 feet squared per day. Horizontal hydraulic conductivity averaged 27 ft/d for the Ogallala aquifer and 1.0 ft/d for the Arikaree aquifer. The vertical hydraulic conductivity averaged 1.4 ft/d for the Ogallala aquifer and 0.004 ft/d for the Arikaree aquifer. Specific yield for the Ogallala aquifer was 0.15 (dimensionless) and averaged 0.02 for the Arikaree aquifer. Specific storage for the Arikaree aquifer was 1.7x10 -6 per foot. Simulated steady-state model inflow and outflow was 459 ft 3 /s. The percentages of inflows were 17 percent from constant-head boundaries, 9 percent from streams, and 74 percent from recharge. Percentages of outflow were 8 percent to constant-head boundaries, 1 percent to wells, 31 percent to streams, and 59 percent to evapotranspiration. Simulated net inflow from the Ogallala aquifer to the Arikaree aquifer ranged from about 22 ft 3 /s in dry years to about 37 ft 3 /s in wet years. Two hypothetical future stress scenarios were simulated using input from the 30-year calibrated simulation of water years 1980–2009. The first hypothetical scenario represented an increase in groundwater withdrawals from 50 hypothetical production wells completed in the Arikaree aquifer. At the end of the 30-year hypothetical increased pumping simulation, water levels declined as much as 66 ft in the Arikaree aquifer, decreased discharge to streams accounted for about 26 percent (2.6 ft 3 /s) of increased withdrawals, and decreased evapotranspiration accounted for about 53 (5.3 ft 3 /s) percent of increased withdrawals. The second hypothetical scenario represented a 30-year period of decreased recharge (drought) by decreasing recharge 0.2 inch (24 ft 3 /s) for each water year. At the end of the hypothetical drought simulation, water levels declined as much as 10.9 ft in the Arikaree aquifer, decreased discharge to streams accounted for about 23 percent (5.5 ft 3 /s) of decreased recharge, and decreased evapotranspiration accounted for about 72 percent (17.3 ft 3 /s) of decreased recharge. The numerical model is a tool that could be used to better understand the flow system of the Ogallala and Arikaree aquifers, to approximate hydraulic heads in the aquifer, and to estimate discharge to rivers, springs, and seeps in the Pine Ridge Reservation area in Bennett, Jackson, and Shannon Counties. The model also is useful to help assess the response of the aquifer to additional stress, including potential increased well withdrawals and potential drought conditions.

South Dakota↗
USGS2015

Areas contributing recharge to production wells and effects of climate change on the groundwater system in the Chipuxet River and Chickasheen Brook Basins, Rhode Island

The Chipuxet River and Chickasheen Brook Basins in southern Rhode Island are an important water resource for public and domestic supply, irrigation, recreation, and aquatic habitat. The U.S. Geological Survey, in cooperation with the Rhode Island Department of Health, began a study in 2012 as part of an effort to protect the source of water to six large-capacity production wells that supply drinking water and to increase understanding of how climate change might affect the water resources in the basins. Soil-water-balance and groundwater-flow models were developed to delineate the areas contributing recharge to the wells and to quantify the hydrologic response to climate change. Surficial deposits of glacial origin ranging from a few feet to more than 200 feet thick overlie bedrock in the 24.4-square mile study area. These deposits comprise a complex and productive aquifer system. Simulated areas contributing recharge to the production wells covered a total area of 0.63 square miles for average well withdrawal rates from 2007 through 2011 (total rate of 583 gallons per minute). Simulated areas contributing recharge for the maximum well pumping capacities (total rate of 3,700 gallons per minute) covered a total area of 2.55 square miles. Most simulated areas contributing recharge extend upgradient of the wells to morainal and upland till deposits and to groundwater divides. Some simulated areas contributing recharge include small, isolated areas remote from the wells. Relatively short groundwater traveltimes from recharging locations to discharging wells indicated that the wells are vulnerable to contamination from land-surface activities; median traveltimes ranged from 3.5 to 8.6 years for the production wells examined, and 57 to 91 percent of the traveltimes were 10 years or less. Land cover in the areas contributing recharge includes a substantial amount of urban and agriculture land use for five wells adjacent to the Chipuxet River; for one well adjacent to a tributary stream, land use is less developed. The calibrated groundwater-flow model provided a single, best representation of the areas contributing recharge to a production well. The parameter variance-covariance matrix from model calibration was used to create parameter sets that reflect the uncertainty of the parameter estimates and the correlation among parameters to evaluate the uncertainty associated with the predicted contributing areas to the wells. A Monte Carlo analysis led to contributing areas expressed as a probability distribution that differed from a single deterministic contributing area. Because of the effects of parameter uncertainty, the size of the probabilistic contributing areas for both average and maximum pumping rates was larger than the size of the deterministic contributing areas for the wells. Thus, some areas not in the deterministic contributing area might actually be in the contributing area, including additional areas of urban and agricultural land use that has the potential to contaminate groundwater. Additional areas that might be in the contributing area included recharge originating near the pumping wells that have relatively short groundwater-flow paths and traveltimes. At the maximum pumping rates, areas associated with low probabilities extended long distances along groundwater divides in the uplands remote from the wells. Climate projections for the Chipuxet River and Chickasheen Brook Basins from downscaled output from general circulation models indicate that mean annual temperature might increase by 4.7 degrees Fahrenheit and 8.0 degrees Fahrenheit by the late 21st century (2070–99) compared with the late 20th century (1970–99) under scenarios of lower and higher emissions of greenhouse gases, respectively. By the late 21st century, winter and spring precipitation is projected to increase by 12 to 17 percent, summer precipitation to increase by about the same as mean annual precipitation (8 percent), and fall precipitation to decrease by 5 percent for both emission scenarios compared with the late 20th century. Soil-water-balance simulations indicate that, although precipitation is expected to increase in three seasons, only in winter do precipitation increases exceed actual evapotranspiration increases. Recharge is projected to decrease in fall and generally change little in spring and summer. By the late 21st century, winter recharge is expected to increase by 13 percent for the lower emissions scenario and by 15 percent for the higher emissions scenario. In fall, recharge is projected to diminish by 13 percent for the lower emissions scenario and by 24 percent for the higher emissions scenario. Although recharge is projected to change seasonally in the 21st century, mean annual recharge changes minimally. Soil moisture is projected to decrease in the 21st century from spring through fall because of increases in potential evapotranspiration, and in fall because of decreases in precipitation in addition to increases in potential evapotranspiration. By the late 21st century, soil moisture for the lower emissions scenario is expected to decrease by 11 percent in summer and 15 percent in fall, and for the higher emissions scenario, decrease by 23 percent for both seasons. These decreases in soil moisture during the growing season might have implications for agriculture in the study area. Predicted changes in the magnitude and seasonal distribution of recharge in the 21st century increase simulated base flows and groundwater levels in the winter months for both emission scenarios, but because of less recharge in the fall and less or about the same recharge in the preceding months of spring and summer, base flows and groundwater levels in the fall months decrease for both emission scenarios. October has the largest base flow and groundwater level decreases. By the late 21st century, base flows at the Chipuxet River in October are projected to decrease by 9 percent for the lower emissions scenario and 18 percent for the higher emissions scenario. For a headwater stream in the upland till with shorter groundwater-flow paths and lower storage properties in its drainage area, base flows in October are projected to diminish by 28 percent and 42 percent for the lower and higher emissions scenarios by the late 21st century. Groundwater level changes in the uplands show substantial decreases in fall, but because of the large storage capacity of stratified deposits, water levels change minimally in the valley. By the late 21st century, water levels in large areas of upland till deposits in October are projected to decrease by up to 2 feet for the lower emissions scenario, whereas large areas decrease by up to 5 feet, with small areas with decreases of as much as 10 feet, for the higher emissions scenario. For both emission scenarios, additional areas of till go dry in fall compared with the late 20th century. Thus projected changes in recharge in the 21st century might extend low flows and low water levels for the year later in fall and there might be more intermittent headwater streams compared with the late 20th century with corresponding implications to aquatic habitat. Finally, the size and location of the simulated areas contributing recharge to the production wells are minimally affected by climate change because mean annual recharge, which is used to determine the contributing areas to the production wells, is projected to change little in the 21st century.

Rhode Island↗