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Blakemore E. Thomas

Publications and source records attributed to Blakemore E. Thomas.

14 recordsLinked to original sources
USGS2015

Hydrology of the middle San Pedro area, southeastern Arizona

In the middle San Pedro Watershed in southeastern Arizona, groundwater is the primary source of water supply for municipal, domestic, industrial, and agricultural use. The watershed comprises two smaller subareas, the Benson subarea and the Narrows-Redington subarea. Early 21st century projections for heavy population growth in the watershed have not yet become a reality, but increased groundwater withdrawals could have undesired consequences - such as decreased base flow to the San Pedro River, and groundwater-level declines - that would lead to the need to deepen existing wells. This report describes the hydrology, hydrochemistry, water quality, and development of a groundwater budget for the middle San Pedro Watershed, focusing primarily on the elements of groundwater movement that could be most useful for the development of a groundwater model Precipitation data from Tombstone, Arizona, and base flow at the stream-gaging station on the San Pedro River at Charleston both show relatively dry periods during the 1960s through the mid-1980s and in the mid-1990s to 2009, and wetter periods from the mid-1980s through the mid-1990s. Water levels in four out of five wells near the mountain fronts show cyclical patterns of recharge, with rates of recharge greatest in the early 1980s through the mid-1990s. Three wells near the San Pedro River recorded their lowest levels during the 1950s to the mid-1960s. The water-level record from one well, completed in the confined part of the coarse-grained lower basin fill, showed a decline of approximately 21 meters. Annual flow of the San Pedro River, measured at the Charleston and Redington gages, has decreased since the 1940s. The median annual streamflow and base flow at the gaging station on the river near Tombstone has decreased by 50 percent between the periods 1968–1986 and 1997–2009. Estimates of streamflow infiltration along the San Pedro River during 1914–2009 have decreased 44 percent, with the largest decreases in the months June–October in the Benson subarea. In the Narrows-Redington subarea, streamflow infiltration has decreased about 65 percent during 1914–2009. The average annual outflow (27.6 hm 3 /year [cubic hectometers per year]) from the Benson subarea aquifer for water years 2001 through 2009 exceeded the inflows (20.0 hm 3 / yr) by 7.60 hm 3 /yr. In the Narrows-Redington subarea for the same period, the average annual outflow (15.7 hm 3 /yr) from the aquifer system exceeded the inflows (13.8 hm 3 /yr) by nearly 2 hm 3 /yr. The largest withdrawals of groundwater in both subareas are for irrigation; these withdrawals peaked in 1973 and have been steadily decreasing since then. Recharge from streamflow infiltration exceeded recharge from the mountain-front and from ephemeral channels in the Benson subarea. In the Narrows-Redington subarea, however, recharge from mountain-front and ephemeral channel recharge exceeded recharge from streamflow infiltration. Evapotranspiration by phreatophytes accounts for the largest outflow of groundwater for both subareas—78 percent of the outflow in the Narrows-Redington subarea and 62 percent of the outflow in the Benson subarea. Precipitation, surface-water, and groundwater chemistry and isotope data indicated the relative age and residence time of groundwater, the amount of interaction between geologic sources and groundwater, and how recharge elevation and season were related to the presence of modern water. The bedrock aquifer receives modern recharge (

Arizona↗
USGS2011

Water availability and use pilot: Methods development for a regional assessment of groundwater availability, southwest alluvial basins, Arizona

Executive Summary: Arizona is located in an arid to semiarid region in the southwestern United States and is one of the fastest growing States in the country. Population in Arizona surpassed 6.5 million people in 2008, an increase of 140 percent since 1980, when the last regional U.S. Geological Survey (USGS) groundwater study was done as part of the Regional Aquifer System Analysis (RASA) program. The alluvial basins of Arizona are part of the Basin and Range Physiographic Province and cover more than 73,000 mi 2 , 65 percent of the State's total land area. More than 85 percent of the State's population resides within this area, accounting for more than 95 percent of the State's groundwater use. Groundwater supplies in the area are expected to undergo further stress as an increasing population vies with the State's important agricultural sector for access to these limited resources. To provide updated information to stakeholders addressing issues surrounding limited groundwater supplies and projected increases in groundwater use, the USGS Groundwater Resources Program instituted the Southwest Alluvial Basins Groundwater Availability and Use Pilot Program to evaluate the availability of groundwater resources in the alluvial basins of Arizona. The principal products of this evaluation of groundwater resources are updated groundwater budget information for the study area and a proof-of-concept groundwater-flow model incorporating several interconnected groundwater basins. This effort builds on previous research on the assessment and mapping of groundwater conditions in the alluvial basins of Arizona, also supported by the USGS Groundwater Resources Program. Regional Groundwater Budget: The Southwest Alluvial Basins-Regional Aquifer System Analysis (SWAB-RASA) study produced semiquantitative groundwater budgets for each of the alluvial basins in the SWAB-RASA study area. The pilot program documented in this report developed new quantitative estimates of groundwater budget components using recent (2000-2007) data and methods of data analysis. Estimates of inflow components, including mountain-front recharge, incidental recharge from irrigation of agriculture, managed recharge from recharge facilities, interbasin underflow from upgradient basins, and streamflow losses, are quantified for recent time periods. Mountain-front recharge is the greatest inflow component to the groundwater system and was estimated using two methods: a basin characteristic model and new precipitation information used in a previously developed regression equation. Annual mountain-front recharge for the study area for 1940-2007 estimated by the two methods is 730,000 acre-ft for the basin characteristic model and 643,000 acre-ft for the regression equation, representing 1.5 percent and 1.3 percent of precipitation, respectively. Outflow components, including groundwater withdrawals, evapotranspiration, and interbasin flow to downgradient basins, are also presented for recent time periods. Groundwater withdrawals accounted for the largest share of the water budget, with nearly 2.4 million acre-ft per year withdrawn from the study area in recent years. Evapotranspiration from groundwater was estimated at nearly 1.3 million acre-ft per year for the study area using a newly developed method incorporating vegetation indices from satellite images and land cover information. For water-budget components with temporal variation that could be assessed from available data, estimates for intervening time periods since before development were also developed. An estimate of aquifer storage change, representing both gains to and losses from the groundwater system since before development, was derived for the most developed basins in the study area using available estimates of groundwater-level changes and storage coefficients. An overall storage loss of 74.5 million acre-ft was estimated for these basins within the study area. Demonstration

Arizona↗
USGS2006

Trends in Streamflow of the San Pedro River, Southeastern Arizona

Total annual streamflow of the San Pedro River at Charleston in southeastern Arizona (fig. 1) decreased by about 66 percent from 1913 to 2002 (fig. 2). The San Pedro River is one of the few remaining free-flowing perennial streams in the arid Southwestern United States, and the riparian forest along the river supports several endangered species and is an important habitat for migratory birds. The decreasing trend in streamflow has led to concerns that riparian habitat may be damaged and that overall long-term water supply for a growing population may be threatened. Resource managers and the public have an interest in learning more about the trend and the possible causes of the trend.

Fact Sheet↗
USGS2006

Hydrogeologic investigation of the middle San Pedro watershed, southeastern Arizona: A project of the Rural Watershed Initiative

In 2005, the U.S. Geological Survey (USGS) began an investigation of the hydrogeology of the middle San Pedro watershed in cooperation with the Arizona Department of Water Resources (ADWR). This project is part of the Rural Watershed Initiative (RWI), which is a program established by the State of Arizona and managed by the ADWR. The primary objective of this project is to improve the understanding of the hydrogeology of the middle San Pedro watershed.

Arizona↗
USGS2006

Trends in streamflow of the San Pedro River, southeastern Arizona, and regional trends in precipitation and streamflow in southeastern Arizona and southwestern New Mexico

This study was done to improve the understanding of trends in streamflow of the San Pedro River in southeastern Arizona. Annual streamflow of the river at Charleston, Arizona, has decreased by more than 50 percent during the 20th century. The San Pedro River is one of the few remaining free-flowing perennial streams in the arid Southwestern United States, and the riparian forest along the river supports several endangered species and is an important habitat for migratory birds. Trends in seasonal and annual precipitation and streamflow were evaluated for surrounding areas in southeastern Arizona and southwestern New Mexico to provide a regional perspective for the trends of the San Pedro River. Seasonal and annual streamflow trends and the relation between precipitation and streamflow in the San Pedro River Basin were evaluated to improve the understanding of the causes of trends. There were few significant trends in seasonal and annual precipitation or streamflow for the regional study area. Precipitation and streamflow records were analyzed for 11 time periods ranging from 1930 to 2002; no significant trends were found in 92 percent of the trend tests for precipitation, and no significant trends were found in 79 percent of the trend tests for streamflow. For the trends in precipitation that were significant, 90 percent were positive and most of those positive trends were in records of winter, spring, or annual precipitation that started during the mid-century drought in 1945-60. For the trends in streamflow that were significant, about half were positive and half were negative. Trends in precipitation in the San Pedro River Basin were similar to regional precipitation trends for spring and fall values and were different for summer and annual values. The largest difference was in annual precipitation, for which no trend tests were significant in the San Pedro River Basin, and 23 percent of the trend tests were significantly positive in the rest of the study area. Streamflow trends for the San Pedro River were different from regional streamflow trends. All seasonal flows for the San Pedro River, except winter flows, had significant decreasing trends, and seasonal flows for most streams in the rest of the study area had either no trend or a significant increasing trend. Two streams adjacent to the San Pedro River Basin (Whitewater Draw and Santa Cruz River), however, had significant decreasing trends in summer streamflow. Factors that caused the decreasing trends in streamflow of the San Pedro River at Charleston were investigated. Possible factors were fluctuations in precipitation and air temperature, changes in watershed characteristics, human activities, or changes in seasonal distribution of bank storage. This study statistically removed or accounted for the variation in streamflow caused by fluctuations in precipitation. Thus, the remaining variation or trend in streamflow was caused by factors other than precipitation. Two methods were used to partition the variation in streamflow and to determine trends in the partitioned variation: (1) regression analysis between precipitation and streamflow using all years in the record and evaluation of time trends in regression residuals, and (2) development of regression equations between precipitation and streamflow for three time periods (early, middle, and late parts of the record) and testing to determine if the three regression equations were significantly different. The methods were applied to monthly values of total flow (average flow) and storm runoff (maximum daily mean flow) for 1913-2002, and to monthly values of low flow (3-day low flow) for 1931-2002. Statistical tests provide strong evidence that factors other than precipitation caused a decrease in streamflow of the San Pedro River. Factors other than precipitation caused significant decreasing trends in streamflows for late spring through early winter and did not cause significant trends for late winter through early spring. Total flows had significant decreasing trends in June through December, low flows had significant decreasing trends in May through December, and storm runoff had significant decreasing trends in July through September. The effects of factors other than precipitation were tested only for July through October for storm runoff. Besides fluctuations in precipitation, the principal factors that could have caused decreasing streamflow trends are (1) changes in watershed characteristics such as changes in riparian vegetation, changes in upland vegetation, and changes in stream-channel morphology, and (2) human activities such as ground-water pumping, construction of runoff-detention structures, urbanization, and cattle ranching (grazing). Changes in upland and riparian vegetation likely were major factors in the decreasing trends in total streamflows and low flows. Total flows and low flows in summer and fall were significantly affected by factors other than precipitation, but late winter flows were not significantly affected. The significant effects coincide with high rates of transpiration from vegetation in the summer and the nonsignificant effects coincide with low rates of transpiration in the late winter. Another piece of evidence that implicates vegetation as a cause is that the upland and riparian vegetation of the San Pedro River Basin changed during the 20th century. The relative proportions of different species changed in upland vegetation (woody plants increased and grasses decreased), and the areal extent and density of riparian vegetation increased substantially. Ground-water pumping in the United States and Mexico had a mixed influence on streamflow trends at Charleston, Arizona; statistical analyses indicate that seasonal pumping from wells near the river for irrigation in the spring and summer was a major factor in the decrease in low flows and that year-round pumping from wells in the regional aquifer away from the river was not a major factor in the decrease in low flows. If regional pumping had caused a trend, the pumping should have affected low flows for all months of the year, but factors other than precipitation did not cause significant trends in low flows for January, February, March, and May. Most of the local pumping near the river was during the spring and summer, and this seasonal pumping probably caused some decreases in summer low flows. These conclusions are for trends from 1913 to 2002, and regional pumping in the United States and Mexico could affect streamflow at Charleston in the future, because regional ground-water pumping often has a delayed effect on streamflows.

Arizona, New Mexico↗
USGS2004

Ground-water, surface-water, and water-chemistry data, Black Mesa area, northeastern Arizona — 2002-03

The N aquifer is the major source of water in the 5,400-square-mile area of Black Mesa in northeastern Arizona. Availability of water is an important issue in this area because of continued industrial and municipal use, a growing population, and precipitation of about 6 to 14 inches per year. The monitoring program in the Black Mesa area has been operating since 1971 and is designed to determine the long-term effects of ground-water withdrawals from the N aquifer for industrial and municipal uses. The monitoring program includes measurements of (1) ground-water pumping, (2) ground-water levels, (3) spring discharge, (4) surface-water discharge, (5) ground-water chemistry, and (6) periodic testing of ground-water withdrawal meters. In 2002, total ground-water withdrawals were 8,000 acre-feet, industrial use was 4,640 acre-feet, and municipal use was 3,360 acre-feet. From 2001 to 2002, total withdrawals increased by 4 percent, industrial use increased by 2 percent, and municipal use increased by 7 percent. Flowmeter testing was completed for 32 municipal wells in 2003. The median difference between pumping rates for the permanent meter and a test meter for all the sites tested was -2.0 percent. Values ranged from -13.7 percent at Hopi High School no. 2 to +12.9 percent at Shonto PM3. From 2002 to 2003, water levels declined in 5 of 13 wells in the unconfined part of the aquifer, and the median change was 0.0 foot. Water levels declined in 8 of 13 wells in the confined part of the aquifer, and the median change was -1.1 feet. From the prestress period (prior to 1965) to 2003, the median water-level change for 26 wells was -8.3 feet. Median water-level changes were -0.4 foot for 13 wells in the unconfirned part of the aquifer and -60.3 feet for 13 wells in the confined part. Discharges were measured once in 2002 and once in 2003 at four springs. Discharge decreased by 16 percent at Pasture Canyon Spring, increased 10 percent at Moenkopi Spring and 90 percent at an unnamed spring near Dennehotso, and did not change at Burro Spring. For the past 11 years, discharges from the four springs have fluctuated; however, an increasing or decreasing trend is not apparent. Continuous records of surface-water discharge have been collected from 1976 to 2002 at Moenkopi Wash, 1996 to 2002 at Laguna Creek, 1993 to 2002 at Dinnebito Wash, and 1994 to 2002 at Polacca Wash. Median flows for November, December, January, and February of each water year were used as an index of ground-water discharge to those streams. Since 1995, the median winter flows have decreased for Moenkopi Wash, Dinnebito Wash, and Polacca Wash. Since the first continuous record of surface-water discharge in 1997, there is no consistent trend in the median winter flow for Laguna Creek. In 2003, water samples were collected from 12 wells and 4 springs and analyzed for selected chemical constituents. Dissolved-solids concentrations ranged from 118 to 642 milligrams per liter. Water samples from 10 of the wells and from all of the springs had less than 500 milligrams per liter of dissolved solids. There are no appreciable time trends in the chemistry of water samples from 7 wells and 4 springs; 7 wells had more than 8 years of data, and the 4 springs had more than 10 years of data.

Arizona↗
USGS2002

Ground-water, surface-water and water-chemistry data, Black Mesa area, northeastern Arizona: 2001-02

The N aquifer is the major source of water in the 5,400-square-mile area of Black Mesa in northeastern Arizona. Availability of water is an important issue in this area because of continued industrial and municipal use, a growing population, and precipitation of about 6 to 14 inches per year. The monitoring program in the Black Mesa area has been operating since 1971 and is designed to determine the long-term effects of ground-water withdrawals from the N aquifer for industrial and municipal uses. The monitoring program includes measurements of (1) ground-water pumping, (2) ground-water levels, (3) spring discharge, (4) surface-water discharge, and (5) ground-water chemistry. In 2001, total ground-water withdrawals were 7,680 acre-feet, industrial use was 4,530 acre-feet, and municipal use was 3,150 acre-feet. From 2000 to 2001, total withdrawals decreased by 1 percent, industrial use increased by 1 percent, and municipal use decreased by 3 percent. From 2001 to 2002, water levels declined in 5 of 14 wells in the unconfined part of the aquifer, and the median change was +0.2 foot. Water levels declined in 12 of 17 wells in the confined part of the aquifer, and the median change was -1.4 feet. From the prestress period (prior to 1965) to 2002, the median water-level change for 32 wells was -15.8 feet. Median water-level changes were -1.3 feet for 15 wells in the unconfined part of the aquifer and -31.7 feet for 17 wells in the confined part. Discharges were measured once in 2001 and once in 2002 at four springs. Discharges decreased by 26 percent and 66 percent at two springs, increased by 100 percent at one spring, and did not change at one spring. For the past 10 years, discharges from the four springs have fluctuated; however, an increasing or decreasing trend is not apparent. Continuous records of surface-water discharge have been collected from 1976 to 2001 at Moenkopi Wash, 1996 to 2001 at Laguna Creek, 1993 to 2001 at Dinnebito Wash, and 1994 to 2001 at Polacca Wash. Median flows for November, December, January, and February of each water year were used as an index of ground-water discharge to those streams. Since 1995, the median winter flows have decreased for Moenkopi Wash, Dinnebito Wash, and Polacca Wash. Since 1997, there is no consistent trend in the median winter flow for Laguna Creek. In 2002, water samples were collected from 12 wells and 4 springs and analyzed for selected chemical constituents. Dissolved-solids concentrations ranged from 96 to 636 milligrams per liter. Water samples from 8 of the wells and from 3 of the springs had less than 300 milligrams per liter of dissolved solids. There are no appreciable time trends in the chemistry of water samples from 9 wells and 4 springs; the 9 wells had more than 7 years of data, and the 4 springs had more than 9 years of data.

Arizona↗
USGS2002

Ground-water, surface-water, and water-chemistry data, Black Mesa Area, northeastern Arizona — 2000–2001, and performance and sensitivity of the 1988 USGS numerical model of the N aquifer

The N aquifer is the major source of water in the 5,400-square-mile area of Black Mesa in northeastern Arizona. Availability of water is an important issue in this area because of continued industrial and municipal use, a growing population, and precipitation of about 6 to 14 inches per year. The monitoring program in Black Mesa has been operating since 1971 and is designed to determine the long-term effects of ground-water withdrawals from the N aquifer for industrial and municipal uses. The monitoring program includes measurements of (1) ground-water pumping, (2) ground-water levels, (3) spring discharge, (4) surface-water discharge, and (5) ground-water chemistry. In 2000, total ground-water withdrawals were 7,740 acre-feet, industrial use was 4,490 acre-feet, and municipal use was 3,250 acre-feet. From 1999 to 2000, total withdrawals increased by 9 percent, industrial use increased by 7 percent, and municipal use increased by 12 percent. From 1999 to 2001, water levels declined in 10 of 15 wells in the unconfined part of the aquifer, and the median change was -0.4 foot. Water levels declined in 8 of 16 wells in the confined part of the aquifer, and the median change was -0.2 foot. From the prestress period (prior to 1965) to 2001, the median water-level change for 33 wells was -17.2 feet. Median water-level changes were -1.2 feet for 15 wells in the unconfined part of the aquifer and -31.0 feet for 18 wells in the confined part. Discharges were measured once in 1999 and once in 2001 at four springs. Discharges decreased by 5 percent and 33 percent at two springs and increased by 3 percent and 81 percent at two springs. For about the past 10 years, discharges did not significantly change in Burro Spring, the unnamed spring near Dennehotso, and Moenkopi School Spring. The record of discharge from a consistent measuring point for Pasture Canyon Spring is too short for statistical analysis of trends. Continuous records of surface-water discharge have been collected from July 1976 to 2000 at Moenkopi Wash, July 1996 to 2000 at Laguna Creek, June 1993 to 2000 at Dinnebito Wash, and April 1994 to 2000 at Polacca Wash. Median flows for November, December, January, and February of each water year were used as an index of ground-water discharge to those streams. There is no significant trend in the median winter flows for Moenkopi Wash from 1977 to 2000. The records for the other three streams are too short for a statistical analysis of trends. The median winter flows for Dinnebito Wash and Polacca Wash, however, appear to have decreased during the last 6 years. There is no apparent trend in the median winter flows for Laguna Creek since 1997. In 2001, water samples were collected from 12 wells and 4 springs and analyzed for selected chemical constituents. Dissolved-solids concentrations ranged from 102 to 628 milligrams per liter. Water samples from 9 of the wells and from the 4 springs had less than 350 milligrams per liter of dissolved solids. Water-chemistry data with sufficient years of record for a statistical analysis of trends over time are available from 7 wells and 4 springs. From about the mid-1980s or early 1990s to 2001 there are no significant trends in the concentrations of dissolved solids, chloride, and sulfate in water samples from 6 of the 7 wells. The concentration of one tested constituent (dissolved solids) in samples from Rocky Ridge PM3 significantly increased from 1990 to 2001. From the late 1980s to 2001, there are no significant trends in the concentrations of dissolved solids, chloride, and sulfate in water samples from Burro Spring, the unnamed spring near Dennehotso, and Pasture Canyon Spring. From 1987 to 2001, concentrations of chloride and sulfate significantly increased in water samples from Moenkopi School Spring and concentrations of dissolved solids did not significantly change. The performance and sensitivity of the 1988 USGS numerical model of the N aquifer were analyzed. The overall performance of the model in steady-state conditions is reasonable for residuals of heads (difference between observed and simulated steady-state heads); 80 percent of the absolute values of residuals are less than 38 feet. Simulated flows are about 40 percent different than estimated flows at two of three discharge areas; however, this comparison is only a rough approximation of performance because the accuracy of the estimated steady-state flows is uncertain. The overall performance of the model for transient conditions is fair for residuals of changes in head (difference between observed and simulated changes in head from steady state to 1999); 80 percent of the absolute values of residuals are less than 31 feet. The model is biased in two areas. In the Tuba City area, simulated changes in head are more negative than observed changes in head; all six residuals are positive, and three residuals are between 75 and 155 feet. In the confined area of the aquifer, observed changes in head are more negative than simulated changes in head; 12 of the 17 residuals are negative, and 8 residuals are between -57 and -20 feet. Analysis of model sensitivity indicates that recharge, transmissivity, and storage coefficient are the most important parameters for estimating heads, changes in heads, and flows. A strong correlation between recharge and transmissivity and a lack of independent and reliable estimates of recharge, transmissivity, and discharge create a uniqueness problem in model calibration. Several models could be constructed and calibrated with different values of recharge or transmissivity and still have similar fits to the observed data. Information from recent data and studies and more advanced modeling techniques could be used to develop a more representative and less uncertain model. Future data collection and studies should focus on obtaining a better definition of recharge, discharge, transmissivity, and storage coefficient.

Arizona↗
USGS2000

Ground-water, surface-water, and water-chemistry data, Black Mesa area, northeastern Arizona—1999

The N aquifer is the major source of water in the 5,400-square-mile area of Black Mesa in northeastern Arizona. Availability of water is an important issue in this area because of continued industrial and municipal use, a growing population, and a precipitation of only about 6 to 12 inches per year. The monitoring program in Black Mesa has been operating since 1971 and is designed to determine the long-term effects of ground-water withdrawals from the N aquifer for industrial and municipal uses. The monitoring program includes measurements of (1) ground-water pumping, (2) ground-water levels, (3) spring discharge, (4) surface-water discharge, and (5) ground-water chemistry. In 1999, total ground-water withdrawals were 7,110 acre-feet, industrial use was 4,210 acre-feet, and municipal use was 2,900 acre-feet. From 1998 to 1999, total withdrawals increased by 0.7 percent, industrial use increased by 4 percent, and municipal use decreased by 4 percent. From 1998 to 1999, water levels declined in 11 of 15 wells in the unconfined part of the aquifer, and the median decline was 0.7 foot. Water levels declined in 14 of 16 wells in the confined part of the aquifer, and the median decline was 1.2 feet. From the prestress period (prior to 1965) to 1999, the median water-level decline in 31 wells was 10.6 feet. Median water-level changes were 0.0 foot for 15 wells in the unconfined part of the aquifer and a decline of 45.5 feet in 16 wells in the confined part. From 1998 to 1999, discharges were measured annually at four springs. Discharges declined 30 percent and 3 percent at 2 springs, did not change at 1 spring, and increased by 11 percent at 1 spring. For the past 10 years, discharges from the four springs have fluctuated; however, an increasing or decreasing trend was not observed. Continuous records of surface-water discharge have been collected from July 1976 to 1999 at Moenkopi Wash, July 1996 to 1999 at Laguna Creek, June 1993 to 1999 at Dinnebito Wash, and April 1994 to 1999 at Polacca Wash. Median flows for November, December, January, and February of each water year are used as an index of ground-water discharge to those streams. Increasing or decreasing trends are not apparent in these median winter flows for the periods of record. In 1999, water samples were collected from 12 wells and 4 springs and analyzed for selected chemical constituents. Dissolved-solids concentrations ranged from 91 to 630 milligrams per liter. Water samples from 10 of the wells and the 4 springs had less than 350 milligrams per liter of dissolved solids. Water-chemistry data are available for nine wells and four springs from about the mid-1980s. For that time period, the data from those sites have remained fairly stable. From 1987 to 1999, concentrations of dissolved solids, chloride, and sulfate may have increased slightly in samples from Moenkopi School Spring.

Arizona↗
USGS1999

Hydrogeologic assessment of the Sequim-Dungeness area, Clallam County, Washington

The Sequim-Dungeness area covers 116 square miles (mi 2 ) on the northern part of the Olympic Peninsula in northwestern Washington. The central part of this area (74 mi 2 ) was designated as a primary study area. During the past two decades, the population has rapidly increased, land use has changed from mostly agricultural to residential, and salmon populations in the Dungeness River have appreciably declined. The increasing competition for water combined with a close relation between ground water, the Dungeness River, and an extensive irrigation system has created a need for a better understanding of ground water and the relation between ground water and surface water in the study area. The Sequim-Dungeness area is underlain by as much as 2,000 feet of unconsolidated Quaternary deposits that are mostly of glacial origin. Interpretation of 10 hydrogeologic cross sections and lithologic logs of about 600 wells led to the delineation of three aquifers, two confining beds, and a lower unit of undifferentiated deposits. A bedrock unit at the bottom is considered the base of the ground-water system. Ground water in the study area is recharged from infiltration and percolation of precipitaton, percolation of unconsumed irrigation water, leakage from irrigation ditches, subsurface inflow through the southern study-area boundary, and leakage from streams. Average annual recharge for the study period (December 1995 to September 1997) was estimated to be 17.7 inches (in.) ( 151 cubic feet per second (ft 3 /s)). The distribution of recharge was 8.6 in. (74 ft3fs) from precipitation, 2.7 in. (23 ft 3 /s) from subsurface inflow, 3.1 in. (26 ft 3 /s) from irrigation, and 3.3 in. (28 ft 3 /s) from leakage from the Dungeness River. The 8.6 in. of recharge from precipitation is much higher than would be expected in an average year because average annual precipitation during the study period was about 28 in., which is 1.35 times higher than long-term average annual precipitation. The long-term average annual recharge from precipitation was estimated to be 5.4 in. (48 ft 3 /s). Ground water discharges as subsurface flow to saltwater bodies, flow to streams, flow to springs, and as withdrawals from wells. Subsurface flow to saltwater bodies and flow to springs were not estimated in this study. Estimated average annual discharge was 3.2 inches (in.) (27 ft 3 /s)) to the Dungeness River and 4.6 in. (39 ft 3 /s) to other streams in the study area. Gross withdrawals from wells in 1996 were estimated to be 1.0 in. (8.4 ft 3 /s). There was a small but statistically significant increase in nitrate concentrations in ground water from 1980 to 1996. Median concentrations in the primary study area were 0.37 milligrams per liter (mg/L) in 1980 and 0.46 mg/L in 1996. The areal pattern of elevated nitrate concentrations has not changed appreciably during the past 15 years. Elevated concentrations were found in a large area east of the Dungeness River and at scattered locations west of the Dungeness River. About 543,200 pounds of nitrogen are estimated to enter the ground-water system in the primary study area each year. Four sources account for about 85 percent of the nitrogen; residential fertilizers, septic systems, mineralization of soil organic matter, and agricultural fertilizers .. It appears that the four major sources are approximately equivalent in amounts of nitrogen. Concentrations of nitrate in the shallow aquifer were significantly higher under residential areas than under natural grasslands or forests. Median nitrate concentrations were 1.3 mg/L under residential areas, 0.55 mg/L under agricultural areas, and 0.12 mg/L under natural grasslands or forests.

Washington↗
USGS1997

Methods for estimating magnitude and frequency of floods in the southwestern United States

Methods were developed for estimating magnitude and frequency of floods at gaged and ungaged sites in the Southwestern United States. Estimating equations for ungaged sites were developed by transferring information from gaged to ungaged sites using multiple regression and a hybrid method developed during this study. The methods and analysis are described, and information from more than 1,300 gaged sites is tabulated.

Arizona, California, Colorado, Idaho, Nevada, New ↗
USGS1997

Water and salt balance of Great Salt Lake, Utah, and simulation of water and salt movement through the causeway

The water and salt balance of Great Salt Lake primarily depends on the amount of inflow from tributary streams and the conveyance properties of a causeway constructed during 1957-59 that divides the lake into the south and north parts. The conveyance properties of the causeway originally included two culverts, each 15 feet wide, and the permeable rock-fill material. During 1980-86, the salt balance changed as a result of record high inflow that averaged 4,627,000 acre-feet annually and modifications made to the conveyance properties of the causeway that included opening a 300-foot-wide breach. In this study, a model developed in 1973 by Waddell and Bolke to simulate the water and salt balance of the lake was revised to accommodate the high water-surface altitude and modifications made to the causeway. This study, done by the U.S. Geological Survey in cooperation with the Utah Department of Natural Resources, Division of State Lands and Forestry, updates the model with monitoring data collected during 1980-86. This report describes the calibration of the model and presents the results of simulations for three hypothetical 10-year periods. During January 1, 1980, to July 31, 1984, a net load of 0.5 billion tons of dissolved salt flowed from the south to the north part of the lake primarily as a result of record inflows. From August 1, 1984, when the breach was opened, to December 31,1986, a net load of 0.3 billion tons of dissolved salt flowed from the north to the south part of the lake primarily as a result of the breach. For simulated inflow rates during a hypothetical 10-year period resulting in the water-surface altitude decreasing from about 4,200 to 4,192 feet, there was a net movement of about 1.0 billion tons of dissolved salt from the south to the north part, and about 1.7 billion tons of salt precipitated in the north part. For simulated inflow rates during a hypothetical 10-year period resulting in a rise in water-surface altitude from about 4,200 to 4,212 feet, there was a net movement of about 0.2 billion tons of dissolved salt from the south to the north part and no salt was precipitated in the north part of the lake.

Utah↗
USGS1995

Ground-water flow and water quality in the sand aquifer of Long Beach Peninsula, Washington

This report describes an investigation of ground-water flow and water quality in the sand aquifer of the Long Beach Peninsula. The peninsula is located in the southwestern corner of the State of Washington, is about 27 miles long, and has an average width of about 1.5 miles. It is surrounded by seawater, by the Pacific Ocean on the west and Willapa Bay on the east. Water supplies on the peninsula are derived mostly from a local water-table aquifer composed largely of sand. The recent growth of population on the peninsula and the projected future growth have created concerns about the quantity and quality of the ground-water resource. Some issues include declining ground-water levels from increased pumpage, and ground-water contamination from seawater intrusion, pesticides or fertilizers from cranberrygrowing areas, and septic-system effluent. The ground-water system of the Long Beach Peninsula consists of a sand aquifer with some lenses of silt and clay that may act as confining beds in local areas. Data are lacking or inconsistent to define a confining bed that extends throughout the peninsula. Hydraulic conductivity calculated from slug tests in 58 shallow wells ranged from 10 to 37 feet per day with a median of 22 feet per day. Average annual ground-water recharge by infiltration and percolation of precipitation is estimated to be about 58 inches or 111,000 acre-feet, which is 72 percent of the average annual precipitation of 80 inches. Average annual ground-water discharge is estimated to be about 30,200 acre-feet to the Pacific Ocean, 56,000 acre-feet to Willapa Bay, and 24,800 acre-feet to surface-water drainage channels. Ground-water movement is generally perpendicular to the spine of the peninsula. A ground-water divide occurs along a north-south line and ground water flows west or east from the divide toward the Pacific Ocean or Willapa Bay. There does not appear to have been any long-term decline of the water table of the sand aquifer from 1974-92. Ground-water levels measured at three east-west cross sections in 1974-75 were at about the same altitude as water levels measured in 1992. Relatively accurate individual regression relations were developed at 45 wells with ground-water altitude as a response variable and cumulative precipitation for 4 months as an explanatory variable. The average coefficient of determination for all individual relations was 0.77, with a range of 0.11 to 0.89. Some empirical frequency or probability relations for precipitation and ground-water levels were used to estimate how often the maximum water levels measured in this study would be expected to occur in the future. These water levels reflected the lower-than-average precipitation that occurred during the study. Assuming that the annual maximum precipitation for 4 consecutive months is random and independent, the historical record of precipitation is representative of the future distribution of precipitation, and the relation between precipitation and water levels is accurate and stationary; a probability analysis of the historical record indicates that in any one year in the future there is a probability of 70 percent that the maximum water levels measured in wells during the winter of 1991-92 would be equaled or exceeded. The shallow ground water had generally low dissolved-solids concentrations in July 1992, with a median concentration of 92 milligrams per liter (mg/L) and a range of 56 to 218 mg/L. Sodium was the dominant cation and bicarbonate was the dominant anion. The distribution of hardness of the water samples was 84 percent with soft water and 16 percent with moderately hard water. The water quality of the shallow ground water was generally good, with a few small to moderate problems. A natural problem is locally high concentrations of dissolved iron. About 30 percent of the water samples had dissolved-iron concentrations of greater than 0.3 mg/L, which is the secondary maximum contaminant level established by the U.S. Environmental Protection Agency. No appreciable amount of seawater has intruded into the sand aquifer. The samples of shallow ground water collected in July 1992 had a median chloride concentration of 15 mg/L and a maximum concentration of 52 mg/L. The heavy average annual precipitation of about 80 inches, large average annual ground-water recharge of about 58 inches or 111,000 acre-feet, and small ground-water withdrawal rate (about 780 acre-feet per year in 1992) combine to maintain a thick freshwater lens of ground water that prevents seawater intrusion throughout the year. Agricultural activities do not appear to have appreciably affected the quality of shallow ground water on the Long Beach Peninsula. The concentration of nitrate in ground water was not significantly higher near cranberry-growing areas, and no sample of ground water or surface water had concentrations of selected pesticides or associated compounds that were above the analytical detection limits. Of the seven ground-water samples in which bacteria were detected, only one sample appeared to be related to agriculture; that sample was from a well located in an area where cattle graze for part of the year. Septic systems probably caused an increase in the concentration of nitrate in shallow ground water in areas of higher population density. Concentrations of nitrate were significantly related to population density. However, the concentrations were not generally high; median concentrations of nitrate increased from less than 0.05 mg/L in areas of low population density to 0.74 mg/L in areas of high density. Septic systems did not cause regional bacterial contamination of the ground water. Bacteria were detected in seven ground-water samples; however, only two of those samples were from wells that are close to septic systems. A limited amount of historical water-quality data is available for the peninsula; therefore, it is difficult to assess long-term changes. From 1968-92, chloride concentrations and values of specific conductance appear to have remained stable. Likewise, it appears that nitrate concentrations did not change from 1987-92.

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