Geology topics

Research about Modesto, California

Source-linked reports with geographic coverage including Modesto, California.

4 recordsLinked to original sources

USGS2008

Simulations of ground-water flow and particle pathline analysis in the zone of contribution of a public-supply well in Modesto, eastern San Joaquin Valley, California

Shallow ground water in the eastern San Joaquin Valley is affected by high nitrate and uranium concentrations and frequent detections of pesticides and volatile organic compounds (VOC), as a result of ground-water development and intensive agricultural and urban land use. A single public-supply well was selected for intensive study to evaluate the dominant processes affecting the vulnerability of public-supply wells in the Modesto area. A network of 23 monitoring wells was installed, and water and sediment samples were collected within the approximate zone of contribution of the public-supply well, to support a detailed analysis of physical and chemical conditions and processes affecting the water chemistry in the well. A three-dimensional, steady-state local ground-water-flow and transport model was developed to evaluate the age of ground water reaching the well and to evaluate the vulnerability of the well to nonpoint source input of nitrate and uranium. Particle tracking was used to compute pathlines and advective travel times in the ground-water flow model. The simulated ages of particles reaching the public-supply well ranged from 9 to 30,000 years, with a median of 54 years. The age of the ground water contributed to the public-supply well increased with depth below the water table. Measured nitrate concentrations, derived primarily from agricultural fertilizer, were highest (17 milligrams per liter) in shallow ground water and decreased with depth to background concentrations of less than 2 milligrams per liter in the deepest wells. Because the movement of water is predominantly downward as a result of ground-water development, and because geochemical conditions are generally oxic, high nitrate concentrations in shallow ground water are expected to continue moving downward without significant attenuation. Simulated long-term nitrate concentrations indicate that concentrations have peaked and will decrease in the public-supply well during the next 100 years because of the low nitrate concentrations in recharge beneath the urban area and the increasing proportion of urban-derived ground water reaching the well. The apparent lag time between peak input concentrations and peak concentrations in the well is about 20 to 30 years. Measured uranium concentrations were also highest (45 micrograms per liter) in shallow ground water, and decreased with depth to background concentrations of about 0.5 microgram per liter. Naturally-occurring uranium adsorbed to aquifer sediments is mobilized by oxygen-rich, high-alkalinity water. Alkalinity increased in shallow ground water in response to agricultural development. As ground-water pumping increased in the 1940s and 1950s, this alkaline water moved downward through the ground-water flow system, mobilizing the uranium adsorbed to aquifer sediments. Ground water with high alkalinity and high uranium concentrations is expected to continue to move deeper in the system, resulting in increased uranium concentrations with depth in ground water. Because alkalinity (and correspondingly uranium) concentrations were high in shallow ground water beneath both the urban and the agricultural land, long-term uranium concentrations in the public-supply well are expected to increase as the proportion of uranium-affected water contributed to the well increases. Assuming that the alkalinity near the water table remains the same, the simulation of long-term alkalinity in the public-supply well indicates that uranium concentrations in the public-supply well will likely approach the maximum contaminant level; however, the time to reach this level is more than 100 years because of the significant proportion of old, unaffected water at depth that is contributed to the public-supply well.

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USGS2004

Hydrogeologic characterization of the Modesto Area, San Joaquin Valley, California

Hydrogeologic characterization was done to develop an understanding of the hydrogeologic setting near Modesto by maximizing the use of existing data and building on previous work in the region. A substantial amount of new lithologic and hydrologic data are available that allow a more complete and updated characterization of the aquifer system. In this report, geologic units are described, a database of well characteristics and lithology is developed and used to update the regional stratigraphy, a water budget is estimated for water year 2000, a three-dimensional spatial correlation map of aquifer texture is created, and recommendations for future data collection are summarized. The general physiography of the study area is reflected in the soils. The oldest soils, which have low permeability, exist in terrace deposits, in the interfan areas between the Stanislaus, Tuolumne, and Merced Rivers, at the distal end of the fans, and along the San Joaquin River floodplain. The youngest soils have high permeability and generally have been forming on the recently deposited alluvium along the major stream channels. Geologic materials exposed or penetrated by wells in the Modesto area range from pre-Cretaceous rocks to recent alluvium; however, water-bearing materials are mostly Late Tertiary and Quaternary in age. A database containing information from more than 3,500 drillers'logs was constructed to organize information on well characteristics and subsurface lithology in the study area. The database was used in conjunction with a limited number of geophysical logs and county soil maps to define the stratigraphic framework of the study area. Sequences of red paleosols were identified in the database and used as stratigraphic boundaries. Associated with these paleosols are very coarse grained incised valley-fill deposits. Some geophysical well logs and other sparse well information suggest the presence of one of these incised valley-fill deposits along and adjacent to the Tuolumne River east of Modesto, a feature that may have important implications for ground-water flow and transport in the region. Although extensive work has been done by earlier investigators to define the structure of the Modesto area aquifer system, this report has resulted in some modification to the lateral extent of the Corcoran Clay and the regional dip of the Mehrten Formation. Well logs in the database indicating the presence of the Corcoran Clay were used to revise the eastern extent of the Corcoran Clay, which lies approximately parallel to the axis of valley. The Mehrten Formation is distinguished in the well-log database by its characteristic black sands consisting of predominantly andesitic fragments. Black sands in wells listed in the database indicate that the formation may lie as shallow as 120 meters (400 feet) below land surface under Modesto, approximately 90 meters (300 feet) shallower than previously thought. The alluvial aquifer system in the Modesto area comprises an unconfined to semiconfined aquifer above and east of the Corcoran Clay confining unit and a confined aquifer beneath the Corcoran Clay. The unconfined aquifer is composed of alluvial sediments of the Modesto, Riverbank, and upper Turlock Lake formations. The unconfined aquifer east of the Corcoran Clay becomes semiconfined with depth due to the numerous discontinuous clay lenses and extensive paleosols throughout the aquifer thickness. The confined aquifer is composed primarily of alluvial sediments of the Turlock Lake and upper Mehrten Formations, extending from beneath the Corcoran Clay to the base of fresh water. Ground water in the unconfined to semiconfined aquifer flows to the west and southwest. The primary source of present-day recharge is percolating excess irrigation water. The primary ground-water discharge is extensive ground-water pumping in the unconfined to semiconfined aquifer, imposing a significant component of vertical flow in the system. A water budget was calculated for water year 2000 using a land-use approach. During water year 2000, the total water supply in the Modesto area was more than 2.5 billion m3 (cubic meter) (2 million acre-ft [acre-foot]). Surface-water deliveries accounted for 60 percent of the total water supply, whereas ground-water pumpage accounted for 40 percent. Ninety-four percent of the water supply was used to meet irrigation demand and approximately 6 percent was used to meet urban demand. The total recharge in the model area was estimated at 1.4 billion m3 (1,100,000 acre-ft). The largest component of recharge is from excess irrigation water (58 percent); precipitation in excess of crop requirements accounted for 41 percent of the recharge. Geostatistical methods were used to develop a spatial correlation model of the percentage of coarse-grained texture in the Modesto area. The mean percentage coarse-grained texture calculated for each depth increment indicates a regional trend of decreasing coarse-grained texture with increasing depth, which is consistent with increasingly consolidated sediments with depth in the study area. The three-dimensional kriged estimates of percentage coarse-grained texture show significant heterogeneity in the texture of the sedimentary deposits. Assuming the hydraulic conductivity is correlated to the texture, the kriged result implies significant heterogeneity in the hydrogeologic framework.

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USGS1990

Monitoring moisture storage in trees using time domain reflectometry

Laboratory and field tests were performed to examine the feasibility of using time domain reflectometry (TDR) to monitor changes in the moisture storage of the woody parts of trees. To serve as wave guides for the TDR signal, pairs of stainless steel rods (13 cm long, 0.32 cm in diameter, and 2.5 cm separation) were driven into parallel pilot holes drilled into the woody parts of trees, and a cable testing oscilloscope was used to determine the apparent dielectric constant. A laboratory calibration test was performed on two sapwood samples, so that the relation between the volumetric water content and the apparent dielectric constant of the sapwood could be determined over a range of water contents. The resulting calibration curve for these sapwood samples was significantly different than the general calibration curve used for soils, showing a smaller change in the apparent dielectric constant for a given change in the volumetric water content than is typical for soils. The calibration curve was used to estimate the average volumetric water content to a depth of 13 cm in living trees. One field experiment was conducted on an English walnut tree ( Juglans regia ) with a diameter of 40 cm, growing in a flood-irrigated orchard on a Hanford sandy loam near Modesto, California (U.S.A.). Rods were driven into the tree at about 50 cm above the soil surface and monitored hourly for the month of August, 1988. The moisture content determined by TDR showed a gradual decrease from 0.44 to 0.42 cm 3 cm −3 over a two week period prior to flood irrigation, followed by a rapid rise to 0.47 cm 3 cm −3 over a four day period after irrigation, then again a gradual decline approaching the next irrigation. A second field experiment was made on ten evergreen and deciduous trees with diameters ranging from 30 to 120 cm, growing in the foothills of the Coast Range of central California. Rods were driven into each tree at 50 to 100 cm above the soil surface and monitored on a biweekly to monthly basis for over a year. Most trees showed an early spring maximum in moisture content determined by TDR associated with leaf growth, and a late summer minimum in moisture content associated with the end of the dry season. Moisture contents ranged from 0.20 to 0.70 cm 3 cm −3 , with an annual percentage change in moisture of 15% to 70% depending on species and environmental conditions. A final field test was performed in northern New Mexico (U.S.A.) to examine the effect of trunk freezing on TDR measurements. This test confirmed that freezing conditions were recorded as a total loss of liquid water by the TDR method. These results suggest that further TDR calibration for wood, plus some understanding of the relation between tree moisture and physiological stress could be useful to several disciplines, ranging from irrigation scheduling to watershed management to forest ecology.

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USGS1977

Guide for data collection to calibrate a predictive digital ground-water model of the unconfined aquifer in and near the city of Modesto, California

The city of Modesto encompasses about 12 square miles in the northeastern part of the San Joaquin Valley, Calif. The ground-water model encompasses about 542 square miles. In the Modesto area, ground water occurs in an unconfined aquifer a confined aquifer. both of which are composed of unconsolidated materials, and a consolidated-rock aquifer. Only the unconfined aquifer was modeled, using several simplifying assumptions concerning hydrologic conditions in the ground-water basin. A program is used that computes the net rate of recharge and discharge under steady-state conditions. The model was then modified until reasonable values of recharge and discharge were computed. Testing of the model indicated that simulated water levels were especially sensitive to tansmissivity, storage coefficient, irrigation return, and riverbed hydraulic conductivity; amond the parameters that affected water levels least were the vertical hydraulic conductivity and specific storage of the confining bed, the so-called E-clay. (Woodard-USGS)

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