Physical subdivision and description of the water-bearing sediments of the Santa Clara Valley, California

Wentworth, Carl M.; Jachens, Robert C.; Williams, Robert A.; Tinsley, John C.; Hanson, Randall T.

doi:10.3133/sir20155017

Cited by 10 publications

(47 citation statements)

References 12 publications

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“…The regional aquifer system is underlain and surrounded by the relatively impermeable bedrock. The Santa Clara Formation, which outcrops along the margins of the valley, is not present in the interior parts of the valley (Andersen et al, 2005;Wentworth and Tinsley, 2005;Wentworth et al, 2015). The Santa Clara Formation was not identified initially in many deeper wells (Tolman and Poland, 1940) and has not been encountered in the recently completed multiple-well monitoring sites completed by the USGS in cooperation with SCVWD.…”

Section: Hydrogeologic Structurementioning

confidence: 94%

“…These underlying deposits form the relatively impermeable base of the regional aquifer systems. The alluvial aquifer systems are composed of a complex sequence of layers of fluvial sand and gravel and fluvial fine-grained silt and clay (California Department of Water Resources, 1967;Wagner et al, 1990;Wentworth, 1997;Wentworth and Tinsley, 2005;Wentworth et al, 2004Wentworth et al, , 2015Knudsen et al, 2000). These layers represent the glacial climate cycles that resulted in the eight cycles of sedimentation over the past 718,000 yr ( Fig.…”

Section: Hydrogeologic Structurementioning

confidence: 96%

“…3B; Wentworth et al, 2015). The confining units in the upper part of the latest cycles represent a regional inundation of the valley during the persistent tectonic subsidence occurring over the past 718,000 yr (Wentworth et al, 2010(Wentworth et al, , 2015. Each cycle represents a fining-upward sequence of predominantly fluvial and alluvial sedimentation that results in coarser-grained basal layers and fine-grained upper layers.…”

Section: Hydrogeologic Structurementioning

confidence: 99%

“…These layers represent the glacial climate cycles that resulted in the eight cycles of sedimentation over the past 718,000 yr ( Fig. 3B; Wentworth et al, 2015). The confining units in the upper part of the latest cycles represent a regional inundation of the valley during the persistent tectonic subsidence occurring over the past 718,000 yr (Wentworth et al, 2010(Wentworth et al, , 2015.…”

Section: Hydrogeologic Structurementioning

confidence: 99%

“…Therefore, the depth of the alluvial-aquifer system and the depth of the effective groundwater flow system were uncertain in some parts of the valley prior to the ongoing studies. Sequence stratigraphic analysis and related hydrostratigraphy are part of the USGS studies (Jachens et al, 2001) that have further delineated the geologic and hydrogeologic framework (Wentworth et al, 2010(Wentworth et al, , 2015Langenheim et al, 2014).…”

Section: Hydrogeologic Structurementioning

confidence: 99%

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Hydrologic framework of the Santa Clara Valley, California

Hanson

2015

Geosphere

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The hydrologic framework of the Santa Clara Valley in northern California was redefined on the basis of new data and a new hydrologic model. The regional groundwater flow systems can be subdivided into upper-aquifer and lower-aquifer systems that form a convergent flow system within a basin bounded by mountains and hills on three sides and discharge to pumping wells and the southern San Francisco Bay. Faults also control the flow of groundwater within the Santa Clara Valley and subdivide the aquifer system into three subregions.After decades of development and groundwater depletion that resulted in substantial land subsidence, Santa Clara Valley Water District (SCVWD) and the local water purveyors have refilled the basin through conservation and importation of water for direct use and artificial recharge. The natural flow system has been altered by extensive development with flow paths toward major well fields. Climate has not only affected the cycles of sedimentation during the glacial periods over the past million years, but interannual to interdecadal climate cycles also have affected the supply and demand components of the natural and anthropogenic inflows and outflows of water in the valley. Streamflow has been affected by development of the aquifer system and regulated flow from reservoirs, as well as conjunctive use of groundwater and surface water. Interaquifer flow through water-supply wells screened across multiple aquifers is an important component to the flow of groundwater and recapture of artificial recharge in the Santa Clara Valley. Wellbore flow and depth-dependent chemical and isotopic data indicate that flow into wells from multiple aquifers, as well as capture of artificial recharge by pumping of water-supply wells, predominantly is occurring in the upper 500 ft (152 m) of the aquifer system. Artificial recharge represents about one-half of the inflow of water into the valley for the period 1970-1999. Most subsidence is occurring below 250 ft (76 m), and most pumpage occurs within the upper-aquifer system between 300 and 650 ft (between 91 and 198 m) below land surface.Overall, the natural quality of most groundwater in the Santa Clara Valley is good. Isotopic data indicate that artificial recharge is occurring throughout the shallower parts of the upper-aquifer system and that recent recharge (less than 50 yr old) occurs throughout most of the basin in the upper-aquifer system, but many of the wells in the center of the basin with deeper well screens do not contain tritium and recent recharge. Age dates indicate that the groundwater in the upper-aquifer system generally is less than 2000 yr old, and groundwater in the lower-aquifer system generally ranges from 16,700 to 39,900 yr old. Depth-dependent sampling indicates that wellbores are the main path for vertical flow between aquifer layers. Isotopic data indicate as much as 60% of water pumped from production wells originated as artificial recharge. Shallow aquifers not only contain more recent recharge but may be more susceptible to anthro...

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Section: Hydrogeologic Structurementioning

confidence: 94%

Section: Hydrogeologic Structurementioning

confidence: 96%

Section: Hydrogeologic Structurementioning

confidence: 99%

Section: Hydrogeologic Structurementioning

confidence: 99%

Section: Hydrogeologic Structurementioning

confidence: 99%

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Hydrologic framework of the Santa Clara Valley, California

Hanson

2015

Geosphere

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An assessment of uncertainties in VS profiles obtained from microtremor observations in the phased 2018 COSMOS blind trials

Asten¹,

Yong

Foti

et al. 2022

J Seismol

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Site response is a critical consideration when assessing earthquake hazards. Site characterization is key to understanding site effects as influenced by seismic site conditions of the local geology. Thus, a number of geophysical site characterization methods were developed to meet the demand for accurate and cost-effective results. As a consequence, a number of studies have been administered periodically as blind trials to evaluate the state-of-practice on-site characterization. We present results from the Consortium of Organizations for Strong Motion Observation Systems (COSMOS) blind trials, which used data recorded from surface-based microtremor array methods (MAM) at four sites where geomorphic conditions vary from deep alluvial basins to an alpine valley. Thirty-four invited analysts participated. Data were incrementally released to 17 available analysts who participated in all four phases: (1) two-station arrays, (2) sparse triangular arrays, (3) complex nested triangular or circular arrays, and (4) all available geological control site information including drill hole data. Another set of 17 analysts provided results from two sites and two phases only. Although data from one site consisted of recordings from three-component sensors, the other three sites consisted of data recorded only by vertical-component sensors. The sites cover a range of noise source distributions, ranging from one site with a highly directional microtremor wave field to others with omni-directional (azimuthally distributed) wave fields. We review results from different processing techniques (e.g., beam-forming, spatial autocorrelation, cross-correlation, or seismic interferometry) applied by the analysts and compare the effectiveness between the differing wave field distributions. We define the M index as a quality index based on estimates of the time-averaged shear-wave velocity of the upper 10 (VS10), 30 (VS30), 100 (VS100), and 300 (VS300) meters and show its usefulness in quantitative comparisons of VS profiles from multiple analysts. Our findings are expected to aid in building an evidence-based consensus on preferred cost-effective arrays and processing methodology for future studies of seismic site effects.

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Widespread aquifer depressurization after a century of intensive groundwater use in USA

Hilton,

Jasechko

2023

Sci. Adv.

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Water supplies for household use and irrigated agriculture rely on groundwater wells. When wells are drilled into a highly pressurized aquifer, groundwater may flow up the well and onto the land surface without pumping. These flowing artesian wells were common in the early 1900s in the United States before intensive groundwater withdrawals began, but their present-day prevalence remains unknown. Here, we compile and analyze ten thousand well water observations made more than a century ago. We show that flowing artesian conditions characterized ~61% of wells tapping confined aquifers before 1910, but only ~4% of wells tapping confined aquifers today. This pervasive loss of flowing artesian conditions evidences a widespread depressurization of confined aquifers after a century of intensive groundwater use in the United States. We conclude that this depressurization of confined aquifers has profoundly changed groundwater storage and flow, increasing the vulnerability of deep aquifers to pollutants and contributing to land subsidence.

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Physical subdivision and description of the water-bearing sediments of the Santa Clara Valley, California

Cited by 10 publications

References 12 publications

Hydrologic framework of the Santa Clara Valley, California

Hydrologic framework of the Santa Clara Valley, California

An assessment of uncertainties in VS profiles obtained from microtremor observations in the phased 2018 COSMOS blind trials

Widespread aquifer depressurization after a century of intensive groundwater use in USA

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