Variable geometry and distribution of stratigraphic sequences of fluvial fans in the eastern San Joaquin Basin, California, were controlled by tectonics, through basin subsidence and basin width, and response to Quaternary climate change, related to the degree of change in sediment supply to stream discharge ratios and local base-level elevation changes. Three fluvial fans -the Kings River, Tuolumne River and Chowchilla River fans -illustrate the influence of these factors on ultimate sequence geometry. In areas with high subsidence rates (e.g. the Kings River fluvial fan) sequences are relatively thick and apices of subsequent sequences are vertically stacked. Areas with relatively low subsidence rates (e.g. the Tuolumne River fan) produced laterally stacked sequences. Rivers that experienced a significant increase in sediment supply to stream discharge ratios due to direct connection to outwash from glaciated portions of the Sierra Nevada developed high accommodation space and relatively thick sequences with deep incised valleys. Conversely, rivers that were not connected to glaciated regions (e.g. the Chowchilla River fan) and, thus, experienced a relatively minor change in sediment supply to discharge ratios during climate change events, produced thinner sequences that lack deep incised valleys. Local base-level connection to sea level, via the axial San Joaquin River, produced deeper incised valleys than those of internally drained rivers. Finally, narrow basin width allowed glacially connected fans to completely fill available accommodation space, thus producing smaller fans that lack preservation of distal, interglacial deposits. Evaluation of these controls allows prediction of sequence geometries and facies distributions for other San Joaquin Basin fans for input into future hydrogeological models.

strontium, and inorganic carbon in water, tritium activities, and carbon-14 abundances) were measured to help identify the sources and ages of the sampled groundwater. In total, Groundwater quality in the 1,850-square-mile North San 207 constituents and water-quality indicators were measured. Francisco Bay Shallow Aquifer (NSF-SA) study unit was Three types of quality-control samples (blanks, replicates, investigated by the U.S. Geological Survey (USGS) from and matrix spikes) were collected at up to 13 percent of April to August 2012, as part of the California State Water the wells in the NSF-SA study unit, and the results for Resources Control Board (SWRCB) Groundwater Ambient these samples were used to evaluate the quality of the Monitoring and Assessment (GAMA) Program's Priority data for the groundwater samples. Blanks rarely contained Basin Project (PBP). The GAMA-PBP was developed in detectable concentrations of any constituent, suggesting that response to the California Groundwater Quality Monitoring contamination from sample-collection procedures was not Act of 2001 and is being conducted in collaboration with a significant source of bias in the data for the groundwater the SWRCB and Lawrence Livermore National Laboratory samples. Replicate samples generally were within the limits of (LLNL). The NSF-SA study unit was the first study unit to be acceptable analytical reproducibility. Matrix-spike recoveries sampled as part of the second phase of the GAMA-PBP, which were within the acceptable range (70 to 130 percent) for focuses on the shallow aquifer system. approximately 91 percent of the compounds. The GAMA NSF-SA study was designed to provide Most of the wells sampled for this study were private a spatially unbiased assessment of untreated-groundwater domestic wells. Private domestic wells are not regulated quality in the shallow aquifer systems and to facilitate in California, and groundwater from these wells is rarely statistically consistent comparisons of untreated-groundwater analyzed for water-quality constituents. Although regulatory quality throughout California. The shallow aquifer system in benchmarks for drinking-water quality do not apply to the NSF-SA study unit was defined as the part of the aquifer private domestic wells, to provide some context for the system that is used by many private domestic wells and is results, concentrations of constituents measured in the shallower than the primary aquifer system used by many untreated groundwater were compared with regulatory and public-supply wells. non-regulatory health-based benchmarks established by In the NSF-SA study unit located in Marin, Mendocino, the U.S. Environmental Protection Agency (USEPA) and Napa, Solano, and Sonoma Counties, groundwater samples California Department of Public Health (CDPH), to nonwere collected from 71 wells. Seventy of the wells were regulatory health-based benchmarks established by the selected by using a spatially distributed, randomized grid-USGS in cooperation with the USEPA, and to non-regulatory based ...

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Recently developed sequence stratigraphic models for fl uvial fans suggest that sequence boundaries in these deposits are marked by laterally extensive paleosols; however, these models were based on paleosol correlations inferred between wells. To test this, we collected ~190 km of ground-penetrating radar (GPR) profi les on three fl uvial fans from the eastern San Joaquin Valley, California, to determine the lateral extent and character of a buried near-surface sequence-bounding paleosol. This paleosol, recognized on GPR by rapid shallow signal attenuation, extends across large areas on all three fl uvial fans. Limited areas of signifi cantly increased signal penetration were also identifi ed, and these zones are interpreted to indicate the absence of the paleosol. The zones where the paleosol is missing likely correspond to paleooutwash channel activity on the fan surfaces that, when active, was able to partially or fully scour through the paleosol and deposit coarse-grained channel sediments in place of the sequence boundary. Erosional breaks are most common on the Kings River fan, while few breaks on the Tuolumne and Merced River fans may indicate less paleochan-nel activity on these fan surfaces during the last outwash event. Differences in channel activity between fans indicate that the Kings River migrated across its fan during the last outwash event, as evidenced by the large number of areas with increased GPR signal penetration and the presence of numerous channel deposits recorded on the soil surveys, while the Tuolumne and Merced Rivers only deposited fl oodplain fi nes, with the channels remaining inside a shallow incised valley, as evidenced by the relatively low number of areas with increased GPR signal penetration and the presence of primarily fi ne-grained material recorded on the soil surveys. Factors controlling these differences may include variable valley subsidence rates and differences in the San Joaquin Basin overall width at each fan location.