Abstract
Stratigraphic and hydrogeologic characterization of Cenozoic strata in the central Sacramento Valley near the Sutter Buttes was completed to establish a conceptual understanding of the subsurface geology and hydrostratigraphy, and provide a foundation for future water resource investigations. The regional stratigraphy and stratigraphic nomenclature for Cenozoic deposits in the study area are synthesized, a subsurface stratigraphic framework is established, the subsurface extent and volume of the Sutter Buttes Rampart is estimated, and the thickness of the freshwater aquifer is delineated.
Subsurface interpretation was achieved by maximizing the use of existing datasets (drillers' logs, Division of Oil, Gas, and Geothermal Resources well files) and integration of new data (logs of recent Department of Water Resources wells) in the Valley. Nine stratigraphic units are defined and make up the stratigraphic framework of the central Sacramento Valley. The units, from oldest to youngest, are the Capay Formation, Ione Formation, Lovejoy Basalt, Sutter formation (containing the Valley Springs, Mehrten, and Tuscan Formations), Rhyolitic Rampart, Andesitic Rampart, Laguna Formation, Turlock Lake Formation, and Alluvial deposits (containing the Riverbank and Modesto Formations). Distinctive subsurface characteristics of each stratigraphic unit were summarized to create a type log that was used for subsurface correlation.
Four regional geologic cross sections along with a chronostratigraphic framework chart illustrate the temporal, genetic, and stratigraphic relationships between the subsurface units, and provide insights into the paleogeography and basin evolution of the study area. The chronostratigraphic units identified suggest the depositional history of the central Sacramento Valley has been dominated by alluvial and fluvial processes from the late Tertiary through the Quaternary with a lacustrine environment during the middle Pleistocene as indicated by the Turlock Lake Formation, a diatomaceous clay interval (~25 feet average thickness) ranging from 0 to -150 feet mean sea level encountered in eleven recent boreholes distributed throughout the Sacramento Valley. In addition, dome eruptions and explosive magmatism occurred at the Sutter Buttes in the early Pleistocene.
A distinctive increase or baseline shift in the natural gamma ray signature coincides with the Sutter Buttes Rampart material in well logs. This distinctive signature enhances the subsurface characterization of the Rampart. The source of the high natural gamma ray values was determined, using the spectral gamma ray log, to be a pronounced increase in potassium and uranium, and a minor increase in thorium. These findings were substantiated by lithologic samples. Volcaniclastic material of the Rampart extends into the subsurface approximately 10 miles (16 km) to the north from the contact between the
Rampart and the Sacramento Valley sediments, 5 miles (8 km) to the south, 4 miles (6 km) to the west near the Sacramento River, and 5 miles (8 km) to the east near the Feather River, and has an estimated minimum volume of 72 km3. The areal extent of the Rampart is elongated to the north and slightly to the south while the areal extent is reduced on the east and west suggesting truncation and reworking by fluvial processes. The coarse-grained nature and considerable subsurface extent of the Rampart forms a significant aquifer in this area. However, analyses of water quality samples collected in the study area suggest there is a connection between elevated arsenic concentrations in groundwater and the presence of Rampart volcaniclastic material. In stratigraphic units above and below the Rampart, arsenic concentrations in groundwater are generally less than 10 μg/1. In contrast, within Rampart material, arsenic concentrations in groundwater samples are found in much higher concentrations (10 to 370 μg/1). Concentrations of arsenic in 27 spatially and depth distributed water samples from DWR monitoring wells ranged from < 0.1 to 370 μg/1. Concentrations in 15 of the samples exceed the US EPA drinking water Maximum Contaminant Level (MCL) of 10 μg/1, and 92 percent of groundwater samples collected from monitoring wells screened in Rampart volcaniclastic material have arsenic concentrations exceeding the MCL.
Fresh groundwater occurs primarily in late Tertiary to Quaternary unconsolidated sediments that extend to depths ranging from Oto greater than -2,000 feet mean sea level. The configuration of the base of fresh groundwater is an uneven surface that commonly reflects the underlying geologic structure of the Valley. The maximum depth of the base of fresh groundwater in the study area is just south of the Sutter Buttes (greater than 2,000 ft. msl). The occurrence of freshwater at this depth may be produced by the combination of the structural complexity related to the uplift of the Buttes and the exposure of Eocene to Cretaceous units (Ione and Kione Formation) and subsequent flushing of meteoric water through these units. On the west side of the study area the base of fresh groundwater reflects the underlying Colusa Buttes, which at the crest of this structure the Eocene-age Capay Formation is within 500 feet of the surface. The base of fresh groundwater boundary in the study area is consistently at or above the top of Eocene marine strata and is observed in Pliocene to Miocene continental deposits. This is likely caused by high artesian pressures and upward vertical gradients in deep aquifers in the Valley, which have been documented in deep (600-1,200 ft bgs) DWR monitoring wells. The brackish water in the lower part of aquifers near the base of fresh groundwater is classified as sodium-chloride (Na-Cl) water. The presence of brackish water in these deeper aquifers combined with upward vertical gradients creates the potential for continued upward migration of brackish water into overlying fresh water aquifers, or upconing beneath areas of pumping. Prolonged well pumping in these areas is likely to result in regional water quality degradation and may ultimately reduce the thickness of the fresh groundwater in Sacramento Valley aquifers.