Aquifer Studies and Recharge Assessment of the Northern California Lower Tuscan Aquifer System
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The Sacramento Valley Groundwater Basin and the Redding Groundwater Basin represent the principal groundwater basins of the broader Sacramento River Hydrologic Region. The Sacramento Valley Groundwater Basin is bound by the Coast Ranges to the west, the Cascade Range to the northeast, and the Sierra Nevada to the east, extending from the City of Red Bluff in the north to the Delta in the south. As described by California Department of Water Resources (DWR), the Sacramento Valley groundwater basin underlies approximately 4,900 square miles of the Sacramento Valley.The Tuscan Aquifer system, a regional aquifer of the Sacramento Valley Groundwater Basin, is among the principal water bearing units in Butte County. The Lower Tuscan is a critical resource to the region. However, there is incomplete data pertaining to aquifer system functions in terms of source, recharge and recovery. Butte County was awarded grant funds from DWR through Proposition 50 (Water Security, Clean Drinking Water, Coastal and Beach Protection Act of 2002) for implementation of the proposed Lower Tuscan Aquifer (LTA) Project.The primary objective of the LTA project was to collect and analyze technical data through development of innovative analytical investigative tools to further the scientific understanding of the LTA system and assess potential recharge sources. Key components of the overall project included development of cost effective methods to conduct and analyze aquifer parameters during the agricultural pumping season using newly installed and existing groundwater monitoring systems; evaluation of LTA recharge from upper reaches of creeks and, analysis of stable isotopes of hydrogen and oxygen from groundwater and surface water samples to assess the source of the recharge water.Aquifer tests were conducted during various agricultural pumping schemes including heavy continous pumping used for flood irrigation and intermittment pumping used for drip and spray irrigation of orchards. Qualitative analysis of drawdown curves from aquifer tests provided significant insights into the interactions between stratigraphically adjacent aquifer systems as well as aquifer stresses to the LTA resulting from various agricultural pumping schemes. The results of this analysis indicated that the LTA is a complex interconnected aquifer system and that a major recharge source to the LTA is through the overlying aquitard and overlying aquifers.Overall, isotopic data for samples collected from groundwater wells throughout the area was indicative of precipitation from elevations that are not far above the valley floor. Moving out along the valley floor isotopic data suggested recharged water may be due to a mixture of lower-elevation precipitation along the basin perimeter and interaction with the Sacramento River and other local streams. The study also found that the upper reach of the creeks contributed little to LTA recharge. Results of water level studies compared to flows recorded from the Sacramento River also suggested a potential connection during high storm events.