Groundwater quality in the Redding–Red Bluff shallow aquifer study unit of the northern Sacramento Valley, California

The Redding–Red Bluff study unit covers approximately 1,200 square miles in Shasta and Tehama Counties, California, at the northern end of the Sacramento Valley. The study unit covers groundwater basins in the Redding area and the northern Sacramento Valley. The Sacramento River flows through the study area. Groundwater aquifers within the regional study area are composed of marine, continental, and volcanic alluvial sediments derived from the surrounding mountain ranges: The Cascade Range to the east, the Klamath Mountains to the north, and the Coast Ranges to the west. The study unit is dominated by natural land use (60 percent), with urban use more common in the Redding study area (36 percent) and agricultural land use more common in the Red Bluff study area (20 percent). This study was designed to provide a statistically representative assessment of the quality of groundwater resources used for domestic drinking water in the Redding– Red Bluff study unit. A total of 50 wells were sampled between December 2018 and April 2019 (Shelton and others, 2020). Domestic wells in the study unit typically are drilled to depths of 80–338 feet (10th–90th percentiles; Shelton and others, 2020), which are shallower (p<0.001) than the depths of public-supply wells in the same area (typically 115–450 feet deep; Bennett and others, 2011). Water levels in domestic wells in the study unit typically are 15–163 feet below land surface (10th–90th percentiles; Shelton and others, 2020). Previous investigations of public supply wells in the study area found relatively low concentrations of inorganic and volatile organic compounds compared to state and national benchmarks, except for arsenic (4.6 percent; Bennett and others, 2011). A State Water Resources Control Board GAMA survey of domestic wells in Tehama County (223 wells), which includes the Red Bluff study area, reported arsenic concentrations above the benchmark (see page 3) in 13 percent of wells, primarily in the southeast part of the study area (California State Water Resources Control Board, 2009). However, these wells were not spatially distributed and do not represent aquifer-scale portions as described herein.