Groundwater samples have been collected in California as part of statewide investigations of groundwater quality conducted by the U.S. Geological Survey for the Groundwater Ambient Monitoring and Assessment (GAMA) Priority Basin Project (PBP). Quality-control samples are routinely collected as part of the GAMA-PBP quality assurance plan. Olsen and others (2010) and Davis and others (2014) previously determined study reporting levels (SRLs) for trace-element results based primarily on field blanks collected in California from May 2004 through March 2013. SRLs are raised reporting levels used to reduce the likelihood of reporting false detections attributable to contamination bias. Davis and others (2014) reevaluated SRLs defined by Olsen and others (2010) by looking for changes in the pattern or magnitude of field blank detections following the implementation of SRLs by Olsen and others (2010). Based on a consistent pattern of decreasing cobalt and manganese concentrations in field blanks from 2009 to 2013 the GAMA-PBP decided to reevaluate all trace element study reporting levels (SRLs) defined by Davis and others (2014). Trace element data from 167 field blanks, 19 source-solution blanks, and 8 equipment blanks collected from October 2009 through October 2018 by the GAMA-PBP were compiled as part of the reevaluation and are presented here. The results of the effort to update the SRLs previously defined by Davis and others (2014) are presented here: Bennett, G.L. V, 2020, Updated study reporting levels (SRLs) for trace-element data collected for the California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priortiy Basin Project, October 2009–October 2018: U.S. Geological Survey Scientific Investigations Report 2020–5034, 24 p., https://doi.org/ 10.3133/ sir20205034.