BACKGROUND While research has shown that small community water systems in the San Joaquin Valley, an agricultural region in California, experience a disproportionate amount of drinking water contamination, little is known about the extent of contamination in other California regions. Additionally, state-wide research on drinking water contamination in areas not served by community water systems (mostly private domestic well users) and research comparing contamination across system size are also limited.METHODS Using a novel method to assign drinking water and groundwater contaminant data to community water system service areas and areas outside of service area boundaries, we conducted a spatial analysis to estimate concentrations of thirteen contaminants and two drinking water standard violations by system size and California region. We developed a cumulative ranking method to evaluate which regions or system size category are most burdened by multiple drinking water pollutants. A trend test was also used to evaluate the influence of system size on contaminant concentrations.RESULTS The San Joaquin Valley, areas not served by community water systems, and small water systems had the highest cumulative rank for multiple high contaminant concentrations, most notably arsenic and nitrate. Large systems and the South Coast region, which includes Los Angeles, had the highest levels of disinfection byproducts and industrial contaminants like tetrachloroethylene. Violations, arsenic, lead, and cadmium had negative trends as system size increased (p<0.05). Industrial contaminants and disinfection byproducts had positive trends as system size increased (p<0.05).CONCLUSIONS Few large-scale studies have examined how geographic region or system size impact drinking water quality. Although not indicative of violating drinking water standards, our results show where efforts for specific contaminants can be targeted in specific regions. The results presented here can help understand where contaminant levels might be elevated, both from an individual contaminant perspective as well as where multiple elevated contaminant levels accumulate.