A detailed review was made of chemical indicators used to identify impacts from septic tanks on groundwater quality. Potential impacts from septic tank leachate on groundwater quality were assessed using the mass ratio of chloride–bromide (Cl/Br), concentrations of selected chemical constituents, and ancillary information (land use, census data, well depth, soil characteristics) for wells in principal aquifers of the United States. Chemical data were evaluated from 1848 domestic wells in 19 aquifers, 121 public-supply wells in 6 aquifers, and associated monitoring wells in four aquifers and their overlying hydrogeologic units. Based on previously reported Cl/Br ratios, statistical comparisons between targeted wells (where Cl/Br ratios range from 400 to 1100 and Cl concentrations range from 20 to 100 mg/L) and non-targeted wells indicated that shallow targeted monitoring and domestic wells (<20 m depth below land surface) had a significantly (p < 0.05) higher median percentage of houses with septic tanks (1990 census data) than non-targeted wells. Higher (p = 0.08) median nitrate–N concentration (3.1 mg/L) in oxic (dissolved oxygen concentrations >0.5 mg/L) shallow groundwater from target domestic wells, relative to non-target wells (1.5 mg/L), corresponded to significantly higher potassium, boron, chloride, dissolved organic carbon, and sulfate concentrations, which may also indicate the influence of septic-tank effluent. Impacts on groundwater quality from septic systems were most evident for the Eastern Glacial Deposits aquifer and the Northern High Plains aquifer that were associated with the number of housing units using septic tanks, high permeability of overlying sediments, mostly oxic conditions, and shallow wells. Overall, little or no influence from septic systems were found for water samples from the deeper public-supply wells.

The Cl/Br ratio is a useful first-level screening tool for assessing possible septic tank influence in water from shallow wells (<20 m) with the range of 400–1100. The use of this ratio would be enhanced with information on other chloride sources, temporal variability of chloride and bromide concentrations in shallow groundwater, knowledge of septic-system age and maintenance, and the use of multiple tracers (combination of additional chemical and microbiological indicators).