a1 ppb = 1 µg/L.
bMedian perchlorate concentration of values above the minimum reporting level (<4 ppb).
cMaximum in United States was measured in Duval County, Florida.
dMaximum in California was measured in San Bernardino County. Abbreviations: MRL, minimum reporting level (<4 ppb); NA, not applicable.
Source: Data from EPA 2004a.