National Water-Quality Assessment (NAWQA) Program
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 98-4222
Sacramento, California, 1999
Figure 6. Annual mean detection frequencies of all 46 target compounds at 50 sites during a 1-year period. Detection frequencies at each site were determined as the mean of 12 monthly detection frequencies. For each compound, the detection frequencies at all 50 sites were averaged.