Fig. 4. Frequencies of herbicide detection in ground water for the multistate studies in relation to reporting limits for (a) atrazine, and (b) metolachlor. CGAS, Ciba-Geigy Atrazine Study (Balu et al., 1998); LUSs, land-use studies; MMS, Metolachlor Monitoring Study (Roux et al., 1991); MWPS, Midwest Pesticide Study (Kolpin et al., 1995); NAWWS, National Alachlor Well-Water Survey (Holden et al., 1992); NPS, National Pesticide Survey (USEPA, 1990, 1992a); SUSs, subunit surveys; t_1/2, half-life for transformation in aerobic soil.