Fig. 5. Frequencies of herbicide detection in shallow ground water for NAWQA land-use studies conducted in agricultural areas, in relation to total agricultural use within a 1-km radius of all sites sampled for each study. Studies with zero use assigned a value of 0.001 kg a.i. per square kilometer to accomodate log scale (see text). a.i., active ingredient; R², coefficient of determination for linear correlation; , Spearman's rank correlation coefficient; t_1/2, half-life for herbicide transformation in aerobic soil. *, ** Statistically significant at the 0.05 and 0.001 probability levels, respectively.

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