Table 2.

Logistic regression models on the probability of observing more than one lead action level exceedance (Pb ALE) per 10 surface drinking water intakes upstream of water quality monitoring stations. The models tested median (1992–2012) chloride concentration (mg L^{− 1}), chloride-sulfate mass ratio (CSMR), and Larson Ratio (LR) as predictor variables.

Predictor variable	Slope	p-Value	Accuracya	Akaike information criterion
Chloride	0.004	0.461	0.436	57.34
CSMR	1.02	0.037	0.667	51.88
LR	− 0.113	0.426	0.538	57.32

^aAccuracy – the sum of true positives and true negatives divided by total observations. True positives are cases with more than one ALE per 10 surface water intakes that have a predicted probability > 0.5. True negatives are the analogous negative case.