Table 4.

Multivariable modeling of land use and bedrock risk factors as related to detection probabilities and concentrations of coliforms and nitrate in private wells.

Sampling period	Contaminant and outcome measurementa (n)b	Risk factor	Univariable model $p$-value	Multivariable model
				Risk factor medianc	Risk factor rangec	Coefficient or trendd,e	$p$-valuef
Groundwater recharge	Coliforms detection (315)	Bedrock depth	0.0090	7.6	0–56.4	Negative	0.0001
		NMP field distanceg	0.036	42	0–723	$-0.002$	0.20
		Manure storage distance	0.14	899	46–3,728	$-0.00008$	0.63
		Agricultural field area within $229\mathrm{m}$	0.072	12.7	0–16.4	$-0.008$	0.77
	${\mathrm{NO}}_3^{-}\text{-N}>10\mathrm{mg}/\mathrm{L}$ detection (318)	NMP field area within $229\mathrm{m}$	0.0013	7.1	0–15.9	0.1	0.024
		NMP field distance	0.14	42	0–724	0.002	0.38
		Manure storage distance	0.082	928	46–3,728	$-0.0002$	0.49
		Bedrock depth	0.0028	7.6	0–56.4	Negative	0.082
	${\mathrm{NO}}_3^{-}\text{-N}$ concentration (200)	NMP field area within $914\mathrm{m}$	0.071	141.7	10.2–235.7	Positive	0.29
		Bedrock depth	0.0063	5.0	0–56.4	Negative	0.0065
No groundwater recharge	Coliforms detection (395)	Manure storage distance	0.0014	878	48–7,054	$-0.0005$	0.0062
		Agricultural field distance, dichotomous	0.15	NA	NA	0.3	0.24
		Agricultural field distance, continuous	0.081	24	0–805	$-0.003$	0.34
		NMP field area within $229\mathrm{m}$	0.059	7.4	0–15.6	0.008	0.75
		Bedrock depth	0.12	12.2	0–61	$-0.006$	0.42
	Coliforms concentration (76)	NMP field distance	0.0026	36	0–554	Negative	0.0050
	${\mathrm{NO}}_3^{-}\text{-N}>10\mathrm{mg}/\mathrm{L}$ detection (399)	Bedrock depth	$<0.0001$	12.2	0–61	Negative	0.021
		NMP field area within $457\mathrm{m}$	0.014	33.3	0–62.4	0.008	0.48
		NMP field distance	0.082	40	0–836	$-0.001$	0.66

Note: NA, Not applicable; NMP, nutrient management plan. Univariable model p-values used for selecting risk factors are included for reference; complete univariable statistics are provided in Tables S8 and S9. Risk factor eligibility for inclusion in multivariable models is described in statistical methods.

^aUnivariable analyses for: a) coliform concentration, groundwater recharge; and b) nitrate concentration, no recharge, showed no eligible variables for multivariable modeling; therefore, these models are missing from the table.

^b$n=\text{number of samples}$ in multivariable model.

^cUnits for distance and depth are meters; area is hectares.

^dIn lieu of reporting multiple coefficients for spline-represented variables, we report the overall trend (positive or negative).

^eInterpretation of coefficient linear terms: change in ln(detection probability) or change in ln(concentration) for a unit change in the risk factor.

^fThe composite p-value is reported for spline-represented variables.

^gFields with NMPs likely receive manure and inorganic fertilizer inputs.