Table 5.

Multivariable modeling of land use and bedrock risk factors as related to detection probabilities and concentrations of genetic markers of host-specific and fecal-associated microbes in private wells.

Fecal marker source and outcome measurement (n)a	Risk factor	Univariable model $p$-value	Multivariable model
			Risk factor medianb	Risk factor rangeb	Coefficient or trendc,d	$p$-valuee
Human marker detection (137)	Drainfield septic systems, count within $229\mathrm{m}$	0.038	2	0–10	0.09	0.11
	Groundwater depth, 14-d antecedent, median	0.0003	1.2	0.3–1.6	$\begin{array}{c}-0.9\end{array}$	0.011
	Rainfall, 2-d antecedent, cumulative	0.0093	14	0–37	Positive	0.69
	Bedrock depth	0.051	6.1	0–46.6	Negative	0.13
Human marker concentration (33)	Bedrock depth	0.011	4.3	0.3–36.6	Negative	0.011
Bovine marker detection (138)	Groundwater recharge, 7-d antecedent, cumulative	0.0041	50	0–60	Positive	0.0092
Bovine marker concentration (41)	Agricultural field area within $229\mathrm{m}$	0.029	11.6	3.7–16.4	Positive	0.024
	Bedrock depth	0.0019	5.2	0–29	$\begin{array}{c}-0.1\end{array}$	0.0006
Any fecal markerf detection (137)	Drainfield septic systems, count within $229\mathrm{m}$	0.0036	2	0–10	Positive	0.036
	Rainfall, 2-d antecedent, cumulative	0.12	14	0–37	Positive	0.19
	Manure storage distanceg	0.94	687	71–3,728	$-0.0004$	0.036
	Bedrock depth	0.027	6.1	0–46.6	$-0.06$	0.0058
	Manure storage distance times bedrock depth interaction	0.045	NA	NA	Negative	0.024
Any fecal marker concentration (77)	Agricultural field area within $229\mathrm{m}$	0.035	12.7	1.1–16.4	Positive	0.097
	Manure storage distance	0.083	762	113–3,728	$-0.0001$	0.76
	Bedrock depth	0.0003	4.6	0–36.6	$-0.08$	0.002

Note: NA, Not applicable. Univariable model p-values used for selecting risk factors are included for reference; complete univariable statistics are provided in Table S10. Risk factor eligibility for inclusion in multivariable models is described in statistical methods.

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

^bUnits for distance and depth are meters; rainfall and recharge are millimeters; area is hectares.

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

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

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

^f“Any fecal marker” includes all microorganisms regardless of host specificity.

^gIncluded in multivariable model because of its significant interaction with bedrock depth.