. 2021 Jun 23;129(6):067004. doi: 10.1289/EHP7813

Table 6.

Multivariable modeling of well construction risk factors as related to detection probabilities and concentrations of coliforms, any fecal-associated marker, and nitrate in private wells.

Contaminant and outcome measurement (n)^a	Risk factor	Univariable model $p$ -value	Multivariable model
Contaminant and outcome measurement (n)^a	Risk factor	Univariable model $p$ -value	Risk factor median^b	Risk factor range^b	Coefficient or trend^c^,^d	$p$ -value^e
Any fecal marker^f detection (83)	Casing depth	0.15	17.7	12.2–48.2	Negative	0.31
	Open interval length	0.13	29.0	2.1–79.6	Positive	0.24
	Bedrock depth	0.027	4.6	0–46.6	$- 0.02$	0.26
Coliforms concentration, recharge (47)	Well depth	0.047	48.8	18.3–100.6	Negative	0.59
	Casing depth	0.057	18.9	12.2–80.2	None	0.91
	Groundwater depth at construction	0.0004	12.2	1.8–36.6	Negative	0.0038
	Well age	0.0042	24	5–49	0.04	0.016
${NO}_{3}^{-} -N > 10 mg / L$ detection, recharge (201)	Casing length below water table	0.040	8.5	–36–58.8	$- 0.02$	0.13
${NO}_{3}^{-} -N > 10 mg / L$ detection, recharge (201)	Bedrock depth	0.0028	6.4	0–55.2	Negative	0.28
${NO}_{3}^{-} -N$ concentration, recharge (124)	Well age	0.15	22	2–80	Positive	0.16
${NO}_{3}^{-} -N$ concentration, recharge (124)	Bedrock depth	0.0063	4.7	0–31.4	Negative	0.11
${NO}_{3}^{-} -N > 10 mg / L$ detection, no-recharge (251)	Casing depth	0.12	18.9	6.1–126.5	0.01	0.65
	Casing length below water table	0.02	8.8	–36–117.3	$- 0.02$	0.33
	Bedrock depth	$< 0.0001$	10.1	0.3–54.3	Negative	0.07
${NO}_{3}^{-} -N$ concentration, no-recharge (127)	Casing depth	0.043	18.0	6.1–126.5	$- 0.008$	0.57
	Casing length below water table	0.054	5.5	–19.8–117.3	Negative	0.74
	Bedrock depth	0.0019	6.7	0.3–49.4	Negative	0.0088

Note: Univariable model $p$ -values used for selecting risk factors are included for reference; complete univariable statistics are provided in Tables S11, S12, and S13. Risk factor eligibility for inclusion in multivariable models is described in statistical methods.

^{^a}

$n = number of samples$ in multivariable model.

^{^b}

Units for length and depth are meters; age is in years.

^{^c}

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

^{^d}

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

^{^e}

The composite $p$ -value is reported for spline-represented variables.

^{^f}

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