|
Variable
|
Nitrate concentration
|
Likelihood of high nitratea
|
|---|
|
Percent change (95% CI)
|
p-value
|
Percent change (95% CI)
|
p-value
|
|---|
|
Percent Black, non-Hispanicb
|
−1.3 (−1.5, −1.0)
|
< 0.0001
|
−4.3 (−5.7, −2.9)
|
< 0.0001
|
|
Percent Hispanicb
|
1.8 (1.6, 2.0)
|
< 0.0001
|
1.9 (1.4, 2.4)
|
< 0.0001
|
|
Percent povertyb
|
−0.8 (−1.2, −0.3)
|
0.0004
|
−2.2 (−3.7, −0.7)
|
0.004
|
|
Percent home ownershipb
|
--f
| |
−0.7 (−1.9, 0.4)
|
0.214
|
|
Percent urban householdsb
|
0.0 (0.0, 0.1)
|
0.14
|
0.4 (0.2, 0.6)
|
< 0.0001
|
|
System size: Smallc
|
9.0 (4.6, 13.7)
|
< 0.0001
|
−21.8 (−31.6, −10.5)
|
0.0003
|
|
System size: Mediumc
|
15.8 (9.2, 22.8)
|
< 0.0001
|
−24.8 (−40.4, −5.1)
|
0.016
|
|
System size: Largec
|
36.2 (27.1, 45.9)
|
< 0.0001
|
−10.0 (−30.6, 16.7)
|
0.426
|
|
System size: V. Largec
|
51.0 (28.2, 77.9)
|
< 0.0001
|
54.1 (−20.4, 199)
|
0.200
|
|
Source water: Groundwaterd
|
33.9 (26.4, 41.9)
|
< 0.0001
|
311 (201, 460)
|
< 0.0001
|
|
Percent croplandb
|
0.6 (0.5, 0.8)
|
< 0.0001
|
3.3 (3.0, 3.7)
|
< 0.0001
|
|
Livestock per 100 acresb
|
0.1 (0.0, 0.2)
|
0.008
|
0.2 (0.0, 0.5)
|
0.107
|
|
Region: Northeaste
|
84.0 (6.0, 219)
|
0.030
|
69.1 (−41.7, 390)
|
0.333
|
|
Region: Southe
|
58.5 (−1.7, 156)
|
0.059
|
106 (−18.5, 418)
|
0.127
|
|
Region: Weste
|
88.1 (13.9, 211)
|
0.014
|
270 (44.7, 849)
|
0.006
|
- Notes: Percent change in nitrate concentration and likelihood of exceeding 5 mg/L associated with a one-unit increase in the county-level demographic data (e.g., 1% increase in percent Hispanic) and agricultural land use data, and for each system size, region, and source water categories relative to the referent group
- N = 39,466. Models include state-specific random intercepts and a fixed intercept term
- aLogistic regression; outcome coded as “1” if system-average concentration ≥ 5 mg/L and “0” otherwise
- bContinuous predictor, table estimates reflect effect of one-unit change in the predictor
- cReferent group: very small systems. dReferent group: surface water. eReferent group: Midwest
- fNot included in final model
