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Scientific Investigations Report 2007–5179

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5179

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Table 9. Volatile-organic compound occurrence in shallow ground water beneath agricultural and urban areas within regional National Water-Quality Assessment Program study units in the arid to semiarid Western United States, 1993–2004.

[The “+” and “-” signs indicate a positive and negative correlation, respectively, of volatile-organic compound occurrence to designated explanatory variable. Abbreviations: DBCP, 1,2-dibromo-3-chloropropane; MCL, maximum contaminant level; USEPA, U.S. Environmental Protection Agency; VOC, volatile organic compound; RIOG, Rio Grande Valley; SANJ, San Joaquin-Tulare Basin; µg/L, microgram per liter; nd, not determined. Symbol: <, less than]

VOC USEPA
MCL (µg/L)
Land
use
Number
of wells
Concentration
range
(µg/L)
Detection
frequency
(percent)
Percent
detections
exceeding
MCL
Important factors relating
to VOC detection
Trichloromethane (chloroform) 180 Agricultural2 235 <0.2–0.34 1 0 Soil with moderate infiltration rate +
Urban 173 <0.2–40.4 29 0 Dissolved oxygen +
Percent households on public sewer +
pH
Percent hydric soils
Dichloro-bromomethane 180 Agricultural 235 <0.2 0 0 nd
Urban 173 <0.2–3.39 5 0 nd
Dibromo-chloromethane 180 Agricultural 235 <0.2 0 0 nd
Urban 173 <0.2–.90 1 0 nd
Tribromomethane (bromoform) 180 Agricultural 235 <0.2 0 0 nd
Urban 173 <0.2 0 0 nd
Tetrachloroethene (PCE) 5 Agricultural 235 <0.2–.421 1 0 nd
Urban2 173 <0.2–89.0 10 3.5 Dissolved oxygen +
Percent industrial land use +
pH
Trichloroethene (TCE) 5 Agricultural 235 <0.2–.80 <1 0 nd
Urban2 173 <0.2–19.19 5 1.2 Percent industrial land use +
Methyl tert-butyl ether (MTBE) 320–40 Agricultural 235 <0.2–.60 <1 0 nd
Urban 168 <0.2–220 8 31.2 Population density +
Percent industrial land use +
Dissolved oxygen
Benzene 5 Agricultural 235 <0.2 0 0 nd
Urban 173 <0.2–5.00 1 0 nd
Toluene 1,000 Agricultural 233 <0.2–.949 1 0 nd
Urban 173 <0.2 0 0 nd
Ethylbenzene 700 Agricultural 235 <0.2 0 0 nd
Urban 173 <0.2 0 0 nd
Xylene 10,000 Agricultural2 234 <0.2–2.80 3 0 RIOG
Urban 172 <0.2 0 0 nd
DBCP 0.2 Agricultural2 235 <0.2–4.0 8 0 SANJ (historical use)
Urban 173 <0.2 0 0 nd

1Total THM MCL.

2A sufficiently strong logistic-regression model was not developed for this volatile-organic compound within areas of this land use. However, the parameters listed were determined to be important in the occurrence of the volatile-organic compound (p < 0.05).

3USEPA drinking-water advisory.

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