Effects of Agriculture and Urbanization on Quality of Shallow Ground Water in the Arid to Semiarid Western United States, 1993

Scientific Investigations Report 2007–5179

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

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

Soil with moderate infiltration rate

Urban

173

<0.2–40.4

Dissolved oxygen

Percent households on public sewer

–

Percent hydric soils

–

Dichloro-bromomethane

¹80

Agricultural

235

<0.2

Urban

173

<0.2–3.39

Dibromo-chloromethane

¹80

Agricultural

235

<0.2

Urban

173

<0.2–.90

Tribromomethane (bromoform)

¹80

Agricultural

235

<0.2

Urban

173

<0.2

Tetrachloroethene (PCE)

Agricultural

235

<0.2–.421

Urban²

173

<0.2–89.0

3.5

Dissolved oxygen

Percent industrial land use

–

Trichloroethene (TCE)

Agricultural

235

<0.2–.80

Urban²

173

<0.2–19.19

1.2

Percent industrial land use

Methyl tert-butyl ether (MTBE)

³20–40

Agricultural

235

<0.2–.60

Urban

168

<0.2–220

³1.2

Population density

Percent industrial land use

Dissolved oxygen

–

Benzene

Agricultural

235

<0.2

Urban

173

<0.2–5.00

Toluene

1,000

Agricultural

233

<0.2–.949

Urban

173

<0.2

Ethylbenzene

700

Agricultural

235

<0.2

Urban

173

<0.2

Xylene

10,000

Agricultural²

234

<0.2–2.80

RIOG

Urban

172

<0.2

DBCP

0.2

Agricultural²

235

<0.2–4.0

SANJ (historical use)

Urban

173

<0.2

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).

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