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Ground-water quality in Orange County, North Carolina

Project Abstract
By W.L. Cunningham and J.M. Fine


Abstract

From November 1998 through January 1999, the U.S. Geological Survey (USGS) sampled 51 domestic wells in Orange County, North Carolina, as part of a more comprehensive evaluation of the county's ground-water resources. Wells were selected among wells inspected by county staff during 1996-1998 based on (a) countywide areal distribution, (b) weighted distribution among hydrogeologic units, and (c) permission from the homeowner. Wells were located by using a global positioning system and plotted on a hydrogeologic unit map.

Water samples were collected from wells in the following hydrogeologic units: felsic metavolcanic (45 percent of the county land area, 41 percent of the samples collected), felsic metaigneous (26 percent area, 26 percent samples), intermediate metavolcanic (14 percent area, 17 percent samples), phyllite (7 percent area, 4 percent samples), mafic metaigneous (3 percent area, 4 percent samples), epiclastic metavolcanic (2 percent area, 6 percent samples), and intermediate metaigneous (<1 percent area, 2 percent samples).

Samples were screened for benzene, toluene, ethylbenzene, and xylene (BTEX), and atrazine by using an immunoassay technique and analyzed by USGS laboratories for major ions (dissolved and total), nutrients, and radon. Samples from 31 wells also were analyzed for dissolved trace metals. Results were compared to State and Federal standards, and compared among hydrogeologic units. None of the BTEX or atrazine compounds were detected in any of the samples. Nitrogen concentrations up to 7.2 milligrams per liter (mg/L) were observed, with a median concentration of 0.49 mg/L. Exceedances of State or U.S. Environmental Protection Agency (USEPA) drinking water standards were observed for iron (3 exceedances of 51 analyses, detection up to 1,100 µg/L), lead (8 of 31, up to 3.5 µg/L), manganese (12 of 51, up to 890 µg/L), and zinc (4 of 31, up to 4,900 µg/L).

Total radon 222 was collected by using a specialized technique to prevent sample degassing. Radon activity ranged from 38 to 4,462 picocuries per liter (pCi/L) countywide, with a median activity of 405 pCi/L. Median radon activities were highest in felsic rocks (487 pCi/L), and lowest in mafic rocks (357 pCi/L). USEPA has proposed a radon maximum contaminant level (MCL) of 300 pCi/L, and an alternative MCL (AMCL) of 4,000 pCi/L. Sixty-seven percent of the samples exceeded the USEPA proposed MCL, and one sample exceeded the proposed AMCL. Radon activities in Orange County were lower than those measured during similar work in nearby Guilford County, where the median activity was 735 pCi/L.


Citation:
Cunningham, W.L., and Fine, J.M., 2000, Ground-water quality in Orange County, North Carolina [abs.], in Proceedings of the Conference, North Carolina Water Resources: The Year of the Hurricanes, March 30, 2000, NCSU McKimmon Center, Raleigh, N.C.: Water Resources Research Institute of The University of North Carolina, IPS no. 14, p. 51.
For more information, contact To order printed copies
North Carolina Water Science Center
U.S. Geological Survey
3916 Sunset Ridge Road
Raleigh, North Carolina 27607
(919) 571-4000
E-mail
Available from the Water Resources Research Institute of The University of North Carolina, North Carolina State University, Raleigh, N.C., IPS no. 14, p. 51.

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