Investigation of Ground-Water Availability and Quality in Orange County, North Carolina

A countywide inventory was conducted of 649 wells in nine hydrogeologic units in Orange County, North Carolina. As a result of this inventory, estimates of ground-water availability and use were calculated, and water-quality results were obtained from 51 wells sampled throughout the County from December 1998 through January 1999. The typical well in Orange County has an average depth of 208 feet, an average casing length of 53.6 feet, a static water level of 26.6 feet, a yield of 17.6 gallons per minute, and a well casing diameter of 6.25 inches. The saturated thickness of the regolith averages 27.0 feet and the yield per foot of total well depth averages 0.119 gallon per minute per foot. Two areas of the County are more favorable for high-yield wells—a west-southwest to east-northeast trending area in the northwestern part of the County, and a southwest to northeast trending area in the southwestern part of the County. Well yields in Orange County show little correlation with topographic or hydrogeologic setting.

Fifty-one sampling locations were selected based on (a) countywide areal distribution, (b) weighted distribution among hydrogeologic units, and (c) permission from homeowners. The list of analytes for the sampling program consisted of common anions and cations, metals and trace elements, nutrients, organic compounds, and radon. Samples were screened for the presence of fuel compounds and pesticides by using immuno-assay techniques. Dissolved oxygen, pH, temperature, specific conductance, and alkalinity were measured in the field. The median pH was 6.9, which is nearly neutral, and the median hardness was 75 milligrams per liter calcium carbonate. The median dissolved solids concentration was 125 milligrams per liter, and the median specific conductance was 175 microsiemens per centimeter at 25 degrees Celsius. Orange County ground water is classified as a calcium-bicarbonate type.

High nutrient concentrations were not found in samples collected for this study. Nitrate was detected in 82 percent of the samples at concentrations ranging up to 7.2 milligrams per liter, although the median concentration was 0.49 milligram per liter; all other samples had a concentration of 2.9 milligrams per liter or less. In general, trace elements were detected infrequently or at concentrations less than State drinking-water standards. However, exceedances of North Carolina drinking-water standards were observed for iron (3 exceedances of 51 analyses, detection up to 1,100 micrograms per liter), manganese (12 exceedances of 51 analyses, detection up to 890 micrograms per liter), and zinc (4 exceedances of 31 analyses, detection up to 4,900 micrograms per liter). Lead was detected in 8 of 31 samples with a concentration up to 3.5 micrograms per liter. Zinc, manganese, iron, and copper were the most frequently detected trace metals at 100, 94, 80, and 61 percent, respectively. Lead, arsenic, bromide, alum inum, and selenium were detected in 13 to 26 percent of the analyses. No benzene, toluene, ethylbenzene, and xylene (BTEX) or atrazine compounds were detected in any of the samples.

Radon activities in ground water can be high because of the rock units present in Orange County. Radon activity ranged from 38 to 4,462 picocuries per liter countywide, with a median activity of 405 picocuries per liter. Median radon activities in Orange County were highest in felsic rocks (487 picocuries per liter) and lowest in mafic rocks (357 picocuries per liter). When evaluated by individual hydrogeologic units, the median radon activity was highest in the phyllite unit (1,080 picocuries per liter in 2 samples) and the felsic metaigneous unit (571 picocuries per liter in 13 samples).

Overall, water-quality data in Orange County indicate few drinking-water concerns. No organic contaminants analyzed (total BTEX and atrazine) or excessive nutrient concentrations were detected, and few exceedances of North Carolina drinking- water standards were detected.