Overview
Dinwiddie and Liu investigated geologic controls on arsenic in private-well groundwater in Orange County, North Carolina. This is upstream groundwater and source-attribution evidence: it does not measure bottled or mineral-water products, but it links private-well arsenic detections to bedrock units, pH, alkalinity, fluoride, and likely oxidation of arsenic-bearing sulfide minerals.
Key numbers
The study used an Orange County dataset containing 1,335 arsenic concentration analyses of private wells geolocated to tax-parcel centroids. A second dataset contained 769 Orange County samples with location, arsenic concentrations, and additional water-quality variables after matching to the statewide well database. The paper states that arsenic concentrations in the groundwater datasets were in mg/L, values below the 0.001 mg/L detection limit were marked <0.001, and the authors converted concentrations to ug/L (ppb) for examination.
The introduction states that the World Health Organization adopted a drinking-water arsenic guideline of 10 ug/L in 1993 and that the United States EPA drinking-water standard is 10 ug/L. The authors note that the EPA standard is not enforced for private wells in the United States.
The results text reports that most wells had arsenic concentrations below detection limit (<1 ug/L), and that most wells with detectable arsenic resided in Neoproterozoic epiclastics or in other Carolina Terrane geologic units. The discussion states that with over 300 well samples in the Neoproterozoic epiclastics unit, the unit’s average groundwater arsenic concentration was 1.25 ug/L.
Table 1 reports whole-rock arsenic (As) and Fe concentrations from 26 collected bedrock samples. Average As and Fe concentrations were: felsic lavas and tuffs, n=8, As 2.8 ppm and Fe 15,963 ppm; felsic plutonic, n=2, As 4.0 ppm and Fe 16,090 ppm; intermediate/mafic plutonic, n=3, As 3.5 ppm and Fe 48,797 ppm; mafic lavas and tuffs, n=3, As 4.0 ppm and Fe 62,743 ppm; Neoproterozoic epiclastics, n=5, As 3.3 ppm and Fe 19,005 ppm; and Triassic sedimentary, n=5, As 1.8 ppm and Fe 18,576 ppm.
Table 2 reports supplementary NCGS whole-rock analyses. Average As and Fe concentrations were: felsic lavas and tuffs, n=15, As 6.7 ppm and Fe 31,780 ppm; felsic plutonic, n=13, As 1.2 ppm and Fe 33,942 ppm; intermediate/mafic plutonic, n=6, As 0.17 ppm and Fe 76,200 ppm; mafic lavas and tuffs, n=13, As 11.9 ppm and Fe 76,000 ppm; and Neoproterozoic epiclastics, n=9, As 6.2 ppm and Fe 41,925 ppm.
The pluton-proximity analysis compared wells located within 500 m of pluton boundaries with wells more than 500 m away. The authors report that wells more than 500 m from pluton boundaries had a statistically greater mean arsenic concentration at the 95% confidence interval; the extracted text does not report the two group means.
The multivariate analysis began with 21 variables from 769 wells and reduced the associated group to six variables: Mg2+ (mg/L), Ca2+ (mg/L), hardness, F- (mg/L), alkalinity, and pH. Q-mode PCA found that the first two principal components explained approximately 64.8% of the variance. Mann-Whitney-Wilcoxon tests showed significant differences at the 95% confidence interval between detected-arsenic and non-detected-arsenic wells for the paired variables the authors tested.
Methods (brief)
The authors combined North Carolina Department of Health and Human Services private-well arsenic data, statewide water-quality records, and a detailed Orange County bedrock map. They used simple kriging to map predicted arsenic concentrations, spatial analysis to compare wells within or beyond 500 m of pluton contacts, hierarchical clustering and Q-mode PCA to evaluate water-chemistry associations, and Mann-Whitney-Wilcoxon tests for non-normal variables. Bedrock samples were cleaned, crushed, powdered, acid-digested, and analyzed by ICP-MS; the reported arsenic is total elemental As, not a separated inorganic-arsenic species.
Related evidence
- Irrigation water and soil amendments as anthropogenic inputs to cropland
- Source attribution and environmental burden apportionment
- Arsenic, Total
- EPA — Maximum Contaminant Level for Arsenic in Drinking Water
Update history
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