GROUNDWATER ARSENIC SUSCEPTIBILITY MAP OF ORANGE COUNTY: COUPLING DETAILED GEOLOGIC DATA AND GROUNDWATER QUALITY DATA TO MODEL ARSENIC SUSCEPTIBILITY

The relationships between naturally occurring groundwater arsenic and geology in Orange County, North Carolina, are complex and are complicated by various factors including well depth and construction, site specific geochemistry (DO, pH, Fe, Mn, SO4, etc.), and other host rock properties. Dissolved groundwater arsenic data overlain on detailed geologic polygons appear to indicate correlation between specific geologic units. Geospatial statistical analyses and tools were used within ESRI ArcMap to examine if the apparent visual correlation could be shown quantitatively.

The groundwater arsenic database (geolocated to the tax parcel centroid) was provided by the Children’s Environmental Health Initiative at Duke University and consisted of 2,125 records. Several records were discarded and included 232 records with elevated detection limits and data from wells that were sampled multiple times. Only the maximum recorded value was kept from wells with multiple analyses. The final dataset was reduced to 1,347 records.

Using ESRI ArcMap, distance tables were generated that tabulated each analysis location from each major geologic unit (Near tool) and a Spatial Join was conducted that combined the geologic unit to the arsenic record. This data was used to develop a simplistic model where the distance between detectable groundwater arsenic data points and major geologic units was examined. Simple linear regression analyses did not produce statistically meaningful results probably indicating non-linear relationships; however, plots of arsenic concentrations and distance from specific geologic units appear to indicate correlations. Data plots of arsenic concentration with respect to certain geologic units generally show higher and more numerous detects of arsenic and as distance increase from these units the concentration and number of wells with detectable arsenic decrease. A simplistic groundwater arsenic susceptibility map was prepared based on this data.