Overview
The North Carolina Geological Survey (NCGS) is providing technical assistance to the DENR groundwater section to ascertain the source(s) of arsenic that occur in domestic water supply wells in selected areas of the Piedmont.
Initial results of investigations were reported to a working group convened at the Mooresville Regional Office in February, 2002.
These pages resulted from a follow-up investigation of available drill hole core in the NCGS core repository. The objective was to determine the whether arsenic-bearing minerals are present in cored intervals and to make general observations about the groundwater levels based on features observed in drill core.
A follow-up stepwise program has been proposed. If funded, it will provide specific information about rock chemistry and mineralogy relevant to the arsenic in groundwater concerns.
Drill core examination
The following two maps show the drill hole core examined from correlative metavolcanic units to the Lake Tillery area and to the stratigraphically underlying metarhyolite. The drill hole cores were from previous gold exploration. Click here for the first map that shows drill hole core from the strike equivalents of the metavolcanic rocks in which the wells of the Lake Tillery area of arsenic concern are located. These drill holes are located some miles to the northwest as there are no other cores in this metasedimentary interval in the NCGS repository. Click here to view the map of the metavolcanic (predominantly very siliceous, rhyolitic rocks that core the antiform across Lake Tillery.
These rock contain small amounts of gold. Pyrite is ubiquitous. Arsenopyrite, a chief host of arsenic, was reported in available drill logs. The depth of groundwater movement can be approximated by the depth of altered rock and iron oxide forming from the primary minerals, and from iron oxide coatings on fractures.
The following table summarizes the drill core examined. More detailed comments are found in the pages that contain the photographs of the drill cores.
| NCGS repository drill hole number | Drill drill hole number | Rock type | Area (north or south) and county | Comments |
| DV-C-1-76 | DV-C-1-76 | Metavolcanic | north (Davidson) | |
| DV-C-2-76 | DV-C-2-76 | Metavolcanic | north (Davidson) | |
| DV-C-2-XX | DV-C-2-XX | Metavolcanic | north (Davidson) | |
| MN-C-01-89 | UW-89-1 | Metarhyolite | south (Montgomery) | |
| MN-C-02-89 | UW-89-2 | Metarhyolite | south (Montgomery) | |
| MN-C-03-89 | UW-89-3 | Metarhyolite | south (Montgomery) |
NURE Geochemistry - groundwater pH
Also the master's thesis, "Geology of the northern half of the Morrow Mountain Quadrangle, North Carolina: A revision of the Albemarle Group," by Sonja L. Ingram, 1999, contains a hand colored geologic map. This map was scanned in its entirety. A copy of this raster map image and thesis is available upon request from Reid (see below for contact information).
The National Uranium Resource Evaluation (NURE) program obtained pH for water sampled in conjunction with stream sediment samples and from domestic groundwater wells for the Charlotte 1 degree x 2 degree geologic map sheet. The scanned Charlotte 1 degree x 2 degree geologic map sheet underlies the two, non-three dimensional maps, that follow. You may have to scroll to see the entire maps.
Water associated with stream sediments shows a pronounced pH low (blue colors denote lower pH) that follows the geologic contact between the stratigraphically underlying metarhyolites and the stratigraphically higher metavolcanics. This trend is shown plotted in three dimensions to highlight the very low pH groundwater values associated with the pyrite along this contact.
The groundwater pH trend is more subdued possibly because of interaction with the country rock over longer residence time.
Project update as of March 2004
Click here for a link to current project images.
For further information
For further information, contact Jeff Reid (jeff.reid@ncmail.net) or Rick Wooten (Rick.Wooten@ncmail.net). The general NCGS telephone number for both is 919.733.2423.
