Paper No. 8
Presentation Time: 3:30 PM

THE LITHOTECTONIC MAP OF THE SOUTHERN APPALACHIANS TO DETAILED 1:24,000 SCALE GEOLOGIC MAPS: TOOLS TO UNDERSTANDING THE OCCURRENCE AND DISTRIBUTION OF NATURALLY-OCCURRING GROUNDWATER CONTAMINANTS IN THE PIEDMONT AND MOUNTAINS OF NORTH CAROLINA


BRADLEY, Philip J., North Carolina Geological Survey, Raleigh, NC 27699-1620, ABRAHAM, Joju, North Carolina Division of Water Quality, 610 E Center Ave, Mooresville, NC 28115 and CAMPBELL, Ted, Division of Water Quality, North Carolina Dept. of Environment and Natural Resources, Swannanoa, NC 28778, pbradley@ncdenr.gov

Published lithotectonic maps of the southern Appalachian orogen separate North Carolina into distinct packages of rocks that broadly share common tectonic origins and generally includes: 1)accreted volcanic arc related rocks spatially associated with the Gondwanan continent; 2) accreted volcanic arc related rocks and clastic wedges spatially associated with the Laurentian continent; 3) rocks spatially associated with Laurentian margin; and 4) clastic and magmatic rocks associated with Mesozoic basins. Together these rocks tell the story of the opening and closing and re-opening of an ocean basin(s). These lithotectonic maps can be used as first order guides to understanding the occurrence and distribution of naturally occurring groundwater contaminants of arsenic (As) and radon (Rn) within the Piedmont and Mountains of North Carolina.

Naturally occurring As in groundwater is present throughout the volcanic island arc related rocks of the Carolina terrane ranging in concentrations from 1 to 800 ppb. However, the detection of As is more pervasive in concentrations above the EPA MCL of 10 ppb in the Albemarle Group associated with the Carolina terrane. On a finer-scale, the Mudstone-member of the Cid Formation is the primary As bearing lithology. Wells hosted in other formations in the Albemarle Group show elevated levels of As to a lesser extent. Conversely, the older sequences of rocks associated with the Carolina terrane (Hyco and Aaron Formations) display sparse detects above the EPA MCL.

Radon is present throughout the state, but is generally spatially associated with felsic plutonic rocks or their metamorphic equivalent. Plutonic rocks in NC can be generally separated into over 10 distinct magmatic events. Some magmatic events are unique to different terranes while some magmatism is shared throughout the State. We are currently attempting to determine if elevated Rn is more associated with distinct magmatic events. Preliminary statistical analyses have proven complicated due to the accuracy of maps and the nature of the non-random data set.

Detailed geologic mapping has helped refine areas of potential As and Rn detections throughout the State. Targeted mapping in more areas would further refine the current relationship noted between lithotectonic units and naturally-occurring groundwater contaminants.