THE RELATION OF OCCURRENCE OF GEOGENIC CONTAMINANTS TO RECHARGE AGE, REDOX CONDITIONS, AND DISTANCE ALONG FLOW-PATHS IN ATLANTIC AND GULF COASTAL PLAIN AQUIFERS, EASTERN USA

Carbon-14 and ³H age tracers from 257 public supply wells were compared to geogenic contaminants of concern (COC) As, Cl, F, Fe, pH, ²¹⁰Pb, ²¹⁰Po and TDS, water quality parameters, and flow-path variables in Atlantic and Gulf Coastal Plain Aquifers, Eastern USA to understand the occurrence of exceedances of pre-assigned thresholds. One half of the MCL (maximum contaminant level) or SMCL (secondary MCL) were chosen as the sentry indicators of the potential to exceed the respective regulatory thresholds. Wells were distributed among six regional aquifer systems with varying hydrogeology, a range of confined and unconfined conditions, within outcropping and subcropping, variably lithified, texturally heterogeneous sedimentary deposits. From northeast to southwest these aquifer systems include: Potomac (PTA) (including the Magothy and Cape Fear Aquifers), Piney Point (PPA), Castle Hayne (CHA), Chattahoochee (CCA), Black Warrior (BWA) and Coastal Lowlands (CLA) Aquifers.

Distributions of groundwater age, water quality parameters and COCs varied within each, and among all, studied aquifer systems. The CLA, CHA, and PTA study areas have young and old (more or less than 50% modern carbon, respectively) water at varying depths and locations indicating multiple recharge zones. The PPA has all old water due to a continuous confining layer and lack of outcrop. The BWA and CCA have young water near the outcrop (upgradient recharge area), and old water deeper and farther along flow paths. Chloride and TDS were strongly correlated with a variety of flow position variables including distance from outcrop. Iron, and ²¹⁰Po correlated with pH and O₂ (redox indicator), but As did not, possibly because of limited availability of As in these aquifers. Arsenic concentrations in groundwater were above the sentry indicator threshold in arid regions, to the west in the CLA and in the PPA, between the upgradient pinch-out area and depths and flow distances near the middle of the aquifer.

Age-distribution modeling completed in the PTA and PPA study areas provided insight into the relationship of very old water (in 10K year bins) with COCs. For example, ²¹⁰Po, and TDS occur above the sentry indicator threshold with 21K to 31K year old water in the PPA study area. Age modeling has the potential to provide further understanding of the occurrence of COCs.