North-Central - 52nd Annual Meeting

Paper No. 23-6
Presentation Time: 10:00 AM

RADIUM MOBILITY AND THE AGE OF GROUNDWATER IN PUBLIC-DRINKING-WATER SUPPLIES FROM THE CAMBRIAN-ORDOVICIAN AQUIFER SYSTEM


STACKELBERG, Paul E., U.S. Geological Survey, 425 Jordan Road, Troy, NY 12180, SZABO, Zoltan, U.S. Geological Survey, New Jersey Water Science Center, 3450 Princeton Pike, Lawrenceville, NJ 08648 and JURGENS, Bryant C., U.S. Geological Survey, California Water Science Center, Placer Hall, 6000 J St., Sacramento, CA 95819

Geochemical conditions that result in elevated concentrations of 224Radium (Ra), 226Ra, and 228Ra in potable parts of the Cambrian-Ordovician (C-O) aquifer system were investigated using raw-water-quality data and environmental tracers of groundwater age from 80 public-supply wells. Mean groundwater ages were estimated by calibration of tracers to lumped parameter models and ranged from less than 20 years in upgradient, unconfined areas to more than 1 million years in downgradient, confined areas. Anoxic, iron-reducing conditions and increased mineralization develop with time and mobilize Ra into solution resulting in the frequent occurrence of 226Ra + 228Ra at concentrations exceeding the U.S. Environmental Protection Agency Maximum Contaminant Level of 5 picocuries per liter. The distribution of the three Ra isotopes differed across the aquifer system. Concentrations of 224Ra and 228Ra were strongly correlated and comprised a larger proportion of the total Ra concentration in samples from upgradient, unconfined areas, where arkosic sandstones provide an enhanced source for progeny in the 232Thorium decay series. 226Ra comprised a larger proportion of the total Ra concentration in samples from downgradient confined regions, where carbonates provide an enhanced source for progeny in the 238Uranium decay series. 226Ra distribution coefficients decreased substantially with anoxic conditions and increasing ionic strength of groundwater (mineralization), indicating that the amount of 226Ra released from solid phases and retained in solution increases relative to the amount of Ra sequestered by sorption processes or co-precipitation with barite as sorption capacity and the concentration of barium decreases. Although 226Ra occurred at concentrations greater than 224Ra or 228Ra, the ingestion exposure risk was greater for 228Ra owing to its greater toxicity. In addition, 224Ra added substantial alpha-particle radioactivity to potable samples from the C-O aquifer system. Thus, monitoring for Ra isotopes and gross-alpha-activity (GAA) is as important in upgradient, unconfined areas as downgradient. GAA measurements made within 72 h of sample collection best capture alpha-particle radiation from the short-lived 224Ra.
Handouts
  • Stackelberg_T10_NC-GSA_2018.pptx (4.1 MB)