Radium Mobility and the Age of Groundwater in Drinking-Water Supplies from the Cambrian-Ordovician Aquifer System

Monday, December 4, 2017: 10:30 a.m.
102 B (Music City Center)
Paul Stackelberg , U.S. Geological Survey, New York Water Science Center, Troy, NY
Zoltan Szabo , U.S. Geological Survey, New Jersey Water Science Center, Lawrenceville, NJ
Bryant C. Jurgens , U.S. Geological Survey, California Water Science Center, Sacramento, CA

Groundwater ages which ranged from modern (< 50 yrs) to ancient (> 1 Myrs) were combined with raw water-quality data from 80 public-supply wells to evaluate the evolution of geochemical conditions that mobilize high concentrations of radium (Ra) in potable portions of the Cambrian-Ordovician (C-O) aquifer system. Findings indicate that it is the combination of anoxic, Fe-reducing conditions and increasing mineralization that favor the mobilization of Ra coupled with the length of time water is in the aquifer system that results in the frequent occurrence of combined Ra (226Ra + 228Ra) at concentrations exceeding the USEPA MCL of 5 pCi/L. Strongly correlated concentrations of 224Ra and 228Ra comprised a larger proportion of the total Ra (Rat = 224Ra + 226Ra + 228Ra) concentration in samples from upgradient recharge areas where arkosic sandstones are common, whereas 226Ra comprised the larger proportion in samples from downgradient confined regions. 226Ra distribution coefficients decreased substantially with anoxic conditions and increasing ionic strength (mineralization) indicating Ra is mobilized to solution from solid phases of the aquifer as sorption capacity is diminished. The rate of release of 226Ra from solid phases by alpha-recoil mechanisms exceeds the rate of Ra sequestration by adsorption processes or co-precipitation with barite. 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 gross alpha-particle radioactivity (GAA) to potable samples from the C-O aquifer system. Thus, monitoring for Ra radionuclides and GAA is equally critical in the recharge area as downgradient, with GAA measurements being completed within 72 h of sample collection to capture alpha-particle radiation from the short-lived 224Ra.

Paul Stackelberg, U.S. Geological Survey, New York Water Science Center, Troy, NY
Paul Stackelberg has worked as a hydrologist with the U.S. Geological Survey since 1988. His research interests have included (1) evaluating natural and anthropogenic factors that affect groundwater quality, (2) developing statistical models for predicting the occurrence of pesticides and other contaminants in groundwater resources, and (3) determining the persistence and fate of pharmaceuticals and other wastewater-related compounds in conventional and advanced drinking-water-treatment facilities. Paul has worked on the National Water Quality Assessment Project since 1994 and is currently coordinating the project’s modeling and mapping team.

Zoltan Szabo, U.S. Geological Survey, New Jersey Water Science Center, Lawrenceville, NJ
Zoltan Szabo, research hydrologist, U.S. Geological Survey (22 years). MSc, Geochemistry, Ohio State University. Current research: Occurrence and mobility of radionuclides, especially radium; of trace elements, especially arsenic and mercury; and wastewater and pharmaceutical compounds.

Bryant C. Jurgens, U.S. Geological Survey, California Water Science Center, Sacramento, CA
U.S. Geological Survey, Hydrologist

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