|Hydrogeology & Hydrology Program|
Radium in Ground Water
|contact: David Bolton (firstname.lastname@example.org)|
A study was conducted of the occurrence and distribution of radium, gross alpha-particle activity, and gross beta-particle activity in ground water in the Magothy Formation and Potomac Group aquifers in the upper Chesapeake Bay area of Maryland. A total of 203 wells were sampled in Prince George's, Anne Arundel, Baltimore, Harford, Cecil, Kent, and Queen Anne's Counties from June through November 1998. This study was a follow-up to a 1997 pilot study of carcinogens in well water in Anne Arundel County, in which radium-226 plus radium-228 concentrations exceeded the U.S. Environmental Protection Agency's Maximum Contaminant Level of 5 picocuries per liter in 15 of the 20 wells sampled in the Magothy and Patapsco Formations. Analysis of the data from this study and the pilot study has resulted in these key findings:
Radium-226 plus radium-228 concentrations exceeded 5 picocuries per liter in 60 of 122 samples (49 percent) from wells in the Magothy, Patapsco, and Patuxent Formations in central and northern Anne Arundel County. Radium-226 plus radium-228 concentrations ranged from less than 1.1 to 66 picocuries per liter. In 55 of 113 samples (49 percent) from wells in this area, short-term (measured within 72 hours of sampling) gross alpha-particle activity exceeded the Maximum Contaminant Level of 15 picocuries per liter. Short-term gross alpha-particle activity ranged from less than 3 to 919 picocuries per liter. Outside of Anne Arundel County, gross alpha-particle activity exceeded 15 picocuries per liter in only 6 of 96 wells (6 percent), including two wells in Prince George's County and one well each in Baltimore, Harford, Cecil, and Kent Counties.
Ground-water radium, gross alpha-particle activity, and gross beta-particle activity were highest in the outcrop and updip areas of the Magothy Formation and the Potomac Group, where the aquifers are generally unconfined; radionuclide concentrations tended to be lower in the deeper, confined parts of these aquifers.
Radium-226, radium-228, gross alpha-particle activity, and gross beta-particle activity tended to increase with decreasing pH. Radium-226 plus radium-228 exceeded 5 picocuries per liter in 65 percent of samples (46 of 71) having pH less than 4.5, while only about 6 percent of samples (3 of 51) with pH 4.5 or more exceeded this concentration.
Radium-226, radium-228, gross alpha-particle activity, and gross beta-particle activity tended to increase with increasing total-dissolved-solids content (as indicated by specific conductance), and with increasing sodium and chloride concentrations in particular. All samples having more than about 10 milligrams per liter sodium and about 15 milligrams per liter chloride had radium-226 plus radium-228 concentrations greater than 5 picocuries per liter.
Radionuclide concentrations tended to decrease with increasing depth to the screened interval. Water from wells with screened intervals greater than 300 feet deep were almost always less than the Maximum Contaminant Levels for gross alpha-particle activity and radium-226 plus radium-228. However, wells that are relatively close to one another may have very different radium and gross alpha-particle activity concentrations, even when screened at the same depths but especially when screened at different depths.
Gross alpha-particle activities measured 30 days after sampling were usually 60 to 90 percent lower than the gross alpha-particle activities in the same samples measured within 72 hours of sampling. This large decrease, along with radium-224 analyses from the pilot study, suggests that radium-224 (which has a half life of 3.64 days) comprises a significant part of the total radium content in ground water in the Magothy, Patapsco, and Patuxent Formations in the upper Chesapeake Bay region.