Preliminary investigation of elevated radioactivity in groundwater in Charles County, Maryland
2015, Andreasen, D.C. and Bolton, D.W.
Open File Report 15-02-02
A review of existing data was conducted to evaluate the occurrence and distribution of gross alpha-particle activity, gross beta-particle activity, radium, and polonium in public water (groundwater) systems in Charles County, Maryland. The study was prompted by the exceedances of the U.S. Environmental Protection Agency’s Maximum Contaminant Level for gross alpha-particle activity at five public water systems.
- Water samples from wells in five public water systems—Chapel Point Woods, Mt. Carmel Woods, Clifton-on-the-Potomac, Waldorf (St. Paul’s Well 9 and St. Charles Well 16), and the Town of Indian Head—all of which are screened in the Upper and Lower Patapsco aquifer systems—exceeded the Maximum Contaminant Level (MCL) of 15 picocuries per liter for gross alpha-particle activity. A water sample from one well at the Brookwood Estates public water system equaled the MCL. The elevated radioactivity (defined as at or above the MCL of 15 picocuries per liter gross alpha-particle activity) tends to occur in the central portion of the county along a line trending approximately north-south.
- The highest gross alpha-particle activity in the Upper Patapsco aquifer system was 122 picocuries per liter at the Chapel Point Woods (main plant) system. The highest gross alpha-particle activity in the Lower Patapsco aquifer system was 53 picocuries per liter from the Waldorf (St. Charles Well 16) system.
- Radium-226, radium-228, and combined radium (Ra-226+Ra-228) concentrations are less than 1.2, 1.7, and 2.5 picocuries per liter, respectively. Combined radium (Ra-226+Ra-228) concentrations are below the Maximum Contaminant Level of 5 picocuries per liter in all six of the public water systems discussed in this report. While there are no analyses for Ra-224 (a known radionuclide in some aquifers in Anne Arundel County, Maryland), the concentrations are likely low since Ra-228 concentrations are low, indicating that another alpha-emitting radionuclide is the source of the elevated gross alpha-particle activity.
- In the Chapel Point Woods and Mt. Carmel Woods public water systems, polonium-210 appears to be the main alpha-emitting radionuclide. At the Chapel Point Woods public water system (main plant Wells 1, 2, and 3 [undifferentiated] and Jude House plant Well 2), polonium-210 concentrations ranged from 6.13 to 46.6 picocuries per liter. All Chapel Point Woods wells are screened in the Upper Patapsco aquifer system. At the Mt. Carmel Woods public water system (Well 1A [Lower Patapsco aquifer system] and Well 3 [Upper Patapsco aquifer system]), polonium-210 concentrations ranged from 2.8 to 26.9 picocuries per liter. The presence of polonium-210 in these systems suggests that polonium may also be the source of elevated gross alpha-particle activity in the other public water systems.
- Elevated radioactivity in the six public water systems occur in aquifer sand layers spanning approximately 900 feet of sediment thickness, which includes both the Upper and Lower Patapsco aquifer systems. Individual sand layers in the Upper and Lower Patapsco aquifer systems are often localized and typically do not extend beyond short distances (less than 1,000 feet) before pinching out. Additionally, since water samples from the public water systems discussed in this report are a composite from multiple well screens tapping discrete sand layers, the existing data do not indicate in which specific stratigraphic horizon the source of the radioactivity is located. Concentrations in groundwater also may vary with time based on pumping intensity and duration.
- There are approximately 57 wells screened in the Upper Patapsco aquifer system and 58 wells screened in the Lower Patapsco aquifer system, within a one-mile radius surrounding the six public water systems identified in this report with elevated radioactivity. Of those wells, 93 are classified as domestic, 14 are public supply wells (from 10 public water systems), five are irrigation, two are test wells, and one is farm supply. Gross alpha-particle activity and combined radium (Ra-226+Ra-228) concentrations in the 14 public supply wells were all below Maximum Contaminant Levels.
- Testing of water wells, in particular domestic wells, for gross alpha- and gross beta-particle activity is recommended to gain better insight into the extent of the radioactivity. Since domestic wells commonly have relatively short, fixed screen lengths they can provide greater definition of radioactivity in specific stratigraphic horizons.