|Hydrogeology & Hydrology Program|
Radium in Ground Water
|contact: David Bolton (firstname.lastname@example.org)|
This study was conducted to provide information on the occurrence and distribution of radium-226, radium-228, and short-term and long-term gross alpha-particle and gross beta-particle activity in the Magothy Formation and Potomac Group aquifers in Anne Arundel County, and to identify other areas in the Coastal Plain of the upper Chesapeake Bay area of Maryland where high radium concentrations may exist in these aquifers. Water samples collected from 203 wells in the study area were analyzed for short-term (measured within 72 hours of sample collection) and long-term (measured 30 days after sample collection) gross alpha-particle and gross beta-particle activity, pH, specific conductance, and dissolved oxygen. Samples from Anne Arundel County were also analyzed for radium-226 and radium-228; additionally, 20 wells were analyzed for major ions.
In Anne Arundel County, radionuclide concentrations in ground water in the Magothy Formation were highest in a three- to four-mile-wide zone extending from the Mountain Road peninsula southwest to Crofton, in or near the outcrop area of the Magothy Formation. All wellwater samples from the Magothy Formation in this zone exceeded the USEPA's MCL of 5 pCi/L for radium-226 plus radium-228. A fairly distinct boundary exists between wells with radium-226 plus radium-228 concentrations greater than 5 pCi/L and those with less than this amount. In the Patapsco Formation, radionuclide concentrations exceeding 5 pCi/L for radium-226 plus radium-228 and 15 pCi/L for short-term GAPA (the MCL) were from wells located in or near the outcrop area, although not all samples from this area exceeded these concentrations. A similar pattern was seen in samples from the Patuxent Formation.
Outside of Anne Arundel County, short-term GAPA and GBPA in the Magothy and Potomac Group aquifers were generally very low. None of the regional wells in the Magothy Formation had short-term GAPA values exceeding 15 pCi/L, although detectable levels (greater than 3 pCi/L) were reported throughout the study area. Short-term GAPA exceeded 15 pCi/L in samples from six regional wells completed in the Patapsco Formation (two wells in Prince George's County and one well each in Baltimore, Harford, Cecil, and Kent Counties). The highest short-term GAPA outside of Anne Arundel County (140 pCi/L) was from a Cecil County well in an area where dissolved solids was considerably higher and pH was lower than ambient ground-water-quality conditions (short-term GAPA from a nearby ambient well was below MCLs). Well KE Ac 20 in Kent County, completed in a deep brackish-water zone of the Patapsco Formation, also had high short-term GAPA, but this well had a near-neutral pH. One of the two wells in the Patuxent Formation exceeded 15 pCi/L for short-term GAPA. The high values of GAPA and GBPA seen in these wells indicates that the potential for high radium concentrations exists throughout the Potomac Group and Magothy Formation aquifers in Maryland.
There was a large percentage decrease in GAPA between the short-term and long-term measurements: the long-term GAPA was between 10 and 40 percent of the short-term GAPA for most samples. This is similar to differences between short-term and long-term GAPA from several samples in the Anne Arundel County pilot study in which radium-224 was detected (Bolton and Hayes, 1999). This suggests that radium-224 comprises a significant percentage of total radium in the Magothy, Patapsco, and Patuxent Formations in the upper Chesapeake Bay area. There was also a significant percentage decrease between the short-term and long-term measurements of GBPA. Individual short-lived beta-emitting isotopes responsible for the decrease were not identified in this study.
Radium, GAPA, and GBPA were negatively correlated with pH. Short-term GAPA exceeded 15 pCi/L in 61 percent of samples with pH values less than 4.5; only about 10 percent of samples with pH values above 4.5 had short-term GAPA above 15 pCi/L. Water samples from many Anne Arundel County wells had pH values less than 4.0, especially those on the Mountain Road peninsula. Potential sources of the acidity are pyrite oxidation and decomposition of organic material. Most samples with specific conductance values greater than 300 ÁS/cm had short-term GAPA above 15 pCi/L; most samples with specific conductance values less than about 50 ÁS/cm had short-term GAPA less than 15 pCi/L.
Radium and short-term and long-term GAPA were positively correlated with specific conductance, TDS, sodium, chloride, magnesium, ammonium, nitrate, and potassium; the strongest correlations were with sodium and chloride concentrations. Radium-226 plus radium-228 concentrations exceeded 5 pCi/L in all samples where sodium concentrations and chloride concentrations were greater than about 10 and 15 mg/L, respectively. For samples with sodium and chloride concentrations greater than these amounts, the sodium:chloride molar ratios were almost exactly 1:1, suggesting that the source has a sodium:chloride ratio of 1:1 as well. An 18-well transect across the Mountain Road peninsula showed that the distribution of radium concentrations corresponded very closely to that of chloride concentrations. Potential sodium chloride sources in the study area include de-icing salts applied to roads and driveways, brine water used to backflush water-softening systems, brackish-water intrusion, and septic-system effluent. None of the Magothy or Patapsco wells in this study appear to have been affected by brackish-water intrusion; however, the high dissolved-solids content of brackish water may be expected to mobilize radium in these aquifers. High chloride concentrations (greater than 80 mg/L) in two wells completed in the Aquia Formation were not associated with high levels of short-term GAPA. The relations between radium concentrations and pH, specific conductance, and major-ion concentrations in the Magothy Formation and Potomac Group aquifers strongly suggest that sorption/desorption processes play a major role in the occurrence of radium in ground water in the area.
Radionuclide concentrations were negatively correlated with depth to the screened interval. Wells with screened intervals less than 100 ft deep had a wide range of radionuclide values, while radionuclides in wells with screened intervals greater than 300 ft deep were almost always below MCLs. For groups of wells adjacent or near one another that had different screen depths, radionuclide concentrations tended to decrease with depth.
Radionuclide concentrations were highest in wells in the updip areas of the aquifers where the wells are shallower, the pH tends to be lower, and specific conductance is often higher than in downdip wells. In the Magothy Formation, the downdip limit of groundwater that exceeds radionuclide MCLs corresponds to an area of decreasing specific conductance, which may correspond to the limit of downgradient migration of human-sourced water-quality constituents. Specific conductance decreased to less than 100 ÁS/cm at this point and then increased downgradient, possibly due to naturally-occurring weathering reactions in the aquifer. pH levels tended to increase from the outcrop area to the deeper, confined parts of the Magothy Formation.