Reports
Formation geochemistry at two boreholes and its bearing on radium content of ground water, Anne Arundel County, Maryland
2003, Duigon, M.T. and Bolton, D.W.
Open File Report 2003-02-15
Abstract
Two boreholes were drilled to sample aquifer materials in order to gain insight into the causes of high radium concentrations in ground water at some locations in the coastal-plain aquifers of Anne Arundel County, Maryland. The first borehole sampled the Magothy and upper Patapsco Formations (in which relatively high radium concentrations—up to 66 picoCuries per liter of 226Ra plus 228Ra—have been found in ground-water samples from some locations). The second borehole sampled the Aquia Formation (in which high radium concentrations have not been found in ground-water samples, although relatively high radon concentrations—median 328 picoCuries per liter compared to median concentration of 180 picoCuries per liter in ground water from the Magothy and Patapsco Formations—have been found).
Based on the results of this study, the following mechanisms are proposed to explain the difference in radium and radon concentrations of the Magothy and Patapsco Formations and the Aquia Formation:
- Detrital zircon from Piedmont source rocks is the main source of the radium progenitor nuclides thorium and uranium
- Uranium and thorium may also be incorporated in the mineral lattice of authigenic glauconite in the Aquia Formation
- Thorium that entered Magothy and Patapsco Formations ground water has been advectively transported owing to low pH of the aqueous environment
- Thorium that entered Aquia Formation ground water has precipitated onto formation sediment grains
- Radium produced in the Magothy and Patapsco Formations by dissolved thorium or ejected by alpha recoil tends to remain dissolved
- Radium produced in the Aquia Formation tends to be removed from solution by cation exchange
- Radium trapped in grain coatings or held at exchange sites in the Aquia Formation is a source for higher radon concentration compared with radon concentration in the Magothy and Patapsco Formations
Evidence in support of these mechanisms includes the presence of zirconium, uranium, thorium, and radium in the formation materials. The lack of correlation between thorium and zirconium concentrations in Aquia sediments, in contrast to the positive correlation in Magothy-Patapsco sediments, is evidence for the presence of thorium in grain coatings and in glauconite as well as being present in zircons. Higher values of effective cation-exchange capacity were measured in samples of the Aquia Formation (median 12.15 milliequivalents per 100 grams) than in samples of the Magothy and Patapsco Formations (median 2.55 milliequivalents per 100 grams and 1.85 milliequivalents per 100 grams, respectively).
