Geologic Characterization to Reduce Natural Radioactivity in a Groundwater Supply
Summary: Identified natural sources of radioactivity in a condominium water supply system in northern New Jersey, developed recommendations for obtaining an alternate or modified supply with lower levels of radioactivity.
Princeton Geoscience evaluated the source of radioactivity in the water supply of condominium development in northwestern New Jersey and recommended alternatives for mitigation of the problem. Groundwater withdrawn from the water supply system exhibited radium-(226+228) and gross alpha activity in excess of the federal Maximum Contaminant Limit (MCL) and NJDEP required correction of the violation. The source evaluation consisted of reviewing published geologic maps of the area, determining well construction and pumping schedule, performing a site inspection, and consulting with the New Jersey Geological Survey.
We determined that several geologic conditions may contribute radioactivity to groundwater, including radioactive minerals associated with certain bedrock units at the site and faults and other rock discontinuities where precipitation and rock-mass volume reduction may cause concentration radioactive minerals. We recommended three options for providing an alternate supply, including geophysical assessment and alteration of the existing well, installation of a new bedrock well in a deeper geologic formation exhibiting lower radioactivity, and installation of a shallow, high-yield well in unconsolidated glacial deposits.
The site is located in a geologically complex area of the State, in which naturally elevated background radioactivity had been documented previously. Because a natural source for the observed groundwater radioactivity was suspected, investigative activities focused on the identification of site-specific conditions contributing to radioactivity in the ground-water supply. The client indicated a desire to avoid the need for disposal of contaminated water treatment wastes (e.g., filtration media), so mitigation strategies were aimed at development of a modified or alternate supply that would comply with MCLs without treatment for removal of radioactivity.
Specific activities completed by Princeton Geoscience included reviewing published maps of bedrock and surface geology, determining well construction and pumping schedule, inspection of geologic conditions at and adjacent to the property and consultation with the New Jersey Geological Survey. Based on these activities, We determined that several geologic conditions may contribute radioactivity to groundwater. These include radioactive minerals associated with hornblende granite and (to a much lesser extent) microcline gneiss bedrock units at the site, and faults identified within the radioactive mineral-bearing rock masses. The faults and other bedrock fractures can function as sources, because radioactive minerals dissolved from the adjacent rock can co-precipitate with iron minerals along the fracture surfaces or may be concentrated due to volume reduction along shear zones. Other geologic units in the area, including the Franklin Marble and surface sand, silt and gravel of glacial origin, do not contain significant radioactive mineral assemblages and do not contribute significantly to the observed groundwater radioactivity.
Based on these findings, Princeton Geoscience identified three alternatives for modifications to the water supply system, including:
- Alternative A - Modifying the construction of the existing well, to allow water entry only from low-radioactivity fracture zones (if such zones exist), following geophysical logging of the well
- Alternative B - Installing a new well in the bedrock at an alternate location on the property (or to greater depth), where zones of radioactive ground-water are less likely to be encountered, if Alternative A cannot be implemented
- Alternative C - Installing a new well in the glacial deposits on the property, if Alternatives A and B cannot be implemented
Alternative C was considered least favorable because it was unclear whether a glacial aquifer of sufficient saturated thickness to provide a yield adequate for the needs of the complex was present, and the only feasible locations for a well in the glacial deposits were distant from the condominium buildings and water distribution system. Alternative B was recommended only as a contingency should Alternative A prove infeasible, due to the greater cost and complexity associated with Alternative B.
For the geophysical logging under Alternative A, we noted that the most useful logs would likely be caliper, neutron density and temperature (for identification of water-bearing fracture zones) and natural gamma (for determining gross lithology changes (e.g., marble vs. gneiss) and identifying highly radioactive zones). An oriented borehole televiewer or video log would also be useful in estimating fracture orientation.
We noted that if significant fractured intervals with low radioactivity are found, the existing well could be modified by grouting off the zones which contribute radioactive water to the well (fractured intervals with high gamma response).