Since the 1950’s it has been known that groundwater from the Cambrian-Ordovician (C-O) aquifer system contained radium (Ra 226 and Ra 228) at concentrations that frequently exceed the US Environmental Protection Agency (USEPA) safe drinking water maximum contaminant level (MCL) of 185 mBq/L; 5 pCi/L. This aquifer provides more than 630 million gallons of water a day for public supply to parts of Illinois, Iowa, Michigan, Minnesota, Missouri, and Wisconsin. In addition, more than half a million people get their drinking water from private wells that tap the Cambrian-Ordovician aquifer.
A newly published U.S. Geological Study, part of the USGS National Water Quality Assessment Project, investigated the conditions that cause these unusually elevated levels of radium in the groundwater from the Cambrian-Ordovician aquifer. Knowing where and how much radium is in groundwater is important because of the health risks associated with drinking water that’s high in radioactive isotopes. Known health risks include an increased incidence of bone cancer and leukemia.
The USGS examined several variables like groundwater age, dissolved minerals, and dissolved oxygen levels in 80 samples collected across the six states. The researchers were able to better understand the conditions that cause radium to leach from the underlying geology into groundwater at higher levels.
They found that water that was recharged into the aquifer long ago, that contains greater amounts of dissolved minerals, and that is low in dissolved oxygen is more likely to leach radium from its surrounding rock. The estimated mean groundwater ages ranged from 19 years to more than 1 million years. Only eight samples had groundwater recharged since about 1950. The USGS found that more than 80% of the groundwater was older than 1,000 years. They also found that the proportion of the three Ra isotopes differed between the regionally unconfined and confined areas of the aquifer system.
Increased concentrations of Ra 226 was found in confined regions of the aquifer. The rate of groundwater flow in the confined area of the aquifer is very slow because relatively stagnant saline water restricts the movement of freshwater into deeper parts of the basins. Total Radium concentrations were significantly lower in HCO3 dominated, oxic samples from the unconfined area of the aquifer as compared to SO4 or Cl dominated, anoxic samples from the regionally confined areas.
Measured concentrations of all three Ra isotopes were significantly correlated with mean groundwater age reflecting the increased mobility of Ra with increasing mineralization and Fe-reducing conditions. The development of anoxic, Fe-reducing conditions and increasing water mineralization with groundwater age favors the mobilization of Ra and results in the frequent occurrence of Rac concentrations greater than 185 mBq/L (5 pCi/L) in the regionally confined area of the aquifer system. Under anoxic, Fe-reducing conditions, Fe- and Mn-hydroxides are dissolved, thereby reducing the adsorptive capacity of the aquifer solids.
No testing was done of the relatively shallower private drinking water wells. These well owners might consider having their water tested for radium as part of their regular well maintenance.
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