Radiochemistry of Uranium-Series Isotopes in Groundwater
Chemical analysis of water from interaquifer connector wells (“recharge wells”) and associated monitor wells in the Central Florida Phosphate District has provided a basis for understanding the chemistry of the Surficial and upper Floridan aquifers and of recharge wells. The data are particularly important because they suggest chemical controls on the mobility of uranium and uranium progeny. Time-series and nearest-neighbor analyses of historical data suggest hydrologic conditions that cause mobilization of iron, sulfur, and radionuclides in ground water and provide a basis for recharge-well location and management to minimize risks of failure of water-quality criteria for gross-alpha radioactivity, iron, and color.
Specific findings can be summarized as follows:
1. Twenty three recharge wells were sampled. A total of 51 water samples from recharge wells were analyzed for 31 chemical variables, including uranium-238, uranium-234, radium-226, lead-210, and polonium-210. Three recharge wells had associated Surficial and Floridan aquifer monitor wells. Eighteen samples were analyzed from the monitor wells.
2. Water of the Surficial aquifer is acidic, reducing, and organic rich. Eh-pH data show that the aquifer water is near the hydrogen sulfide-sulfate stability boundary, and sulfides are the major sulfur species in the water. Ferrous iron is the stable iron species. Polonium and, to some degree, radium may be present in activities sufficient to be of concern.
3. Floridan aquifer water is neutral to slightly alkaline and slightly reducing. Sulfate is the dominant sulfur species, and ferrous iron is stable. Activities of radionuclides are low in the Floridan aquifer where sampled, although radium has been shown to be a problem in other studies.
4. Surficial aquifer water is highly variable and ranges from a sodium-sulfate to sodium-calcium-bicarbonate- sulfate compositions. Upper Floridan aquifer water compositions do not vary significantly and the water is a calcium-magnesium-bicarbonate solution.
5. In recharge wells, sulfur is oxidized to sulfate and iron may be oxidized to the ferric state. Ferric hydroxides precipitate if the system is oxidizing, but most of the ferric hydroxide precipitate appears to form on well casings and screens, not in the turbulent flow of the well water. In general, recharge-well water closely reflects the chemistry of Surficial-aquifer water, from which it is derived.
6. Sampling trials indicate that much of the ferric hydroxide and biomat that causes color and turbidity violations in water quality is adherent on well casings. Sampling with a tripod to prevent contact of the sampling vessel with the walls of the well avoids contamination of the sample with color- and turbidity-causing casing ” slimes ” .
7. Sulfate reduction is initiated at reduction-oxidation (redox) potentials less than -150 mv.
8. Gross-alpha radioactivity measurement techniques are not reliable measurements of true alpha activity because of short-lived radon daughters that may be present in the sample. Gross-alpha radioactivity should only be used as a screening technique to determine if there is a potential for radiation problems.
9. Uranium was low in all samples and is not a problem. Uranium is mobilized as uranyl ion in oxidizing conditions.
10. Radium was not a problem in most samples. Four recharge wells had samples in excess of the 5 pCi/l standard. Radium appears to be mobilized as a sulfate complex. Strontium, and perhaps barium, appears to enhance nucleation of radiocolloids of radium sulfate. All samples were well below chemical equilibrium with respect to particulate radium sulfate.
11. Lead-210 is present in all systems in minor quantities. Soluble lead-210 does not support its daughter polonium-210, so the aqueous polonium is derived from lead sorbed or precipitated on aquifer materials. Isotherms indicate that lead is strongly adsorbed on clays and is precipitated in contact with ferruginous quartz sand and limestone. Lead is slightly mobile in the Surficial aquifer owing to the low pH of the water. In the Floridan aquifer it is controlled by precipitation of lead carbonate (cerussite). Lead-210 activity is not correlated to total lead concentration.
12. Polonium-210 is most abundant in the Surficial aquifer where it constitutes most of the gross-alpha radioactivity. Polonium mobility is controlled by solubility of the radiocolloids formed by complexing of polonium with hydroxyl. In acid, low OH-1 water, polonium is mobilized, while in alkaline, high OH-1 waters, it forms the radiocolloid. The polonium-hydroxide radiocolloid coexists and co-precipitates with iron-hydroxy complexes. Movement of the radiocolloid and iron-hydroxy colloids is mechanically inhibited in aquifer systems.
13. High gross-alpha radioactivity is spatially related to fracture traces. Fracture traces are characterized by enhanced vertical leakance, thickened “leached zone”, and extreme temporal variability in saturated-zone thickness. Recharge wells within 0.3 km of a fracture trace are most prone to violation of the 15 pCi/l MCL for gross-alpha radioactivity. Therefore, placement of recharge wells over 0.3 km from fractures should minimize water-quality violations.
14. Gross-alpha (polonium) radioactivity in recharge wells is seasonally related to water-table fluctuations (as reflected in precipitation data at a nearby gage). High recharge and water-table conditions lead to dilution of the radioactivity on the scale of weeks to a month. On the longer term, it appears that years with maximum unsaturated zone development (low water table elevations or wide variations in water-table elevation) have low gross-alpha radioactivity in recharge wells. This is because the particulate, polonium-hydroxide complexes (and other metal-iron-hydroxide complexes) are stable, filterable, and, therefore, not transported. In wet years, when the saturated zone is thick (high water table, little variation in water-table elevation), acid conditions develop that break up the hydroxy-complexes and polonium is mobilized to produce high gross-alpha radiation in the wells.
15. It appears that recharge wells can be utilized in the Central Florida Phosphate District if simple precautions are taken. First, they should be located to avoid fracture traces or other locations where the “leached zone” is thick and water-table conditions favor acid water. Second, sampling protocols should be carefully designed, in conjunction with regulatory agencies, to avoid contamination from casing slimes.
16. Chemical modeling and raw data clearly indicate that conditions for transport of polonium and lead exist in the Surficial aquifer, but not in the Floridan aquifer. Therefore, it appears that, with respect to the parameters measured, the quality of receiving Floridan aquifer waters is not jepardized by the practice of interaquifer connector (recharge) wells.