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Fort Greely SM-1A Nuclear Reactor


From the information supplied to ATSDR, there appears to be no recent valid radiological sampling of ground water that meets the appropriate quality control standards [4]. The last valid sampling results submitted to ATSDR were from 1973 when the AEC discharge limits were 3 x 10-3 microcuries per milliliter (3 million picocuries per liter, 1.1 x 105 Bq) [3]. At that time, the recharge well was sampled and the concentration of tritium detected was 1 x 10-5 microcuries per gram [5]. Since one gram of water occupies an approximate volume of 1 milliliter, this is equal to 1 x 10-5 microcuries per milliliter (10,000 picocuries per liter [pCi/L], 370 Bq/L). There is no information in this report [5] as to the depth of the sample; however, the army has informed ATSDR that the water enters the aquifer about 100 feet deeper than the drinking water supply wells [4].

Tritium (H 3) has a radioactive half-life of 12.3 years which means that every 12.3 years, the amount of H 3 is reduced by 50% of the amount initially present during that time period. Since the last validated sampling occurred in 1973; that is, 25 years from 1998, the amount of H 3 remaining in the recharge well would be about 25% of the amount present in 1973. This would not take into account dilution within the aquifer and naturally occurring flushing of the aquifer. The calculated concentration based only on radioactive decay is 2.5 x 10-6 microcuries per milliliter (2,500 pCi/L, 92.5 Bq/L). Under the Safe Drinking Water Act, the maximum contaminant level for H 3 in drinking water is 20,000 pCi/L (740 Bq/L) [6].

The other radiological contaminants detected in the recharge well in 1973 included cesium-134 which has now decayed through 12 half-lives and is essentially non-existent. The other contaminant detected was listed as gross beta radiation that could have consisted of numerous radiological components of reactor operations including cesium-137 (Cs 137) and strontium-90 (Sr 90), each with about a 30 year half-life. Therefore, they could be present at concentrations similar to the concentrations at the time of disposal. The concentration of gross beta radiation reported in 1973 was 3 x 10-8 microcuries per gm (0.03 pCi/L; 1.1 x 10-3 Bq/L). Using the most conservative scenario that this measurement of gross beta radiation is composed entirely of Cs 137 (Cs 137 decays by beta radiation, the resulting radionuclide, barium-137 emits the gamma radiation) or Sr 90, the current (1998) concentration in the recharge well would be about half of the 1973 value; that is, about 0.015 pCi/L (5.5 x 10-4 Bq). The current drinking water standards for Cs 137 and Sr 90 are 200 pCi/L (7.4 Bq/L) and 8 pCi/L (0.3 Bq/L), respectively.

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