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Colorado Division of Mining Reclamation and Safety February 17, 2015 <br />Eric Scott 6 1400159 <br />Based on the available regional data and consideration of the factors described above, current levels of <br />tritium in precipitation for the Site are conservatively estimated to be between 6.2 TU and 8.1 TU. Tritium <br />levels in groundwater that are greater than current precipitation values represent waters that have <br />recharged between the onset of nuclear testing in the 1950s and the present. Tritium levels in <br />groundwater that are between approximately 0.4 TU and current precipitation values (6.2 to 8.1 TU) <br />would represent a mixture of pre -1952 and post -1952 recharge. <br />Based on these assumptions and calculations, the groundwater sampled in MW -1 (8.5 ±0.45 TU) <br />recharged relatively recently. However, tritium levels for groundwater measured in the other Site wells <br />range between 0.6 TU and 1.7 TU and represent mostly pre -1952 water with some post -1952 water. The <br />potential mixture for pre -1952 and post -1952 waters is a non - unique solution, as a range of combinations <br />may be realistic. Additionally, the values may represent a continuum of recharge to groundwater over <br />time form the open mine pit, from the onset of mining in the 1930s until the A2 pit was backfilled in 2001. <br />For the purposes of this discussion, a simple calculation has been performed to only assess a two <br />component system, assuming a modern water and a pre -1952 water, To provide a lower bound to the <br />estimation of pre -1952 versus post -1952 recharge the estimated range of 6.2 TU to 8.1 TU for modern <br />precipitation, a percentage of pre -1952 (pre -bomb) water was calculated for each well (Table 2, below). <br />Well MW -2 and the more recently installed wells MW -6 and MW -7 have a calculated percentage of pre - <br />1952 water between approximately 80% and 90 %. Well MW -4 has the lowest percentage of estimated <br />pre -1952 water at 78% to 83 %, whereas MW -3 has the highest at 97 %. The results presented in Table 1 <br />are conservative as in they are calculated with the assumption that the post -1952 has the tritium levels <br />equal to modern precipitation. If however, any part of the post -1952 portion of the water recharged prior <br />to present day, tritium levels of the post -1952 portion of the water would be greater and a higher <br />percentage of pre -1952 water would be calculated. For example, if a tritium level of approximately <br />12.6 TU representing precipitation from 1975 is used in the calculation for MW -2, the percentage of pre - <br />1952 water calculated would be 93 %, and this percentage is even greater (97 %) if water from 1965 is <br />used in the calculation. <br />Table 2: Calculated Percentages of Pre -1952 Water <br />Well <br />Tritium Concentration TU <br />Percentage of re -1952 Water <br />MW -1 <br />8.5 <br />0% <br />MW -2 <br />1.2 <br />86%-90% <br />MW -3 <br />0.6 <br />97%-97% <br />MW-4 <br />1.7 <br />78%-83% <br />MW -6 <br />1.5 <br />81%-86% <br />MW -7 <br />1.2 <br />86%-90% <br />- percentages calculated based on estimated level of tritium in precipitation of 6.2 TU to 8.1 TU <br />1' `Golder <br />is \14\ 1400159 \0100 \0122 \tm- boettchadimestone uar fnl- 17feb15 \1400159 tm- boettcherlimestone ua fnl- 17feb15.docx 4- Associates <br />9 rY_ 9 QTY_ <br />