Scientific Investigations Report 2006–5232

Scientific Investigations Report 2006–5232

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Comparability of water-quality data determined from both the qualitative evaluation (97.5 percent comparable) and the quantitative evaluation (97.9 percent comparable) indicates that the change from purging 3 to 1 wellbore volumes had no discernible effect on comparability of water-quality data at the INL. However, the qualitative evaluation was limited by a small data set. Only October-November 2003 data were available for comparison to historical data.

This comparability of water-quality data is not surprising considering the character of the ESRPA. The ESRPA is mostly an unconfined aquifer and chemical equilibrium with atmospheric gases (such as dissolved oxygen and dissolved carbon dioxide) is maintained in the ground water. Carbon dioxide establishes a buffering process that stabilizes pH in the ESRPA (figs. 3-32). Similarly, the ESRPA is saturated with dissolved oxygen except for areas where organic compounds were introduced by waste disposal. Dissolved oxygen maintains oxidizing conditions in the ground water and stabilizes oxidation-reduction processes in the aquifer, which favors development of the oxidized species such as sulfate and nitrate. The combination of these conditions decreases the tendency of ground water to undergo rapid chemical change because water standing in an open borehole in the unconfined ESRPA is chemically the same as water in the aquifer, and both are in equilibrium with the atmosphere.

Barcelona and others (2005) found that water chemistry from wells in high-permeability materials stabilized after purging only one-half wellbore volume of water from a well. For both the high- and low-permeability wells, stabilization of field water-quality parameters was the principal indicator of chemical stability, one of the ESRPA sample-collection criteria. The ESRPA generally is a high-permeability aquifer, which likely accounts for the comparability of water-quality data. In the Barcelona and others (2005) study, small flow rates (<0.5 L/min) were used for purging and sampling. As a result, their conclusions are not directly comparable to the conclusions of this study.

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