Effects of Increased Atmospheric Carbon Dioxide on Environmental Transport of Radionuclides, 20-R8091
Printer Friendly VersionPrincipal Investigators
English Pearcy
Amy Glovan
David Turner
Deborah Waiting
Inclusive Dates: 09/25/09 – Current
Background - Global atmospheric CO2 concentrations have been increasing for some time, and this change can potentially affect many Earth systems and processes, including environmental transport of radionuclides and heavy metals. Increases in atmospheric CO2 are expected to continue such that CO2 concentrations at long times in the future will be much higher than present values. These increased CO2 concentrations may have particular impact on radioactive waste disposal systems, which must be capable of limiting environmental transport of radionuclides for thousands of years. This project will evaluate the potential significance of such changes.
Approach - The technical approach for this project is to identify and acquire measurements of groundwater compositions from a selected groundwater system spanning recent decades and corresponding measurements of atmospheric CO2 concentrations. The Edwards Aquifer system has been selected for this work because of the comprehensive data available (e.g., thousands of wells and numerous springs sampled and measured for more than 50 years over a large geographic area). Further, because carbonates like the Edwards Aquifer respond quickly to recharge and are the aquifer host rocks most sensitive to changes in groundwater chemistry produced by variation in atmospheric CO2, the atmospheric chemistry signal is anticipated to be clearest for a carbonate aquifer. Whatever effects are found in the Edwards Aquifer will be applicable to other types of carbonate aquifers and will be bounding for noncarbonate aquifers.
Accomplishments - The project team identified, obtained, and regularized a set of data containing 23,394 individual water chemistry records spanning from 1913 through 2009. These data were screened sequentially to identify records that were analytically complete, charge balanced, and representative of the bulk of the aquifer. Water records that did not meet these criteria were eliminated from further consideration, leaving 3,385 records. This resulting set was further refined to focus on individual wells for which there are data spanning 30 years or more and for which there are three or more analyses. This reduced the set to 1,074 individual water analyses on 89 wells. These remaining wells span six counties (with the preponderance of the wells in Bexar County) and include the years 1942 through 2009. Statistical analyses were used to evaluate potential trends in water chemistry for these records, with particular attention to bicarbonate because it is the aqueous component that will change most as a result of increasing atmospheric CO2 and it is potentially important for increasing contaminant transport. Fifty-four of the 89 wells were found to have statistically significant positive 30+ year trends for bicarbonate concentration. This predominance (61 percent) of positive bicarbonate trends among those wells with long-term records is consistent with what was expected from interaction with increasing atmospheric CO2. Geochemical speciation modeling of the waters also was initiated under a variety of conditions. This modeling will provide a basis for projecting from the historical water chemistries toward potential future conditions and interpreting those conditions for contaminant transport.
