Effects of Increased Atmospheric Carbon Dioxide on Environmental Transport of Radionuclides, 20-R8091
Principal Investigators
English Pearcy
Amy Glovan
David Turner
Deborah Waiting
Inclusive Dates: 09/01/09 – 04/01/11
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 evaluated the potential significance of such changes.
Approach — The technical approach for this project was 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 was 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 was anticipated to be clearest for a carbonate aquifer. Whatever effects were found in the Edwards Aquifer would be applicable to other types of carbonate aquifers and would be bounding for noncarbonate aquifers.
Accomplishments — SwRI researchers 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 those 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. Researchers used statistical analyses 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 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 would be expected from interaction with increasing atmospheric CO2.
Geochemical modeling of ranges of historical geochemical speciation as well as projecting future water chemistries showed that there would be systematic effects of atmospheric forcing that could influence and reduce hypothetical radionuclide sorption. These effects are small, however, and are generally masked by complicating factors such as biological activity, equilibration with limestone, and natural spatial variability. This relatively small effect is especially true for the historical data, where changes in atmospheric CO2 are on the order of 20 percent, but also is true even for projections of up to 2,000 ppm CO2.
This project was successful in identifying and quantifying past groundwater chemistry responses to changes in atmospheric CO2 concentrations over a period of decades. These changes were evaluated temporally and spatially, and ranges of potential effects were projected for the future.
Potential weakness of the atmospheric chemistry signal as reflected in the historic groundwater chemistries was recognized during planning for this project. Quantification of the signal confirmed that weakness. Starting with a very large historical dataset, screening the data for those records of highest quality, and carefully analyzing the resulting information, researchers found that though changes in groundwater chemistry resulting from increases in atmospheric CO2 were discernable and statistically significant, the changes have limited effect on the potential for contaminant transport.
The Edwards Aquifer system chemistry is robust in response to atmospheric chemistry forcing; even larger increases in atmospheric CO2 projected for the future are likely to produce only modest changes in contaminant transport capability. The improved quantification of the degree to which effects of changes in atmospheric CO2 on groundwater chemistry are reflected in historic data demonstrates the resilience likely to be present in other such systems, even in response to large changes in atmospheric chemistry. As the broader climate change debate has evolved nationally and internationally in recent years, many potential climate change effects have been postulated. Some of those projections invoke dramatic scenarios. Whereas strong changes appear to be occurring in some natural systems, information from this project documents the substantial buffering capacity present in major groundwater aquifers and provides an improved basis for related decisions.
