Age-Dating Groundwater to Assess Contamination
Quantitative characterization of groundwater replenishment (known as recharge) and flow paths is essential to water resource management and environmental remediation. In response to a request for assistance from California’s Water Resources Control Board, LLNL is assessing the vulnerability of the state’s 16,000 municipal water wells to contamination by conducting focused studies on the nitrate and emerging contaminants in groundwater.
Both radioactive and stable isotopes can provide information about a groundwater’s age, which refers to the last time the water was in contact with the atmosphere. Younger water is more susceptible to contamination; therefore, it requires close monitoring.
Livermore is one of only two facilities in the world with high-throughput capability for age-dating analyses and is unique in using isotopic tracers to characterize groundwater travel times and flow paths.
Relevance to PLS Research Themes
Mass spectrometry using noble gases, which are chemically inert, was originally developed at LLNL for nuclear test diagnostics. Over the years, its applications have been extensive in research for our national-security mission. The technology is also used to understand radionuclide sources and transport in the ground at the Nevada Test Site.
|Figure 1. Groundwater age and travel distances are shown for a recharge facility in Orange County, California. Tritium–helium age dating is used for groundwater measured in years, and 136Xe is used as a tracer for groundwater with ages in days.|
These programs are closely aligned with our mission to tackle environmental
issues that affect water safety and nuclear waste cleanup. Noble gas methods
are also used in other research, including studies on groundwater flow and
pathogen transport, subsurface nitrate transport and biogeochemistry, and transport
of xenobiotics during land application of wastewater.
Major Accomplishments in 2004
Our unique approach uses noble gas spectrometry to quantify groundwater flow through tritium–helium age dating and xenon tracer studies (Figure 1). Tritium–helium age dating measures groundwater in units of years, and isotopically enriched xenon tracers measure groundwater with ages in days.
One drawback of using age to assess vulnerability is that “old” groundwater may contain a small fraction of very young water that is highly contaminated. To address this possibility, LLNL, in collaboration with the United States Geological Survey (USGS), also measures recently released volatile organic compounds such as MTBE (methyl tert-butyl ether) at very low levels.
|Figure 2. A map of the Los Angeles Basin shows the levels of the organic contaminant MTBE found in groundwater. MTBE is found only in shallow, young groundwater, demonstrating that deep groundwater is protected from surface contamination. (Images courtesy of the USGS.)|
We have found that coastal aquifers, particularly in the Los Angeles Basin and Santa Clara County, are well protected by extensive low-permeability zones that act as barriers to vertical contaminant transport (Figure 2). Although their young water shows low levels of common organic contaminants, their old (deep) water is free of anthropogenic contaminants. The absence of contamination in deeper groundwater is remarkable considering that these basins are located in some of the most highly compromised areas in the state with respect to contaminated surface sites.
We have found the reverse situation for California’s Central Valley, especially northern California where the principal aquifers are not well protected. Common solvent contaminants are present in both young and old groundwater in the Sacramento Valley, a distribution requiring more than one transport path for contamination. California’s Central Valley is rapidly developing, and significant effort will be necessary to prevent surface contamination from affecting the drinking water supply.
The ability to characterize groundwater flow is essential to understanding contaminant and pathogen transport. We have developed a new method to calibrate and validate high-resolution simulations of groundwater flow and contaminant transport to rapidly determine groundwater ages for large numbers of samples. By using noble gas mass spectrometry, we have characterized the flow of water within a basin to determine the spatial distribution of groundwater age. We are also using noble gas methods to determine the water temperature and the abundance of dissolved gases above the equilibrium solubility limit in water. These parameters are directly related to the elevation, seasonality, and soil conditions during natural replenishment of groundwater. Our large data set has revealed strong regional differences in water recharge that have not been previously well understood.
J.F. Clark, et al., “Geochemical Imaging of Flow near an Artificial Recharge Facility, Orange County, California,” Ground Water 42(2), 167–174 (2004).
J.E. Moran, et al., “A Contamination Vulnerability Assessment for the Sacramento Area Groundwater Basin,” Lawrence Livermore National Laboratory (http://www.waterboards.ca.gov/gama/docs/cas_llnl_sac.pdf).
L.K. Rademacher, J.F. Clark, and G.B. Hudson, “Temporal Changes in Stable Isotope Composition of Spring Waters: Implications for Recent Changes in Climate and Atmospheric Circulation,” Geology 30(2), 139–142 (2002).