The use of isotopes as a tool in conducting groundwater studies has increased significantly in popularity in recent years, as was evident by the solid attendance of this symposium. Although the utility of oxygen, hydrogen, and helium isotopes has been documented for decades, particularly for tracing recharge and age dating, isotopes of nitrogen, carbon, boron, silicon, sulfur, chloride, and various metals have also become widely accepted and friendlier to project budgets. In this symposium, their combined application was repeatedly shown to add a new layer of understanding to hydraulic or biogeochemical studies.
Tom Mohr, GRA President and
Symposium presenter Saikat Sengupta of India
The groundwater studies symposium highlighted the use of both stable and unstable (radioactive) isotopes in many typical investigations. The first day was dedicated to a course to illustrate isotope fundamentals, dispense valuable reference material, and review several case studies, demonstrating the application of isotopes for water and nitrate provenance determination, and for groundwater age dating. The symposium consisted of six sessions, covering various applications of isotope data, their use in forensic investigations, and their value in delineating recharge areas or as tracers.
Isotope Methods for Groundwater Investigation Course
Carol Kendall of the U.S. Geological Survey and Jean Moran of Lawrence Livermore National Laboratory worked tirelessly to prepare an original curriculum for this one-day course. Carol presented six chapters of information including references, general isotope fundamentals, water isotope fundamentals, isotope hydrology, isotope biogeochemistry, and ecological applications of stable isotopes. Jean followed with an introduction to groundwater age dating, which focused on the tritium-helium method but touched on other age-dating isotopes, such as 85Krypton and 14Carbon. Together, Carol and Jean presented a detailed summary of theory and applications along with an invaluable compilation of reference materials
Carol has provided a complete set of references associated with her presentation, which can be accessed at http://wwwrcamnl.wr.usgs.gov/isoig/res/refs.html. Additionally, she has provided her presentation slides, which can be accessed at http://wwwrcamnl.wr.usgs.gov/isoig/res/presentations.html.
Symposium Keynote Speaker: Niel Plummer, U.S. Geological Survey
Niel Plummer presented work he conducted in investigating groundwater resources in the Middle Rio Grande basin in central New Mexico. He showed how isotopes were applied to delineate not only water resources associated with the active river channel but also those waters associated with paleo channels and other sources not obviously associated with the Rio Grande. Analytical data relied upon included isotopes (2H, 18O, 3H, 14C, 13C, and 34S), environmental tracers (CFCs and sulfur hexafluoride), major cations and anions, atmospheric gases, and temperature. This approach was an excellent example of how thoroughly a groundwater investigation can be conducted using multiple sources of related data.
The overall objective of this investigation was to better understand aspects of the groundwater flow system, including sources of recharge, groundwater flow paths, groundwater travel times, variability in characteristics with depth, and historical recharge rates. Groundwater was sampled from nearly 300 wells and springs in addition to monthly surface water samples collected over a two-year period. Maps of these data clearly depicted influence of the active river with a depleted hydrogen isotopic signature, but also illustrated paleo-channels located west of the active channel (i.e. the inner valley) that correlate with historic groundwater elevation maps. Up to 13 hydrogeochemical zones were identified using a combination of chemical, isotopic, and tracer data, each related to a particular source of groundwater or geologic unit. Distinctions between these zones are sharp enough to be identified in individual municipal wells serving the City of Albuquerque.
Results from this investigation changed the conceptual model of the Middle Rio Grande basin in terms of quantifying a reduced basin-wide recharge rate, clarification that the primary source of water to the City of Albuquerque is the Rio Grande rather than the surrounding mountains, determining that historical depressed groundwater elevations reflect transient recharge rates rather than a zone of high transmissivity, and finding that groundwater ages range up to 40,000 years before present. These data were further used to calibrate the numerical groundwater flow model, illustrate variations in water quality with depth, and characterize the importance of paleo-recharge patterns to modern-day groundwater demand requirements.
Niel’s work exemplified the use of chemical and isotopic tools that can be used at other areas to improve our understanding of the extent of non-renewable water resources, modern rates of recharge, and the associated estimate of the total volume of the renewable resource. Data collected together in this fashion are invaluable for refining hydrogeologic groundwater flow models that can be used to better manage water resources.
Examples of Isotope Uses
Sessions 1 and 2, moderated by William Motzer of Todd Engineers and Michael Taraszki of MACTEC Engineering and Consulting, consisted of seven talks discussing the use of isotopes for evaluating groundwater recharge and as tracers for groundwater flow paths.
Dr. Julie Sueker of ARCADIS, BBL, Inc. gave the first presentation, titled Isotope Applications for Environmental Forensic Investigations. Dr. Sueker gave three examples of the use of stable isotopes in environmental forensic investigations. Her first study involved the use of δ 13C of petroleum hydrocarbons in evaluating the source of hydrocarbon sheen in river water at a bulk oil terminal. Compound-specific δ13C and biomarkers showed that the sheen was clearly different from up gradient LNAPLs and residuals. The next case study was of a historical accidental gasoline release and 2,500-foot plume of benzene and 1,2-DCA for which an in-situ bioremediation pilot program had been initiated in October 2003. Delta 13C values of benzene and 1,2-DCA indicated reasonable microbial growth rates were occurring. The third case study involved the δ13C analysis of TCE, DCE, VC, and ethane to evaluate microbial degradation. The analyses indicated that TCE and DCE degradation had occurred.
A presentation on the Use of Stable Isotopes as a Forensic Tool to Determine Sources of Perchlorate in Groundwater in the Chino Basin was given by Dr. Neil Sturchio of the University of Illinois – Chicago. Dr. Sturchio’s research focused on δ37Cl and δ18O of perchlorate (ClO4–) as source signatures. The data collected and analyzed from groundwater in the Chino Basin pointed toward a source that had the same signature as natural perchlorate from the Chilean Atacama Desert, which comes from nitrate fertilizers once used in the Chino Basin to fertilize citrus crops.
Matthew K. Landon of the USGS presented Use of Multiple Isotopic Tracers to Constrain Understanding of Processes Affecting Ground-Water Quality of a Sub-Regional Scale, Central-Eastside San Joaquin Valley. This study was done in conjunction with the State Water Board’s Groundwater Ambient Monitoring and Assessment (GAMA) program and involved groundwater samples collected from 78 wells in the Merced, Turlock, and Modesto basins. Samples were analyzed for up to 350 chemical, isotopic, and microbial constituents. Results from δ18O and δD showed that there were variations in water recharge sources with mixing of high elevation and local source waters. Delta18O and δ15N of nitrate showed nitrate attenuation by denitrification in the western part of the study area.
Michael Singleton, of Lawrence Livermore National Laboratory, continued demonstrating examples of isotope use with Isotopic Tracers of Human Wastewater in Groundwater Systems. This study was conducted in conjunction with the GAMA program and evaluated analytical results from groundwater samples impacted by septic tanks in Chico and Livermore, California. Data evaluated included isotopic compositions of nitrate and water, dissolved major ions and noble gases, 3H/3He age dating, and trace organic compounds. Results reflect ambiguity in formerly agricultural and urban areas due to the isotopic signature overlap of nitrate sources. However, when combined with other data, such as trace organic compounds and estimates of groundwater age, nitrate sources can be appropriately identified. Younger waters consistently illustrated higher nitrate concentrations and evidence of partial denitrification that occurs in the vadose zone near leach fields.
Also discussing how isotopes can help delineate anthropogenic impacts, Megan Young of the USGS presented The Oxygen Isotopic Composition of Phosphate: A Tool for Tracing Nutrient Sources in Aquatic Ecosystems. This presentation described a novel method for culling additional information from phosphate data, particularly because phosphorous has only one stable isotope. To obtain additional data, this method takes advantage of the fact that phosphorous is typically bound to four oxygen atoms (i.e. orthophosphate). Because oxygen has three stable isotopes, the oxygen isotope signature of phosphate (δ18OP) can reflect mixing of different phosphate sources or biologic cycling when phosphate-oxygen is exchanged with water-oxygen. Future work will include developing a better understanding of δ18OP sources and how they may change with migration through terrestrial and aquatic environments.
Speaking to a recently very popular topic, Paul Hatzinger of Shaw Environmental, Inc. presented Stable Isotope Fractionation in Nitrate and Perchlorate During In Situ Biodegradation in Groundwater. This presentation discussed the advantageous use of an ongoing remediation effort to demonstrate in situ fractionation of both chlorine and oxygen in perchlorate during biodegradation. The study area has been remediated using vegetable oil substrate, and a push-pull test was conducted to inject contaminated groundwater into the study area. Groundwater samples subsequently withdrawn indicated that nitrate reduction by indigenous bacteria fractionated nitrogen and oxygen at a ratio of approximately 1.2:1 (ε15N/ε18O), consistent with heterotrophic denitrification. The fractionation ratio for chlorine and oxygen isotopes in perchlorate was 0.4:1 (ε37Cl/ε18O), which is indistinguishable from their laboratory studies using pure cultures of two different genera. These results are useful for assessing natural attenuation of perchlorate and for evaluating perchlorate forensics in environments where biodegradation may be occurring.
Rounding out the session, Steve Carle of Lawrence Livermore National Laboratory presented: A New Modeling Framework for Simulation of Apparent 3H-3He Groundwater Age and Mixing. Steve demonstrated his use of tritium-helium (3H-3He) data to calibrate regional groundwater flow simulations, particularly at a study area near a dairy farm in the San Joaquin Valley, California. Because 3H-3He data provide estimates of a mean age, particular care is needed to account for the effects of mixing by pumping, and dispersion. These data, however, can be used for flow calibration if the flow model can simulate the mixing mechanisms involved. A new modeling framework for simulating apparent groundwater age was presented through the use of the two-phase flow model NUFT. Results of Steve’s work in the San Joaquin Valley are promising and demonstrate the successful simulation of both spatial and temporal flow patterns based upon isotopic data.
Sessions 3A and 4A, moderated by Michael Singleton of Lawrence Livermore National Laboratory and Stephanie Moore of Daniel B. Stephens & Associates, Inc, consisted of seven talks discussing the use of isotopes as tools for conducting forensic investigations related to natural and anthropogenic constituents.
Dimitri Vlassapolous of S.S. Papadopulos & Associates presented: Source Identification and Allocation of Chlorinated Solvent Contamination Among Multiple Sources: Use and Limitations of Compound-Specific Isotope Analysis. The use of compound-specific analysis of carbon isotope ratios in chlorinated solvents was discussed, specifically in case studies where the δ 13C values of TCE and PCE were used to identify source areas and detect the presence or absence of biodegradation. The VOC isotope signature was evaluated in conjunction with δ 18O signatures of groundwater to establish recharge patterns and corroborate groundwater flow model results. The use of multiple lines of evidence, including water isotopes, VOC signatures, and δ 13C of TCE and PCE, was instrumental in settling ongoing litigation.
In another demonstration of forensic identification of VOCs, Alan Jeffrey of Zymax Forensics presented: Use of Carbon Isotope Ratios to Distinguish Perchloroethylene Plumes in Soil and Groundwater. The use of solid phase micro extraction to concentrate chlorinated solvents for GC-IRMS analysis was described, in addition to discussion of the kinetic isotope effect for carbon. The first of two case studies included multiple onsite dry cleaning facilities that, according to similar δ 13CPCE isotope ratios in soil and groundwater, apparently used PCE from the same manufacturer, making source allocation difficult. Compound-specific δ 13C values from a second site with both onsite and offsite dry cleaning facilities, however, clearly distinguished an offsite source of the PCE plume with mixing occurring onsite.
Stephanie Moore of Daniel B. Stephens & Associates presented Geochemical and Isotopic Tracers to Define Salinity Sources in the Lower Rio Grande Valley, New Mexico. Multiple isotopic signatures (including δ 11B, δ 34S, δ 18O, δ 2H, and 87Sr/86Sr) were used to identify salinity sources that are degrading groundwater quality. Of particular concern was whether the sources were anthropogenic or natural, a question that required identification of known salinity sources with a specific sampling plan. Isotopic results augmented chemical data and confirmed the hypothesis that the dominant salinity contributions originated from deep groundwater inflow to the Rio Grande. Importantly, results indicated that it is not possible to reproduce observed salinization by evapotranspiration and agricultural processes alone.
In another example of using isotopes to forensically identify recharge sources, Saikat Sengupta from the Indian Institute of Technology presented The Role of Surface Water-Groundwater Interaction in Releasing Arsenic from Shallow Aquifer of Southern West Bengal: An Isotope Geochemical Approach. The threat of arsenic contamination to groundwater used to supply drinking water in the Bengal Delta is significant, but stable isotope compositions of oxygen and hydrogen in water have been successfully applied to determine its source. Results confirm the long-held belief that infiltration from ponds has mobilized arsenic naturally occurring in soils. Arsenic is released by the FeOOH originating from organic wastes found within the anthropogenic recharge.
Patrick Longmire of Los Alamos National Laboratory presented Application of Nitrogen Isotopes in Distinguishing Different Sources of Nitrate in Groundwater at Los Alamos National Laboratory, New Mexico. The longtime use of neutralized nitric acid (negative but variable δ 15N –NO3 ratios) and a nearby source of nitrate from sewage effluent (positive variable δ 15N –NO3 ratios) provided a unique opportunity to monitor subsurface nitrate transport downgradient of the source. Mixing between these isotopically distinct sources has been confirmed to occur within the vadose zone and ratios have been used to calculate transport velocities from 2 to 5 m/yr between depths of 35 and 75 m.
Also discussing the impact of nitrate, Chris Reuhl of the University of California, Santa Cruz presented Nitrate Dynamics Within the Pajaro River, A Nutrient-Rich Losing Stream. Concentrations of nitrate in the Pajaro River are consistently higher during the dry season of the year and, although most solutes behave conservatively through the study area, nitrate concentrations are observed to decline by up to 30 percent. This decline correlates with net loss of channel discharge and extensive surface-subsurface exchange. Stable isotopes of nitrate, total dissolved phosphate concentrations, and streambed chemical profiles suggest that denitrification is the most significant nitrate sink along the lower portion of the reach. Denitrification efficiency is expressed by downstream enrichment of δ 15N –NO3 and this efficiency is lower when discharge is higher.
Ted Koelsch of K2 Enviro, Inc., presented The Use of Tritium in Understanding the Migration Pathways and Fate of Organic Contaminants in Heterogeneous Hydrological Flow Environments. Throughout the Midwest, carbon tetrachloride was a commonly used fumigant associated with grain silos, densely located particularly in Kansas and Nebraska. Tritium isotopes were used to concisely characterize the hydrogeologic continuity of sites and to refine conceptual models of groundwater and contaminant transport. Two case studies presented benefited from this approach in which the sites were successfully characterized with respect to dominant versus minor infiltration pathways, the degree of connection between aquifers, and the influence of nearby surface drainages.
Recharge and Tracers
Sessions 3B and 4B, moderated by Julie Sueker of ARCADIS BBL and Alan Jeffrey of ZymaX Forensics, consisted of eight talks discussing the use of isotopes for evaluating groundwater recharge and as tracers for groundwater flow paths.
Arun Wahi of Daniel B. Stevens & Associates presented Geochemical Quantification of Semiarid Mountain Recharge, showing results of a study where geochemical and isotopic tracers were employed to discern dominant mechanisms of natural recharge to an aquifer in the semiarid San Pedro River basin located in southeast Arizona. Stable isotopes of the water molecule indicated that mountain recharge is the dominant recharge mechanism and that approximately 65% of the recharge is derived from wintertime precipitation. Multiple tracers were evaluated to assess the relative importance of several possible flow and transport scenarios. .
Dr. C. John Suen of California State University, Fresno, presented Applications of Stable Isotopes as Tracers in Groundwater Studies – Some Examples from the San Joaquin Valley. Three examples of stable isotope applications in groundwater studies conducted in private wells from the southern San Joaquin Valley were presented. Results from the studies indicate that the chemistry of the groundwater is primarily a product of mixing between Sierran water and various components of irrigation and waste waters, and that agricultural and industrial activities have adversely impacted groundwater quality in the region.
Michael Taraszki of MACTEC Engineering and Consulting, Inc. presented the Use of Stable Oxygen Isotopes to Delineate Recharge from CVP-Source Percolation Ponds in Llagas Subbasin, Santa Clara County, California. The isotopic composition of CVP source water, distinct from locally-derived recharge water, was used to illustrate the lateral and vertical migration patterns of groundwater derived from the percolation pond ‘source’ areas. Depth-discrete groundwater data define a three-dimensional ‘plume’ of imported water extending several miles downgradient and to depths of hundreds of feet and augment previous estimates of rapid groundwater flow rates of over 2 feet/day.
In a another example of the international acceptance of isotope data, Sangram Shrestha of the University of Yamanashi, Kofu City, Japan presented Stable Isotopes as Indicators of Groundwater Recharge System in Kathmandu Valley, Nepal: A Preliminary Study. In Kathmandu, groundwater abstraction rates are currently greater than recharge rates and this study and future related studies are designed to apply isotopic tracers to define recharge mechanisms for the valley aquifer. Mr. Shrestha regaled us with a story of the challenges of traveling along busy streets on a motor scooter to reach the various sampling points.
Joe Iovenitti of Weiss Associates presented Groundwater Relationships Between the Fall River Springs and Medicine Lake Volcano-Evaluation of All Available Data, where hydrogen and oxygen isotope ratios were used to investigate the source of waters in Northern California. Local wisdom placed the source of the water in Medicine Lake Highlands, a Pleistocene to Holocene volcano. Stable isotope ratios of water in the Medicine Lake caldera were similar to the water at Fall River Springs. However, the recharge volume in the caldera was insufficient to account for the flow rate in the Fall River Springs. Other high elevation sources for the water were proposed.
Matthew Waterman of Bechtel Corporation showed how hydrogen and oxygen isotope ratios could be used to investigate the source of water infiltrating a large tunnel in Riverside County, Southern California, in his presentation, Application of Environmental Isotopes in Understanding Recharge in Semi-Arid Areas. Stable isotope ratios of the infiltrating water indicated a local source that was altered by evaporation in the unsaturated zone. The isotope ratios also allowed the recharge rate of the inflow water to be estimated.
Victor Madrid of Lawrence Livermore National Laboratory presented A Stable Isotope Tracer Experiment Using Hetch Hetchy Reservoir Water, and showed how the hydrogen and oxygen isotope ratios from this legendary reservoir could be used as a tracer in a remediation study. The isotopically-distinct Hetch Hetchy water was injected upgradient of a PCE-contaminated site near Tracy, California, and its movement through the site was monitored by observing changes in the stable isotope ratios from the local groundwater values.
Jordan Clark of the University of California, Santa Barbara discussed the use of tracers, including the noble gas isotopes 3He, 124Xe, and 136Xe, in managed aquifer recharge (MAR) studies in his presentation Successes and Failures of Gas Tracers Experiments Near Spreading Ponds. Trapped air beneath spreading basins can retard the tracers, complicating the calculation of the aquifer recharge rates. The use of sulfur hexafluoride (SF6) was specifically illustrated as a useful tracer for large volumes of groundwater due to its affordable analysis and low background concentrations.
Seven technical posters were also presented at the symposium including four student posters that were evaluated and awarded prizes by GRA. All of the student posters presented high quality research in an exemplary manner. Jenny Druhan, from University of California, Berkeley, received first prize for her research titled “Use of Sulfur Isotopes to Identify Bacterial Sulfate Reduction Processes during In Situ Stimulated Bioremediation of a Uranium Contaminated Aquifer”. Rachel Andrus, from California State University, Los Angeles, was awarded second prize for her poster presenting her Master’s research titled “Selenium Isotopes: A Viable Tracer for Geologic Processes?” Glenn Shaw, of University of California Merced, presented his doctoral research titled “Determination of Groundwater Contribution to the Merced River During Snowmelt Using 36Cl/Cl, 222Rn, and 3He/4He, Yosemite National Park” and was awarded third place. Nelson Bernal, from California State University, Fresno, received fourth place for a poster displaying his Master’s research titled “Application of Stable Isotopic Data to Characterize the Water Flow Through a Fractured Terrain of the Sierra Nevada Foothills”.
Special thanks belong to sponsors of this groundwater studies symposium, including major sponsors ARCADIS, Locus Technologies, Inc., RSI Drilling and Microseeps, Inc., and meal sponsors Daniel B. Stephens and Associates, Inc. and Todd Engineers.
Michael Taraszki is with MACTEC Engineering and Consulting, Inc.; Julie Suekeris with ARCADIS BBL; Michael Singleton and Jean Moran are with Lawrence Livermore National Laboratory; Bill Motzer is with Todd Engineers.
Exhibitors: ARS Technologies, Inc. |
Laboratory Data Consultants (LDC) | Los Gatos Research |
Instrumentation Northwest | Precision Sampling, Inc. | ZymaX Forensics