This document is also available in format: fs Information about the age of ground water can be used to define recharge rates, refine hydrologic models of ground-water systems, predict contamination potential, and estimate the time needed to flush contaminants from ground-water systems. CFCs also can be used to trace seepage from rivers into ground-water systems, provide diagnostic tools for detection and early warning of leakage from landfills and septic tanks, and to assess susceptibility of water-supply wells to contamination from near-surface sources. During the past 50 years, human activities have released an array of chemical and isotopic substances to the atmosphere. In the atmosphere, these substances have mixed and spread worldwide.
In the early s, USGS scientists Busenberg and Plummer, developed a method to date ground water on the basis of chlorofluorocarbon CFC content of the water that is practical, cost-effective, and applicable to most shallow ground-water systems. The feasibility of using CFCs as tracers of recent recharge and indicators of ground-water age was first recognized in the s see Plummer and Busenberg, and references therein.
What is Carbon-14 Dating? Part 1
CFCs have been increasingly used in oceanic studies since the late s as tracers of oceanic circulation, ventilation, and mixing processes. USGS scientists Busenberg and Plummer, adapted analytical procedures developed by the oceanographic scientific community for ground-water studies and designed sampling equipment and procedures for collection and preservation of water samples in the field.
Water samples for CFC analysis are now routinely collected from domestic, irrigation, monitoring, and municipal wells, and from springs. A closed path is established between the well or pump to a valve system that is used to fill glass ampoules with water, creating a headspace with CFC-free, ultra-pure nitrogen gas.
The samples are then transported to the U. Ground-water dating with CFC, CFC and CFC is possible because 1 their amounts in the atmosphere over the past 50 years have been reconstructed, 2 their solubilities in water are known, and 3 concentrations in air and young water are high enough that they can be measured.
These age-dating tracers can help water-resource managers to develop management strategies for shallow groundwater systems that contain mostly young groundwater. Old groundwater is defined as water that entered the aquifer before and more commonly refers to water older than 1, years. hydrologists specializing in age dating, on the other hand, determine groundwater age from the concentration of a radioactive or radiogenic isotope, or of an isotope or molecule that marks an anthropic event, such as nuclear weapons testing. They ?gure age from the rate the isotope decays or accumulates, or the time elapsed since. analyses performed on the sample revealed that the age of the groundwater was about 24 years. Groundwater flow calculations confirmed that the travel time between the lagoons and the well for TCE and metals is between 20 andCited by: 3.
As with any environmental tracer, age applies to the date of introduction of the chemical substance into the water, and not to the water itself. The accuracy of the determined age depends in part on how perfectly the CFCs are transported with the water.
Chemical processes, such as microbial degradation and sorption during transit, can also affect the concentration of CFCs and other compounds used in dating. USGS scientists have investigated some of the most important factors that can affect CFC concentrations in ground water and the resulting age interpretations Busenberg and Plummer, ; Plummer and others, ; Dunkle and others, ; Ekwurzel and others, ; Cook and others, ; Plummer and others a, b; Plummer and Busenberg, These include 1 uncertainty of the temperature at the water table during recharge, 2 entrapment of excess air during recharge, 3 uncertainty in recharge elevation, 4 thickness of the unsaturated zone, 5 effect of urban air where CFC values may exceed regional values, 6 contamination from a specific local source, 7 microbial degradation in anaerobic environments8 sorption onto organic and mineral surfaces, and 9 mixing of younger and older water in a well.
Because of the effect of these factors on CFC concentration, collection of additional data is often needed to determine the apparent age of ground water.
Classify groundwater age with a single tritium measurement
For example, measurements of concentrations of dissolved gases, such as dissolved oxygen, help to define the potential for microbial degradation. Measurements of dissolved methane are useful in recognizing environments where all three CFCs can be degraded. Measurements of dissolved nitrogen and argon can be used to help determine recharge temperature and excess air and to recognize environments undergoing denitrification.
Chlorofluorocarbons are stable, synthetic organic compounds that were developed in the early s as safe alternatives to ammonia and sulfur dioxide in refrigeration and have been used in a wide range of industrial and refrigerant applications.
CFC and CFC were used as coolants in air conditioning and refrigeration, blowing agents in foams, insulation, and packing materials, propellants in aerosol cans, and as solvents. CFC has been used primarily by the electronics industry in semiconductor chip manufacturing, in vapor degreasing and cold immersion cleaning of microelectronic components, and surface cleaning.
Probably better known to the public as Freon TMCFCs are nontoxic, nonflammable and noncarcinogenic, but they contribute to ozone depletion.
This agreement, the Montreal Protocol on Substances That Deplete the Ozone Layer, was strengthened in and again in when was established as the cut-off date for CFC production in industrialized countries.
Tritium half-life Although it is difficult to evaluate age information from tritium data alone, age commonly can be reliably determined from data on tritium 3 H and its decay product, helium-3 3 He. Several conditions are necessary to solve the calculation and interpret the age: 1 The sample must contain detectable 3 H greater than approximately 0.
Krypton 85 Kr has also been used to date ground water. The source for atmospheric input of 85 Kr is reprocessing of fuel rods from nuclear reactors. Because of difficulties in collection and analysis, 85 Kr is not yet a practical dating tool for ground-water studies.
CFCs and nitrate concentrations were measured between June and January on a section of the Delmarva Peninsula, in the Fairmount watershed. The method-tritium-based age classification, or TBAC-requires just a single measurement of tritium, along with knowledge of sample date and location. Knowing groundwater age can help water-resource managers determine which contaminants are anticipated to be present or absent in groundwater, allowing more cost-effective use of monitoring dollars.
For example, if the TBAC method determines that groundwater is premodern, that groundwater is less likely to contain chemicals, such as current-use pesticides, that came into use after The TBAC method can also be used as a screening tool to determine if other, more complicated age-dating methods are needed to resolve questions of groundwater age. The TBAC method can be used retroactively to assign age categories to any sample previously analyzed for tritium, provided that the sampling date and location are known.
When this was done for existing USGS data, the results using the TBAC method very closely matched those from studies that used multiple tracers of groundwater age and complex analyses. Citation: Lindsey, B. Geological Survey Scientific Investigations Report - 18 p. Learn about the quality and availability of groundwater for drinking, where and why groundwater quality is degraded, and where groundwater quality is changing. Is groundwater the source of your drinking water?
When mixing is ignored, as it often is, tritium alone can overestimate the mean age of the modern (less than 50 year) age component. Tritium/helium-3 age dating is not affected by dilution with old (or pre-modern) groundwater and yields both the mean age of the modern age component and, when compared with tritium in precipitation curves, the mixing ratio between modern and pre-modern water. Other chemical dating tools. Tritium (half-life years) provides another useful tracer of young ground water. Although it is difficult to evaluate age information from tritium data alone, age commonly can be reliably determined from data on tritium (3 H) and its decay product, helium-3 (3 He).The 3 H/ 3 He age is based on a calculation that determines the amount of 3 He derived from. Groundwater samples will be collected and analyzed to fill important data gaps in existing groundwater availability studies and to demonstrate age-dating techniques that could be used in future studies to understand recharge conditions. Tracers of modern recharge: Groundwater samples will be analyzed for 3 H/ 3 He, CFCs, and SF 6.
The USGS is assessing the quality of groundwater used for public supply using newly collected data along with existing water-quality data. Learn more about this invisible, vital resource so many of us depend on. The age of groundwater is key in predicting which contaminants it might contain. There are many tracers and techniques that allow us to estimate the age-or mix of ages-of the groundwater we depend on as a drinking water supply.
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Tracing and Dating Young Ground Water
Featured Study: Shale-gas production and groundwater quality Groundwater age indicates that it's too soon to fully assess effects of Marcellus Shale gas production on groundwater quality in the upland aquifer zone used for domestic supply. Learn More. Featured Study: Simple method to classify groundwater age A new USGS report documents a simple method to classify groundwater age as premodern recharged beforemodern recharge in or lateror a mix of the two.
Try the Tools Within the same aquifer, groundwater that is shallow and near the recharge area is younger than groundwater that is deep or that has moved far from the area where recharge occurs.
Groundwater Age Mixtures and Contaminant Trends Tool : Use the GAMCTT tool to explore how basic aquifer properties and well configurations affect groundwater age mixtures in groundwater discharge and on contaminant trends from nonpoint-source contaminant input scenarios.
Age dating of groundwater
This tool will be released in Date published: March 2, Filter Total Items: Year Select Year Apply Filter. Contacts: Bruce Lindsey. Attribution: Water Resources. Date published: March 1, Contacts: Laura M Bexfield. Contacts: Leslie A DeSimone. Date published: February 28, Contacts: Paul Stackelberg. Contacts: Sandra Eberts. Date published: February 27, Follow the links below to publications that discuss research that uses groundwater age dating.
Year Published: Tritium as an indicator of modern, mixed, and premodern groundwater age Categorical classification of groundwater age is often used for the assessment and understanding of groundwater resources. Lindsey, Bruce D. View Citation. Lindsey, B.
Sep 12, Groundwater age-dating simplified Release Date: September 12, A new USGS report documents a simple method to classify groundwater age as premodern (recharged before ), modern (recharge in or later), or a mix of the two. 3 H/ 3 He dating complements existing capabilities within the U.S. Geological Survey for dating of young ground water, such as, uses of chlorofluorocarbons and sulfur hexafluoride, and can be applied to dating water recharged since about If the mixture contains multiple fractions of young water, the groundwater age is groundwater as a mean age of the young fraction s in the mixture. Effects of dating can be more significant when waters are sampled from relatively large intervals in aquifers.
Geological Survey Scientific Investigations Report - 18 p. Stackelberg, Paul E. Year Published: Using groundwater age distributions to understand changes in methyl tert-butyl ether MtBE concentrations in ambient groundwater, northeastern United States Temporal changes in methyl tert-butyl ether MtBE concentrations in groundwater were evaluated in the northeastern United States, an area of the nation with widespread low-level detections of MtBE based on a national survey of wells selected to represent ambient conditions.
Year Published: A partial exponential lumped parameter model to evaluate groundwater age distributions and nitrate trends in long-screened wells A partial exponential lumped parameter model PEM was derived to determine age distributions and nitrate trends in long-screened production wells. Jurgens, Bryant C. Year Published: Educational webtool illustrating groundwater age effects on contaminant trends in wells No abstract available.
McMahon, P. Use of classes based on redox and groundwater age to characterize the susceptibility of principal aquifers to changes in nitrate concentrations, to ; ; SIR; ; McMahon, P. Year Published: Estimates of tracer-based piston-flow ages of groundwater from selected sites-National Water-Quality Assessment Program, This report documents selected age data interpreted from measured concentrations of environmental tracers in groundwater from 1, National Water-Quality Assessment NAWQA Program groundwater sites across the United States.
Hinkle, Stephen R. Year Published: A comparison of recharge rates in aquifers of the United States based on groundwater-age data An overview is presented of existing groundwater-age data and their implications for assessing rates and timescales of recharge in selected unconfined aquifer systems of the United States.