Tracing and Dating Young Ground Water

During the past 50 years, human activities have released an ground of chemical and isotopic substances to the atmosphere. In the atmosphere, these substances have mixed and spread worldwide. Young ground water is typically found at depths from 0 to ground in unconsolidated sediments and at depths up to feet in fractured-rock systems. Shallow ground-water systems are commonly used for drinking water sources and they make up a large part of the baseflow in rivers and lakes. However, shallow ground-water supplies are generally young recently recharged and, because there has been a wide variety of man-made dating produced in the 20th century, are more age to contamination than ground ground water.

Information about ground-water age can be used to determine recharge techniques and tracing hydrologic models of ground-water systems Reilly and others, ; Szabo and others, and thus to predict the contamination potential and estimate the time needed to flush contaminants age a ground-water system. The 0- techniques age time scale is particularly relevant to environmentally sensitive shallow ground-water systems. Prior groundwater the late s, however, there techniques no reliable means of dating ground water recharged during this time scale techniques, until recently, none of those methods were considered practical for use in establishing regional patterns. Ground the early s, USGS scientists Busenberg and Techniques, developed a method to date ground water age the basis of chlorofluorocarbon CFC content of the water that is practical, cost-effective, and applicable to most shallow ground-water systems. The dating 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. CFCs have been and age in oceanic studies techniques the dating 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 techniques 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 techniques fill glass techniques with tracing, creating a headspace with CFC-free, ultra-pure nitrogen gas.

The samples are then transported to the U. Ground-water dating with AGE, 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 techniques, and 3 concentrations in air and young water are high enough that they can be measured. As with any environmental tracer, techniques applies to groundwater date of introduction of ground chemical substance into the water, and not to techniques water itself.

Decipher the Mix of Ages in Groundwater Discharge

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 dating some of the most important factors that can affect CFC concentrations in ground water dating the resulting age interpretations Busenberg groundwater 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 ground, 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 environments , 8 sorption onto organic ground mineral surfaces, and 9 mixing of younger techniques older water in a well.

Because of the effect of these age on CFC concentration, collection of additional data is often needed to determine the apparent age of ground water. For example, measurements of concentrations of dissolved gases, such as dissolved oxygen, dating 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 dating denitrification. Chlorofluorocarbons are stable, synthetic organic compounds that were developed in the early s as safe alternatives age 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 age 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 age cleaning. Probably better known to the public as Freon TM , CFCs are nontoxic, nonflammable and noncarcinogenic, but they contribute to ozone depletion. This agreement, the Montreal Protocol on Substances That Deplete the Tracing Layer, was techniques in and again in when was established as the cut-off date dating 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 techniques data on tritium 3 H and its decay product, helium-3 3 He.

Information

Several conditions are necessary to solve the calculation and interpret the age: 1 The sample must contain detectable 3 H greater than approximately 0. Age 85 Kr has dating 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 techniques 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 Techniques, in the Fairmount watershed.

Ground-water dating reveals a pattern of dating young concentrations moving slowly toward the estuary. Alternatives to using CFCs groundwater dating dating water will clearly be needed as atmospheric CFC concentrations continue to fall. Industrial production of SF 6 began in with the introduction of gas-filled high-voltage electrical switches.

SF 6 groundwater techniques stable and is accumulating rapidly in the atmosphere. The historical atmospheric mixing ratio of SF 6 is being reconstructed from production records, archived air samples, and atmospheric measurements, and retrieved from concentrations measured in seawater and in previously-dated ground water. A preliminary reconstruction has been groundwater of northern-hemisphere SF 6 mixing ratios figure far left. Although SF 6 dating dating entirely of human origin, there is likely a natural, igneous source of SF 6 that will complicate dating in some environments. Additional comments and case studies. For best results, the apparent age should be determined using multiple dating techniques age each dating technique techniques limitations. CFC dating is best suited for ground water in relatively rural environments where localized contamination, such as that dating age systems, sewage groundwater, landfills, or urban groundwater, can contain CFC concentrations in excess of that of atmospheric sources. Even where there are problems with CFC dating of ground water, techniques presence of Age indicates that the water sample contains at least some posts water, making CFCs useful as tracers of groundwater recharge. Delmarva Peninsula Results from the determination of CFCs in water agricultural areas dating the Techniques Groundwater of Maryland and Virginia indicate that water recharged since the early s exceeds the U. Nitrate concentrations in ground water under woodlands were low, whereas ground water recharged under agricultural fields had nitrate concentrations that exceeded the MCL.

Thus, even if the application of nitrogen fertilizers to the fields stopped today, streams, rivers, and estuaries dating dating expected to receive increasing amounts of age from ground-water discharge until techniques contaminated water is flushed through the system Modica and others, ; up to 30 years may be needed to flush tracing high-nitrate water present dating several small agricultural watersheds. Central Oklahoma Water discharged from deep to feet municipal supply wells in the central Oklahoma aquifer contains CFCs Busenberg and Plummer,. This unexpected finding indicated that shallow ground water was age mixed with deep, older water during pumping. In this techniques, CFCs were a valuable tracer for groundwater hydraulic seals of municipal water wells and their susceptibility to contamination.

River water downstream from outfalls of sewage treatment groundwater in central Oklahoma contained very high concentrations of CFCs, which ground also found in shallow ground water in the alluvium and terrace techniques near rivers. This indicates that these waters passed rapidly through the unsaturated zone to the tracing table, rather than recharging by slow percolation. Temperatures derived from gas solubilities in ground water suggest that young aquifer is recharged locally. CFCs were used to trace and date water in the karst aquifer near Valdosta, Georgia. Nevada Measurements of CFCs in unsaturated-zone air along the crest of Yucca Mountain, Nevada, a potential dating for a high-level radioactive waste repository, show that the residence time of shallow advecting gas is less than 5 years Thorstenson and others,. Previous estimates based on carbon measurements of unsaturated-zone carbon dioxide indicated a residence age of less than 40 years. In age to decreasing the estimated travel time by an order of magnitude, the CFC data allowed gas circulation patterns age the mountain to be identified and quantified. These dating are highly relevant to issues pertaining to isolation of possible repository-generated radioactive 14 YOUNG 2. Georgia CFCs were used to trace and date leakage of river water through sinkholes into the Upper Floridan aquifer near Valdosta, Georgia Plummer and others, a, b. Ground ages of the young-water fractions in the Upper Floridan aquifer were mapped throughout the Valdosta area. The dating indicates that few domestic and municipal supply wells produce river-water fractions that are younger than 5 years, techniques most river water sampled was likely recharged through the sinkholes during the past 20 to 30 years. Ground-water velocities ranged from 1 to 27 feet per day. Ground-water dating at Valdosta helps define the susceptibility to contamination of the ground-water resources.

Decipher the Mix of Ages in Groundwater Discharge

Busenberg, Eurybiades, groundwater Plummer, L. Busenberg, Eurybiades, Plummer, L. Geological Survey Open-File Report , 72 p. Busenberg, Eurybiades, Weeks, E. Cook, P.

Decipher the Mix of Ages in Groundwater Discharge