OGW Tech Memo 95.02 Attachment To: " , WRD Archive File, Reston, VA "
cc: "Velvie E Stockdale, Office Automation Assistant, Reston, VA " Subject: OGW Tech Memo 95.02 Attachment Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Date: Fri, 28 Feb 1997 13:56:02 -0500 From: "Velvie E Stockdale, Office Automation Assistant, Reston, VA " NOTE: The ^ that is throughout this attachment means to superscript whatever follows, some of the numbers that were supposed to be subscript could not be shown as such. Attachment 1 Request for CFC Analyses Today's date:__________ Name of contact:__________________ E-mail:________________________ Phone No:__________________ FAX:_______________________ Description of job (where?. project?, purpose of CFC dating?): ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Shipping Address:_________________________________ _________________________________ _________________________________ _________________________________ _________________________________ Account number for billing:________________________ Dates of sampling:____________________________ When to ship equipment:__________________________ When equipment and ampules well be returned:__________________ Estimated number of wells:___ Type of wells:____monitor;____domestic;____municipal;____other Well diameters:____2';____4';____6';____other;______ Range of well depths:_________________feet Materials in well construction:____PVC;____metal;____other Pump(s) to be used:___________________________________________ Material in discharge line of pump:__________________ Equipment to be sent: ____CFC sampler;____Spare parts;____Trap;____Ampule holder ____Ampules; ____Connector to well; ____other:____________ Number of ampules to ship: __ Are dissolved gases (N^2 and Ar) to be sampled: ___; How many?____ Will CFC-surface water samples be collected?_____; If so, how many?____ Will CFC-air samples be collected?____; If so, how many? __ Other field samples and analyses:____^3H,____DO,____H2S ____CH4;____pH, cond. temp.; ____stable isotopes Do you need training on CFC sampling procedures? ___yes; ___no Do we need to send someone to help with training, sampling? ___yes; ___no Address and contact for Reston CFC Laboratory: Julian E. Wayland (703-648-5847) Bonnie Hower (secretary - 703-648-5838) FAX: 703-648-5832 U.S. Geological Survey 12201 Sunrise Valley Drive 432 National Center, Rm. 5B127A Reston, VA 22092 Attachment 2 PLANNING FOR CHLOROFLUOROCARBON STUDIES This document provides (1) basic information on uses of chlorofluorocarbons in hydrologic studies, (2) guidance on site selection and sampling procedures, and (3) policy and procedure for obtaining analytical services from the Reston CFC Laboratory. BACKGROUND Chlorofluorocarbons (CFCs) are relatively stable volatile organic compounds that can be used for tracing and age dating young ground water. CFCs were first produced in the 1930s as the refrigerant trichlorofluoromethane (CCl2F2, or CFC-12) followed by production of trichlorofluoromethane (CCl3F, or CFC-ll) in the 1940s, and trichlorofluoromethane (C2Cl3F3, or CFC-113) in the 1960s. Annual production of CFCs presently exceeds 0.4xl0^9 kg. CFCs have been produced for use as refrigerants, aerosol propellants, cleaning agents, solvents, and blowing agents in the production of foam rubber and plastics. No known natural sources of CFCs exist (Lovelock, 1971). All CFCs produced are eventually released to the atmosphere, and through the atmosphere they are partitioned into the hydrosphere by gas- liquid exchange equilibria. The 1990 atmospheric volume fractions of CFC-12, CFC-11, and CFC-113 were approximately 485,268, and 77 parts per trillion, respectively (on the NOAA scale). The first measurements of CFCs in ground water were reported by Thompson (1976) and Thompson and Hayes (1979), who used a gas chromatograph with electron capture detector (ECD) to analyze on site the CFC-11 concentration of ground water from southern New Jersey, Arkansas, and south-central Texas. Schultz (1979) confirmed the findings of Thompson and Hayes (1979) and used the CFC-11 observations to model the hydrology of the Edwards aquifer. CFCs have been used extensively in oceanographic studies to trace ocean currents and date water masses (Gammon and others, 1982; Bullister, 1984; Wisegarver and Gammon, 1988; Smethie, and others, 1988). The Water Resources Division began investigating uses of chlorofluorocarbons as tools for dating and tracing ground-water movement in 1988. The initial studies were conducted within the Central Oklahoma and Delmarva pilot NAWQA studies. These studies (Busenberg and Plummer, 1992; Dunkle and others, 1993; Ekwurzel and others, 1994) have demonstrated applications of CFCs as very sensitive tracers of young ground water (water recharged since the 1940s) and as tools for determining date of recharge of young ground water. Ground-water age based on CFCs has been used recently by USGS hydrologists to check and refine ground-water flow models (Hinkle and Snyder, 1993; Reilly and others, 1994; Szabo and others, 1996). CFCs are currently being used in a wide range of hydrologic investigations throughout the U.S. Plummer and others (1993) review principles and applications of CFCs and other environmental tracers for dating young ground water. Tracer applications CFCs are invaluable in a qualitative sense as tracers of recent recharge. The analytical detection limit of approximately 1 pg/kg for CFC-11 and CFC-12 in water allows identification of post-1940 water using CFC-12 and of post-1947 water using CFC-11. Because most ground-water samples are regarded as mixtures, the presence of detectable CFC-12 or CFC-11 indicates that the water contains at least some post-1940 or post-1947 water, respectively. Because of the extremely low detection limit for CFCs, it is possible to detect mixtures of as little as 0.01 percent modern water in pre-1940 ground water, and, if the source of recent water has been contaminated with CFCs from anthropogenic sources in addition to the atmosphere, as is observed in some urban areas, modern waters containing CFC concentrations at least several orders of magnitude less than 0.01 percent can be detected. Important uses of CFCs as hydrologic tracers are demonstrated by Thompson and others (1974), Schultz and others (1976), Randall and Schultz (1976), Davis and others (1980), and Davis and Bentley (1982). Schultz and others (1976) and Busenberg and Plummer (1992) show that CFCs are excellent tracers of effluent from sewage treatment plants. Dating applications Dating ground water by using CFCs takes advantage of the known solubility of CFCs in water, and historical data on past atmospheric concentrations of CFCs. Northern latitude tropospheric CFC concentrations have been reconstructed for the continental U.S. from 1940 to the present from manufacturing production data and atmospheric measurements (since the mid-l970s). In most cases gases dissolved in recharge waters are preserved in ground water due to low rates of molecular diffusion in water relative to recharge rate. Field and theoretical studies have shown that for relatively thin unsaturated zones (< 10 m), gas transport is rapid and the composition of unsaturated-zone air is virtually that of the troposphere. The ground-water recharge data can be determined independently for CFC-11, CFC-12, and CFC-113 from the measured concentration in ground water by comparing the calculated air equilibrium concentration with the known atmospheric concentrations of the CFCs. Gas solubilities in recharge waters depend on recharge temperature, which is the temperature at the base of the unsaturated zone when recharge occurs. As a first approximation, the recharge temperature can be taken as the local mean annual temperature. More accurate recharge temperatures can be determined from measurements of other dissolved gases, such as gas chromatographic measurements of dissolved nitrogen and argon or mass spectrometric measurements of dissolved noble gases. The recharge date refers to the date when the recharge water became isolated from the unsaturated-zone air. The ground-water age is the elapsed time between date of sampling and date of recharge. Under the best conditions, ground-water ages can be determined within +/-2 years or better. Ground-water ages determined from CFCs have been compared with ground-water travel times computed in MODPATH as part of the refinement of ground-water flow models (Hinkle and Snyder, 1993; Reilly and others, 1994; Szabo and others, 1996). Among the many potential uses of ground-water dating, ground-water ages determined from CFCs have also been used to determine local recharge rates (Dunkle and others, 1993; Reilly and others, 1994; Solomon and others, 1995), to determine the date of introduction of other solutes into a ground-water system (Bohlke and Denver, 1993), determine rates of degradation of pesticides and herbicides (Denver, and others, 1993), and test recharge mechanisms through deep unsaturated zones (Busenberg and others, 1993). FIELD SAMPLING A special apparatus that excludes contact with air is used for collecting water samples for CFC analysis. The samples are then flame-sealed into borosilicate-glass ampules for transport and laboratory storage prior to analysis. Using this sampling apparatus, water samples free of CFCs have been collected in the field, and after months of storage remained free of CFCs. The collection, transport and storage of water samples without contamination is a critical step to reliably date waters with CFCs, particularly for old waters that contain very low concentrations of CFCs. Special precautions are needed in the selection of the sampling pump, the pump discharge line, and the well-construction materials. Materials not suited for CFC-sampling include Tygon tubing, silicone rubber tubing, and in general most plastics, rubbers, greases and oils. Good results have been obtained from all-metal casings and PVC casings (when the latter casing sections have been threaded together without use of glue or other adhesives). In addition to sampling of monitoring wells with portable pumps, CFCs also have been successfully sampled from a variety of domestic, municipal and industrial wells. It is recommended that pump discharge lines be of metal construction. For portable pumps, best results have been obtained using refrigeration grade 1/4 inch copper or aluminium tubing threaded onto the pump. Other materials needed for CFC sampling include ultra-pure nitrogen (99.999%), welding-grade oxygen, and MAPP gas. Approximately 5 minutes is required for collection of a single sample. Setup and take down of sampling equipment totals about 30 minutes at a well. It is recommended that a total of 5 ampules be filled and sealed for each well. The Reston CFC Laboratory can provide further details and training on CFC sampling procedures, including methods of collection of surface waters, air, and unsaturated zone gas samples. Site selection Some environments are not well suited for CFC tracer or dating applications. For example, waters cannot be dated if concentrations of all three CFCs have been altered from that determined by equilibration of recharge water with air (for example, water recharged in 1992 at 9^oC would contain about 850 pg/kg of CFC-11, 360 pg/kg of CFC-12, and 85 pg/kg of CFC -113). Environments impacted by seepage or disposal of industrial wastes, or sewage effluent can have CFC concentrations that exceed ambient air-water equilibrium values by 3 to 5 orders of magnitude or more. Such waters cannot be dated using chlorofluorocarbons, and risk contamination of the analytical system. Although waters cannot be dated using CFCs if the CFC concentrations exceed that of ambient air-water equilibration, the mere detection of CFCs indicates that the water contains at least a portion of post-1940s water. It is often observed that although one CFC may be contaminated locally, other CFCs may be unaltered from air-water equilibrium yielding provisional recharge dates based on only one or possibly two of the CFC tracers. CFC-11, CFC-12 and CFC-113 are stable under aerobic conditions. Although still being investigated, it appears that CFC-11 can be degraded under sulfate-reducing conditions in ground-water environments and all CFCs may be degraded under methanogenic conditions. Potential for microbial degradation of CFCs under anaerobic conditions is in the order CFC-11>CFC-113>>CFC-12. Potential for sorption of CFCs on particulate organic matter is preferentially in the sequence CFC-113>CFC-11>>CFC-12. CFC ages will appear too old if the tracer is degraded or removed. Reliable CFC-12 ages have been obtained from several sulfate reducing environments (Dunkle and others, 1993; Katz and others, 1995; Cook and others, 1995). In one case, parts of a methanogenic environment (unpub. data from Valdosta, Georgia) have been dated using CFC-12. Environments rich in particulate organic matter can cause sorption of CFCs, particularly CFC-113 and CFC-11. Although some reducing and partially contaminated environments have been dated successfully using CFCs, the environments best suited for CFC dating have the following properties: . Located in rural settings where CFC concentrations are most likely controlled by air-water equilibrium only. . Relatively thin unsaturated zone, probably less than 10 m, where unsaturated-zone air is mixed rapidly and is similar to atmospheric air composition. . Oxic ground water where there is no evidence for microbial degradation of CFCs. After denitrification, microbial degradation of CFCs (particularly CFC-11) may begin. . Unsaturated zones with low organic matter content which minimizes the sorption of CFCs on soil organic matter during dry periods. . Waters that contain detectable concentrations of CFCs (that is, waters that contain at least a portion of post-1940 recharge). Samples that should not be submitted to the Reston CFC Laboratory The Reston CFC Laboratory is not licensed or equipped to handle radioactive samples and such should not be submitted for CFC analysis. Samples containing more than 0.1 ppm of any CFC have concentrations that are 4 to 5 orders of magnitude greater than the possible dating range. Such samples can contaminate the analytical system causing days of down time for cleaning the columns. Unfortunately, it is not always possible to know in advance whether waters contain very high concentrations of halocarbons. Potentially contaminated samples should be collected as normal VOC samples and submitted to the National Water Quality Laboratory for analysis, prior to submission for CFC analysis. Many other volatile halocarbons are also detected when running samples for CFC analysis, and high concentrations of these compounds can also contaminate the analytical system. These compounds include vinyl chloride, methyl chloride, methyl chloroform, methyl bromide, methylene chloride, chloroform, trichloroethylene, carbon tetrachloride, and tetrachloroethylene. Samples containing more than several ppb of any of these compounds should not be submitted to the Reston CFC Laboratory. ANALYTICAL PROCEDURES CFCs in ground water are determined in the Reston CFC Laboratory by use of a purge-and-trap gas chromatography procedure with ECD using methods modified from Bullister (1984) and Bullister and Weiss (1988). The gas chromatograph is calibrated at the beginning and end of each day using an air standard from Niwot Ridge, Colorado that has been calibrated against gravimetric standards prepared by NOAA (Elkins, 1989; Elkins, 1993). The sample is introduced into stripping chamber, and the CFCs are purged with ultra-pure N2 and quantitatively collected in a trap cooled to -30^oC. The CFCs are released when the trap is heated to 95^oC. The CFCs are separated from other halocarbons in a precolumn. After the CFCs have entered the analytical column, the precolumn is back flushed to remove all other highly retentive halocarbons. The concentrations of CFC-11 CFC-12 and CFC-113 in the water sample are calculated from the concentrations in the water sample, the water-sample temperature, and the volumes of water and headspace in the ampules. The detection limit is approximately 1 pg/kg. The uncertainty in the CFC analyses is approximately 3 percent for concentrations greater than 50 pg/kg. Analytical uncertainties increase for waters with CFC concentrations of less than 50 pg/ kg, approaching 50 percent at the detection limit of 1 pg/kg. A detailed description of the procedures can be found in Busenberg and Plummer (1992). SCHEDULING SAMPLING It is necessary to schedule CFC sampling at least several months in advance to assure availability of sampling equipment and to allow time to obtain cylinders of compressed ultra-pure nitrogen and welding-grade oxygen locally. The attached form 'Requests for CFC analyses' (Attachment 1) can be returned to schedule sampling. When making arrangements for CFC sampling of ground water, it is recommended that preliminary samples be taken from a deep presumably old water source in the study area in order to check the pump and sampling equipment that are used. This should be done more than 1 week in advance of full sampling. The ampules from the old well should be shipped to the Reston CFC Laboratory via overnight courier. These preliminary samples will be analyzed within several days to determine if the pump and sampling equipment produce suitable blanks. Costs The price is currently $200.00 per well. This price includes use of sampling equipment and ampules, consultation, analytical services, and preliminary interpretation of the data. The Reston CFC Laboratory will provide the borosilicate-glass ampules that have been sized to fit the sampling apparatus. It is recommended that 5 ampules be sealed per well. The Reston CFC Laboratory will analyze and report results for 3 of these ampules, and all 5, if warranted. For single ampules of surface water (only), the price is $70.00 per ampule. Special arrangements need to be made for air samples, such as unsaturated zone air, or local atmospheric air samples. Reporting of results Samples will be analyzed in the order they are received. Results will be reported to the originating project office electronically and by paper copy. On request, a Lotus 123 version 4.0 spreadsheet will be provided that allows testing of sensitivity of data to estimates of the uncertainty in recharge temperature and elevation. Normal reporting includes dissolved concentrations of CFC-11, CFC-12 and CFC-113 in pg/kg, and preliminary estimation of recharge date, assuming no processes other than that of air-water equilibration interfere with the CFC concentrations in the sample. Billing Charges for all samples received prior to approximately September 4, will be charged by Standard Voucher to current fiscal year funds. Samples received on or after September 4, will be charged to the next fiscal year. Standard vouchers will be prepared in Reston on or before September 8, with copy to the field office. Responsibilities of the Reston CFC Laboratory The Reston CFC Laboratory will provide the following: - Loan of sampling apparatus and spare parts. - Loan of molecular sieve trap with nitrogen regulator, used with ultra-pure nitrogen provided by the requester. - Use of shipping boxes for equipment. - Borosilicate glass ampules sized to fit sampler. - Dissolved gas sampling tubes, for determining recharge temperatures based on dissolved nitrogen and argon (optional). Cost per sample is $150. - Provide training in sample collection, if needed. - Analysis of CFC-11, CFC-12, CFC-113. - Preliminary interpretation of data and assignment of age, if possible. - Consultation on sampling procedures, use of equipment, and data. - Special arrangements can be made for on-site training in sampling procedures. Responsibilities of project requesting service - Purchase field gases. (1) Ultra-pure nitrogen. Two C-size tanks of ultra-pure, carrier grade, 99.999 percent pure or better nitrogen. Only one tank is necessary, the second tank being used as a backup. Several months may be required to procure C-size tanks of ultra-pure nitrogen. (2) Welding-grade oxygen. One C-size or 2 D-size tanks. Or, if using Benzomatic welding outfit, use canisters (camp stove type fittings available from larger hardware stores). (3) MAPP gas. Two canisters, available from hardware stores or plumbing supply stores. Warning: MAPP gas is recommended because it is fully in the gaseous state in the canister. This is a safety precaution that prevents the possibility of flammable liquids accidently being transferred into the hose to the torch. Although propane produces a suitable flame, it is liquified in the canister and is therefore not recommended for field use. This can create a very dangerous situation if the canister turns over during flame sealing of ampules. - Obtain necessary field tools. Essential tools include: 12 inch adjustable wrench (for attaching regulators to gas cylinders), 6 inch adjustable wrench (for attaching gas and water lines to sampling apparatus), needle nose pliers (for removing waste glass from sampler after sealing), Teflon tape (goes around mouth of ampule, if loose fit in sampler), set of Allen wrenches (if repairs are needed on sampler apparatus), spark igniter for torch, water squirt bottle for cleaning chips of broken glass from sampling apparatus. Other tools and supplies that are useful: misc. phillips and regular screwdrivers, electrical tape, 1/8 inch and 1/2 inch tube cutter, pliers, metal file. (Occasionally, it will be necessary to cut off worn ends of metal tubing connecting well water and ultra-pure nitrogen to the sampler, to make better seals.) - Pay all Federal Express shipping of equipment and ampules. All ampules are to be double-boxed, padding on top of ampules and shipped next-day delivery (Federal Express). The equipment can be shipped via 2-day Federal Express. The equipment and samples are too fragile to risk normal mailing. - Provide pump with refrigeration-grade 1/4 inch copper tubing, for field sampling from observation wells. Refrigeration- grade, 1/4 inch copper tubing is available from most plumbing- supply stores. Warning: use only refrigeration grade tubing, other grades of tubing have oil coatings inside and will surely contaminate CFC samples. The tubing is available in 50 foot rolls which need to be connected with 1/4 inch Swedgelok fittings. Alternatively, 1/4 inch refrigeration-grade aluminum tubing many be used. One supplier of 1/4 inch refrigeration-grade aluminum tubing is Supelco, located at Supelco Park, Bellefonte, PA 16823-0048. Phone 814-359-2784. Order 1/4 inch refrigeration-grade AL tubing, Cat. no. 20524, $24./50 feet. You will also need Swedgelok fittings to connect sections of 1/4 inch AL tubing. 1/4 inch refrigeration-grade copper tubing can be obtained at lower cost and is readily available from most plumbing supply stores. The Teflon tubing, standard on most sampling pumps, has memory effects, and once contaminated with CFCs, can be unusable for very long periods. When sampling for CFCs using portable pumps with existing Teflon discharge lines, it is best to replace the Teflon discharge line with 1/4 inch refrigeration-grade copper or aluminum tubing prior to sampling. Many of the commonly used portable pumps have been used successfully to obtain CFC samples. The Reston CFC Laboratory can provide some guidance in selection of appropriate sample pumps. - Provide torch, regulators, etc for sealing ampules. Suggest Benzomatic welding kit Model OX 2500 or equivalent (contains torch, hoses valves for canisters of oxygen and propane or MAPP gas, even a sparker). It may be advantageous to purchase an oxygen regulator and attach it to the oxygen line of the Benzomatic torch swap. In this way it is possible to use larger tanks of welding-grade oxygen which are readily available from welding supply stores and last a lot longer. Welding setups can also be put together from the following: 1) welding hose, which is available from welding supply stores, 2) Veriflo Corp. Air Gas torch, Cat. no. 2073B10 available from Thomas Scientific, or equivalent (warning other torches may not work with MAPP gas and oxygen, or may not make suitable size flame for sealing ampules), 3) MAPP (MPS) regulator (part no. 806/7014) made by Control Corp. of America, Virginia Beach, VA. (call them (804-422-8330) for location of supplier nearest you), 4) standard oxygen regulator available from a welding supply store, and 5) probably best to have all this equipment assembled by a professional welding supply store. - Pay travel/per diem for in-field training. - Return sampling equipment promptly, so it can be checked out and sent to another user. - Return unused ampules. - Replace any damaged field sampling equipment (N2 regulator, valves in sampler, shipping boxes). COLLECTION OF SUPPORTING DATA Whenever CFC studies are conducted, it is highly recommended that additional data be obtained that can be used to help interpret the CFC results. These include: (1) During well purging and CFC sampling, temperature, specific conductance, dissolved oxygen and temperature should be recorded at approximately 5 minute intervals. (2) Because CFCs can be degraded microbially under anaerobic conditions, it is important to obtain some indication of redox conditions. Field measurement of dissolved oxygen is particularly important. Field personnel should at least note if hydrogen sulfide odor is detected, or preferably measure dissolved hydrogen sulfide concentrations in the field using a spectrophotometer. Measurements for dissolved methane can also be used as a further indication of redox conditions. (3) Tritium measurements are extremely useful in separating ambiguities in ages among the various CFCs and confirming the results. (4) If recharge temperatures are uncertain, they should be determined by gas-chromatographic measurement of dissolved nitrogen and argon in the sample. An analytical service for dissolved gas analyses and determination of recharge temperature can be obtained through the Reston CFC Laboratory at a cost of $150 per sample (plus shipping costs). (5) Tritium/helium-3 age dating provides an additional tool for checking the CFC results. This dating tool is especially useful in environments where CFC concentrations deviate from that established by air-water equilibration. For example, tritium/helium-3 measurements can be used to date waters contaminated with CFCs or highly reducing waters where the CFCs may have been removed by microbial degradation or sorption processes. The National Water Quality Laboratory (NWQL) maintains a contract for tritium/helium-3 dating. Contact Ann Mullin at the NWQL for the information on tritium/helium-3 dating at 303-467-8235. CONTACT AND ADDRESS FOR RESTON CFC lABORATORY: Julian E. Wayland (703-648-5847) Bonnie Hower (secretary - 703-648-5838) FAX: 703-648-5832 U.S. Geological Survey 12201 Sunrise Valley Drive 432 National Center. Rm. 5B127A Reston. VA 20192 REFERENCES CITED Bohlke, J.K., Revesz, K., Busenberg, E., Deak, J., Deseo, E., 1993. A 5O-year record of halocarbon contamination of ground water by the Danube River, NW Hungary. Geol. Soc. Am. Abstract with Program, 25, A-365. Bullister, J.L., 1984, Atmospheric chlorofluoromethanes as tracers of ocean circulation and mixing: Studies in the Greenland and Norwegian seas, Ph.D. dissertation, 172p., Univ. Calif., San Diego, La Jolla. Bullister, J.L., and Weiss, R.F., 1988, Determination of CCl3F and CCl2F2 in seawater and air, Deep Sea Res., 35, 839-854. Busenberg, E., and Plummer, L.N., 1992, Use of chlorofluorocarbons (CCl3F and CCl2F2), as hydrologic tracers and age-dating tools: Example - The alluvium and terrace system of central Oklahoma. Water Resources Research, v. 28, 2257-2283. Busenberg. E., Weeks, E.P., Plummer, L.N., and Bartholemay, R.C., 1993, Age dating ground water by use of chlorofluorocarbons (CCl3F and CCl2F2], and distribution of chlorofluorocarbons in the unsaturated zone, Snake River Plain aquifer, Idaho National Engineering Laboratory, Idaho. U.S. Geological Survey Water-Resources Investigations 93-4054, 47p. Cook, P.G., Solomon, D.K., Plummer, L.N., Busenberg, E., and Schiff, S.L., 1995, Chlorofluorocarbons as tracers of groundwater transport processes in a shallow, silty sand aquifer. Water Resources Research, 31(3), 425-434. Denver, J.M., Shedlock, R.J., and Bohlke, J.K., 1993, Application of ground-water dating to water-quality assessment of the Delmarva Peninsula, Delaware, Maryland and Virginia, United States. Geol. Soc. Am. Abstract with Program, 25, A-365. Dunkle, S.A., Plummer, L.N., Busenberg, E., Phillips, P.J., Denver, I.M., Hamilton, P.A., Michel, R.L., and Coplen, T.B., 1993, Chlorofluorocarbons (CCl3F and CCl2F2) as dating tools and hydrologic tracers in shallow ground water of the Delmarva Peninsula, Atlantic Coastal Plain, United States. Water Resources Research, v. 29, no. 12. p. 3837- 3860. Ekwurzel, B., Schlosser, P., Smethie, W.M., Jr., Plummer, L.N., Busenberg, E., Michel, R.L., Weppernig, R., and Stute, M., 1994, Dating of shallow groundwater: Comparison of the transient tracers ^3H/^3He, chlorofluorocarbons and ^85Kr. Water Resources Research, v. 30, No. 6, p. 1693-1708. Elkins, J.W. ed., 1989, Geophysical Monitoring for Climatic Change, -Summary Report (1988) Boulder, Colo., National Oceanic and Atmospheric Administration, No. 17, 142 p. Elkins, J.W., Thompson, T.M., Swanson, T.H., Buffer, J.H., Hall, B.D., Cummings, S.0., Fisher, D.A., and Raffo, A.G., 1993. Decrease in the growth rates of atmospheric chlorofluorocarbons 11 and 12. Nature, 364, 780-783. Gammon, R.H., Cline, J., and Wisegarver, D., 1982, Chlorofluoromethanes in the Northeast Pacific Ocean: Measured vertical distributions and application of transient tracers of upper ocean mixing. J. Geophys. Res., 87C, 9441-9454. Hinkle, S.R., and Snyder, D.T., 1993, Comparison of chlorofluorocarbon dating with particle-tracking results of a regional ground-water model: Portland Basin, Oregon and Washington. Geol. Soc. Am. Abstract with Program, 25, A-366. Katz, B.G., Lee, T.M., Plummer, L.N., and Busenberg, E., 1995, Chemical Evolution of groundwater near a sinkhole lake, northern Florida. 1. Flow patterns, age of groundwater, and influence of lakewater leakage. Water Resources Research 31(6), 1549-1564. Lovelock, J.E., 1971, Atmospheric fluorine compounds as indicators of air movements: Nature, v. 230, p. 379. Plummer, L.N., Michel, R.L., Thurman, E.M., and Glynn, P.D., 1993, Environmental Tracers for age-dating young ground water, in Alley, W.M., ed., Regional Ground-water Quality, Chap. 11, Van Nostrand Reinhold, New York, p. 255-294. Randall, J.H., and T.R. Schultz, Chlorofluorocarbons as hydrologic tracers: A new technology, Hydrol. Water Res. Ariz. Southwest, 6, 189-195, 1976. Reilly, T.E., Plummer, L.N., Phillips, P.J., and Busenberg. H., 1994, Estimation and corroboration of shallow ground-water flow paths and travel times by environmental tracer and hydraulic analyses - A case study near Locust Grove, Maryland, 1994, Water Resources Research, v. 30, No. 2, p. 421-433. Schultz, T.R., J.H. Randall, L.G. Wilson, and S.N. Davis, 1976, Tracing sewage effluent recharge- Tucson, Arizona. Ground Water, 14, 463-470. Schultz. T.R., 1979, Trichlorofluoromethane as a Ground-Water Tracer for Finite-State Models. Ph.D. Dissertation, Tucson, AZ, University of Arizona. Smethie, W.M., Jr., Chipman, D.W., Swift, J.H., and Koltermann, K.P., 1988, Chlorofluoromethanes in the Arctic Mediterranean seas-Evidence for formation of bottom water in the Eurasian Basin and deep-water exchange through Fram Strait. Deep-Sea Research, v. 35, p. 347-369. Szabo, Z., Rice, D.E., Plummer, L.N., Busenberg, E., Drenkard, S., and Schlosser, P., Age-dating of shallow groundwater with Chlorofluorocarbons tritium/helium-3, and flow-path analysis, southern New Jersey coastal plain. Water Resources Research, 32(4), 1023-1038. Thompson, G.M., 1976, Trichloromethane: A New Hydrologic Tool for Tracing and Dating Groundwater, Ph.D. Dissertation, Dept. of Geol., 93 pp., Indiana Univ., Bloomington, Indiana. Thompson, G.M., and J.M. Hayes, 1979, Trichlorofluoromethane in groundwater: A possible tracer and indicator of groundwater age. Water Resources Research, v. 15, p. 546-554. Thompson, G.M., Hayes, J.M., and Davis, S.N., 1974, Fluorocarbon tracers in hydrology. Geophysical Research Letters, v. 1, p. 177-180. Wisegarver, D.P., and Gammon, R.H., 1988, A new transient tracer: Measured vertical distribution of CCl2FCClF2 (F- 113) in the North Atlantic subatic gyre. Geophys. Res. Letter, 15, 188-191.