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
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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:__________________


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


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.


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 

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

   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

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 

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.


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.


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 

  - Consultation on sampling procedures, use of equipment, and 

  - 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).


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

   (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.

                 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-

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.

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