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:__________________
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
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