The objectives were to develop a non-volatile tracer to use in gas exchange experiments in laterally unconfined systems and to study applications of deliberate tracers in limnology and oceanography. Progress was made on both fronts but work on the development of the non-volatile tracer proved to be more difficult and labor intensive that anticipated so no field experiments using non-volatile tracers was performed as yet. In the search for a suitable non-volatile tracer for an ocean scale gas exchange experiment a tracer was discovered which does not have the required sensitivity for a large scale experiment, but is very easy to analyze and will be well suited for smaller experiments such as gas exchange determinations on rivers and streams. Sulfur hexafluoride, SF6, was used successfully as a volatile tracer along with tritium as a non-volatile tracer to study gas exchange rates from a primary stream. This is the first gas exchange experiment in which gas exchange rates were determined on a head water stream where significant groundwater input occurs along the reach. In conjunction with SF6, Radon-222 measurements were performed on the groundwater and in the stream. The feasibility of using a combination of SF6 and radon is being studied to determine groundwater inputs and gas exchange of rates in streams with significant groundwater input without using a non-volatile tracer

Bopp, R.

Broecker, W. S.

Ledwell, J. R.

English

NASA Technical Reports Server

w.* -+* 
The Effect of Wind and Currents on Gas Exchange in an Estuarine System 
Final Technical Report 
W.S. Broecker 
J.R. Ledwell 
R. Bopp 
Period covered: 
August 1, 1986 to July 31, 1987 
Trustees of Columbia University 
City of New York 
Office of Projects and Grants 
Box 20 Low Memorial Library 
New York N.Y. 
10027 
Grant: NAGW 667 (supplement 2) 
[NASA-CR-Y81424) THE EFFECT OF UIND A l i )  N88-10468 
CURRENTS 01 G A S  EXCHANGE IN AN ESTUARINE 
SPSTER Final T e c h n i c a l  Report,  1 Ang., 1986 - 
31 Jnl. 1987 ;Columbia Univ.)  11 p A v a i l :  Unclas 
NTXS HC A O S I R P  A O t  CSCL 08C G3/48 0104401 
https://ntrs.nasa.gov/search.jsp?R=19880001086 2020-03-20T08:44:52+00:00Z
Final technical report for NASA grant NAGW-667 "The Effect of Wind and 
Currents on Gas Exchange in an Estuarine System 
INTRODUCTION 
The objectives of the proposal were to develop a non-volatile 
tracer to use in gas exchange experiments in laterally unconfined 
systems and to study new applications of deliberate tracers in limnology 
and oceanography. Progress has been made on both fronts but work on the 
development of the non-volatile tracer proved to be more difficult and 
labour intensive than anticipated so no field experiments using non- 
volatile tracers have been performed as yet. In our search for a 
suitable non-volatile tracer for an ocean scale gas exchange experiment 
we have discovered a tracer which does not have the required sensitivity 
for a large scale experiment but is very easy to analyze and will be 
well suited for smaller experiments such as gas exchange determinations 
on rivers and streams. 
Sulfur hexafluoride, SF6, was used successfully as a volatile 
tracer along with tritium as a non-volatile tracer to study gas exchange 
rates from a primary stream. To our knowledge this is the first gas 
exchange experiment in which gas exchange rates were determined on a 
head water stream where significant groundwater input occurs along the 
reach. In conjunction with SF6, 222Radon measurements were performed on 
the groundwater and in the stream. We are currently studying the 
feasibility of using a combination of SF6 and radon to determine 
groundwater inputs and gas exchange rates in streams with significant 
groundwater input without using a non-volatile tracer. 
DISCUSSION 
For gas exchange experiments in systems which are laterally 
unbounded, a non-volatile tracer is needed in addition to a volatile 
tracer in order to distinguish between concentration decrease due to 
lateral dispersion and concentration decrease due to gas exchange. Open 
systems which require further study are rivers and streams where 
biological oxygen demand can be increased drastically due to release of 
organic wastes and in which gas exchange of oxygen is the major 
mechanism of keeping the water "healthy". 
over the ocean are not well constrained and the parameters controlling 
the rates are poorly understood. Better knowledge of the gas exchange 
is essential to predict the impact of the ocean on the atmospheric level 
of "greenhouse gases" such as CO2 and CH4 . Our long term goal is to 
determine gas exchange rates in open ocean experiments and to relate the 
gas exchange to environmental forcing functions. 
Short term gas exchange rates 
A non-volatile tracer has to be stable in the water column, non- 
toxic, and measurable down to concentration of 10 -15 mol/L for ocean 
work. Although tritium would be ideal for such work it would be 
difficult to field this tracer since bomb produced tritium is still used 
extensively as an oceanic tracer, and unfavorable public opinion about 
using radio tracers in the environment. 
Our approach is to use a halogenated acid, extract it from the 
water phase, alkylate it to make it volatile, and analyze it by electron 
capture detection gas chromatography. This method of analysis is orders 
of magnitude more sensitive fo r  halogenated compounds than measuring the ' 
compounds in a liquid matrix. Moreover, using the appropriate reactants 
can increase the sensitivity of the derivitized compound for analyses. 
The initial compound which was tried was pentafluorobenzoic acid, 
PFBA which has been used successfully as a groundwater tracer 
(Stetzenbach et al., 1982). Esters were formed with methanol, using a 
BF3/methanol mixture (figure la) ; with Bromo-2,3,4,5,6- 
pentafluorotoluene, using an extractive alkylation technique with 
tetrabutylamonium hydrogen sulfate (figure lb); and using trichloro 
ethanol. The last two methods will yield esters with additional halogen 
atoms thereby increasing the sensitivity of the derivative. Since 
alkylation with a halogenated reactants will make most organic acids 
present in natural waters sensitive to electron capture analyses, many 
more peaks appear on chromatograms with associated problems of peak 
interference. The yields of the alkylation reactions appeared to be 
very low for all the reactants which is attributed to the strong acidity 
of pentafluorobenzoic acid acid ( pK=1.7). 
One of the compounds which was tried as an alternative was 
decafluorobenzhydrol (DFBH). DFBH can be extracted quantitatively from 
water with hexane and it can be analyzed directly in the organic phase 
without prior synthesis. DFBH has a distribution coefficient between 
sediments and water of about 20, and thus particle reactive which would 
be a problem for long term experiments but for short term tracer 
releases the loss rate will be negligible. DFBH can be detected down to 
moles. Due to the strong partitioning into the organic phase 
samples can be concentrated by extraction into a small volume of hexane 
and further concentration can be accomplished by partial evaporation of 
the hexane. Since decafluorobenzhydrol is not very soluble in water, 
the compound is dissolved in methanol prior to injecting a spike into 
water. 
The low solubility of the compound in water, the tendency to adsorb 
onto particles, and lack of extreme sensitivity limits the use of this 
tracer to short term lake or stream experiments. For ocean experiments 
we are currently investigating pentafluorophenylacetic acid as a 
possible choice. It is not as strong an acid as pentafluorobenzoic acid 
and the alkylation reaction appears to yield a higher and more 
reproducible yield. Although preliminary work has been very 
encouraging, more work remains to be performed before we can adequately 
assess the suitability of this tracer for oceanic purposes. 
A collaborative effort between Dr. P. Mulholland and Dr. G .  Elwood 
of the environmental science division of Oak Ridge National Laboratories 
and our group has shown the feasibility of using SFg for determining gas 
exchange rates in shallow head water streams. Prior stream work has 
concentrated on larger, slower moving streams with little groundwater 
1 
I 
i n f low a long  t h e  reach  us ing  85Krypton and t r i t i u m  (Tsivoglou, 1967) or 
propane and dye (Rathbun, 1979) a s  v o l a t i l e  and non-vo la t i l e  tracers, 
r e s p e c t i v e l y .  Although t h e  f i r s t  t r a c e r  p a i r  works very  w e l l ,  c u r r e n t  
use  i s  s e v e r e l y  limited due t o  t h e  radio'  a c t i v i t y  of  t h e  compounds. 
E s p e c i a l l y  "Kr i s  cons idered  an environmental  hazard .  
i sop ropano l  are n o t  completely conserva t ive  and t h e  compounds do n o t  
have a v e r y  low s e n s i t i v i t y  f o r  d e t e c t i o n .  I n  t h e  experiment i n  t h e  
W e s t  Fork of  t h e  Walker Branch i n  Oak R i d g e ,  Tn w e  used  SF6 and t r i t i u m  
a s  v o l a t i l e  and non-vo la t i l e  t r a c e r  and used t h e  222Rn concen t r a t ion  i n  
t h e  s t r eam t o  l o c a t e  groundwater i n p u t s .  Our u l t i m a t e  g o a l  i s  t o  use  
SF6 and 222Rn t o  de te rmine  gas  exchange c o e f f i c i e n t s  and groundwater 
i n f low r a t e  i n  streams without  having t o  r e s o r t  t o  a non-vo la t i l e  
tracer. 
Dye and 
I n  t h e  experiment  4 l i t e r  of water con ta in ing  1 0  x C i  of 
t r i t i u m  and 5 x moles of SF6 was i n j e c t e d  i n t o  t h e  s t ream over  a 
p e r i o d  of  f o u r  hours  such t h a t  a dynamic e q u i l i b r i u m  between inf low,  
d i l u t i o n  and escape of SF6 was reached. Samples w e r e  t aken  a t  10 rn 
i n t e r v a l s  a long  t h e  300 m reach.  F igure  2 shows t h e  i n c r e a s e  i n  f low of 
t h e  r i v e r  deduced from t h e  d i l u t i o n  of t h e  t r i t i u m  downstream of  t h e  
i n j e c t i o n  p o i n t .  The radon concent ra t ion  p r o f i l e  i n  t h e  s t r e a m  which i s  
i n f l u e n c e d  by h igh  radon groundwater input  and g a s  exchange shows 
i n c r e a s e s  a t  p o i n t s  of  h igh  inf low ( f i g u r e  3 ) .  Groundwater samples 
t aken  a t  s p r i n g s  had an average concen t r a t ion  of 495 p C i / L  wi th  a 
s t a n d a r d  d e v i a t i o n  of  25 p C i / L  which i n d i c a t e s  t h a t  a l l  t h e  groundwater 
i n p u t s  had s imilar  concen t r a t ions .  
I n  t h i s  experiment  t h e  gas  exchange c o e f f i c i e n t ,  k w a s  c a l c u l a t e d  
from t h e  change i n  r a t i o  between t h e  v o l a t i l e  and t h e  non-vo la t i l e  
tracer accord ing  t o :  
where h is  t h e  d e p t h  of t h e  stream; R i  and Rf a r e  t h e  i n i t i a l  and f i n a l  
r a t i o s  of v o l a t i l e  t o  non-vo la t i l e  t racer  i n  t h e  stream (see figure 4 ) ;  
and A t  is  t h e  time i n t e r v a l .  
change s i g n i f i c a n t l y  a long  t h e  reach al though t h e  s l o p e  and the  geometry 
d i f f e r e d  f o r  d i f f e r e n t  p a r t s  of  t h e  stream. 
c o e f f i c i e n t  w a s  27 cm/hr which i s  an o rde r  of  magnitude greater than  t h e  
ave rage  observed  i n  l a k e s  (Wanninkhof et  a l .  1987) .  S ince  t h e  i n p u t  
c o n c e n t r a t i o n  of  Rn i s  cons t an t  and t h e  ground water  does n o t  c o n t a i n  
any SF6, it shou ld  be p o s s i b l e  t o  deduce t h e  g a s  exchange ra te  and 
in f low of ground water  i f  t h e  inf low r a t e  of  water  s p i k e d  wi th  SF6 and 
2 2 2 ~  and  SF^ c o n c e n t r a t i o n s  a r e  measured a t  c l o s e l y  spaced i n t e r v a l s  
d u r i n g  t h e  experiment .  
(see f i g u r e  5 ) .  The inf low i n t o  t h e  "box' can be determined from t h e  
SFg i n  t h e  s p i k e  con ta ine r ,  t h e  inf low rate  of t h e  s p i k e  i n t o  t h e  s t r e a m  
and t h e  c o n c e n t r a t i o n  i n  t h e  s t r e a m .  The concen t r a t ions ,  area, and 
stream v e l o c i t y  can be measured which l eaves  t h r e e  independent  equa t ions  
and t h r e e  unknowns: t h e  ground water input ,  t h e  outf low,  and t h e  gas 
The gas  exchange c o e f f i c i e n t  did n o t  
The average  g a s  exchange 
This  can be i l l u s t r a t e d  wi th  a s i m p l e  box model 
. exchange ra te .  
f 
1 '  
This method was not used for the experiment in the Walker Branch 
since not enough radon measurements were performed in the stream and the 
radon measurements were taken two days prior to the spike release to 
avoid excessive handling of the tritiateh water. 
release a cloud burst caused the flow of the west Fork to double making 
comparison of gas exchange coefficients and concentrations before and 
during the spike experiment very ambiguous. 
The night before the , 
CONCLUSION 
Steady advances have been made in development of the non-volatile 
tracer. The synthesis and chromatography of the fluorinated compounds 
have been worked out. Although our initial choice of tracer, 
pentafluorobenzoic acid did not fulfill our  objective, the work has 
proved worthwhile in chosing other compounds. One of the compounds, 
decafluorobenzhydrol will be fielded shortly in a stream release to test 
the stability of the compound in nature. Work has started on another 
halogenated acid, pentafluorophenyl acetic acid with encouraging 
results. 
interference for natural water samples has not been fully resolved. 
Suitable extraction methods and chromatographic peak 
O u r  volatile tracer, SF6, has been used successfully in conjunction 
It appears to be feasible that by using 
with tritium to determine gas exchange coefficients in a primary stream 
with large ground water inputs. 
radon and SFg simultaneously the groundwater inputs and gas exchange 
coefficients in such streams can be determined without elaborate 
measurements of stream flow or  use a non-volatile tracer. Since non- 
volatile tracers do not degrade while volatile tracers escape to the 
atmosphere it is probable that opposition to injecting volatile tracers 
will be less than using non-volatile tracers. Furthermore this method 
gives a direct measure of the radon burden to the local atmosphere from 
streams. 
Continuation of the development of tracer methods for an open ocean 
experiment will be sponsored by the NSF chemical oceanography division 
and NASA oceanographic processes division with emphasis on establishing 
a relationship between gas exchange coefficients and radar back scatter 
form satellite scatterometers. 
PUBLICATIONS ACKNOWLEDGING GRANT: NASA-NAGW-667: 
Wanninkhof, R., J.R. Ledwell, W.S. Broecker, and M. Hamilton. 1987. Gas 
exchange 
on Mono Lake and Crowley Lake, California. In Press, J A g d I -  
REFERENCES 
Rathbun, R.E. 1979. Estimating the gas and dye quantities for modified 
tracer technique measurements of stream reaeration coefficients. 
Y.S.G.S. Water Resources 15 KFU 79 - 27. 
. Stetzenbach, K.J., S.L .  Jensen, and G.M. Thompson. 1982. Trace 
enrichment of fluorinated organic acids used as groundwater tracers 
by liquid chromatography. Emiron. Sci. Te-1. 16 , 250-254. 
Tsivoglou, E.C. Tracer measurements of stream reaeration. USDI, Federal 
water pollution control administration. Washington D.C., 1967. 
I 
I 
l l  
Y 
c 
- 
A 
n 
Figure la. Chromatograms of the PFBA methyl ester. 
A. Slank, extractton of organic molecules fron 1.66 nl  tap 
water and subseauent esterfication. 
E. Extraction of 1.7 ml of lo-' m l / l  PFBA in Nexane 
C. Extraction of 2 5  
2 u l  injections. Column 4 X SE-30. 6 X CN-210 cn A0/100 
Chromosorb W R .  Column tenperature 100 OC. flov rate 40 
nllmin. The ester elutes at 2.3 dnuter. Assuning I00 X 
yields and linear response the peak area on Chrooatogram C 
should he I n  times that of Chromatogram 8. The lower 
observed response in B is attributed to incomnlete reaction 
of PFBA in presence of excess hexane. 
of IO-' nol/l PFBA i n  Hexane 
01 
I- 
- ? ?  
0 - T  
Figure 1b. ChronatoErnm of the PFBA pentafluorohenzpl ester. The 
ester elutes at 4.6 mfnutes. 
interference described in the text since the solution is 
diluted down a factor of I O 3  after the reaction. 
at 3.2 ntnuten is excess a-hronn pentafluorotoluene. 
2 U I  injection. Column 4 Z SE-30, 6 X 01'-210 on BOllO0 
Chronosorh W R .  Colunn t-emperature 160 'C. flow rate 40 
nl/mln. 
This sample does not show the 
The peak 
N 
L 
I 
I 
10 I 
“9 
i 
\ 
fl\ u flow (11s) 
0 100 200 300 400 
dis tame( m) 
Figure 2. Flow of West Fork of Walker Branch, Oak Ridge Tn. during 
the tracer experiment. The flow is determined from the 
dilution of the tritium spike. The distance is measured 
upstream from a weir. The tracers were injected at 342 m. 
'C 
0 
C 
p! 
* 
I 
Walker Branch Radon 
500 1 
400 
0 
a 
0 
distance 
Figure 3 .  Radon concentrations in the stream. The concentrations 
increase at locations with high groundwater input. 
C 
, 
' i  
v)  w 
C c 
i 
-3 4 
I 
I n L'
I t l  - 0  
0 I 
I I 1 I 
, I 
0 .0 0.2 0.4 0.6 0.8 1 .0 t .2 1 .4 
travel time (hours) 
Figure  4 .  Change i n  r a t i o  of  SF6 t o  t r i t i u m  i n  t h e  s t ream. 
s l o p e  of t h e  l i n e  and t h e  average depth,  t h e  gas  exchange 
c o e f f i c i e n t  can be determined (see t e x t ) .  To o b t a i n  t h e  
t r a v e l  t i m e  of a water p a r c e l  an average stream flow of  225 
m/hr was used determined from t h e  r a t e  a t  which t h e  f r o n t  of  
t h e  t r a c e r  pa t ch  moved downstream. 
From t h e  
r b 
4 k x A x Rnav 
5 k x A x S F  6,8V 
+ 
I 0 
SF 
Rnl 
6,o SF 
3 91 Rn 
x d  
Flow balance I + G = 13 
Radon  balance I X Rn, + G x Kn, = 0 X Rn,, + k X A X RrlR,, 
SF6 ba lance  i x S F -  = 0 x SF6,@ + K x A x SF6,aV 
b,l 
x d t  
t
Figure 5. Diagram of how gas exchange coefficients can be determined 
using a gaseous deliberate tracer, such as SF6 and radon. 


The effect of wind and currents on gas exchange in an estuarine system

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