Environ. Sci. Technol. 1987, 2 1 , 909-916
F. Enuiron. Sci. Technol. 1984, 18, 428. Pierson, W. R. Lawrence Berkeley Lab Report LBL-9037; Lawrence Berkeley Laboratory: Berkeley, CA, 1979; pp
Eichmann, R.; Neuling, P.; Ketseridis, G.; Hahn, J.; Jaenicke, R.; Junge, C. Atmos. Enuiron. 1979, 13, 587. Eichmann, R.; Ketseridis, F.; Schebeske, G.; Jaenicke, R.; Hahn, J.; Warneck, P.; Junge, C. Atmos. Environ. 1980,14, 695. Simoneit, B. R. Atmos. Enuiron. 1984, 18, 51. van Vaeck, L.; Broddin, G.; van Cauwenberghe, K. Biomed.
216-220.
“EPRI-1092Identification and Quantification of Polynuclear Organic Matter on Particulates from a Coal-Fired Power Plant”; Oak Ridge National Laboratory: Oak Ridge, TN, June 1979. Dutcher, J. W.; Sun, J. D.; Lopez, J. A,; Wolf, I.; Wolff, R. K.; McClellan, R. 0. A m . Znd. Hyg. Assoc. J. 1984, 7,491. Warnatz, J. In Soot in Combustion Systems; Lehaye, J., Prado, G., Eds.; Plenum: New York, 1983. Faegri, K.; Iverson, J. Textbook of Pollen Analysis; Hafner: New York, 1964. Begeman, C. R.; Collucci, J. M. S A E Trans. 1970, 79, No.
Mass Spectrom. 1980, 7 , 473. “1982 Air Quality Plan for the St. Louis Metropolitan
Area-Transportation Element. Appendices”;East-West Gateway Coordinating Council: St. Louis, March 1982. Caw, G. R.; Boone, P. M.; Macias, E. S. In Particulate Carbon: Atmospheric Life Cycle; Wolff, C. T., Klimisch, R. L., Eds.; Plenum: New York, 1982; pp 207-243. “1982 Point Source Emission Inventory for St. Louis City and Country”;Missouri Department of Natural Resources, Division of Air Quality: St. Louis, MO, 1982. “Compilationof Air Pollutant Emission Factors”, 3rd ed, including Supplements 1-7; US.EPA Research Triangle Park, NC, 1977; Document AP-42. Boyer, K. W.; Laitinen, H. A. Enuiron. Sci. Technol. 1975, 9, 457. Henderson, T. R.; Sun, J. D.; Li, A. P.; Hanson, R. L.; Bechtold, W. E.; Harvey, T. M.; Shabanowitz,J.; Hunt, D.
700469.
Asinger, F. In Paraffin Chemistry and Technology; Pergamon: New Yofk, 1968. MacKay, D. Bobra, A.; Chan, D. W.; Shiu, W. Y. Environ. Sci. Technol. 1982, 16, 645. Received for review August 19, 1986. Accepted May 22, 1987. This work was partially funded by NZH Grant RR00954 for the Washington University Mass Spectrometer Facility.
Tritium as a Tracer for the Movement of Surface Water and Groundwater in the Glatt Valley, Switzerland Peter H. Santschi,
*pt
Eduard Hoehn,t Alfred Lueck,? and Klaus Farrenkothent
EAWAG, Swiss Federal Institute for Water Resources and Water Pollution Control, CH-8600 Dubendorf, Switzerland, and EIR, Swiss Federal Institute for Reactor Research, CH-5303 Wurenlingen, Switzerland
A pulse of tritiated water (-500 Ci) accidentally discharged by an isotope processing plant in the Glatt River Valley, northern Switzerland, allowed us to observe the migration of a contaminant pulse through a sewage treatment plant, rivers, and various wells of infiltrated groundwater. The accident pointed to various memory effects of the tritium, which acted as a conservative tracer. Tritium concentrations in surface water and groundwater were used to test predictions for the transport of conservative anthropogenic trace contaminants accidentally discharged into the sewer system. Mass balance calculations indicate that about 2-10% of the tritium pulse infiltrated to the groundwater and about 0.5% of the total reached eight major drinking water wells of this densely populated area. In spite of the complex hydrogeology of the lower Glatt River Valley, tritium breakthrough curves could be effectively simulated with modeling approaches developed from an experimental well field. Introduction Controlled releases of small amounts of radioactivity into the environment are common. However, accidental releases are relatively rare. In many instances, the fate of radionuclides inadvertently released is not adequately monitored. Exceptions to this can be found in ref 1-3, which modeled the distribution of such isotopes as 3H, 93r, or 13’Cs in aquifers contaminated from waste disposal sites. The purpose of this paper is to document an accidental release of tritium into the aquatic environment of a densely populated catchment basin in northern Switzerland. An ’EAWAG. EIR. 0013-936X/87/0921-0909$01.50/0
isotope processing plant accidentally released a pulse of tritiated water (400-600 Ci), providing an opportunity to study the migration of this conservative trace in the environment. From the plant, the contaminant migrated through a sewage treatment plant and subsequently entered river water. Eventually, the tritium partially infiltrated into an aquifer that underlies the river. This incident caused no significant health consequences in either the human population or the ecosystem as a whole. Calculations of the mass balance and migration rates of the tritium plume in surface water and groundwater will be useful in two ways. (1)Any anthropogenic trace contaminant accidentally released into the environment would show a behavior either similar to that of the tritium pulse (conservative substances) or slower (nonconservative substances). (2) It shows the usefulness of extending modeling approaches and parameters derived from a simpler and more defined system (i.e., an infiltration test field) to the more complex situation of various drinking water wells situated along the lower Glatt River Valley. Of general interest is the question of the types of simple models that can be used to simulate the migration of the tracer plume in the different reservoirs. Since present-day water resources are increasingly endangered by pollution from anthropogenic activities, more detailed information is needed on water flow and water exchange parameters. For surface waters, these parameters can often be derived directly from water budgets. In the case of river water infiltration into groundwater systems, the mean linear with x = infiltration groundwater flow velocity V , (=x/tW, distance and t w = mean residence time) and the macroV,, with D = hydrodynamic disdispersivity AL (=D/ persion coefficient) are essential modeling parameters that are often derived from observations of natural or artificial
0 1987 American Chemical Society
Environ. Sci. Technol., Vol. 21, No. 9, 1987
909
A T M 0
‘-I
S
P H E R E
I 1
R I V E R
I
t
Figure 1. Schematic of a typical situation after an accidental release of a contaminant into the environment. The contaminant pulse WIIIbe modulated by a number of reservoirs coupled in series or in parallel to the main flow path. This memory effect is a non-steady-state feature: at 0 < t < t w of the system, y = x - x’ z - z’ u - u’ v - v’ w - w’ > 0; at t >> t , of the system, y = 0.
+
+
+
+
tracer concentrations. Unfortunately, for groundwater residence times on the order of months to years, there exists no suitable natural radiotracer. Those traveling with the water itself have radioactive half-lives that are too long to be useful on the given time scale (4). Thus, experiments using artificial radioactive or nonradioactive tracers are needed. The accidentally discharged tritium pulse provided such an experimental tracer. Tracer pulses are carried away from the injection point by surface waters and will be modulated by any reservoir through which they are transported (Figure 1). Diffusion losses to sediments and atmosphere as well as exchange processes with additional reservoirs coupled in series may act on any chemical substance, further delaying the response in the water downstream. This can lead to memory effects in parts of the system, which are caused by the slow leakage a t later times from some of these reservoirs, where parts of the tracer pulse had been retained (Figure 1). These memory effects, if deconvoluted, can reveal system behavior. Such effects were observed following the accidental discharge of tritium described here. Methods Most of the tritium was released to surface waters in the form of tritiated water (HTO), which could be measured by a standard liquid scintillation technique, with proper corrections for background and quenching. Background activities in the natural waters of Switzerland are 100-200 pCi/L, predominantly determined by the atmospheric bomb tests conducted between the early 1950s and 1962. Description of Incident On December 13, 1983, during a cleanup operation of a storage room a t an isotope processing plant, 30 supposedly empty tritium (3H2) gas bottles of 3.3-L volume were cut open and rinsed with water. Tritium, which might have been deposited as HTO on the oxidized walls of the bottle or on the valve during the 15-20 years of storage, was released into the processing plant’s holding tanks. Routinely taken samples of the holding tanks later allowed the company to estimate accurately the quantity of tritium released. The significant quantities of tritium found 1-2 days later on floor and furniture surfaces and in the air of the processing plant’s storage room indicated that considerable quantities of tritium were also liberated into the room directly and subsequently vented to the outside. The person who cut open the 30 bottles had incorporated a total of 10 mCi during the few hours of the cleanup operation. Urine samples of other workers indicated that they had incorporated tritium as well, but to a lesser extent (Figure 3a). 910
Environ. Sci. Technol., Vol. 21, No. 9, 1987
The extent of this unexpected tritium release was not detected until 1 day after the spill. By that time, the tritium, first discharged into the processing plant’s holding tanks, had already reached the sewage treatment plant and the surface waters. Most of the discharged tritium was then quickly carried away by the Glatt River. However, small amounts of tritium remained in the two anaerobic digester tanks, as they exchange with the three clarifier basins of the treatment plant a t considerably smaller rates. Water sampling started 3-6 days following the tritium released, and the pulse was strong enough to be traced in the drainage basin of the Glatt and Rhine Rivers for 1-2 months within a reach of more than 100 km and in groundwater for more than 2 years. Description of the Catchment Basin The drainage basin of the Glatt River and the locations of the sampling stations are shown in Figure 2. The area of the catchment basin is 240 km2, the average discharge of the river is 8.5 m3 s-l (4-15 m3 s-l) ( 5 ) , and its flow velocity is 0.4-1.2 m s-l. The discharge rate of the Glatt River on December 12-19, 1983, was 4.5 m3 and was fairly constant. A total of 15-20% of this discharge is effluent from a number of secondary sewage treatment plants (6). The Glatt River drains into the Rhine River near Rheinsfelden. The average discharge of the Rhine River in Base1 is 1000 m3 s-l. The Glatt River meanders down the valley; on its way it infiltrates continuously and permanently to the groundwater between wells 1 and 8, including an existing groundwater test field of EAWAG (7). No significant lateral inflow exists from other tributaries along the 12-km reach between wells 1 and 8. From the concentration of some trace organic contaminants it was known, for example, that the groundwater of infiltration origin makes up a significant fraction of the total groundwater a t well 8 (Ruteli 11, Glattfelden). The hydrogeology of the lower Glatt Valley was examined by Freimoser and Locher (8) (see Table I) and features a perialpine granular outwash aquifer of glaciofluvial origin and quaternary age. The aquifer is heterogeneous and consists of layers of well-permeable gravel and sand. The grains are poorly sorted. The lower confining bed consists of poorly permeable fine-grained molassic sediments, glacial till, or lacustrine clays. South of well 1 (Hofstetten, Oberglatt) the aquifer intercalates with less permeable terminal moraines and pinches out. North of well 8 (Ruteli 11, Glattfelden) the aquifer enters the Rhine Valley (Figure 2 and Table I). Results and Discussion Sewage Treatment Plant. The main pulse of tritium passed quickly through the sewage treatment plant, giving an exponential decrease in activity with an initial resldence time of 1 day (Figures 3d and 4c). However, residual activity was released for a much longer time than what would be expected from the initial washout rate. We therefore attempted to model the tritium concentration in the outflow of the treatment plant using a simple twobox model consisting of the treatment plant’s three clarifier basins as box one (with subscript I; box one has a total volume VI of lo4 m3 and a flow rate QI of 0.9 X lo4 m3 day-l) and the two anaerobic digester tanks as box two (with subscript 11; box two has a total volume VI, of 3400 m3), coupled in parallel. Water from the digester tanks is continuously recycled into the clarifier basins at a constant flow rate of 58 m3 day-l (QII) (Figure 4); such a rate would fill box two in about 60 days.
Table I. Hydrogeological Data of t h e Wells” well
b, m
d, m
K , m/d
1
(1) Hofstetten Oberglatt (2) Hofstetten, Niederglatt (3) Hori, Saali (4) Schulhaus, Hori (5) Hirslen 11, Bulach (6) Hochfelden (7) Herrenwies, Bulachb (8) Ruteli 11, Glattfeldend
11
1.2 1.6
220 670
1.2 1.2 1.0
86 40 40
0.0024 0.002 0.0013 0.002
2 1.6
300 300
5 18 20 12 54 14 19
Qn,
m3/d
0.01
7 11 2 2 5
0.0025 0.01
20 90
x, m
350 350 1000 100
200 40 45OC 500
t60,
130 52 1800 250 75 120 33
a Interpreted from results of Freimoser and Locher (8). b = average saturated aquifer thickness; d = well diameter; F = bd; K = vertically averaged horizontal aquifer hydraulic conductivity, calculated from single-well pumping tests; i = regional groundwater hydraulic gradient; q = iK,Q, = Fq (Darcy’s law) = natural flow through the well; x = length of flow path in groundwater between the losing river and the well, estimated from flow lines; tbO= estimated mean residence time = x p / q ; p = porosity 0.2. Well with horizontal screens. cThe flow distance in the saturated aquifer was estimated from the lines of equal potential. However, the horizontal distance from the river is only about -50 m. dIdentical with well 9 in Hoehn and Santschi (9).
-
The differential equations describing the concentrations of tritium in the output (Gout) of the treatment plant and in the digester tanks (CIJ are
with k, = water exchange coefficient for digester tanks = QII/VI, = 0.017 day-l, k2 = kl(VII/VJ, k, = (k,+ kJ/2, and X = water exchange coefficient for clarifier tanks = QI/VI = 0.9 day-’ (Figure 4b). The solution to these equations assuming a Dirac A function input (i.e,, a very sharp input pulse) can be found with standard techniques for solving linear differential equations (e.g., matrix algebra or Laplace transformations). The dilution curve [Cout/Cout(0)]becomes a function of the exchange rates A, kl, and k2 of the two boxes: (3) C,ut(t) = C O U t ( 0 ) ( D + E )/ (N - M) with
D = N exp(-Nt) - M exp(-Mt) E = kl[exp(-Mt) - exp(-Nt)]
(4)
(5)
and N = A + S
M=A-S
+ k,,J2 - XhJl” A = X/2 + k,
S = [(X/2
(8)
(9)
By using the appropriate parameters for water exchange rates and volumes for this treatment plant, a satisfactory fit could be reached to the tritium data in the outflow. Since X >> k, > h2, C ( t ) decreases approximately with exp(-At) a t small values of t; a t large values of t, C ( t ) decreases approximately with exp(-k,t) (Figure 4c). The initial residence time of about 1day for the passage of the first tracer pulse increased later to about 60 days. This increase is, of course, the consequence of the slow release of tritium initially trapped by the two anaerobic digester tanks. Deviations of up to a factor of 2 from the model curve are the result of (a) irregular flow rates causing a further dilution of the tritium input from the digester tanks during peak flow rates and (b) a second but smaller tritium input of about 5 Ci from the waste discharge of the isotope processing plant on December 26-27 (Figure 3 and modeled in Figure 4c). A value of 520 f 100 Ci can be obtained for the total amount of tritium passing through a sewage treatment plant (Table 11) calculated from the
Table 11. Mass Balance of Accidentally Discharged Tritium in Different Reservoirsn Ci from analysis of activities of holding tanks outflow of sewage treatment plant Glatt River at Rheinsfelden Rhine River at Reckingen Rhine River at Rheinfelden/Laufenburg Rhine River at Basel (Village-Neuf) O 1
480 f 10 520 f 100 450 f 50 410 f 90 660 f 160 400 f 110
Ci = 3.7 X l O l o Ba.
daily tritium measurements (Figure 3c) and flow records after December 16, 1983. Transport through Surface Waters. Tracing of the tritium pulse through the watershed (Figures 1 and 3) was accomplished through the use of the continuous monitoring stations of the Swiss National River Surveillance Project (NADUF) and of the Swiss National Radioactivity Surveillance Project (KUER). Routine sampling by these two projects allowed us to establish a mass balance for tritium (see below). In the Glatt River, the tritium pulse was carried downstream a t flow rates similar to those described in Gujer et al. (5): 10-20 h to travel the 30-km distance from the injection point to the confluence with the Rhine River. As this time period is shorter than the sampling intervals of these surveillance projects, our measured tritium response curves are not sensitive to short-term variations in this rate. However, the slow exchange in additional reservoirs coupled in series delayed the response in the Rhine River downstream from Rheinfelden. For example, the concentrations of tritium in flow-proportional weekly samples, taken from the Rhine River at Village-Neuf near Basel, were nearly constant for December 12-26,1983 (Figure 3). Several impoundments for hydroelectric plants along the Rhine River must have caused this delayed response. By using the integrated activities and the flow rates a t the different sampling stations along the Glatt and Rhine River, the total amounts passing through these two rivers were calculated as 410-660 Ci (Table 11). Infiltration of Tritiated Water into Groundwater. The tritium concentrations shown in Figure 3f depict the breakthrough responses in eight of the monitored drinking water wells. Most responses were found to be bi- or multimodal, probably because of the flow through aquifer layers of different hydraulic conductivity. The range of the mean residence times of tritium between the Glatt River and the wells between 4 months and 2 years (Table 111) reflects an engineering practice of searching for the optimum between (a) the requirement of maximum steady yield of groundwater of infiltration origin and (b) the reEnviron. Sci. Technol., Vol. 21, No. 9, 1987
911
REKlNG EN (13km)
t--o
SE
NW RiVER GLATT
(100 km)
-
DIRECTION OF GROUND WATER FLOW
0
5
GROUND WATER LEVEL
*
10m
SAMPLING STATIONS: 0 WASTE WATER
0SURFACE
WATER
0 PRECIPITATION 10 krn
1 - 0
0
ISOTOPE ROCESSING PLANT
Flgure 2. Selected sampling stations in the drainage basin of the Glatt River in northern Switzerland: (1) Oberglatt (Hofstetten), (2) Niederglatt, (3) Hoeri (Saali), (4) Hoeri (Schulhaus), (5) Buelach (Hirslen I, 11) (6) Hochfelden, (7) Buelach (Herrenwis), (8) Glattfelden (Rueteli 11), (9) Glattfelden (Rueteli I), and (10) Glattbrugg.
Table 111. Analysis of Selected Breakthrough Curves”
vw,
well
tw, d
tw2, lo3 d2
m d-’
(1)Hofstetten Oberglatt (2)Niederglatt (3)Hori Saali (4)Schulhaus Hoeri,* (5)Hirslen I1 Bulach (6)Hochfelden (7) Herrenwis Bulach (8) Ruteli I1 Glattfeldenc
278 352 461 554 240 482 134 158
77.3 124 213 307 57.6 232 18.0 25.0
1.3 1.0 2.2 (0.2Id 0.8 (O.l)d 3.4 3.2
U?,
mlQet
lo3 d2
pCi d m-3
21.6 44.8 25.6 33.9 29.9 20.7 6.4 13.0
135 236 1011 691 640 640 421 240
m3 d-’
m , Ci
f = 100(m/M)/(Q,/Q,), % infiltrated river water
354 877 278 8.45 79.9
0.05 0.21 0.28 0.006 0.05
10 20 80 55 50
1.06 0.22
35 20
QW
2530 914
“tw = mean travel time, determined from t at d t / S t D dt = 0.5;m / Q , = concentration amplitude (area under the breakthrough curve, eq 12);ut2 = temporal variance of the tritium breakthrough; Q, = effective discharge at the wells = eQ, + [Q,(l- e ) ] ;e = fractional pumping time; Qp = pumped groundwater, (m3 d-l); Q, = natyral flow, see Table I; m = total tritium activity appearing in the well = ( m / Q e ) Q eM; = total tritium activity in river = QrJ”CrlVeI dt; Q, = effective discharge of river (m3 d-l). *Extrapolated to 1380 days after December 13,1983. Well 9 in Hoehn and Santschi (9). dValue for x (Table I) is most likely too low.
quirement of absence of harmful surface-water bacteria. Comparison of Residence Times: Darcy’s Law vs. Tritium Breakthrough. The tritium-derived residence times (Table 111) lead one to ask how well one would have predicted the residence time by using existing hydrogeological data and applying Darcy’s law. 912
Environ. Sci. Technol., Vol. 21, No. 9, 1987
The residence times resulting from the tritium breakthrough (Table 111) did not compare well with those estimated by Darcy’s law (Table I) except for well 7. The tritium values for wells 1 and 5 are about twice the maximum estimates from Table I, possibly because the value for the hydraulic conductivity ( K )was estimated too low.
URINE
' Ib '
2b
Wwxer Yo
'
Ib ' January
' 30 December 1983
lo6'
WASTE WATER TREATMENT PLANT
o
1
2b
1
infloi
' 3b
I
'
lb
' 2b
IO2' '
'
lb
' o; ' 30 December 1983
I
' Ib
'
2b
' 30
1
January
'
2b
lb ' February
'
February
G R O U N D WATER WELLS
"t'
Hofstetten (Oberglatt) Saali (Hoerl) Hirslen (Bulachj Herrenwis (Bulach) Rueleli 2 (Glattfelden)
0
o A
x t
GROUND WATER WELLS 0
Niederglatt Schulhaus (Hoeri)
A
Hochfelden
0
4.
..
SURFACE WATER
l
n Glatt
R ' v e , , I! R h e i P s l e l d e n
0
Rhino R , r e r , a l R e i ngeP
A
R h i n e River, a! V'Ilage-Neul
a
U V
10"
'
io ' 20 ' December 1983
io
I
'
lb ' 20 January
'
3b 1
'
lb
8
' 20
February
Flgure 3. Time dependency of the measured tritium concentrations in (a) human urine, (b) precipitation around the isotope processing plant, (c) waste water treatment plant, (d) surface waters, (e) groundwater wells (with smooth breakthrough responses), and (f) groundwater wells (others) at the sampling stations shown in Figure 2.
The misfit for well 3 remains unexplained. The value for K for well 8 given in Table I probably represents the aquifer layer responsible for the distinct first peak of the breakthrough curve. An average value for the groundwater flow velocity of wells 1, 3, 5, 7, and 8, where the data on hydraulic conductivities are more reliable, has been calculated to be 2.2 m/day (Table 111). Analysis of Infiltration of Tritium-Containing River Water to Drinking Water Wells. In an attempt
to simulate the tritium breakthrough in the drinking water wells, we adapted an earlier analysis of the tritium data from the nearby experimental well field (9). At this site, the recharging river is regarded as a line source in the horizontal plane for the freshly infiltrated water. Concentration gradients in the horizontal tranverse and in the vertical direction could be neglected, which allowed for a one-dimensional analysis of the breakthrough responses. We assumed the hydrogeological situation at the well sites Environ. Sci. Technol., Vol. 21, No. 9, 1987
913
,NFL@%
SCREEN
1
PRIMARY CLARIFIER
BIOLCGmCAL REACTOR
Y
W RFMnVAl GRIT
..
. SLUDGE PRIMARY+
"
I AhAEROEIC DIGESTERS
0
CIGESTEC PRIMARY
SAMPLING POlhTS
A
0 9 d-'
k, = 0.017 d-', k2 = 0.006d-l
TRITIUM A C T I V ' T I
- - j
- F L O W ?ATE
10'
1 10 '981
Io'
1984
Flgure 4. Tritium pulse through the sewage treatment plant: (a) simplified flow diagram of the waste water treatment plant, (b) two-box model to simulate the resulting tritium concentration in the treatment plant's effluent, and (c) model simulations of the tritium concentrations in plant's effluent. The model simulations assume constant flow (solid line) or variable, actual flow conditions and new inputs of 5 Ci of tritium on December 29, 1983 (broken line).
to be similar to the one a t the experimental site. The model prediction of ref 9 is based on the second temporal moments. One advantage of the method of moments (10-12) lies in the fact that the dispersivity is given independently from the advection-dispersion theory (14) and no curve-fitting procedure is necessary. A relation between the temporal variance of the breakthrough and a constant dispersivity is given in terms of a temporal differentiation as (10) AL = (Vw/2)(dut2/dt) (loa) Assuming Vw to be constant and neglecting second-order terms a t large values of t, eq 10a is equivalent to ut2 = 2Dtw/Vw2 = 2Dtw2/(xVw) (10b) The observation of tritium breakthrough in the test field revealed a linear dependence of ut2 on tW2,rather than on tw (9). From the slope of a plot of ut2 vs. tw2,one can calculate the macrodispersivity AL. From eq 10b it follows that AL is linearly related to the residence time tw by AL = ntw (Ild with n = (Vw/2)(dut2/dtw2)= Vw/Pe (llb) P e = x/AL = Peclet number (ref 13) (llc) ( d u t 2 / d t 2 ) = 2D/(Vwx) = 2AL/x = 2/Pe ( l l d ) 914
Environ. Sci. Technol., Vol. 21, No. 9, 1987
The observations from the drinking water wells agree with the observations from the test field. We simulated the breakthrough responses with a solution to the advection-dispersion equation (14), corresponding to the initial and boundary conditions as discussed previously (9): C ( x t ) = (rn/Q)(x/tW)(4nDt)-l/*exp[-(x - Vw)2/4Dt] (12) The model curves of the breakthrough in the drinking water wells in Figure 5 used two different values of AL. A slope n in eq l l a of 0.37 (equivalent to Pe = 6) was given by ref 9 for the test field. A slope n of 0.11 (equivalent to Pe = 20) was calculated from a combination of the values of the test field with those of the drinking water wells. The acceptable match of the simulations with the data verifies the model. The responses a t wells 4 and 6 have not been analyzed because they did not exhibit smooth responses. The length of the flow path between the river and the wells was taken from Table I. These values, estimated from flow lines given in ref 8, are probably too low for wells 2, 4, and 6. The peak values of the model curves are too high for wells 7 and 8, probably an effect of the model's inability to simulate early peaks. The tracer mass constituting for early peaks in wells 7 and 8 is of the same order of magnitude as the peak overshoot of the model. The problematics of modeling multimodal tritium distributions in the layered aquifer is discussed in more detail in Hoehn and Santschi (9). Mass Balance Calculations. As was previously mentioned, the calculated total amount of released tritium of 400-600 Ci (Table 11) agrees well with the value determined by the company itself, which used the tritium activities in the holding tanks and their volumes, i.e., 480 Ci (15). As is often the case, budgeting is most accurate a t the source and becomes less precise the farther away from it the samples are taken. Most of that tritium activity must have left Switzerland in Base1 via the Rhine River. Small amounts of tritium were also measured in the urine of factory workers (1-10 mCi each, Figure 3a), in the rain and snow in 100-m radius around the isotope processing plant (5100 mCi, Figure 3b), and in the two anaerobic digester tanks of the sewage treatment plant (51Ci, Figure 4a). Loss of tritium by vapor exchange and by exchange with various groundwater reservoirs cannot be determined from the river mass balance as those losses are likely within the errors of our calculation. The amounts that had infiltrated into the groundwater, i.e., 10-50 Ci, could be determined directly from the integrated activities of tritium in the various wells, assuming a total groundwater recharge of 0.01-0.04 m3 m-2 h-l (7). Well pumping rates and integrated activities were used to calculate the total tritium activities in the drinking water wells (Table 111). The total tritium activity of the seven monitored drinking water wells was equivalent t o about 0.4% of the total tritium activity in the river (Table 11). The highest amount was found in well 7 where it amounted to about 1 Ci, which is about 0.2% of the total amount of tritium. This implies that the drinking water supply stations of these communities in the lower Glatt valley are quite exposed to dissolved contaminants from the river. The effective discharge by pumping water from the wells is 1-2 orders of magnitude higher than the natural flow through the monitored wells, resulting in a large fraction of water originating from the infiltrating river. Drinking water wells can be recharged by the river or by more remote sources. Our tritium data allow us to estimate the importance of the river recharge to the
HOFSTETTEN
@ (OBERGLATTl
TIME RUETELI 2
@ (GLATTFELDEN)
TIME
[days]
Figure 5. Observed tritium breakthrough and model predicted curves in selected drinking water wells. Solid line = model curve using A , = 0.37tw and broken line = model curve using A , = O . l l t w (see eq 1l a , and text). (a) Hofstetten Oberglatt (1); (b) Hoeri Saali (3); (c) Hirslen I1 Buelach (5); (d) Herrenwis Buelach (7); (e) Rueteli I 1 Glattfelden (8).
drinking water supply, under water flow conditions similar to those that have occurred during the incident. If we compare the integral of the breakthrough curves in the wells and in the river, we can estimate the fraction of drinking water that originates from the river This is equivalent to the appropriate ratios of the respective total tritium masses to their respective flow rates or, more specifically, the ratio of the amount of tritium found in each well (m)to the total amount in the river (M) divided by the ratio of the water pumped in that well (Q,) to the total water discharge of the Glatt River a t the point of infiltration (QJ:
(n.
f
= (m/M)/(Qe/Qr)
(13)
While Q, applies to the time (years) of tritium infiltration into drinking water wells, Q, has to be taken for the time (month) of tritium transport by the river. We thus calculate that up to 1 0 4 0 % of the drinking water in these wells originate from the river (Table 111). These estimates o f f are probably upper limits for other conservatively behaving pollutants under steady-state conditions, as the river flow rate during the incident was below average. However, measurements of the concentrations of anthropogenic tri- and tetrachloroethylene (16) indicate also that
more than 40% of the drinking water must be of river origin in some of these drinking water wells. Nevertheless, the consequences and the dose rates to the general population resulting from this accidental release were found to be insignificant (15). Dispersion and admixture of tritium-poor water with the contaminated effluent resulted in dilutions of HTO to activities that never exceeded 1%of drinking water standards.
Conclusions and Implications for t h e Fate of Soluble Trace Contaminants Infiltrating to Groundwaters From an analysis of tritium concentrations in various reservoirs following an accidental pulsed release of tritiated water into a sewer line, we conclude the following: (1)Three different reservoirs (coupled in series or parallel) along the flow path took up some of the tritium and released it more slowly later after the main pulse had passed through. Such memory effects were most noticeable for (a) the sewage treatment plant (initial residence time of 1 day), where the two anaerobic digester tanks retained about 1 Ci (residence time of about 60 days), (b) the groundwater aquifer in the drainage basin of the lower Glatt valley, retaining 10-50 Ci (residence times of about 2 months to 2 years), and (c) several empoundments for Environ. Sci. Technol., Vol. 21, No. 9, 1987 915
hydroelectric plants that delayed the tritium pulse by 1-2 weeks. (2) Three different estimates of the amounts of tritium released into surface waters agree on a value of approximately 500 Ci: (a) holding tanks of the isotope processing plant (480 Ci), (b) outflow of sewage treatment plant (520 Ci), and (c) four different sampling stations along the Glatt and Rhine Rivers (410-660 Ci). (3) The drinking water supply stations of these communities in the lower Glatt valley are quite exposed to dissolved trace contaminants from the river because a large fraction of the water in the wells is derived from infiltrating river water (i.e., 10-80%). The well activities account for about 0.4% of the total tritium activity in the river (m in Table 11). (4) The quantities of infiltrating tritium (under pulse input conditions) and other conservative trace contaminants (under steady-state conditions) in the various drinking water wells could be on the order of 0.01-0.2% of the total tritium or contaminant mass being transported by the river, a t similar water flow and pumping rates as have occurred during the incident. (5) Tritium concentrations in the outflow of the sewage treatment plant and the water that infiltrated into the aquifer could be effectively simulated with simple modeling approaches. The modeling of the drinking water wells benefited from current and previous experiments in the more controlled situation a t the infiltration test site a t Glattfelden. Flow velocities ranged from 0.8 to 3.4 m/day, with an average of 2.2 m/day, similar to those in the experimental test field (9). The one-dimensional modeling approach consisted of using (1)the second temporal moment of the concentration distribution in combination with the advection-dispersion equation and (2) the linear dependency of the macrodispersivity (AL) on the residence time (tw) as revealed by our data and those of Hoehn and Santschi (9) from the experimental test field, which is a consequence of the constancy of dat2/dtw2. The latter is equivalent to 2/Pe (Pe = Peclet number = 20 here). Provided hydrogeologic conditions remain the same, this model approach allows predictions for longer distance and time scales than those observed here. Acknowledgments We gratefully acknowledge the assistance of T. Laufenburger (SUVA), who related to us the exact circum-
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stances of the incident, E. Werth, H. J. Hueppi, and S. Bollhalder, who provided support for some of the tritium measurements, and 0. Wanner and P. Reichert, who reintroduced P.H.S. into the technique of Laplace transformations. The critical suggestions to early versions of the manuscript by H. R. von Gunten, D. Imboden, B. Honeyman, and P. Huggenberger and the critical comments of two anonymous reviewers were helpful. Registry No. HTO, 13670-17-2. Literature Cited Robertson, J. B. In Isotope Techniques i n Groundwater Hydrology; IAEA: Vienna, 1974; Vol. 2, pp 451-478. Ryan, B. J.; Kipp, K. L. Low-Level Radioactive Contamination f r o m a Cold-Scrap Recovery Operation; U.S. Geology Survey: Wood River Junction, RI, 1985; Open File Report 84066. Pickens, J. F.; Jackson, R. E.; Inch, K. J. Water Resour. Res. 1981, 17(3),529-544. Moser, H.; Rauert, W. Isotopenmethoden in der Hydrologie; Matthess, G., Ed.; Gebr. Borntrager: Berlin, 1980; 400 pp. Gujer, W.; Krejci, V.; Schwarzenbach, R. P.; Zobrist, J. Gas, Wasser, Abwasser 1982, 62(7),298-311. Schwarzenbach, R. P.; Giger, W.; Hoehn, E.; Schneider,J. P. Enuiron. Sci. Technol. 1983, 17(8),472-479. Hoehn, E.; Zobrist, J.; Schwarzenbach, R. P. Gas, Wasser, Abwasser 1983, 63(8), 401-410. Freimoser, M.; Locher, Th. Eclogae Geol. Helu. 1980, 73(1),
251-270. Hoehn, E.; Santschi, P. H. Water Resour. Res. 1987,23(4), 633-640.
Fischer, H. B. Air Water Pollut. 1966, 10, 493-452. Fischer, H. B.; List, E. B.; Koh, R. C. Y.; Imberger, J.; Brooks, N. H. Mixing in Inland and Coastal Waters; Academic: New York, 1979. Fried, J. J.; Combarnous, M. A. In Advances in Hydroscience; Chow, V. T., Ed.; Academic: New York, 1971; p 169.
Sauty, J. P. Water Resour. Res. 1980, 16(1), 145-158. Bear, J. Hydraulics of Groundwater; McGraw-Hill: New York, 1979.
KUER “Bericht der Eidenossischen Kommission zur Ueberwachung der Radioaktivitat der Schweiz”;Bundesamt fuer Gesundheitswesen: Bern, 1984. Giger, W.; Schwarzenbach, R. P.; Hoehn, E.; Schellenberg, K.; Schneider,J. K.; Wasmer, H. R.; Westall, J.; Zobrist, J. Gas, Wasser, Abwusser 1983, 63(9),517-531. Received for review June 23, 1986. Revised manuscript received March 24, 1987. Accepted J u n e 10, 1987.
