Environ. Scl. Technol. 1993, 27, 2420-2428
Evidence of Terrestrial Discharge of Deep Groundwater on the Canadian Shield from Helium in Soil Gases Me1 Gascoyne’ and Marsha I. Sheppard AECL Research, Whiteshell Laboratories, Pinawa, Manitoba, Canada ROE 1LO
Assessment of the impact of deep geological disposal of nuclear fuel wastes at a site in the Canadian Shield requires knowledge of the location and size of areas of discharge of deep groundwater from the vicinity of the underground disposal vault. A strong He anomaly has been detected in soil gases in a 10 X 10 m area of wetland on the banks of Boggy Creek, near Lac du Bonnet, Manitoba. The area has He concentrations in near-surface soils as high as 360 nL.L-1 and is assumed to indicate discharge of He-rich groundwater through a permeable subsurface bedrock fracture. Elevated C1- concentrations in groundwater and its use as a “deer lick” support this interpretation. A He flux density of -2.1 L-m-2-a-1is determined from a depth profile of He concentrations at one location in the site. A total He flux of 270 Loa-’ is determined for the entire site, which corresponds to a deep groundwater discharge of about 26 000 Loa-1. This estimate is comparable with He fluxes and calculated groundwaterdischarges for two other lake-bottom locations on the Canadian Shield. Introduction
Canada’s concept for the disposal of nuclear fuel waste in a vault deep in the plutonic rock of the Canadian Shield (1)is being evaluated using an assessmentmodel (2).These models trace the potential release of radionuclides from the vault to the biosphere to assess possible health and environmental impacts. The understanding of the processes occurring at the geosphere/biosphere interface is crucial in these assessmentsbecause released radionuclides may be discharged to aquaticand possibly terrestrial areas. The two types of discharge involve different transfer pathways and can differentially influence predicted health and environment impacts. Evidence for discharge of deep groundwater to terrestrial areas, causing saline seeps or animal licks, has been reported (3). Most environmental assessments of deep disposal of nuclear fuel wastes presently being made worldwide assume that some (2)or all (4) of the groundwater discharges to a terrestrial environment. Clearly, the fraction of deep groundwater discharging terrestrially is an important model parameter which, unfortunately, is not well known. One reason for this is that techniques for detecting terrestrial groundwater discharges are imprecise and poorly developed. Here, we introduce a method for detecting terrestrial discharge of deep groundwater and apply it to show how groundwater discharge rates, data essential in establishing the fraction of terrestrial versus aquatic discharge in assessment models, can be estimated. Helium and radon (Rn) are the main gases produced in the subsurface in igneous rock formationsand are a product of radioactive decay of natural U- and Th-series isotopes. Helium is stable and accumulates with time, whereas Rn has a 3.8-d half-life and reaches a constant concentration in a closed system containing parent 2z6Ra. Both gases tend to diffuse out of their host minerals and, because of 2420 Envlron. Scl. Technoi., Vol. 27, No. 12, 1993
their solubilityunder pressure, accumulatein groundwater. In plutonic rocks, groundwater flow occurs almost exclusively along fractures, and extremely high levels of He and Rn have been observed in groundwaters at depth, especially in U- and Th-rich plutonic rocks (5, 6). Depending on flow conditions, both gases will tend to diffuse upward to the atmosphere where the fractures subcrop. In a groundwater recharge situation, the upflow of gases is limited, but in a discharge environment, high-concentration flows of He and Rn may enter the atmosphere or mix with soilgases. The processes of subsurface generation of He and Rn, and their migration and transport to the surface, are illustrated in Figure 1. The above principles have led to the use of gases in soil and shallow groundwater as a method of detecting subsurface bedrock fractures and for determining local hydrogeological conditions (7-9).Recent application of these methods at groundwaterrecharge and discharge sites near Lac du Bonnet, Manitoba (10) has shown that He is more useful than Rn in detecting subsurface fractures that discharge groundwater. This is because overburden in the area contains high levels of Rn, produced in situ by decay of U (and, hence, 226Ra),and He accumulates with time, unaffected by radioactive decay. During a program of research involving analysis of lakeand river-bottom waters to detect deep groundwater discharge (1I), elevated He concentrations were observed at a location in Boggy Creek, in the Whiteshell Research Area (WRA), near Lac du Bonnet. An adjacent wetland site, which was seen to be heavily used by deer, was subsequently surveyed using standard soil-gas sampling methods (12). Initial results indicated that He concentrations in the soil gases were up to eight times atmospheric levels in the center of the site. This anomaly was many times the size of the largest He anomaly previously observed in the WRA [up to two times atmospheric levels (1311. The general lack of bedrock outcrop and surface lineaments in the area prevented the application of geological information to infer discharge of deep (He-rich) groundwater at the site. Hence, a detailed geophysical, hydrogeological, and soil-gas study was performed to define the extent, magnitude, and cause of the anomaly. This paper describes the results of the soil-gas study to define the extent and magnitude of the anomaly. A comprehensivepaper describing all the geological aspects of the site is in preparation. Site Description and Methods
The study area is a small expanse of peat overlying clay, about 100 m X 100m in extent. It is covered by tall marsh grasses (Calamagrostis canadensis) and lies on a hummock sedge (Carex sp.) flood plain forming the east bank of Boggy Creek (Figure 2). The site is frequented by wildlife that appear to be eating soil and surrounding roots in a bare peat area at the center. Hence, the area is known as the “deer lick” [after the “moose licks” of northwestern
0013-936X/93/0927-2420$03.00/0
Publlshed 1993 by the American Chemical Society
Aquatic Discharge Area
Terrestrial Discharge Area
sol1 (I h,gh He(An) boltom
overburden (high Rn,noHel
He
+
gas
low He groundwal
t
I
Flgure 1. Schematic diagram showing the generation of He and Rn gases at depth in bedrock and their migration to the surface.
c4\
waterfall scattered
trees
:Igure 2. Map showlng location of the helium anomaly study site near Boggy Creek.
Ontario where the animals seek salt from groundwater springs (3)l. The center of the deer lick is generally very soft and wet, indicating that groundwater may be coming to the surface a t this point.
The preliminary soil-gas sampling of the site was performed using standard probe and gas pumping equipment (12). In 1991, however, the site remained close to saturation throughout the year, and this method could not be used. An alternative technique using ping-pong balls was tested instead (14). This technique was first developed by Dyck and Da Silva (15) for He analysis of lake sediments in the laboratory. It has been used successfully for in situ He analysis of lake sediments by Stephenson et al. (16). The technique involves placing a ping-pong ball at a standard depth in saturated soils or sediments for 4-7 d. Helium dissolved in soil pore water surrounding the ball equilibrates with the air inside the ball and attains a concentration representative of that of the saturated soil. The ball is then removed, immediately sealed in a water-filled container that is impervious to He, and transported to the laboratory for analysis. In this study, ping-pong balls were mounted in a hole bored through a 0.8-m-long 5 X 10 cm stake and retained by a nylon mesh stapled to the wood on each side of the hole. The stake was driven into the overburden to a depth of 0.5 m and left in place for at least 7 d. It was removed by pulling upward on a steel bar inserted through a small hole near the top of the stake. The equilibrated ball was stored in a tightly-capped, water-filled glass container (a ‘Mason’ jar) until analysis. This method of sampling is described in detail elsewhere ( 2 4 ) . To determine the extent of the He anomaly, two grids were laid out: an inner grid with stakes at 5-m intervals across the deer lick and surrounding grasses and an outer grid with stakes at 15-m intervals, extending to Boggy Creek and north and south across the whole site (Figure 3). Following the survey of the grids and examination of the results, a further sampling was made to determine the variation in He content with depth in the deer lick and to test the reproducibility of the ping-pong ball method. To do this, three long stakes (each consisting of two 2.5 m X Environ. Scl. Technol., Vol. 27, No. 12, 1993
2421
.
I I
N
60
I
break in slope r j
t
I I
,
outer grid \B
1
:A:+ FF8
EEB
HH8
008
t
e
b
b
b
i
I 1 1 1
b
.
bare p at area
AA6 b
inner grid + b
88% b
HS4
A.
b
b
b
b
b
a,
AA4
1
. jA . ....... . : I
b
b
b
b
e
884
CC4
I
4
'
HS1,2,3
2d
b
I
.
*
b
b
b
b
I
.
Dl
A1
HS5,6
01
4
I
b
'w
v
*
a
I I
V b
*
b
I
I
I I
)r
I
cn
I
0
0
I
m
I I I
I
(m)
n
0
15
30
Figure 3.
4 cm X 9 cm studs lashed together) were driven in to refusal in a 1m X 1m area surrounding site C4 (Figure 3). The stakes were drilled with cavities for ping-pong balls at intervals of 0.1 m in the upper 0.5 m, increasing to about 0.3 m for the remainder of the stake. Each stake was left in place for 7 d and then removed by a combination of digging and pulling with a tripod and pulley. The balls were removed and analyzed within 1-2 d. Helium concentration in the balls was determined by mass spectrometric analysis using a Veeco MS 18AB helium leak detector, converted to measure absolute He abundance with a specially designed gas inlet system (14). Laboratory air ([He] = 5.24 ppm) and a He standard in Nz (9.7 ppm) were used to bracket 10-mL samples of soil gas. Samples were withdrawn for analysis from the pingpong ball using a 10-mLplastic hypodermic syringe fitted with a 23-gauge needle inserted into the ball while held under water. An open-ended 18-gaugehypodermic needle was pushed into the bottom of the ball to allow water to replace the withdrawn gas sample. The response of the mass spectrometer to the atomic mass 4 He peak was obtained on a chart recorder and as an eye-averaged, digital voltmeter reading, connected in parallel with the recorder. Two 10-mLgas samples were obtained from each ball and analyzed together with standard gases in sequence. Analytical details are described in Gascoyne et al. (14). Analytical precision is typically within f0.1 ppm.
-
2422
Envlron. Scl. Technol., Vol. 27, No. 12, 1893
Helium concentration in the ping-pong ball was determined by comparing the relative peak heights of the chart traces (or voltmeter readings) and using the following equation, which corrects for drift and changes in sensitivity during the measurements: He,,
(ppm) = m(Z - S) + c,
(1)
and
m = (cs - ca)/(S
(2)
-(E)
where m is the slope of the relationship between machine response and He concentrations in the standard and air samples; ca and ca are He concentrations in standard gas (9.7 ppm) and in air (5.24 ppm), respectively, and $3, and & are the average peak heights of the sample, standard, and air traces, respectively. Helium concentration in the soil pore water surrounding the ping-pong ball is determined as follows: Hedisa(nL0L-I) = 0.00899Hebd
X
1000
(3)
The factor 0.00899 is derived from the ratio of the concentration of He in water in equilibrium with air (47.1 nL-L-' a t 10 "C) and He in air (Ca). For an unsaturated environment, He concentration in the soil gas is the same as in the ball and is determined as in eq 1.
If the sample was first diluted with air, as described above, the measured He concentration in the ball was corrected to allow for the fact that the diluent used (laboratory air) contains 5.24 ppm He, as follows: x 155) - (145 x 5.24) (4) 10 Samples of groundwater in the overburden were obtained from each of the piezometers (Figure 3) installed earlier for hydrogeologic measurements (17). The groundwater samples were filtered (0.45 pm) in the laboratory and split into aliquots for major anion and cation analysis (14). COIT
He,,
=
'
N 4
A7
D7
5 , q 3s;
11:
49
53
G7
(UnCOlT He,,
Results The results of analyses of soil-gassamples obtained from the inner and outer grids and the variations in He concentration are shown in Figure 4. With few exceptions, above-background He concentrations are confined to the area of the exposed peat and attain almost 360 nL-L-l water at site C4, the wettest part of the site. Helium concentrations in the soil pore water underlying the surrounding grassed area are close to that expected from equilibrium with the atmosphere (47 nL0L-l water), except for seven sites in the outer grid where He exceeds 100 nL-L-l water. Three of these sites, EE2, GG2, and HH6 (Figure 4b) are unrelated to one another or to the deer lick site and may be very localized He anomalies [duplicate analyses of the same ball are in good agreement (14)l.The remaining four sites are grouped to the northwest of the wetland area and are adjacent to three other sites where He lies between 50 and 100 nL-L-l water. Measurements of He concentrations at depth in the most anomalous part of the site, at C4, are shown in Figure 5. Comparable He concentrations to the inner grid survey at the 0.5-m depth can be seen at all three locations, but below this depth He concentrations increase dramatically. The south stake recorded He values about an order of magnitude greater than those at the north and west locations around C4. A maximum He concentration of 73 500 nL.L-1 water (- 1500times equivalent atmospheric levels) was observed near the base of this stake. Depth probing in the inner grid showed that depths to bedrock range from 1.8 to 2.6 m, with a gradual fall in bedrock elevation towards the west (14). Other than a shallow ridge running from southeast to northwest through the deer lick, there is no clear topographic feature in the underlying bedrock surface. The groundwaters sampled from piezometers in the area are all Na-Ca-Mg-HCO&O&pe waters, which are typical of overburden deposits in this area. However, they differ from other local overburden waters in that they contain significant levels of C1- (50-140 mpL-9. Discussion The anomalous He levels in soil gases and groundwaters at the 0.5-m depth in the Boggy Creek site are most clearly observed in the unvegetated part of the site, where they are as much as eight times greater than equivalent atmospheric levels. The anomaly at this site is considerably greater than the anomaly at the groundwater discharge location detected at a fully terrestrial location 4 km to the northeast, where He concentrationswere about twice atmospheric concentration in unsaturated ground (13). Similarly, anomalies of this magnitude are rarely
bare peat area
47
5
1
A1
4
57
8
4
8
5
'"1 51
0
5
8
7
7.2
tl
48
50
4
D1
4
8
G1 I---------(
1Om
Figure 4. Variatlons in He concentration (as nL.L-') in soil water at 0.5 m depth in (a) the inner grid, shown as point values and (b) the outer and inner grids, as contours.
found in the literature. In fact, most previous studies indicate that He anomalies in soil gases are very weak and are a t most 0.5 ppm above atmospheric levels, even above U ore bodies (summarized in ref 18). Adjacent to hot springs, however, significant soil-gas He anomalies have been found [up to 1000 ppm He in Colorado (1913, indicating that strong He anomalies are associated with deep groundwater discharge pathways rather than bedrock fracturing or U-Th mineralization. This interpretation best accounts for the He anomaly seen in this study and is supported by several other lines of evidence: (1)Deep groundwaters in the Lac du Bonnet granite have significant amounts of dissolved He, between 1and 56 mL-L-1 water, or up to 106 times the equilibrium level attained with the atmosphere (10). They are enriched in Envlron. Scl. Technol., Vol. 27, No. 12, I993 2423
O r 0 20
II
-.
-
40-. I-
2
,
IO
-
100
1
"
-
i f 2 0 1 140
-
180
-
180
-
200
-
t
2 2 0 " '
-
.. _
j
-
-
c _
-
. .
5
-
-
E n 8
-
40i 50
60
70
80
c
110 "
14
I
-12
-10
-8
-6
-4
-2
0
2
He FLUX DENSITY (L.m.2.al)
Figure 6. Varlatlon in He flux density with depth for two adjacent sampling positions In the deer Ilck. Vertical bars lndlcate the depth interval sampled.
Therefore,
F = -D@ff( d C / dZ)
(7)
For the Boggy Creek deer lick, the depth of unsaturated peat at the time of sampling (September 1991 was about 1.5 m, and so the diffusion model may be applied for this zone. Values for porosity of the peat are not available for this site but can be determined from data for peat given in refs 26 and 24: e = 1- p/PD X 100% = 72.6% (8) where p = bulk density (0.23 p ~ m - ~and ) , PD = particle density (0.84 pcm-3). Similarly, values of a are not known for this site but are likely to range from 0.5 to 1.0 (23) for a highly porous material. Helium flux densities have been calculated for the north and west stake data (Figure 5) using eqs 6 and 7, for the cases where a = 0.5 and 1.0. The south stake data are not used in this analysis because the results indicate highly variable and reversing concentration gradients in the upper 1 m of overburden. The results are shown for the case a = 1 in Figure 6 against the depth of unsaturated overburden. It can be seen that the calculated He flux density for both the north and west stake sites is fairly constant over the top 1m of overburden and, using a = 1,averages 0.0067 nL.cm-2.s-1 or 2.1 L.m-2.a-l. If a = 0.5, the average flux density would be half this value. The relatively constant values with depth supports the initial assumption of diffusion for gas movement, without influence of advective processes. However, a lower He flux may be occurring if He is significantlyretarded by moisture present on soil particles in the unsaturated zone. Current studies at the site are attempting to directly measure this flux. In the present work, the flux calculated below represents a maximum value, assuming no retardation by soil moisture. Using a mean flux density (F') of 2.1 L-m-2*a-1as representative of the C4 site, with a He concentration at 50-cm depth of 357 nL-L-l water (Figure 4a), we can calculate flux densities for all other measurement sites (i) within the deer-lick area using the ratio Hei/He,r as a scaling factor. The average He discharge for the entire deer lick (He) may then be determined by summation of flux density X area products, as follows:
where A is the area of the measurement site (m2). It is 2424
Envlron. Scl. Technol., Vol. 27. No. 12, 1993
Table I. Calculation of Total Helium Flux from Deer Lick Site
sample site B3 B4 B5 c3 c4 c5 D3 D4 D5 E3
E4 E5
He concn (nL.L-1) 54 131 85 223 357 133 202 131 122
151 181 56
He flux (L-a-l) 8.0
19.4 12.6 33.0 52.8 19.7 29.9 19.7 18.0 22.3 26.8 8.3 total flux 270
assumed that each measurement site is representative of a 5 m X 5 m square and that the inclusion of the grassed area within these areas (Figure 4a) has a negligible effect on the He flux. Results of the application of eq 8 are shown in Table I. For the case when a = 1, the total calculated He flux from the deer lick area is 270 L-a-1. A very approximate calculation of the deep groundwater discharge rate to the deer lick may be made if the He concentration of the groundwater is known. If the average He concentration of saline groundwater in the Lac du Bonnet granite is about 10 mL-L-l (IO),then groundwater discharge to the deer lick is about 26 000 L-a-l. Variation in He concentration of the deep groundwater and retardation of He diffusion by pore water in the unsaturated zone are among the influences that could increase or decrease this result by as much as an order of magnitude. These volumes are comparable to calculated groundwater discharges through lake-bottom sediments in the Experimental Lakes Area in northwestern Ontario (16), where a total He flux of -380 L-a-l was determined for Lake 239. Stephenson et al. (11)also calculated a He flux of 128 L-a-l into Boggy Lake, near the site, on the basis of a 4-mon accumulation of He in bottom waters under winter ice. The source of the He was believed to be groundwater from an inclined fault zone discharging into or close to the lake. Stephenson et al. (11)estimated that the He flux corresponded to a deep groundwater discharge of 9600 L-a-l on the basis of the average He content of Lac du Bonnet granite groundwater. N
Conclusions
The use of measurements of the He concentration in soil gases and soil waters is a valuable method for indicating locations of deep groundwater discharge to the surface and, correspondingly, the presence of subsurface bedrock fractures. Estimates of He flux and groundwater discharge rates can be made if concentration-depth data are available. These methods, shown to be successful in both terrestrial and aquatic environments, allow us to evaluate the partitioning of deep groundwaters dischargingto terrestrial and aquatic areas offering a unique opportunity to better understand and model the processes at the geospheref biosphere interface. Acknowledgments
The landowner of the study site, Mr. Maurice Boulanger, is gratefully acknowledged for allowing access to the site
and permission to install sampling and monitoring equipment. M. Stephenson is acknowledged for providing suggestions and experience in He sampling in saturated environments and numerous discussionsand adviceduring the course of this work. The comments of an unnamed reviewer and J. Eikenberg have greatly improved the content of this paper. This work was performed for the Canadian Nuclear Fuel Waste Management Program which is jointly funded by AECL and Ontario Hydro under the auspices of the CANDU Owners Group. Literature Cited Hancox, W. T.; Nuttall, K. The Canadian approach to safe, permanent disposal of nuclear fuel waste. Nucl. Eng. Des. 1991, 129, 109-117. Davis, P. A.; Zach, R.; Stephens, M. E.; Amiro, B. D.; Bird, G. A.; Reid, J. A. K.; Sheppard, M. I.; Sheppard, S. C.; Stephenson, M. The biosphere model, BIOTRAC, for postclosure assessment for Canada's nuclear fuel waste management concept. Atomic Energy of Canada Limited Report, 1993, in preparation. Frape, S. K.; Fritz, P.; Blackmer, A. J. Hydrochemical Balances of Freshwater Systems. IAHS Publ. 1984, No. 150,369-379. Baeyens, B.; Grogan, H. A.; van Dorp, F. Biosphere modelling from a deep radioactive waste repository: Treatment of the groundwater-soil pathway. Nagra Technical Report 89-20, Wettingen, Switzerland, 1990. Andrews, J. N.; Giles, I. S.;Kay, R. L. F.; Lee, D. J.; Osmond, J. K.; Cowart, J. F.; Fritz, P.; Barker, J. F.; Gale, J. Geochim. Cosmochim. Acta 1980,46, 1533-1543. Bottomley, D. J.; Ross, J. D.; Clark, W. B. Geochim. Cosmochim. Acta 1984, 48, 1973-1985. Larocque, J. P. A.; Gascoyne, M. A survey of the radioactivity of surface water and groundwater in the Atikokan area, northwestern Ontario. Atomic Energy of Canada Limited Technical Record TR-379,1986. These technical reports are available from SDDO, AECL Research, Chalk River Laboratories, Chalk River, Ontario KOJ 1J0, Canada. Gregory, R. G.; Durrance, E. M. J. Geophys. Res. 1987, 92(B12), 12567-12586. Banwell, G. M.; Parizek, R. R. J . Geophys. Res. 1988, 93, 355-366. Gascoyne, M.; Wuschke, D. M.; Durrance, E. M. Fracture detection and groundwater flow characterization using He and Rn in soil gases. Appl. Geochem. 1993, 8, 223-233. Stephenson, M.; Schwartz, W. J.; Evenden, L. D.; Bird, G. A. Can. J. Earth Sci. 1992,29, 2640-2652. Gascoyne, M.; Wuschke, D. M. Fracture detection and groundwater flow characterization in poorly exposed ground using helium and radon in soil gases. Atomic Energy of Canada Limited Report AECL-10370,1991. Gascoyne, M.; Wuschke, D. M.; Brown, A.; Hayles, J. G.; Kozak, E. T.; Lodha, G. S.; Thorne, G. A. A gas migration test in saturated fractured rock. Atomic Energy of Canada Limited Report AECL-10542, 1991. Gascoyne, M.; Hawton, J. J.; Watson, R. L.; Sheppard, M. I. The helium anomaly in soil gases a t the Boggy Creek site, Lac du Bonnet, Manitoba. Atomic Energy of Canada Limited Technical Report TR-583/COG-92-376, 1993. Dyck, W.; Da Silva, F. G. J. Geochem. Erplor. 1981, 14, 41-48. Stephenson, M.; Schwartz, W. J.; Melnyk, T. W.; Motycka, M. F. Measurement of advective water velocity in lake sediment using natural helium gradients, J. Hydrol., accepted for publication. Thorne, G. A. Hydrogeology of surficial materials of Permit AreaD and F and the Lee River study area in the Whiteshell Research Area. Atomic Energy of Canada Limited Technical Record TR-498, 1990. Butt, C. R. M.; Gole, M. J. J. Geochem. Explor. 1985,24, 141-173. Environ. Sci. Technol., Vol. 27, No. 12, 1993 2425
(19) Roberts, A. A.; Friedman, I.; Donovan, T. J.; Denton, E. H. Geophys. Res. Lett. 1975, 2, 209-210. (20) Gascoyne, M.; Elliott, L. C. M. A simple well-water dilution model. Atomic Energy of Canada Limited Technical Record TR-391, 1986. (21) Gascoyne, M.; Davison, C. C.; Ross, J. D.; Pearson, R. Saline Water and Gases in Crystalline Rocks. Geol. Assoc. Can. Spec. Pap. 1987, No. 33, 53-68. (22) Gascoyne, M. Reference groundwater composition for a depth of 500 m in the Whiteshell Research Area-Comparison with synthetic groundwater WN-1. Atomic Energy of Canada Limited Technical Record TR-463, 1988.
2426
Environ. Scl. Technol., Vol. 27, No. 12, 1993
(23) White, R. E. Introduction to the principles and practice of soilscience;Halsted Press: New York, 1979;pp 101-102. (24) Handbook of Chemistry and Physics, 66th ed.; CRC Press: Boca Raton, FL, 1985. (25) Forsythe, W. E. Smithsonian Physical Tables, 9th ed.; Smithsonian Institute: Washington, DC, 1964. (26) Sheppard, M. I.; Thibault, D. H. J.Environ. Qual. 1988,17, 644-653.
Received for review December 28, 1992. Revised manuscript received May 14, 1993. Accepted May 21, 1993.
