From: ed@titipu.meta.com (Edward Reid)
Newsgroups: rec.backcountry
Subject: Water treatment article from Am J Pub Health (1989)
Date: 23 Aug 92 18:32:11 GMT
Reply-To: ed@titipu.meta.com (Edward Reid)
Organization: Accuracy, Ltd.
X-Mailer: uAccess - Macintosh Release: 1.6v0

This article was discussed on rec.backcountry a couple of months ago.  I finally
got a copy, along with the time to play with OCR and the ensuing correcting,
proofreading and additional typing.  Here it is.  If you notice any typos, let
me know so I can correct my master.  I'll save my comments on the content for a
separate posting; the following comments only describe the alterations required
to cram the information through the ascii needle's eye.

Italics and boldface: changed to roman.

Subscripts: raised to in-line.

Superscripts (exponents): in-line preceded by ^.

Superscripts (references): in-line enclosed in ().

Greek letter mu (micro): changed to u.

Figures: described in narrative, as translation to ascii graphics would be
difficult.  Figure 2 is the most important.  It's out of the question for me to
email the TIFFs to individuals, and I don't have a way to make them available
for FTP, but if someone wants to offer to put the TIFFs up for FTP, I will
gladly mail them to that person.  However, I would really recommend looking the
article up in your library if you want to see the figures.

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Backcountry Water Treatment to Prevent Giardiasis
=================================================

Jerry E. Ongerth, PhD, PE, Ron L. Johnson, Steven C Macdonald, MPH, Floyd Frost,
PhD, and Henry H. Stibbs, PhD

American Journal of Public Health December 1989, Vol 79, No 12, pp 1633-1637.

Copyright 1989 AJPH 0090-0036/89$1.50 [used without permission]

Abstract

This study was conducted to provide current information on the effectiveness of
water treatment chemicals and filters for control of Giardia cysts in areas
where treated water is not available.  Four filters and seven chemical
treatments were evaluated for both clear and turbid water at 10C.  Three contact
disinfection devices were also tested for cyst inactivation.  Filters were
tested with 1-liter volumes of water seeded with 3x10^4 cysts of G. lamblia
produced in gerbils inoculated with in vitro cultured trophozoites; the entire
volume of filtrate was examined for cyst passage.  Chemical treatments were
evaluated at concentrations specified by the manufacturer and for contact times
that might be expected of hikers (30 minutes) and campers (eight hours, i.e.,
overnight).  Two of the four filter devices tested were 100 percent effective
for Giardia cyst removal.  Of the other two filters, one was 90 percent
effective and the other considerably less effective.  Among the seven
disinfection treatments, the iodine-based chemicals were all significantly more
effective than the chlorine-based chemicals.  None of the chemical treatments
achieved 99.9 percent cyst inactivation with only 30-minute contact.  After an
eight-hour contact each of the iodine but none of the chlorine preparations
achieved at least 99.9 percent cyst inactivation.  None of the contact
disinfection devices provided appreciable cyst inactivation.  Heating water to
at least 70C for 10 minutes was an acceptable alternative treatment.

--------------------------------------------------------------------------------

Introduction

Giardia lamblia is the most commonly identified human intestinal parasite in the
United States.  Giardiasis is commonly transmitted between humans, especially
among small children.  lt is also transmitted in water, particularly in the
mountainous regions of the U.S.  Since 1965, over 80 waterborne outbreaks of
giardiasis have occurred in community water systems, affecting more than 20,000
persons (1).  Giardiasis in hikers and campers has also been documented (2,3);
indeed, it is commonly considered a backpackers' illness.  Giardia cysts in
concentrations as high as four per gallon have been detected in untreated
surface water in northeastern and western states (4).

Concern over waterborne transmission of Giardia has led to development of a
variety of chemical disinfectants and portable filters for individual use in the
backcountry.  Although some information on such methods has been reported
(2,5,6), there is no comprehensive guide to their reliability in actually
removing or inactivating Giardia cysts.  We tested four commercially available
portable filters and one contact disinfection device for their ability to remove
Giardia cysts from water.  We also evaluated the cysticidal effectiveness of
seven chemical disinfectants and three contact disinfection devices.

--------------------------------------------------------------------------------

Methods

Cysts of G. lamblia were prepared for use in both the filtration and
disinfection tests by propagation in gerbils inoculated with trophozoites from
sterile culture.  Trophozoites were of two isolates: one from a beaver (Be-4
isolate from Alberta) and one from a human (H-2 CSU isolate from Colorado). 
Cysts were concentrated from crushed, filtered gerbil feces by flotation on zinc
sulfate (sp. gr. 1.18), cleaned, and stored in distilled water at 4C for up to
10 days before use.  Similarly, G. muris cysts of an isolate originally obtained
from hamsters (7) were purified from feces of infected athymic (nu/nu) mice and
stored before use.  Cyst concentrations were determined with a Coulter Counter
(Model ZBI, Coulter Electronics, Hialeah, FL) and a haemacytometer.  Except
where noted, cysts were added to water samples in concentrations of about
3x10^4/ml.  Cyst viability was assayed by fluorogenic staining (8) and in vitro
excystation (7).  In the former method, live cysts are distinguished by two
fluorescing dyes.  One dye is fluorescein diacetate (FDA), which when absorbed
by cysts produces a fluorescent green only in live cysts; the second dye, either
propidium iodide (Pl) or ethidium bromide (EB), is excluded efficiently by live
cysts but absorbed by dead cysts, resulting in red fluorescence.

Filter testing

The following backpacker-type water filters were purchased from local retailers:
First Need Water Purification Device (First Need), General Ecology Inc.,
Lionville, PA; H2OK Portable Drinking Water Treatment Unit Model No. 6 (H2OK),
Better Living Laboratories Inc., Memphis, TN; Katadyn Pocket Filter (Katadyn),
Katadyn Products Inc., Wallisellen, Switzerland; and Pocket Purifier, Calco Ltd,
Rosemont, IL.  Also noted in this category is the Water Tech Water Purifier
(Water Purifier), Water Technologies Corp., Ann Arbor, Ml.  Although it is not
advertised as a filter and was not specifically tested for Giardia cyst removal,
we report qualitative observations made during disinfection testing (see below)
because its configuration and mode of operation suggest that particle removal
may occur.  Physical and operating information provided in the filter packaging
is summarized in Appendix A.  Each device was tested when it was new.  Devices
that removed all cysts when new were retested after a period of use
approximating several months for a regular weekend user.

Each filter was prepared for testing by filtering four liters of tap water to
purge loose carbon particles or debris.  The cyst removal performance of each
filter was determined by filtering one liter of spring water, turbidity of 0.1
NTU, to which formalin-fixed G. lamblia cysts had been added.  The entire
filtrate volume was passed through a 25-mm dia., 5-um pore size, polycarbonate
membrane (Nuclepore, Pleasanton, CA).  stained with EB (100 ug/ml), and mounted
under a cover slip.  Cysts were counted at x250 magnification with the aid of
epifluorescence microscopy.  A representative portion of each filter was
examined to quantify cyst recovery as described previously (9).  The area
examined was inversely proportional to the number of cysts found and ranged from
3.5 percent of seeded positive control filters to 25 percent (one quadrant) of
filters with cyst densities less than one per field.  Total numbers of cysts
present were estimated by extrapolation in direct proportion to the area
examined.  In extensive work on recovery of Giardia cysts using the procedures
described above, cyst retention on the 5-um polycarbonate membrane in a single
filtration step has routinely averaged 80 to 90 percent (Ongerth JE:
unpublished).  Accordingly, the ability to identify high levels of cyst removal,
which would result in passage of very few or no cysts, is excellent.  This
ability is unaffected by the factors that contribute to lack of precision in
counting large numbers of cysts on filters; such inaccuracies usually occur when
only small representative subareas are examined and the total numbers are
estimated by extrapolation.  A seeded positive control and an unseeded negative
control were processed with each batch of filter evaluations.  The cyst removal
performance evaluation was replicated three times for each filter device, with
results expressed as the arithmetic average and corresponding standard
deviation.

Contact Disinfection Testing

The Water Purifier is described in packaging information as a contact
disinfection device.  Likewise, the H2OK and Pocket Purifier devices are
described as providing disinfection as well as removing cysts by filtration. 
These devices were therefore tested for their effect on cyst viability in
addition to filtration efficiency.  A single 500-ml sample for each device was
seeded with approximately 2.5 x 10^4 cysts and passed through the device. 
Filtrate was collected and filtered as described above to recover cysts.  The
viability of cysts was then assessed by FDA and EB staining as described below.

Disinfectant Testing

The cysticidal effects of seven commercially available and commonly used
disinfectant preparations were tested with identical procedures.  Four of the
products were iodine based: Polar Pure Water Disinfectant (Polar Pure), Polar
Equipment, Saratoga, CA; Coghlan's Emergency Germicidal Drinking Water Tablets
(CEGDWT).  Coghlan's Ltd, Winnipeg. Canada; Potable Aqua Drinking Water
Germicidal Tablets (Potable Aqua), Wisconsin Pharmacal Inc., Jackson, WI; and 2
percent iodine prepared from I2 reagent grade (Baker, Phillipsburg, NJ).  The
remaining three products were chlorine-based: Sierra Water Purifier (Sierra), 4
in 1 Water Co., Santa Fe, NM; Halazone, Abbott Laboratories, North Chicago, IL;
and commercial liquid bleach (5.25 percent sodium hypochlorite).  Disinfectant
solutions were characterized by pH and total halogen concentration (Appendix B),
the latter being determined colorimetrically using the DPD method.

Two water sources were used, one to reflect clear high-mountain conditions, the
other to reflect downstream, more turbid conditions.  Water sources were
characterized by pH, turbidity, and free chlorine demand (Appendix C).  The
upstream source was from a small, spring-fed tributary to the Snoqualmie River
near North Bend, Washington.  Samples were taken from the stream approximately
50 yards downstream from the spring.  The downstream source was the discharge
from Lake Washington in Seattle, Washington.  Samples were taken in midstream at
the entrance to Portage Bay, adjacent to the University of Washington campus. 
Water samples were prepared for testing by adding disinfectant, according to
manufacturers' instructions, to one liter of water in stoppered glass bottles
(Appendix B).

Cysticidal properties of the chemical treatments were determined as follows.

1) Water was put in 50-ml disposable plastic centrifuge tubes and placed in a
10C incubator.

2) G. lamblia cysts were added to each test sample at time zero.

3) Tubes were vortex-mixed, sampled, and returned to the incubator.

4) At each sampling time, i.e., time 0, 30 minutes and 8 hours, a 10-ml sample
was withdrawn; a portion was used for measuring disinfectant concentration, and
in the remainder the disinfectant was quenched with 0.1-mM sodium thiosulphate.

5) Cysts in the quenched sample portion were exposed to aqueous solutions of the
viability indicators, FDA (25 ug/ml) and EH (100 ug/ml), filtered on to a 13-mm
dia. 5-um pore-size filter membrane, and rinsed with distilled water (10 ml).

6) Filters were mounted on glass slides, sealed under coverslips and examined by
epifluorescence microscopy at x250 magnification (Model 16, Carl Zeiss, Inc.,
Thornwood, NY) to enumerate proportions of red and green fluorescing cysts
indicating dead and live status, respectively.  The viability baseline of the
cysts was established by running a control sample of untreated water seeded with
cysts through each test, using procedures identical to those for disinfectant-
treated samples.  Data are presented in terms of percent survival relative to
the controls (Figure 2).  The effectiveness of each disinfectant for killing
cysts in both upstream and downstream water was determined in triplicate, with
results expressed as the arithmetic average and corresponding standard
deviation.

The Water Tech Water Purifier, a contact disinfectant, was also tested as a
chemical disinfectant.  The test water was 100 ml of spring-source water seeded
with Giardia cysts.  The treated water was filtered, stained, and examined for
cyst viability as described in steps 5 and 6 above.  Three replicates were
assayed.

Heat Inactivation

Inactivation of G. lamblia and G. muris cysts by heating was established as
follows.  Cysts were added to distilled water in 15-ml glass test tubes.  The
seeded tubes were incubated for 10 minutes at temperatures ranging from 10C to
70C.  Afterwards, cyst suspensions were cooled immediately by swirling in 10C
water for one minute.  Cyst viability was determined either by excystation or by
staining.  If by the latter, FDA and EB were added to the samples, the tubes
were vortex-mixed, and a 1-ml aliquot was filtered through a 13-mm dia. 5-um
pore-size filter membrane.  Filters were rinsed, mounted, and examined as
described above to enumerate the live and dead cysts.

--------------------------------------------------------------------------------

Results

Filter Device Tests

The four filters differed significantly in their ability to remove Giardia cysts
(Figure 1).  The number of cysts recovered from water having passed through the
filter devices ranged from zero to greater than 10^4 in individual tests.  The
performance of individual devices was consistent as indicated by the standard
deviations for each of the three replicate test sets (Figure 1).  The percentage
of cysts removed by the devices, corresponding to 100 minus the percent of cysts
recovered from the filtrate, was 100 percent for the First Need and Katadyn
filters and approximately 90 percent for the H2OK filter.  The concentration of
cysts in the Pocket Purifier effluent was not statistically different from the
seed concentration.

The First Need and Katadyn filters were then subjected to a period of moderate
use and then retested.  The volume of water processed during the simulated use
period was not the same for the two filters owing to differences in their
operation.  The difference in volume had no apparent effect on performance of
the two filters.  A total of 88 liters of tap water (turbidity of 0.3 NTU) was
filtered with the First Need.  During the process it was back-flushed, as
recommended in package instructions, because the filtration rate decreased after
50, 71, and 75 liters had been filtered.  After 88 liters had been processed,
the filtration rate was about 25 percent lower than when the filter was new, and
it was retested in that condition.  The Katadyn filter was subjected to use by
filtering one liter of tap water four times a day for five days.  At the end of
each day, the filter was cleaned according to package instructions by
disassembling, brushing the filter element, and allowing it to air-dry overnight
before reassembly.  After the respective periods of use, these two filters were
tested in triplicate for efficiency of cyst removal.  Performance of these
filters was the same, 100 percent cyst removal, when they were retested.

Cyst Inactivation

Contact Disinfection Devices - The effect of each of the contact disinfection
devices on G. lamblia cyst viability was limited.  The Water Purifier
inactivated about 15 percent of the cysts added in 100 ml of upstream (low
turbidity) water; the H2OK filter inactivated about 5 percent of the cyst
challenge, and the Pocket Purifier inactivated about 2 percent of the cyst
challenge.

Chemical Disinfectants - The effectiveness of seven disinfecting chemical
preparations ranged from only a few percent to greater than 99.9 percent,
depending on the chemical and its concentration, the contact time, and the
disinfectant demand of the water (Figure 2).  None of the disinfectants was more
than 90 percent effective after a contact time of 30 minutes.  After eight-hour
contact, the four iodine-based disinfectants, each caused a greater than 99.9
percent reduction in viable cysts.  The chlorine-based disinfectants were
clearly less effective than the iodine-based ones at both contact times.

Heating in Water - Experiments conducted with cysts of G. lamblia and of G.
muris indicated that the two species have virtually the same sensitivity to
inactivation by heating.  Cysts at both species were completely inactivated by
heating to 70C for 10 minutes.  Heating to 50C and 60C for 10 minutes produced
95 and 98 percent inactivation, respectively (Figure 3).

--------------------------------------------------------------------------------

Discussion

To remove Giardia cysts from water, one must use a filter with sufficiently
small pores to trap the cysts and sufficiently large capacity to produce a
useful volume of treated water before backwashing or replacement is necessary. 
Although a number of manufacturers advertise that their filters remove Giardia
cysts, the only previously published account of filter performance was for the
Katadyn unit (6).  Our filter evaluation study showed that only the First Need
and the Katadyn filters removed cysts with at least 99.9 percent effectiveness. 
Under the same test conditions, the H2OK filter was approximately 90 percent
effective and the Pocket Purifier was less than 50 percent effective for cyst
removal.  The analysis of viability for the cysts collected in the effluent of
the Water Purifier, H2OK, and Pocket Purifier indicates that passage through the
device did not significantly reduce the percentage of viable cysts.

The current study showed that none of the chemical treatments could inactivate
more than 90 percent of cysts with 30 minutes of contact time at 10C.  At both
30 minutes and eight hours of contact time, the iodine-based disinfectants
inactivated a higher fraction of cysts than did the chlorine-based products. 
All methods inactivated a lower percentage of cysts in cloudy or turbid water
than in clear water.  All disinfectants performed better with eight hours of
contact time than with 30 minutes.  Only the iodine-based compounds inactivated
99 to 99.9 percent of cysts, within eight hours of contact time for both turbid
and clear water.  As observed by Jarroll, et al (5), the 2 percent tincture of
iodine was less effective than the other iodine preparations with 30 minutes of
contact time, but it was as effective as the others at eight hours.  Comparison
of our results with those of Jarroll, et al (5), is complicated by differences
between test conditions used.  However, our results generally indicate more
stringent requirements for effective inactivation of Giardia cysts.  Differences
between cyst populations used in the two studies could account for the observed
differences, even though both were G. lamblia.  Cysts produced in our
trophozoite - gerbil system had consistently high intrinsic viability (>80
percent), excysted efficiently when fresh (80 to 90 percent), and have appeared
more resistant to halogen disinfectants than reported previously (Ongerth J.E.:
unpublished).

The results of heat inactivation in our study correspond to previous reports
indicating that heating to between 60C and 70C kills Giardia cysts efficiently. 
In addition, our data illustrate the correspondence between the fluorogenic
staining and in vitro excystation procedures for assessing cyst viability.  When
applied to cysts of the same condition.  Staining indicates a slightly higher
proportion of viable cysts than does excystation.  Overall, however, the two
procedures provide comparable information.

--------------------------------------------------------------------------------

Figure 1 - Effectiveness of Four Portable Water Filters for Removal of Giardia
Cysts from One-Liter Volumes of Water Each containing approximately 3x10^4 Cysts
(dotted line).  [A bar chart showing the positive and negative controls and
results from the filters, on a log scale.  The First Need and Katadyn results
and the negative control were all zero.  The Pocket Purifier and the positive
control were approximately the same - i.e. the Pocket Purifier did not remove
cysts at all.  The H2OK results were somewhat below the positive control,
actually -- due to the log scale -- indicating 90% removal.]

Figure 2 - Effect of Time and Disinfectant Concentration of Seven Chemical
Disinfectants on Survival of G. lamblia Cysts in Turbid and in Clear Water.  [A
rather striking bar chart comparing chemical treatments under varying
conditions.  The chlorine compounds were basically ineffective, with no
significant effect at 30 minutes; at 8 hours the Sierra was still totally
ineffective, the bleach killed about half the cysts, and the Halazone killed 70-
90% of the cysts (better in clear water).  The iodine compounds were poor at 30
minutes in turbid water (half killed), only a little better at 30 minutes in
clear water (70-90% killed, with Potable Aqua the best), but completely
effective (100% killed) after 8 hours.]

Figure 3 - Inactivation of Giardia Cysts as a Function of Temperature (10-minute
exposures) as Indicated by Ethidium Bromide Staining and by in vitro
Excystation.  [A line chart showing cyst survival at different temperatures. 
Four combinations of Giardia species, source, and laboratory technique are
shown, but all show approximately the same results.  40C kills no cysts; 50C
kills a lot of cysts, 60C kills most cysts, 70C kills all cysts.]

--------------------------------------------------------------------------------

Acknowledgements

References to commercial products shall not be construed to represent or imply
the approval or endorsement by project investigators or sponsors.

Grant support was provided in part by the REI Environment Committee which
assumes no responsibility for the content of research reported in this
manuscript.

--------------------------------------------------------------------------------

References

(1) Craun GF: Waterborne outbreaks of giardiasis: current status.  In: Erlandsen
SL, Meyer EA (eds): Giardia and Giardiasis.  New York: Plenum Press, 1984; 243-
262.

(2) Kahn FH, Visscher BR: Water disinfection in the wilderness.  West J Med
1975; 122:450-453.

(3) Barbour AG, Nichols CR, Fukushima T: An outbreak of giardiasis in a group of
campers.  Am J Trop Med Hyg 1980; 25:384-389.

(4) Ongerth JE, Butler R, Donner RG, Myrick R, Merry K: Giardia cyst
concentrations in river water.  In: Advances in Water Treatment and Analysis,
Vol 15.  Denver: Am Water Works Assoc, 1988; 243-261.

(5) Jarroll EL, Bingham AK, Meyer EA: Giardia cyst destruction: effectiveness of
six small quantity water disinfection methods.  Am J Trop Med Hyg 1980; 29:8-11.

(6) Schmidt SD, Meier PG: Evaluation of Giardia cyst removal via portable water
filtration devices.  J Freshwater Ecol 1984; 2:435-439.

(7) Schaefer FW III, Rice EW, Hoff JC: Factors promoting in vitro excystation of
Giardia muris cysts.  Trans R Soc Trop Med Hyg 1984; 78:795-800.

(8) Schupp DG, Erlandsen SL: A new method to determine Giardia cyst viability:
correlation of fluorescein diacetate and propidium iodide staining with animal
infectivity.  Appl Environ Microbiol 1987; 53:704-707.

(9) Ongerth JE, Stibbs HH: Identification of Cryptosporidium oocysts in river
water.  Appl Environ Microbiol 1987; 53:672-676,

(10) American Public Health Assoc: Chapter 408E In: Standard Methods for the
Examination of Water and Wastewater, 15th ed.  Washington, DC: Am Public Health
Assoc, 1980; 309-310.

--------------------------------------------------------------------------------

Appendix A: Water Filter characteristics Listed by Manufacturers on Packaging or
Instruction Insert

[Manufacturer column omitted.  See text for this information.]

Name        Filter Type            Operating Operating Useful     Restrictions
                                   Mode      Rate      Life       /Limitations

First Need  0.4 um microscreen     hand pump 1 pt/min  up to 800  A
            plus adsorber                              pints

H2OK        6 um mesh, 3 in.       gravity   1 qt/min  2000 gal   A, B
            activated carbon w/Ag

Katadyn     0.2 um ceramic,        hand pump  1 qt/min many years A
Pocket      Ag-impregnated
Filter

Pocket      10 um (nominal), halo- mouth     -         -          A
Purifier    genated resin (38% I), suction
            Ag-impregnated carbon

Water Pur-  Polystyrene resin bed  gravity   -         100 gal    A, C
ifier (a)   (46% I2 as I5)

A - Does not desalinate; not for saltwater or brackish water.
B - Pretreat with I2 for bacterially contaminated water.
C - Not for use with muddy water.
(a) Not described as a filter by package information.

--------------------------------------------------------------------------------

Appendix B: Characteristics of Disinfectant Preparations

[Manufacturer column omitted.  See text for this information.]

Name        Active Chemical        Recommended Application  Total Halogen    pH
                                                            Concentration    (b)
                                                            (a), (mg/liter)

Polar Pure  Crystalline iodine,    1-7 capfuls per quart    2.4 (1           6.1
            99.5%                  depending on temperature cap/quart)

CEGDWT      Tetraglycine hydro-    1 tablet per liter or    4.5 (1           5.6
            periodate 16.7% (6.68% quart                    tab/quart)
            titrable iodine)

Potable     Tetraglycine hydro-    1 tablet per liter or    5.3 (1           5.6
Aqua        periodate 16.7% (6.68% quart                    tab/quart)
            titrable iodine)

2% Iodine   Iodine                 0.4 ml per liter         4.5              6.5

Sierra      Calcium hypochlorite & 100 crystals (50 mg)     11.6             6.7
            hydrogen peroxide      Ca(OCl)2 + 6 drops H2O2
                                   per gallon

Halazone    p-dichloro-sulfamoyl   5 tablets per quart      7.5              6.7
            benzoic acid, 2.87%

Chlorine    sodium hypo-chlorite,  5 ml per gallon          3.9              7.1
bleach      5.25%

(a) As prepared according to package instructions.
(b) In water treated according to package instructions.

--------------------------------------------------------------------------------

Appendix C: Characteristics of Disinfectant Test Water

Source               pH     Turbidity (NTU)   Chlorine Demand (a)
                                              (mg.liter)

Spring-fed           6.8    0.09              0.3

Lake Washington      7.1    0.75 - 0.80       0.7

(a) 30 minutes, free chlorine demand (5).

--------------------------------------------------------------------------------

The authors

Address reprint requests to Jerry E. Ongerth, PhD, PE, Assistant professor,
Department of Environmental Health, SB-75, University of Washington, School of
Public Health and Community Medicine, Seattle, WA 98195.  Dr. Stibbs is with the
Department of Pathobiology, also at the School, and Mr. Macdonald is with the
Department of Medical Education, School of Medicine, both at the University of
Washington; Mr. Johnson is with the Department of Biological Chemistry, Johns
Hopkins School of Medicine, Baltimore; Dr. Frost is with the Office of
Environmental Programs, Department of Social and Health Sciences, Olympia, WA. 
This paper, submitted to the Journal January 12, 1289, was revised and accepted
for publication June 22, 1989.

Edward Reid  (8*}>
  eel:   ed@titipu.meta.com  or  nosc.mil!titipu.meta.com!ed
  snail: PO Box 378/Greensboro FL 32330