756 CAN. GEOTECH. J . VOL. 26, 1989

of scale on N,,. It can be argued (see, for example, Graham and Hovan 1986) that the locally mobilized + varies from point to point in a failing sand domain, and with the size of the footing. It is one of the particular features of the method of stress characteristics that it can accommodate systematically varying +. For reasons given in the paper, we chose on this occasion to use a simpler constant-+ model and not a more complex variable-+ one.

We would be interested in suggestions that would effect improvements in modelling footings near the crests of slopes

and, also, additional model tests and strength tests that would provide data for the dependency of footing capacity on size or pressure.

GRAHAM, J., and HOVAN, J.-M. 1986. Stress characteristics for bearing capacity in sand using a critical state model. Canadian Geotechnical Journal, 23: 195-202.

GRAHAM, J., and STUART, J.G. 1971. Scale and boundary effects in foundation analysis. ASCE Journal of the Soil Mechanics and Foundations Division, 97(SMll): 1533-1548.

A hydrochemical study of urban landslides caused by heavy rain: Scarborough Bluffs, Ontario, Canada:' Discussion

DAL HUNTER Sea, Incorporated, Consulting Engineers, 950 Industrial Way, Sparks, NV 89431, U.S.A.

Received January 24, 1989

Accepted February 6, 1989

Can. Geotech. J. 26, 756-757 (1989)

The authors of this paper are to be commended for demonstrating that the origin of pore water can be a critical factor in evaluating certain landslides. The geochemical data presented is reasonable, well analyzed, and clearly supports their hydrogeological model.

Major and minor ion chemical analysis has been used for many years as a method of distinguishing waters from varied sources (Hem 1970). In more recent years, however, these methods have been supplemented and even replaced by isotopic techniques. The stable isotopes of oxygen and hydrogen are most commonly used, often in conjunction with tritium, the radioactive isotope of hydrogen. Carbon-14 can sometimes be used to age date carbon dioxide dissolved in the water and demonstrate that two water samples are of significantly different ages.

Analysis of stable isotopes will normally yield distinctive isotopic "fingerprints" for water from different sources or with different hydrogeological histories. This is largely due to kinetic factors whereby processes such as evaporation, condensation, and freezing act preferentially on either the lighter or the heavier isotope.

For example, evaporation is generally the dominant con- trol on isotopic fractionation. During evaporation, a higher percentage of the lighter isotope (160) is partioned into the vapor phase, thereby changing the isotope ratio (ls0/160) of the residual water. Hydrogen isotopes are affected pro-

'paper by N. Eyles and K. W. F. Howard. 1988. Canadian Geotechnical Journal, 25: 455-466.

portionately to those of oxygen. For this reason, a graph of hydrogen isotope ratio ( 2 ~ / ' ~ ) versus oxygen isotope ratios for water samples from around the world generally plot along a straight line. Different waters will normally plot in different locations along this meteoric line or trend.

Minor tritium ( 3 ~ ) is produced in the atmosphere by interaction of cosmic rays with oxygen and nitrogen. However, the major source of tritium for use in water analysis was produced by atmospheric testing of thermo- nuclear bombs between 1952 and 1969. With a half-life of only 12.3 years, the most important use of tritium is in distinguishing waters that entered an aquifer prior to 1953 (pre-bomb) from water that was in contact with the atmo- sphere after 1953 (post-bomb).

Isotopic analysis is an exceptionally powerful tool for use in distinguishing different waters and would likely have pro- duced excellent results for this study. Furthermore, isotope data could have demonstrated whether the lower aquifer is indeed part of a deep regional system of groundwater flow. Additional information about mixing of two primary waters might also have been revealed. Costs for an isotopic approach would have been comparable to that of major and minor ion chemical analysis.

Numerous references are available regarding the use of stable and radioactive isotopes for hydrogeological evalua- tion. The interested reader is referred to Krauskopf (1979), Drever (1982), and Hunter (1989) for broad overviews of techniques and potential applications. A much more thorough discussion is presented in Fritz and Fontes (1980).

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DISCUSSIONS 757

DREVER, J.I. 1982. The geochemistry of natural waters. Prentice- HUNTER, D. 1989. Applications of stable isotope geochemistry to Hall, Inc., Englewood Cliffs, NJ. engineering geology. In Proceedings, 25th Annual Symposium

FRITZ, P., and FONTES, J.-C., editors. 1980. Handbook of on Engineering Geology and Geotechnical Engineering, Reno, environmental isotope geochemistry. Vol. 1, The terrestrial NV, in press. environment. Elsevier Science Publishing Co., New York, NY. KRAUSKOPF, K. 1979. Introduction to geochemistry. McGraw Hill

HEM, J.D. 1970. Study and interpretation of the chemical Book Company, New York, NY. characteristics of natural water. United States Geological Survey, Water Supply Paper 1473.

A hydrochemical study of urban landslides caused by heavy rain: Scarborough Bluffs, Ontario, ~anada : ' Reply

N. EYLES AND K. W. F. HOWARD Department of Geology, University of Toronto, Scarborough Campus, 1265 Military Trail, Scarborough, Ont.,

Canada MIC IA4

Received May 15, 1989

Accepted May 16, 1989

Can. Geotech. J . 26, 757-758 (1989)

We appreciate the favorable comments made by D. Hunter and the additional information he provides on the potential role of environmental isotopes in groundwater studies. He is correct in his statement that environmental isotopes have provided a useful supplement to traditional major and minor ion chemical methods. There are many examples in which both stable and unstable isotopes of hydrogen, oxygen, carbon, nitrogen, chlorine, sulfur, and uranium have contributed much to the understanding of groundwater sources, flow paths, and hydrogeochemical processes (Fritz and Fontes 1980, 1986; Kronfeld and Rosenthal 1981; Fritz 1982; Faure 1986; Drever 1982).

However, while we recognize the potential value of isotopic methods for hydrogeologic investigation, we feel it is important to dispel the common notion that isotope techniques represent a hydrogeologic panacea. In particular, Hunter's implication that environmental isotopes can be used in isolation to "replace" major and minor ion chemical techniques requires comment.

Responsible use and interpretation of environmental isotopes requires careful consideration of isotope geochemistry and an awareness of the limited range of appli- cation of the various isotope techniques. For example, tritium isotope investigations have a limited applicability if the groundwaters of a region consistently predate the early 1950's. In addition, "O/D will not distinguish between recent waters and 5000 year old waters if both were recharged under similar climatic and hydrologic regimes.

iscu cuss ion by D. Hunter. 1989. Canadian Geotechnical Journal, 26, this issue.

14c dating techniques also are unable to differentiate between waters that are 500 000 and 70 000 years old, and cannot reliably separate 1000 year old from 50 year old water. The 14c technique could prove effective for waters dating 30 000 to 1000 years before present, but then only on the condition that significant mixing has not taken place. Given these constraints, some understanding of the ground- water flow regime and the geologic, hydrogeologic, and climatic history of the region is a precondition for detailed, reliable, and cost-effective isotope studies.

In our hydrochemical study of urban landslides along the Scarborough Bluffs, major and minor ion chemical data distinguished between groundwaters within two deltaic sand aquifers separated by a glacial diamict aquiclude. The upper, shallow aquifer contains elevated concentrations of chloride and nitrate, which have likely been introduced by local, recent recharge in the heavily urbanized catchment. The lower aquifer shows significantly less contamination, suggesting a groundwater flow path that originates beyond the urbanized region. Despite Hunter's assertion that isotopic data "would likely have produced excellent results for this study'' and "could have demonstrated whether the lower aquifer is indeed part of a deep regional system of groundwater flow," the use of isotopes is subject to severe constraints. For example, high regional hydraulic gradients within the permeable unconsolidated sands generate high groundwater flow velocities (> 100 m/year) that, given the small scale of the groundwater catchments (typically in the range 10-50 km) are responsible for short groundwater residence times. It follows that even the oldest groundwaters would unlikely provide a reliable positive 14c age date.

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