Nuclear techniques for glaciological studies in Canada

F. A. Prantl and H. S. Loijens

Abstract. During the years 1969 to 1971 two large drainage basins, origins of the North Saskatchewan River and its tributary, the Mistaya, Rocky Mountains, Alberta, Canada, were subject of an an extensive hydrological and tritium study. The basins cover 1900 km2, 15 per cent of which is glacierized. Nuclear signatures were determined of all waters contributing to the fluvial discharges. They were used in a simple model to estimate the amount of runoff entering the rivers during the spring melt of the seasonal snow cover, and to demonstrate the importance of glacial melt for the stream flow regimes. The results were that spring runoff from snowmelt contributed as much as 60 per cent, summer melting of glacial ice as much as 40 per cent.

Techniques nucléaires pour des études glaciologiques au Canada Résumé. Les niveaux de tritium et les caractères hydrologiques des bassins versants de la rivière North Saskatchewan et de son affluent, la Mistaya, Montagnes Rocheuses, Alberta, Canada, ont été suivis de 1969 à 1971.15 pour cent de la surface totale des bassins de 1900 km2, est recouverte de glaciers. Le traitement mathématique des résultats a permis d'estimer les quantités relatives d'eau provenantes de la fonte des neiges et de la fonte des glaciers. Au printemps les eaux de fonte des neiges peuvent représenter jusqu'à 60 pour cent des eaux des rivières, alors qu'en été la portion d'origine glaciaire peut atteindre 40 pour cent.

INTRODUCTION In recent years tritium has become recognized more and more as a tool for hydrological studies to trace water movements, to determine mixing ratios of waters of different origin, or their age—largely unsolved questions to which classical hydrological procedures do not always provide the means for answers. Ambach et al. (1973) suggested using tritium for runoff studies in small glacierized basins of the Austrian Alps to separate the relative contributions of meltwaters from the accumulation and the ablation zone to the total glacial discharge. Martinec et al. (1974) and Martinec (1975) showed that tritium could be used in small drainage basins in Czechoslovakia and in Switzerland to separate the components of direct runoff from the snow pack and recession flow in the water budgets. Our study, initiated in 1969, was aimed at finding out whether tritium could be used on a much larger scale to improve our knowledge of water budgets of Canadian rivers that originate from glacierized areas and are important sources of water supply and hydroelectric power. The field programme consisted of hydrological and tritium surveys of the headwaters of the North Saskatchewan and the Mistaya rivers. The mean annual flows were about 40 m3/s and 6 m3/s, respectively. The basins cover 1900 km2; 15 per cent is glacierized. Elevations range from 1400 m a.s.l. at the river confluence to 3490 m a.s.l. at the western boundary, the Continental Divide. Glacier mass balance data were collected on Peyto Glacier in the Mistaya River basin. Precipitation, river and tributary waters were sampled at 19 locations at 2-week intervals. Additional firn and glacier ice samples were collected. The field programme lasted until the beginning of 1972. The samples were shipped to the laboratory for tritium analyses where after electrolytic enrichment they were counted by liquid scintillation techniques. Analytical errors were less than 10 per cent. From these measurements we determined, firstly, the levels and temporal variations of tritium in waters contributing to the river flows. Secondly, by establishing a suitable, simple runoff model to accommodate the tritium data, 237 238 F. A. Prantl and H. S. Loijens we attempted to estimate the relative contributions of runoff from melting (a) of the seasonal snow cover and firn in the glacier accumulation zones and (b) of ice in the glacier ablation zones. The last step was then to compare the results with the available hydrological evidence.

NUCLEAR SIGNATURES The North Saskatchewan and Mistaya rivers and their tritium concentrations originate from mixing of basin waters of different origin and history : runoff from precipitation, melting of the seasonal snow cover and of firn in the glacier accumulation zones, of ice in the ablation zones, and groundwater contributions. Our first step led to identifying the nuclear signatures of these waters and their changes in both basins during the study. May to September precipitation showed relatively high tritium concentrations, averaging about 354 TU in 1970 to 395 TU in 1971 (1 TU = 10 ~18 T/H). They were about 3.5 times higher than the average tritium concentrations in the seasonal snow packs of the corresponding winters. During each summer we observed a peak of the tritium concentrations in June and July precipitation, 572 TU in 1970 and 728 TU in 1971. These were the highest tritium values amongst all the samples. The seasonal variation with summer highs and winter lows is typical of the global atmospheric tritium transport and was superimposed on the annual trends. Compared to the 12 per cent increase of tritium concentrations in May to September precipitation from 1970 to 1971, that in snow from the winters of 1969-1970 to 1970-1971 was 21 per cent. We could also obtain tritium values in precipitation from before the start of our study in 1969 by sampling underlying, older firn in the accumulation area of Peyto Glacier. We found decreasing values from 1967-1968 to 1968-1969. Similar trends have been observed globally over these years. The 1969 increase which persisted until the end of our study in 1972 has been attributed to the resuming of nuclear weapons testing in the atmosphere. We observed no significant difference between the tritium concentrations of snow packs at different altitudes until about the end of April each year. Until then, they averaged about 105 TU in 1970 and 127 TU in 1971. After April, however, we found an altitude effect of about 0.17 TU/m. The pack at high altitudes became isotopically heavier than in the valleys. Isotope fractionation during ripening of the pack, melting or evaporation at temperatures that were altitude dependent could only have played a minor role. The major reason, however, for the existence of this effect was that the packs at different altitudes represented different time periods. At high altitudes the accumulation period is longer in general than in the valleys, extending into the time of isotopically heavier spring precipitation. Ice from the glacier ablation zones was virtually free of tritium. It had formed before 1954 when thermonuclear weapons testing started to introduce man-made tritium into the hydrosphere, and has been affected by at least 20 years of radioactive decay. In groundwater the tritium concentrations were also relatively high, about 353 TU and 364 TU in 1970 for the North Saskatchewan and the Mistaya River basins, respectively. They decreased by about 12 per cent the following year. These values were determined from the river baseflow concentrations of tritium, before the onset of spring melt. In the fall of 1970 the values were lower in both basins, 270 TU and 280 TU, respectively. In the fall of 1971 we had no values for the North Saskatchewan River basin ; those of the Mistaya River basin were Nuclear techniques for glaciological studies in Canada 239 300 TU which was higher than in the fall of 1970. We interpolated values between consecutive years. These relatively high tritium concentrations indicate that the groundwater must be replenished substantially each year by the May to September precipitation. The mean annual decrease in groundwater tritium from 1970 to 1971 was in contrast to the rise in precipitation tritium over these years, and thus must reflect storage releases from before 1968. From these data, one can estimate water residence times and basin retardations. This we have not yet done.

RIVERS AND TRIBUTARIES In the rivers and tributaries tritium concentrations between April and August each year were lower than in precipitation and groundwater. This must reflect the additions of meltwaters from the seasonal snow cover, firn and glacial ice, but trends were similar with 1970 levels lower than in 1971. In tributaries we observed diurnal tritium variations of an amplitude of as much as 160 TU. High tritium concentrations occurred in general during morning hours when streamfiows were low due to low additions of meltwaters from glacial ice. Low tritium concentrations, on the other hand, were found during the hours of peak flows in the afternoons and evenings when glacial meltwaters contributed significantly to the flows. Ambach et al. (1973) reported similar observations. In the lower basin regions diurnal tritium variations were quenched and small compared to seasonal. The latter were similar for both rivers in 1970 and 1971 : fairly constant values between the end of September until November, then slightly increasing until March for the North Saskatchewan River, and until April for the Mistaya. After these dates which coincided with the onset of spring melting of the seasonal snow cover in each basin, the tritium concentrations of both rivers declined. Until the end of June 1970 this trend reached 42 per cent for the North Saskatchewan River and 33 per cent for the Mistaya. The corresponding values in 1971 were 33 per cent and 26 per cent. This decline was interrupted temporarily each year by a peak in mid-July with values of up to 80 TU above the June lows. The decline then continued in both rivers until about mid-September each year when a sharp rise started towards the relatively high fall and winter values. This river cycle, with summer lows and winter highs of the tritium concentrations, is the opposite of what one expects from the tritium cycle in precipitation. It indicates that the direct runoff from the relatively highly tritiated May-September precipitation was small compared to the amount meltwaters contributed to the rivers at low tritium concentrations. It confirms our result of the groundwater tritium study, namely that May-September precipitation was used preferentially to replenish the groundwater storage of the basins.

QUANTITATIVE EVALUATION By establishing a suitable, simple model using nuclear signatures and their temporal variations we attempted to complement the hydrological data. The model consisted of the four compartments, May-September precipitation, seasonal snow and firn, glacial ice and groundwater, which feed directly or indirectly into the fifth, the river. Each compartment, i, was characterized by its nuclear signature, cj^ where q denoted the tritium concentration in each compartment, /, during Ar;/denoted the fraction of water that compartment i = 1,... 4 contributed to the river flow,/6 = 1, during the period, àt. 240 F. A. Prantl and H. S. Loijens Evaporation losses from the basins during At were ignored. The relative contributions of runoff from melting (a) of seasonal snow and firn and (b) of glacial ice to the river flows were estimated from

4 4 S/A = c5 with ~Zfi =f5 = 1 i=l i=l These equations could be simplified by two assumptions that followed from our observations. Firstly, the relative contribution of meltwaters from glacial ice was negligible until mid-June 1970, and the end of June 1971 when snow line started to move upwards towards the glacier regions. Secondly, runoff from May to September precipitation was small compared to the contributions other compartments made to the river flows during this time. Our detailed calculations which cannot be presented here gave, in summary, the following results : From March or April until June runoff from melting of the seasonal snow cover contributed increasingly to the rivers. From our calculations we noticed a difference between the two basins. Contributions started and terminated earlier and were higher in the North Saskatchewan River basin. They peaked at an estimated 60 per cent for the latter and 50 per cent for the Mistaya River basin in June 1970, and were slightly lower in 1971. This result agrees with what one would expect from the basin conditions, since the North Saskatchewan valley is lower in altitude. Throughout July, August and September meltwater from glacial ice in the ablation zones contributed significantly to the river flows. The North Saskatchewan River had received only slightly higher contributions than the Mistaya. They both peaked in August at an estimated 40 per cent; in September they were in the order of 20 per cent each year. In 1970 they started and peaked at least two weeks earlier than in 1971 which agrees with out climatological records. The mid-July tritium peak in the rivers was estimated to have consisted of about 10 per cent delayed meltwater runoff from firn at high altitudes, and about 70 per cent storage release. The remaining 20 per cent was estimated to represent the relative contribution of meltwater from glacial ice. The results obtained from this simple model that can accommodate nuclear basin signatures and their temporal variations were in good general agreement with those from a hydrological glacier runoff simulation model (Derikx and Loijens, 1971) and hydroglaciological analyses of the study area (Loijens, 1974).

CONCLUSIONS The study demonstrated that tritium measurements of the waters in large drainage basins can be used to estimate spring runoff from the seasonal snow cover and summer melt of glacial ice. These components of the water budgets of the rivers could so far not be estimated separately by classical hydrograph analyses. The nuclear technique provided this information in large, remote areas without having to operate extensive, costly networks of hydrological and meteorological field stations and is suggested as a powerful tool for water resources research and management in Canada.

Acknowledgements. We greatly appreciated the assistance in the field by Mr T. Bellaar- Spruyt and several summer students and the cooperation of the Banff National Park Administration. We should like to thank the Saskatchewan Research Council for the tritium analyses of our samples. NOTE: Not enough space was available for figures and tables. They will be included in a more detailed report under preparation. Nuclear techniques for glaciological studies in Canada 241 REFERENCES Ambach, W., Eisner, H. and Url, M. (1973) Seasonal variations in the tritium activity of runoff from an Alpine glacier (Kesselwandferner, Oetztal Alps, Austria). In Hydrology of Glaciers (Proceedings of the Cambridge, England, Symposium, September 1969), pp. 199-204: IAHS Publ. no. 95. Derikx, A. L. and Loijens, H. S. (1971) Model of runoff from glaciers. In Hydrology Symposium no. 8: Runoff from Snow and Ice (Symposium held in Quebec City, May 1971), pp. 152-199: Inland Waters Branch, Department of Energy, Mines and Resources, Ottawa, Canada. Loijens, H. S. (1974) Streamfiow formation in the Mistaya river basin, Rocky Mountains, Canada. Paper presented at the Western Snow Conference, Anchorage, Alaska, April 1974, pp. 86-95. Martinec, J., Siegenthaler, U., Oeschger, H. and Tongiorgi, E. (1974) New insights into the runoff mechanism of environmental isotopes. In Symposium on Isotope Techniques in Groundwater Hydrology (Proceedings of the Symposium held in Vienna, Austria, March 1974), vol. 1, pp. 129-143: IAEA Proceedings series SM-182/9. Martinec, J. (1975) Subsurface flow from snowmelt traced by tritium. Wat. Resour. Res. 11, no. 3, 496-498.