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The variability of stream chemistry in a coast mountain watershed, British Columbia

University of British Columbia

A glacierized 24 km² watershed in the Coast Mountains of British Columbia was studied during the 1976 melt season in order to investigate the naturally occurring spatial and temporal variations of stream water chemistry. The chemical species measured were those that have been shown to be the major products of chemical weathering: calcium, magnesium, potassium, sodium, and silica. Detailed analysis of errors in field and laboratory procedures were an integral part of the research design and it was shown that errors on individual determinations were similar to those of university and government laboratories. The two major components of the research design were: 1) comparison of the chemistry of four major tributaries within the study area, and 2) an analysis of the temporal variability of stream chemistry at the basin outlet. The results of paired comparisons of tributary chemistry were consistent with geologic differences between sub-basins. For example, potassium concentration was greater in streams draining granodiorite than in a stream draining only quartz diorite, reflecting the lesser abundance of K-feldspar in quartz diorite. The highest solute concentrations were observed in spring water draining a metamorphic roof pendant while glacier meltwater had the lowest concentrations of all terrestrial water. Four models were investigated for describing the relationship between stream chemistry and stream discharge at the basin outlet. The best predictive model for calcium concentration (log Ca = a + b log Q, Ca = 10[sup a] Q[sup b]) explained 60% of the calcium variance. However, subsequent analysis showed that the unexplained variance contained information about temporal changes in runoff sources. In particular, the time-dependent behavior of the residual variance was interpreted as the result of an increase in the ratio of glacier meltwater to ground water discharge through the summer. Furthermore, the sub-basin approach in the research design made it possible to objectively identify the two glacierized sub-basins as the source of this phenomenon. All solutes behaved similarly except potassium which generally varied little through time or with changing discharge. This was attributed to the high availability of K⁺ in vegetation and soils and an apparent buffering of its concentration in the weathering zone. In a glacierized sub-basin almost lacking well-developed soils and forested slopes, potassium behaved more like the other solutes. All terrestrial water was shown to be in equilibrium with kaolinite in the Ca-plagioclase weathering system. Within the kaolinite stability field, water samples from different sources plotted in positions that were consistent with the availability of plagioclase, residence time, and the availability of C0₂ as a source of acidity during hydrolysis.

Text

2010-03-03

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http://hdl.handle.net/2429/21480

Geography

University of British Columbia

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