A Comparison of Chemical and Isotopic Hydrograph Separation

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Abstract
As part of the construction of a simulation model to test an acid precipitation neutralization mechanism, the stream hydrograph was separated into its base flow and event water components using stable environmental isotopes of water, naturally occurring conservative tracer. Three snowmelt events and one storm event during the winter and spring of 1984 were studied at an instrumented watershed in the Hubbard Brook Experimental Forest, New Hampshire. Conditions for use of the isotopic tracer were not always met, however. During the latter part of the snowmelt and the storm the isotopic content of the groundwater and event water were not distinguishable. Furthermore, the isotopic content of the meltwater varied considerably over time, thereby reducing the precision of the hydrograph separation. Frequent sampling of the meltwater is mandatory to assess this variability. Because the concentration of major cations and anions was measured as well, chemical tracers could be compared to the isotopic tracer, when the isotopic hydrograph separation was reliable, to test whether the chemical tracer was conservative. Dissolved silica was found to act as a conservative tracer for this watershed.