Application of environmental isotopes in storm-discharge analysis of two contrasting stream channels in a watershed

Abstract

Oxygen and deuterium isotopes were used as tracers to identify the rainwater and pre-storm water components of storm runoff in the Cedar River Watershed of northeastern Iowa, U.S.A. Instantaneously discharged water in the main channel, the Cedar River, showed steady chemical characteristics during the storm event. Hydrograph separation by oxygen and deuterium isotopes demonstrated that the discharge water predominantly consisted of ground water (83%). It suggested that the Cedar River is in good hydrologic connections with the upper bedrock aquifer. The rainwater effectively infiltrated into the subsurface forcing the contaminated ground water to discharge into the main channel. In contrast, the Dry Run Creek, a tributary of the Cedar River, demonstrated increased dominance of overland flow. The water in the creek was considerably impacted by the rain event through dilution, and the instantaneously discharged water predominantly consisted of rainwater (54%). The remaining 46% was the subsurface water derived from the shallow alluvial aquifers of sand and gravel deposits. The Dry Run Creek is in hydrologic connections with the alluvial aquifers only, and the less significance of pre-storm water in the creek is attributed to the discontinuous nature of the sand and gravel deposits in the valley area. This isotopic investigation demonstrated that the origin of water components in storm runoff and its geochemical evolution can vary considerably from channel to channel within the same watershed.