CONTROLS AND CONSEQUENCES OF ENHANCED CHEMICAL REACTIONS AT THE STREAMBED INTERFACE

Chemical reactions occur throughout natural waters however the reaction rates often are enhanced just below the sediment-water interface compared with surface water or deeper groundwater. Hydrologic exchange across the sediment surface brings reactive solutes and fine particulates from surface waters into contact with the abundant reaction sites associated with microbial colonies and reactive geochemical coatings on granular sediments. Furthermore, hydrologic transport across the interface may itself enhance reaction rates by relieving potential transport limitations through the re-supply of reactive substrates from surface water. Thus the chemical processing of metals, nutrients, and organic compounds at streambed interfaces may be enhanced either because of high rates of delivery of reactive solutes across the interface and/or because of the high intrinsic reaction capacity of the benthic biolayer. Here we highlight new understanding of enhanced reaction rates and their microbial and geochemical controls at streambed interfaces. Our approach distinguishes and quantifies reaction limitation and transport limitation both at the centimeter-scale within streambed sediment and at the river network scale where the effect of streambed reactions accumulates and influences downstream water quality.