THE EXPANDING SCOPE OF HYPORHEIC-ZONE INVESTIGATIONS IN STREAMS, RIVERS, AND WETLANDS: CONNECTING SMALL-SCALE STORAGE PROCESSES WITH REACH-SCALE CUMULATIVE EFFECTS

The movement of surface water into and out of storage in streams, rivers, and wetlands delays downstream transport of solutes, increases contact time with reactive surface coatings on sediments and vegetation, and stimulates biogeochemical reactions that can remove contaminants from flowing water. Past investigations of the storage processes (bed sediments, aquatic vegetation, periphyton mats, and zones of slow-moving surface water) fall into two categories. One has emphasized cumulative effects through a reach-averaged analysis of solute tracer movement. The other style of investigation emphasizes fundamental processes through fine-scale measurements of tracer fluxes into and out of individual storage areas. Solute tracer injections have the advantage of providing an integrated picture of storage, but they lack the specifics about processes controlling storage that are necessary to transfer results to other flow regimes or channels with different geomorphic characteristics. Process-based measurements of hyporheic flow and reaction rates at small scales (as fine as centimeters in the streambed) are specific about the controlling processes, but are themselves too variable to estimate cumulative effects at larger scales. Our approach has been to conduct simultaneous centimeter-scale and kilometer-scale measurements during in-stream tracer experiments. This multi-scale approach has allowed the relative contributions of different types of storage areas to be determined, while at the same time quantifying the cumulative effects of storage on transport and reaction at the scale of kilometers. Results led to improved models that distinguish the major components of storage and reaction in surface water, within dense vegetation, and in both shallow and deeper hyporheic flow paths. These improvements are helping us to generalize beyond our site-specific experiments to other systems. Examples include determination of the role of hyporheic zones in denitrification in agricultural streams in Indiana, and determining the relative importance of emergent vegetation and hyporheic zones in storage and reaction of 1) dissolved metals in a semi-arid stream receiving acidic mine drainage, and 2) phosphorus in the floodplain wetlands of the Everglades.