Water Resources Research Act Program

Details for Project ID 2009WA265B

Influence of Large Wood Addition on Nitrogen Transformations at the Surface water/groundwater interface

Institute: Washington
Year Established: 2009 Start Date: 2009-03-01 End Date: 2010-02-28
Total Federal Funds: $27,500 Total Non-Federal Funds: $56,462

Principal Investigators: Clay Arango, Carey Gazis

Project Summary: Stream restoration projects often include the addition of large wood to streams to diversify in-stream habitat, retain organic matter, and promote surface water infiltration into the floodplain for late season release to streams. Although wood addition is a common method of stream restoration, relatively little is known about how this approach changes surface water quality by affecting nutrient transformations at the surface water/groundwater interface. In gravel-cobble bed streams, which are common in the western United States, epilithic biofilms release ammonium by mineralizing dissolved organic matter carried from the surface water into shallow floodplain aquifers. This ammonium can be nitrified and released as nitrate at downstream upwelling sites. Because nitrate is a much more mobile form of nitrogen, it can be exported much longer distances prior to biotic removal from the water column, and it frequently contributes to eutrophication in downstream water bodies. Therefore, increasing surface water/groundwater interaction by large wood addition could increase the transformation of ammonium to nitrate and could have the unintended consequence of lowering water quality. We propose a Before-After Control-Impact study design to assess how wood addition affects nitrification in shallow floodplain aquifers before and after wood addition. We have four primary objectives in our study: 1) characterize hydraulic properties of substrate and measure patterns of downwelling and upwelling before and after wood addition, 2) measure changes in transient storage and hydrologic retention (as channel friction) before and after wood addition, 3) measure hyporheic nitrification rates before and after wood addition, and 4) measure whole-stream nitrification rate and uptake of ammonium and nitrate before and after wood addition. To meet the first objective, we will use a network of piezometers located in upwelling and downwelling zones of the upstream control and downstream treatment reaches to measure vertical hydraulic gradient. To meet the second objective, we will use conservative tracer releases and the OTIS model to estimate transient storage parameters in the stream. We will also calculate channel friction (Darcy-Weinbach friction factor) as an estimate of hydrologic retention. To calculate hyporheic nitrification for our third objective, we will incubate sediments in situ to measure net nitrate accumulation, and we will incubate a second set of sediment to measure changes in dissolved oxygen to calculate respiration of organic matter. In our final objective, we will measure whole-stream ammonium and nitrate uptake rates to determine the transport distance of inorganic nitrogen. We will also measure whole-stream nitrification to be able to determine the relative contribution of sediment nitrification and stream nitrification to whole-stream nitrate dynamics. Our proposal has the support of the Yakama Nation, who anticipates that our project will clarify how wood addition in association with stream restoration projects will influence water quality in stream ecosystems. Our proposal also has the support of the Department of Ecology because our study will compliment their upcoming large-scale study of nutrient levels in the Yakima River basin. Further, we anticipate that our results will answer some novel ecological questions that will advance our general understanding of stream nutrient cycling and surface/groundwater interactions.