Water Resources Research Act Program

Details for Project ID 2019GA058B

Impact of beaver dams and beaver dam analogues on hydrologic and nitrate retention in Atlanta, GA

Institute: Georgia
Year Established: 2019 Start Date: 2019-06-01 End Date: 2020-05-31
Total Federal Funds: $18,000 Total Non-Federal Funds: $48,566

Principal Investigators: Dr. Sarah H. Ledford

Abstract: Overall ecosystem retention of solutes is related to the hydrologic retention of the water and the biological or chemical retention of the solute. By replacing pervious land surfaces with impervious surface cover, urban areas have drastically decreased the hydrologic residence time of water in streams [Soulsby et al., 2014] by increasing hydrologic connection between the land and stream, resulting in increased flashiness during storms and often decreased baseflow between precipitation events, symptoms termed the Urban Stream Syndrome [Walsh et al., 2005]. More specifically for nutrient retention, channelization of streams, which sometimes includes replacing the streambed with concrete or boulders, reduces surface water-groundwater interactions, decreasing the size of the hyporheic zone, a location of vital importance for increased hydrologic residence time that also allows solutes to interact with microbes and sediment reaction sites in the subsurface, resulting in overall ecosystem retention. One of the goals of stream restoration in urban areas has been to increase hydrologic residence time of water in systems that had previously been engineered to move water out as quickly as possible [Bernhardt et al., 2005; Craig et al., 2008; Palmer et al., 2014]. The City of Atlanta has an active ecological community of beaver, who have built dams in urban streams across the city. However, pressure from development, along with limited resources due to small, disconnected pockets of forest, have driven beaver out of some areas where they used to reside. Because of this, the City’s Department of Watershed Management is piloting the installation of beaver dam analogues (BDAs) to reconnect an incised stream to its surrounding floodplain. Understanding the success of this investment of public dollars in restoration infrastructure in improving hydrologic residence time and nutrient retention is vital. By slowing down the surface water flow through damming, both natural and man-made dams will increase hydrologic retention. It is also expected that the increased hydrologic head behind dams will increase surface water flow into the groundwater, resulting in increased hyporheic exchange and thus increased potential for biologic retention of nutrients, specifically nitrate. Nutrient contamination of surface water in the city of Atlanta is widespread, due to the City’s many wastewater treatment plants, combined sewer system, sanitary sewer overflows, along with other non-point sources including lawn fertilizer. This study aims to address two hypotheses: H1: Both beaver dams and beaver dam analogues significantly increase the hydrologic residence time of surface water in cities, both in dry and wet antecedent moisture conditions. H2: Both beaver dams and beaver dam analogues induce hyporheic exchange below the dam, resulting in increased nitrate retention during baseflow. Two sites will be used for this study- a beaver dam on an unnamed tributary to Shoal Creek in the South River basin, and beaver dam analogues that have been installed on a small tributary of Nancy Creek in the Chattahoochee River basin, both in the metropolitan Atlanta region. A network of piezometers will be installed above and below each dam and injections of deuterium and salt will be conducted during dry and wet antecedent moisture conditions. Samples will be analyzed for stable isotopes of water, along with chloride, to evaluate hydrologic residence times including evaporation rates in ponded water behind the dams. Samples will be analyzed for concentrations of dissolved oxygen, nitrate, nitrite, and ammonium to evaluate nitrate retention in the hyporheic zone. Measurement of vertical hydraulic gradient between the surface and hyporheic water, along with hydraulic conductivity of the subsurface, will allow for the calculation of vertical flux, and loadings of different nitrogen species. Comparison of hydrologic residence time and nitrate retention between the sites and in the stream reaches immediately above each site will help us identify if investment of public funds in BDAs will help improve streams in the Atlanta region