Description
Eutrophication is a common problem for surface waters, causing increases in phytoplankton and algal blooms that can negatively impact the environment. Stream ecosystems can remove significant fractions of nutrient loads, due to both removal by riparian vegetation, and to microbial activity in the hyporheic zone, which is the subsurface zone located beneath the streambed where surface water and groundwater mix. Urbanization and channelization can significantly reduce nutrient retention in streams by reducing or eliminating hyporheic recharge and vegetation uptake. To identify how effective riparian zones are at retaining nutrients in a highly urbanized watershed, storm sampling, tracer addition experiments and modelling of the system was done on two experimental reaches in San Diego, CA. Storm samples were taken upstream and downstream of riparian zones on two reaches, and analyzed for nitrogen. Tracer addition experiments using Bromide (Br-) and nitrate (NO3-N) were done during the dry season on two different reaches of Alvarado Creek (channelized, 122.08 m; natural 281.31 m). Samples were collected during the addition at various distances downstream from the injection site (17 – 281 m). Retention was calculated for storm sample and tracer addition sample data for each reach. The breakthrough curve was then modelled for each sub-reach in the One-Dimensional Transport with Inflow and Storage (OTIS) model. Transient storage metrics and error statistics were then calculated to determine ideal parameter sets. Despite OTIS and OTIS-P model limitations, it was determined that both hyporheic exchange rate and size are small in riparian sections of urban watersheds, with hyporheic exchange primarily occurring in transient storage zones based on the Br tracer data. Uptake of NO3-N during the tracer injection experiment was limited (<5%) in the concrete channel, but was up to 7% over a short (120 m) earthen reach, suggesting that an entire stream network, covering several km, could remove a significant percentage of the nitrate load. During stormflows, high discharge and flow velocities significantly decrease residence time and limit nutrient retention. Potential future research could use 15N-isotopic NO3-N over a longer injection period in order to identify and quantify specific species in the riparian zone contributing to NO3-N uptake.
