Changes in hydrology and pollutant loads from stream restoration in an urban headwater catchment

Abstract

Stream restoration structures can alter the hydrology and biogeochemical processes of urban headwater streams in important ways. As a stream restoration design, regenerative stormwater conveyance (RSC) structures are built to reconnect floodplains to stream channels, raise the groundwater table, and increase streamwater retention times. Altered hydrology and flow dynamics within restored stream reaches can affect solute concentrations and loads in baseflow and stormflow runoff. We monitored interannual changes in precipitation, groundwater and stream runoff before and after the implementation of a stream restoration using an RSC design in order to evaluate changes in hydrology and to estimate changes in solute loads of an urban headwater catchment in the Rock Creek watershed of Washington DC. A Before After Control Impact (BACI) experimental design was used. Compared to the control catchment, average recharge of the groundwater system of the RSC riparian area increased by 0.76 m in the post-restoration period and contributed to a 6 % increase in baseflow as a percentage of total runoff (compared to a decrease of 6 % in the control catchment). Area yields from the pre- to the post-restoration monitoring period decreased in both the RSC and control catchments, but significantly larger decreases in total nitrogen (TN), total phosphorus (TP) and total suspended solids (TSS) occurred in the RSC catchment (i.e., −59 % vs −23 %, −54 % vs −28 %, and −76 % vs −40 %, respectively). Results of this study indicate that RSCs as stream restoration structures in degraded headwater catchments can result in important hydrological and biogeochemical changes that significantly reduce nutrient and sediment loads.