Headwater management alters sources, flowpaths, and fluxes of water, carbon, and nitrogen in urban watersheds
Abstract
Increased urbanization has altered watershed hydrology and increased nutrient pollution, leading to eutrophication and hypoxia in downstream coastal ecosystems. Due to urban stream degradation, there have been efforts to restore streams and reduce peak-flow discharges and contaminant export through stormwater management and stream restoration. However, there have been relatively few studies comparing watershed scale impacts of contrasting headwater management practices on sources and fluxes of water, carbon, and nutrients across space and time. In this study we compared sources and fluxes of water, carbon (C), and nitrogen (N) along 4 watersheds of contrasting headwater management: 2 urban degraded watersheds with minimal or no stormwater management and 2 managed urban watersheds with stormwater controls and stream restoration. Surface water samples were collected biweekly at USGS gauging stations located within each watershed over 2 years. Spatially, watersheds were sampled longitudinally during 4 seasons. Sources of water, nitrate, and carbon were investigated using isotopic and spectroscopic tracer techniques. Indicator anions (F-, Cl-, I-, SO42-) were also used to trace anthropogenic vs. natural water sources. Hydrologic flowpaths (groundwater vs. overland flow) were assessed with longitudinal synoptic surveys using stable water isotopes of H and O. Annual fluxes of water, C, and N, were estimated using the USGS program LOADEST. H and O isotope data showed that the source of stream water is primarily groundwater during summer months, with greater contributions from stormflow during winter months for all 4 watersheds. Elevated levels of indicator anions (F-, Cl-, I-, SO42-) as well as greater "pulses" of C and N over time in the degraded vs. managed watersheds indicate potential sewage sources due to leaky sanitary sewers and greater stormdrain inputs. Unlike the managed watersheds where hydrologic flowpaths were from groundwater in headwaters, the longitudinal H and O isotope data indicated that degraded watersheds had greater overland flow sources due to stormdrain infrastructure and engineered headwaters. The degraded urban watersheds consistently showed highly variable "pulsed" fluxes for C, N, P and indicator anions than the managed watersheds. While the managed watersheds showed lower total annual export for C, the annual N and P exports were not consistently lower than the degraded watersheds. Most of the C, N, and P was exported during higher flows in the degraded watersheds, while most of the nutrient export for the managed watersheds was during baseflow. Our results suggest that watershed restoration strategies have the potential to alter sources fluxes, and flowpaths of water, carbon and nitrogen. Along the urban watershed continuum, there may be differences in the potential for stormwater management vs. stream restoration to alter sources and fluxes of nutrients, which has implications for management of important biogeochemical processes in urban streams and rivers.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B43H0534P
- Keywords:
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- 0428 BIOGEOSCIENCES / Carbon cycling;
- 0469 BIOGEOSCIENCES / Nitrogen cycling;
- 0470 BIOGEOSCIENCES / Nutrients and nutrient cycling;
- 0493 BIOGEOSCIENCES / Urban systems