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Stream restoration and sewers impact sources and fluxes of water,carbon, and nutrients in urban watersheds
Citation:
Pennino, M., S. Kaushal, P. Mayer, R. Utz, AND C. Cooper. Stream restoration and sewers impact sources and fluxes of water,carbon, and nutrients in urban watersheds. HYDROLOGY AND EARTH SYSTEM SCIENCES. EGS, 20:3419-3439, (2016).
Impact/Purpose:
Nitrogen and co-pollutants such as phosphorus and carbon play a significant role in water quality, eutrophication, and harmful algal blooms. Urban watersheds differ significantly in their sources and fluxes of nitrogen and co-pollutants based on the structure and age of cities and on the extent and type of stormwater management. In order to assess the variability in pollutant flux, especially the fate and transport of nitrogen and carbon, we examined four watersheds in the Chesapeake Bay area near Baltimore, MD. We found high variability in annual exports and a high contribution of nitrogen from wastewater flow. Atmospheric nitrogen sources were important during storms. Annual C, N, and P exports increased with watershed impervious surface cover, and NO3- export decreased with greater stormwater management. Our results suggest that watershed urbanization exacerbates pulses of nutrient pollutants to streams and that urban stream corridor can be an important nonpoint source of carbon and nutrient loads to streams. Our data have significant implication for effective watershed management to stem pollutant loads including better wastewater management (i.e. leaky pipes), improved stormwater capture (i.e. green infrastructure and restoration), and improved planning of urban growth to limit pervious surfaces.
Description:
An improved understanding of sources and timing of water and nutrient fluxes associated with urban stream restoration is critical for guiding effective watershed management. We investigated how sources, fluxes, and flowpaths of water, carbon (C), nitrogen (N), and phosphorus (P) shift in response to differences in stream restoration and sanitary infrastructure. We compared a restored stream with 3 unrestored streams draining urban development and stormwater management over a 3-year period. We found that there was significantly decreased peak discharge in response to precipitation events following stream restoration. Similarly, we found that the restored stream showed significantly lower monthly peak runoff (9.4 ± 1.0 mm/d) compared with two urban unrestored streams (ranging from 44.9 ± 4.5 to 55.4 ± 5.8 mm/d) draining higher impervious surface cover. Peak runoff in the restored stream was more similar to a less developed stream draining extensive stormwater management (13.2 ± 1.9 mm/d). Interestingly, the restored stream exported most carbon, nitrogen, and phosphorus loads at relatively lower streamflow than the 2 more urban streams, which exported most of their loads at higher and less frequent streamflow. Annual exports of total carbon (6.6 ± 0.5 kg/ha/yr), total nitrogen (4.5 ± 0.3 kg/ha/yr), and total phosphorus (161 ± 15 kg/ha/yr) were significantly lower in the restored stream compared to both urban unrestored streams (p < 0.05) and similar to the stream draining stormwater management. Although stream restoration appeared to potentially influence hydrology to some degree, nitrate isotope data suggested that 55 ± 1% of the nitrate in the restored stream was derived from leaky sanitary sewers (during baseflow), similar to the unrestored streams. Longitudinal synoptic surveys of water and nitrate isotopes along all 4 watersheds suggested the importance of urban groundwater contamination from leaky piped infrastructure. Urban groundwater contamination was also suggested by additional tracer measurements including fluoride (added to drinking water) and iodide (contained in dietary salt). Our results suggest that integrating stream restoration with restoration of aging sanitary infrastructure can be critical to more effectively minimize watershed nutrient export. Given that both stream restoration and sanitary pipe repairs both involve extensive channel manipulation, they can be considered simultaneously in management strategies. In addition, ground water can be a major source of nutrient fluxes in urban watersheds, which has been less considered compared with upland sources and storm drains. Goundwater sources, fluxes, and flowpath should also be targeted in efforts to improve stream restoration strategies and prioritize hydrologic “hot spots” along watersheds where stream restoration is most likely to succeed.
URLs/Downloads:
Stream restoration and sanitary infrastructure alter sources and fluxes of water, carbon, and nutrients in urban watersheds