Print PageAfter decades of research on the dynamics and controls over retention and transformation vs. transport of nutrients, we still know surprisingly little about these processes in urban landscapes, where land cover, hydrologic flowpaths, nutrient loads, and capacity for nutrient retention are profoundly altered. We established a network of stormwater monitoring sites in hierarchically nested catchments of Scottsdale, Arizona that vary in size from 5 to 17,000 ha and feature different stormwater infrastructure designs. Complete sampling through storm hydrographs of autumn (monsoon) and winter (frontal storm) events in 2010-2011 allowed us to examine changes transport of dissolved nitrogen (N), phosphorus (P), and organic carbon (DOC) through each event and as a function of size and infrastructure.
Results/Conclusions
Although particulate nutrient forms dominated transport (per the Part II presentation by L.Turnbull), concentrations of dissolved N and P species and DOC were often high. Among catchments, those with infrastructure designs that favored hydrological retention (i.e., dry retention basins) had higher concentrations than those that were highly connected (street drainage, or street-pipe infrastructure), likely because the latter flowed more often and nutrient accumulation between storms was consequently lower. Most sites showed clockwise hysteresis, indicating that nutrients were flushed from the catchments over the course of the events. Hydrologically retentive catchments exhibited counter-clockwise hysteresis, though, suggesting they were transport-limited rather than supply-limited. Changes in speciation of dissolved nitrogen through the course of individual events hinted that nutrient processing in the catchment may have been stimulated by the onset of rainfall, leading to increases in nitrate (nitrification) or decreases in organic nitrogen (mineralization) later in the event. Isotopic data on nitrogen and oxygen in nitrate revealed differential sources of this N species throughout the events, and supported the hypothesis that some processing of N was occurring in the catchment after the initiation of the storm. Overall, infrastructure had minor but detectable influences on dissolved nutrient transport. Washes, the most recent design for handling stormwater in neighborhoods, were not discernibly different from street-pipe infrastructure. This suggests that these designed ecosystems have a limited capacity for nutrient retention, despite similarity in appearance to non-urban desert washes. Thus, we caution that design of urban stormwater infrastructure should be evaluated for its efficacy in nutrient retention as well as hydrologic modulation.
