Urbanization dramatically modifies the movement and transformations of nitrogen (N) compounds in semi-arid ecosystems. In particular, nitrate contamination of drinking water is a growing concern in urban areas, especially in arid and semi-arid environments, where urban runoff is actively-managed to recharge groundwater and augment water supplies. Water managers and urban planners, however, lack information on what ecosystem characteristics are most important in controlling the quality of this recharged water, especially its nitrate concentrations. This research will quantify how sources, transport, and fate of nitrate in storm runoff vary with the density and type of urban land use in Tucson and Phoenix (CAP LTER), Arizona watersheds. Seasonal patterns of nitrate export will be characterized, and new isotopic tracer techniques will be used to understand nitrate sources and mechanisms controlling nitrogen transformations along semi-arid urbanization gradients. These mechanisms will be modeled and integrated into interactive visualization products that will aid in decision-making regarding urban development patterns and storm water management approaches.
This research will help identify sources of surface water and groundwater nitrate contamination in arid and semi-arid deserts. Water is precious in these regions, yet increasing incidences of contamination of ground and surface waters threaten this vital resource. This research coordinates local (Tucson, Phoenix), state (Arizona), and federal (National Atmospheric Deposition Program/Environmental Protection Agency) resources to focus on a problem that has local, regional and global implications. The project will engage citizen-scientist volunteers, train graduate and undergraduate students in policy-relevant research, foster interactions between scientists and decision makers, and develop transferable visualization tools.
This research has sought to understand how urbanization alters the hydrological and biogeochemical processes that govern the movement and transformation of materials through ecosystems. Numerous studies have documented that runoff volume, peak flows, and pollutant loads increase in storm runoff when cities develop, and the traditional perception is that the extent of impervious surface cover-- hard surfaces like pavement, rooftops, etc. -- determines the amounts of water flow and material transport. However, we hypothesized that it is the interplay among the intensity, configuration, and type of urbanization and the type and configuration of stormwater infrastructure, together with climate variability, that govern these processes in aridland cities. This study investigated the influence of watershed and storm characteristics on the origins of nitrogen in storm flows, the potential for removal of that pollutnat, and stormwater runoff (i.e., amount of flow) in the greater Phoenix metropolitan area. Study watersheds spanned a gradient of size and infrastructure type from highly connected (i.e., those drained primarily by streets and/or pipes) to highly disconnected (i.e., featuring retention basins) infrastructure systems, facilitating an assessment of how this variation influenced water and pollutant movement to downstream systems (delivery). Results demonstrate a profound influence of stormwater infrastructure type on water and pollutant delivery: catchments with highly connected stormwater infrastructure generated runoff in response to limited rainfall, whereas the runoff response was less frequent and less flashy in catchments that are hydrologically disconnected. Runoff from catchments occurs less frequently and runoff coefficients (proportion of rain that runs off) decrease with increases in spatial scale, which is analogous with scaling behavior observed in non-urban aridland catchments. A key factor governing runoff response at the largest scales of observation is spatial variability in rainfall. Nitrogen concentrations (specially nitrate, which is a pollutant in high concentration) in urban stormwater runoff were often high. Infrastructure designs that favor hydrological retention (i.e., retention basins) yielded higher concentrations than those that are highly connected (street drainage, or street-pipe infrastructure), likely because the latter flowed more often and nitrogen accumulation between storms was consequently lower. Overall, infrastructure had minor but detectable influences on nitrogen transport. These designed urban ecosystems thus have a limited capacity for nitrogen retention. Isotopic analysis of nitrate in rainfall and in stormwater runoff indicates that urban yards are major sources. Patterns across many storms suggest that hydrology is the dominant control on nitrogen delivery from urban watersheds. However, urban watersheds are not merely passive conduits for nitrogen; they may instead retain the majority of nitrate that enters watersheds as atmospheric deposition. It seems likely that most of this retention occurs in residential yards, rather than in stormwater infrastructure features, given the large area of yards and the high rates of biogeochemical processing within them. These findings contrast with earlier work that suggested that stormwater features may be hotspots of biogeochemical transformations at the watershed scale. Stormwater infrastructure features may indeed be hotspots of nitrogen removal, but our research suggests that the mechanisms are hydrologic rather than biogeochemical. Particulate nutrient forms often dominate nutrient transport. In catchments with highly connected infrastructure, the high frequency of runoff means that particulate materials are also flushed frequently from the catchment. In contrast, in catchments with disconnected stormwater infrastructure, a larger amount of rainfall is required to generate a runoff response at the catchment outlet and runoff is less flashy than in highly connected catchments, meaning that flow has a lower capacity to entrain and transport particulate materials. The high contribution of particle-bound forms of nutrients to total catchment nutrient export within arid urban catchments has not been realized previously, and export may be underestimated with without quantification of particle-bound forms of nutrients. The transport of nutrients attached to sediment could have significant implications for nutrient cycling at longer time scales, since a high fraction of nutrients bound to sediment tend not to be immediately bio-available, and thus the deposition of sediment within stormwater features when flow subsides could contribute to the formation of biological hot spots. The most compelling finding of our work is that the ecosystem services provided by systems designed to either shunt water out of the system as rapidly as possible or to retain water within the system sometimes have consequences for the provision of ecosystem services related to water quality modulation. Thus, an important outcome of this research is to have demonstrated that the design of urban stormwater infrastructure should be evaluated for its efficacy in material retention as well as hydrologic modulation. The project has contributed directly to the education of a Ph.D. student, a post-doctoral scholar, and numerous undergraduate students, including students from underrepresented groups. Project findings have enhanced the scientific community's understanding of stormwater dynamics in urban, arid ecosystems, and project data and infrastructure have laid the groundwork for further investigations.
