The University of Arizona

Urban Runoff

Collaborative Research: Impacts of urbanization on nitrogen biogeochemistry in xeric ecosystems

Urbanization is dramatically altering hydrology and biogeochemistry that govern material transformations and movement of materials through ecosystems. In arid to semi-arid environments, such as much of Arizona, runoff from urban areas is increasingly managed as a source of active and/or focused recharge to limited groundwater resources. Urbanization increases both the volume and velocity of runoff during storm events that may in turn, enhance recharge to groundwater. However, it also increases the concentration and variety of pollutants that have may have detrimental impacts on the quality of this water. In particular, nitrate contamination in surface and groundwater is a growing concern in arid and semi-arid ecosystems in the southwestern U.S., yet little is known how urbanization alters nitrate accumulation, mobilization, transport and processing of nitrate in surface runoff.

In response to these knowledge gaps, researchers are quantifying how sources, transport, and fate of nitrate in storm runoff vary with degree of urbanization in a semi-arid environment. Nitrogen is both a direct threat to groundwater and an indicator of ecosystem function and thus allows us to address critical hydrological and ecological questions. Specifically, three questions are being addressed:

  1. How does urbanization impact the processes controlling delivery of nitrate from upland to lowland parts of the desert landscape?
  2. What are the dominant nitrate sources in arid urban watersheds? and
  3. How does wash substrate type modulate in- stream/wash processing of nitrate?

 

Researchers are characterizing seasonal patterns of nitrate export from a set of urbanizing watersheds that represent a gradient of land use density in Tucson and Phoenix (CAP LTER). They will perform simultaneous measurement of !17O, "#$O, "15N in nitrate on extracted from precipitation and surface water samples collected in these watersheds. In concert with water isotopes ("D,"#$O) and conservative geochemical tracers, the isotopic data is being utilized in conjunction with mixing models and response function representations of nitrate concentration and mass flux. Combined these methods will illuminate the underlying processes and mechanisms controlling nitrogen biogeochemistry along an urbanization gradient. The response functions will also be combined with a decision support system for urban flood hydrology to communicate to decision makers the implications of differing patterns of urban development

Broader impacts of this research lie in the potential to identify sources of surface and water nitrate contamination in arid and semi-arid deserts. Deserts and semi-arid regions occupy more than 30% of the Earth and are under increasing stress due to regional land use change such as urbanization and agricultural development. Water is precious in these regions, yet increasing incidences of contamination of ground and surface waters by nitrate have been observed and threaten this vital resource. The research will be carried out by new investigators who will coordinate local, state and federal resources to focus on a problem that has local, regional and global implications. Four graduate students are being trained in using sophisticated research methods, conducting interdisciplinary research, and communicating their understanding of the urbanization of water resources. At a local level, we are engaging citizen scientist volunteers to assist in collection of rainfall and rainfall chemistry. This program and other extension activities will promote a higher level of knowledge exchange.

See also UA News article: UA-Led Team Studying Nitrate in Urban Areas