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.
How do we maintain clean water and a healthy environment in rapidly growing desert cities? We assembled a team of undergraduate, graduate students, interested "citizen scientists", city managers, and University professors to address this question in Tucson, Arizona. Findings from our study are being used by these communities to help manage stormwater runoff and land use. The Tucson metropolitan area is home to over 1 million people and relies heavily on groundwater for drinking water. Groundwater can be replenished by rainwater infiltrating at high elevations in the mountains but also infiltrate in concentrated portions of the landscape like dryland rivers or ephemeral channels where streams flow only following rainfall events of a given size. In the western US, these ephemeral systems represent most of the streams (89% for western US and 94% in Arizona). As cities are built in these desert environments, humans are modifying these stream channels and contributing landscapes with little consideration for how they may affect surface water quality and quantity and the amount and quality of the water that ends up in the groundwater. Our results show that increasing the amount of asphalt and concrete speeds the removal of rainfall from roads, and also increases the frequency and duration of runoff events by lowering the rainfall size needed to produce runoff. This increase in runoff also increases the concentration of potential pollutants in the surface water. Increasing runoff to ephemeral channels may thus increase the water that can be recharged to groundwater in these dry cities, but the trade off is likely decreased water quality. Our research also shows that by managing the ephemeral streams in a more natural state –with pervious substrate like sand or grass, we can promote the natural processes that remove pollutants from the runoff waters before they get groundwater. However, this also introduces tradeoffs for greenhouse gas emissions –those gases that are particularly good at trapping long wave radiation and holding heat in the atmosphere. We found that greenhouse emissions were surprisingly high from urban ephemeral washes following small rainfall events and simulated flood pulses and that these fluxes varied with channel structure indicating that human modification of channel structure such as texture and carbon inputs hold import for nitrogen, storm water, and groundwater management. These findings indicate that ephemeral channels are important as concentrated areas of nutrient cycling and gas fluxes that can remove pollutants, particularly nitrate, from water before it is delivered to groundwater. Increases in frequency and duration of runoff associated with urbanization as described above will likely enhance these losses. Collectively, we show tradeoffs of human modifications of channels for greenhouse gas emissions and aqueous losses in ephemeral streams in Tucson, Arizona. Our integrated findings indicate human modification of storm water management systems to impervious channels increases delivery of water and N to downstream navigable waters and areas of focused recharge whereas importation of clay materials leads to higher trace gas losses as N and reduced delivery of nitrate. Management will need to address long-term challenges of balancing tradeoffs of greenhouse N gases and N in water supplies, intermediate scale issues of enhancing recharge, and short-term issues of removing water and preventing flooding. As such, our findings have been incorporated into recommendations of storm water management practices of ephemeral washes and land use by the Environmental Protection Agency as well as local communities and organizations in Arizona.
