Forecasting Nitrogen Concentrations

FAYETTEVILLE, Ark. – Tracking pollutants in urban environments presents a challenge for researchers, but predicting pollution levels is necessary to ensure sustainability of urban areas. University of Arkansas researcher Steven Burian has developed a linked air-water modeling system that can predict concentrations of nitrogen compounds from a Los Angeles watershed.

"The different elements of a city are coupled, interacting and feeding back with each other in nonlinear, sometimes counterintuitive ways," explained Burian. "We are developing a system of linked models to look at sustainability and vulnerability issues for cities."

Burian, assistant professor of civil engineering, is working with Timothy McPherson, Michael Brown, Jerry Streit and H.J. Turin of Los Alamos National Laboratory on a project funded through the Urban Security Initiative. This multi-disciplinary effort examines the relationship between urban infrastructures — such as power, transportation, water and wastewater systems — and the natural environment.

Developed for the Ballona Creek watershed in urban Los Angeles, Burian’s air-water modeling framework links the CIT airshed model and the U.S. EPA Storm Water Management Model (SWMM). The input and output datasets are managed in a Geographical Information System (GIS) environment. The modeling framework can be operated in a high-fidelity mode to look at processes in detail and a low-fidelity mode for planning-level assessmetns.

Ballona Creek is located in an extremely urbanized catchment that drains a substantial part of Los Angeles. It has been shown to be the greatest source of non-point source pollution for Santa Monica Bay. Running approximately 12 kilometers (7.5 miles) from downtown Los Angeles to Santa Monica Bay, about half of Ballona Creek is a concrete-lined trapezoidal channel, but the section from Centinela Avenue to Santa Monica Bay has a dirt/sand bottom and rock-lined sides. All of the tributaries to Ballona Creek are man-made, primarily concrete channels or pipes.

During wet weather the flow rates in Ballona Creek can increase by a factor of at lease 100. Modeling of these events is complicated by the effects of the high tide in Santa Monica Bay, which can hold up the discharge despite high flow rates and substantial water volume.

Burian’s model incorporates land use data from the Southern California Association of Governments, elevations from the U.S. Geological Survey and road data from the U.S. Census. In addition, the team collected data on the location, size, slope and length of storm drains by physically verifying data from the Los Angeles Department of Public Works. Nearly 3,000 storm drains with a length of more than 365 km (587 mi) were characterized to develop the model. Parameters for the water quality portion of the model were developed with historical water quality provided by researchers at the University of California- Los Angeles and the Los Angeles County Department of Public Works.

Although his model accurately predicted the nitrogen levels in a storm event on Dec. 4, 1999, Burian and his team are seeking to further validate and extend the model. Additional data are being collected to calibrate the model and replace preliminary estimates with more site-specific data.

Burian will present his results at the Restoring Urban Wetlands Conference in Los Angeles on May 17. The conference is sponsored by the Ballona Wetlands Foundation, the Los Angeles and San Gabriel Rivers Watershed Council, Santa Monica Bay Restoration Project, the UCLA Institute of the Environment and the Trust for Public Land.