Modeling the atmospheric deposition and stormwater washoff of nitrogen compounds
Introduction
The earth's population is increasing at a rate of approximately 1.45% annually and is tending to concentrate more and more in coastal and riparian areas (LANL, 2000). In 1960, approximately 80 million people lived in coastal areas of the United States. As of 1990, 112 million people lived in US coastal areas, and projections indicate 127 million people will live on the US coast by 2010. Southern California Counties are projected to be among the leaders in absolute population change (Culliton et al., 1990). Increased human population and population density in coastal areas have placed increased pressure on coastal ecosystems. One impact that has been well studied in the past decade in the United States and elsewhere is direct and indirect inputs to water bodies from atmospheric deposition (US EPA, 2000, Hicks, 1997). In particular, research has focused on quantifying the atmospheric inputs of nitrogen compounds to coastal water bodies (e.g. Weaver et al., 1999, Scudlark et al., 1998, Fisher and Oppenheimer, 1991, Hinga et al., 1991).
The input of biologically available nitrogen compounds to surface waters can produce several deleterious water-quality impacts (Jickells, 1998, Novotny and Olem, 1994). Excessive nitrogen inputs in the form of ammonium (NH4+), nitrite (NO2−), nitrate (NO3−), and organic nitrogen compounds to sunlit and quiescent water bodies that are nitrogen limited could accelerate the growth of phytoplankton and macrophytes. Increased frequency of phytoplankton blooms could lead to the development of hypoxic conditions (reduced oxygen levels) or anoxic conditions (complete lack of oxygen) in water bodies. Moreover, altered nutrient dynamics could result in loss of species diversity, fundamental changes to ecosystem structure, and toxic algal blooms (US EPA, 2000, Hicks, 1997). These conditions can adversely affect aquatic life and the associated beneficial uses of water bodies.
Nitrogen compounds can enter a water body via tributaries, overland flow, subsurface flow, atmospheric dry and wet deposition, storm drainage systems, and discharges from industrial and municipal wastewater treatment facilities. The effective management of water bodies receiving this variety of inputs requires accurate knowledge of pollutant sources and transport pathways. The problem is inherently multimedia requiring an integration of atmospheric, watershed, and water body processes. Information produced by integrated airshed–watershed–water body monitoring and modeling studies is imperative to understand and analyze such complex problems and to develop and evaluate linked airshed–watershed management plans.
Because of its size and economic and political importance, Chesapeake Bay is probably the most studied large water body from an integrated airshed–watershed–water body perspective. Research has focused on the quantification of direct atmospheric deposition loadings and indirect loadings from deposition onto the watershed (Garrison et al., 1999, Dennis, 1997, Fisher and Oppenheimer, 1991, Fisher et al., 1988), identification of nitrogen sources (Russell et al., 1998), and linked airshed–watershed modeling (Wang et al., 1997, Linker and Thomann, 1996). Results from these studies and others have indicated that atmospheric deposition accounts for approximately 27% of the nitrogen load to the bay. The atmospheric deposition component includes the loading from direct wet and dry deposition onto the bay and the indirect loading from dry and wet deposition onto the bay watershed.
Atmospheric deposition has also been found to account for a significant fraction of the total nitrogen loading to other coastal water bodies. In Florida, studies have determined that approximately 26% of the nitrogen load to Sarasota Bay and 28% of the nitrogen load to Tampa Bay comes from atmospheric deposition and indirect loading from deposition onto the watershed (Valigura et al., 1996). In the northeast United States, atmospheric deposition accounts for approximately 20% of the total nitrogen load to Long Island Sound (Hu et al., 1998, Valigura et al., 1996).
Besides monitoring studies, several integrated airshed–watershed–water body modeling studies have been performed in the past decade. The Chesapeake Bay Program has been especially active in performing integrated modeling efforts to identify the sources of nutrient inputs to the bay and to determine the relative importance of atmospheric deposition. For example, Wang et al. (1997) determined the wet deposition of nitrate and the dry deposition of nitrate, organic nitrogen, organic phosphorus, and dissolved organic phosphorus to the Chesapeake Bay watershed. These deposition values were used as inputs to the Phase IV Chesapeake Bay Watershed Model. In another modeling analysis, E.H. Pechan and Associates, Inc. (1996) evaluated the effects of various NOx emission reductions on the nutrient inputs to the Chesapeake Bay. They utilized a projected 2005 emissions inventory developed as input to the US Environmental Protection Agency's (EPA) Regional Oxidant Model (ROM). Nitrate deposition by bay subwatershed was estimated by converting NOx emissions to deposition using source-receptor coefficients determined by repeated runs of the Regional Acid Deposition Model (RADM). The Chesapeake Bay Watershed Model then estimated nitrate loading to the bay from the nitrate deposition amounts.
Studies of pollution inputs to the Great Lakes have also used integrated airshed–watershed–water body modeling. The mass balance studies of the EPA Great Lakes Program (e.g. Lake Michigan Mass Balance Study; EPA Great Lakes Program, 2000) link transport and transformation models to study the changes in concentrations in the air, water, soil, and biota that would result from changes in loading. The goal of the effort is to provide a scientific basis for the determination of load reductions to meet their desired quality objectives.
One of the goals for linking atmospheric deposition models and watershed models is to determine the relative contribution of the total loading to water bodies resulting from atmospheric deposition. Pollutants can essentially follow two transport pathways from the atmosphere to the water body; one is through direct deposition or diffusion and the other is through deposition/diffusion to the watershed and subsequent washoff during rainfall events. By determining the direct and indirect contribution from atmospheric deposition, appropriate integrated air–water quality management plans can be formulated and implemented. The research efforts described above have produced significant advances in integrated airshed–watershed–water body modeling, but there is still a need for further research to reduce uncertainty and improve the accuracy of integrated modeling efforts (Hicks, 1998).
This paper describes a research effort to link an air chemistry model and an urban runoff model to simulate the fate and transport of nitrogen compounds through the atmosphere and storm drainage system of the Los Angeles Basin. The goal of the project was to develop an operational linkage between the CIT airshed model (Russell et al., 1988, McRae et al., 1982) and the US EPA Storm Water Management Model (SWMM) (Huber and Dickinson, 1988). Nitrogen compounds were selected for this demonstration because they are important in both the air and water environments (Ellis, 1986), they have been shown to be detrimental to coastal water quality (Jickells, 1998), and data were available to test the model linkage for the Los Angeles Basin.
In the linkage described here, the CIT model, using an air emissions inventory and meteorological data, simulates the fate and transport of nitrogen compounds and other photochemical pollutants in the atmosphere. SWMM simulates the stormwater runoff flow rates and relevant nitrogen compound concentrations from drainage subcatchments and the flow and transport of the nitrogen compounds to the outlet of the storm drainage system. In this paper we describe CIT and SWMM, we discuss the steps we took and assumptions we made to provide a preliminary linkage, and we apply the CIT–SWMM linkage to a demonstration problem. For the demonstration we determine the relative contributions of pollutant loading to Santa Monica Bay from: (1) direct dry deposition, (2) direct wet deposition, (3) dry deposition onto a watershed washed into the bay by surface runoff, and (4) wet deposition onto a watershed that becomes part of the surface runoff.
Section snippets
Model descriptions
CIT is an Eulerian-based model and solves the transport and chemical reactions of pollutants in the atmosphere using a numerical solution scheme for a set of 35 reacting chemical species (Russell et al., 1988, McRae et al., 1982). It contains 106 chemical kinetic equations and implements the Lurmann, Carter, and Coyner lumped molecule chemical mechanism. CIT requires (1) land use and an emissions inventory defined over the modeling grid and (2) wind, temperature, and atmospheric boundary layer
CIT–SWMM linkage issues
The transport of atmospherically deposited chemical compounds through the terrestrial biosphere is presently an area of great uncertainty (Hicks, 1998, Valigura et al., 1996). Potentially, an accurate representation of the airshed and watershed in CIT and SWMM and a physically precise linkage between the two models will reduce the degree of uncertainty. A significant amount of research is still required to accurately describe the retention/transfer of a contaminant from the airshed through the
Case study
We selected the Ballona Creek watershed in Los Angeles, California, USA, to demonstrate the CIT–SWMM interface. Ballona Creek drains approximately 300 km2 (116 mi2) of the 3500 km2 (1350 mi2) heavily urbanized Los Angeles Metropolitan Area. Ballona Creek has been found to be the most significant source of nonpoint source pollution to the Santa Monica Bight (Suffet et al., 1997, Wong et al., 1997). The creek begins as a covered storm drain in downtown Los Angeles and it eventually becomes an
Results
CIT calculated dry deposition over its entire grid cell structure. We extracted the hourly dry deposition amounts for the 22 grid cells that cover the Ballona Creek watershed for the August 27, 1987 and the December 3, 1987 simulations. The dry deposited amounts of nitrogen compounds were converted into NO3− and NH4+ based on how the various nitrogen compounds would react with rainfall and stormwater (see Table 1). For the August 27th simulation, the average NO3− and NH4+ loads available for
Summary
This paper described our initial work to integrate CIT and SWMM to model the urban atmosphere, watershed, and storm drainage system. The first stage of this linkage involved five issues: (1) reconciling the different modeling domain sizes, (2) accounting for dry deposition due to plant uptake, (3) estimating the fraction of deposited contaminant available for washoff, (4) defining wet deposition inputs to SWMM, and (5) parameterizing the SWMM washoff algorithm. We devised a procedure to
Acknowledgements
This project was supported as part of the Urban Security Initiative at Los Alamos National Laboratory. We thank the Urban Security Principle Investigator Grant Heiken for his continued support. We also thank Professor Michael K. Stenstrom from the University of California, Los Angeles, and one of his graduate students, Andy Lee, for their cooperation during the development of the Ballona Creek watershed stormwater model.
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