Modeling the Impacts of Diffuse Pollution on Receiving Water Quality

Shanahan, P. & Somlyody, L. (1995). Modeling the Impacts of Diffuse Pollution on Receiving Water Quality. IIASA Working Paper. IIASA, Laxenburg, Austria: WP-95-002

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Abstract

Nonpoint or diffuse pollutants represent a major cause of water-quality degradation of rivers, estuaries, lakes, and reservoirs and have become increasingly significant in countries where point sources of pollution are largely controlled. Nonpoint sources cause eutrophication, oxygen depletion, sedimentation, acidification, and salinization in receiving water bodies, introduce pathogenic organisms and other pollutants, and through shock loads of pollutants, cause mortality and morbidity of aquatic organisms. The major sources of nonpoint pollution include agriculture, silviculture, construction, and urban runoff.

The potential effects of nonpoint source pollutants on receiving water may be evaluated using water-quality models. A recommended procedure is to: 1) establish study objective and constraints; 2) determine pollutant interactions; 3) perform a screening analysis; and 4) select a water-quality model and complete the analysis. In the screening analysis, simple analytical tools are used to evaluate the potential severity of impacts and the time scales at which impacts occur. We describe a screening-level model that evaluates the response to a conceptual spike load of unit strength. Water-quality models may be either relatively simple analytical models or more complex numerical models. For many situations, an analytical model will provide sufficient analysis of a problem and no further evaluation is required. For more complex problems, numerical water-quality models can provide a detailed and rigorous analysis.

Analytical models are available to describe a variety of pollutant and receiving water situations. We describe models for the following: streams and rivers -- simple conservative and nonconservative pollutant inflows, probabilistically described conservative pollutant inflows, biochemical oxygen demand and dissolved oxygen, suspended sediment, and adsorptive micropollutants; lakes and reservoirs -- nutrient loading (eutrophication), and micropollutant loading; and estuaries -- nonconservative pollutant inflows. A table of available numerical models and their applicability and capabilities is also provided.

A case study of Lake Balaton, Hungary -- which formed the subject of a major policy oriented research of IIASA and the Hungarian Academy of Sciences at the late seventies, early eighties -- illustrates the procedure for assessing nonpoint source pollutant impacts. Lake Balaton is a large but shallow lake which is experiencing water-quality degradation due to nutrient inflows and consequent eutrophication. Approximately 70% of the nutrient load comes from nonpoint sources. The largest single load is the inflow from the Zala River which varies over time with precipitation. Analysis of historical data shows that a monthly time scale should be used to capture the effects of this variability on the response in the most western, hypertrophic basin of Lake Balaton. Analysis with the unit-load screening model illustrates that short time scales must be considered to evaluate the response to nonpoint source loads of the Zala River itself while much longer time scales, on the order of a year, suffice for evaluating the response of the entire lake.

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