10.2190/J050-F9MR-P68A-BNUF J050F9MRP68ABNUF 1 A Mass Balance Accounting Procedure for Estimating Contributions to Water Quality 1 16 20020509 20020509 20020509 20020509 J050F9MRP68ABNUF.pdf http://baywood.metapress.com/link.asp?target=contribution&id=J050F9MRP68ABNUF 1 D. C. Wilkin R. C. Flemal Department of Landscape Architecture, University of Illinois Department of Geology Northern, Illinois University A computerized procedure allows the use of existing State water quality data, collected in a fixed, diffuse grab sample network, to estimate the approximate relative contributions to water quality attributable to defined and undefined upstream inputs. The routine determines an empirical assimilation rate for any water quality constituent whose assimilation can be modeled, as a rough approximation, using first-order die away. After estimating the relative contributions of all defined contributors, mostly NPDES permit holders, the magnitude of the total undefined contribution, largely nonpoint, is estimated by difference. Because the system is constrained to operate using only existing water quality monitoring data, the results are admittedly of low resolution. Nonetheless, for synoptic analyses of water quality at the regional level, the system provides highly usable management information. Institute for Environmental Studies, University of Illinois at Urbana-Champaign, <i>Feasibility of a Systematic Approach to Water Quality Management in Illinois</i>, Illinois Institute for Environmental Quality Document No. 77/35, 133 pp., 1977. U. S. Environmental Protection Agency, <i>Methods for Identifying and Evaluating the Nature and Extent of Non-Point Sources of Pollutants</i>, Report No. EPA-430/9-73-014, Washington, D. C., 261 pp., 1973. D. C. Wilkin and R. C. Flemal, <i>The Feasibility of Water Quality Improvement in Three Illinois Rivers</i>, Manuscript in preparation, 1978.