CE 397: Environmental Risk Assessment Department of Civil Engineering The University of Texas at Austin
Reading: Handbook of Hydrology, Chap 14: pp. 14-1 to 14-18 on Contaminant Transport in Surface Water
Water Pollution Sources
A water body such as a river, lake or bay, can be contaminated by inflow sources, such as rivers, by deposition from the atmosphere, and by exchange of contaminants with bed sediments. The inflowing sources can themselves be partitioned into point sources or nonpoint sources, according to whether or not the contaminant entered as a concentrated flow at a particular pipe outfall, such as from a treatment plant or an industrial outfall, or whether the contamination just arises from the general collection of runoff from the land surface which carries a considerable volume of chemical and biological matter with it.
Point sources arise from municipalities and from industry. They are normally estimated by using measured concentrations in the outfall discharges, or by using "Typical Pollutant Concentrations" which tabulate expected concentrations for particular kinds of industry and municipal discharge.
Nonpoint sources are estimated as Load = k x Runoff x Concentration x Area where the load (kg/day) is the mass loading rate into the water body, the runoff (mm/yr) is the mean annual runoff from the area, usually estimated as Runoff = Precipitation x Runoff Coefficient, Concentration is the Expected Mean Concentration (mg/l) or EMC of the contaminant (the ratio of the mass of contaminant to the volume of water in a runoff event or in the sum of all runoff events over a year), Area is the land area (km2) from which the pollution occurs, and k is a factor to convert the units to make them consistent. EMC's are tabulated for particular kinds of land use and are usually represented by the median concentration of a set of samples taken from runoff draining from land with that particular land use. An Exercise on Estimating Nonpoint Source Pollution in the Mission Basin in South Texas illustrates how this type of pollutant loading can be estimated using GIS.
Atmospheric deposition directly on a water body can contribute a significant mass of some kinds of constituents, in particular, Nitrogen. The rate of deposition can be estimated using data from the National Atmospheric Deposition Program. Data from this web site http://nadp.nrel.colostate.edu/NADP/ report measurements at many atmospheric sampling stations in the United States and also allow the construction of maps of particular constituents for some regions.
Contaminants are partitioned between bed sediments and the overlying waters. It can occur that historical contamination of water has resulted in a large reservoir of contaminants accumulating in sediments which can then be resuspended in water if the sediments are disturbed. Here is an example of metals concentrations in the sediments of the Great Lakes.
An example of a study looking at all sources of loadings is the Total Loadings to the Corpus Christi Bay System done at CRWR by Ann Quenzer, Ferdi Hellweger and David Maidment. Here is some interesting information the Aquatic Health of the Great Lakes.
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