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Image of a boy standing at a water fountain.Water Quality Assessment: Overview
Water quality is determined by assessing three classes of attributes: biological, chemical, and physical. There are standards of water quality set for each of these three classes of attributes. The national standards for drinking water are developed by the federal government's Environmental Protection Agency (EPA). All municipal (public) water supplies must be measured against these standards.

Some attributes are considered of primary importance to the quality of drinking water, while others are of secondary importance. Therefore, the EPA drinking water standards are categorized as primary drinking water standards and secondary drinking water standards. Primary drinking water standards regulate organic and inorganic chemicals, microbial pathogens, and radioactive elements that may affect the safety of drinking water. These standards set a limit--the Maximum Contaminant Level (MCL)--on the highest concentrations of certain chemicals allowed in the drinking water supplied by a public water system. Photo 1999

Secondary drinking water standards regulate chloride, color, copper, corrosivity, foaming agents, iron, manganese, odor, pH, sulfates, total dissolved solids, and zinc, all of which may affect qualities of drinking water like taste, odor, color, and appearance. The concentration limit of these contaminants is referred to as the Secondary Maximum Contaminant Level (SMCL). Examine the EPA's current drinking water standards.

State agencies are responsible for monitoring public water supplies and enforcing the primary and secondary drinking water standards set by the EPA. Local water districts must test and treat drinking water and maintain the EPA standards for quality. These districts are also responsible for informing the public when any water quality standards have been violated.

Given these standards, stream- and groundwater supplies should be of high quality. Generally, one compares the values for the various measures of stream- and groundwater quality at a given time and location to the average of those values across the entire watershed. This "average" water quality across a watershed is referred to as the watershed's "baseline."


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