Water Quality

The term “water quality” refers to the fitness of water for a defined use, such as human consumption and is measured in terms of chemical, physical, and biological parameters. Standards against which the quality of water is compared are neither universal nor constant and depend upon who has established the standards and for what purpose.

Chemical Analysis of Water

Pure water, composed only of molecules of dihydrogen oxide (H₂O) and nothing else, does not exist in nature. Even rainwater contains measurable amounts of dissolved constituents in the range of 10 milligrams per liter. Water is of acceptable quality for a defined use or purpose if, on analysis, constituents in it do not exceed prescribed concentrations. Water quality describes the fitness or suitability of water for a specific use and is based on chemical, physical, and biological parameters. Significant differences have arisen among scientific and engineering disciplines and government agencies as to which indicators are appropriate for the evaluation of water quality. The results of an analysis of a water sample are commonly presented as indicators of water quality without the provision of a context of use or standards.

Standards against which the quality of water is compared are neither universal nor constant and depend on who has established the standards and for what purpose. Legal standards prescribed by regulation are binding. They prescribe conditions not to be exceeded for a water resource and allow penalties to be assessed for violations. Standards are subject to change over time as more becomes known about the health effects of exposure to various contaminants, as analysts are able to detect ever smaller quantities of contaminants, and as governments apply standards to more situations.

This methodology of listing constituents and concentrations that must not be exceeded has undergone constant evolution. Prior to the establishment of formal criteria, odor, color, taste, turbidity, and temperature were measures commonly used in assessing the desirability of a drinking water source. By 1784, less direct chemical criteria involving the ability of a sample to dissolve soap without forming lumps or a residue were applied to establish whether water was drinkable. Chemical criteria alone, however, are insufficient for establishing a water supply as both drinkable and safe. The relationship between disease and pollution by human and animal wastes had been recognized for centuries. An early method of measuring the bacterial quality of water, for example, relied on observing the length of time that a stored water sample remained free of visible growths. This was eventually replaced by techniques to identify and count bacteria and to compare measurements to a standard.

US Geological Survey and US Public Health Service

The US Geological Survey (USGS) first reported chemical analyses of natural waters in 1879 and by 1901 had published more than twenty-five reports on the geologic control and on the chemical and physical properties of natural water. At that time, many water sources were pristine and unaffected by human or animal activity. The composition (quality) of waters from lakes, rivers, and wells resulted solely from reactions with gases in the air and minerals in soil and rock. The major dissolved constituents, those generally with concentrations of more than 1 milligram per liter, are calcium, magnesium, sodium, potassium, bicarbonate, carbonate, chloride, sulfate, nitrate, and oxygen; these constitute up to 99 percent by weight of the dissolved matter in pristine waters. Under less common natural situations, other constituents such as iron or fluoride may exceed 1 milligram per liter concentration. Minor constituents are detected in pristine natural waters if sought by analysis. Only some of the major constituents, such as sulfate and chloride, are included in either the US primary (enforceable) or secondary (nonenforceable) drinking water standards. Canadian standards are generally the same as US standards, with only minor differences. Other major constituents are listed in the standards of other organizations, such as the World Health Organization or the European Community (EC).

After 1901, the USGS expanded its activities to include pollution studies, recognizing that sewage and industrial wastes were degrading the quality of water and adversely affecting municipal water supplies. In 1905, the USGS began the first monitoring program to assess the quality of streams and lakes, efforts that led to estimates of the amounts of dissolved and suspended matter carried to the oceans by rivers.

The US Public Health Service (USPHS) was established in 1912 and was directed to study sanitary water quality. The USGS continued to study water supplies for public use and for agricultural and industrial purposes, and maintained limited networks to evaluate trends in water quality. In the United States, formal chemical water-quality standards originated by action of the USPHS in 1914. Dissolved constituents believed to be harmful to humans were identified, and maximum allowable concentrations were established. Water exceeding the limits could not be used for food preparation or for drinking water on passenger trains (interstate carriers) crossing state lines. The USPHS standards were widely, although unofficially, regarded as the basis for the acceptability of a water supply for human consumption. The standards were expanded both in the number of constituents covered and in lowered permissible concentrations in 1925, 1942, 1946, and 1962. The final set of standards promulgated by the USPHS, issued in 1962, considered bacterial quality; physical characteristics of turbidity, color, and odor; chemical characteristics, some mandatory and others recommended; and radioactivity. In earlier times of pristine sources in protected watersheds, water that required no extensive treatment was available, and one set of standards was sufficient for both raw water and drinking water. By 1962, modern civilization had affected most water sources, and raw water from degraded sources had to be treated so that the standards for drinking water were met at the point of delivery.

Environmental Protection Agency

By the 1960s, industrial manufacturing of new chemical products and by-products, as well as a generation of waste and wasteful practices, had resulted in contamination and pollution, with many materials found to be persistent in the environment and affecting surface and groundwater supplies. Efforts to restrict the degradation of water sources were underway in specific geographic areas ranging from small drainage basins to interstate commissions for river basins. The Environmental Protection Agency (EPA) was established in 1970 and given the task of setting goals and standards and identifying sources of polluting effluents. Federal legislation, including the US Water Quality Act and the Clean Water Act and its addendums, led to broadened activities in documenting quality, mandating treatment and recovery, and requiring the establishment of standards for discharged waters. The National Pollutant Discharge Elimination System (NPDES) was established for all point source discharges into US waters, and required permits, reporting, and effluent limitation. For situations where the limitations were not stringent enough to improve the quality of receiving water, treatment was required. The EPA was charged with developing a wide array of water-quality standards. An understanding of the degree to which the nation’s waters are contaminated and what the water quality trends are was seen as essential to management decisions. The USGS was thus brought back into the process of monitoring surface and groundwater supplies, looking at contamination to a greater extent than before. The agency established the National Water Quality Assessment Program(NAWQA) to systematically study the quality of US water, stream basin by stream basin.

The first set of drinking-water standards to be established by the EPA became effective in 1977 under the mandates of the 1974 US Safe Drinking Water Act. Among the inorganic contaminants, fluoride and mercury were newly added to the earlier USPHS standards, nitrate became a mandatory rather than recommended standard, and radioactive sources were redefined. This came about in large part because of better information about the risks and health effects of those contaminants. The standards have undergone changes and extensions since 1977, including modifications authorized by the Safe Drinking Water Act Amendments of 1986 and 1996. A prior mandate of the EPA to regulate twenty-five new contaminants every three years has been set aside, replaced by a mandate to review at least five contaminants for possible regulation every five years, in consideration of the following criteria: whether the contaminant adversely affects human health, whether it is known or substantially likely to occur in public water systems with a frequency and at levels of public health concern, and whether regulation of the contaminant presents a meaningful opportunity for health-risk reduction. In addition, there are provisions for monitoring unregulated contaminants, creating an accessible database, and disclosing violations to the public served by water systems within twenty-four hours.

Domestic use, including drinking water, is considered by many to be the most essential use of water, and standards for drinking water receive the greatest attention. Water quality requirements for other purposes may be more restrictive (for example, for high-pressure boiler water) or less restrictive (for example, for hydraulic cement manufacture) than standards for domestic use.

Application of Water-Quality Studies

In the earth sciences, water-quality studies generally have a focus beyond determining whether a water source is suitable for a given purpose. Larger questions of regional water quality, such as determining how and why water with certain characteristics is associated with a specific rock type, or explaining how and why groundwater evolves chemically as it slowly moves through an aquifer, are examples of applications that center on understanding natural processes. The contamination and pollution of water resources have drawn considerable attention, leading to studies of the transport of pollutants by water. One example of applications of the methodology from the earth sciences should illustrate the disciplinary perspective.

Of the major constituents in natural waters, only sulfate is listed in the national primary drinking water standards. Sulfate in drinking water produces laxative effects and an unpleasant taste. It is listed in the nonenforceable national secondary drinking water standards for taste and laxative effects at a concentration of 250 milligrams per liter and is prohibited by the primary standards at 400 to 500 milligrams per liter. The sources of the sulfate are listed as “natural deposits.”

As a result of the Safe Drinking Water Act and using data from the US Geological Survey, a series of state maps was compiled to show the regional variation in concentration of major dissolved constituents in well water. A zone of high-sulfate water extends from Lake Erie across northwest Ohio and into east-central Indiana. Water from wells in this area is of generally unacceptable quality. From the perspective of the earth sciences, it was important to build upon the recognition of this zone to determine probable cause. Obviously, such a study cannot exclude areas from which groundwater just passes the standards. Results from this study provide an understanding of the geologic controls on the system. High-sulfate water is recovered from shallow wells in glacial deposits and also from wells in a deeper bedrock aquifer. The situation is explained in part by the presence of gypsum and anhydrite, both calcium sulfate minerals, in a sedimentary terrain characterized by shale and dolomite units. Glacial activity eroded formerly exposed bedrock of shale and anhydrite, moved it, and redeposited it as unconsolidated material over undisturbed bedrock, also containing anhydrite and dolomite. High values of sulfate in the water, at some wells exceeding 1,000 milligrams per liter, would ordinarily not be possible because of the solubility limits of calcium sulfate minerals. In such waters, magnesium values were elevated, and calcium values were lower than expected. The conclusion is that in order for such high values of sulfate to exist in the water, a process known as dolomitization is taking place, in which dolomite, a calcium magnesium carbonate mineral, is dissolved, and calcite, a calcium carbonate mineral, is deposited, in effect adding magnesium and removing calcium from the water. Without this process, such high values of sulfate would not normally be possible due to the very low solubility of many sulfate minerals.

Contaminated Waters

Contaminated waters may be safe to drink but typically are not. In addition to the constituents derived from natural sources, these waters contain (by definition) constituents derived from a variety of human activities, such as improperly treated sewage, storage tanks that have leaked, improper disposal of hazardous wastes, and agricultural practices. Many of the chemicals involved, such as various herbicides and pesticides, are manufactured synthetically and do not occur in nature. Standards for safe drinking water for most of these constituents range generally from tenths to several hundred micrograms per liter.

Maximum contaminant levels (MCLs) are derived from studies of risk to humans consuming two liters of water a day over a lifetime of seventy years. Based upon extrapolations of animal studies for a given contaminant, an increased cancer risk to humans of one in one million over a lifetime leads to the standard. By statute, no cancer risk is to be tolerated, but the technology does not exist to totally eliminate the very small concentrations of many contaminants in water resources. Thus, the maximum contaminant level goal is generally zero for MCLGs but a small actual concentration for MCLs.

Other water-quality standards, including national effluent standards for limiting the discharge of pollutants into surface waters, have been established or enforced as a result of a number of federal laws and resultant regulations, including the Clean Water Act, which addresses water pollution from point and nonpoint sources; the Comprehensive Environmental Response, Compensation and Recovery Act (the “Superfund” Act), which mandates cleanup of hazardous-waste sites and leaking tanks; the Resource Conservation and Recovery Act, which defines and requires tracking and proper disposal of hazardous wastes; the National Environmental Policy Act, which establishes the need for environmental impact statements for federally controlled or subsidized actions; the Endangered Species Act, which protects some habitats; and the Wild and Scenic Rivers Act, which limits development along some rivers. States, in some instances, have established water-quality standards similar to or more restrictive than the ones put forth by federal agencies.

Management of water quality for effluents takes place through regulations applied to discharges from or into three distinct sites. End-of-pipe standards specify levels that must be met at the point from which effluent is discharged from an industrial facility. Technology-based standards impose regulations on a discharging facility. Assimilative capacity standards define the water-quality conditions that a receiving body of water (stream or lake) must not exceed. Dischargers to the stream or lake are identified, and those causing the stream or lake to exceed standards are required to manage the quality of their discharges.

In the 2020s, several environmental non-profit groups filed lawsuits against the EPA, citing the agency’s lack of acceptable water pollution limits in relation to various industries. These groups also claimed water quality standards were antiquated and needed updating to stop pollution from industry and retain the safety of the water. The agency updated its water quality standards in 2024, but continued to face lawsuits from environmental groups as well as utilities, industry groups, and jurisdictions who objected to the updated standards. The US Supreme Court agreed to hear a one such case, City and County of San Francisco v. EPA, and review the agency's wastewater regulations.

Principal Terms

concentration: amount of a specific substance present in a given volume of sample water; commonly used units include milligrams per liter or micrograms per liter

contaminants: solutes introduced into the hydrologic environment as a result of human activity, without regard to degree of degradation

detection limit: the lowest concentration of a constituent that can be reliably detected

maximum contaminant level goals (MCLGs): nonenforceable health goals based on the levels of contaminants that cause no negative health effects

maximum contaminant levels (MCLs): enforceable standards for drinking water established by government regulatory agencies, such as the U.S. Environmental Protection Agency under the Safe Drinking Water Act

national primary drinking water standards: the list of MCLs and MCLGs for organic and inorganic constituents; the list also includes various standards for asbestos fibers, turbidity, bacteria, viruses, and radioactive-emitting constituents, established using less common concentration units

pollution: the presence of environmental components that are not naturally present; contaminant levels at objectionable concentrations

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