primary standards regulate substances that potentially affect human health;^[8]

[7]

secondary standards prescribe aesthetic qualities, those that affect taste, odor, or appearance.

[9]

The first problem is the extent to which the sample may be representative of the water source of interest. Water sources vary with time and with location. The measurement of interest may vary seasonally or from day to night or in response to some activity of man or natural populations of aquatic and animals.^[22] The measurement of interest may vary with distances from the water boundary with overlying atmosphere and underlying or confining soil. The sampler must determine if a single time and location meets the needs of the investigation, or if the water use of interest can be satisfactorily assessed by averaged values of sampling over time and location, or if critical maxima and minima require individual measurements over a range of times, locations or events. The sample collection procedure must assure correct weighting of individual sampling times and locations where averaging is appropriate.^{[23]: 39–40} Where critical maximum or minimum values exist, statistical methods must be applied to observed variation to determine an adequate number of samples to assess the probability of exceeding those critical values.^[24]

plants

The second problem occurs as the sample is removed from the water source and begins to establish with its new surroundings – the sample container. Sample containers must be made of materials with minimal reactivity with substances to be measured; pre-cleaning of sample containers is important. The water sample may dissolve part of the sample container and any residue on that container, and chemicals dissolved in the water sample may sorb onto the sample container and remain there when the water is poured out for analysis.^[23]: 4 Similar physical and chemical interactions may take place with any pumps, piping, or intermediate devices used to transfer the water sample into the sample container. Water collected from depths below the surface will normally be held at the reduced pressure of the atmosphere; so gas dissolved in the water will collect at the top of the container. Atmospheric gas above the water may also dissolve into the water sample. Other chemical reaction equilibria may change if the water sample changes temperature. Finely divided solid particles formerly suspended by water turbulence may settle to the bottom of the sample container, or a solid phase may form from biological growth or chemical precipitation. Microorganisms within the water sample may biochemically alter concentrations of oxygen, carbon dioxide, and organic compounds. Changing carbon dioxide concentrations may alter pH and change solubility of chemicals of interest. These problems are of special concern during measurement of chemicals assumed to be significant at very low concentrations.^[22]

chemical equilibrium

The (WHO) published updated guidelines for drinking-water quality (GDWQ) in 2017.^[3]

World Health Organization

The (ISO) published regulation of water quality in the section of ICS 13.060,^[57] ranging from water sampling, drinking water, industrial class water, sewage, and examination of water for chemical, physical or biological properties. ICS 91.140.60 covers the standards of water supply systems.^[58]

International Organization for Standardization

– United Nations environment program

Global Freshwater Quality Database (GEMStat)

Water policy in the European Union

– Drinking water quality and testing (United States)

U.S. Centers for Disease Control and Prevention (CDC)

– Water Data and Tools of the USEPA

U.S. Environmental Protection Agency

– National Water Quality Assessment Program of the USGS

U.S. Geological Survey

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