Some properties of the geometric mean and its use in water quality standards (original) (raw)

1978, Water Resources Research

The geometric mean is often used to express acceptable levels of fecal coliform counts in federal and state water quality criteria or standards. The expected value of the geometric mean is shown to be a function of the sample size and to be very sensitive to the skew as well as to the form of the underlying distribution function. Implications of these findings are discussed with regard to standards. In particular, it is noted that great care must be taken in specifying the minimal sample size to be used. 'Not to exceed a median fecal coliform bacterial concentration of 14 MPN per 100 ml with not more than 10 percent of samples exceeding 43 MPN per 100 ml for taking of shellfish.' These criteria can be contrasted with the report of the National Academy of Sciences, National Academy of Engineering [1972] 'Water Quality Criteria 1972,' in which 'no specific recommendation is made concerning the presence or concentrations of microorganisms in bathing water,' but 'it is recommended that the geometric means of fecal coliform and total coliform densities in raw surface water sources (for public supply) not exceed 2,000/100 ml and 20,000/100 ml, respectir•ely. ' Furthermore, with regard to the harvesting of shellfish the National Academy of Sciences and the National Academy of Engineering recommended that the water quality meet the National Shellfish Sanitation Standards, which require that 'the coliform median most probable number (MPN) of the water does not exceed 70 per 100 ml, and not more than 10 per This paper is not subject to U.S. copyright. Published in 1978 by the American Geophysical Union. cent of the samples ordinarily exceed an MPN of 230 per 100 ml for a five-tube decimal dilution test .... '