The Environmental Protection Agency (EPA) requires that cities monitor over 80 contaminants in their drinking water. Samples from the Lake Huron Water Treatment Plant gave the results shown here. Only the range is reported, not the mean. For each substance, estimate the standard deviation by using one of the methods shown in Table 8.11 in Section 8.8.

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MCLG Range Allowable MCLG Substance Chromium Barium Fluoride Origin of Substance Detected 0.47 to 0.69 0.004 to 0.019 1.07 to 1.17 100 Discharge from steel and pulp mills, natural erosion Discharge from drilling wastes, metal refineries, natural erosion Natural erosion, water additive, discharge from fertilizer and aluminum factories 4.0