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 (presumably the mean would be the midrange). (a) For each substance, estimate the standard deviation σ by using one of the methods shown in Table 8.11 in Section 8.7. (b) Why might an estimate of σ be helpful in planning future water sample testing?
Substance | MCLG Range Detected | Allowable MCLG | Origin of Substance |
Chromium | 0.47 to 0.69 | 100 | Discharge for steel and pulp mills, natural erosion |
Selenium | 0 to 0.0014 | 50 | Corrosion of household plumbing, leaching from wood preservatives, natural erosion |
Barium | 0.004 to 0.019 | 2 | Discharge from drilling wastes, metal refineries, natural erosion |
Fluoride | 1.07 to 1.17 | 4.0 | Natural erosion, water additive, discharge from fertilizer and aluminum factories |
MCLG = Maximum contaminant level goal
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