Math statistic- estimates and sample sizes

safe lumber co. hires an independent contractor to monitor the groundwater within a one mile of its factory to ensure that no toxic levels of arsenic are being released. The contractor takes a sample of groundwater from selected sites within the one mile radius. Safe lumber will need to pay for an expensive clean-up if a confidence of the average arsenic level in groundwater lies completely above 50 parts per billion ( for example, if the confidence interval is (75,95).