Creasey and Flegal pump the water out of the well at its natural rate of seepage - perhaps a cup a minute. Commercial consultants typically pump 5 to 10 gallons a minute. You may also be fracturing the rocks around the well, causing more metal to leach into your sample. We used to use mercury to preserve nutrient samples. We have to be aware of this possibility if we're testing for mercury.
Transportation is a weak link in commercial monitoring of contaminated sites.
By contrast, a lab whose primary business is monitoring organic chemicals, like benzene, might not have such facilities available. Perhaps most important, the workers in Flegal's lab spend at least half of their time analyzing blanks - containers of ultrapure water that are brought along on the collection trips, and treated in exactly the same way as the containers of well water. No matter how many precautions Flegal and Creasey take, there is still the possibility of contamination from unexpected sources. By measuring the blanks, they can determine just how much contamination occurred, and compensate for it. Finally, Creasey and Flegal take each measurement three separate times, so that they can get an estimate of their experimental error.