How to Get a Clean Sample of Contaminated Water

(Excerpted from main article.)

It may sound like the easiest thing in the world to dip a bottle in a well and come out with a pure sample of well water--until you hear all the ways your sample can be contaminated.

Let's start with that bottle. Is it made out of glass? "One of the dirtiest materials you can use" for measuring metals, Coale says. "The only glass suitable for trace metal analysis is quartz, which is really expensive." All other glasses contain metals that may leach into your sample. Flegal's lab uses only plastic containers.

Suppose your bottle is plastic. Did you use it straight out of the manufacturer's sterile packaging? If so, it is far too dirty. Flegal's lab washes all the containers at least five times, using two different kinds of acid baths and "ultrapure" water that has been through six chemical filters. The process of cleaning the bottles can literally take months.

How did you keep your bottle from floating on the surface of the well? Maybe you used a lead weight? Too bad: if even an imperceptible fleck of lead breaks off that weight, it can ruin your sample.

In reality, hydrologists donít get their samples just by dipping a flask in a well: they pump the water out of the well. That poses additional problems: if the water ever comes in contact with the metal of the pump, then it may become contaminated. Flegal and Creasey use a peristaltic pump (go to Perfect Pump commercial site) instead, an ingenious contraption in which the water never touches the pump, only the plastic tubing. The pump operates on a similar principle to a straw. If you squeeze a tube filled with water between your fingers, and then move your fingers along the tube, the water will follow along. Replace your fingers with a rotating head, and you have a peristaltic pump.

Still, collecting the sample is a messy business. Soil and water make mud, and mud is the last thing you want in your sample. So it takes two people to collect the sample: the "dirty hands" person who operates the pump, and the "clean hands" person (usually Creasey) inside the van. Each sample is collected in two bags, one inside the other. The "dirty hands" person is only allowed to touch the outer bag, the "clean hands" person is only allowed to touch the inner one.

"Contamination consciousness is more important than any procedure," says Charles Alpers, a chemist with the United States Geological Survey. "If you're the clean-hands person, and you've got to scratch your nose, you've got to ask the other guy to do it."

The rate of pumping is also critical. 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. "When you do that, you're re-suspending things that have drifted to the bottom," Creasey says. You may also be fracturing the rocks around the well, causing more metal to leach into your sample.

How did you drive your sample back to the lab? "We used to use mercury to preserve nutrient samples. So we have traces of mercury all over our vehicles," says Alpers. "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. Commercial testers usually ship their samples to an outside lab after collecting them, so the samples pass through three sets of hands: the collector, the courier, and the lab technician.