By PAUL HETZLER

Ever have a strong desire to know what happens to stuff you pour down the drain? I didn’t think so. But if your water comes from a well and your waste goes to a septic tank, this may interest you.

A while back, my job dealt with groundwater contamination (cleaning it, not causing it). When polluted groundwater is found in the vicinity of a domestic well, it’s standard practice to place test wells, also called monitoring wells, between the presumed source of pollution and the house.

A test well is a sort of early warning device in case contaminants migrate toward a home. Water from these new sentinel wells is analyzed immediately to prove they are clean to begin with, and tests are done periodically thereafter. The frequency of sampling varies according to the situation.

Sadly, now and then harmful chemicals unrelated to the problem at hand turn up in a new, presumably uncontaminated, test well. Constituents of paint thinners and degreasers are common, and also from auto fluids and cleaning products. It is a case of what went down to the septic tank comes back up to the tap—and in these cases, people are unknowingly drinking it. As you might expect, fecal coliform bacteria are usually found in the water, too.

Nearly a quarter of US households—even more in northern New York—have septic systems. Sewage flows into a septic tank where solids settle out and are partially broken down by bacteria. Liquid flows out the top to the leach, or drain, field where it percolates into the soil, a critical element in septic systems. The ideal soil is deep loam with enough porosity for “good guy” bacteria to get oxygen they need but not so coarse that liquid flows away too fast for bacteria to break down remaining waste.

Unless you’re building your own house, you can’t choose the soil for the drain field. But it’s good to keep in mind that clay soil or very coarse soil reduces the effectiveness of a septic system, potentially allowing sewage effluent to leach—and be washed by rain—down to groundwater. If a drain field in clay or gravel soil needed replacing, one could expand it to improve septic system performance.

Water use greatly impacts septic system function. The less water a household uses, the better the system works and the less likely that unwanted stuff will make it to groundwater. Also, as water use increases, so does the area from which the well draws (zone of influence). Say 500 gallons per day lowers the water table in a thirty-foot radius and 5,000 gallons/day draws down 200 feet away. If the well is 100 feet from the drain field, groundwater beneath the field is constantly moving toward the well at that higher pumping rate.

A word on wells. OK, several. Deep wells are less vulnerable to contamination than shallow ones, but none are immune. Boreholes are preferential flow pathways. As many of us, myself included, do sometimes, water takes the path of least resistance. It will flow merrily down the outside of a casing no matter how deep the casing goes. And nobody’s well is in “solid rock.” if that was true, they would have a very dry, very useless hole in the ground, not a water supply. Groundwater flows through fractures in rock to reach a well, in some cases traveling miles per week.

If you are interested in having your well checked for contaminants, think of water testing like school testing. Acing your Algebra test doesn’t necessarily mean you’ll do great on your History exam. Let’s say you (wisely) test your well water annually for E. coli and other coliform organisms that could indicate leakage from septic systems or manure runoff. Getting an “A” on that test will not tell you if the water has trace amounts of pesticides or petroleum products. That test requires a very different set of analytical equipment, possibly even a separate lab.

Farms are especially vulnerable. They may have shop floor drains, overfills from fuel deliveries, leaking bulk storage tanks and/or piping, as well as incidental slops and spills, all of which can threaten groundwater. Even backyard mechanics can inadvertently pollute their water by flinging auto fluids out back. Common wisdom says oil and water don’t mix, and for the most part that is true. However, a small amount does dissolve in water, and it is more than enough to be problematic.

For example, the carcinogen benzene, a “light” (volatile) constituent of both gasoline and diesel, is 0.18 percent soluble in water. That doesn’t sound like much, right—how bad can it be? But consider that New York State’s maximum limit for benzene in drinking water is 0.7 parts per billion. If my math is right—and someone please check it—0.18 percent translates to one million, eight-hundred thousand parts per billion. It takes a heck of a lot of dilution to get from 1, 800,000 to 0.7.

The next time you test for bacteria, think about testing for volatiles and semi-volatiles, too. EPA Method 503.1 covers many of the compounds one might find around farm and home shops. The most important thing you can do to protect your well is to avoid putting chemicals down the drain, because what goes down very well might come up.

Paul Hetzler is a forester and a horticulture and natural resources educator with Cornell Cooperative Extension of St. Lawrence County.