In the largest watershed study of its kind, Michigan State University researchers have sampled 64 river systems in the state for E. coli and the human fecal bacteria B-theta and found that, in a nutshell, septic tanks aren't working.

At least not as well as experts thought. The researchers say that "sample after sample" shows bacterial concentrations are "highest where there were higher numbers of septic systems in the watershed area," water expert Joan Rose tells Phys.org.

Freshwater contamination from septic tanks stems from one key miscalculation, the team reports in the Proceedings of the National Academy of Sciences: soil filtration. It's long been assumed that soil filters human sewage, acting as a sort of natural treatment system, and thus so-called "discharge-to-soil" methods like digging holes in the ground beneath outhouses has long been common.

But these methods clearly aren't keeping pathogens from water supplies, at least not as well as hoped. "For years we have been seeing the effects of fecal pollution, but we haven't known where it is coming from," Rose says.

Her team was able to harness advances in source tracking in what she calls "CSI for water." Now that we know the source, she adds, states like Michigan, Florida, and South Carolina, not to mention resorts near lakes throughout the US, need to rethink how heavily they rely on septic tanks for human sewage.

It's "imperative," she warns, "that we get this right." (On the subject of poo: Here are 5 ways to poop better.)

This article originally appeared on Newser: Think Septic Tanks Keep Poo Out of Our Water? Er, No

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