WILTON — Every other Tuesday at precisely 7:30 a.m. during the summer months Curt Schnare walks down to the Horseshoe, a popular swimming hole on the Souhegan River in Wilton, with a cooler — but he isn’t planning a picnic. He fills a small bottle with river water, places it in the cooler, and then brings the sample to the Milford Wastewater Treatment Facility, where it is tested for E. Coli bacteria and dissolved oxygen levels. His test is just one of dozens by other Souhegan Valley Watershed Association volunteers who collect samples a various sites at the same time to assess the overall health of the river.

The popularity of the Souhegan River as a swimming, fishing and paddling destination has increased over the last few decades, but as recently as 30 years ago, it wasn’t rare for visitors to the river to see feces floating downstream around them. By all accounts the river is in much better shape today, but without the help of volunteers like Schnare, the river could look more like a cesspool than the clear, bubbling waterway it is today.

For Schnare, the tests are his way of making sure the river is healthy and can continue to support the activities, like swimming, fishing and kayaking, that he and his children enjoy.

“It’s nice to know where you are swimming is a clean spot,” he said. “The river is a resource and I don’t want to see it degraded in any way. It makes me feel like I’m contributing to my community.”

Most people give little thought to water quality, as long as it looks clear, but E.Coli levels higher than 88 colonies of bacteria per 100 milliliters can cause ear infections or stomach upset in swimmers, and low dissolved oxygen levels can be stressful to, and even kill, fish. High E. Coli levels are not usually seen at the Horseshoe, but in other areas of the river it isn’t uncommon to see levels as high as 300 colonies. In rare cases heavy rainfall can create runoff that sends agricultural fertilizers into the river, pushing E. Coli counts into the thousands.

Schnare always hopes the water comes back relatively clean, but sometimes tests that reveal high levels of bacteria can help lead the SVWA to pollution sources.

“If there are any changes starting to occur, they don’t happen overnight,” Schnare said. “If you do have a spike, you can start looking for the source. If you can pinpoint the source, you can nip it in the bud before it really destroys the water quality.”

Since the SVWA began consistently testing the river’s water in 1996, volunteers have been able to use test results to pinpoint several pollution sources and clean up the river. Finding a specific pollution source sometimes takes an army of volunteers testing the river every 20 feet in a problem area. The association has even used DNA testing on bacteria to determine whether it had come from human or animal excrement.

“When you pick up pollution, it really is hard to figure out exactly where it is coming from,” SVWA president George May said. “We pay particular attention to places like the Horseshoe, or any place where people swim.”

In some cases pollution comes from natural manure fertilizers running off farmland and into the river or even a portable toilet that has fallen into the river. In other cases — including one discovered recently in Wilton — the SVWA has found pipes dumping raw sewage from toilets directly into the Souhegan, May said.

“If we weren’t doing any of this testing, the river would probably get tested only once every 10 years. As soon as we see the problem, we can bring in the experts and we can get these things cleaned up,” May said.

When a pollution source is found, the SVWA notifies organizations like the Environmental Protection Agency or the USDA for further investigation or enforcement. The river still has its problems — a spill at a Milford gas station last year sent more than 1,500 gallons of diesel fuel flowing into the river, and the SVWA is actively investigating abnormal levels of pollution entering the river from a Greenville brook — but, all things considered, the Souhegan is probably the cleanest it has been this century.

“We certainly are not seeing feces floating down the river. That kind of thing doesn’t happen anymore, and I would say almost all of the pipes coming into the river that were doing the serious polluting have been stopped,” May said.

The Souhegan River never saw pollution on the level of the Nashua River, which was famously known to change a variety of brilliant colors after waste discharges from paper manufacturing companies along the river. Nevertheless, restoring the river has taken, and will continue to take, decades of work by dedicated volunteers.