By Cat Holmes
University of Georgia
The method, called targeted sampling, is simple. Instead of using statistical models and fixed locations, targeted sampling uses shoe leather and common sense.
Targeted sampling is based on what used to be called a sanitary survey. For a sanitary survey, a person walked along sewer pipes and sampled for leaks. Targeted sampling is the same, except a person samples for sources of fecal contamination in creeks and waterways.
“It’s akin to the children’s game of hot and cold,” said Peter Hartel, the UGA crop and soil scientist who devised the method. “You sample the water until you find areas where high numbers [of fecal bacteria} are present. Then you look around. It’s commonsensical: Are there cows in the water? A broken sewer pipe?”
The current methods used to track down sources of fecal contamination in water are time consuming and expensive. That’s because scientists often use set sampling locations and databases instead of their eyes and legs to determine where a problem has likely occurred.
Unfortunately, set sampling locations are often chosen because they are easy to get to, like bridges, but they may not be anywhere near the source of the problem.
“If you actually go out and walk the waterways and sample everything that looks suspicious – every pipe, every tributary – you generally uncover [the problem] quickly and easily,” Hartel said.
Targeted sampling greatly enhances the accuracy of bacterial source tracking. Chemical and DNA-based tests for bacterial source tracking are typically 65-85 percent accurate; when the same tests are combined with targeted sampling, they are 95-99 percent accurate.
“It’s like looking for a needle in a haystack, except you’re looking for a needle with a great big magnet,” Hartel said. “For example, if targeted sampling identifies a hotspot of fecal contamination and I know that there’s a nearby dog park and septic field, then I only have to sample the dog park, septic field and water source to determine how much the dogs or septic tank are contributing to the overall fecal contamination.”
So far, Hartel has been able to find fecal contamination quickly and easily with targeted sampling. When he tried it on the Sapelo River, he found that half the fecal contamination in the river was due to a malfunctioning private wastewater treatment facility. It took one day to find the source of the problem.
“I have yet to come across a case where targeted sampling and common sense didn’t work,” Hartel said. He is preparing to use the method along the beach of a coastal island.
“Because the coast has a high water table, failing septic tanks are a common problem,” Hartel said. With targeted sampling, a hotspot can be quickly identified and the site can be double- checked for optical brighteners common in laundry detergent. “If we find optical brighteners, we know what the problem is. It sure isn’t the deer doing their laundry.”
The method does have a couple of drawbacks, Hartel said. One is that it requires a lot of manpower. He has used volunteers and there could be liability issues if a volunteer were to get injured or hurt. Another potential problem is trespassing. However, Hartel is confident that these issues could be addressed.
“I wanted to find a method to identify sources of fecal contamination that’s fast and cheap,” he said. “This method works fine. And the beautiful part is it’s common sense.”
Cat Holmes is a news editor for the University of Georgia College of Agricultural and Environmental Sciences.