The system Columbus water officials use to measure atrazine levels begins with an old, battered plastic bottle wrapped in duct tape and tied to a rope.

Every two weeks, a utilities worker lowers the bottle in 11 spots in streams that run past farms on their way to the city’s three main reservoirs. The worker labels the water samples with the date and location and returns them to the city’s lab.

It’s part of a warning system to measure how much atrazine and other farm pollutants are washed off fields and into Columbus’ main sources for drinking water. They also check for six other farm herbicides – two more than the feds suggest -frequently found in water: acetochlor, alachlor, cyanazine, metolachlor, metribuzin and simazine.

Knowing how much is in the water helps officials estimate how much powdered carbon they will need to filter atrazine down to levels the federal government considers safe. The city has a separate program that pays farmers to reduce chemical use and runoff.

Heavy storms during planting season usually are a bad sign.

Columbus uses more than 10 tons of powdered carbon a day, costing taxpayers about $10,800 each day during spring and summer months when atrazine levels rise. The city spent more than $1.5million for the carbon and the tests in 2009.

“Over the years, we’ve gained a pretty good feel for when these events occur and what to look for,” said Rick Westerfield, the city’s Water Division administrator.

Federal regulations that date to 1992 require Columbus to look for atrazine in treated drinking water once every three months and once every month in the summer. Other cities are required to look more often or less often for the same contaminants, depending on levels detected by previous water tests.