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Secchi test reveals the ‘health’ of Lodi Lake

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Posted: Friday, July 27, 2012 12:00 am | Updated: 9:32 am, Fri Jul 27, 2012.

There's a lot to be learned about the life of a river, and a town, with one simple tool. A secchi (pronounced seck-y) disk can tell the right observer just how clean the water is.

Lodi Lake docent coordinator Kathy Grant and her student team of Storm Drain Detectives went out to three testing sites on Lodi Lake and the Mokelumne River to test water clarity on July 15. The results showed some improvement on one testing site, but two others worsened.

The test is called the secchi dip-in. The secchi disk is weighted and dropped into the water on a marked rope to measure depth. Students mark the depth of the disk when they can no longer see it. It takes about an hour to test all three sites.

Water transparency is affected by several factors, from algae to water color to pollution. Most pollution is caused by runoff or human activity in the testing area. Checking the clarity of the water can show how healthy or unhealthy the body of water is.

Water clarity worsened on the lake this year. On the first site north of the boathouse, the disk was visible up to 4.95 feet. Last year, it was visible at 6.50 feet.

On the second site near the center of the lake, the disk could be seen at 4.50 feet. In 2010 it was visible at a depth of 6.0 feet.

But clarity improved on the Mokelumne. Students tested a site upstream of the Woodbridge Dam and recorded visibility up to 11.45 feet. That is a decent jump from the 2011 measurement of 8.20 feet.

In the report summary, the students found the water visibility in the lake has not changed drastically since monitoring began in 2001.

"It bounces up and down every year. I'm not sure what causes that," she said, adding it might be interesting to factor yearly rainfall into the secchi disk measurements.

Grant said water quality generally improves in years with less rainfall and runoff, because less muck and fewer chemicals are rinsed down storm drains.

But even in those drier years, more pollution comes during the summer, not the rainy winter.

"More people are hosing down driveways, washing their cars and fertilizing lawns. It all goes in the water," she said.

Volunteers test water clarity in rivers and lakes around California. Statewide averages are expected to fall within the same range as the 2011 median transparency for volunteer-monitored lakes in California, which was 8.30 feet.

Fallen Leaf Lake, in the El Dorado National Forest, has the best clarity in all of California, with measurements of over 55 feet. Pinole Creek, in Contra Costa County, has the worst clarity in California, with measurements under one foot.

Contact reporter Sara Jane Pohlman at sarap@lodinews.com.

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