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News of the weir

May 18, 2009


William Arnold and Chris Wennen.

William Arnold (left) and Chris Wennen, a graduate student in water resources science, examine water monitors at a weir on a stormwater pond.
Photo: Patrick O'Leary

Researchers use remote-controlled sensors to track pollutant loads from storms

Deane Morrison

The next cup of stale coffee you pour down the drain may end up as evidence. Not in a courtroom, but in a study of how well Twin Cities sewers and waterways handle the loads of pollutants washed into them by storms.

Armed with a network of five wireless sensors stationed near the Twin Cities' Minnehaha Creek, researchers from the University's Water Resources Center are monitoring—in real time—when and where storms wash road salt, lawn chemicals, and other pollutants into area waterways.

Study leaders William Arnold and Miki Hondzo, both professors of civil engineering, hope to have 100 stations in the next five to 10 years. An expanded system could feed up-to-the-minute data to a Web site that recreational users of lakes and streams could use to plan their outings. It could also help urban designers tailor their plans to minimize the runoff of chemicals in local watersheds during a rain, or allow farmers to decide the best times to apply fertilizers.

Flush with data

Levels of nitrate from fertilizers and chloride from road salt tend to be low until rain washes them into waterways. The effects of rain can be dramatic, as shown by data from Minnehaha Creek and another metro-area stream, Shingle Creek, both of which empty into the Mississippi River.

"Concentrations of nitrate go up from about 200 micrograms per liter of water up to about 400 in Shingle Creek and from about 100 to 250 micrograms per liter in Minnehaha Creek," Arnold says.

But with the much larger volumes of water sweeping through the creeks, these concentrations add up to even more dramatic increases in the load, or amount of a pollutant passing a point on the stream bank every second.

The nitrate load is two to five milligrams per second in dry periods and increases to 50 to 200 milligrams per second when it rains," Arnold states. And "a student found that during a one-month period, two-thirds of the nitrate that entered Shingle Creek entered over just four days."

"There should not be caffeine in the water at all, as storm and sanitary sewers are supposed to be separated."

In contrast to the situation with nitrate, "concentrations of chloride actually drop when it rains," Arnold says, "but loads increase from about one gram per second in dry periods to 10 to 50 grams per second during rainfall."

Of the five stations now monitoring water that drains into Minnehaha Creek, two are in "stormwater ponds" that collect street runoff and feed into the creek. Another monitors a channel connecting a small lake—Lake Pamela—to the creek. The last two monitor water above and below the point where the channel empties into the creek.

Mounted on tripods, the stations draw power from solar panels. Besides nitrate and salt, they monitor pH, the amount of oxygen dissolved in the water (high is good), temperature, turbidity, and depth. Four of the sensors radio their data to a base station, which compiles the data and sends it overnight by cell phone to the U's St. Anthony Falls Laboratory for analysis.

"When a storm breaks, students can program the system remotely to take readings as often as once a minute to be sure to catch the movement of pollutants at its peak.

"The sensors give better estimates of pollutant loads than traditional sampling," says Arnold. "We now have data every one to 30 minutes instead of twice a month."

In addition to the sensors, automatic samplers collect water as often as every 30 minutes during storms. Researchers take the samples back to the lab to analyze them for fecal coliforms, pesticides, and caffeine.

Which brings us to that coffee you tossed down the drain. 

"There should not be caffeine in the water at all, as storm and sanitary sewers are supposed to be separated," says Arnold. "The presence of caffeine means there are either cross connections between the storm and sewer systems or leaky sewer pipes—and when it rains, the sewage makes it into creeks along with the stormwater."

Unfortunately, caffeine does get into the creeks. Levels jump from less than 20 nanograms per liter in dry periods to 70 (Minnehaha) or 500 (Shingle) nanograms per liter during a rainfall, Arnold reports.

On the bright side, the researchers have found that stormwater ponds in the Shingle Creek watershed remove pollutants from the water.

"This project is a great example of research that will move environmental monitoring to the next level and improve our understanding and management of water resources," says Deb Swackhamer, director of the Water Resources Center.

The research team also includes civil engineering associate professors Paige Novak and Raymond Hozalski.

Read about a University study of road salt.

Related Links

Department of Civil Engineering

St. Anthony Falls Laboratory