WORTHINGTON - When the Okabena-Ocheda Watershed District’s board of managers contracted with a firm to study the chemistry in Lake Okabena and the cause of its summertime smelly algal blooms, they already knew the turquoise green filaments were caused by excessive nutrients in the lake. 

Now, as Wenck Associates finalizes its report, the proof is in the numbers - production agriculture is deemed the largest contributor of nutrients ending up in Lake Okabena.

The 72-page diagnostic study completed by the Maple Plain-based firm reviews the lake’s ecology and includes both an internal and external source assessment. The study details how phosphorus and sediment are ending up in the lake, whether from wind erosion or rainfall, the city’s stormwater system, streambank erosion or runoff from agricultural lands.

In its conclusion, the report explains that nearly 85 percent of the watershed is located outside of the city of Worthington, and “runoff from rural areas is the largest contributor of sediment and phosphorus to Okabena Lake.” It pointed to field erosion (65 percent), streambank erosion (7 percent) and animal agriculture as contributing to the sediment load.

“It was estimated that over 300,000 pounds of phosphorus is produced by livestock in the Okabena Lake watershed each year,” the report stated. “While this study was not able to determine the exact amount of livestock phosphorus that reaches the lake, these results suggest manure spreading is likely a significant source and local farmers should continue implementing responsible manure management practices.”

But agricultural practices aren’t the sole reason for the lake’s presence on Minnesota’s Impaired Waters list since 2010.

Soil transported by wind erosion accounts for approximately 18 percent of the annual sediment load going into Lake Okabena and 3 percent of the phosphorus load; while rainfall is believed to contribute about 2 percent of the phosphorus load in the lake. Residents within the city of Worthington are also contributing to the unhealthy water body, with 10 percent of sediment and 8 percent of phosphorus reaching the lake through the city’s storm sewer system and direct runoff.

Shifting from percentages to real numbers, Wenck reported that more than 1,068 tons (2,137,400 pounds) of sediment is flowing into Lake Okabena in an average year along with nearly 7,481 pounds of phosphorus.

To put some perspective on those numbers, Okabena-Ocheda Watershed District Administrator Dan Livdahl said one ton of soil is the thickness of a dime over one acre. Considering the size of Lake Okabena and the amount of sediment, the annual load would amount to a coating of sediment in the lake of three or four dimes thick.

“It’s coming off the streets, off our landscape - it’s a lot of deposition that’s occurring,” he said.

Gathering samples

The Okabena-Ocheda Watershed District, along with the city of Worthington and the Minnesota Pollution Control Agency (MPCA) hired Wenck to conduct the study as part of a much broader study currently under way on the entire Missouri River Basin. As Livdahl explained, the city and OOWD wanted to know, as closely as possible, where the pollution is coming from so that no sector would feel they were treated unfairly.

“If we had stayed with the other study, the information would have been based on a lake like ours,” he said. While the city and the MPCA contributed financially to the study, the watershed district purchased water monitoring equipment and provided the labor.

The study is the first step in the process of developing a Total Maximum Daily Load (TMDL), which is basically a rulebook for watersheds and communities to follow. Once completed, the TMDL will specify the total amount of pollution that can enter the lake every single day, Livdahl said.

The goal is, by following the recommendations in the TMDL, Lake Okabena will someday return to a healthy state and eventually removed from the impaired waters list.

While MPCA’s target date to complete the draft TMDL for Lake Okabena isn’t until June 30, 2016, water quality monitoring will continue not only in the lake, but in Whiskey Ditch south of Oxford Street and in a stream that flows into Sunset Bay. Livdahl said monitoring has been done on Lake Okabena every year since 1998, while the stream monitoring was done last year for the first time.

Those monitoring efforts will likely be expanded as a result of the study, as Wenck recommends surface water samples be collected in Sunset Bay during the summer months, and that water quality be monitored in at least two or three city stormwater ponds during the summer for the next year or two.

“They have suggested looking at stormwater ponds and seeing how effective they really are,” said Livdahl. “We’re not done (monitoring) by any means, but I think we have a really good baseline.”

Much accomplished, more to do

While a lot of work has been done in the watershed north and west of Lake Okabena in the past 20 years - from the construction of a dam to wetland restorations, the addition of filter strips and generally improved farming practices - the report proves more effort is needed.

“With all of that in place, that’s still the highest source of pollution to the lake,” Livdahl said. “We need to target land that’s in row crops. If we’re going to make progress, the next things are not easy. It’s going to be talking to farm operators about management on their property.”

With a relatively small area of the watershed targeted, Livdahl said there are just a few families that farm most of the land in the area where improvements are needed. That helps in terms of reaching all of the farmers, but at the same time, Livdahl said whether it’s one, three or five families, the decisions they make will have a big difference on the lake’s health.

He said conservation practices, such as leaving more residue on the surface through conservation tillage, would help keep more of the sediment and phosphorus on the land instead of being blown or washed into Okabena Creek. Another option for farmers would be to plant cover crops.

“When the land is bare, it’s susceptible to wind and water runoff,” Livdahl explained. “How you work that into your farming operation is another issue. I don’t think that is something we can come out, through regulation, and encourage people to do. We may come up with some education and incentives to help people work it into their operation.

“People have made a lot of progress,” he added. “We have too much pollution coming off the landscape as a whole.”

For city residents, the messages remain the same - keep grass clippings off the street and keep soil on construction sites and on your yard.

“Those are practices that we’ve talked about for 20 years too, but we do need to do a better job,” Livdahl said. “We still have polluted runoff coming off our streets and hills and ... polluting the lake.”

In addition to residents doing their part to keep the city storm drains clear of yard waste and debris, Livdahl said the city may consider upgrades to and construction of new stormwater ponds, the addition of more rain gardens and possibly more floating islands like those being tested on the E.O. Olson stormwater pond on the Minnesota West campus.

Any of the projects identified can begin before the TMDL study is completed.

“We already know what we need to do,” said Livdahl. “We know where we need to work and what practices we need to do.

“Ultimately we’re going to have to talk to people in the agricultural community - and we’re going to have to talk to people in the urban community about how we can reduce pollution,” he added.

And if people aren’t willing to do their part to help the lake?

“I guess if we do nothing to reduce the rate of sediment and nutrient loading to (Lake) Okabena, we can expect water quality to remain about the same as today and the lake to be a foot shallower in 80 to 100 years,” Livdahl said.

A Wenck Associates representative will attend the Okabena-Ocheda Watershed District’s March 3 meeting to present information from the report. That meeting, slated for 4 p.m. in the Nobles County Public Works facility, will include watershed board members and representatives from the city of Worthington.