Phosphorus removal milestone is reached

CELINA - Dredging's primary purpose is to create safe and navigable waterways, but according to a new study presented by Dr. Stephen Jacquemin of Wright State University-Lake Campus, the amount of nutrient-laden sediment removed by dredging is surpassing the amount entering the lake.

Grand Lake has a large amount of legacy phosphorus that has built up over the years, Jacquemin said during April's Lake Improvement Association meeting, so when tackling the issue of water quality, the community must find ways to not only manage external nutrients coming in, but also the internal nutrients already present.

"We understand that we have a harmful algal bloom problem. We also understand that that harmful algal bloom problem is being fueled by excess nutrients, and we understand that those excess nutrients are not just coming in this year. They came in last year, and the year before, and they've been coming into our lake for well over a century. And so given this, we have a huge amount of legacy phosphorus in the bottom of the lake."

According to sediment core samples taken from around the lake by the Ohio Environmental Protection Agency, an average 1,100 milligrams of phosphorus per kilogram of sediment is in the first 12 inches of muck on the lake bottom. According to Jacquemin, phosphorus levels were higher in some areas, including the St. Marys portion of the lake and in some channels, but the internal nutrient load was still high overall throughout the lake.

There's not a lot of strategies for tackling the nutrients that already have built up, but dredging is one of them, Jacquemin said. According to measurements provided by the Ohio Department of Natural Resources, over the past decade the dredging program has removed between 2.9 million and 3 million cubic yards of material, including 1.1 million cubic yards that has been relocated to littoral wetland sites and roughly 1.9 million cubic yards moved to dredge material relocation areas.

DMRAs are separate sites that hold dredge material, while littoral wetland sites are still connected to the lake. The goal of littoral sites is to eventually build the sediment high enough to plant wetland plants that take up excess nutrients, Jacquemin explained. There are two active littoral sites being used by the dredging program, and 13 total DMRA sites, though some of the latter are inactive because they are full.

If you were to dry out the total amount of dredged material from the past decade into solids and then considered the average load of phosphorus in the sediment of Grand Lake, Jacquemin calculated the dredging program has moved the equivalent of 181 tons of phosphorus to DMRAs and 112 tons of phosphorus to littoral sites.

Based on Ohio EPA data from the past decade, Jacquemin estimated an average of 12,000 tons of suspended sediments washes into the lake each year.

"Our most recent 2018 dredge totals are about 34,000 dry tons of sediment," Jacquemin said.

Jacquemin also noted that while he thought dredging was a solution for Grand Lake, it wouldn't necessarily be a solution for every lake suffering from water quality issues.

"It does create a more homogenous bottom, it stirs up sediment, it sometimes clouds water quality, it sometimes clouds the water a bit. But in a situation in a lake like Grand Lake, the benefits I would argue outweigh the costs."