A settling basin and wetland near Parma, Idaho, is cleaning sediment and phosphorus from irrigation runoff water before it goes into the Snake River.

Developers expect their findings will serve them well in a much larger project they plan upstream.

“What it brings is tangibility,” said Hal Anderson, managing partner of Idaho Water Engineering and a partner in project developer Clean Water Partners.

Called the North Alkali Drain Water Quality Improvement Demonstration Project, it returns irrigation water diverted from the Boise River to the South Boise Drain and eventually to the Snake River. The project prevents substantial volumes of sediment and phosphorus from entering the river.

Anderson said it removes 60-90% of suspended sediments.

CWP partner Doug Jones said variations reflect changes in crop rotations and thus sediment loads. Planting more broadcast grains and less corn and beans — grown in rows with more loose dirt — reduces sediment, for example. North Alkali findings include that removing a ton of sediment takes out about two pounds of attached phosphorus.

Here’s how it works.

Water is pumped into a basin at 1 cubic foot per second. Phosphorus attached to sediment particles settles on the basin bottom over about four hours. Phosphorus still dissolved in the water then goes into the constructed wetland, where plants including cattails, hard-stem bullrush, willows and fescue consume it.

Devices at both ends measure how well it works.

Sediment is excavated periodically, providing soil enhancements and compost, and the plants are harvested every year or two. New plants are “hungrier,” and thus more efficient at taking up phosphorus, Anderson said.

“We are attempting to close the loop,” CWP partner and ecologist Rob Tiedemann said. “We don’t want there to be any waste. We want to return the soil to the agriculture community.”

Concepts proved through the North Alkali project could be applied on a larger scale, he said.

The group in the past month secured a provisional pending patent on its system, CWP partner Dave Tuthill said.

Jones said CWP aims to sell credits. Because ag-return water does not come from a single, identifiable source and is not regulated, “it needs an incentive.” The U.S. Environmental Protection Agency “realizes having clean water must include participation from the non-point-source community.”

EPA Regional Administrator David Ross, in a Feb. 6 memo to regional administrators, said the agency supports using water-quality trading and other market-based programs to maximize pollutant reduction and improve water quality.

He called for accelerating adoption of programs that carry an incentive to reduce non-point pollution through land-use practices and technology.

To the east, on Boise River tributary Fifteen Mile Creek south of Middleton, CWP plans to build a 3-4-acre basin and a 23-acre wetland. Siting it to take full advantage of elevation-change gradient to move water will be important, but designers will look to North Alkali to determine how fast they can deliver water and still remove phosphorus most effectively in a larger-scale project.

“If you deliver too much water too fast, plants drink from a firehose and don’t take up phosphorus efficiently,” Tiedemann said.

The North Alkali project in 2013 received a $100,000 Section 319 grant: $60,000 from the Lower Boise Watershed Council and the rest in matching funds from the grantee and partners. CWP seeks continued funding.

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