Gov. Mark Dayton’s reluctant decision last week to exclude private ditches from buffer strip mapping was an obvious setback to his hard-fought 2015 water-quality law.

But the head of Minnesota’s Board of Water and Soil Resources (BWSR) said Tuesday that private ditches account for only a “small portion” of surface drainage and that the state is forging ahead with its plan to thwart chemical runoff from soils in farm country.

BWSR Executive Director John Jaschke said the state won’t attempt to calculate the percentage of drainage now flowing from private ditches. The data collection would take months and the Department of Natural Resources (DNR) has said there isn’t a good method to quantify it.

Instead, the DNR will continue to map public ditches in need of 16.5-foot-wide strips of natural vegetation under the new law. Dayton pushed for the requirement in response to a state finding that half of Minnesota lakes and rivers in southern Minnesota are too polluted much of the time to allow swimming or fishing.

The strips of grasses and other vegetation help to block runoff of fertilizer, pesticides, other chemicals and harmful bacteria that would otherwise flow into streams, rivers and lakes. Buffers also are intended to aid fish habitat by slowing erosion and curtailing riverbank sloughing.

Dayton backed off on the mapping of private ditches, saying Republican leaders threatened not to fund other work for the environment. The issue has pitted the governor against some farmers, who don’t want to lose land that could be used to plant crops.

Going forward, Jaschke said the BWSR’s job will be to work with local authorities to install missing buffer strips on a rolling schedule.

“Nothing has changed in that respect,” Jaschke said.

He said there shouldn’t be any disputes over whether a ditch is public or private. They are classified in legal descriptions and public records as one or the other. He also said the normal collective process of establishing public ditch systems will continue to convert private ditches to public ones.

Jaschke said Renville County, with its flat, extensively drained farm fields, is an example of a place with few private ditches. The further west you go, to places such as Lac qui Parle and Yellow Medicine counties, there is less uniformity, he said. And in one south-central watershed district, rough estimates show that nearly 10 percent of the drainage is by private ditch.

“It’s very different place to place,” Jaschke said.