Buffers, which are filter strips planted in perennial vegetation (i.e. not corn and soybeans) are most effective at controlling phosphorus and sediment loads, which tend to come as “sheet flow” over the land. They are less effective at controlling nitrate pollution, because nitrates tend to drop down into the groundwater or into installed drain tiles and flow into surface waters that way.
For decades, Minnesota law has required minimum 50-foot buffers on all “public waters” in agricultural areas. The responsibility for implementing and enforcing that requirement has, however, been diffuse. The state department of natural resources adopted the 50-foot requirement as a “shoreland standard” in the 1980’s, but, under the shoreland law, that only triggered the requirement that local units of government adopt that rule as part of their local zoning ordinances. Most, if not all, ordinances contain that provision today, but often the only enforcement tool is some kind of formal civil or criminal action by a county or city attorney, which rarely if ever happens. Local governments also have fairly free rein to grant variances from the rule, or to “grandfather in” violations as permitted “nonconforming uses.”
Minnesota’s drainage law has also contained a buffer requirement for public ditches. Ditches that do not fall within the definition of “public waters,” e.g. do not drain at least two square miles, require one-rod, or 16.5-foot buffers, but only following a “redetermination of benefits.”