The following is part of a series featuring the results of student research on water quality in the Red Cedar Watershed:

For my research done over summer 2017, we analyzed the Minnesota Phosphorus-Free Lawn Fertilizer Law’s effects on surface water quality. Essentially, we set out to determine if Minnesota’s statewide law had produced its intended effect of reducing phosphorus content in surface waters like lakes, rivers, streams.

We wanted to find out whether a policy of the Minnesota law’s caliber should be implemented in the state of Wisconsin. The Wisconsin state legislature enacted a similar statewide policy in 2010. But the Wisconsin law pales in comparison to the stringency of the Minnesota law. For example, it is still possible to purchase phosphorus fertilizer in Wisconsin, the sale of which has been completely eliminated in Minnesota.

We used total phosphorus content data from the National Water Information System (NWIS) and EPA’s Storage and Retrieval System (STORET) for all counties in Minnesota and Wisconsin. The model incluuded a difference-in-differences estimator to account for the “treatment” group in Minnesota, and the “control” group in Wisconsin.

Variables used

Our analysis uses what is called in the social sciences a “natural experiment”. Similar to medical trials, one group (Minnesota) receives a treatment in the form of the law, while the other group (Wisconsin) does not, thus acting as the placebo (i.e., control).

Fixed effects regression allows us to control for variables that do not change over time like geographical incongruity as well as variables that may change over time but not between entities such as national level legislation). Our model uses location as the entity variable and month as its time variable. Finally, our difference-in-differences approach enables us to estimate a causal impact of the law on Minnesota surface water quality.

In our model, we included two control variables to produce the most robust results possible — population density and mean precipitation by county. Three models were created in total, each with increased inclusion of fixed effects and control variables.

Significant effects

The third, and most complete model, including all fixed effects and control variables, produced results showing that the Minnesota Phosphorus-Free Lawn Fertilizer Law was significantly and directly responsible for reducing phosphorus content by 0.096 mg/l, on average, in Minnesota surface waters.

If Dunn County or the state of Wisconsin as a whole were to adopt a lawn fertilizer law as stringent as that of Minnesota, there would be significant economic benefits. Using the most recent average water quality measure in Dunn County, we can simulate a 0.096 mg/l reduction in phosphorus content as a result of a similar law’s enactment. This decrease in phosphorus content would result in property value increases of $3,174 per lakefront or near-lakefront property in Dunn County.

Implementing a policy like the Minnesota Phosphorus-Free Lawn Fertilizer Law appears to have relatively no cost. Putting similar law in place in Wisconsin — or modifying the existing policy — seems to be extremely beneficial not only to the people of Dunn County, but also to the rest of the state.

Our goal now is to present this information to Wisconsin policymakers with the aim of bringing real change to fruition.

Andrew Hutchens of the University of Central Florida took part in the Linked Applied Knowledge in Environmental Sustainability Research Experience For Undergraduates through University of Wisconsin-Stout last summer.