Not on My Lawn: The Effects of Minnesota's Phosphorus-Free Fertilizer Law on Surface Water Quality Andrew Hutchens is an Eco
University of Wisconsin--Stout
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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 empirically determine if Minnesota’s statewide law had produced its intended effect of reducing phosphorus content in surface waters, e.g. lakes, rivers, streams. Doing so would allow us to conclude 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. However, 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, while the sale has been completely eliminated in Minnesota. We utilized 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. Our analysis utilized a fixed effects regression model with a difference-in-differences estimator included to account for the “treatment” group, Minnesota, and the “control” group, Wisconsin. Our analysis uses what is called a “natural experiment” in the social sciences. 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 time-invariant variables (things that do not change over time- e.g., geographical incongruity), as well as entity-invariant variables (things that may change over time but not between entities- e.g., 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. 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 equally 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 be relatively costless. Thus, implementing a similar policy in Wisconsin, or modifying the existing Wisconsin 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.
Economics at University of Central Florida