First-season results from the lawn fertilizer experiment
(Note: This data is provisional, and is based on flow data derived from Manning's equation; in-situ flow data will be collected in Spring 2002. These results are prior to our distributing P-free fertilizer to Plymouth residents, but reflect in part the consequences of the existing ordinances.)

This graph summarizes the total phosphorus run off (on a per acre basis) resulting from the major rain events of July through November 2001 for the six study watersheds in Plymouth and Maple Grove. Overall, Plymouth had roughly half the phosphorus runoff as Maple Grove.

There are certainly sources of phosphorus to aquatic systems other than fertilizer, such as grass clippings or tree leaves that find their way into storm systems, garden fertilizers, etc; however, there are no strong reasons that these alternate sources differ among the study watersheds.

Other potential confounding factors are urban street sweeping policies. The city of Plymouth sweeps streets once/year, in spring to remove sand. Maple Grove sweeps in spring and during fall leaf drop. In addition, areas adjacent to lakes get swept an additional 2-3 times during the summer. Maple Grove thus appears to have a more aggressive street sweeping policy than Plymouth.

Other watershed factors are quite similar between the municipalities, with no strong differences in soil types (predominantly loams and clay loams), topography, precipitation patterns, storm sewer design, street width, lawn and house size and design, or lot size. We are in the process of compiling this comparative information for the two municipalites and will post this information when complete. While no two watersheds are identical, this design allows statistically robust comparisons among municipalities with and without fertilizer ordinances - note that it passed an initial peer review by the EPA panel that reviewed the grant application, and will be subject to further professional review as results are submitted for publication.

This graph shows phosphorus run off (on a per acre basis) for the individual rain events from July through November 2001. The Maple Grove watersheds delivered a higher phosphorus load to downstream systems in 17 of the 21 rain events.

Further analyses of this data are in progress; this site will be updated as new results become available.