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.
are certainly sources of phosphorus to aquatic systems other than
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.
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.
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.
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.