The month of September was selected based on Somviz results to represent the effects from activites during the post-harvest season. The spatial distribution and magnitude of average total E. coli concentrations at specific sampling sites are shown using proportional symbols. The basemap is a choropleth map showing the distribution of total swine animal units per square mile within each watershed. This map answers the following questions:
In September, there are less sampling locations displayed on the map compard to June, but the sites on the eastern edge of the state remain high in their average E. coli concentrations. The sites with high concentrations on the eastern edge of the state are stacked on top of one another, following close to where the Mississippi river is located. These sites may be showing increased contamination from areas upstream of Iowa, in Minnesota or Wisconsin. This hypothesis could be further explored by extending the study area. Additionally, sites with increased concentrations are also seen near the center of the state directly below a watershed unit with high swine density.
These proportional symbol maps show the importance of not only looking at aggregated data, but also at point location data if available, since they may tell different stories. In this case, although there wasn’t a large relationship between swine density and changes in E. coli concentration at the watershed level, it is clearly shown that high concentrations are found near the bottom of watershed units, indicating that the downstream flow of water as an important consideration.