Candice you have some great thoughts here and in my head I see them as two different "MWEEs."

Certainly we are overwhelmed by all of the single-use objects in our world and many do not consider the enormous impact that they have. Getting students to the point that they are more thoughtful about their choices is a noble goal and fits within the civic engagement/stewardship component of the MWEE. I think the most challenging part is making the watershed connection and will require some thinking around how to best organize around this idea. 

The nitrate issue is an interesting one because there is a concern that we might see increased nitrates in waterways because of changes in agricultural practices and land use in response to climate change. I just read this article on Nitrogen Pollution which you might find interesting and applicable to what you're considering. Theres a nice call to action for reducing high-animal-protein diets in there too.