Choose one with and one without a medium to high density of purple loosestrife. The sites should be similar in as many ways as possible to control for intervening variables such as impact of nearby human activity or amount of water and depth. Obtain permission to use the site for sampling. Create data sheets to record chosen water quality tests in several locations at each site. Assign groups of students to each location at both sites.
Groups assign each student tasks to perform during the sampling.
Set parameters for sampling so that each group uses the same techniques. Do some research on the meaning of results for each water test. Record also an estimate of the amount of different kinds of vegetation, animals etc. If it is not possible for the class to visit the site, individual students could visit the site outside of class and bring observations and samples in for analysis. Collect water samples and biological samples for testing.