Talk about ground water filtration. Use 2 liter bottles with the sides cut off to make a 'pan' for your substrate. Bottle 1: soil with grass planted. Bottle 2: wood chips, pebbles, leaves and soil. Bottle 3: Soil and sand. Cut the bottom off of 3 more, 2 ltr bottles and tie strings to them and hang them from the substrate bottles from the bottle tops to collect the run off water. Now water each of your substrate bottles and observe the results.
