Studying the effects of pollution on the environment from a text book is one thing. Seeing those effects first hand is a different experience altogether. You can duplicate the effects without actually polluting the environment by building a model watershed. Building a model watershed will showcase the negative effects of pollution while providing a fun school project.
If you don't want to use gelatin, you can use dry dirt or small pebbles to represent the pollution.
Perform your demonstration in a clear area.
Avoid spraying too much, otherwise the water might run over the borders of your model.
Pack the foam into the plastic bin so that it resembles a nature scene. One side of the bin will be higher than the other to represent mountains or other high elevation areas.
Press a small basin into the foam on the lower side to create a minter lake. Push down hard enough so that the foam stays in place and does not resume its original shape.
Wedge a small river running from the higher side of the basin down to the miniature lake by compressing the foam with your fingers. Alternatively, you may want to cut a small river out of the foam with a knife. Be careful to always cut away from your own body.
Cover the entire surface of your miniature watershed with aluminum foil. You may need to use more than one sheet. Glue another scrap of aluminum foil over any areas which tear while covering your model.
Glue your model buildings to the aluminum foil. Place the models in between the lake and the mountains. Verify that your buildings do not block the flow of the river.
Place bits of colored gelatin between the buildings and the river. Just put a little bit by each building. This represents pollution.
Spray water onto your model with the spray bottle. This represents rain. Watch how the water flows into the river and carries the gelatin into the lake.