Restoring ground water is not easy, it's not cheap, and it's not always effective. The two most common approaches are containment and extraction. Containment keeps the polluted water away from the rest of the aquifer. If the residents of the Upper Cape want to contain their polluted water, they could have underground retaining walls built or have the water's natural flow direction altered by pumps. The downside of containment? It doesn't do anything to purify the water - it just keeps the contaminated water from moving into the rest of the aquifer. Extraction involves getting the water out of the ground, cleaning it by aeration and filtration, and then returning it to the aquifer. The downside of extraction - it requires a long time, even decades. A final option is to do nothing - just let the contamination flow into the ocean. The downside? You decide.