Availability of shallow ground water and lack of control of subdivision development has resulted in use of on-site water and sewerage systems by some 400,000 people in rapidly expanding suburbs of Minneapolis-St. Paul. Surveys have demonstrated that nearly half of individual wells are affected by sewage contamination, as evidenced by presence of detergents and elevated nitrates. Extent of contamination is affected by well depth, population density, soil character, age of development, and direction and rate of ground water movement.