Because of concern for water quality and public health, cities built sewage treatment plants where wastewater is treated before it is discharged. New sewer lines were constructed to carry household wastewater to these treatment plants. Diversion dams were built in the old sewer lines to divert sewage into the new system and to prevent sewage from discharging into the river except during rain. Since combined sewers also carry stormwater, rain increases the volume of water in the system, which then overflows the diversion dams allowing raw sewage to flow into the rivers. These overflows are referred to as Combined Sewer Overflows, or CSOs.
Raw sewage can carry a variety of human bacteria and viruses.
Depending on the amount and concentration of the sewage and on how people are exposed to it, these bacteria and viruses can cause illness. Combined sewer overflows also contain a variety of chemicals, oils, and other wastes picked up by stormwater as it flows across parking lots, roads, lawns, etc. Sewage overflows occur during rain and are therefore diluted by rain and river water, but the potential for health and environmental hazards is still present. People most likely to be affected are water skiers, swimmers, and others involved in water sports. There are water quality standards in Kentucky that limit the amount of certain bacteria in water.