Montana Earth Science Picture of the Week

This photo shows the “septic tank” portion of a septic system for a house near Montana City. Eventually the tank was covered with dirt. Once completed, gravity will pull wastewater from the house (through a pipe) into the concrete box where solids will settle to the bottom. Helpful bacteria will beak down the sewage into a material referred to as “sludge”. Lighter solids such as hair and grease will float to the top. Periodically both of these layers will need to be removed by a septic pumping business.

The liquid portion of the wastewater, called “effluent” flows out of the tank through a pipe that takes it to a drain field. The drain field (a.k.a. leach field) consists of a series of underground pipes that are surrounded by gravel so that the effluent can easily trickle out and then soak into the soil. The soil acts as a natural filter, removing disease-causing bacteria, nitrates, and phosphates.

Septic systems are effective in most situations. As long as there are several feet of the right kind of soil between the drain field and the water table, groundwater contamination should not be a problem. However, in some situations where the water table is high, nitrate contamination from human waste may be a concern. Water tables tend to be near the surface near lakes and streams, and near the lower parts of valleys. Nitrates that seep into lakes or rivers from septic systems can contribute to algae blooms, leading to lower levels of dissolved oxygen in the water. High levels of nitrates in drinking water can contribute to a type of cancer and “blue baby syndrome”.

NOTE: That is my hat on the tank. I put it there to give you a sense of how big the tank is.

Terms: blue baby syndrome, nitrates