The typical sources of waste water entering a septic system are toilets (approximately 38%), laundry (25%), showers/baths (22%) and sinks/other (15%). Therefore, the potential contaminants must all be introduced into the system from one of these sources. The principal contaminant-type of concern is microbiological (pathogenic bacteria and viruses).

Soils which are very permeable (have a rapid percolation rate), also have a very small capacity to absorb effluent from the leach field and this capacity may be quickly exceeded if the system is not designed to take this into account. Not allowing for soils with little capacity to absorb moisture is a prime reason groundwater contamination occurs, because pollutants tend to move rapidly through the soil with little chance for decomposition.

The typical leach field will be perpetually wet (remember that several hundred gallons of liquid a day enter the tank and thus the field). This moisture encourages the growth of a "slime mat" composed of a variety of microscopic plants (algae) and animals (bacteria, etc.). This slime mat is the final clarifier of the waste water, pulling out left-over nutrients for their own use. They will also decompose, to varying degrees, certain synthetic organic chemicals such as some pesticides and solvents.

Many environmental factors (rainfall, soil moisture, temperature and pH, and availability of organic material in the soil) influence the movement and fate of microbes from the septic system through the soil to groundwater. Once out of the French drains in the leach field, pathogenic bacteria will have to compete for food with soil microbes and the microbes in the slime mat underlying the leach field.

Phosphorous, a contaminant introduced from many laundry detergents, typically is not a groundwater contamination problem because it is readily taken up by iron, aluminum and calcium naturally occurring in the soil. Urea is converted by the septic system flora into nitrite, nitrate and ammonium. Nitrate may be a groundwater contaminant particularly in soils which are very permeable. Nitrate moves readily through most soils dissolved in water.

Metals pose interesting problems. Possible contaminants include lead (from lead water pipes or lead solder- on water pipes), arsenic (found as a contaminant in phosphate detergents), iron, tin, zinc, copper and cadmium. They are not typically a concern in septic systems.

Movement of many organic contaminants such as solvents, cleaners, degreasers and pesticides, through soils is not well understood. There is certainly the possibility for organics, such as solvents, to move with water through the soil to groundwater. Also possible are adsorption onto soil, decomposition by soil microbes or uptake by microbes or plants. The environmental fate of most pesticides has been closely examined, but not from the prospective of subsurface introduction via a septic system.