| The typical sources of waste water entering a septic | | | | groundwater. Once out of the French drains in the |
| system are toilets (approximately 38%), laundry (25%), | | | | leach field, pathogenic bacteria will have to compete |
| showers/baths (22%) and sinks/other (15%). | | | | for food with soil microbes and the microbes in the |
| Therefore, the potential contaminants must all be | | | | slime mat underlying the leach field. |
| introduced into the system from one of these sources. | | | | Phosphorous, a contaminant introduced from many |
| The principal contaminant-type of concern is | | | | laundry detergents, typically is not a groundwater |
| microbiological (pathogenic bacteria and viruses). | | | | contamination problem because it is readily taken up |
| Soils which are very permeable (have a rapid | | | | by iron, aluminum and calcium naturally occurring in the |
| percolation rate), also have a very small capacity to | | | | soil. Urea is converted by the septic system flora into |
| absorb effluent from the leach field and this capacity | | | | nitrite, nitrate and ammonium. Nitrate may be a |
| may be quickly exceeded if the system is not | | | | groundwater contaminant particularly in soils which are |
| designed to take this into account. Not allowing for soils | | | | very permeable. Nitrate moves readily through most |
| with little capacity to absorb moisture is a prime reason | | | | soils dissolved in water. |
| groundwater contamination occurs, because pollutants | | | | Metals pose interesting problems. Possible |
| tend to move rapidly through the soil with little chance | | | | contaminants include lead (from lead water pipes or |
| for decomposition. | | | | lead solder- on water pipes), arsenic (found as a |
| The typical leach field will be perpetually wet | | | | contaminant in phosphate detergents), iron, tin, zinc, |
| (remember that several hundred gallons of liquid a day | | | | copper and cadmium. They are not typically a concern |
| enter the tank and thus the field). This moisture | | | | in septic systems. |
| encourages the growth of a "slime mat" composed of | | | | Movement of many organic contaminants such as |
| a variety of microscopic plants (algae) and animals | | | | solvents, cleaners, degreasers and pesticides, through |
| (bacteria, etc.). This slime mat is the final clarifier of the | | | | soils is not well understood. There is certainly the |
| waste water, pulling out left-over nutrients for their | | | | possibility for organics, such as solvents, to move with |
| own use. They will also decompose, to varying | | | | water through the soil to groundwater. Also possible |
| degrees, certain synthetic organic chemicals such as | | | | are adsorption onto soil, decomposition by soil |
| some pesticides and solvents. | | | | microbes or uptake by microbes or plants. The |
| Many environmental factors (rainfall, soil moisture, | | | | environmental fate of most pesticides has been |
| temperature and pH, and availability of organic material | | | | closely examined, but not from the prospective of |
| in the soil) influence the movement and fate of | | | | subsurface introduction via a septic system. |
| microbes from the septic system through the soil to | | | | |