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. 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. 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.