In the soil, biological processes, filtration, and
adsorption remove most pathogens and some
nutrients. However, conventional septic systems
are not adequate for removing nitrate, and only
partly remove phosphorus, certain pathogens, and
certain other compounds, especially where soils or
ground water conditions are marginally suitable, or
where septic system densities are too high (EPA,
2002). Anything that is not removed by the soil
under the drainfield will end up in groundwater.