Evaluation of Denitrification Systems for Improving Groundwater Quality From On-Site Waste Disposal Systems

Introduction: The Central Wisconsin Sand Plains region contains some of the largest and most productive aquifers in the state of Wisconsin. The sandy glacial outwash soils of the area are highly permeable and the terrain is relatively flat. These factors combined with a relatively shallow unconfined aquifer make this region particularly susceptible to contamination from various land use practices.

One of the land use practices which is increasingly becoming a concern involves the residential development of unsewered areas. Trends in the 1970’s and 1980’s saw population increases in suburban areas. As a result, suburban expansion quickly exceeded beyond the reaches of municipal water and sewage and thus private sewage systems became common.

The primary purposes of current private sewage systems are the disposal of wastewater and the removal of bacteria. Only recently has consideration been given to the level of chemical treatment which can be expected from private sewage systems and their potential for groundwater pollution.

Soil absorption systems are designed to receive wastewater from a septic tank and dispose of it below ground where it is hopefully treated before it reaches the groundwater. It is generally recognized that three feet of unsaturated soil is required to properly treat sewage effluent to allow adequate removal of disease causing bacteria, viruses, suspended solids, and some organic materials.

Treatment efficiency in private sewage systems for other chemical constituents may be less than ideal or simply unknown in the present. Because nitrate, a breakdown of organic nitrogen compounds, is very soluble and does not adsorb to soil, it often reaches groundwater from what are considered well functioning septic systems. When used on sandy soils, many properly functioning septic systems have been shown to result in significant nitrate-N concentrations in groundwater.