Volatile Organic Compound Attenuation in Unsaturated Soil Above and Below an On-site Wastewater Infiltration System

Onsite wastewater infiltration systems are utilized in rural, unsewered areas for the disposal of wastewater from single and multiple-family homes, small communities, and businesses. In Wisconsin, there are approximately 750,000 onsite systems. The onsite wastewater infiltration system consists of a septic tank and a drainfield (soil absorption system) as shown in Figure 1. Wastes are partially decomposed anaerobically in the septic tank. Some solids settle out of the wastewater and remain as sludge in the bottom of the tank, while some float as scum. The effluent from the tank is discharged to a gravel-filled bed or trench in the drainfield. The biological mat that develops at the soil infiltration surface serves to further degrade the wastewater.

The soil beneath the drainfield serves as the final treatment medium prior to discharge to groundwater beneath the site. Wastewater infiltration systems represent a significant source of groundwater pollution (Miller and Scalf, 1974; Canter and Knox, 1984). Included as probable pollutants are volatile organic chemicals (VOCs). VOCs are groups of organic compounds characterized by high vapor pressures and fairly low solubilities in water. They include chemicals such as toluene, chloroform, and 1,,1-tri chioroethane. The sources of VOCs in residential systems are with septic tank and drainfield are household products such as disinfectants, cleaners, paint thinners, and gasoline (Hathaway, 1980).

The increased utilization of household products that contain VOCs increases the potential for negative impacts of wastewater infiltration systems on the soil and groundwater beneath these sites. There is relatively little information regarding VOCs in septic tanks, and there was no information found regarding VOCs in the unsaturated soil surrounding the drainfield in a soil absorption system. This research project was conducted in response to the lack of this type of essential information.

OBJECTIVES

The primary objective of this study was to determine the fate of toluene, chloroform, and I,i,l-trichloroethane in unsaturated Plainfield sand in a soil absorption system. Several goals were identified in order to achieve the primary objective:

1. To monitor VOC concentrations in soil gases above and to the sides of the drainfield and in gas above the soil surface;
2. To determine VOC transport 95 cm (approximately 3 feet) beneath the drainfield to what could potentially be a water table;
3. To determine a difference in the fate of VOCs added to a drainfield dosed with tapwater or septic tank effluent; and
4. To compare the movement of water and VOCs through a soil absorption system.

A secondary objective was to design and operate a laboratory model of a soil absorption system that simulates an in-situ system. Several goals were identified in order to achieve this secondary objective:

1. To monitor tapwater and septic tank effluent quality and compare it to septic tank effluent leachate and tapwater leachate collected 95 cm beneath the drainfield;
2. To monitor bromide movement through the soil absorption system and relate it to water movement; and
3. To monitor soil tensions above, below, and to the sides of the drainfield.

It is anticipated that the information obtained from this study will be used by the Wisconsin Departments of Natural Resources and Industry, Labor and Human Relations. This information will be used in developing programs for the proper disposal of household products and management of onsite wastewater infiltration systems with regard to VOCs.

Final report was submitted as a thesis:
Sauer, P. A. (1991). Fate of volatile organic chemicals (VOCs) in unsaturated soil surrounding a drainfield in a soil absorption system. [Unpublished master’s thesis]. University of Wisconsin-Madison.