February 1999

New Study Demonstrates LAS Biodegrades In Septic System

A recent study by the U.S. Soap and Detergent Association concludes that linear alkylbenzene sulfonate (LAS) is readily removed in septic system leaching fields. The study demonstrates that LAS biodegrades in the subsurface soil system and is adsorbed to soil particles, thereby preventing potential migration to drinking water aquifers.

The five-part study, “Investigation of an Onsite Wastewater Treatment System (OWTS) in Sandy Soil,” examined a private septic system in Jacksonville, Florida to assess the fate and transport of detergent ingredients, including LAS, in septic tank effluent and groundwater. They chose a site with a very high watertable where potential chemical migration likely would be greatest (the worst case scenario).

Part 1: Subsurface Soil and Groundwater Characterization. Researchers installed an extensive groundwater monitoring network with 90 discrete sampling points at depths from 1.8 to 9 meters below ground surface. Aquifer “slug” tests and bromide tracer tests also were conducted to help define the flow of groundwater.

Part 2: Chemical Transport Characterization. Researchers identified the migration patterns of various indicators of groundwater quality, including chlorine, total organic carbon, nitrogen, phosphorus and methylene blue active substance. Researchers collected groundwater samples during the peak seasonal fluctuations: when the soil immediately below the septic system was saturated with groundwater (seasonal highwater) and when saturated soil measured 4 centimeters below the system (seasonal low water). The study concluded that migration generally was slightly greater under highwater conditions.

Part 3: Fate of Anionic and Nonionic Surfactants. Researchers used highly specific analytical methods to examine recovered groundwater and concluded that LAS, alcohol ether sulfate (AES) and alcohol ethoxylates (AE) are readily removed in septic system leaching fields, even during highwater conditions.

Part 4: Adsorption and Biodegradation of LAS. Researchers examined the fate and transport of household cleaning products in septic systems and indicated that LAS was removed by biodegradation and adsorption to soil particles during migration in the subsurface soil system.

Part 5: Predictive Model for Cleaning Product Ingredients. Researchers also developed a mathematical model for predicting the fate and transport of household cleaning product ingredients in septic systems. The model can be used by product manufacturers and government authorities for predicting potential exposure.
 
The findings of the study, which were presented at the November, 1998 Society for Environmental Toxicology and Chemistry (SETAC) conference, demonstrate that LAS and other detergent ingredients do not migrate into drinking water aquifers even under worst-case conditions, such as sandy soil conditions where the watertable is barely below ground surface.