Investigation of an Onsite Wastewater Treatment System in Sandy Soil: Site Characterization and Fate of Anionic and Nonionic Surfactants

Abstract

This study reports on the fate of linear alkylbenzene sulfonate (LAS), alcohol ethoxylate (AE), and alcohol ether sulfate (AES) surfactants in a home septic system near Jacksonville (FL, USA) that has been used since 1976. The drainfield at this site resides in fine sand (< 6% silt and clay) with an unsaturated zone that ranges from 0 to 1.3 m. During the wettest times of the year, it is likely that effluent from the septic system passes directly into the groundwater without exposure to an unsaturated zone of soil. Groundwater was collected during two sampling events, representing seasonal high and low groundwater table levels, and analyzed for the surfactants LAS, AES, and AE. During the wet season, the unsaturated zone was approximately 0.01 m beneath the drainfield. During the dry season, the unsaturated zone was about 0.4 m below the drainfield. Alcohol ethoxylate was not detected in any groundwater samples during either sampling. Alcohol ether sulfate was not found in the dry season sampling, but traces of AES had migrated downgradient about 4.7 m horizontally and 1.8 m vertically in the wet season. Linear alkylbenzene sulfonate was detected in some dry season samples and had moved downgradient some 11.7 m horizontally and 3.7 m vertically in the wet season. These observations demonstrate that these surfactants were removed to a great extent; otherwise, they would have traveled more than 260 m downgradient, which is the calculated distance that a conservative tracer like bromide would have moved downgradient over the life of the system. The most likely removal mechanisms for these surfactants were biodegradation and sorption. Therefore, this study indicates that LAS, AE, and AES are readily removed from groundwater in soils below septic system drainfields even in situations with minimal unsaturated soil zones.

Investigation of an Onsite Wastewater Treatment System in Sandy Soil: Sorption and Biodegradation of Linear Alkylbenzene Sulfonate

“…This study was part of a program that investigated the fate and transport of household cleaning product surfactants in onsite wastewater treatment systems (OWTS) [1,2]. Sorption and biodegradability characteristics are two important parameters for determining the fate of a material in the soil environment.…”

Investigation of an Onsite Wastewater Treatment System in Sandy Soil: Modeling the Fate of Surfactants

“…A more detailed characterization of the surficial geology at the study site can be found in Nielsen et al [6]. Briefly, the soils are characterized as Adamsville fine sand.…”

“…The surfactants investigated (LAS, AES, and AE) were all measured in the STE and saturated zone discharge plume during November 1993 and May 1995 [6]. These determinations were used to test the predictability of the model.…”

“…The groundwater concentrations of LAS used in testing the model were corrected for analytical field spike recoveries, and the highest measured concentration at each wellpoint was used regardless of depth because the groundwater plume does not travel at a uniform depth below the ground surface. Nondetectable surfactant concentrations were assumed to equal 0 mg/ L. The surfactant data used in the modeling exercise are presented in Nielsen et al [6].…”

“…Groundwater velocity was measured in the field using a bromide tracer technique, while groundwater gradients were determined by measuring water table elevations at selected wellpoints [6]. Bromide concentrations were measured at selected wellpoints and time intervals by means of an ion-selective electrode.…”