CHEMICAL FATE AND TRANSPORT IN A DOMESTIC SEPTIC SYSTEM - SORPTION AND TRANSPORT OF ANIONIC AND CATIONIC SURFACTANTS

The sorption and transport of linear alkylbenzene sulfonate (LAS) and dialkyl quaternary ammonium compounds [ditallow dimethyl ammonium chlo ride (DTDMAC) and distearyl dimethyl ammonium chloride (DSDMAC)] were determined within the upper soil horizons and in the aquifer below a s eptic tank tile field at a study site in Ontario. Sorption of LAS was greatest immediately below the tile field (K-d = 17 L/kg) within the u pper unsaturated soil horizons and lowest in the aquifer (K-d = 1 L/kg ). LAS sorption appeared to be controlled by a hydrophobic mechanism a nd was positively correlated with the organic carbon (r = 0.9) and the clay content (r = 0.9) in the subsurface environment. The sorption of DTDMAC/DSDMAC (K-d = 25-62 L/kg) was an order of magnitude higher tha n LAS but showed no trend throughout the subsurface environment. The r etention of DTDMAC/DSDMAC was believed to be by ion exchange within th e clay interlayer basal spaces. Samples collected from the field showe d concentrations of DTDMAC to range from approximately 60 mg/kg in the tile field gravel to <2 mg/kg within 10 cm of soil below the tile fie ld gravel. Concentrations of LAS were approximately 20 mg/kg in the ti le field gravel and diminished to <1 mg/kg within 5 cm of soil below t he tile field gravel. Soil solution and aquifer concentrations of LAS were always <50 mu g/L and usually below the detection limit of 10 mu g/L in the aquifer. Concentrations of DTDMAC in the aquifer were below the limit of detection (4 mu g/L). Mathematical calculations illustra ted the importance of biodegradation and sorption as removal mechanism s of these materials in subsurface environments.