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Movement of Methylene Blue Active Substances from Septic Tank Effluent through Two Coastal Plain Soils¹

ABSTRACT

Methylene blue active substances (MBAS) concentration as a function of distance and depth in soils with high perched water tables were monitored as part of a study, supported by the Virginia Department of Health, to determine movement of septic pollutants through natural soil systems. The soils studied were Varina and Goldsboro. Piezometers were placed at selected distances and depths from the drainfield in the direction of ground-water flow to determine both vertical and lateral movement. Also, soil adsorption studies were conducted on selected horizons from these soils to determine the influence of adsorption on distance traveled and biodegradation. Horizontal movement of MBAS in both soils were generally limited to 3.0 m or less. In the Varina, decreases in MBAS above the plinthic horizon could be described adequately as a semilogarithmic function. MBAS in this zone decreased from 0.80 and 0.51 to 0.49 and 0.29 mg/liter from 0.15 to 3.0 m from the drainfield for the winter and summer seasons, respectively. At the 6.1 m distance, concentrations were reduced to 0.10 and 0.12 mg/liter, respectively. In the plinthic horizon, MBAS concentrations were greater adjacent to the drainfield, but decreased more rapidly with distance. The decrease in MBAS in this horizon could be described by a logarithmic function.

At the Goldsboro location, MBAS was concentrated at the 152 cm depth sampled for both the winter and summer seasons. During the winter, concentrations at the 152 cm depth remained at approximately 0.50 mg/liter through 1.5 m distance then decreased significantly. During the summer MBAS concentrations were approximately three times higher and extended to the 3.0 m distance before decreasing significantly. There was no indication of movement of MBAS at the 432 cm depth in this soil.

Soil adsorption studies indicated that these soils were capable of adsorbing large quantities of linear alkylbenzenesulfonate (LAS), but inefficiently under saturated conditions as indicated by adsorption of approximately 65% of the original for all three levels employed.

Key Words: linear alkylbenzenesulfonate • anionic surfactants • LAS adsorption • in situ movement • semilogarithmic and logarithmic functions • surface active agents

¹ Contribution from the Department of Agronomy, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. This work was supported by the Virginia Department of Health. Presented in part before the Southern Branch of ASA at Atlanta, Ga., 6 Feb 1973.

² Assistant and Associate Professor of Agronomy, respectively, Virginia Polytech. Inst. and State Univ.

Received for publication November 15, 1974.