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Chemical Characterization of the Gaseous and Liquid Environments of Subsurface Drain Systems¹

ABSTRACT

Chemical composition of subsurface drain effluents was determined in the field as a function of location (various depths and soil textures), entry point of the solution into the drainpipe, and distance from outlet (with or without a water trap). Liquid samples were analyzed for Fe, Mn, NO₃-N, HCO₃, organic carbon, electrical conductivity, pH, and dissolved oxygen, and gas samples were analyzed for O₂ and CO₂.

Manganese ranged from < 0.1 to 6.1 ppm and Fe ranged from < 0.1 to 14.2 ppm which illustrated the wide range of oxidation-reduction conditions present in the soils drained by subsurface drain lines. Chemical composition changed little during the season.

Water entering the upper portion of the drain lines was lower in Mn, which was the result of short flow paths through soil zones with higher O₂ levels, than the water moving through longer flowlines which entered the bottom of the drains. The solutions in the drain lines were undersaturated with respect to the drain line atmosphere, which indicated that O₂ flows downward through the drain trench backfill. This is an important factor in the oxidation of Fe and Mn in subsurface drain lines. However, longitudinal movement of O₂ into drain lines, via the outlets, is a minor factor contributing to the precipitation of Fe and Mn and does not account for the extensive amount of precipitation of Fe or Mn within the drain system.

¹ Contribution from the Imperial Valley Conservation Res. Center, Sci. and Ed. Admin. USDA, Brawley, CA 92227.

² Soil Sci., Imperial Valley Conservation Res. Center, Brawley, Calif. Arnold MacKenzie retired from the Sci. and Ed. Admin. in July 1975.

Received for publication April 14, 1978. Accepted for publication June 6, 1978.