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Changes in Inorganic Nitrogenous Compounds from Septic Tank Effluent in a Soil with a Fluctuating Water Table¹

ABSTRACT

Changes in NH₄⁺, NO₂^–, and NO₃^– were monitored in situ during 1972, 1973, 1974, and 1975 in a Virginia Coastal Plain soil with a fluctuating water table. Samples of soil solution above and in a very slowly permeable plinthic horizon were analyzed for the above-mentioned inorganic N fractions. Ammonium-N in solution above the plinthic horizon decreased with increased distance from the drainfield in the direction of ground water flow. This decrease ranged from 23 to 4 µg/ml at 0.15 and 12.0 m from the drainfield, respectively. Decreased NH₄⁺ was attributed to the processes of adsorption and nitrification. Nitrite and NO₃^– concentrations did not change significantly with distance above the plinthic horizon, but did accumulate in the plinthic material beginning at a 1.27-m distance from the drainfield. Nitrification adjacent to the drainfield was possibly inhibited by the high oxygen demand, high NH₄⁺ content, and general anaerobic conditions present. Nitrification above the plinthic horizon was apparently related to distance from the drainfield and fluctuations in the water table level. In this zone nitrification appeared to be followed by denitrification when NO₃^– moved into a more anaerobic zone or when higher water table levels resulted in inundation of the original zone of nitrification. Areas where denitrification was postulated to occur had redox potentials of approximately +200 mV (adjusted to 25C and pH 7.0). Nitrite and NO₃^– that moved into the plinthic horizon apparently did not undergo denitrification since conditions in this horizon are unfavorable for the biological denitrification process.

Key Words: redox potential • nitrification • biological denitrification • NH₄⁺ adsorption • perched water table • aerobic-anaerobic zones

¹ Contribution from the Dep. of Agronomy, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061. This work was supported by the Virginia Dep. of Health.

² Assistant Professor of Agronomy, Virginia Polytechnic Inst. and State Univ.

Received for publication March 8, 1976.