NUTRIENT TRANSPORT AND TRANSFORMATION BENEATH AN INFILTRATION BASIN

Field experiments were conducted to examine nutrient transport and tra nsformation beneath an infiltration basin used for the disposal of tre ated wastewater. Removal of nitrogen from infiltrating water by denitr ification was negligible beneath the basin, probably because of subsur face aeration as a result of daily interruptions in basin loading. Ret ention of organic nitrogen in the upper 4.6 m of the unsaturated zone (water table depth of approximately 11 m) during basin loading resulte d in concentrations of nitrate as much as 10 times that of the applied treated wastewater, following basin ''rest'' periods of several weeks , which allowed time for mineralization and nitrification. Approximate ly 90% of the phosphorus in treated wastewater was removed within the upper 4.6 m of the subsurface, primarily by adsorption reactions, with abundant iron and aluminum oxyhydroxides occurring as soil coatings. A reduction in the flow rate of infiltrating water arriving at the wat er table may explain the accumulation of relatively coarse (>0.45 mu m ), organic forms of nitrogen and phosphorus slightly below the water t able. Mineralization and nitrification reactions at this second locati on of organic nitrogen accumulation contributed to concentrations of n itrate as much as three times that of the applied treated wastewater. Phosphorus, which accumulated below the water table, was immobilized b y adsorption or precipitation reactions during basin rest periods.