Phosphorus Concentration—Water Flow Interactions in Tile Effluent from Manured Land

Phosphorus concentrations in tile effluent during drainage events increased with increasing flow and decreased as flow diminished. Nitrate and chloride concentrations were inversely related to flow rate, which is consistent with simple dilution and depletion effects. Piezometers demonstrated the presence of a water table perched above high bulk density material with low vertical permeability just below the plow layer (0–25 cm). Nitrate/chloride ratios of tile effluent during the course of drainage events, coupled with NO 3 /Cl ratios in soil samples from the plow layer and subsoil, suggested the flow of water to the tile was dominated by downslope movement in the plow layer and by downward movement to the tile in the relatively permeable backfill material above the tile. Calcium or salt concentration in soil solution and tile effluent during the course of a drainage event influenced the P concentration. Increasing Ca concentrations decreased P, whereas decreasing Ca concentrations increased P concentrations. The phosphate chemistry coupled with different flow components reaching the tile provided an acceptable explanation of P concentration changes during drainage events.