Hydraulic Efficiency in Biofilm Affected Tile Drain.

Hydraulic efficiency of subsurface tile drainage can be reduced by biofilms growing on the inside of the tile lines. This study was conducted to evaluate the effects of a manganese reducing biofilm on tile line hydraulic efficiency. Field studies were conducted using monitoring wells to measure water table levels and using a conservative non-reactive tracer to measure transport near the drains. A series of these measurements were done before and after the biofilm was removed from the drain using a water jet. Also, the model, DRAINMOD, was used with 18 years of field-measured data to determine the tile drainage parameters most sensitive to the biofilm accumulation. The tracer transport data was analyzed using moment analysis to obtain flow velocities near the drain. Prior to drain cleaning, the water table and tracer results provided flow velocities of 0.68 - 0.77 cm hr-1. The post-cleaning tracers indicated that velocities increased initially (0.92 - 1.08 cm hr-1), but then returned to 0.69 cm hr-1 within two months. Flow within the tile drainage line was reversed as a result of large rainfall events, and/or restricted drainage outlet conditions (e.g. plugging from ice and snow). In addition, water table levels elevated when the outlet was plugged in the winter suggesting lateral groundwater flow was contributing to drainage discharge. The DRAINMOD combined drainage routine simulated water table levels within 10 cm of measured values; however, prediction of drainage volumes was poor, which was likely due to the lateral influxes of groundwater. The drainage coefficient was the most sensitive model parameter, which agreed with the sensitivity of the field studies to outlet conditions. The biofilm in the drains was found to have little effect on reducing hydraulic efficiency compared to the restriction of drainage outlet conditions and lateral influxes of groundwater.