Tile drains are one of several best management practices whose purpose is to reduce the export ofcontaminants to receiving waters. It is conventionally assumed that contaminants are effectively retained in the soil priorto subsurface water entering the drain. Two conditions may result in significantly elevated contaminant loading in tileeffluent: preferential flow through macropores, and steep slopes which increase drainage in shallow, permeable soils on ahardpan. This article presents field monitoring data on phosphorus and fecal coliforms measured in New York Statewatersheds under both conditions described, and examines the mechanisms for increased tile drain contaminant deliveryfrom manure-applied fields to streams. Soluble phosphorus concentrations peaked at 1.17 mg/L, and as much as 37% ofsoluble phosphorus was exported from the field site via subsurface drains. Fecal coliform concentrations peaked at71,000 organisms/100 mL. The total number of fecal coliforms discharged from the drain during the flow event wouldhave required about 75 fold dilution to bring the fecal coliform concentration to a municipal wastewater treatment plantdischarge effluent standard of 400 organisms/100 mL. Under some conditions, the contaminant discharge from subsurfacedrains may also have significant water quality impacts to receiving waters.
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Scott et al. (1998) studied this question.