Field drainage installations called blind inlets have been adapted byU.S. Department of Agriculture(USDA) scientists for managing agricultural pollutants in the Lake Erie watershed. These systems, which were developed byAgricultural Research Service(ARS) scientistDoug Smithand others, are so effective that farmers who install them are eligible for financial assistance through USDA'sEnvironmental Quality Incentives Program. ARS is USDA's chief intramural scientific research agency.
Prairie potholes in crop fields have no natural outlets and are prone to ponding, so farmers often install tile risers to drain away excess water. These risers are perforated vertical pipes that extend a foot or more above the soil and connect directly to subsurface tile drainage networks.
Water drained from potholes by the risers is discharged via the subsurface drains into the nearest field ditch without any filtration or processing.
Smith, who works at the ARSNational Soil Erosion Research Laboratoryin West Lafayette, IN, studied how water quality differed between pothole drainage that passed through tile risers and drainage that passed through blind inlets. Often agricultural runoff drained from the potholes in northeast Indiana's St. Joseph River watershed eventually reach Lake Erie. A blind inlet is similar to a French drain that channels excess water away from a building foundation. Smith constructed blind inlets by digging a square pit three feet deep at the lowest point of each pothole and then strategically arranging layers of gravel, tile lines, and related drainage materials in each pit.
Water samples collected from drainage channeled through risers consistently showed the highest levels of total phosphorus lost from the fields—as high as 1.73 ounces per acre. In addition, almost all these samples had higher levels of sediment, soluble phosphorus, total phosphorus, and nitrogen than water that drained through blind inlets. Phosphorus loads in samples from the blind inlets were 78% lower on average than phosphorus loads in samples from the tile risers, and average sediment loads were 79% lower.