New information from the U.S Department of Agriculture (USDA) shows that while surface runoff is a big contributor to phosphorus in watersheds, there is a significant amount coming from tiles. This article from the USDA says scientists in two studies for the agency found the runoff from farm fields into the Lake Erie Basin is a major contributor to algal blooms that can contaminate drinking water supplies.
Between 2008 and 2013, [Doug Smith, a soil scientist at the Agricultural Research Service (ARS) Grassland, Soil and Water Research Laboratory in Temple, Texas] found that 49 percent of dissolved phosphorus and 48 percent of total phosphorus in the watershed was discharged via tile drains.
From 2005 to 2012, Kevin King, an agricultural engineer at the ARS Soil Drainage Research Unit in Columbus, Ohio, monitored phosphorus levels in the discharge from six tile drains and the outlet of a headwater watershed in central Ohio. He found that tile drains contributed 47 percent of the phosphorus discharge.
Farmers in the region are generally careful to apply only as much fertilizer as needed, and King’s measurements indicated that only around 2 percent of that phosphorus was lost through runoff. But phosphorus concentrations in the tile drainage and the watershed discharge often exceed concentrations recommended for preventing algal blooms, the researchers say. King’s team concluded that reducing phosphorus losses will require practices that mitigate losses via tile drainage in the late fall, winter and early spring, when most of the phosphorus loading occurs.
USDA is trying to find ways to reduce phosphorus levels in the Lake Erie Watershed, helping the U.S. Environmental Protection Agency meet its announced goal of reducing phosphorus fertilizer runoff into the Great Lakes by more than 1,400 tons by 2019.