Tile drainage increases aeration in the root zone of poorly drained soils by
accelerating water movement from the subsurface. Water movement is the main agent
of chemical transport, either transporting soluble materials directly (e.g., nitrate) or by
transporting the soil particles that bind adsorptive compounds (e.g., pesticides). The
detection frequency of field-applied chemicals in surface water and groundwater near
agricultural lands prompts the study of chemical fate on tile-drained fields.
Tile effluent was monitored on four Willamette Valley fields from October 2000-
May 2002. All fields were planted in perennial grass seed, except corn was planted in
one field in 2000, and in another field in 1999. Field areas were 30, 20, 2.86, and 2.86
acres. At the outlet of each tile system, effluent flux was monitored and samples were
frequently collected for analysis of nitrate, tracer, and selected pesticides. A transect
of piezometers was installed to monitor water table dynamics. Field-scale saturated
hydraulic conductivity (Ks) was estimated by inputting tile flux data into the
Polubarinova-Kochina solution to the Boussinesq equation for tile drains, and these
values were compared to core-scale K values for each field.
Two fields had a peak drainage rate of over 25 mm day⁻¹. Because of the rapid tile
response, the water table never reached within 0.5 m of the soil surface, and dropped
below the depth of the tiles (about 1 m) within a day, causing tile flow to be
intermittent. The tile drains on these two fields intercepted 4% and 10% of the
precipitation. The two other fields had slower drainage (5-9 mm day⁻¹) and tile-drained 28% and 33% of the precipitation. The water table reached within 10 cm of
the soil surface and took 7-14 days to drop below the level of the tile drains.
Flow-weighted average nitrate concentrations ranged from 1 mg NO₃-N L⁻¹ on the
well-established tall fescue field to 14 mg NO₃-N L⁻¹ on the first-year tall fescue field
that had been in corn the previous year. Mass losses ranged from 1-6.7% of applied
nitrate. Concentrations peaked during periods of low flow, while mass losses peaked
during high-flow periods. Diuron, metolachior, and chlorpyrifos losses through the
tile lines were less than 0.25% of that applied pesticide, and never exceeded EPA
drinking water advisory limits. Concentrations of metolachior and chlorpyrifos were
similar in tile effluent and surface runoff. The highly adsorptive pesticide diuron lost
3% of the applied mass through surface runoff and exceeded EPA drinking water
advisory limits for diuron for three months.
Average soil core and field-scale K values on each field were within a factor of
two. The soil core data varied over several orders of magnitude, and the model data
varied over one order of magnitude for each field. A tracer study verified that
preferential flow accounts for less than 0.5% of solute transport to tile drains.
On perennial grass seed fields in the Willamette Valley, the loss of highly
adsorptive pesticides through surface runoff may be the greatest environmental
concern. By reducing surface runoff, tile drains could improve surface water quality
in the Willamette Valley. Tiles could also be diverting nitrate from groundwater, and
delivering it to surface water during peak flow periods when it is most diluted.
Because the water table dropped slowly and nitrate losses followed EPA standards
after the first year of grass stand establishment, Water Table Management (WTM) is
not advised at this time.