Chemistry of subsurface drain discharge from an agricultural polder soil

Protecting groundwater and surface water quality in drained agricultural lands is aided by an understanding of soil physical and chemical processes affecting leaching of plant nutrients and other chemical constituents, and discharge from subsurface drains. Our objectives were to determine which chemical processes most affected discharge of redox-active components and macroelements (mainly base cations) from a subsurface drain, and to assess soil characterization needs for more accurate reactive-transport modeling. We measured the chemical composition of discharge water samples collected for 150 days from a 0.075-ha agricultural field plot on a calcareous polder soil. In light of previously reported two-dimensional, hydrological modeling of subsurface NO3 and Cl discharge, variations in the dissolved concentrations of Mn, Fe, Ca, Mg, Na, K, HCO3, and S could be qualitatively understood in terms of hydrology along with oxidation-reduction (redox), precipitation/dissolution, and cation exchange processes. Elevated concentrations of Mn and Fe in the presence of NO3 during periods of peak flow were consistent with mixing of porewaters from reduced and oxidized soil zones. Discharge samples were supersaturated with respect to pure calcite, and responded only weakly to changes in pH over the measured range of pH 7.2-8.2. Dissolved concentrations of Na, Mg, K, and S depended on their relative affinities for the soil exchanger phase and initial NH4Cl-extractable concentrations throughout the soil profile. Quantitative prediction of reactive components in drain discharge would require coupling of two-dimensional hydrological modeling with modeling of depth-dependent precipitation/dissolution and cation exchange processes, and redox kinetics.