The Impact of Clay Mineralogy on Nitrate Mobility under Unsaturated Flow Conditions

Transient unsaturated horizontal column experiments were conducted to assess clay mineralogy impacts on electrostatic processes affecting nitrate (NO₃ ⁻) mobility. Replicated tests were conducted on quartz sand, mixtures of the sand and kaolinite, illite, and montmorillonite, and two natural soils with organic matter removed. In each test, a 200 mg L⁻¹ nitrate–nitrogen (NO₃ ⁻–N) solution was injected at the inlet of dry soil columns. Comparison of corresponding NO₃ ⁻–N concentration and volumetric water content profiles from the column tests provided valuable information regarding soil mineral composition impacts on NO₃ ⁻ transport. With the exception of a small peak at the wetting front, NO₃ ⁻–N concentrations for the quartz sand were consistently near the 200 mg L⁻¹ injection level within the wetted portion of the columns, indicating that NO₃ ⁻ electrostatic interactions were negligible. Anion adsorption processes in the 25% kaolinite–75 % sand mixture produced a result in which the NO₃ ⁻–N concentrations adjacent to the inlet of the columns were approximately 20% greater than that of the injected solution. Anion exclusion was the dominant electrostatic interaction affecting NO₃ ⁻ mobility in the 25% illite–75% sand, 25% montmorillonite–75% sand, and 15% kaolinite–7.5% illite–7.5% montmorillonite–70% sand mixtures and in the two natural soils. Evidence of anion exclusion in these artificial and natural soils includes NO₃ ⁻–N concentrations near the column inlet that were 11 to 19% less than the injected solution concentration, and NO₃ ⁻–N concentrations near the wetting front that were greater than the injected solution concentration by factors of 1.7 to 5.4. These results indicate that anion adsorption is an important process affecting NO₃ ⁻ mobility in low pH soils, with limited amounts of organic matter, and having a clay-size fraction dominated by kaolinite, while anion exclusion is a key electrostatic interaction influencing NO₃ ⁻ mobility in near-neutral to high pH soils, especially if significant amounts of montmorillonite are present.