Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2007
Publication Date: 4/9/2007
Citation: Allred, B.J., Bigham, J.M., Brown, G.O. 2007. The impact of clay mineralogy on nitrate mobility under unsaturated flow conditions. Vadose Zone Journal. 6(2):221-232. Interpretive Summary: Because of growing concerns over environmental problems such as nitrate (NO3-) contamination of aquifers in agricultural areas and the Gulf of Mexico hypoxic zone, there is a need for better understanding of the processes affecting nitrate transport through the soil profile. Therefore, laboratory experiments were conducted to assess the impacts on NO3- mobility, due to various clay minerals commonly found in soil. These clay minerals impact NO3- mobility in one of two ways, anion adsorption and anion exclusion. Anion adsorption occurs when negative NO3- ions are attracted to positively charged soil particle surfaces, thereby becoming attached on or concentrated next to these soil particle surfaces. If anion adsorption is a dominant process, NO3- mobility in soil is reduced. Anion exclusion occurs when negative NO3- ions are repelled from negatively charged soil particle surfaces. If anion exclusion is a dominant process, NO3- mobility in soil is increased. The types of clay minerals present can greatly influence whether anion adsorption or anion exclusion is a dominant mechanism affecting NO3- movement through the soil profile. Experimental results indicate that if conditions are acidic and the clay mineral, kaolinite, is present in sufficient quantities, then NO3- anion adsorption will occur. If conditions are not acidic and the clay minerals, illite and montmorillonite, are present in significant quantities, then NO3- anion exclusion will occur. Furthermore, given dry enough conditions, anion exclusion processes can produce high concentration “pulses” of NO3- that move through the soil profile. The results of this research can be used to evaluate the environmental NO3- contamination risks associated with different soils.
Technical Abstract: Due to environmental concerns, there is a need for better understanding of processes affecting nitrate transport through the soil profile. Transient unsaturated horizontal column experiments were therefore conducted to assess clay mineralogy impacts on the electrostatic processes affecting nitrate (NO3-) mobility. Replicated tests were conducted on a quartz sand, mixtures of the sand and one or more clay minerals (kaolinite, illite, and montmorillonite), and two natural soils (with organic matter removed). In each test, a 200 mg/L nitrate-nitrogen (NO3--N) solution was injected at the inlet of dry soil columns. Comparison of corresponding NO3--N concentration (reported with respect to soil solution) 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, concentration profile values for the 100% sand were consistently around 200 mg/L NO3--N elsewhere within the wetted portion of the columns, thereby indicating extremely weak electrostatic interactions involving NO3-. Anion adsorption processes in the kaolinite(25%)/sand(75%) 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 illite(25%)/sand(75%), montmorillonite(25%)/sand(75%), and kaolinite(15%)/illite(7.5%)/ montmorillonite(7.5%)/sand(70%) mixtures and in the Slaughterville and Teller series soils from Oklahoma. Evidence of anion exclusion in these clay/sand mixtures and natural soils includes soil solution NO3--N concentrations near the column inlet that were 11% to 19 % less than the solution concentration originally injected at the inlet. Further strong anion exclusion evidence includes peak soil solution NO3--N concentrations that were greater than 200 mg/L, by factors of 1.7 to 5.4, found at column wetting fronts for the clay/sand mixtures and natural soils containing significant amounts of illite and montmorillonite. Overall research results indicate anion adsorption is an important process affecting NO3- mobility in low pH soils, with limited organic matter, having a clay-sized 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.