|Laboratory Investigation of Anion Exclusion Effects on Nitrate Mobility in Unsaturated Soil|
Summary: Nitrate derived from fertilizers applied at the surface of farm fields can leach downwards and contaminate underlying aquifers. Percolating water containing nitrate can also be intercepted by shallow subsurface drainage pipe systems and then discharged offsite into streams, rivers, and lakes; in turn potentially having an adverse impact on surface water quality at local, regional, and national scales. Therefore, solving environmental problems associated with nitrate (NO3-) requires a better understanding of how NO3- moves through the soil profile under unsaturated flow conditions.
To a large extent, NO3- mobility in soil is governed by electrostatic processes, particularly anion exclusion. Nitrate ions have a negative charge (i.e. nitrate is an anion), and anion exclusion processes occur when mineral/organic soil particle surfaces also have a net negative charge, which is typically the case. Pore-scale (micro-scale) anion exclusion processes begin with negatively charged NO3- anions being electrostatically repelled from the sides of pores comprised of negatively charged soil particles (Figure 1). The NO3- anions are then concentrated in the centers of the pores, where the fastest pore water is present (Figure 1). Consequently, where anion exclusion processes dominate, the NO3- anions move at a rate that is greater than the average pore water velocity. This is why the movement of NO3- in the subsurface is often quicker than expected. Various factors related to the solid soil matrix and the soil water can have a significant impact on NO3- anion exclusion processes.
Figure 1. Pore-scale conceptualization of anion exclusion processes under unsaturated flow conditions.
Consequently, a laboratory investigation with transient unsaturated horizontal column experiments was carried out to assess factors affecting the anion exclusion processes that substantially control NO3- transport in soil. A computer controlled syringe pump was used to inject water containing NO3- into the inlet of a horizontally mounted soil column (Figure 2). The computer controlled syringe pump maintained a constant water content at the inlet as the wetting front progressed into the column. Unsaturated conditions prevailed within the soil column during testing, and all tests were stopped before the wetting front reached the end of the column. Test results include water content and nitrate-nitrogen (NO3--N) concentration profiles plotted versus column distance (from inlet) or the Boltzmann transform (distance from the column inlet divided by the square root of the test duration time) (Figure 3).
Figure 2. (a) Schematic of computer controlled syringe pump apparatus, (b) photo of computer controlled syringe pump apparatus, (c) close-up photo of syringe pump and horizontal soil column with controller in background, and (d) close-up of syringes inserted into column inlet
Figure 3. Example of water content (left) and NO3--N concentration (right) profile results from two transient unsaturated horizontal column experiments conducted with initially dry Teller loam soil. Profiles are plotted versus the Boltzmann transform. Note: As shown in these example results, and as is often observed for initially dry soils, anion exclusion processes concentrate NO3- at the wetting front edge. In this case, NO3--N concentration at the wetting front was more than twice its injected concentration.
1) For the experiments conducted, regardless of the initial soil water content, anion exclusion processes result in the measured NO3--N concentration at the column inlet having a value less than the injected NO3--N concentration. Because the inlet water content and NO3--N concentration remain constant during testing, an effective excluded water content for NO3- can be calculated at the inlet. If the soil is initially dry, anion exclusion processes concentrate NO3--N at the wetting front edge (see Figure 3). Therefore, given initially dry soil conditions, such as is common in arid climates, anion exclusion not only makes NO3- more mobile, but can potentially produce high concentration NO3- “pulses” that move through the soil profile.
2) For a particular soil, if the injected NO3--N concentration is not varied, then the NO3--N concentration measured at the column inlet tends to remain the same, regardless of the water content value maintained at the inlet. This result indicates that the NO3- excluded water content at the inlet is linearly dependent on the total inlet water content.
3) Soils that have a high cation exchange capacity (CEC) will exhibit a much greater NO3- anion exclusion effect that soils with a low CEC.
4) Anion exclusion processes influencing NO3- mobility in the soil are more affected by the total amount of anions and cations present in the soil solution (ionic strength) and not as much by the actual NO3- concentration.
5) The type of accompanying cation can greatly affect the magnitude of the NO3- anion exclusion effect. Monovalent cations increase the NO3- anion exclusion effect, while trivalent cations suppress the NO3- anion exclusion effect.