Assessment of nitrate transport parameters using the advection-diffusion cell

Abstract

This study aimed to better understand nitrate transport in the soil system in a part of the state of North Rhine-Westphalia, in Germany, and to aid in the development of groundwater protection plans. An advection-diffusion (AD) cell was used in a miscible displacement experiment setup to characterize nitrate transport in 12 different soil samples from the study area. The three nitrate sorption isotherms were tested to define the exact nitrate interaction with the soil matrix. Soils varied in their properties which in its turn explain the variations in nitrate transport rates. Soil texture and organic matter content showed to have the most important effect on nitrate recovery and retardation. The miscible displacement experiment indicated a decrease in retardation by increasing sand fraction, and an increase in retardation by increasing soil organic matter content. Soil samples with high sand fractions (up to 94 %) exhibited low nitrate sorption capacity of less than 10 %, while soils with high organic matter content showed higher sorption of about 30 %. Based on parameterization for nitrate transport equation, the pore water velocity for both sandy and loamy soils were significantly different (P < 0.001). Pore water velocity in sandy soil (about 4 × 10 m/s) was about 100 to 1000 larger than in loamy soils (8.7 × 10 m/s). On the other hand, the reduction in nitrate transport in soils associated with high organic matter was due to fine pore pathways clogged by fine organic colloids. It is expected that the existing micro-phobicity increased the nitrate recovery from 9 to 32 % resulting in maximum diffusion rates of about 3.5 × 10 m/s in sandy soils (sample number CS-04) and about 1.4 × 10 m/s in silt loam soils (sample number FS-02).

Mass Transport of Nitrate in Soil by Utilizing the Optimized Diffusion Cell and Emission-Transmission-Immission Concept

“…This could be attributed to the disinfection process that has been performed on the equipment used. This result is consistent with what was found in a previous study [21] that reported that if no biological inhibitor was added, then the nitrate flux decreased after a short time of test and that refers to the phenomenon of the formation of a microbial film that consumes nitrate as a growth media and causes some biological reduction.…”

“…Furthermore, a higher concentration of nitrate solution entering the soil specimen in the ODC does not necessarily mean that soil specimens will spend less time to reach equilibrium state [34,47], hence the sorbed amount of nitrate is higher; the enclosed area between the two curves is bigger. This finding is directly in line with what is stated by Aljazzar and Al-Qinna [21], who named the plotted area between the two nitrate fluxes as a confined area representing the amount of nitrate adsorbed within the soil sample and thus indicating nitrate retardation in each sample.…”

“…Once the exchangeable sites on soil surface are filled, nitrate will pass out the setup without retardation. A similar pattern of results was reached by AlJazzar and Al-Qinna [21], showing that the nitrate outflux in different initial concentrations (50, 75 and 100 mg/l) increased with time.…”

“…However, unfavourable small amounts of nitrate output flux for fine-textured soil samples were obtained due to the formation of microbial films over the surface layer of many soil samples inside the advection-diffusion cell. This difficulty led to underestimating the maximum sorption capacity of studied soils during certain sorption phases [21].…”

Modelling groundwater flow and nitrate transport: a case study of an area used for precision agriculture in the middle part of the Vistula River valley, Poland

“…The combination of these processes is a key factor in changes of nitrate concentration in the soil-water system. Aljazzar & Al-Qinna (2016) reported that nitrate transport in coarsely textured soils was dominated by advection supported by low sorption capacity, while nitrate transport in loamy soils was primarily through diffusion. Moreover, it was detemined in our laboratory studies that cohesive soils tended to adsorb nitrate onto their surface.…”