Coupled Transport of Nitrate and Chloride in Soil Columns

Abstract

Nitrate (NO₃⁻) is known as one of the most common groundwater contaminants especially in areas exposed to high nitrogen input because of agricultural activity. Because the vadose zone is the pathway for contaminants into the groundwater, understanding the transport mechanism of NO₃⁻ in different soil types is required to generate information to prevent groundwater pollution. The objectives of this study were to examine the individual and coupled transport of NO₃⁻ and chloride (Cl⁻) and determine the effect of pore-water velocity on solute transport parameters under saturated flow conditions. Acrylic plastic columns (10 cm long and 10 cm inner diameter) were repacked with sand and loam. A pulse type input of 200 mL of three solutions (0.1 M CaCl₂, 0.1 M Ca[NO₃]₂ and 0.1 M CaCl₂-Ca[NO₃]₂ mixture) was individually applied to the soil columns. The CXTFIT program was used to determine the two region nonequilibrium model parameters from 32 breakthrough curves under four different pore-water velocities. The retardation factor and dispersion coefficient values for NO₃⁻ and Cl⁻ from their individual and coupled transport increased with pore-water velocity. The mobile water content also increased with pore-water velocity for both anions. Nitrate and Cl⁻ exhibited similar transport behavior. No differences were observed between their individual and coupled transport in both soils. The results obtained in this study showed that Cl⁻ could be used to predict the mobility of NO₃⁻ in areas where NO₃⁻ is not suceptible to denitrification.