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Soil Physics

Monitoring and Modeling Lateral Transport through a Large In Situ Chamber

^a USDA-ARS Environmental Quality Lab., Bldg. 007, 10300 Baltimore Ave., Beltsville, MD 20705
^b USDA-ARS Environmental Microbial Safety Lab., Beltsville, MD 20705

^* Corresponding author (sadeghia{at}ba.ars.usda.gov)

Accurate characterization of lateral transport components is an important step toward a more quantitative assessment of the fate and transport of nutrients and the functionality of riparian/wetland systems. Our specific objectives were: (i) to design an in situ chamber for studying lateral flow under shallow watertable and riparian zone conditions; (ii) to monitor predominantly horizontal transport of non-conservative (NO₃) and conservative (Br) tracers in shallow saturated zone of the soil monolith; and (iii) to obtain reaction and transport parameters, and additional insights about the flow and transport inside the soil monolith. HYDRUS-2D model was used to simulate flow and transport of Br and NO₃, and to evaluate the applicability of this model to the observed flow and transport. Advective-dispersive equation (ADE) and mobile-immobile zone model (MIM) options were tested using the Br data. The breakthrough curves (BTCs) of NO₃ and Br were similar while the concentrations rose, then became distinctly different with NO₃ concentrations decreasing much faster. The calibrated denitrification rate of 0.713 ± 0.211 d^–1 was about an order and a half of magnitude larger in the loam layer (25–35 cm) than in the overlaying sandy loam layer (0–25 cm) and in the sandy clay loam layer (35–65 cm) below. Up to 60% of the introduced NO₃ was lost to denitrification. The methodology presented here allowed the in situ estimation reaction and transport needed for modeling; and it showed a potential to provide detailed information critical for the interpretation of the modeling outcomes performed at field and watershed levels.

Abbreviations: ADE, advective-dispersive equation • BTC, breakthrough curve • CV, coefficient of variation • D_L, longitudinal dispersivity • D_T, transversal dispersivity • K_fs, field saturated hydraulic conductivity • MIM, mobile-immobile zone model • REMM, Riparian ecosystem management model • SSC, stainless steel chamber