Transport of Inorganic and Natural Organic Tracers Through an Isolated Pedon in a Forest Watershed

Abstract

Abstract The transport of several inorganic ions (Mg 2+ , NH + 4 , Br ‐ , and NO ‐ 3 ) and natural dissolved organic C (DOC) through the upper 1.2 m of an isolated in‐situ forest pedon was investigated. The purpose of the study was to investigate the spatial heterogeneity and the rate of nutrient transport in a forest soil. The tracers were added as a pulse to the soil surface during saturated flow in three separate experiments sequentially involving MgBr 2 , NH 4 NO 3 , and DOC, respectively. The temporal patterns of NO ‐ 3 and Br ‐ concentration observed at selected profile locations in the pedon were described by the convective‐dispersive (CD) equation using flux‐type boundary conditions with effluent concentrations treated as flux‐averaged concentrations. Skewed breakthrough curves (BTC) indicative of preferential flow were obtained at all measurement positions. Modelfitted velocities ( v ) agreed well with saturated infiltration rates established in‐situ for soils of the adjacent forest watershed. Although the CD equation could describe the temporal mobility of the nonreactive Br ‐ and NO ‐ 3 tracers at any given point in the pedon, it could not adequately describe the tracer concentration profiles with depth at selected times after tracer addition. Observed BTCs for Mg 2+ , NH + 4 , and DOC indicated that these tracers were reactive with the soil but also demonstrated that a significant quantity of these tracers moved through the soil via preferential flow. The temporal patterns for reactive tracer movement through the pedon could be modeled with the CD equation when adsorption was reversible.