Finite element modeling of phosphorus leaching through floodplain soils dominated by preferential flow pathways

Phosphorus is a critical nutrient in soils, providing both positive and negative effects to different systems. While optimum crop growth requires a range of P above 0.2 mg/L, preventing surface water enrichment generally requires P to be below 0.03 mg/L. Proper application and control of phosphorus is important to increase farming efficiency and to protect freshwater systems from toxic algal growth. While the movement of phosphorus through many soil types has been well-documented, the presence of highly conductive, gravel outcrops and macropores in soil can have a significant, poorly-documented effect on phosphorus movement. In the Ozark ecoregion, for example, the erosion of carbonate bedrock (primarily limestone) by slightly acidic water has left a large residuum of chert gravel in Ozark soils, with floodplains generally consisting of coarse chert gravel overlain by a mantle (1 to 300 cm) of gravelly loam or silt loam. Highly conductive gravel outcrops and macropores may create preferential flow pathways for water moving through the soil column, along with any solutes in solution. In previous research, floodplain sites in Oklahoma and Arkansas were chosen due to the presence of cherty gravel outcrops that reached near the soil surface. Soil properties were evaluated, and two-dimensional electrical resistivity data were collected and correlated to hydraulic conductivity. Water was then applied to several plots (1, 10, and 100 m ²) with known concentrations of phosphorus, Rhodamine WT, and chloride for up to 52 hours, and flow towards a nearby stream was monitored with observation wells. The objective of this research was to use finite element modeling to develop a long-term model for this phenomenon for future predictions. Results from the previous research were modeled with HYDRUS-3D, a three-dimensional, finite-element model for flow and contaminant transport (both equilibrium and physical/chemical nonequilibrium transport) through soils. HYDRUS-3D was setup to simulate the 1 m² infiltration plot at the Barren Fork Creek site, with initial hydraulic conductivity data calculated from the plot scale infiltration experiment for the upper silt loam soil and from 2D geophysical data for the underlying gravel. The mobile-immobile (MIM) phase model within HYDRUS was also utilized, and MIM solute transport parameters were found iteratively using chloride tracer data taken from two wells. Results from this research will be used to predict phosphorus transport parameters and solve for long-term phosphorus transport through these soil profiles.