Modeling Preferential Fluid Flow through Deep Tropical Soils

Deforestation and the expansion of agricultural land is occurring at a rapid pace throughout much of South America. Understanding the impacts of this trend on local hydrogeology is key to mitigating potential negative effects on ecosystem health and sustaining future agricultural opportunities. Tanguro Ranch is located in a Brazilian region of extensive land use conversion, and it’s deep, well-weathered soils are primary controls on the regional hydrogeology. In the last two decades, approximately half of the ranch land has been converted to soy and maize crop that requires heavy fertilization with both phosphate and nitrate. Previous studies at Tanguro have observed higher groundwater solute concentrations in agricultural watersheds, but no significant increases in stream solute concentrations. This has been attributed to vertical preferential flow paths that enhance nutrient transport to groundwater and reduce direct lateral surface and subsurface transport to nearby stream channels. This study quantifies the role of these vertical flow paths by pairing multi-year soil moisture and meteorological data with soil macropore data to develop detailed subsurface transport models. Soil moisture data are collected on a daily basis from 9-meter deep pits using time-domain reflectometers. Local meteorological data are collected from two weather stations located on the ranch. Macropore distribution and preferential flow data were assessed using tension infiltration and dye tracers. Modeled fluid fluxes will be integrated with soil pore water and groundwater chemistry to develop a regional model of flow from agricultural fields to groundwater to the stream network. These models contribute to our understanding of subsurface nutrient fate and transport through deep, tropical soils, and also help assess potential groundwater quality impacts from future climate and land use changes.