Water Flow Characteristics of a Shrink-Swell Soil from a Wetting and Drying Cycle.

Soils native to the Red River Valley are high in smectitic shrink-swell clay content. The shrink-swell process of these soils causes varied hydrological characteristics that are not well understood. As a result, water flow and solute transport is difficult to model and predict. This study was conducted to monitor water flow and salt transport through a Fargo silty clay soil. Six large (20 cm-dia. by 121 cm-length), undisturbed soil monoliths were harvested from a field near Mooreton, ND. Three of these cores were considered saline and three were considered non-saline. A laboratory experiment was designed to accelerate field conditions of a wet/dry cycle native to North Dakota. Five liters of water was applied to the dry soil and leachate was collected for cation analysis to represent salt removal. Elbow tensiometers were installed at four different depths (12 cm, 39 cm, 67 cm, and 93 cm) to monitor water content through the cores over time. Results from two-time leaching events indicate varied hydrological response in regards to falling head infiltration. Average time for complete infiltration of the high salinity cores was 15 minutes and 45 days for the low salinity cores. HYDRUS 1D modeling of water flow using van Genuchten parameters and experimental water content measured by the tensiometers illustrate variance between the two results. Variance between the modeled and experimental results increase with depth, indicating deep water flow through cracked clay that is not well accounted for in a basic HYDRUS 1D water flow model. Solute removal from the two-time leaching events indicate an estimated 131 leaching events with a 0.25% salt removal required to remove excess cations from a high salinity soil to a low salinity soil status. Water flow through the dry soil is preferential through macropore cracks. The short time span for infiltration in the high salinity cores indicate an increase presence of macropores. The long time span for infiltration in the low salinity cores indicate a decrease presence of macropores. This response in infiltration between the high and low salinity cores may suggest that the presence of excess salts may induce cracking in these shrink-swell soils.