Coastal Watershed Climate Adaptation: Increasing Coastal Watershed Resilience to Storm-Induced Soil Salinization Using a Novel Concept for Capillary Valves

Abstract

Coastal farms are increasingly affected by soil salinization. Saline water can enter these farms through surface flooding, drainage ditches, and shallow groundwater. Once below the ground, salt may remain in groundwater and later move upward into the root zone. This dissertation examines how salt enters coastal agricultural fields, how it moves through soil and groundwater, and how its effects may be reduced. Field surveys were conducted on a coastal farm in Hyde County, North Carolina. Electromagnetic induction mapping showed patchy areas of high electrical conductivity across the field, which were often associated with drainage ditches. Electrical resistivity tomography identified transient conductive zones below a drainage ditch, even though no salt damage was visible at the surface. Hydrologic and geophysical modeling showed that saline ditch water could produce a similar subsurface response. These results support the interpretation that drainage ditches may contribute to hidden salt accumulation below coastal farmland. MODFLOW 6 simulations showed that saline water can move from a losing ditch into the shallow subsurface and leave salt behind after the water level falls. HYDRUS-1D simulations showed that evapotranspiration can then move water and salt upward from shallow groundwater. This process was strongest when groundwater was close to the surface and varied with soil texture. Together, these results explain why soil salinity can develop in patches. A possible treatment, called a capillary valve, was tested using laboratory soil columns and HYDRUS-1D. The valve consisted of a thin, coarser sediment layer placed within a finer sediment profile. It reduced upward water and salt movement and lowered salt accumulation above the layer during drying. It also allowed downward drainage under wet conditions. However, soil above the valve dried faster, and some salt remained stored near the layer. An economic analysis evaluated the impact of soil salinization on revenues for crops common to North Carolina, including corn, soybeans, and cotton. Corn had the greatest potential revenue loss, given that it is the crop most sensitivity to salt, followed by soybeans and cotton. For a representative 10 ha farm, implementing a capillary valve as a remediation strategy to reduce soil salinity from 8.00 to 3.57 dS/m was estimated to avoid about $10,915 per year in corn revenue loss. Reducing salinity to 1.50 dS/m increased this amount to about $16,225 per year. These estimates translate into a break-even cost for the installation of a capillary valve of between $420/acre - $8186/acre, depending on the crop value. Initial estimates of the material costs to construct a capillary valve were on the order of $13,170/acre, indicating that it may not be an economically viable option unless higher value crops are considered.. Overall, this dissertation shows that coastal soil salinization can result from both lateral salt movement from drainage ditches and upward salt movement from shallow groundwater. It also shows that capillary valves may reduce this risk, although their water-management and economic tradeoffs require further study.