UC Davis

Understanding the fate and transport of nitrogen through the vadose zone is vital to reduce nitrate leaching, protecting groundwater quality, and enhancing resource use efficiency. Currently, there is limited data on the continuous monitoring of nitrate transport through the deep vadose zone. The lack of high-quality data makes it difficult to evaluate the effectiveness of conservation practices aimed at reducing nitrate leaching. The vadose-zone monitoring system (VMS) serves as an innovative technology for near real-time monitoring of nitrate and other contaminants as they travel through the shallow and deep vadose zone to groundwater. The objective of this study was to evaluate the performance of the VMS technology at three sites using bromide (Br-) as a tracer and the unsaturated flow model HYDRUS 1D to understand underlying vadose zone water flow and solute transport processes. Site 1 was a field crop site near Esparto, CA, with a heavy Capay-clay soil and a groundwater depth of approximately 10 m. Site 2 was an almond orchard near Modesto, CA, with a moderately homogenous sandy loam in the top 2 m and a sandy clay loam down to 6 m and a water table at approximately 8 m. Site 3 was a citrus orchard near Orange Cove, CA, with a sandy loam soil at the shallow depths (0-2 meters) followed by sandy clay loam down to 7 m and a groundwater depth of approximately 25 m. The VMS was installed at all sites to collect soil pore water samples at approximately 1 m intervals to about 7 m depth. To test the system performance, approximately 380 L of a 500 ppm Br- solution was applied as a conservative tracer at the three sites. The applied tracer was either percolated by rain, irrigation, or a combination of the two. The site HYDRUS models demonstrated a complete breakthrough of Br- at each vadose sampling port depth. Measured pore-water from the VMS exhibited similar solute breakthroughs with varying time and concentrations. The bromide tracer results confirm that the VMS is capable of monitoring flow and transport processes in the deep vadose zone.