SOIL VERSUS GROUNDWATER SALINITY AT NAS LEMOORE IN WESTERN SAN JOAQUIN VALLEY, CALIFORNIA

The San Joaquin Valley is one of the most productive agricultural regions in the world. Although practice of irrigation has helped make this possible, it has also caused severe salinization of soil in this region. Due to strong evapotranspiration, salt ions accumulate in the soil, which is then leached to groundwater during rainfall or irrigation events. Conversely, the groundwater table may rise to levels close to soil surface, thereby contributing salt to the surface soil by soil capillarity. To best manage soil salinity, it is essential to understand the relationship between soil salinity and groundwater salinity as well as the long-term impacts of irrigation farming.

The shallow saline aquifer at NAS Lemoore is chemically characterized in this study in conjunction with a larger study that was conducted to evaluate soil and groundwater conditions. Soil and groundwater salinity parameters such as electrical conductivity (EC), pH, and total dissolved solids (TDS) were measured either in the field or from 450 soil cores, drilled to depths of 20-25 ft. Soil paste solutions were prepared and measured to obtain soil pH and EC data.

By comparing the new data with historical records, it has been found that the surface soil of the region has progressed from saline and severely saline levels (EC = 4-12 dS/m) in 1999 to non-saline (EC < 2 dS/m) in 2010 due to land leasing for irrigated crop production. The soil chemistry profile shows that the top 5-ft of soil is non-saline or slightly saline. Soil salinity sharply increases at the depth of groundwater table. Thus, in order to use the soil sustainably in the future, the groundwater table must be closely monitored and maintained at depths well below 5-ft bgs through management and engineering measures.