Shallow Groundwater Salinity in the Western San Joaquin Valley

This study used a method for mapping salinity that accounts for uncertainty using regional‐scale, high‐resolution airborne geophysical data, leading to spatially comprehensive 3‐D maps of likely fresh (low salt) and saline (high salt) groundwater. Shallow groundwater is shown to have relatively high salinity in much of the study area, and a clay layer protects lower salinity water in the underlying Tulare aquifer.

Some key findings:​

• Groundwater in the alluvial aquifer is typically higher salinity (1) than in the underlying Tulare aquifer (2).
• Clay layers form the regional boundary between these two aquifers and help protect the underlying fresher water.​​
• Saline groundwater is found in the alluvial aquifer near historical produced water disposal ponds (3).​
• Shallow saline groundwater is also observed in other areas not near oil and gas development (4), likely caused by natural or other land-use activities.​
• Downgradient from some unlined canals, the groundwater is markedly fresher. This is probably due to water seeping out of the canal over time.​

These maps also show spatial patterns in shallow, saline groundwater. A zone downgradient of produced water disposal ponds (historical) has been characterized by monitoring wells and shows up in this 3d map as a deep pink area in the alluvial aquifer [1] Shallow, saline groundwater is also observed in other areas [2]—here the cause is either natural or related to irrigation because they are not near areas of oil and gas development. Another regional feature can be clearly seen on this map as well. This line [3] is an unlined surface water canal. To the east, the groundwater is markedly fresher than to the west of the canal. This is probably due to water seeping out of the canal.