Nitrate-nitrogen in groundwater is the most widespread contaminant causing up to ten times as many well closures in the state of California as all other industrial contamination combined. While large amount of research has focused on nitrogen cycling in the root zone (to depths of 6-10 feet), little is known about the fate of nitrogen between the root zone and the groundwater table. Unlike other agricultural regions of the United States, however, groundwater levels in many areas of Central and Southern California and elsewhere in the southwestern United States are from 30 feet to over 100 feet deep. Therefore, the deep vadose zone is a critical link between agricultural sources and groundwater. Few studies have surveyed the hydrology and the fate of nitrogen at such depths or monitored leaching of nitrogen to a deep water table. Field-scale spatial viability of nitrate due to natural viability of soils and vadose zone sediments also remains unaccounted for in most work on groundwater quality impacts of agricultural nitrogen management.

In this study, field, laboratory, and modeling research was carried out in conjunction with an eastern San Joaquin Valley orchard site (near Reedley, Fresno County). The site was subject to a unique long-term nitrogen fertilizer study that investigated crop impacts of several alternative management practices, with fertilization rates ranging from 0 to 325 pounds of nitrogen per acre. Groundwater levels at the orchard site are approximately 50 feet below the surface, which is typical for many ares in the southern and eastern San Joaquin Valley. The unsaturated zone at the site has a heterogeneous profile that is characteristic for many alluvial soils and sediments found in the San Joaquin Valley and other alluvial basins in the southwestern United States.

The sustainability of water resources is key to continued prosperity in the San Joaquin Valley and California. The vadose zone is an often-ignored layer of wet but unsaturated sediments between the land surface and the water table. It plays an important role in groundwater recharge and in controlling the flux and attenuation of nitrate and other potential groundwater contaminants. In a former orchard at the UC Kearney Research and Extension Center, we investigated the processes that control the movement of water, nitrate and other contaminants through the deep vadose zone. These processes were found to be controlled by the alluvial sedimentary geology of the vadose zone, which is highly heterogeneous. This heterogeneity should be considered when interpreting soil and deep vadose zone monitoring data and assessing of the leaching potential of agricultural chemicals. The transport of contaminants through the vadose zone may be significantly faster than previously assumed, while denitrification is likely limited or insignificant in the oxic, alluvial vadose zone of the eastern San Joaquin Valley.