Little empirical evidence exists about the spatial distribution of NO3–N in deep vadose zones and about the associated fate and transport of NO3–N between the root zone and the water table. We investigated NO3–N occurrence in a deep alluvial vadose zone and its relation to geologic site characteristics, hydraulic properties, and fertilizer application rates via an intensive three‐dimensional core‐sampling campaign beneath an irrigated orchard in semiarid Fresno County, California. Statistical and geostatistical analyses were used to determine spatial variability of NO3–N and water content, to estimate total NO3–N mass in the vadose zone beneath each of three fertilizer treatments, and to compare NO3–N occurrence with that predicted from standard agronomic analysis of N and water flux mass balances. Vadose zone NO3 was highly variable and lognormally distributed. Fertilizer treatment had a significant effect on NO3–N levels in the vadose zone. In all cases, deep vadose zone N mass estimated by kriging measured data totaled only one‐sixth to one‐third of the mass predicted by the N and water flux mass balance approach. Vadose zone denitrification estimates could not account for this discrepancy. Instead, the discrepancy was attributed to highly heterogeneous flux conditions that were not accounted for by the mass‐balance approach. The results suggest that spatially variable vadose zone flow conditions must be accounted for to better estimate the potential for groundwater NO3 loading.

Statistical analysis and interpretation of heterogeneous sediment hydraulic properties is important to produce reliable forecasts of water and solute transport dynamics in the unsaturated zone. Most field characterizations to date have focused on the shallow 2‐m root zone. We characterized the geologic and hydraulic properties of a 16‐m‐deep, alluvial vadose zone consisting of unconsolidated sediments typical of the alluvial fans of the eastern San Joaquin Valley, California. The thickness of individual beds varies from <5 cm for some clayey and silty floodplain material to >2.5 m for large sandy deposits associated with buried stream channels. Eight major geologic units (lithofacies) have been identified at the site. Unsaturated hydraulic properties were obtained from multistep outflow experiments on nearly 100 sediment cores. Multivariate analysis of variance and post hoc testing show that lithofacies and other visual‐ and texture‐based sediment classifications explain a significant amount of the spatial variability of hydraulic properties within the unsaturated zone. Geostatistical analysis of hydraulic parameters show spatial continuity of within‐lithofacies variability in the horizontal direction in the range of 5 to 8 m, which is approximately an order of magnitude larger than spatial continuity in the vertical direction. Low nugget/sill ratios suggest that 1‐ to 10‐m sampling intervals are adequate for detection of horizontal spatial structure. The existence of thin clay or silt layers within lithofacies units results in only moderate spatial continuity in the vertical direction, however, suggesting inadequate sampling frequency for hydraulic parameter variogram development in that direction.

Heterogeneity in unsaturated soils and sediments is well known to exist at different scales, from microscopic scale to macroscopic scale. Characterization of different types of heterogeneity in deep vadose zones is challenging because of the usual lack of information at such sites. In this paper, we considered a site with detailed geological, chemical, and hydraulic properties measurements throughout an approximately 16‐m deep vadose zone consisting of unconsolidated, alluvial deposits typical for the alluvial fans of the eastern San Joaquin Valley, California. At the agricultural site, data were also available for a 7‐yr long field fertilization experiment that we used to independently estimate the amount of nitrate stored within the vadose zone at the end of the experiment. Simple mass balance calculations were performed and compared to six conceptually different two‐dimensional and three‐dimensional vadose zone numerical models that were implemented to represent varying degrees of hierarchical details of heterogeneity. Despite widely differing structure and heterogeneity of unsaturated flow, all models resulted in a narrow range of estimated nitrate storage in the deep vadose zone for near‐cyclically repeated water and nitrogen fertilizer applications. Simulated nitrate storage was found to be approximately six to eight times larger than the measured storage at the field. Simulated nitrate variability, while qualitatively similar in pattern, was considerably lower than measured, despite the large simulated hydraulic variability. This study underscores that physical heterogeneity of deep vadose zones may have limited effects on the transfer of conservative contaminants applied repeatedly to the land surface. It also raises questions about our understanding of the chemical fate of nitrate in the vadose zone; and suggests the presence of a significant immobile moisture domain within the deep vadose zone that is not explainable by heterogeneity of Richards equation parameters, yet needs to be considered for simulating nitrate transport under conditions of cyclical infiltration with gravity dominated convective flux.

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