Geologic and geochemical variability across an alluvial vadose zone and shallow groundwater underneath an irrigated agricultural almond orchard site

Abstract:

Nitrate concentrations in groundwater are rising around the world and facing regulatory scrutiny. One of those is the Irrigated Lands Regulatory Program (ILRP), developed to assess, control, and regulate nitrate leaching from irrigated crops. To comply, growers must implement N management plans, improve N use efficiency, and reduce N leaching to groundwater. At the watershed scale, nitrate concentrations were found to be related to agricultural practices. However, neither a mass balance approach nor root zone monitoring were able to consistently account for the nitrate concentration found in groundwater at this scale. Commercial orchard scale (10-100 ha) implementation with direct measurements of resulting groundwater quality immediately underneath the orchard is lacking.

Our project provides the first comprehensive assessment of groundwater nitrate impact from a best practice using three monitoring approaches to assess nitrate impact to groundwater: (1) Groundwater monitoring is the regulatory gold standard to assess pollution sources, but is expensive. (2) Vadose zone monitoring provides immediate feedback on potential groundwater nitrate discharge but can be labor-intensive. (3) The nitrogen balance is a tool familiar to growers under the ILRP but its relationship to actual groundwater nitrate discharge is poorly understood. This project demonstrates the link between these approaches.

Soil cores collected in a 56 ha commercial almond orchard were characterized for geological and geochemical properties, from the topsoil across the vadose zone (7 m) and shallow groundwater (7 m). The alluvial sediments underlying the orchard were found to be highly heterogeneous in all parameters of core sediments and groundwater chemistry. Geostatistical realizations of the texture in the field made it possible to understand the observed geochemical variability. Mass balance approach and upscaled vadose zone monitoring show similar estimates of the fluxes out of the root zone, varying between the sites in the orchard due to the high soil heterogeneity.