Measuring nitrate leaching across the critical zone at the field to farm scale

Abstract

AbstractResearch is lacking monitoring beyond the root zone for programmatic or regulatory assessment and for onsite monitoring of field‐ or farm scale agricultural nitrate leaching to groundwater. Here, we investigate the relationship between measurements of N at the sediment core scale, the groundwater monitoring well scale, and the field or farm management scale. Importantly, we monitor across the vertical continuum of root zone (RZ, to 3 m), deep vadose zone (DVZ, to 7 m), and upper saturated zone (SZ, to 14 m) in a highly heterogeneous alluvial sediment system, common to many agricultural groundwater basins. Twenty 14‐m‐deep sediment cores collected across a 56‐ha commercial almond orchard were characterized for texture, water content, nitrate, and ammonium. Groundwater nitrate samples (GW) were also obtained from monitoring wells installed and screened across the SZ at the coring sites. Measured parameters were found highly variable at the 0.05‐m core sample scale. Monitoring well nitrate concentrations also span over a half order of magnitude despite the uniform agronomic practices across the orchard. Laterally aggregated, orchard‐scale parameter means exhibited significant vertical trends and differed in variability between RZ, DVZ, and SZ. Within the SZ, coarse texture facies nitrate concentrations were most closely correlated to GW nitrate, confirming the significance of preferential flow paths within the SZ. The data also indicate significant correlation between DVZ and GW nitrate concentrations. Our findings have important implications to developing appropriate onsite monitoring tools of field‐ or farm‐scale N emissions to groundwater.