Richards Equation–Based Modeling to Estimate Flow and Nitrate Transport in a Deep Alluvial Vadose Zone

Abstract

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.

Real-time monitoring of nitrate transport in the deep vadose zone under a crop field – implications for groundwater protection

“…Here, as well as in previous studies in the literature, nitrate fluxes in the unsaturated zone underlying agricultural land uses were associated with nitrogen application rates and soil physical properties (Green et al, 2008;Botros et al, 2012;Turkeltaub et al, 2015b). Therefore, to attenuate nitrate leaching to aquifers, a search should be dedicated to locate the "hot spots" where these conditions prevailed (Liao et al, 2012).…”

“…Nitrate fate in the subsurface has been investigated by various approaches, such as (i) isotopic signature analysis in groundwater systems (Oren et al, 2004;Wassenaar et al, 2006;Showers et al, 2008;Baram et al, 2013), (ii) cropmanagement strategies, which combine crop production and nitrate leaching to the subsurface (Hanson et al, 2006;Doltra and Muñoz, 2010;Beggs et al, 2011), and (iii) studies based on data from the deep vadose zone (Dann et al, 2010;Nolan et al, 2010;Botros et al, 2012;Kurtzman et al, 2013;Dahan et al, 2014;Turkeltaub et al, 2015b). Nevertheless, estimates based on data obtained from excavated soil profiles and pore-water sampling during a short period of time represent a snapshot in time of the sediment's chemical state rather than dynamic temporal variations.…”

Assessing the effectiveness of drywells as tools for stormwater management and aquifer recharge and their groundwater contamination potential

“…A multi-year study conducted at a site in the Eastern San Joaquin Valley, California, found that varying permeabilities and layering in the vadose zone, among other things, leads to the creation of preferential flow paths that extend to the water table separated by areas of more stagnant flow (Harter et al, 2005;Botros et al, 2011). While the study was conducted specifically in reference to nitrate and other agrochemicals, preferential flow paths and rapid lateral flow occurring adjacent to low permeability materials has the potential to both shorten the arrival times of peak contaminant concentrations while also prolonging transport of the total mass for many groundwater contaminants owing to sequestration in the slower zones (Harter et al, 2005).…”

Numerical Homogenization

“…The classical Richards equation models the flow pressure head in unsaturated media and is often combined with a mass transportation equation in order to simulate pollutant distribution in unsaturated soil [59]. In the literature on numerical simulations for the Richards equation, the main study and experiments are usually carried out for single-scale problems.…”