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

Abstract

Abstract. Nitrate is considered the most common non-point pollutant in groundwater. It is often attributed to agricultural management, when excess application of nitrogen fertilizer leaches below the root zone and is eventually transported as nitrate through the unsaturated zone to the water table. A lag time of years to decades between processes occurring in the root zone and their final imprint on groundwater quality prevents proper decision-making on land use and groundwaterresource management. This study implemented the vadosezone monitoring system (VMS) under a commercial crop field. Data obtained by the VMS for 6 years allowed, for the first time known to us, a unique detailed tracking of water percolation and nitrate migration from the surface through the entire vadose zone to the water table at 18.5 m depth. A nitrate concentration time series, which varied with time and depth, revealed -in real time -a major pulse of nitrate mass propagating down through the vadose zone from the root zone toward the water table. Analysis of stable nitrate isotopes indicated that manure is the prevalent source of nitrate in the deep vadose zone and that nitrogen transformation processes have little effect on nitrate isotopic signature. The total nitrogen mass calculations emphasized the nitrate mass migration towards the water table. Furthermore, the simulated pore-water velocity through analytical solution of the convection-dispersion equation shows that nitrate migration time from land surface to groundwater is relatively rapid, approximately 5.9 years. Ultimately, agricultural land uses, which are constrained to high nitrogen application rates and coarse soil texture, are prone to inducing substantial nitrate leaching.

Sources and fate of nitrate in groundwater at agricultural operations overlying glacial sediments

“…Denitrification in the unsaturated zone is limited by low water contents and oxic conditions, resulting in substantial stores of NO3 -in vadose zones (Turkeltaub et al, 2016;Ascott et al, 2017). NO3 -in water that is removed rapidly from site is 5 also unlikely to be substantially attenuated by denitrification due to oxic conditions and rapid transit times (Ernstsen et al, 2015).…”

Transport and degradation of perchlorate in deep vadose zone: implications from direct observations during bioremediation treatment

“…Each monitoring unit included: (a) a flexible time-domain reflectometer (FTDR) sensor for continuous measurement of variations in the sediment water content Rimon et al, 2007), and (b) vadose zone sampling ports (VSPs), which enable frequent sampling of the vadose zone pore water for chemical analysis (Baram et al, 2012a;Dahan et al, 2009;Rimon et al, 2011b;Turkeltaub et al, 2016). The VMS flexible sleeve was installed in a 0.16 m diameter uncased borehole drilled slanted at a 55 • angle (to the horizon) to a vertical depth of 37 m. In addition to the 11 monitoring units that were installed with the VMS, four additional monitoring units were installed directly in the soil at depths of 0.5 and 1.5 m. It should be noted that the slanted installation is preferred to ensure that measurements carried out by each monitoring unit take place in separate undisturbed sediment columns.…”

“…Continuous measurements of the hydrological and chemical properties of the unsaturated zone may be achieved with a vadose zone monitoring system (VMS) . The VMS provides highresolution measurements of variation in sediment water content Rimon et al, 2007) and evolution of the pore water's chemical composition across the unsaturated profile (Rimon et al, 2011a;Dahan et al, 2014;Turkeltaub et al, 2014Turkeltaub et al, , 2016.…”

Estimating Nitrate Leaching to Groundwater from Orchards: Comparing Crop Nitrogen Excess, Deep Vadose Zone Data‐Driven Estimates, and HYDRUS Modeling

“…Leaching of N applied to agricultural land below the effective root zone is of great concern to groundwater quality worldwide (Green et al, 2008; Hallberg, 1987; Huang et al, 2011; Viers et al, 2012). However, large spatial and temporal variability in leachable N concentrations (mainly nitrate N [NO 3 − –N]) below the effective root zone, at the scale of typical field vadose zone instrumentation, makes it difficult to accurately quantify NO 3 − losses to groundwater (Baram et al, 2016; Kurtzman et al, 2013; Onsoy et al, 2005; Turkeltaub et al, 2016). In view of emerging regulatory programs in Europe (Drevno, 2016; Tsakiris, 2015), California (Dowd et al, 2008; Harter et al, 2005), and elsewhere, there is considerable interest in the assessment and the development of methods that would enable more accurate estimates of NO 3 − losses from agricultural fields to groundwater.…”

“…In some, mass balance along with in situ pore water sampling from below the effective root zone was used to evaluate NO 3 − losses under different N management practices (Li et al, 2007). In others, point measurements of changes in water content and NO 3 − concentrations across the entire vertical domain of a deep (>15 m) vadose zone were used to calibrate flow and transport models (Turkeltaub et al, 2016, 2015). However, such studies fail to address the large degree of spatial horizontal and vertical variability in NO 3 − distribution, fate, and transport below the root zone at the field or orchard scale (>0.10 km 2 ) (Baram et al, 2016; Botros et al, 2012; Onsoy et al, 2005).…”