Three-dimensional modeling of nitrate-N transport in vadose zone: Roles of soil heterogeneity and groundwater flux

Simulation of spatial and temporal variation of nitrate leaching in the vadose zone of alluvial regions on a large regional scale

Excessive use of fertilizers presents a significant threat to groundwater safety. To mitigate nitrate leaching and ensure the sustainable utilization of groundwater resources, it is crucial to quantify the spatial heterogeneity of nitrogen leaching and its drivers. Therefore, accurate modeling of deep nitrate leaching at large regional scales is necessary. In this study, we have created a computational framework to analyze the transport of unsaturated zone water and nitrate at a regional scale. The framework is based on a process-oriented, watershed-scale computational model that segments the study area into a grid system, with each grid modeled using Richards-based advection-diffusion equations for water and solutes. The research model estimated nitrate nitrogen leaching, accumulation, and denitrification in the vadose zone of agricultural fields in the Baiyangdian watershed, which is a typical agricultural region with complex land use and soil deposition conditions in the North China Plain. The results showed that there were significant spatial differences in nitrate N leaching, denitrification and accumulation with values of 0-388 kg/ha/year, 30-177 kg/ha/year and 75-4778 kg/ha. Groundwater recharge in the wheat/maize, vegetable, and cotton area exhibited a negative correlation with nitrate N accumulation while showing a positive correlation with nitrate N leaching. Nitrate nitrogen distribution indicated spatial heterogeneity, attributable mainly to the heterogeneity in soil texture, structure, and land use. With nitrate nitrogen leaching and denitrification levels reaching 327-388 kg/ha/year and 133-175 kg/ha/year, respectively, vegetable fields pose a direct threat to groundwater. Meanwhile, wheat/maize fields showed the greatest nitrate nitrogen accumulation, ranging from 624 to 4778 kg/ha. This excessive buildup of nitrate in these fields presents a potential hazard to groundwater quality. Soil texture in the root zone had a greater influence on the amount of nitrate leaching and denitrification than soil texture below the root zone. Deeper soil texture (>2 m) was found to mainly control total nitrate accumulation in the vadose zone. To assess nitrate leaching, denitrification, and accumulation at a regional scale within the deep vadose zone, a process-oriented model was developed, considering the intricate associations among land usage, soil texture, and biochemical reactions.

Long-term numerical modeling of nitrate leaching into groundwater under surface drip irrigation of corn

Proper management of fertigation is necessary to deal with the harmful impacts of fertilizers. This research aimed to investigate the nitrate leaching rate into groundwater in different fertigation management under the climate change impact in drip irrigation of corn. For this purpose, HYDRUS-2D was calibrated by performing field experiments. Plant water requirement and rainfall were projected until 2050 using LARS-WG6 under the RCP85 scenario. Then, nitrate leaching up to groundwater at the depth of 5 m was simulated in the growing season of corn and the like until 2050 in three fertigation scenarios, including S1 (three regional fertigation splits with irrigation efficiency of 85%), S2 (weekly fertigation with irrigation efficiency of 85%), and S3 (optimum fertigation with irrigation efficiency of 100%). Finally, the annual nitrate leaching rate to groundwater and leached amount were compared in the studied scenarios. The results demonstrated that nitrate penetrated to the depth of 117 and 105 cm at the end of the first year in S1 and S2 scenarios, respectively. In these scenarios, nitrate will reach groundwater in 2031, but nitrate concentrations will not be the same. In the S3 scenario, the nitrate will reach a depth of 180 cm by 2050. Total leached nitrate to groundwater up to 2050 will be 1740, 1200, and zero kg/ha in S1, S2, and S3 scenarios, respectively. Based on the approach of this study, the vulnerability of groundwater to nitrate contamination in different agricultural areas can be evaluated, and appropriate strategies with minimum environmental impacts of fertilizer abuse can be selected accordingly.

Nitrate transport in a fracture-skin-matrix system under non-isothermal conditions

Subsurface leaching of agricultural runoff has been identified to pose a serious hazard to the soil-water ecosystem and human health, mostly due to the associated contamination with nitrate. Our understanding of the nature of contaminant spread in the vadose and aquifer zones has been improved from recent mechanistic models on the flow and transport of contaminants through fractured porous media. The present study aims to explore the impacts of skin formation in a fracture-matrix aquifer system onto the nitrogen species transport under non-isothermal settings using numerical modeling. A finite-difference scheme was employed to capture the nitrogen concentration profile and kinetics of transformation by solving the derived partial differential equations. The results show evidence of an additional mass transfer from fracture to skin so as to reduce the migration of nitrogen species (NO₃-N and N₂) at the fracture-matrix interface thereby reducing the peak concentration of N₂ by nearly 1.5 times in fracture after denitrification. Although the thermal conductivity of the rock matrix has a direct impact on the temperature distribution in fracture-skin-matrix profiles, the presence of skin has a cooling effect for a high-temperature influent (45 °C), which also deteriorates the propagation of organic N₂ and NO₃-N, within the fracture. An increase in the temperature coefficient of skin has resulted in an apparent reduction in nitrogen species migration, indicating the thermo-chemical feasibility of an intermediate skin favoring the mass transfer processes. The findings of this study can be extended toward realistic estimation of groundwater contamination risks and for the design of biological filters for in situ remediation.

Impacts of Future Climate Variability on Atrazine Accumulation and Transport in Corn Production Areas in the Midwestern United States

Atrazine is one of the most prevalent herbicides that has been widely applied to agricultural lands in the U.S. Understanding the transport and accumulation of atrazine in the subsurface under future climate scenarios is essential for future agriculture and water management. Here, we predict atrazine transport and accumulation under an intensive corn production land based on 20 projected global climate model (GCM) realizations, while considering uncertainties of transport parameters. Our study predicted continuous groundwater table declination and atrazine mass accumulation on the study site. We show that atrazine mass accumulation in corn production areas is subject to total precipitation in the atrazine application season, whereas atrazine plume movement is controlled by the sequence of annual precipitation. Atrazine mass transport and accumulation are more sensitive to climate variation on the field sites with low sorption and atrazine degradation rate. Under the extreme condition, the atrazine plume can migrate as far as five meters from the ground surface in only three years. While annual mean precipitation in the Midwestern U.S. is projected to increase in the future, groundwater vulnerability to atrazine and associated water quality impacts may rise in the U.S. Corn Belt, especially in sites with low atrazine degradation and sorption.

Effects of Underground Coal Mining on Soil Spatial Water Content Distribution and Plant Growth Type in Northwest China

The impact of coal mining subsidence on surface ecology involves the influence of several ecological elements such as water, soil, and vegetation, which is systematic and complex. Given the unclear understanding of the synergistic change patterns of the water-soil-vegetation ecological elements in the influence of coal mining in the west, this paper investigates the impact of coal mining on the surface ecology, especially the distribution of soil water content (SWC). In 2020, this study collected 3000 soil samples from 60 sampling points (at depth of 0-10 m) and tested the SWC. All samples come from three different temporal and spatial areas of coal mining subsidence in the desert mining area of Northwest China where soil types are mainly aridisols. At the same time, the interactions among deep SWC and surface soil physical and chemical properties, surface SWC and soil fertility, and pH were analyzed. The spatial variability of soil moisture is reflected by kriging interpolation, and SWC values at different depths are predicted as a basis for monitoring the environmental impact of different coal mining subsidence years. The research has shown that the ground subsidence leads to a decrease in SWC value and changes in surface soil pH, physical and chemical properties, and covering vegetation, which have occurred from the beginning of coal mining. The impact of coal mining on the SWC of the unsaturated zone is mainly at the depth of 0-6 m, where SWC is not directly related to the nutrient content of the surface soil. The overall settlement of the ground will stir up simultaneous decline in the quality of deep SWC and topsoil. The findings of this investigation suggest that changes in the soil structure caused by coal mining subsidence are the key factor in SWC loss. Timely monitoring and repairing 0-6 m ground fissures, as well as selecting shrubs on the surface is the best choice for the restoration of the ecological environment and prevention of soil erosion in this area.

Denitrification in the vadose zone: Modelling with percolating water prognosis and denitrification potential

Transport and transformation processes of nitrogen in the soil are an essential part of understanding the relationship between agricultural input and nitrate (NO₃^-) concentrations in groundwater. The presented study describes these transformation processes around NO₃^- degradation at a water catchment in the Lower Rhine Embayment, Germany. Despite intensive agriculture, extracted groundwater at a depth of 21 to 22 m shows unexpectedly very low NO₃^- levels, below 3 mg/L NO₃^- for all wells. The local water supplier therefore carried out investigations in this area and generated soil data from 22 representative areas (142 soil samples from 82 drilling meters from the surface to a max. depth of 5.5 m) and groundwater analyses from 17 groundwater monitoring wells (from 3 to 5 m below ground surface). Soil types are predominantly luvisol and gleysol. The substrate in the topsoil is mainly clayey silt; underneath there are mostly medium-grained sands with partial silt intercalations which appear as a separate layer. Based on this dataset, the percolating water residence times and the NO₃^- leaching potential were calculated in this study. Together with the nitrogen surplus and with the help of reactive transport modelling, the denitrification potential in the vadose zone was simulated. The comparison of simulation results with laboratory-measured data shows a high correlation. Substantial NO₃^- reduction in the vadose zone was observed: dependent on soil type, reduction capacity and water residence time, up to 25% of the NO₃^- was reduced here. The applied modelling is considered an improvement in NO₃^- degradation potential assessment because it considers many relevant variables such as precipitation, soil parameters (grain size, field capacity, available water capacity, coarse fragments) and nitrogen input. Therefore, a transfer to other sites with comparable hydro(geo)logical conditions is possible, also due to relatively easily determinable input data. This assessment of nitrogen degradation in the vadose zone will be a useful tool for NO₃^- levels forecast in groundwater.

Influence of transient porosity in a coupled fracture-skin-matrix system at the scale of a single fracture

The subsurface leaching of soluble chemicals in a fractured porous medium poses long-term risk of groundwater contamination. Tracing the occurrence, movement and consequences of such hydro-geo-chemical interactions is the fundamental process for an effective remediation plan. However, the complexity of geomorphology and mass transfer mechanisms makes it challenging while addressing these issues in a real field scale. The present study focuses on simulating the concentration profile of nitrate elution in a pseudo two-dimensional coupled fracture-skin-matrix system under active biodegradation using an implicit finite difference numerical technique. The interface between the fracture and rock matrix is assumed to possess a skin with time-varying porosity imitating the effect of bio-clogging. The results indicate that denitrification is significant in reducing the dissolved nitrate concentration for initial skin porosity of 10% in the presence of an unlimited oxygen and primary substrate. When the rate of change of skin porosity remains lower with a minimal variation, the nitrate concentration provided a considerable reduction in the vicinity of the fracture inlet. A similar trend is observed for dissolved oxygen concentration as well. The concentration profile of nitrate showed a higher rate of reduction with an increase in initial skin porosity value from smaller to significantly larger values. The present study clearly indicates the role of skin interface in depicting the solute concentration profile in fracture, especially during the washout of bio-clogged membrane (biofilm) attached to the rock matrix.

Fertilizers and nitrate pollution of surface and ground water: an increasingly pervasive global problem

Nitrate pollution of ground and surface water bodies all over the world is generally linked with continually increasing global fertilizer nitrogen (N) use. But after 1990, with more fertilizer N consumption in developing countries especially in East and South Asia than in the industrialized nations in North America and Europe, nitrate pollution of freshwaters is now increasingly becoming a pervasive global problem. In this review it has been attempted to review the research information generated during the last two decades from all over the world on different aspects of nitrate pollution of natural water bodies. It is now evident that not more than 50% of the fertilizer N is directly used by the crops to which it is applied. While a small portion may directly leach down and may reach ground and surface water bodies, a large proportion ends up in the soil organic N pool from where N is mineralized and is taken up by plants and/or lost via leaching during several decades. Present trends of nitrate pollution of freshwaters, therefore, reflect legacies of current and past applications of fertilizers and manures. Tools such as simulation models and the natural variation in the stable isotopes of N and oxygen are now being extensively used to study the contribution of fertilizers and other sources to nitrate enrichment of freshwaters. Impacts of agricultural stewardship measures are being assessed and nitrate enrichment of water bodies is being managed using modern digital models and frameworks. Improved water and fertilizer management in agroecosystems can reduce the contribution of fertilizers to nitrate pollution of water bodies but a host of factors determine the magnitude. Future research needs are also considered.

Climate change impacts the subsurface transport of atrazine and estrone originating from agricultural production activities

Climate change will impact soil properties such as soil moisture, organic carbon and temperature and changes in these properties will influence the sorption, biodegradation and leaching of trace organic contaminants to groundwater. In this study, we conducted a modeling case study to evaluate atrazine and estrone transport in the subsurface under current and future climate conditions at a field site in central Nebraska. According to the modeling results, in the future, enhanced evapotranspiration and increased average air temperature may cause drier soil conditions, which consequently reduces the biodegradation of atrazine and estrone in the water phase. On the other hand, greater transpiration rates lead to greater root solute uptake which may decrease the concentration of atrazine and estrone in the soil profile. Another consequence of future climate is that the infiltration and leaching rates for both atrazine and estrone may be lower under future climate scenarios. Reduced infiltration of trace organic compounds may indicate that lower trace organic concentrations in groundwater may occur under future climate scenarios.

Evaluation and simulation of spatial variability of soil property effects on deep percolation and nitrate leaching within a large-scale field in arid Northwest China

Variability of soil properties within large-scale fields not only exists in the horizontal domain, but also in the vertical direction, causing spatial variability in yield. Three yield zones were delineated based on measured yield in 2017 and 2018 within a large field in northwest China. The Soil Water Heat Carbon Nitrogen Simulator (WHCNS) model was calibrated and used to simulate yield, nitrogen uptake (N_u), water use efficiency (WUE), fertilizer N (nitrogen) use efficiency (FNUE), deep percolation (DP), nitrate leaching (NL) and residual nitrate (RN) at each sampling point in different yield zones. Based on the simulations, there were significant differences in N_u, WUE, FNUE, DP, NL and RN in 0-100 cm and 100-160 cm soil layers among the three yield zones. DP, NL and RN in the layers were strongly determined by the interaction of zone and year (p < 0.05), thus yielding consistent patterns mainly determined by soil properties and meteorological factors. The modelled ranges of DP, NL, and RN (0-160 cm) were 25-119 mm, 15-94 kg ha^-1, and 178-476 kg·ha^-1 respectively, across the field. Soil texture in the maize main root zone (0-100 cm) has a great influence on yield and N_u, and in the 100-160 cm layer upon DP and NL. RN was abundant after harvest and should be taken into account to determine the nitrogen fertilization demand for the following crop. The study showed that the process of delineating zones can be based on historical yield, making it feasibly easier than mapping soil properties. In view of the fact that there were large losses of water and nitrogen with uniform irrigation and fertilization management, the effects of vertically variable soil properties should be considered in future precision agriculture research, to achieve higher economic benefits and utilization efficiency.

Prediction of nitrate accumulation and leaching beneath groundwater irrigated corn fields in the Upper Platte basin under a future climate scenario

Understanding the impacts of future climate change on soil hydrological processes and solute transport is crucial to develop appropriate strategies to minimize the adverse impacts of agricultural activities on groundwater quality. To evaluate the direct effects of climate change on the transport and accumulation of nitrate-N, we developed an integrated modeling framework combining climatic change, nitrate-N infiltration in the unsaturated zone, and groundwater level fluctuations. The study was based on a center-pivot irrigated corn field at the Nebraska Management Systems Evaluation Area (MSEA) site. Future groundwater recharge (GR) and actual evapotranspiration (ET_a) rates were predicted via an inverse vadose zone modeling approach by using climatic data generated by the Weather Research and Forecasting (WRF) climate model under the RCP 8.5 scenario, which was downscaled from the global CCSM4 model to a resolution of 24 km by 24 km. A groundwater flow model was first calibrated on the basis of historical groundwater table measurements and then applied to predict the future groundwater table in 2057-2060. Finally, the predicted future GR rate, ET_a rate, and groundwater level, together with future precipitation data from the WRF climate model, were used in a three-dimensional (3D) model to predict nitrate-N concentrations in the subsurface (saturated and unsaturated parts) from 2057 to 2060. The future GR was predicted to decrease in the study area, as compared with the average GR data from the literature. Correspondingly, the groundwater level was predicted to decrease (30 to 60 cm) over the 5 years of simulation in the future. The nitrate-N mass in the simulation domain was predicted to increase but at a slower rate than in the past. Sensitivity analysis indicated that the accumulation of nitrate-N is sensitive to groundwater table elevation changes and irrigation rates.