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

Contamination of groundwater from nitrogen fertilizers in agricultural lands is an important environmental and water quality management issue. It is well recognized that in agriculturally intensive areas, fertilizers and pesticides may leach through the vadose zone and eventually reach groundwater. While numerical models are commonly used to simulate fate and transport of agricultural contaminants, few models have considered a controlled field work to investigate the influence of soil heterogeneity and groundwater flow on nitrate-N distribution in both root zone and deep vadose zone. In this work, a numerical model was developed to simulate nitrate-N transport and transformation beneath a center pivot-irrigated corn field on Nebraska Management System Evaluation area over a three-year period. The model was based on a realistic three-dimensional sediment lithology, as well as carefully controlled irrigation and fertilizer application plans. In parallel, a homogeneous soil domain, containing the major sediment type of the site (i.e. sandy loam), was developed to conduct the same water flow and nitrate-N leaching simulations. Simulated nitrate-N concentrations were compared with the monitored nitrate-N concentrations in 10 multilevel sampling wells over a three-year period. Although soil heterogeneity was mainly observed from top soil to 3m below the surface, heterogeneity controlled the spatial distribution of nitrate-N concentration. Soil heterogeneity, however, has minimal impact on the total mass of nitrate-N in the domain. In the deeper saturated zone, short-term variations of nitrate-N concentration correlated with the groundwater level fluctuations

Modeling Fate and Transport of Contaminants in the Vadose Zone: Vapor Intrusion and Nitrate-N Leaching Under Future Climate Scenarios

Understanding and predicting water flow and contaminant transport through the vadose zone is one of the major challenges in the field of hydrology. Lack of experimental data and theoretical understanding about the vadose zone has prevented accurate calculations of water flow and solute transport in this zone and reduced the efficiency of water management practices. Throughout the past few decades, conceptual and mathematical modeling approaches have been applied as useful tools to describe the complex process of flow and transport influenced by physical, chemical, and microbiological interactions within the vadose zone. In this work, three-dimensional numerical models were developed to simulate water flow and solute transport in variably saturated porous media and were integrated with well-controlled field measurements to validate their performance. This dissertation has two major parts. In the first part, the goal was to develop a 3-D numerical model to quantify vapor intrusion pathways into a slab-on-ground building under different pressure and ventilation site conditions. Mechanisms controlling vapor intrusion pathways were identified through comparisons between modeled and measured indoor air concentration, subsurface contaminant and oxygen distribution profiles, and diffusive and advective fluxes. The dependency on oxygen concentration in the biodegradation modeling was found to be very important to describe the biodegradation of volatile hydrocarbons. In the second part, effects of climate change on the fate and transport of nitrate-N beneath a center pivot-irrigated corn field were evaluated. This study utilized a rich historic data set collected from 1993 to 1996 to develop a 3-D numerical model based on realistic sediment lithology to simulate water flow and nitrate transport in the variably saturated porous media. Using this model, future groundwater nitrate-N concentration was predicted from 2057 to 2060 using future climate data. Future groundwater recharge was predicted to decrease at the study area compared to the average historical groundwater recharge data. Nitrate-N leaching was predicted to decrease under the future climate scenario due to increasing evapotranspiration and decreasing mineralization rates

SIMULATION AND PREDICTION OF THE GROUNDWATER LEVEL IN THE SURROUNDED AREA OF THE NEBRASKA MANAGEMENT SYSTEM EVALUATION AREA SITE (MSEA).

An efficient water budget is necessary to develop sustainable practices in irrigated lands and determine future trends. The groundwater level (GWL) can rise or fall depending on the time of the year. When winter ends, and spring begins, accumulated snow starts to melt, and rainfall starts to fall. Therefore, water infiltrates and raise GWL. This research predicts the groundwater table from 2056 to 2060 in the surrounding area of the MSEA. Visual MODFLOW Flex was used to simulate the real groundwater-level and forecast the future GWL. Future predictions show that the GWL will increases in a non-irrigated season (winter season) and decreases in a irrigated season. Nevertheless, the decreasing rate is higher than the recharge rate and is approximately 1.02 feet

Pore-Scale Investigation of Nanoparticle Transport in Saturated Porous Media Using Laser Scanning Cytometry

Knowledge of nanoparticle transport and retention mechanisms is essential for both the risk assessment and environmental application of engineered nanomaterials. Laser scanning cytometry, an emerging technology, was used for the first time to investigate the transport of fluorescent nanoparticles in a microfluidic flow cell packed with glass beads. The laser scanning cytometer (LSC) was able to provide the spatial distribution of 64 nm fluorescent nanoparticles attached in a domain of 12 mm long and 5 mm wide. After 40 pV of injection at a lower ionic strength condition (3 mM NaCl, pH 7.0), fewer fluorescent nanoparticles were attached to the center of the flow cell, where the pore-scale velocity is relatively higher. After a longer injection period (300 PV), more were attached to the center of the flow cell, and particles were attached to both the upstream and downstream sides of a glass bead. Nanoparticles attached under a higher ionic strength condition (100 mM NaCl, pH 7.0) were found to be mobilized when flushed with DI water. The mobilized particles were later reattached to some favorable sites. The attachment efficiency factor was found to reduce with an increase in flow velocity. However, torque analysis based on the secondary energy minimum could not explain the observed hydrodynamic effect on the attachment efficiency factor

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

Contamination of groundwater from nitrogen fertilizers in agricultural lands is an important environmental and water quality management issue. It is well recognized that in agriculturally intensive areas, fertilizers and pesticides may leach through the vadose zone and eventually reach groundwater. While numerical models are commonly used to simulate fate and transport of agricultural contaminants, few models have considered a controlled field work to investigate the influence of soil heterogeneity and groundwater flow on nitrate-N distribution in both root zone and deep vadose zone. In this work, a numerical model was developed to simulate nitrate-N transport and transformation beneath a center pivot-irrigated corn field on Nebraska Management System Evaluation area over a three-year period. The model was based on a realistic three-dimensional sediment lithology, as well as carefully controlled irrigation and fertilizer application plans. In parallel, a homogeneous soil domain, containing the major sediment type of the site (i.e. sandy loam), was developed to conduct the same water flow and nitrate-N leaching simulations. Simulated nitrate-N concentrations were compared with the monitored nitrate-N concentrations in 10 multilevel sampling wells over a three-year period. Although soil heterogeneity was mainly observed from top soil to 3m below the surface, heterogeneity controlled the spatial distribution of nitrate-N concentration. Soil heterogeneity, however, has minimal impact on the total mass of nitrate-N in the domain. In the deeper saturated zone, short-term variations of nitrate-N concentration correlated with the groundwater level fluctuations

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