Groundwater nitrate pollution risk assessment of the groundwater source field based on the integrated numerical simulations in the unsaturated zone and saturated aquifer
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Abstract:
Groundwater pollution risk assessment in the groundwater source field (GSF) is crucial to ensure groundwater quality safety. A systematic method of assessing groundwater pollution in the GSF was established by combining the numerical models of groundwater flow and solute transport in the vadose zone and aquifer. It is featured by revealing the paramount fate of contaminant from the surface to receptor “well (wells)” via the pathway of vadose zone and aquifers. The method was verified in the phreatic and semi-confined aquifers of a vital GSF, Beijing-Tianjin-Hebei region (BTHR) in China. Nitrate was selected as the model pollutant. The results indicated that the groundwater pollution risk of the phreatic aquifer was dominated by the mediate level (45.27%), and that the second semi-confined aquifer was mainly ranked as relatively low (30.29%) and mediate (38.17%) levels. The groundwater pollution risk maps of the two aquifers were similar. The high and relatively high risk areas were affected by the high intensities of groundwater pollution sources (GPSIs) or short distances from the pollution sources to the pumping well. The low and relatively low risk areas were controlled by low GPSIs and adequate attenuation and denitrification of nitrate in the aquifer. The groundwater pollution risk in the semi-confined aquifer was lower than that in the phreatic aquifer. The groundwater pollution risk mapping provides a valuable scientific reference for the groundwater pollution prevention and control with the focus on the “pollution source” and “groundwater source field”. The proposed method can be further applied to the protections of the GSFs in the BTHR.Keywords:
Phreatic
Groundwater Pollution
Groundwater model
MODFLOW
Abstract. The current generation of large-scale hydrological models does not include a groundwater flow component. Large-scale groundwater models, involving aquifers and basins of multiple countries, are still rare mainly due to a lack of hydro-geological data which are usually only available in developed countries. In this study, we propose a novel approach to construct large-scale groundwater models by using global datasets that are readily available. As the test-bed, we use the combined Rhine-Meuse basin that contains groundwater head data used to verify the model output. We start by building a distributed land surface model (30 arc-second resolution) to estimate groundwater recharge and river discharge. Subsequently, a MODFLOW transient groundwater model is built and forced by the recharge and surface water levels calculated by the land surface model. Results are promising despite the fact that we still use an offline procedure to couple the land surface and MODFLOW groundwater models (i.e. the simulations of both models are separately performed). The simulated river discharges compare well to the observations. Moreover, based on our sensitivity analysis, in which we run several groundwater model scenarios with various hydro-geological parameter settings, we observe that the model can reasonably well reproduce the observed groundwater head time series. However, we note that there are still some limitations in the current approach, specifically because the offline-coupling technique simplifies the dynamic feedbacks between surface water levels and groundwater heads, and between soil moisture states and groundwater heads. Also the current sensitivity analysis ignores the uncertainty of the land surface model output. Despite these limitations, we argue that the results of the current model show a promise for large-scale groundwater modeling practices, including for data-poor environments and at the global scale.
MODFLOW
Groundwater model
Hydrological modelling
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MODFLOW
Groundwater model
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The thirteen gypsum solution caves in north central Texas here described occur within the Permian Blaine Formation. Four are of vadose origin, four were developed by solution in the phreatic zone, and five were not studied in sufficient detail to determine their origin. Diagnostic features and histories of two caves, one representative of those having a vadose origin and one representative of a phreatic origin, are discussed.
Phreatic
Phreatic eruption
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Abstract The “HYDRUS package for MODFLOW” is an existing MODFLOW package that allows MODFLOW to simultaneously evaluate transient water flow in both unsaturated and saturated zones. The package is based on incorporating parts of the HYDRUS‐1D model (to simulate unsaturated water flow in the vadose zone) into MODFLOW (to simulate saturated groundwater flow). The coupled model is effective in addressing spatially variable saturated‐unsaturated hydrological processes at the regional scale. However, one of the major limitations of this coupled model is that it does not have the capability to simulate solute transport along with water flow and therefore, the model cannot be employed for evaluating groundwater contamination. In this work, a modified unsaturated flow and transport package (modified HYDRUS package for MODFLOW and MT3DMS) has been developed and linked to the three‐dimensional (3D) groundwater flow model MODFLOW and the 3D groundwater solute transport model MT3DMS. The new package can simulate, in addition to water flow in the vadose zone, also solute transport involving many biogeochemical processes and reactions, including first‐order degradation, volatilization, linear or nonlinear sorption, one‐site kinetic sorption, two‐site sorption, and two‐kinetic sites sorption. Due to complex interactions at the groundwater table, certain modifications of the pressure head (compared to the original coupling) and solute concentration profiles were incorporated into the modified HYDRUS package. The performance of the newly developed model is evaluated using HYDRUS (2D/3D), and the results indicate that the new model is effective in simulating the movement of water and contaminants in the saturated‐unsaturated flow domains.
MODFLOW
Groundwater model
Subsurface Flow
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The greater difference between day and night temperatures in arid and semi-arid areas influences water movement and heat transport in the vadose zone, and further influences the water and heat fluxes between the water table and the atmosphere. An evaporator and lysimeter, designed by the authors, and combined numerical simulation technology were used to study water movement and heat transport in the vadose zone, and the evaporation of phreatic water under the influence of surface temperature for different groundwater depths. The differences between water movement of the vadose zone and phreatic water evaporation calculated by isothermal and anisothermal models were also compared. The results of experiment and numerical simulations show that the surface temperature has a great influence on both water movement and heat transport in the vadose zone, as well as on evaporation intensity and the evaporation depth of phreatic water when the surface temperature is more than 25°C. The influential depth for the soil water content of vadose zone and the temperature of unsaturated and saturated zones is about 70 cm, but the greatest change is in the top 35 cm. The limited evaporation depth of phreatic water was about 70 cm for the experimental medium (silt/fine sand). The evaporation intensity of phreatic water was found to be maximum for a groundwater level of 20–40 cm (about 0·096 cm/h for silt/fine sand). The error of more than 8% was due to water movement of the vadose zone and the evaporation intensity of phreatic water calculated using an isothermal model. A coupled water and heat model was used to simulate water movement of the vadose zone and the exchange flux between the water table and atmosphere for surface temperatures higher than 25°C. For surface temperature below 25°C, the results of the isothermal and anisothermal models were coherent. There is thus no need to consider the influence of surface temperature on water movement of the vadose zone or the flux between the water table and the atmosphere.
Phreatic
Capillary fringe
Silt
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