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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.

Sorption is a necessary process influencing the subsurface solute or contaminant transport. There are many adsorption isotherms, and selecting one to represent the system is likely to remain an elusive goal. Also, the lack of observed data and its associated certainty may hinder the determination of sorption parameters. This study investigated the influence of different isotherms – linear adsorption assumption, Freundlich isotherm, Langmuir isotherm, and Redlich – Peterson isotherm, on chloride concentrations. The numerical model also gauged the extent of recharged waters migrating from the injection well. Temporal analysis of groundwater resident volumes helped to understand better the change in chloride concentrations with the selected isotherm. The residual solute concentrations from the isotherms followed the order: Redlich – Peterson < Freundlich < Langmuir. The results highlighted the importance of obtaining a range of sorption values rather than a single value from one fitted isotherm for the productive aid in groundwater decision making. [ABSTRACT FROM AUTHOR]

Capture zone analysis has been a prominent aspect in managing water supply wells. The study of aquifer vulnerability and protection also employs groundwater age modeling. Therefore, a comprehensive analysis of capture zones and groundwater age was used to propose a new method to understand the age fractions of water reaching pumping wells. The concept was derived from a volume-weighted approach for finding the mean age of a multi-component groundwater mixture. Numerical analysis of groundwater flow, age transport, and capture zone delineation was carried out to obtain volume-weighted mean groundwater age at the well. Thus, the groundwater age reaching a pumping well was easily assessed by computing age fractions. An additional recharge well rendered the groundwater age 25 days younger than that with a pumping well alone at 450 days. The example application described how the results could estimate the mean groundwater age at the sink point, contributed from the various sources. [ABSTRACT FROM AUTHOR]