Simulations of freshwater lens recharge and salt/freshwater interfaces using the HYDRUS and SWI2 packages for MODFLOW

Abstract

Abstract:The paper presents an evaluation of the combined use of the HYDRUS and SWI2 packages for MODFLOW as a potential tool for modeling recharge in coastal aquifers subject to saltwater intrusion. The HYDRUS package for MODFLOW solves numerically the one-dimensional form of the Richards equation describing water flow in variablysaturated media. The code computes groundwater recharge to or capillary rise from the groundwater table while considering weather, vegetation, and soil hydraulic property data. The SWI2 package represents in a simplified way variable-density flow associated with saltwater intrusion in coastal aquifers. Combining these two packages within the MODFLOW framework provides a more accurate description of vadose zone processes in subsurface systems with shallow aquifers, which strongly depend upon infiltration. The two packages were applied to a two-dimensional problem of recharge of a freshwater lens in a sandy peninsula, which is a typical geomorphologic form along the Baltic and the North Sea coasts, among other places. Results highlighted the sensitivity of calculated recharge rates to the temporal resolution of weather data. Using daily values of precipitation and potential evapotranspiration produced average recharge rates more than 20% larger than those obtained with weekly or monthly averaged weather data, leading to different trends in the evolution of freshwater-saltwater interfaces. Root water uptake significantly influenced both the recharge rate and the position of the freshwater-saltwater interface. The results were less sensitive to changes in soil hydraulic parameters, which in our study were found to affect average yearly recharge rates by up to 13%.

Estimation of Conservative Contaminant Travel Time through Vadose Zone Based on Transient and Steady Flow Approaches

“…One advantage of numerical models of flow and transport in unsaturated zones is that both water and contaminant fluxes reaching the groundwater table can be estimated. Additionally, one dimensional model for unsaturated zone processes can be integrated with 3D models for flow and transport in the saturated zone, such as MODFLOW or MT3D, which is currently undertaken by the authors [18,38,39].…”

“…In this way, one obtains estimations of both water and solute fluxes at the water table on condition that detailed weather, soil and vegetation data are available as the model input. This approach is gaining increasing popularity; however, the results were shown to depend on many factors, including the time resolution of the weather data [16][17][18]. Recently, Vero et al [19] presented a comprehensive framework for estimating travel time based on simulations with the HYDRUS-1D computer code [20].…”

A comprehensive quasi-3-D model for regional-scale unsaturated–saturated water flow

“…8a in October 2014. Based on two sampling locations in October 2015 and 2016, the downward recharge is estimated according to the movement distance of the tracer peak and the average water content from the initial position of the tracer to the final position (Tan et al, 2014). The soil water content at the depth of 1 m is relatively stable according to the measurements and the results of Peng (2015), which ensures the reliability of the experiment.…”

Numerical Analysis of Recharge Rates and Contaminant Travel Time in Layered Unsaturated Soils

“…These two tasks are closely related, since they both require knowledge of water velocity in the vadose zone, which is generally variable in space and time. In order to achieve this goal, numerical models of unsaturated flow and transport are often used [1][2][3][4][5][6][7][8][9][10][11][12]. They can be considered as an approach alternative or complementary to other, more costly and time-consuming methods (e.g., lysimeters, tracer experiments).…”