Basic Example of Seawater Intrusion Modeling with MODFLOW, Model Muse and SWI2

Seawater intrusion is an issue in coastal aquifer management especially on arid environments. Overexplotation of groundwater resources by high production wells on intensive irrigation schemes could lead to an intrusion of saline water from sea and an impact to the quality of water sources.

Available options to assess the behaviour and impact of seawater intrusion are limited in both open source and commercial software; there is also a need of highly skilled groundwater modelers to understand the complex model setup and model output that can provide useful information about the groundwater flow regimen and the risks to water quality posed by seawater intrusion.

Seawater Intrusion Package 2 (SWI2) is a modeling software developed by the USGS and coupled on MODFLOW-2005. SWI2 allows three-dimensional variable-density groundwater flow and seawater intrusion in coastal aquifer systems. This tutorial deals with a basic example of the implementation of SWI2 on a MODFLOW model contructed on Model Muse. The tutorial show the whole procedure of model setup, datasets implementation, conceptualization of boundary conditions and result evaluation.

For further implementation of SWI2 on your own modeling work, it is strongly recommended to read the MODFLOW SWI2 documentation available on the:

• MODFLOW Online Guide

• Documentation of the seawater intrusion (SWI2) package for MODFLOW

Model conceptualization

The model involves two stress periods in steady-state conditions:

• 100 days of unstressed conditions divided in 100 time steps of 1 day

• 100 days of pumping extraction in 2 wells divided in 100 time steps of 1 day

Model thickness is around 60 meters and consist of 1 layer con 53 columns and 50 rows, the aquifer layer was conceptualized as convertible (free/confined). Each cell has a dimension of 10m x 10m. A visualization of the model model grid and dimensions can be seen below.

Two types of boundary conditions were selected for this tutorial:

• General Head Boundary (GHB): For the conceptualization of regional flow and sea level interaction.

• Well (WEL): For the simulation of groundwater extraction in wells.

A figure of the distribution of the GHB cells (dark blue) and WEL cells (green) is show below.

Model output

The distribution of the water table at the end of the second stress period can be seen below.

The following figure shows the seawater intrusion distribution on unstressed conditions, at the end of the first stress period.

The final stage of seawater intrusion and the interaction with WEL cells at the end of the second stress period can be seen in the following figure.

Tutorial

Model files

You can download the model files of this tutorial here.