Deepak Kumar & Pankaj Gupta


Groundwater resources are one of the biggest sources of freshwater on earth. The percent contribution of groundwater among all freshwater sources is approximately 30 per cent. In contrast, the contribution of surface water is merely 1 per cent. All around the world, groundwater is the main source of irrigation water supply for agriculture practices and for various industrial and domestic uses. So, even a layman can understand the importance of groundwater resources both from the point of view of quality as well as quantity. A nation will stand healthy if groundwater resources are free of contamination. But, if contamination exceeds permissible limits, serious health issues can slowly and gradually capture the affected regions in the long run. Due to various industrial and domestic disposal by anthropogenic activities, the contamination of metals, heavy metals, organic as well as inorganic pollutants may have percolated in groundwater. Also, to satisfy the increasing demand for water at the domestic and industrial levels, quantification of groundwater needs to be done at various administrative scales.

Now, the question arises, how to do it? How can one analyse the amount of groundwater present in an aquifer? What is the depth of groundwater? What is the drawdown at spatial and temporal scales when pumping of water from various locations is done? How much recharge of groundwater is possible for a given rainfall condition? What contamination is present in groundwater? How to remediate it? What is the spatial and temporal distribution of these contaminations? How do various point and nonpoint sources of surface water contamination affect groundwater pollution? The questions may be many more. The answers to these questions may be one word only. It may be done using groundwater modelling.

When water infiltrates from the surface of earth to the soil-water system beneath the surface of the earth, it passes through the vadose zone and it may further reach to the aquifer systems. The movement of water in the vadose zone can take place in three dimensions and the process is very complex. The movement of soil water may result in the recharging of groundwater systems. Groundwater modelling softwares can be used to predict the recharge of groundwater, to study movement of soil water in vadose zone, to understand movement of soil water and solute transport. Groundwater models may be used to know the temporal and spatial variations of the hydraulic heads. These models are also used to calculate water as well as contamination transport from ponds to aquifers, or industrial sites to groundwater, or agricultural, domestic, or landfill leachate to aquifer systems. Groundwater models may also be used to understand surface-groundwater interactions. But it is highly difficult to accurately predict the actual natural phenomena due to its complex nature. Although soil water movement is a complex phenomenon, groundwater modelling provides vast information with output close to real scenarios. Groundwater modelling softwares is developed by various groups of brilliant researchers and has been proven to be helpful in understanding the soil-water movement beneath the surface of the earth. A brief enlisting and overview of software used for groundwater modelling has been discussed in the following paragraph.

Groundwater models may be physical or mathematical. Physical models are generally laboratory-based and the outcome of physical models depends upon the parameters considered for modelling at the lab scale. For an example of physical models, let us suppose, a scientist wants to know the adsorption of chromium by activated charcoal mixed in a sand aquifer system, certainly, he has to take degradation data from the lab conditions. So, physical models are usually developed at a small scale and the results obtained from these models can’t be generalised at a large scale.

For a large-scale study of aquifer systems, mathematical models are generally used. Mathematical models might be analytical or numerical. Analytical models are an oversimplified form of the true phenomenon. For example, analytical models can be developed for understanding the drawdown from an aquifer system taking aquifer parameters constant everywhere. If the variability of various aquifer systems needs to be incorporated in groundwater modelling, then numerical models should be used.
Numerical models provide more clear pictures of actual natural phenomena. Numerical models in advanced form can be developed as Graphical user interface (GUI), so that end users can develop better concepts. Thus, these types of models may be called conceptual models.

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Groundwater models may be categorised as commercial or open- source software. Most of the open source software may be downloaded from the United States Geological Survey website (https://on.doi. gov/3jPVS8I). Some of the examples of open software used in groundwater modelling are initial versions of the Modular three-dimensional finite- difference ground-water flow model (MODFLOW, MODFLOW-2005), Modular 3-Dimensional Transport model (MT3D-USGS), SEAWAT (SEAWAT program was developed to simulate three-dimensional, variable- density, transient ground-water flow in porous media. The source code for SEAWAT was developed by combining MODFLOW and MT3DMS into a single programme that solves the coupled flow and solute-transport equations), TracerLPM, RRAWFLOW (R code has been developed for groundwater flow modelling), HYDROTHERM, SUTRA, Integrated Water Flow Model (IWFM), etc.

Examples of commercial softwares are Visual MODFLOW (VMOD), Groundwater Vistas, Model MUSE, Waterloo HYDROGEOLOGIC, Groundwater Modelling System (GMS), etc. Visual MODFLOW has a graphical interface. It was developed by Waterloo Hydrogeologic. It was the first commercially available GUI for MODFLOW and was released in 1994. Since then various updates on VMOD have come and users can further update themselves through weblink (https://www. waterloohydrogeologic.com/). Groundwater Vistas has been developed by Environmental Simulations, Inc. (ESI) located in the USA. More updates on it can be found at (https://on.doi.gov/3jPVS8I). Model MUSE has a graphical user interface and it is developed by USGS. More information on model MUSE can be obtained from (https://on.doi. gov/3K70thw) Waterloo HYDROGEOLOGIC also provides hydrogeologic software solutions. – GMS is a product of AQUAVEO, which offers state-of-the-art software for civil engineering works.

The end users should not use groundwater models blindly. Before using these models, it is important to know the exact purpose or application. Since different models have different levels of accuracy and the demand for data type also varies, thus end users should also perform ground truthing.

Views expressed by: Deepak Kumar, Department of Soil & Water Conservation Engineering, GBPUA&T, Pantnagar & Pankaj Gupta, Center for Rural Development and Technology, IIT Delhi

 

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