Development of Ground-Water Models

Paul Hsieh

Ground-water models are computer programs used to simulate the flow of water in the subsurface. Today, ground-water models are routinely used to analyze a wide variety of environmental issues, such the impact of ground water pumping and the spread of contaminants leaked into the underground, as well as geologic issues such as the role of fluids in triggering earthquakes, landslides, and volcanic eruptions. Many ground water models in use today result from long-term development and refinement within the U.S. Geological Survey. Because these models are placed in the public domain and freely available via the Internet, they are widely used in industry, academia, and government.

USGS scientists at Menlo Park have made significant contributions to model development. Two pioneers in this field are John Bredehoeft and Jacob Rubin. Bredehoeft wrote computer programs to evaluate ground-water aquifers as early as the 1960s, while other scientists laboriously constructed analog models by wiring together resistors and capacitors on a large panel. Now as a private consultant, Bredehoeft continues to participate in scientific discussions on the application of ground-water models. Rubin, now retired from the USGS, devoted a major part of his career to developing models that can handle many dissolved chemicals in ground water and their mutual interactions. His "Feed Forward" method made it possible to solve complex equations of solute transport and chemical reactions that were previously impractical to solve even on a computer.

During the past 20 years, USGS scientists took advantage of the steady increase in computing power to develop models to simulate a broad range of increasingly more complex processes. Computer programs developed by Menlo Park scientists include SHARP, BIOMOC, RATEQ, HYDROTHERM, and Model Viewer. The SHARP model simulates the fresh-water/salt-water interface in coastal aquifers, where inland intrusion of salt water is always a risk from ground-water pumping. The BIOMOC model simulates how dissolved chemicals are transported by ground water and transformed by microbes--these processes strongly control the spread of pollutants at sites where contamination such as an oil spill has occurred. The recently developed RATEQ model enhances this analysis by incorporating more efficient computing algorithms and a more detailed description of how organic and inorganic chemicals react with surfaces of aquifer grains.

For geothermal application, the HYDROTHERM model was developed to simulate the flow of water and heat over a temperature range of 0 to 1,200 degrees Celsius and a pressure range of 0.5 to 10,000 bars. Over this temperature and pressure range, water may occur as liquid, steam, a mixture of liquid and steam, or in supercritical phase. The HYDROTHERM model enables simulating high temperature/pressure processes such as the intrusion of magma to the shallow subsurface in volcanic regions.

As models increase in complexity, it becomes more important to adequately visualize the model results. The Model Viewer software was developed to provide three-dimensional visualization of results from commonly used models such as MODFLOW, MT3DMS, and SUTRA. The user may rotate the model to view it from different directions, crop the model to examine the interior structure, and animate the time evolution of simulated quantities.

As computing power continues to increase in the 21st century, models will continue to grow in capability, complexity, and sophistication. Models will be larger, more detailed, more automated, and including more processes (for example, including both ground water and surface water in the same model). Model development will continue to be an important area of work in the UGSG.

These and other models can be found at http://water.usgs.gov/software/ground_water.html.

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