Methyl tert-butyl ether (MTBE) is a volatile organic compound (VOC) derived from natural gas that is added to gasoline either seasonally or year round in many parts of the United States to increase the octane level and to reduce carbon monoxide and ozone levels in the air. In 1993, production of MTBE ranked second among all organic chemicals manufactured in the United States. Currently, the U.S. Environmental Protection Agency (EPA) tentatively classifies MTBE as a possible human carcinogen. Health complaints related to MTBE in the air were first reported in Fairbanks, Alaska in November 1992 when about 200 residents reported problems such as headaches, dizziness, eye irritation, burning of the nose and throat, disorientation, and nausea. Similar health complaints have been registered in Anchorage
This paper discusses a method for overcoming the problem of weak sinks representing wells that result from spatial discretization effects when using MODPATH, the particle-tracking postprocessor for the ground water flow model MODFLOW. Weak sink cells are model cells that represent a well that does not discharge at a sufficiently large rate to capture all of the flow entering the cell; therefore, flowpaths within these cells cannot be uniquely defined because it is impossible to know whether a given water particle discharges to the well or passes through the cell. Creating a submodel of the well cell by using the nested rediscretization method can eliminate this ambiguity by converting the weak sink cell into a strong sink cell. The method is designed to be run manually for each well and for steady-state conditions. Other advantages, disadvantages, technical considerations, and limitations of the method are presented. Software created for the method consists of five Fortran programs that are operated using a set of instructions. A practical application of the method is presented by using an example wellhead-protection problem that demonstrates that nested rediscretization can provide more accurate particle-tracking results than those obtained by using a coarsely discretized model alone.
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