V. 3 Issue 1 April, 2004
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SEVIEW 6.2 Capabilities
SEVIEW is the most advanced and comprehensive transport and fate modeling package available. SEVIEW includes the SESOIL, AT123D, SUMMERS and BIOSCREEN models. It now also includes a link between SESOIL and MODFLOW. The MODFLOW link is versatile enough that it can be easily used with any version of MODFLOW and any of the associated transport and fate models. The SESOIL, AT123D and SUMMERS models have been used by regulatory agencies across the country to develop baseline cleanup objectives. Using SEVIEW you can quickly develop site-specific cleanup objectives. The MODFLOW link now lets you incorporate advanced numerical modeling capabilities when developing risk-based cleanup objectives.

Our SESOIL - MODFLOW link incorporates both groundwater recharge and contaminant load. This means that variations in the water table in MODFLOW can be used to calibrate SESOIL recharge rates. In fact, SESOIL can now also be used to calibrate MODFLOW. This is because groundwater recharge rates are not necessarily positive or the same for each month. Seasonal variations and other factors such as depth to groundwater can have a significant influence on recharge rates. SESOIL incorporates these and other vadose zone processes. SEVIEW now allows MODFLOW users to take advantage of the advanced SESOIL capabilities.

1.2 Not a One Way Process
The interaction between groundwater and the unsaturated zone is not necessarily a simple one-way process. It is intuitively obvious that precipitation produces groundwater recharge. However, it may not be as obvious that moisture from the water table can also migrate upward through the vadose zone. This is because moisture in the vadose zone moves from wet areas to dry areas instead of simply flowing downgradient. Unfortunately many vadose zone models ignore this and assume a simple downward movement. Furthermore, recharge rates can vary from month to month due to variations in temperature and precipitation. The SESOIL Hydrologic Cycle sub-model was designed to simulate these processes. Now you can use SEVIEW to incorporate these processes when using MODFLOW.

1.3 AT123D vs. MODFLOW
Under most scenarios AT123D and MODFLOW produce comparable results. However, this is not true if groundwater recharge accounts for a significant portion of the overall groundwater flow. SESOIL generates both groundwater recharge and contaminant load to groundwater. However, AT123D only uses the contaminant load data. This link is based on the assumption that recharge accounts for only a small fraction of the overall groundwater flow. While true for most sites, this assumption can break down under certain climatic conditions and with low permeability aquifers. Under this scenario AT123D will produce groundwater concentrations even higher than the SESOIL leachate concentrations. Unlike AT123D, MODFLOW utilizes both groundwater recharge and contaminant load. Resulting in a more accurate estimation of the resulting groundwater concentration.

1.4 Advanced Capabilities
Although SESOIL produces monthly results MODFLOW does not need to be run using a uniform monthly stress period. This is because the SEVIEW link can combine any number of monthly SESOIL results together. The link even contains provisions for non-uniform stress periods in MODFLOW. Using MODFLOW it is now also possible to simultaneously simulate multiple SESOIL source loads.

1.5 Putting It All Together
In most cases soil cleanup objectives should not be developed based solely on SESOIL leachate concentrations. This is true even if the point of compliance is directly below the contaminated soil. This is because leachate entering groundwater will be diluted. There are several ways of determining a resulting groundwater concentration. Simple equations such as the dilution factor or SUMMERS model produce conservative results. The SESOIL - AT123D link offers a powerful easy to use method which can be used for most sites. However, AT123D results may also be too conservative in low permeability aquifers. That's where the SESOIL - MODFLOW link comes in.

You can learn more about the MODFLOW link on our www.seview.com webpage. You can contact us at (608) 240-9878 or at info@seview.com.

Not a One Way Process
Advanced Capabilities
Putting It All Together

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