NJDEP Guidance
The New Jersey Department of Environmental Protection (NJDEP) requires that a site-specific impact to ground water (IGW) soil remediation standard be developed for contaminated soil. NJDEP guidance describes four methods that can be used.

1. Soil-Water Partition Equation
2. Synthetic Precipitation Leaching Procedure
3. SESOIL Modeling
4. SESOIL/AT123D Modeling

There is one caveat, that sufficient site-specific data has been obtained on which to base the standard. But that so often the problem. The cost of characterizing the site could outweigh the benefit of reduced remedial action based on the higher IGW standard. You never know until after the detailed site investigation is complete. But, it would be a waste of funds to characterize a site only to find the site-specific IGW failed to accomplish much of anything.

Testing the Waters
At first this seems to be a catch-22 situation. But, you do not need to fully characterize the site to see if it is worth it. Estimated parameters can be used to determine if it is worth the cost of characterizing the site. The guidance only applies to submittals to the NJDEP. Remember, you are not developing IGW standards for submittal to the NJDEP, but to determine if any method looks promising. Preliminary modeling should be done early in the site investigation so the results can be used to assist in the design of sampling plans.

1. Soil-Water Partition Equation
I recommend starting with the Soil-Water Partition Equation as it requires the least site-specific data. All you need is a site-specific soil organic carbon content. A Dilution-Attenuation Factor (DAF) is used to simulate mixing within the aquifer.

The problem with this method is it does not simulate contaminant transport. In fact it assumes the contaminated soil is located beneath the water table. So not surprisingly, this is often not a very useful method. Still occasionally, it is sufficient to close a site. Most likely some of the site-specific IGW standards will be high enough that a few of the contaminants of concern will fall off.

2. SESOIL Modeling (SESOIL/AT123D Modeling)
At its core, SESOIL is based on the Soil-Water Partition Equation. But, unlike the Soil-Water Partition Equation, SESOIL simulates contamination migration through the vadose zone. Transport and fate processes include: volatilization, soil adsorption, and biodegradation. Simulation of these processes can significantly reduce the predicted groundwater concentration.

As with the Soil-Water Partition Equation it is important to use a measured soil organic carbon content. Other soil properties can be estimated using information obtained from the site investigation or based on regional trends.

3. Synthetic Precipitation Leaching Procedure
The Synthetic Precipitation Leaching Procedure (SPLP) method does not simulate contaminant transport. It assumes the contaminated soil sits directly above the water table. A Dilution-Attenuation Factor (DAF) is applied to simulate groundwater mixing. It is essentially a real world approximation of the Soil-Water Partition Equation.

The main difference is the use of a site-specific contaminant adsorption. This has several benefits. First because regulatory agencies use extremely conservative assumptions for contaminant adsorption. Second, because the SPLP method includes adsorption to soil no matter the process. This means adsorption is not solely based on organic carbon. Finally, because it includes residence time. The longer a contaminant is in contact with the soil, the more tightly it is bound to the soil. In other words, the longer the time between the release and the collection of the SPLP sample the higher the adsorption to soil is likely to be. The combination of these processes means adsorption values produced by the SPLP method can be significantly higher.

Still the SPLP method alone may be enough to reduce the leachate concentration. If it does not work the SPLP adsorption parameter can be calculated using the NJDEP SPLP spreadsheet. Adsorption values produced by the SPLP method tend to be significantly higher than the default values. Much like obtaining a site-specific soil organic carbon content. The calculated SPLP adsorption values should first be applied to the Soil-Water Partition Equation. Then SESOIL for any of the remaining contaminants.