One of the most important steps in the planning of a bioremediation, chemical oxidation, or carbon adsorption treatment is the estimation of contaminant mass present. Skipping this step is the most common cause of these treatments failing to accomplish their cleanup goals. Calculation of the contaminant mass present should be included in all site assessments and taken into consideration in feasibility studies.
There are many methods to determine the contaminant mass present, the most accurate for organic chemicals being calculated from soil borings. This provides the mass present in the vadose, smear zone, or saturated soil which accounts for, in the case of petroleum, up to 99% of the contaminant mass present. Multiply the concentration in soil by the soil volume and it will be the most accurate mass estimate available.
Groundwater can also be used to infer soil contamination using the equation Csoil = Cgw * Koc * Foc. This is often the only method available in the years following the initial assessment that included soil borings. There are several sources of error that are introduced when using this method. First, for pollutants like gasoline or diesel, there are over 100 constituents that all adsorb to the organic fraction of soil differently and vary from blend to blend in the percent composition of the fuel. There is no “standard” Koc for gasoline or diesel further complicating the calculation. Also, during the initial assessment, fraction of organic content in soil must be analyzed or else the Foc portion of the calculation is a guess. These samples would ideally be taken from the smear zone from an uncontaminated boring at a petroleum impacted site.
Bioremediation, chemical oxidation, and carbon adsorption treatment are not chemical specific to the regulated contaminants!
Another potential source of error is the lack of sampling for the entire organic mass present, or in the case of fuels, testing for TPH, TPH-G, or TPH-D simply because the regulators do not require it. This is very important for calculating the mass estimate and ultimately the product dosing. Bioremediation, chemical oxidation, and carbon adsorption treatment are not chemical specific to the regulated contaminants! Many states only require sampling for BTEX and a few other contaminants that make up 5-20% of gasoline and diesel. If these chemicals are the only petroleum constituents quantified a lot of errors are introduced into the mass estimate. ETEC recommends sampling TPH in both soil and groundwater at all petroleum projects to obtain the highest accuracy in determining the mass estimate.
Contaminant mass estimates should be the starting point for any treatment selection process, especially bioremediation, chemical oxidation, or carbon adsorption. We will explore product dosing and its importance in subsequent blog posts.