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Vol. 10  No. 3

CONTAMINANT TRANSPORT CASE STUDY

&
THE RISK BUNDLE

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CONTAMINANT TRANSPORT CASE STUDY

A small source of Toluene and Benzene was found 10 m from the edge of an industrial property. The source concentration is 5 mg/L, and 1.5 mg/L for Toluene and Benzene, respectively. Previous hydrological studies have shown that the subsurface is predominantly sand, with a hydraulic conductivity of 3 x 10^-3 cm/s and a porosity of 0.38 (equivalent to effective porosity as well). The subsurface in the source zone is completely saturated, and the hydraulic gradient is 0.00002 towards the edge of the property. The owner has been monitoring Toluene (because it has the highest source concentration) at the edge of the property for five years and has found no traces of it in the water leaving the property. The owner would like to assume that the groundwater flow is too slow for contamination off property to be a concern in his/her lifetime, and would like you to do a few quick checks to confirm or deny this assumption given the information below:

• log K_oc (Toluene) = 2.69

• log K_oc (Benzene) = 2.13

• H (Henry’s Law – Toluene) = 0.0066

• H(Henry’s Law – Benzene) = 0.0055

• f_oc (fraction of organic carbon) = 0.004

• Dry bulk density = 2.65 g/cm³

• Dispersion = 1 x 10-7 m²/s

Questions to be answered:

1. How fast is the groundwater flowing?

2. Will the contaminants move as quickly as the groundwater?

3. Should the owner assume that these contaminants will not reach the edge of the property within the next 5 years?

4. Should the owner be concerned about these contaminants in any other phases?

1.  How fast is the groundwater flowing?

For this calculation, we will use the Groundwater Flow Calculator using the following data:

• Hydraulic gradient = 0.00002

• Hydraulic conductivity = 3 x 10^-3 cm/s = 3 x 10^-5 m/s

• Effective Porosity = 0.38

Therefore, the average groundwater velocity is 0.000000157895 m/s (will use 1.58 x 10^-7 m/s)

2.  Will the contaminants move as quickly as the groundwater?

For this calculation, we will first calculate the distribution coefficient using:

• f_oc = 0.004 = 0.4%

Toluene

• Log K_oc = 2.69

            Benzene

• Log K_oc = 2.13

             Therefore, the distribution coefficient for Toluene is 1.96, and for Benzene is 0.54.

Next, we will calculate the retardation coefficient for each contaminant using:

• Bulk density = 2.65 g/cm3

• Porosity = 0.38

Toluene

• K_d = 1.96

Benzene

• K_d = 0.54

Therefore, the retardation coefficients are 14.67 and 4.76 for Toluene and Benzene, respectively.

3.  Should the owner assume that these contaminants will not reach the edge of the property within the next 5 years? Could they have reached the edge of the property already?

For this calculation we will use the Ogata Banks Calculator for 1D contaminant transport. Concentrations at the edge of the property will be calculated for both 5 years and 10 years after initialization of the source zone; this corresponds to present time, and 5 years from now. The following input parameters will be used:

• Distance = 10 m

• Time of interest = 5 years (157680000 seconds) and 10 years (315360000 seconds)

• Groundwater velocity = 0.000000158 m/s

• Dispersion = 0.0000001 m²/s

Toluene

• Source concentration = 5 mg/L

• Retardation = 14.67

Benzene

• Source concentration = 1.5 mg/L

• Retardation = 4.76

Therefore, the concentration of Toluene at present is calculated to be 0.0 mg/L, and will be 0.0038 mg/L in 5 years. In addition, the concentration of Benzene at present is calculated to be 0.048 mg/L, and will be 0.83 mg/L in 5 years.

They should be monitoring Benzene despite its lower source concentration as the retardation coefficient is much lower, and is therefore more mobile. Benzene has most likely already reached the edge of the property, and within another 5 years Toluene will also begin to appear in these waters as well.

4.  Should the owner be concerned about these contaminants in any other phases?

Another concern may be the concentration within the soil at the source zone. The concentration in the soil at the source can be estimated using the Soil/Water Partitioning Calculator:

• f_oc = 0.004

• P_w = 0.38

• P_a = 0.00

• P_b = 2.65 g/cm3

Toluene

• C_w = 5 mg/L

• H_d = 0.0066

• K_oc = 10^2.69 = 490

Benzene

• C_w = 1.5 mg/L

• H_d = 0.0055

• K_oc = 10^2.13 = 135

Therefore, the estimated concentration of Toluene and Benzene in the soil around the source zone is 10.52 mg/kg and 1.02 mg/kg, respectively. It is evident that the sorbed contaminants on the soil may also cause some environmental risks. In addition, should the area become unsaturated, there is the possibility of volatilization into the atmosphere. There are several tools available to calculate the risk of these, and many other possibilities. Two of these tools are included in the Risk Bundle.

References

Domenico P.A. and Schwartz, F.W. (1998). Physical and Chemical Hydrogeology. Published by John Wiley & Sons, Inc. New York, NY.

Fetter, C.W. (1994). Applied Hydrogeology; Third Edition.  Published by Prentice-Hall, Inc., Englewood Cliffs, NJ.

Freeze, R.A. and Cherry, J.A. (1979). Groundwater. Published by Prentice-Hall, Inc., Upper Saddle River, NJ.

Sawyer, C.N., McCarty, P.L. and Parkin, G.F. (1994). Chemistry for Environmental Engineering; Fourth Edition. Published by McGraw-Hill Inc., NY.

 

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THE RISK BUNDLE

 

The GroundwaterSoftware.com risk bundle includes two risk based software packages; RBCA Toolkit for Chemical Releases and RISC.

RBCA Toolkit for Chemical Releases

The RBCA Tool Kit for Chemical Releases is a comprehensive modeling and risk characterization software package designed to meet the requirements of the ASTM Standard Guide for Risk-Based Corrective Action (E-2081) for Tier 1 and Tier 2 RBCA evaluations for chemical release sites in addition to traditional risk assessment calculations. RBCA Tool Kit for Chemical Releases combines contaminant transport models and risk assessment tools to calculate baseline risk levels and derive risk-based cleanup standards for a full array of soil, groundwater, surface water, and air exposure pathways. The ease-of-use features and streamlined graphical interface features of RBCA Tool Kit for Chemical Releases make it an essential tool for handling RBCA and generic risk assessment calculations for both simple and sophisticated problems.

RISC

RISC 4 is a software package for performing fate and transport modeling and human health risk assessments for contaminated sites. A unique feature of RISC 4 is its ability to perform a backward risk calculation as well as the conventional forward risk calculation. The backward risk calculation in RISC 4 refers to calculating a cleanup level for an input value of risk. Fate and transport models are available in RISC 4 to estimate receptor point concentrations in groundwater and indoor and outdoor air. No other RISK package offers all this! RISC 4 can be used to estimate the potential for adverse human health impacts (both carcinogenic and non-carcinogenic) from up to nine exposure pathways. Additional pathways and other non-human health impacts may be considered in future revisions of RISC

For more info or to purchase click here: RBCA Toolkit for Chemical Releases and/or RISC (both include RISK bundle purchase option)

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