CE 397 Environmental Risk Assessment

Subsurface Modeling and Risk Based Exposure Level Calculations

Homework #5



The intent of this homework is for you to do an environmental risk assessment of a hypothetical site based on a case study described at: http://www.ce.utexas.edu/stu/haywilli/term/progress.html The site is a chemical manufacturing facility where there have been releases in the past. The property is to be divided into an industrial area that will continue to operate and an area for redevelopment. The homework has four parts. The first two deal with determining acceptable concentrations in groundwater using pathways of ingestion of groundwater as a drinking water source, and by inhalation of vapors produced by volatilization of the groundwater through soil into a house. Parts 3 and 4 deal with determining risk based exposure limit concentrations for individual chemicals and pathways, and then considering the cumulative risk of several chemicals acting together.

Chemical properties data are available in the TNRCC Table Appendix VII (the one that Dr. Corsi provided) and in the BP RBDP Guidance Document Table A2.1b. The TNRCC document also includes a table of toxicological parameters, Table Appendix VI, the BP document includes toxicological data for selected chemicals in Table A-2.1c. Other toxicity data sources are included in the course homepage web sites list, (e.g., IRIS database, Envirofacts Master Chemicals Database). Where input parameters are not given in the problem statement and are not given in the case study database, you should use the TNRCC default values. In your solution, please state the source of all input values.

1. Risk of Ingestion of Contaminated Groundwater

1.a. Calculate the groundwater seepage velocity for the case study site.

The total soil porosity in the saturated zone is given for the case study as n = 0.40. As a first assumption this total porosity can be used for the effective porosity for groundwater flow. The saturated hydraulic conductivity is given as K = 4 x 10-4 cm/sec. The hydraulic gradient, I, can be determined from the attached groundwater flow map. The seepage velocity in groundwater is given by:

Vgw = K*I/n

b. Calculate the velocity for the movement of benzene, chlorobenzene, naphthalene and trichloroethylene in groundwater. Assume that the soil particle density rs = 2.65 kg/m3. Which COC moves fastest? Which moves slowest?

 For this calculation, the distribution coefficient, Kd (which is Kp in Corsi's notes), bulk density, rband effective porosity, n, are needed. The fraction organic carbon value given in the case study is foc = 0.01. The organic carbon partition coefficient, Koc should be calculated using the methods presented in Dr. Corsi's notes or Chapter 16, p. 16.23 in the Handbook of Hydrology chapter on Groundwater Contaminant Transport. R is the retardation factor for each COC. The following equations can be used.

Kd = Koc*foc

 rb = rs(1-n)

 R = 1 + (rb*Kd)/n

Vcoc = Vgw/R

 c. Estimate the travel time for groundwater and for benzene, chlorobenzene, naphthalene and trichloroethylene from source area #2 to the groundwater monitoring well located at the point of demonstration #3. For this problem the point of demonstration given in the case study is the same as the point of exposure as described in the TNRCC guidance. The distance is given in the case study database document is 450 ft.

d. The steady state long term concentration in groundwater at point of demonstration #3 must meet the following RBEL (or if you prefer, RBSL) for groundwater ingestion.

COC RBEL Concentration (mg/L)
Benzene 5
Chlorobenzene 780
Naphthalene 1600
Trichloroethylene 5

What are the appropriate PCL (or if you prefer source area SSTL) for each of the COC? Are the current measured concentrations in groundwater in the vicinity of MW-39, given in the attached table, above the calculated PCL values?

Use the TNRCC NAF algorithms, or others if you prefer. Please document your calculation method.

The following equations can be used.



 PCL = RBEL *1/(equation LT-1a).

Here DAF means Dilution Attenuation Factor which is the same thing as NAF or Natural Attenuation Factor.

The following values may be assumed:

There is no first order decay of the COC, i.e., l = 0 for all COC.

Groundwater source term width Sw = 300 ft (based on the case study)

Groundwater source term depth Sd = 10 ft

Based on EPA's land disposal regulations the following relationships can be used to estimate the groundwater dispersivities:

Longitudinal dispersivity ax = 0.1* (distance to the point of exposure) m

Transverse dispersivity ay = 0.33ax (m)

Vertical dispersivity az = 0.05ax (m)

2. Risk of Inhalation of Vapors from Groundwater Through Soil in a Building.

Looking at the indoor air inhalation exposure pathway, suppose a house is planned to be constructed on at the case study site in the vicinity of MW 5. This location is away from soil sources and is in the proposed new residential area.

a. Calculate the vapor phase concentration of benzene and naphthalene just above the water table in the vicinity of MW 5 using Henry's law. The groundwater concentration data are given in the attached table.

 b. The steady state long term concentration in indoor air at the house must meet the following RBEL (or if you prefer, RBSL) for residential indoor air inhalation:

COC RBEL Concentration (mg/m3)
Benzene 3.2e-01
Naphthalene 290

Using the TNRCC NAF algorithms, or others if you prefer, please document your calculation method, what are the appropriate PCL (or if you prefer source area SSTL) for each of the COC in groundwater in the vicinity of MW 5? Are the current measured concentrations in groundwater at MW 5 above the calculated PCL values?

 First compare the vapor concentrations calculated in part a above, if the vapor concentrations immediately above the water table are lower than the vapor RBEL, then the current concentrations in groundwater are acceptable and the rest of the calculation is not necessary.


PCL = RBEL *1/(Equation CM-6:VFwesp)

The following values can be assumed:

For building parameters use default values given in the TNRCC manual page 6-56.

For effective diffusivity values use equations presented on page 6-55.

Diffusion coefficients in air and water given in TNRCC Table VII

Lgw = 8 ft = 244 cm (this depth is from the bottom of the basement to the top of the groundwater table, with an 8 foot basement and a depth to groundwater of 16 feet, the Lgw = 8 ft)

hcap = 5 cm

hv = 239 cm

qT = 0.48 (for the purpose of this problem, we are assuming that the soil in the vadose zone is homogeneous, even though two soil types have been identified in the case study, we have chosen this value to be the representative soil porosity for the vadose zone soils)

qacrack = 0.25

qwcrack = qT - qacrack

qacap = 0.044

qwcap = qT - qacap

qas = 0.25

qws = qT - qas

3. Computation of Risk Based Exposure Limits

Calculate soil ingestion RBELs (or RBSLs, if you prefer) for naphthalene and trichloroethylene for a construction worker involved in the redevelopment of the case study site and for a child resident. Exposure factors from the BP RBDP guidance document (Table A3.1) should be used for the construction worker and exposure factors from the TNRCC guidance (Table 6.6) for the child. Toxicity values from the TNRCC Table VI should be used.

 4. Cumulative Risk

At the case study site ten COC have been detected in groundwater. Nine of the chemicals (excluding benzene) have reference dose values for quantifying non-carcinogenic effects. First calculate the residential groundwater ingestion PCLs (these would be at a source area so the PCL = RBEL) assuming that the groundwater well is used for drinking water only. Use the TNRCC tables for input values. If for each COC the HQ is set equal to 1 for the PCL, then we are not explicitly accounting for cumulative effects. (Implicitly in the conservative values being used for the calculations we are providing a factor of safety that address the potential cumulative effects, see TNRCC guidance Appendix I Section 3). If we assume that the PCL adjustment method presented by TNRCC, see section 6.3.2 , is appropriate, but because of the sensitivity of the case study site the regulatory agency wants to see the HI = 1, not 10 as TNRCC specifies (this is often the case), how would you go about implementing the adjustment method? What would your adjusted PCL values be?

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