CE 397 Environmental Risk Assessment

Homework #5 Solution

Lesley Hay Wilson



The problems for this homework are based on the case study site 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.

Chemical properties data from the TNRCC Table Appendix VII were used in developing this homework solution. The TNRCC document also includes a table of toxicological parameters, Table Appendix VI and it was used to collect the necessary toxicology data. Input values are as stated in the problem statement and identified in the Excel file prepared for the solution. The excel file can be downloaded here hmwk5sol.xls.

 1.a. The groundwater seepage velocity for the case study site was calculated.

 The hydraulic gradient was estimated based on the change in groundwater elevation between MW 39 and MW 3. The distance is 626 feet and the groundwater elevation change is 3.13 feet.

I = 3.13/626 = 0.005 ft/ft

Vgw = K*I/n

Vgw = 5.17 ft/yr


b. The velocities for the movement of benzene, chlorobenzene, naphthalene and trichloroethylene in groundwater were calculated.

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 was calculated in the solution using equation 16.5.19 in Chapter 16, Groundwater Contaminant Transport. R is the retardation factor for each COC. The following equations were used.


Kd = Koc*foc

 rb = rs(1-n)

 R = 1 + (rb*Kd)/n

Vcoc = Vgw/R

 Based on the R values presented in the attached Excel sheet, benzene travels fastest and naphthalene travels slowest.

 c. 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 were calculated. 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.

Travel time is calculated as:

t= distance/velocity

Groundwater travel time for the given distance is 87 years.


COC Travel Time (yr)









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


The appropriate PCLs (or if you prefer source area SSTL) for each of the COC were calculated using the TNRCC calculation method and formula for the natural attenuation factor (NAF). The assumptions in the problem statement were used.




 PCL = RBEL *1/(equation LT-1a)


COC PCL Concentration (mg/L)
Benzene 35
Chlorobenzene 5506
Naphthalene 11294
Trichloroethylene 35


 All of the concentrations at MW-39 are above the calculated PCL values. The solution however assumes that a steady state concentration has been reached at the point of exposure (this is inherent in the derivation of the NAF formula). Given the travel times calculated for each of the COC, reaching a steady state does not appear likely, so a time-dependent model that accounts for biodegradation should be used to evaluate the groundwater concentrations and derive the PCLs.

2.a. The vapor phase concentrations were calculated using the MW-5 groundwater concentrations and the Henry's law values given in the TNRCC Table VII.

 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

 The vapor concentrations calculated in part a above, were compared to the RBEL and both the benzene and naphthalene values were above the indoor air RBEL values, so the remainder of the transport calculations were completed for both chemicals.

The TNRCC NAF algorithms and effective diffusion coefficient equations were used to calculate the PCLs in groundwater. All of the default values were taken from the problem statement and the TNRCC manual page 6-56.


 PCL = RBEL *1/(Equation CM-6:VFwesp)


COC PCL Concentration (in groundwater) (mg/l)
Benzene 26.7
Naphthalene 3e+5


The current measured concentrations in groundwater at MW 5 are above the calculated PCL value for benzene and below the PCL for naphthalene.


3. Soil ingestion RBELs (or RBSLs, if you prefer) for naphthalene and trichloroethylene for a construction worker and for a child resident were calculated. Exposure factors from the BP RBDP guidance document (Table A3.1) were used for the construction worker and exposure factors from the TNRCC guidance (Table 6.6) were used for the child. Toxicity values from the TNRCC Table VI were used. The RBSL algorithms presented in the toxicology lecture were used.

Since trichloroethylene exhibits both non-carcinogenic behavior and carcinogenic behavior, PCL values were calculated for both toxicity values. Naphthalene only exhibits non-carcinogenic behavior, so only one values is derived. It is interesting to note that the non-carcinogenic value for trichoroethylene actually results in the lower RBEL value (one might assume that the cancer effects would represent the more sensitive exposure).


COC RBEL-carcinogen (mg/kg) RBEL non-carcinogen (mg/kg)
Naphthalene na 3276
Trichloroethylene 869 491
Construction Worker    
Naphthalene na 81760
Trichloroethylene 5203 12264



 4. 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 the residential groundwater ingestion PCLs (these are at a source area so the PCL = RBEL) were calculated using child exposure factors. The explicit consideration of cumulative effects was then included using the TNRCC ratio method. The Excel sheet includes two different procedures for deriving the adjusted PCL. The first is based on  making adjustments for the COC with the highest PCL first and then reducing the other values. This is assuming that if remedial action is required then, the concentrations of the higher PCL COC would be reduced in the efforts to meet the more restrictive PCLs. Also the PCLs for the chlorinated compounds and MTBE were reduced the least since in general these are the more difficult and costly to remediate. The second method is based on the original relative proportions of the COC. The levels at source area #2, represented by MW 39, were used to calculate relative proportions in the source area of each COC. The adjustment fractions for the PCL were based on these original proportions. A third method, not shown in the solution, is to ratio the PCL evenly, by the number of COC involved. In this case each PCL would be multiplied by 1/9 or 0.11. Depending on the site, each of these methods has its strengths and weaknesses. If a remedial action system is necessary the first method may be most appropriate, if it is unclear what actions will be taken at a site the even ratios may be acceptable. If only the COC with the highest concentrations will warrant remedial action, then the second method may be most appropriate.