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The Imperial chemical Industries Plc established in 1926 was Britains premier chemical manufacturing company. They manufacture products ranging from chemotherapy drugs for cancer patients to insulating materials and polymers which were used in Britain and in other parts of the world. For more than 50 years now, the chemical plant site at Runcorn owned by ICI has been landfilling its waste by products such as large amounts of highly toxic mercury, sodium, perchlorethylene, trichlorethylene, carbide and lime slurry, rubble, lime, chlorinated solvents, chemical catalysts, general industrial waste and thousands of unmarked steel drums whose contents are unknown (The Guardian, 2000). in the obsolete quarries in Weston; believing it was secure, they capped it when it was full and landscaped it.
RISK ASSESSMENT PROCESS
A risk assessment of Hexachloro- 1, 3-butadiene was conducted using CalTox to quantitatively predict its environmental concentration in the various compartments, to determine exposure pathways to receptor organisms and to assess it potential effects upon comparison with toxicity assessment information. The risk assessment process involves: Problem formulation, Exposure assessment, Toxicity Assessment and Risk characterisation.(figure 1)
HAZARD IDENTIFICATION AND EFFECT ASSESSMENT
Aspects of Weston village and problem
Weston village has a population of over 1000 people who have been living in a serene environment until they discovered the presence of volatile Hexachloro- 1, 3-butadiene (HCBD) in some of their homes, especially those close to the quarry sites. The presence of HCBD is believed to have occurred as a result of the presence of the landfill since this chemical isnt commercially produced within that locality and also not found naturally in the environment. A number of boreholes have been dug and this also revealed elevated levels of HCBD vapor concentrations. So far, 19 out of 104 properties in the street tested near the landfill have proved positive for HCBD, and more than 50 people have been evacuated to local hotels (The Guardian, 2000). Eventually, Only 21 of Weston's 467 houses are affected by the toxic gas (The Independent, 2000). HCBD is a known toxin that could concentrate in the kidney when ingested, causes reproductive problems, shortens lifespan and lowers fertility (The Guardian, 2000). Laboratory tests have shown that HCBD can cause kidney and liver cancer and affect foetal developments in rats while checks on some of the residents by the health authority showed that some were suffering from minor kidney abnormalities (BBC, 2001). The International Agency for Research on Cancer (IARC) has determined that Hexachloro- 1, 3-butadiene is not classifiable as to its carcinogenicity in humans, but indicated that there was limited evidence that Hexachloro- 1, 3-butadiene was carcinogenic in rats; While USEPA has determined that Hexachloro- 1, 3-butadiene is a possible human carcinogen.
Hexachloro- 1, 3-butadiene
Hexachloro- 1, 3-butadiene is a halogenated aliphatic compound, that is formed as a by- product during the processing of chlorinated aliphatic compounds (most likely tri- and tetrachloroethene and tetrachloromethane and carbon tetrachloride). It is used in the manufacture of rubber, as an agricultural fumigant, as an additive in lubricants and transformer oils, as a fluid for gyroscopes, a heat transfer liquid, and/or a hydraulic fluid (ATSDR, 1994)
Figure 1: The risk assessment process (Landcare research, 2007)
Physical-chemical Property Information (USEPA, 2003 and Chem Spider)
Name: Hexachloro- 1, 3-butadiene
Systemic Name: 1,1,2,3,4,4-Hexachloro-1,3-butadiene
Other names: Perchlorobutadiene, Dolen-Pur
Odor: Turpentine-like odor
Solubility: Not very soluble in water
ChemSpider 2D Image | HEXACHLOROBUTADIENE | C4Cl6 Figure2: Structure of HCBD (chemspider)
Chemical Formula: C4Cl6
Molecular Weight: 260.76
Average mass: 260.760803 Da
Monoisotopic mass: 257.81311 Da
Boiling Point (at 760 Pa): 215oC
Melting Point : -21oC
Vapor Pressure (at 25oC): 0.15 Pa
Density (at 20oC): 1.55 g/cm3
Water Solubility (at 20oC) 2- 2.55 mg/L
Organic Solvents: Ethanol, Ether
Partition Coefficients: Log Kow 4.78, Log Koc 3.67
Odor Threshold (air): 12.00 mg/m3
Odor Threshold (water): 0.006 mg/L
Vapour pressure (at 25o): 20 Pa
Atmospheric half-life: 356 days
Toxicity information for HCBD
Studies have been carried out on the toxicity of HCBD using various animals. Toxicity test carried on rats show that for non-carcinogenic effects the No Observed Adverse Effect level (NOAEL) is 0.2 mg/kg b.w per day (UK, DOH)
Oral Slope factor was already determined by the Integrated Risk Information System as 7.8 x 10-2 (mg/kg-day)-1 (USEPA, 1993)
Reference Dose (RfD) or Tolerable daily Intake (TDI) is an estimate of the amount of a substance in food or drinking-water, expressed on a body weight basis (mg/kg or µg/kg of body weight), that can be ingested daily over a lifetime without appreciable health risk (WHO, 1991)
TDI or RfD = NOAEL
Uncertainty Factor (1000)
The Uncertainty/Safety factor (1000) is used for humans to ensure that exposure concentrations never exceed NOAEL, since there may be uncertainties that may have been excluded during toxicity assessment.
TDI = 0.2
1000 = 0.0002mg/kg b.w per day
Caltox is an eight (8) compartment multimedia multipathway exposure risk assessment model that carries out stochastic simulations.
It pathways include:
root-zone soil and
CalTox is based on the principles of conservation of mass and chemical equilibrium (Asante-Duah, 1998). Caltox incorporates a fugacity level III model and stochastic risk assessment process that evaluates time dependent movement of contaminants in various environmental media. It calculates gains and losses by accounting for transport from one compartment into another and chemical biodegradation and transformation (Asante-Duah, 1998). This model was developed by the Californian EPA for assessing human risk through exposure to organic chemicals in the environmental media from contaminated landfill sites.
Caltox mathematically predicts theoretically the dose a person may take from the concentration of a chemical (HCBD) in the soil. It makes a distinction between environmental concentration of the chemical and the exposure concentration of a person. CalTOX was designed in an excel spread sheet to have both variability and uncertainty analyses incorporated by allowing the presentation of both risk and calculated clean up goals as probability distributions.
CalTox was used in the assessment of HCBD in Weston. Below are the required Inputs to run the model.
Physical -Chemical properties of HCBD (Appendix II)
Landscape properties for Weston quarry (water, plant and soil properties) (Appendix III)
Exposure factors for the Population (people living in Weston village)
Information on Toxicity assessments (Cancer slope factor) (Appendix I)
Time average concentration in on-site environmental media
Time average Population daily dose
Exposure media concentration
Model input data stated above were put into the model. The blank boxes were filled in the input section of the model; the 'input value columns were filled with data in the chemical properties, landscape properties and human exposure factors sections of the model. Then the 'exposure pathway-include-exclude toggle section of the model was switch on or off depending which pathway was relevant to the assessment. After required data on HCBD has been inputted into CalTox, predicted environmental concentrations for the various media and exposure pathways were given.
Time average Concentration in on-site environmental media
Table 1: Predicted Environmental Concentrations in the different media
MEDIA AND CORRESPONDING POTENTIAL DOSES IN mg/kg-d (averaged over the exposure duration)
Table 2: Potential doses in corresponding media
At this stage of the risk assessment process the risk was characterized using the information provided from both the exposure assessment and the toxicity assessment tests. Risk of HCBD was characterized both for carcinogenic and non-carcinogenic risk.
The carcinogenic risk probability was calculated using the low-dose linear model. The linearized multistage model is a model used to extrapolate carcinogenic responses observed at high doses to responses at low doses to humans. It is recommended by USEPA and it is highly conservative and assumes linearity at low doses.
P(d) = q * d
Where P(d) = Lifetime probability of developing cancer (µg/m3)
q = Slope potency factor (mg/kg.day)-1
d = Dose (mg/kg b.w per.day)
P(d) = 7.8 x 10-2 (mg/kg.day)-1 * 7.73x10-3 mg/kg b.w per day
Therefore P(d) = 6 x 10-4
P(d) = 6
10000 = 6 in 10,000
Therefore the probability of carcinogenic risk is 6 out of 10000 people. This is a very high value as compared to the standard value of 1 out of 1000,000.
From the model, it was deduced that the inhalation pathway is the most significant with respect to cancer risk because it has the highest percentage potential dose of 68.08%. The cancer probabilities for the key pathway (Air) was calculated
P(d) = Air concentration (µg/m3) x Inhalation unit risk(µg/m3)-1
Inhalation unit risk = 2.2 x 10-5 (µg/m3)-1 (USEPA:IRIS, 1993)
Air concentration = 5.9x10-3 (mg/m3)
Convert Air concentration unit value to µg/m3 = 5.9x10-3 (mg/m3) * 1000
Air concentration = 5.9 (µg/m3)
P(d) = 5.9 (µg/m3) * 2.2 x 10-5 (µg/m3)-1
P(d) = 13x 10-5
P(d) = 13
Therefore the probability of cancer risk through inhalation is 13 out of 100,000 people or 1 out of 10,000 people; which is a very high value as compared to the standard value of 1 out of 1000,000.
The Ingestion pathway is also important because it shows a percentage potential dose of 31.92%. But it was not calculated because the exposure pathway toggles were switched off. (See Appendix).
The dermal exposure pathway displayed very low chances of exposure through dermal uptake; with a percentage of 0.01%.
Pathways in order of significance
Non- carcinogenic probability
Non- carcinogenic risk probability was also assessed using the Hazard Index. The Hazard index is the ratio of the average daily dose of a chemical (HCBD) to the tolerable daily intake (TDI) of the chemical (HCBD).
Hazard Index = Exposure Dose (mg/kg b.w per day)
Tolerable Daily Intake (mg/kg b.w per day)
Tolerable daily intake = NOAEL
Uncertainty Factor (1000)
TDI = 0.2
1000 = 0.0002mg/kg b.w per day
Hazard Index = 7.73x10-3 mg/kg b.w per day
0.0002mg/kg b.w per day
Hazard Index = 38.65
The Hazard index is greater than 1, indicating that the probability of developing non-carcinogenic effects of HCBD is quite high.
DISCUSSION OF FINDINGS
It is worth noting that the toxicity assessment tests were carried out on only rats and mice and not a range of other organisms.
Verification of the model
The indoor/outdoor air concentration measured in Weston village was compared with the concentration given by the model.
Average concentration measured in Weston village = 6.8ppb
Model concentration = 5.89 x 10-3 mg/ m3
Convert model concentration to parts per billion
Parts per billion by volume (ppb) = mg/m3 x (23.61Lmol-1 / 260.7gmol-1) x 1000
5.89 x 10-3 mg/ m3 x (23.61Lmol-1 / 260.7gmol-1) x 1000 = 0.53ppb
Therefore model concentration is 0.53ppb while measure concentration is 6.8ppb. Measure concentration is higher.
Advantages of Caltox
Caltox is suitable model for this assessment because it presents an increased accuracy in the evaluation of human health risk from the hazardous waste sites and permitted facilities in Weston. It also incorporates an appropriate fate and transport model and a stochasitic risk assessment process. CalTox has also been able to provide a clear distinction between the risk assessment process of Westons landfill and risk management steps in remediation decisions. It is also able to tell us distinctly the various exposure pathway concentration of the chemical (hexachloro 1,3-butadiene) and its concentration in the environment. CalTox quantitatively addresses both uncertainty and variability by allowing the presentation of both risk and clean up goals as probability distribution (Asante-Duah, 1998).
Disadvantages of CalTox
The risks calculated are only for on-site receptors and for one organic chemical at a time.
Caltox is mostly applicable at sites where the primary source of contamination is the soil.
The model is limited to predicting long term exposures (several months to years) not short term. (Asante-Duah, 1998).
CalTOX is designed for modeling very low concentrations of contamination; when contaminant concentration exceeds the solubility limit in any phase, the results of the model are no longer valid. There is a warning in the spreadsheet model to advise the user when this happens. CalTOX should not be used as substitute for measured data, where it is available. (T.E. McKone, 1993)
Recommendations for the Management of HCBD risk in Weston
At the end of the risk assessment process, recommendations on the management of HCBD risk in Weston were suggested.
Residents living closest to the landfill with the more exposure to HCBD should be moved out of their houses in order to reduce their exposure to the chemical.
The company (ICI) should buy up the properties closest to the landfill in order to compensate the inhabitants of those buildings
The company (ICI) should also carry out medical treatment and checkup for all inhabitants of Weston village especially those with high indoor exposure because the Log Kow value for HCBD is 4.78 which is close to 5, therefore it would bio-accumulate.
Some Inhabitants of Weston village may refuse to leave their homes and relocate to another town; ICI should be able to supply them with HCBD free drinking water to reduce ingestion exposure. This is for inhabitants with little or no exposure in their homes and boreholes.
Since HCBD has been assessed and proven to be a Persistent organic pollutant, ICI could open up the landfill and seal it properly with an impermeable layer to prevent HCBD from leaking out of the landfill and persisting in the atmosphere.
HCBD is a possible human carcinogen as well as a non-carcinogen. The high values obtained from the risk assessment of HCBD with the use of CalTox indicate that a high level of concern should be placed on it. HCBD has also been assessed as a possible POP since it meets all the POP criteria (CCM, 2007). HCBD may need to be considered for ban to reduce exposure.