2.1 Study Area Khulna is the third-largest metropolitan city of Bangladesh, stands on the banks of the Rupsha and the Bhairab rivers. Geographically, Khulna lies between 22°47´16´´to 22°52´0´´north latitude and 89°31´36´´ to 89°34´35´´ east longitude. At present, Khulna city has a population of about 1.5 million with an area of 47 square kilometers and 31 Wards. The MSW of Khulna city is dumped in the disposal site at Rajbandh, Khulna. Therefore, the open dumping site at old Rajbandh was selected as a case study to assess soil quality and human health risk and hence discussed in the following articles.
2.2 Sample Collection In this study, fifteen soil samples were collected from different selected locations of waste disposal sites. In this study, a GPS device has been used to record the latitude and departure of each sampling point. The soil samples were collected at a depth of around 0-30 cm from the existing ground surface within the disposal site.
2.3 Laboratory Investigations In this study, an acid digestion procedure has been performed on soil samples to convert into a liquid for measuring the concentrations of heavy metals in soil. To these attempts, at first 10 g of each soil sample was taken into a 100 ml conical flask washed with deionized water. Then 6 ml of the solution prepared with HNO3 /HClO4 acid and deionized water at a ratio of 2:1 was added and left overnight. Each sample was heated at a temperature of 150°C for about 90 minutes. Later on, the samples were heated at a temperature of 230°C for 30 minutes. Subsequently, HCl solution prepared with HCl and deionized water at a ratio of 1:1 was also added to the digested sample and re-digested again for another 30 minutes. Then the digested sample was washed into a 100 ml volumetric flask and obtained mixture was cooled down to room temperature. Then the concentration of heavy metals such as Arsenic (As), Cadmium (Cd), Cobalt (Co), Chromium (Cr), Copper (Cu), Iron (Fe), Mercury (Hg), Manganese (Mn), Nickel (Ni), Lead (Pb) and Zinc (Zn) of digested soil samples was measured in the laboratory with the help of Atomic Adsorption Spectrophotometer (AAS).
2.4 Risk Assessment Methods The human health risk assessment comprises problem identification (contaminated site), exposure assessment (exposure pathways) toxicity assessment (reference doses, slope factor), and risk assessment (cancer and noncancer risks) and hence discussed in the following articles.
2.4.1 Exposure Assessment: Exposure assessment for human health risk of waste disposal sites has become progressively more important due to the emission of toxicological heavy metals from contaminated soil. According to the US.EPA (1989) guidelines humans can be contaminated through three pathways including direct ingestion, dermal contact, and inhalation through the nose from contaminated soil. In this study, all three pathways were considered for soil samples. In addition, chronic daily intake (CDI) (mg/kg/day) in case of non-carcinogen risk for ingestion, dermal and inhalation of soil were computed using Equation 1, 2, and 3, respectively, was taken from exhibit 6- 18 in the Risk Assessment Guidance for Superfund. Volume I, Human Health Evaluation Manual (Part A): Interim Final (RAGS) (US.EPA, 1989).
2.4.2 Toxicity Assessment The risk is divided into two parts from a toxicity point of view: non-cancer risk and cancer risk. The chemical with high enough doses can cause non-cancer health effects. However, when the dose is sufficiently low, typically no adverse effect is observed. The reference dose (RfD) and carcinogenic slope factor (CSF) are considered for non-carcinogenic and carcinogenic risks, respectively, and were followed by RAGS (US.EPA, 1989). Therefore, the carcinogen and non-carcinogen thresholds are assigned from the historical database and numerous experiments. According to RAGS (USEPA, 1989), risk models (Eq. 4) for evaluating the non-cancer risk of soil were considered. Potential non-carcinogenic risks were assessed from each exposure pathway with the reference dose (RfD) in order to produce the hazard quotient (HQ),
2.4.3 Health Risk Assessment In the study, the potential health risk of inhabitants in the vicinity of the selected waste disposal site was assessed based on non-carcinogenic and carcinogenic risks and hence discussed in the following articles.
2.5 Uncertainty Analysis For assessing human health risk using exposure and risk models considering of variability and uncertainty is a very important parameter (Kilic and Aral, 2008; US.EPA, 1989). The basic aim of an MCS is to characterize, quantitatively the uncertainty and variability in estimates of exposure or risk model. An MCS analyses the model hundreds or thousands of times and each time the values were selected randomly. For exposure and risk models, distinguishing between variability and uncertainty is very important because it is directed affected the final outputs of risk values (Morgan and Henrion, 1990). In this study, the variability and uncertainty of risk values were analyzed using MCS through @RISK 7.5.× software with 10000 iterations.