Modelling for pollutant migration in the tannery belt, Dindigul, Tamil Nadu, India N. C. Mondal* and V. S. Singh
National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, India
Groundwater in and around Dindigul town in Tamil Nadu, India, is polluted due to discharge of untreated effluents from 80 functional tanneries. Total dissolved solids (TDS) in about 100 km2 area are observed ranging from 2000 to 30,573 mg/l in open dug wells. A mass transport model was constructed to study pollutant migration. The study area covering 240 km2 was chosen to construct the groundwater flow model in the weathered part of unconfined aquifer system. The shallow groundwater potential field computed through the flow model was then used as input to the mass transport model. MT3D computer code was used to simulate mass transport in groundwater system. The mass transport model was calibrated with field observations. The available database was, however, quite sparse. Notwithstanding, efforts were made to arrive at reasonable guesstimates of the characteristic parameters. Sensitivity analysis, an integral part of calibration was carried out whereby model parameters, viz. transmissivity, dispersivity, etc. were altered slightly and the effect on calibration statistics was observed. This study clearly indicates that transmissivity plays a more sensitive role than dispersivity, indicating that the migration phenomenon is mainly through advection rather than dispersion. The study also indicated that even if the pollutant sources were reduced to 50% of the present level, TDS concentration level in the groundwater, even after 20 years, would not be reduced below 50% of present level. Keywords: Dindigul, groundwater pollution, pollutant migration, tannery industry, weather zone. THE study area, a granitic rock formation in Dindigul district, Tamil Nadu, India possesses poor groundwater potential. Serious contamination of both surface water and groundwater has been reported in this area as a result of uncontrolled discharge of untreated effluents by 80 tanneries for the last three decades1–4. The health of the rural farming community and people working in the tanning industries has been seriously affected. They suffer from occupational diseases such as asthma, chromium ulcers and skin diseases5. About 100 km2 area of fertile land has lost its fertility. Total dissolved solids (TDS) concentration6 in groundwater at some pockets varies from 17, 024 to 30, 575 mg/l. As the discharge of effluents continues, a prognosis of fur-
ther pollutant migration is carried out using a mathematical model. A numerical model of the area was developed using the finite difference technique coupled with method of characteristics and it was also used to predict TDS migration for the next 20 years. Sensitivity analysis was carried out to identify the parameters which influence contaminant migration. Sensitivity analysis shows that advection and not dispersion is the predominant mode of solute migration. There are a large number of reports and papers available to describe the solute transport models to study contaminant migration in the industrial belts, coastal aquifer, etc. (C. P. Gupta et al., unpublished)7–14. The computer software MOC developed by Konikow and Bredehoeft10 based on finite difference coupled with the modified method of characteristics is used for the present study. The area is a hard-rock, drought-prone region situated in Dindigul District, Tamil Nadu, India (Figure 1) and it lies between 10°13′44″–10°26′47″N lat. and 77°53′08″– 78°01′24″E long. It is spread over an area of about 240 km2 and is characterized by undulating topography with hills located in the southern parts, sloping towards north and northwest15. The highest elevation (altitude) in the hilly area (Sirumalai Hill) is of the order of 1350 m amsl, whereas in the plains it ranges from 360 m amsl in the southern portions to 240 m amsl in the northern...