The present study was undertaken with a serious concern over vast environmental degradation being caused by the metal pollutants emanating from local tannery effluents. The chief tanning industries of Pakistan located in Kasur, Mian Channun, Peshawar and Multan were selected for the analysis of effluent, relevant groundwater and soil samples, all collected in triplicate, from thirty eight tanneries of Kasur, six tanneries of Mian Channun, ten tanneries of Multan and fifteen tanneries of Peshawar. The effluent samples were grab sampled from the main discharge outlets of the tanneries, while the soil samples were collected from peripheral distances of 50, 100 and 150m from the effluent discharge point to conduct a distance based metal distribution study. The water sampling was undertaken from relevant industries, and the background samples were collected from remotely located sites around the tanneries.
Flame atomic absorption spectrophotometry, coupled with automatic background compensation facility, was used for the estimation of Na, Ca, K, Mg, Fe, Mn, Cr, Co, Cd, Ni, Pb and Zn in the three media. The estimation of physico-chemical parameters such as pH, temperature, conductivity, alkalinity, chloride, sulphate and sulphide was also included in the domain of the study. The metal data obtained were subjected to univariate and multivariate statistical analysis pertaining to basic statistical parameters, linear correlations and regression analysis, while multivariate analysis was included for source identification through Principal Component Analysis (PCA) and Cluster Analysis (CA).
In the case of Kasur tanneries the following mean metal levels were recorded, respectively for effluent, groundwater and soil samples: Cr (391 mg/L, 2.12 mg/L and 16.7 mg/kg) and Na (25519 mg/L, 211 mg/L and 33683 mg/kg). Overall, the decreasing metal concentration order in various samples was: Na> Cr > Mg> Ca> K in effluents; Na> Ca> K > Mg > Cr in soil and Na > Ca> Mg > K> Fe > Cr in groundwater. A number of strong positive correlations were identified between the pairs of various metals in the three media. In groundwater, both Na and Mg were strongly correlated with Cr, at r = 0.553 and 0.535, respectively. The screening of effluent versus soil metal content relationship confirmed that chromium in effluent was strongly correlated with chromium in soil indicating the probable contamination of the soil in the area by the metals being flushed out in tannery effluents. The study revealed that the Cr, Pb and Fe levels were 21-42 fold higher than those recommended by international agencies as safe levels.
For Mian Channun tanneries, Na, Ca, K and Mg emerged as the dominant metals In effluent, ground water and soil samples. The effluents contained highest mean concentration for Na, at 627 mg/L, followed by 304 mg/L K, 247 mg/L Ca and 82.7 mg/L Mg. Since the Mian Channun tanneries have traditionally adopted the vegetable tanning process, low Cr concentrations (0.387 mg/L) were encountered in effluents. However, in the groundwater samples the levels of the metals Na, Ca, Mg and K were several times higher than safe permissible levels and background levels. The' estimated concentrations stood at 275 mg/L Na, 79.8 mg/L Ca, 51.4 mg/L Mg and 35.9 mg/L K. The decreasing concentration order for these metals in the Mian Channun tanneries soil samples was K > Na> Ca> Mg. Moreover, Cr and Fe showed enhanced concentrations in the relevant soil samples over those of the background samples. The noted high metal levels in groundwater and soil samples evidenced a significant buildup in the metal concentration arising from the interaction with effluents - a situation not different for Multan tanneries as well, where Na, Ca, K, Cr and Mg were present at dominant levels in the three media. The overall distribution of the metals in effluent from various industries was found to be non-Gaussian as suggested by high standard deviation values and enhanced skewness and kurtosis parameters marking random distribution.
The Peshawar tanning industry was a parallel case in that it exhibited again a divergent and non-normal distribution of metals in the three media, with Na at the highest mean concentration of 1277 mg/L, 882 mg/L, and 11347 mg/kg in the effluent, groundwater and soil samples, respectively. Among other metals Cr and Ca were notable: the former exhibited 51.7 mg/L, 0.089 mg/L and 12.3 mg/kg and the latter 103 mg/L, 43.9 mg/L and 814 mg/kg in effluent, groundwater and soil samples, respectively. For samples from this industry, some significant correlations were observed between effluent and soil samples in terms of Na, Cr, Ni, Co and Pb. The groundwater-soil interrelationship suggested that Na levels in the soil and groundwater were significantly correlated (r = 0.486, p< 0.001); similarly Cr in the soil was strongly correlated with Ca in groundwater at r = 0.486, at p< 0.01.
The overall scenario of trace metal distribution in effluents projected Cr, Fe, Co Ni, Pb and Zn as dominant metals contributing considerable adverse toxic effects on the environment in the vicinity of the tanneries investigated. The PCA and CA identified a heavy toxic metal burden in the effluents being discharged from the tanneries terminating into gross pollution of groundwater and soil by Cr and other metals at levels far exceeding the safe limits laid down for the safe discharge of tannery effluents by world health bodies. The study on distance based metal distribution arising from effluents also revealed that the soil adjacent to the tannery units was being badly contaminated by the effluents. The chromium pollution situation caused by Multan tanneries, the oldest tannery. cluster in Pakistan, was more severe than observed for other tanneries. The chromium pollution was meager in case of Mian Channun tanneries due to adoption of vegetable tanning process. Keeping in view the state of gross environmental pollution arising from this industrial sector, the present study presents deliberations on policy issues related to metal pollution control.