In this study, contaminant transport behaviour in the aquifer system (140 m × 180 m × 5 m) was analyzed using a 3-D groundwater flow and contaminant transport model viz. MODFLOW2005 and MT3DMS. The impacts of hydrodynamic dispersion parameters on the conservative contaminant plume dynamics were analyzed for homogeneous and heterogeneous aquifer systems with low permeability porous media (LPPM). The spatio-temporal distribution of contaminant concentration and breakthrough curves (BTCs) at 12 observation wells were used to analyze the transport dynamics due to conservative contaminant released from a single point source over a hypothetical study area for a period of 1 year (365 days). Results from MODPATH show a significant variation in the pathway of groundwater for homogeneous and heterogeneous aquifer systems. During the source loading period, a very low value of concentration of order 10-9 mg/m3 was observed in the LPPM region. The spatial distribution of contaminant plume for aquifer system with LPPM varied largely as compared to homogeneous aquifer system. The maximum value of concentration in the aquifer with LPPM was found to be ~40% higher than the homogeneous system after source removal. After the source removal, the maximum value of 1.98 mg/m3 was observed for the homogeneous system at a location away from pumping and extraction well after 730 days; however, for a heterogeneous system with LPPM, the maximum value of 2.57 mg/m3 was observed. An early breakthrough was observed for αL= 54 m as compared to αL= 9 m for homogeneous aquifer system, clearly depicting the effect of longitudinal dispersivity on BTC. However, effects of dispersivity on the rising and falling limbs of the BTC were negligible for heterogeneous aquifer system with LPPM. Further, an impact of LPPM and longitudinal dispersivity on the peak concentration value at observation well (OBS-7) was undistinguishable. The numerical simulations carried out in this study mimic the realistic heterogeneous aquifer conditions and highlighted the relevance of LPPM and associated transport processes on contaminant transport dynamics at field-scale, which was usually overlooked.

The human health risk assessment (HHRA) of groundwater system in the vicinity of Chandigarh dumping site was conducted, assuming oral ingestion and dermal contact exposure scenarios. Observed data of lead (Pb) concentration in the leachate was used to compute cancer risk (CR) by integrating unsaturated 1-D leaching model with probabilistic HHRA framework. The 99 percentile and maximum value of lead (Pb) concentration at the water table was estimated as 0.089 mg/L and 0.506 mg/L, respectively, for pre-monsoon season, higher than the safe limit of 0.050 mg/L. In contrast, for the post-monsoon season, only the maximum value of Pb concentration exceeded the safe limit. Results from 10,000 Monte Carlo simulations showed that the 99 percentile and maximum value of CR for all the sub-populations during pre-monsoon exceeded the safe limit (>10 ) via oral ingestion exposure to Pb-contaminated groundwater. The 95 percentile value of CR for adult sub-population was estimated as 1.05 x 10 for premonsoon; however, for the post-monsoon season, only maximum values of CR exceeded the safe limit. The cancer risk estimates for the pre-monsoon and post-monsoon seasons via skin dermal contact exposure were found to be lower than the safe level, posing no danger to human health. Among sub-populations, the order of posing CR was found to be in the order as adults (>18 years) > child I (1-5 years) > teen (11-18 years) > child II (6-10 years). Uncertainty analysis showed that the lead concentration (>95% variance contribution), as a major contributor towards uncertainty in the risk estimates, while event duration (t ), exposure duration (ED), and ingestion rate (IR) were observed as minor contributors. The approach presented in this study considered the uncertainty in the unsaturated leaching model parameters along with uncertainty in the exposure model parameters, thus can help decision-makers in estimating risk from open dumping sites with minimal data availability.