ASSESSMENT OF NAPL AND HEAVY METAL CO-CONTAMINATION IN SOIL-WATER SYSTEM

Abstract

Soil-water pollution is among the major environmental problems faced across the globe today. Contaminants in the soil-water system mainly exist in mixtures, and this co-existence of different classes of pollutants seems more harmful and complex to manage than their individual presence. Heavy metals and non-aqueous phase liquids (NAPLs) are two commonly found contaminant classes in most of the problematic soil-water sites having distinct behaviour. Managing soil-water resources co-contaminated with these pollutants gets complicated by the fact that, although the two components are treated differently, they can mutually affect each other’s fate and transport processes. Accordingly, effective management of NAPLs and heavy metals polluted resources requires investigating the challenges of co-contamination. Further, managing these resources require accurate monitoring of contaminants before designing the restoration measures. Physico-chemical monitoring alone may not provide complete information as it cannot characterize the associated impact of target contaminants on the ecosystem at different spatial and temporal resolutions, leading to inadequate screening of polluted sites. Thus, appropriate monitoring techniques are needed for accurate detection, prevention, and remediation of target pollutants and their co-contaminants. Current pollution monitoring practices in many geographical locations are primarily based on conventional physico–chemical assessments, which do not always reflect the potential toxicity of contaminant cocktails. On the other hand, biomonitoring provides a range of sensitive techniques to characterise the eco–toxicological effects of chemical contamination. This study highlights the need to integrate biomonitoring tools alongside physico–chemical monitoring techniques to provide holistic information on the ecological impairment of contaminated soil-water systems. A bio-integrated monitoring framework is proposed here to identify toxicity drivers by utilising ‘effect–based’ and ‘risk–based’ monitoring approaches. This framework will help address the current challenges and address knowledge gaps about existing environmental resource quality and ecosystem degradation. The proposed framework is first used to investigate an industrial site polluted by heavy metals before studying the chromium (Cr) and NAPLs (mineral oil or MO and toluene) co-contaminated system through a series of practical experiments and numerical analysis. Bio-integrated monitoring of heavy metals is first carried out at the Baddi-Barotiwala-Nalagarh (BBN), an industrially polluted site in Himachal Pradesh, India. The study characterizes the major heavy metals present in the soil-water system and investigates their bioaccumulation potential in four native macrophyte species, namely 1) Nasturtium officinal R. Br., 2) Canna Indica, 3) Ageratum Conyzoides, and 4) Ranunculus Sceleratus. The level of the twelve most toxic heavy metals (Cr, Fe, Co, Ni, Cu, Zn, As, Mo, Se, Cd, Hg, Pb) in water and within native plant biomass (roots, stems, and leaves) is measured and used for estimating bioconcentration and translocation factors, BCF and TF, respectively. Results show that all macrophytes hyperaccumulate (i.e., BCF > 1000) Fe, Se, Cd, and three of the four species hyperaccumulate Cu and Zn. Nine (Fe, Co, Ni, Cu, Zn, Mo, Se, Cd, Pb) of the twelve heavy metals are hyperaccumulated by Ranunculus Sceleratus, highlighting its potential for phytoextraction in the study area. High concentration in plant biomass and comparatively lower concentration in the root zone water for heavy metals like Co, Se, and Cd is detected either due to past pollution episodes or high bioaccumulation within the plant organs. Since physico-chemical assessments alone cannot derive information on historical pollution events, biomonitoring is imperative to acquire additional knowledge and therefore, is crucial in managing polluted sites effectively. Thereafter, a series of hydroponic experiments are conducted to analyse the biomonitoring potential of Eichhornia crassipes macrophyte for hexavalent chromium, Cr (VI), subjected to different concentrations 0, 10, 20, 40, 60, 80, 100 mg L-1. Changes in the Cr (VI) concentration is measured in the growth media and in different plant tissues, i.e., roots, stems, and leaves. Bioaccumulation and toxicity monitoring are carried out by examining the uptake and translocation of Cr (VI) within plant tissues and through morphological changes in the macrophyte. A similar set of experiments is then conducted in the co-presence of Cr (VI) and MO (representative light NAPL) for their concentration level of 5 and 25 mg L-1. Bioaccumulation of Cr (VI) in Eichhornia crassipes is compared with and without the presence of MO. Similarly, morphological changes in the selected macrophyte are studied and compared for different concentrations of both contaminants. The results show significant bioaccumulation of Cr (VI) in roots, stems, and leaves of Eichhornia crassipes with noticeable indications of morphological damages (stagnant growth and decrease in the number of leaves) at greater exposure concentration (> 40 mg L-1) of the heavy metal. The results also demonstrate the restricted uptake and accumulation of Cr (VI) by Eichhornia in the presence of MO in a dose-dependent manner. The morphology of the macrophyte is adversely affected due to co-contamination with MO causing more prominent effects than Cr (VI) alone.Finally, the impact of toluene, another light NAPL, is investigated on the fate and transport of Cr (VI) in a continuous soil-water system using a one-dimensional vertical column setup having dimensions of 20 cm length and 4.5 cm diameter. Preliminary microcosm experiments are conducted before upscaling the study to this column setup. For the microcosm study, Cr (VI) removal efficiency by a popular adsorbent, nano zerovalent iron (nZVI), is analysed under varying Cr (VI) concentrations (25, 50, 75, 100, 150, 200 mg L-1) and pH (3, 5, 7, 9). The experiments are also performed to study the impact of toluene on Cr (VI) fate under the application of nZVI adsorbent. For the 1-D column experiment, the column is filled with fine (0.5 – 1.0 mm) and coarse (1.0 – 2.0 mm) sand layers at the top and bottom of the adsorbent packed in the middle. Cr (VI) solution of 10 mg L-1 is allowed to flow continuously at a constant rate of 50 mL/hr for 21 days. The results show that ~ 100 % Cr (VI) is removed by the adsorbent during the first 80 hours of the experiment. Thereafter, the outlet Cr (VI) concentration starts rising and reaches a maximum level (9.9 mg L-1) at equilibrium. When toluene is co-present in equal concentration in the inlet water, Cr (VI) concentration in the outflow starts increasing quickly as compared to the earlier case. This shows a noticeable shift in the fate and transport processes, particularly in the adsorption of Cr (VI), taking place in the soil-water system due to NAPL co-contamination. The results of the column study are then numerically analysed using Hydrus 1-D inverse modelling for deducing the adsorption isotherms. To sum up, this thesis highlights some of the key issues associated with managing heavy metal and NAPL co-contaminated soil-water resources. It is found that inadequate monitoring using conventional physico-chemical techniques generates partial/biased data set for complex contaminant mixtures, and thus, reduces the efficiency of site restoration measures. The overall results of this research decipher the need to upgrade the current strategies of polluted site assessment by focussing on biomonitoring and detailed co-contamination studies. The proposed bio-integrated monitoring framework along with the co-contamination studies is capable of investigating the pollutants impact on the surrounding ecosystem and can significantly help society in combatting the issue of soil-water contamination.