It is often fascinating to sit back and wonder how we got into what we are doing currently. It might seem a very futile exercise in the beginning. But, when you start connecting the dots, it can be quite an interesting story; in fact, an important one that shapes the future direction of one’s life’s trajectory. Cleaning up the environment works on a similar philosophy. It is important to be aware of the legacy of the environmental contaminants one is dealing with, know where their source lies and understand their movement patterns to be able to formulate methods of eradicating them.
I left home for the first time in 2012 to attend the Miranda House college at the University of Delhi, India where I did my Bachelor’s in Chemistry. It was the first time I was making my own decisions independently. Chemistry was not my first choice at the time and for a very long time towards the end of high school I felt a certain pull towards pursuing Physics. The decision was influenced by the fact that I could secure in my XII standard board exams, the university-set cut-off grades, only in Chemistry, to get admission in the college of my choice. My grades in Physics did not make the high cut-offs. With passing years, as I got into research, I have realized that in research you need a diverse range of knowledge that can cross the boundaries of a single discipline. It is often helpful to have diverse interests and there is no reason to believe that you cannot change your path to pursue something different from where you started your journey. In the third year of my Bachelor’s, we were taught Inorganic Chemistry by Dr. Bani Roy.
Inorganic Chemistry has a certain whim. There are rules and there are exceptions. Most often it was the exceptions to the rules that stumped me in the exams. One of the examples that I distinctly remember was calculating the ionization enthalpy of atoms. Ionization enthalpy (IE) which is defined as the amount of energy required to remove an electron from an atom has the general trend of decreasing as we move down within a group (a column in the periodic table). This is a result of the increasing distance between outermost electrons and nucleus with the addition of extra shells. But the exception to this rule is when there is poor shielding offered by d-orbitals which is why, IE of Gallium (Ga) is more than that of Aluminum (Al). I never scored high in the Inorganic Chemistry exams. But I found it interesting.
I liked how Dr. Roy made the quirks of Inorganic Chemistry look less overwhelming for her students by explaining the justifications for the exceptions to the rules, instead of blankly stating them. Nevertheless, it did get overwhelming towards the end of the semester when we had to remember all the rules and exceptions for the exams. But in my opinion, that was a problem of procrastination more at our end as students.
It was towards the end of this class that we learnt about the arsenic (As) heavy metal pollution in the groundwater wells of the Indo-Gangetic plains of West Bengal and Bangladesh. Part of the reason why it made a special impact on me could be because I learnt it from Dr. Roy, a Bengali herself just like me. Arsenic is a heavy metal pollutant that is responsible for the highest risks of mortality worldwide because of its toxicity and the number of people exposed. [1] Arsenic pollution contaminates the soil impacting a staple source of the diet of the Indo-Gangetic region, rice and groundwater that is a major source of water for drinking and irrigation purposes in the region.
The long-term toxic effect of inorganic arsenic (iAs) has been known since the nineteenth century and was classified as a carcinogen by the International Agency for Research on Cancer (IARC) in 1980[2]. The Indo-Gangetic alluvium is the largest alluvial plain in the world and studies concluded that the source mineral responsible for the releasing of arsenic in the groundwater of the area is arsenolite (As2O3)[3]. An agrarian economy like India with agriculture-related sectors representing half the labor market[4], has 14% of its rice production coming from the state of West Bengal[5]. The wet-dry cycle of this area quickens the movement of As throughout the environment. Rice is grown in water-logged paddy fields. Arsenic exists in the pentavalent (As V) oxidation state under aerobic (i.e., with oxygen) conditions and in the trivalent state (As III) under anaerobic (i.e., oxygen-less) conditions. In the monsoon season when paddy fields are flooded, the waterlogged conditions near the roots of the paddy lead to an anaerobic condition, converting pentavalent As, otherwise present near the soil surface exposed to the air, to its more toxic trivalent state. Trivalent As is easily soluble in water and is readily taken up by the plant roots and accumulated in the paddy. Further contamination of the shallow groundwater is brought about by the infiltration of arsenic into deeper soil layers during monsoon flooding.[6] It was estimated that in 2012 about 39 million people in Bangladesh alone were still exposed to arsenic concentrations above WHO provisional guideline value of 10 μg/L.[7] While the plight of the people from my home state suffering from the adverse effects of the contamination deeply impacted me, I was fascinated by the complexity of the chemistry of heavy metals and their pathways to get into the human diet. I felt the need to explore this more.
I started my master’s in environmental sciences at the Nalanda University, India. I was exposed to the opportunity of choosing from a diverse range of electives from Geohydrology and Nanoparticles to Environmental Sociology. The freedom to choose a future direction of work helped me explore and find my specific interest in quantitative research. In the summer of 2016, second year of my masters, I was offered an internship at the Department of Earth Sciences in the Indian Institute of Science, Education and Research (IISER) Kolkata.
I worked with Dr. Niharika Anand, a PhD student at the time working with Dr. Sujata Ray, to review articles to examine the trends in reported organochlorine pesticide (OCP) levels in human milk. Regulated by the Stockholm Convention on Persistent Organic Pollutants, India has banned the use of organochlorines such as DDT in agriculture but continues restricted use for controlling the spread of malaria. Human milk is a sensitive measure of the general exposure of a population. Dr. Anand was comparing the contamination status of OCPs, pyrethroids and the neonicotinoids, between a semi-urban location with both agricultural and built-up areas (Nadia) and an urban location (Kolkata) in West Bengal, India.[8] Unlike heavy metals like As, OCPs are often hydrophobic (dislikes water) and lipophilic (loves fats and other hydrocarbons). The lipophilic nature of these molecules is what makes them dangerous for aquatic organisms and humans. The affinity to the lipids in animals makes OCPs hard to eliminate, leading to its accumulation after prolonged exposure, termed bioaccumulation. The bioaccumulation potential of a hydrophobic chemical like organochlorine pesticides poses a risk to the ecological and human health. A large portion of the human diet consists of freshwater and estuarine fish in the state of West Bengal in India and Bangladesh and has been attributed as one of the bioaccumulation pathways for these organochlorine pesticides by several studies.[9] [10] [11] [12] The fundamental processes that guide the transport of heavy metals and pesticides into different environmental substrates and finally into organisms, are interestingly different from one another. While pesticides are hydrophobic and tend to bind with anything but water, heavy metals can readily form an aqueous solution to be available for bio-uptake. The differences in the exposure pathways for different chemicals can be key to determining targeted cleanup (technically called remediation) methodologies.
I continued working with hydrophobic organic contaminants (HOCs) in my doctoral research at the University of Maryland Baltimore County, focused on Polychlorinated Biphenyls (PCBs). PCBs were domestically manufactured between 1929 and 1979 and used extensively in the industry for their non-flammability, chemical stability, and high boiling point which made them perfect for electrical insulation. Their use was banned in the year 1979 by the Toxic Substances Control Act (TSCA) and the USEPA established by the late 1990s that PCBs are probable human carcinogens.[13] Because of their stable chemical structures, PCBs persist in the environment several decades after their ban, interacting with the different environmental substrates to cause adverse ecological and human health impacts. At UMBC, I am working to understand the existing methods of sampling environmental levels of PCBs to improve ways of monitoring them more efficiently.
Efficiently monitoring contaminant levels in the environment is critical to decision making for setting reasonable remediation goals and performing targeted remediation to reach those set goals. Remedial efforts are incredibly extensive and ridiculously expensive, and sometimes they don’t work. An interesting case study in this aspect is the story of the cleanup of PCB contamination in the Hudson River, USA. In 1984, 200 miles of the 315-mile-long river ended up being the country’s one of the largest superfund sites and placed on EPA’s National Priorities List. The river that served as a major transportation route during the Industrial Revolution, was contaminated with approximately a million pounds of PCBs that were discharged from General Electric (GE) capacitor manufacturing plants located in New York. In February 2002, the EPA issued a Record of Decision (ROD) for the Hudson River PCBs Superfund Site that called for targeted environmental dredging of approximately 2.65 million cubic yards of contaminated sediments[14]. Removing contaminated sediments by dredging can often lead to re-suspension and partial re-dissolution of the PCBs in the water column. The first five-year review that came out in 2017 reported elevated levels of PCBs in surface sediments with re-contamination in certain dredged areas and higher than target concentrations of the 2002 ROD in fish tissue. [15]
The one take-away that I have gathered over the years of being associated with the world of environmental contaminants is that the story of toxins is a complex one. The lack of understanding of the fundamental scientific processes that drive these environmental contaminants through the environment is costly. Improving the ways to predict and measure the outcome of contaminant pollution and remediation is crucial to moving towards a cleaner, more functional world.
References:
[1] C. Hopenhayn, Arsenic in drinking water: impact on human health, Elements 2 (2006) 103–107. [2] International Agency for Research on Cancer (IARC), Monographs on the evaluation of carcinogenic risks to humans. Lyon, France 84, 526 (2004) [3] Goel, P. (2018). Identification of the Source Mineral Releasing Arsenic in the Groundwater of the Indo-Gangetic Plain, India. In: Hussain, C. (eds) Handbook of Environmental Materials Management. Springer, Cham. https://doi.org/10.1007/978-3-319-58538-3_129-1 [4] https://www.trade.gov/country-commercial-guides/india-food-and-agriculture-value-chain [5] https://ipad.fas.usda.gov/countrysummary/Default.aspx?id=IN&crop=Rice [6]Shrivastava, A. et al. (2017) ‘Arsenic contamination in agricultural soils of Bengal deltaic region of West Bengal and its higher assimilation in monsoon rice’, Journal of Hazardous Materials, 324, pp. 526–534. Available at: https://doi.org/10.1016/j.jhazmat.2016.11.022. [7] https://www.who.int/news-room/fact-sheets/detail/arsenic [8] Anand, N. (2019). Pesticide Residues in Urban and Semi-Urban Regions of West Bengal, India: Risk Assessment in Human Milk and Water (Doctoral dissertation, Indian Institute of Science Education and Research Kolkata). http://eprints.iiserkol.ac.in/821/ [9] Aktar, M.W., Paramasivam, M., Sengupta, D. et al. Impact assessment of pesticide residues in fish of Ganga river around Kolkata in West Bengal. Environ Monit Assess 157, 97–104 (2009). https://doi.org/10.1007/s10661-008-0518-9 [10] Agnihotri, N.P., Gajbhiye, V.T., Kumar, M. et al. Organochlorine insecticide residues in Ganga river water near Farrukhabad, India. Environ Monit Assess 30, 105–112 (1994). https://doi.org/10.1007/BF00545617 [11] K. K. Vass, S. K. Mondal, S. Samanta, V. R. Suresh, P. K. Katiha; The environment and fishery status of the River Ganges. Aquatic Ecosystem Health & Management 15 November 2010; 13 (4): 385–394. doi: https://doi.org/10.1080/14634988.2010.530139 [12] Mohapatra, S.P., Gajbhiye, V.T., Agnihotri, N.P. et al. Insecticide pollution of Indian rivers. Environmentalist 15, 41–44 (1995). https://doi.org/10.1007/BF01888888 [13] https://www.epa.gov/pcbs/learn-about-polychlorinated-biphenyls-pcbs [14] https://www.epa.gov/hudsonriverpcbs/hudson-river-cleanup [15] https://scenichudson.org/wp-content/uploads/legacy/Technical-Supplement-Second-Five-Year-Review.pdf
