Accounting for black carbon relationship with organochlorines in Lesser Himalaya, Pakistan

Abstract

Persistent Organic Pollutants (POPs) including organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) are important group of chemicals that may be released into the environment accidently or as a result of intentional anthropogenic activities and can cause various ecotoxicological and human health hazards. Their long range atmospheric transport potential could lead them to reach high altitude cold regions where they become deposited and trapped on surface media. Intriguingly, another pollutant, black carbon (BC) shows strong association with these trace chemicals and could sturdily affect the environmental distribution of these contaminants. The Himalaya is globally highest mountain range of 2400 km2 with an altitude of < 100-8844 masl that separates the Indian Subcontinent from the Tibetan Plateau (TP). Of particular importance is its proximity to industrialized regions of China, India and Pakistan. Due to influence of wind patterns (monsoon and westerlies) and anthropogenic activities, lower stretch of the Himalaya is at direct exposure to POPs. Current study was designed to monitor soil, sediment, water and atmospheric concentrations of long lived OCPs and PCBs in the Lesser Himalayan Region (LHR) of Pakistan. Polyurethane foam passive air samplers (PUF-PAS) were deployed for air samples, while soil, sediment and water samples were collected according to the defined protocols. Further, total organic carbon (TOC) and BC were analyzed in soils and sediments. Chemo-Thermal Oxidation (CTO-375) method was used for BC analysis, whereas OCs were analyzed by Gas Chromatography-Mass Spectrometry (GC-MS) system. Soil BC and TOC ranged between 0.16– 1.77 and 6.8−41.3 mg g-1, while sedimentary BC and TOC varied between 0.3−43.5 mg g−1 and 1.7−65.4 mg g−1, respectively. OCPs in soil, air, sediment and water samples from the LHR ranged between 0.69−5.77 ng g−1, 3.77−247 pg m−3, 0.59−3.64 ng g−1 and 0.07−41.4 ng L−1, respectively. PCBs concentrations ranged between 0.12–2.55 ng g-1, 8.49–458 pg m–3, 0.01−1.31 ng g−1 and 0.671−84.5 ng L−1, respectively. Spatially, Zone C (altitude range of 737−975 masl) have shown higher OCs levels in case of all matrices. Though, air mass trajectories over the LHR indicated long range transport as atmospheric source input, which was further explained by Clausius– Clapeyron plots between ln P and inverse of temperature (1000/T; K) where all OCPs and most of the PCBs have shown insignificant relationship (r2 = 5E-06–0.41; p-value = 0.06–0.99). However, local source emissions and valley transport may also implicate based on spatial distribution and altitudinal patterns. The results of linear regression analysis revealed potential input of BC in soil distribution of OCs concentrations in the region. Additionally, soil-air partitioning of OCs was assessed using octanol-air partition (KOA) and black carbon-air partition (KBC) based models. Regression results indicated combined influence of both organic matter (r2 = 0.3−0.85) and black carbon (r2 = 0.31−0.86) via absorption and adsorption, respectively in soil-air partitioning of OCs in the LHR. The relationship of sedimentary BC and TOC with OCPs and PCBs was evaluated using Principal Component Analysis (PCA) and Pearson Correlation Analysis that indicated higher sorptive influence of BC over TOC in distribution status of OCs in the LHR. Sedimentwater partitioning of OCPs and PCBs were deduced using field data by employing one carbon (fOCKOC) and two carbon Freundlich model (fOCKOC + fBCKBC-WCW nF−1). Results suggested improved measured vs predicted model concentrations when BC was induced in the model and suggested adsorption to be the dominant mechanism in phase partitioning of OCs in the LHR. The results of this study sheds light on the environmental concentrations of OCs in lower stretch of the Himalaya and help in better understanding of the processes involved in fate and transport of organic pollutants in the Himalayan region. Further investigations are required to understand the role of carbon fractions in fate and transport of other groups of organic pollutants at higher altitudes of the Himalayan region.