Spatio-temporal variations in total carbon content in contaminated surface waters at East Kolkata Wetland Ecosystem, a Ramsar Site

Abstract

Everyday nearly 1.1 million m3d−1 composite wastewaters from different source points, both municipal and industrial, pass through the East Kolkata Wetlands (EKW) via a web of canals which is productively used as irrigation water and in pisciculture. These composite wastewaters carry huge amount of total organic carbon (TOC) and total inorganic carbon (TIC), which have profound effect on wetland biogeochemistry and point out wetland carbon sequestration potential. However, at EKW (Ramsar Site; Group A; Criterion 1), which has been designated as a unique waste dependent resource recovery ecosystem for the past several decades, no comprehensive work has been done on the ecosystem service with special reference to carbon sequestration. Therefore, the aim of the present investigation is to estimate the TOC and TIC contents in composite wastewater and wastewater-fed fish ponds at EKW ecosystem at spatio-temporal scales. Different physico-chemical factors of these waters are also estimated in the same scales to record their influence on TOC and TIC contents in EKW. ANOVA results point out significant spatial and temporal variations of TOC and TIC besides other physico-chemical factors. Most of the measured factors including TOC and TIC contents are higher during pre-monsoon than in monsoon and post-monsoon periods. It is apparent from the correlation matrix that there are significant positive correlation between TOC and TIC and factors like TSS, TDS, conductivity, salinity, total hardness, F, Cl, SO4, Na, K, Ca and Mg. Principle component analysis (PCA) also suggests the significant influence of these physico-chemical factors that changed seasonally. Despite the mean level of TOC (348.05±154.98mgL−1) in composite wastewaters at the EKW areas are higher than the USEPA safe limit (2–4mgL−1), a significant amount of carbon is sequestered in the wetland ecosystem. Between source point and final discharge site nearly 63% of carbon of TOC and 71% carbon of TIC in wastewater have been reduced. Interestingly, comparable level of reductions, i.e., 61% for TOC and 62% for TIC have also been noted between the source point and the wastewater-fed fish ponds. Efficient partitioning of carbon in bottom sediments and in aquatic biota has been possible for the quintessential flow-rate of wastewater in the carrying canals, long residence time in the wastewater-fed fish ponds and repeated use of wastewater in irrigating agricultural fields at the EKW ecosystem.