Inter-estuarine and seasonal to decadal variations of heavy metal pollution in the Gulf of Cambay, India.

Toxic heavy metals adsorbed preferentially onto suspended sediments enter our food chain by bio-assimilation in coastal ocean organisms. To decipher metal pollution status in the Gulf of Cambay (food hub of India) under rising anthropogenic pressure, we present seasonal abundances of Ti, Cr, Co, Ni, Cu, Zn, Cd, and Pb in > 150 samples of suspended sediments (> 0.45 µm) collected in four seasons (2016-2017) from two large estuaries (Narmada and Tapi). The suspended sediments of both the estuaries generally show low heavy metal pollution (Igeo < 1). The hotspots of moderate-to-high pollution of Pb (Igeo < 4), Cd (Igeo < 3), and Zn (Igeo < 2) are found at salinity < 2, and those of Co (Igeo < 2) at salinity ~ 20-30 in the Tapi estuary during the non-monsoon seasons indicating their decoupled sources in Surat. The PLI values show no or little seasonality in the overall metal pollution status of both the estuaries. A comparison with the literature data suggests that suspended sediments efficiently capture active metal pollution in Indian estuaries. Furthermore, a recent decline (2004-2017) in estuarine metal pollution in the Gulf of Cambay found in this study could be due to enhanced organic matter supplies by enhanced urban sewage discharge and/or more trapping of contaminated riverine sediments upstream of the newly built large dam reservoirs counteracting the growth of anthropogenic metal inputs in the Narmada and Tapi watersheds. The data scarcity of heavy metal concentrations in suspended sediments limits reporting unambiguously the current pollution status of other major Indian estuaries.

Geochemical and isotope tracing of groundwater salinity in the coastal Gujarat alluvial plain, India.

This study attempts to learn the groundwater salinization mechanism in the large and well-populated coastal Gujarat alluvial plain under growing anthropogenic pressures. For this purpose, geochemical compositions and 87Sr/86Sr were analyzed in 53 groundwater samples collected in the three different seasons during 2016-2017. Additionally, surface water samples collected in each season from two nearby large rivers (Narmada and Tapi) and urban sewage discharge of Surat city were also analyzed. Excluding a few salinity hotspots (EC >8-18 mS/cm), the seasonal average of EC (~2.3-2.5 mS/cm) in groundwater shows little fluctuations negating evaporation as the major salinization process. Localized anthropogenic inputs as traced using NO3 also fail to explain the widespread groundwater salinity. A strong marine influence on the groundwater salinity is revealed by an average Cl/Br of ~655 ± 104 mol/mol in groundwater (except a few outliers) falling close to the seawater composition. The lateral seawater intrusion at shallow depths is generally not supported by the freshening of coastal groundwater samples and generally seaward movements of shallow groundwater as indicated by regional piezometric heads. However, few (Ca)-Mg-Cl type groundwaters collected from north of the Narmada River fall in the piezometric depressions and hint at seawater intrusion and/or upconing of deep saline aquifers. The Sr isotope budget in most of the alluvial groundwater samples is controlled by the seepage groundwater-seawater mixing. Exceptional 87Sr/86Sr values owing to Sr inputs from different age terrains are found in the northeastern groundwater and a few groundwater samples collected downslope of ancient deposits in the central alluvium. The upconing of salty waters having marine signatures partly altered by aquifer-water interactions primarily governs the regional groundwater salinity, which makes the study area vulnerable to continued salinization by excessive groundwater extraction.