Atmospheric Deposition: An Important Determinant of Nutrients and Heavy Metal Levels in Urban Surface Runoff Reaching to the Ganga River.

Excessive loading of water bodies with surface runoff-driven nutrients and heavy metals has become a serious concern worldwide. We investigated the surface runoff quality for nutrients and heavy metals being flushed to the Ganga River, as influenced by atmospheric deposition (AD). We selected three city sites in India, Haridwar, Varanasi, and Howrah, which differ widely with respect to population density and anthropogenic activities. We found distinct spatio-temporal trends in AD input of nutrients and heavy metal with values being highest in Varanasi region followed by Howrah and Haridwar. The runoff nutrients and metals showed strong synchrony with their respective levels in AD input. The concentrations were higher in the first flush. We found strong correlations (R2 = 0.83-0.93; p < 0.001) between AD metals and nutrients with their respective concentration in runoff. For all the studied metals, except Cd, the major proportions were in particulate form. The Cd was present in almost equal proportions in particulate and dissolved fractions. Metals in runoff were found in order as: Zn > Pb > Cu > Ni > Cr > Cd. In general, the concentrations of metals were higher than those reported in other studies. The contamination factor and geo-accumulation index show that the Cd was a major pollutant in the runoff. The pollution load index (PLI) indicates that all three sites are highly polluted. Our study indicates that there is a need to reduce particulate loads. Furthermore, because of the high concentrations of pollutants in the first flush, strategies may be developed to enhance the efficiency of treatment of the first flush of runoff.

Heavy Metal Pollution in the Ganga River Enhances Carbon Storage Relative to Flux.

This study evaluated the relationships between metal pollution and carbon production at six sites along a 285 km length of the Ganga River. Metal contaminated sites did show a significant reduction in microbial biomass, substrate induced respiration, fluorescein diacetate hydrolytic assay (FDAase) and ß-D-glucosidase. Concordantly, despite a high concentration of total organic carbon at these sites, CO2 emission at the land-water interface remained low. We found a strong positive correlation between CO2 emission and TOC (r = 0.92; p < 0.001). However, this relationship weakens when the sum of total heavy metal (∑THM) exceed 400 µg g-1. Also, CO2 emission did show a positive correlation (r = 0.85; p < 0.001) with FDAase. The study shows that metal accumulation in riverbed sediment could potentially lead to better carbon sequestration on account of reduced microbial/enzyme activities. This carries significance for riverine carbon budget and modeling.

Metal Contamination in Seven Tributaries of the Ganga River and Assessment of Human Health Risk from Fish Consumption.

We investigated the distribution of Zn, Cu, Ni, Pb, Cr, and Cd in water, sediment, and two dietary fish (an omnivore, Labeo rohita and a benthic carnivore, Clarias batrachus) and potential health risk to human consumers during summer low flow (2017-2018) at 28 sites across 7 tributary confluences of the Ganga River. We selected Devprayag, an upper reach site, as a reference for data comparison. We found significant spatial variations in the distribution of study metals and the concentrations remained higher in tributaries, confluences, and downstream cities. The pollution load index showed all sites except Devprayag in the polluted category. Ecological risk analysis indicated 1 site with very high risk, 7 with considerable risk, and 10 with moderate-risk category. The Zn did appear the most, and Cd the least accumulated metal in the fish. The metal accumulation was higher in C. batrachus. The levels of Cd, Cr, and Pb in the study fishes were higher compared with the international standards. The health risk analysis indicated safe levels for individual metals except for Cd where the target hazard quotient (THQ) did exceed 1 for C. batrachus at the Ramganga and Varuna confluences. When all metals were considered, the THQ was > 1 (> 2 for C. batrachus), indicating the full possibility of adverse health effects to human consumers. Our study highlights the importance of tributaries in creating a mosaic of metal-rich habitats in the Ganga River and food chain associated with a health risk to human consumers.

Assessment of heavy metal pollution in water and surface sediment and evaluation of ecological risks associated with sediment contamination in the Ganga River: a basin-scale study.

We investigated eight heavy metals (Cr, Cd, Cu, Ni, Pb, Zn, Mn, and Fe) in water and bed sediment at 9 study sites along with 2320 km stretch of the Ganga River. Principal component analysis (PCA) and indices such as geo-accumulation index (Igeo), contamination factor (CF), enrichment factor (EF), pollution indices, and sediment quality guidelines were used to assess source apportionment and magnitude of contamination. Concentrations of Cr, Cd, Pb, Ni, Cu, and Fe in water have exceeded their respective standards in the middle and lower reaches of the river. Sediment Cr and Ni have reached probable effective concentration (PEC) at Kannauj, imposing likely threats to sediment dwellers. Highest Igeo values were recorded for Cr, Cd, and Pb at Kannauj, Rajghat, and Howrah. We further tested ecological risks (Er) and potential ecological risks (PERI) to assess individual and cumulative effects and found the Kannauj, Rajghat, and Howrah sites under the high-risk category. The modified pollution index (MPI) and the modified degree of contamination (mCd) also revealed the middle and lower river reaches under moderately to the heavily polluted category. Our study provides the first detailed watershed-scale database on heavy metal concentration in water and bed sediment, the magnitude of contamination, and likely ecological risks to aquatic organisms in the Ganga River. Given that the Ganga water is used for drinking and irrigation and the river harbors a diversity of habitats for fisheries, the study merits attention from a human health perspective as well.