The pristine nature of river Ganges: its qualitative deterioration and suggestive restoration strategies

A systematic approach to understand hydrogeochemical dynamics in large river systems: Development and application to the River Ganges (Ganga) in India

Large river systems, such as the River Ganges (Ganga), provide crucial water resources for the environment and society, yet often face significant challenges associated with cumulative impacts arising from upstream environmental and anthropogenic influences. Understanding the complex dynamics of such systems remains a major challenge, especially given accelerating environmental stressors including climate change and urbanization, and due to limitations in data and process understanding across scales. An integrated approach is required which robustly enables the hydrogeochemical dynamics and underpinning processes impacting water quality in large river systems to be explored. Here we develop a systematic approach for improving the understanding of hydrogeochemical dynamics and processes in large river systems, and apply this to a longitudinal survey (> 2500 km) of the River Ganges (Ganga) and key tributaries in the Indo-Gangetic basin. This framework enables us to succinctly interpret downstream water quality trends in response to the underpinning processes controlling major element hydrogeochemistry across the basin, based on conceptual water source signatures and dynamics. Informed by a 2019 post-monsoonal survey of 81 river bank-side sampling locations, the spatial distribution of a suite of selected physico-chemical and inorganic parameters, combined with segmented linear regression, reveals minor and major downstream hydrogeochemical transitions. We use this information to identify five major hydrogeochemical zones, characterized, in part, by the inputs of key tributaries, urban and agricultural areas, and estuarine inputs near the Bay of Bengal. Dominant trends are further explored by investigating geochemical relationships (e.g. Na:Cl, Ca:Na, Mg:Na, Sr:Ca and NO₃:Cl), and how water source signatures and dynamics are modified by key processes, to assess the relative importance of controls such as dilution, evaporation, water-rock interactions (including carbonate and silicate weathering) and anthropogenic inputs. Mixing/dilution between sources and water-rock interactions explain most regional trends in major ion chemistry, although localized controls plausibly linked to anthropogenic activities are also evident in some locations. Temporal and spatial representativeness of river bank-side sampling are considered by supplementary sampling across the river at selected locations and via comparison to historical records. Limitations of such large-scale longitudinal sampling programs are discussed, as well as approaches to address some of these inherent challenges. This approach brings new, systematic insight into the basin-wide controls on the dominant geochemistry of the River Ganga, and provides a framework for characterising dominant hydrogeochemical zones, processes and controls, with utility to be transferable to other large river systems.

Engineered zinc oxide nanoparticles: an alternative to conventional zinc sulphate in neutral and alkaline soils for sustainable wheat production

Zinc oxide nanoparticles (ZnONP) were synthesized and characterized using SEM, EDAX, DLS and UV-Vis spectra. Its use as a nanofertilizer as an alternative to conventional zinc sulphate (ZnSO₄.7H₂O) was evaluated in five Zn-deficient soils with a variable pH range (7.2-8.7). For this, the carbon of the soil microbial biomass (SMBC), the bacterial population, the nutrient dynamics and the biometric parameters of the wheat crop were assessed. The varying dosages (0, 100, 200 and 500 mg/L), sizes (30-100 nm), and the spherical shape of ZnONPs were evaluated in comparison to ZnSO₄.7H₂O levels. Results showed the maximum SMBC and bacterial population at 100 mg/L of ZnONPs but a sharp decline at higher concentrations. In addition, soil application of ZnONPs at 5 mg/kg produced a higher root elongation (4.3-8.8%), shoot elongation (3.5-4.0%), total chlorophyll (4.9-5.6%), grain yield (1.7-2.3%) and grain Zn-content (1.6-2.1%) in comparison to the conventional ZnSO₄.7H₂O at 10 mg/L. ZnONPs at 100 mg/L produced a higher soil microbial biomass carbon (3.9-4.6%), bacterial population (7.2-9.0%), germination (22%) and grain Zn-content (17.9-20%) as compared to the conventional ZnSO₄.7H₂O at 0.5%. The higher grain Zn-contents could be attributed to the small size and high surface area of ZnONPs resulting in easy entry into the plant system either through root or foliar by penetrating the pores present in the cell membranes. Conversely, the conventional ZnSO₄.7H₂O, due to its larger size and higher solubility as compared to ZnONPs, has low retention in plant systems, high surface run-off and low fertilizer efficiency. Thus, the authors concluded to apply spherically synthesized ZnONPs (average size-36.7 nm) at 5 mg/kg in the soil application and 100 mg/L in the foliar application for maintaining SMBC and bacterial population, improving total chlorophyll, and grain Zn-contents and overall sustaining wheat production in Zn-deficient neutral and alkaline soils.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02861-1.

Nutrient and microbial water quality of the upper Ganga River, India: identification of pollution sources

The Ganga River is facing mounting environmental pressures due to rapidly increasing human population, urbanisation, industrialisation and agricultural intensification, resulting in worsening water quality, ecological status and impacts on human health. A combined inorganic chemical, algal and bacterial survey (using flow cytometry and 16S rRNA gene sequencing) along the upper and middle Ganga (from the Himalayan foothills to Kanpur) was conducted under pre-monsoon conditions. The upper Ganga had total phosphorus (TP) and total dissolved nitrogen concentrations of less than 100 μg l^-1 and 1.0 mg l^-1, but water quality declined at Kannauj (TP = 420 μg l^-1) due to major nutrient pollution inputs from human-impacted tributaries (principally the Ramganga and Kali Rivers). The phosphorus and nitrogen loads in these two tributaries and the Yamuna were dominated by soluble reactive phosphorus and ammonium, with high bacterial loads and large numbers of taxa indicative of pathogen and faecal organisms, strongly suggesting sewage pollution sources. The high nutrient concentrations, low flows, warm water and high solar radiation resulted in major algal blooms in the Kali and Ramganga, which greatly impacted the Ganga. Microbial communities were dominated by members of the Phylum Proteobacteria, Bacteriodetes and Cyanobacteria, with communities showing a clear upstream to downstream transition in community composition. To improve the water quality of the middle Ganga, and decrease ecological and human health risks, future mitigation must reduce urban wastewater inputs in the urbanised tributaries of the Ramganga, Kali and Yamuna Rivers.