The ponds are the important water resources for the developing nations including rural India. They not only supply the water during the non-monsoon period but also acts as a flood absorber during the monsoon period. However, with the increase in population, modernization, and alternate water supply schemes, the ponds are getting polluted mainly due to the discharge of wastewater from residential areas, sewage outlets, solid waste dump sites, livestock rearing sites (e.g., dairy, poultry), and fertilizers and pesticides from farmers’ fields. The sludge accumulated in these water bodies may contain an appreciable amount of toxic metals that may leach to the groundwater and long-term use of contaminated water, either directly from these water bodies or the nearby handpumps or tubewells, can cause serious threats to human health. This study investigated nine heavy metal (Pb, Cu, Ni, Zn, Cr, Cd, Fe, As, and Mn) contamination in the pond sludge, pond soil, and groundwater at twenty-one villages in Western Uttar Pradesh (India), and provides a sound case for examining the toxic metal accumulation in the terrestrial water bodies. The mean concentration of Pb, Cu, Ni, Zn, Cr, Cd, Fe, As and Mn in pond sludge was 0.64, 0.53, 0.94, 3.34, 0.62, 0.010, 151.70, 0.32, and 35.02 mg/kg sludge, respectively, and in pond soil the concentration of Pb, Cu, Ni, Zn, Cr, Cd, Fe, As and Mn were 0.13, 0.45, 1.21, 0.82, 3.22, 0.06, 2055.04, 0.16, and 457.84 mg/kg soil, respectively. The geo-accumulation index (I-geo) in the pond sludge and soil ranged from unpolluted to extremely polluted. The average I-geo values of metals in pond sludge and soil were in order Pb (4.93) > Zn (4.36) > Ni (0.70) > Cu (0.40) > As (0.32) > Cd (-0.58) > Cr (-1.96) > Fe (-4.24) > Mn (-5.28) and Cu (2.76) > Cr (2.35) > Pb (2.17) > Ni (2.01) > Zn (1.53) > Cd (-0.22) > Fe (-0.49) > As (-0.78) > Mn (-1.41), respectively. The combined ecological risk due to metal accumulation extended from low risk to very high risk. The groundwater samples were observed to non-comply the drinking water standards due to high levels of Pb, Cu, Ni, Fe, and Mn, and the human health hazard index ranged from 0.831 to 21.76. This study highlights the value of baseline monitoring and provides a sound case for examining the pond ecosystem for heavy metal toxicity before and during the rejuvenation of such water bodies in India and other sub-tropical countries. 

Appraisal of heavy metal pollution in the water resources of Western Uttar Pradesh, India and associated risks

Salt-affected soils contain high levels of soluble salts (saline soil) and exchangeable sodium (alkali soil). Globally, about 932 million ha (Mha), including 831 Mha of agricultural land, is salt-affected. Salinity and sodicity adversely affect soil microbial diversity and enzymatic activities, and thereby carbon and nitrogen dynamics and greenhouse gas (GHG) emissions from soils. In this review article, we synthesize published information to understand the impact of salinity and sodicity on GHG production and emissions from salt-affected soils, and how various reclamation amendments (gypsum, phosphogypsum, organic manure, biochar, etc.) affect GHG emissions from reclaimed soils. Nitrous oxide (N2O) and methane (CH4) emissions are of greater concern due to their 298 and 28 times higher global warming potential, respectively, compared to carbon dioxide (CO2), on a 100-year time scale. Therefore, CO2 emissions are given negligible/smaller significance compared to the other two. Generally, nitrous oxide (N2O) emissions are higher at lower salinity and reduced at higher salinity mainly due to: (a) higher ammonification and lower nitrification resulting in a reduced substrate for denitrification; (b) reduced diversity of denitrifying bacteria lowered down microbial-mediated denitrification process; and (c) dissimilatory nitrate reduction to ammonium (DNRA), and denitrification processes compete with each other for common substrate/nitrate. Overall, methane (CH4) emissions from normal soils are higher than those of salt-affected soils. High salinity suppresses the activity of both methanogens (CH4 production) and methanotrophs (CH4 consumption). However, it imposes more inhibitory effects on methanogens than methanotrophs, resulting in lower CH4 production and subsequent emissions from these soils. Therefore, reclamation of these soils may enhance N2O and CH4 emissions. However, gypsum is the best reclamation agent, which significantly mitigates CH4 emissions from paddy cultivation in both sodic and non-sodic soils, and mitigation is higher at the higher rate of its application. Gypsum amendment increases sulfate ion concentrations and reduces CH4 emissions mainly due to the inhibition of the methanogenesis by the sulfate reductase bacteria and the enhancement of soil redox potential. Biochar is also good among the organic amendments mitigating both CH4 and N2O emission from salt-affected soils. The application of fresh organic matter and FYM enhance GHG emissions for these soils. This review suggests the need for systematic investigations for studying the impacts of various amendments and reclamation technologies on GHG emissions in order to develop low carbon emission technologies for salt-affected soil reclamation that can enhance the carbon sequestration potential of these soils.

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Greenhouse Gas Emissions from Salt-Affected Soils: Mechanistic Understanding of Interplay Factors and Reclamation Approaches

In the modern era, due to urbanization, industrialization, and anthropogenic activities in the catchment, greenhouse gas (GHG; CO2, CH4, and N2O) emissions from freshwater ecosystems received scientific attention because of global warming and future climate impacts. A developing country such as India contributes a huge share (4% of global) of GHGs from its freshwater ecosystems (e.g., rivers, lakes, reservoirs) to the atmosphere. This is the first comprehensive review dealing with the GHG emissions from Indian freshwater bodies. Literature reveals that the majority of GHG from India is emitted from its inland water, with 19% of CH4 flux and 56% of CO2 flux. A large part of India’s gross domestic product (GDP) is manipulated by its rivers. As a matter of fact, 117.8 Tg CO2 year−1 of CO2 is released from its major riverine waters. The potential of GHG emissions from hydropower reservoirs varies between 11–52.9% (mainly CH4 and CO2) because of spatio-temporal variability in the GHG emissions. A significant contribution was also reported from urban lakes, wetlands, and other inland waters. Being a subtropical country, India is one of the global GHG hotspots, having the highest ratio (GHG: GDP) of 1301.79. However, a large portion of India’s freshwater has not been considered yet, and there is a need to account for precise regional carbon budgets. Therefore, in this review, GHG emissions from India’s freshwater bodies, drivers behind GHG emissions (e.g., pH, mean depth, dissolved oxygen, and nutrients), and long-term climatic risks are thoroughly reviewed. Besides research gaps, future directions and mitigation measures are being suggested to provide useful insight into the carbon dynamics (sink/source) and control of GHG emissions.

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India’s Contribution to Greenhouse Gas Emission from Freshwater Ecosystems: A Comprehensive Review

: The accumulation of nutrients (eutrophication) in water bodies generally produces increased concentrations of organic matter that eventually are deposited in sediment, and partially mineralized, generating greenhouse carbon gases (GHCG). The application of eutrophication control methods in-cludes the application of phosphate adsorbing materials such as Phoslock (PHOS), and hypolimnetic oxygenation systems (HOS). We evaluated the generation of GHCG in sediment subject to these eutrophication control methods. Combined water and sediment samples from the Valle de Bravo reservoir in Mexico, were incubated in reactors, where the following eutrophication control methods were applied: HOS, PHOS, HOS + PHOS, and compared to a reactor without treatment (CONTROL). Redox potential (Eh), pH, redox-sensitive ions, and GHCG emissions were monitored, observing the following rates: CONTROL (15.6 mmol m − 2 d − 1 ) > HOS (12.8) > HOS + PHOS (11.0) > PHOS (9.7 mmol m − 2 d − 1 ), with the CONTROL rate within values determined from published sediment core data. The GHCG emissions increased with time as Eh decreased, and sulfate reduction increased. Application of eutrophication control methods in the Valle de Bravo reservoir, would most probably result in lower GHCG generation and emission rates. This is due to the repression of sulfate-reduction in water-sediment systems where HOS and PHOS were applied both individually and combined.

Effect of Eutrophication Control Methods on the Generation of Greenhouse Carbon Gases in Sediment

The accumulation of nutrients (eutrophication) in water bodies generally produces increased concentrations of organic matter that eventually are deposited in sediment, and partially mineralized, generating greenhouse carbon gases (GHCG). The application of eutrophication control methods includes the application of phosphate adsorbing materials such as Phoslock (PHOS), and hypolimnetic oxygenation systems (HOS). We evaluated the generation of GHCG in sediment subject to these eutrophication control methods. Combined water and sediment samples from the Valle de Bravo reservoir in Mexico, were incubated in reactors, where the following eutrophication control methods were applied: HOS, PHOS, HOS + PHOS, and compared to a reactor without treatment (CONTROL). Redox potential (Eh), pH, redox-sensitive ions, and GHCG emissions were monitored, observing the following rates: CONTROL (15.6 mmol m−2 d−1) &gt; HOS (12.8) &gt; HOS + PHOS (11.0) &gt; PHOS (9.7 mmol m−2 d−1), with the CONTROL rate within values determined from published sediment core data. The GHCG emissions increased with time as Eh decreased, and sulfate reduction increased. Application of eutrophication control methods in the Valle de Bravo reservoir, would most probably result in lower GHCG generation and emission rates. This is due to the repression of sulfate-reduction in water-sediment systems where HOS and PHOS were applied both individually and combined. 

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Inland water bodies (particularly ponds) emit a significant amount of greenhouse gases (GHGs), particularly methane (CH4), carbon dioxide (CO2), and a comparatively low amount of nitrous oxide (N2O) to the atmosphere. In recent decades, ponds (<10,000 m2) probably account for about 1/3rd of the global lake perimeter and are considered a hotspot of GHG emissions. High nutrients and waterlogged conditions provide an ideal environment for CH4 production and emission. The rate of emissions differs according to climatic regions and is influenced by several biotic and abiotic factors, such as temperature, nutrients (C, N, & P), pH, dissolved oxygen, sediments, water depth, etc. Moreover, micro and macro planktons play a significant role in CO2 and CH4 emissions from ponds systems. Generally, in freshwater bodies, the produced N2O diffuses in the water and is converted into N2 gas through different biological processes. There are several other factors and mechanisms which significantly affect the CH4 and CO2 emission rate from ponds and need a comprehensive evaluation. This study aims to develop a decisive understanding of GHG emissions mechanisms, processes, and methods of measurement from ponds. Key factors affecting the emissions rate will also be discussed. This review will be highly useful for the environmentalists, policymakers, and water resources planners and managers to take suitable mitigation measures in advance so that the climatic impact could be reduced in the future.

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Jhlaesh Kumar

Papers

Greenhouse Gases Trade-Off from Ponds: An Overview of Emission Process and Their Driving Factors