Zone Salinity Research Articles

Hydrogeological Resolution Using Composite Geophysical Methods: A Case Study of Inland Salinity in Part of Agra, Uttar Pradesh India

ABSTRACT This manuscript is an attempt at utilizing a combination of varied Geophysical methods (n = 4) – Vertical Electrical Sounding (VES), Electrical Resistivity Tomography (ERT), Time Domain Electromagnetic (TEM), and Borehole logging for finer resolution of potential fresh groundwater and saline groundwater aquifer zones in marginal alluvial plains of Agra district, Uttar Pradesh state. A conceptual model of sub-surface aquifer disposition displaying zones of fresh and saline groundwater zones was prepared using an interpreted and processed multi-parameter dataset. A positive correlation was found between TEM (44.6 m), Well log data (43.0 m), VES data (47.2), and ERT data (45.0 m) demarcating the boundary up to which fresh groundwater potential lies from the ground surface. Conventional methods rely on resistivity values to identify groundwater potential zones, and a rough estimation of salinity can be carried out. Instead of the conventional VES method, the combination of aforesaid methods results in finer resolution and easier demarcation of fresh groundwater zones from those of saline zones. This method can be successfully reproduced to demarcate the extent of saline water ingress in coastal areas and finding freshwater lenses within inland saline aquifers.

Ecological impact of artificial barrage on calanoid copepods in the Vembanad Lake, Southwest Coast of India

The Vembanad Lake (VL) is a brackish water lagoon, an integral part of Ramsar site, Vembanad Kol Wetland ecosystem on the southwestern coast of India. VL faces escalating anthropogenic impact, notably worsened by the establishment of the Thanneermukkom barrage (TMB) in 1976. This study hypothesizes that the TMB has led to significant ecological changes in VL, specifically altering the distribution and abundance of calanoid copepods, which are vital as a food source for higher trophic levels, particularly fish. To test this hypothesis, zooplankton samples were collected from various sites along the lake, and environmental parameters such as salinity and chlorophyll-a concentration were measured. This investigation reveals a distinct limnetic zone in the southern region, in contrast to the oligo-mesohaline zone in the north, attributed to the TMB. The study identifies the prevalence of calanoid copepod species such as Heliodiaptomus cinctus, Allodiaptomus mirabilipes, and Acartiella gravelyi in the TMB-South region, reflecting a freshwater-dominated ecosystem. The low salinity and high chlorophyll-a content in this area create an optimal environment for these species. In contrast, the dominance of Acartia (Odontacartia) spinicauda, Pseudodiaptomus serricadatus, and Paracalanus sp. in the TMB-North indicates higher salinity levels, with species in this region adapting to a different ecological niche. Thus, the installation of the TMB has significantly altered the ecological balance of VL by dividing the water body into two distinct salinity zones. This separation has led to changes in the distribution and abundance of calanoid copepods, which are essential for energy transfer to higher trophic levels in the VL food web, consequently affecting fish populations and overall biodiversity.

Assessing Vulnerability to Cyclone Hazards in the World’s Largest Mangrove Forest, The Sundarbans: A Geospatial Analysis

The Sundarbans is the world’s largest contiguous mangrove forest with an area of about 10,000 square kilometers and shared between Bangladesh and India. This world-renowned mangrove forest, located on the lower Ganges floodplain and facing the Bay of Bengal, has long served as a crucial barrier, shielding southern coastal Bangladesh from cyclone hazards. However, the Sundarbans mangrove ecosystem is now increasingly threatened by climate-induced hazards, particularly tropical cyclones originating from the Indian Ocean. To assess the cyclone vulnerability of this unique ecosystem, using geospatial techniques, we analyzed the damage caused by past cyclones and the subsequent recovery across three salinity zones, i.e., Oligohaline, Mesohaline, and Polyhaline. Our study also examined the relationship between cyclone intensity with the extent of damage and forest recovery. The findings of our study indicate that the Polyhaline zone, the largest in terms of area and with the lowest elevation, suffered the most significant damage from cyclones in the Sundarbans region, likely due to its proximity to the most cyclone paths. A correlation analysis revealed that cyclone damage positively correlated with wind speed and negatively correlated with the distance of landfall from the center of the Sundarbans. With the expectation of more extreme weather events in the near future, the Sundarbans mangrove forest faces a potentially devastating outlook unless both natural protection processes and human interventions are undertaken to safeguard this critical ecosystem.

Impact of lining of irrigation canals on groundwater recharge: A case study from Punjab province in Indus River basin of Pakistan

The Canal irrigation network of Pakistan's Indus River Basin (IRB) is one of the world's largest gravity-flow networks, with a significant portion located in Punjab province. The conveyance, delivery, and application of this water to the irrigated crops in the basin result in the loss of more than 50% of its water. This loss of water from the irrigation system is a major source of recharge for the large alluvial aquifer underlying the IRB. Canal lining is carried out as a measure to reduce seepage losses from the conveyance system of irrigation water. On the other hand, canal lining is considered a barrier to aquifer recharge. A study titled “Impact assessment of canal lining in Punjab,” funded by JICA, has been carried out in order to interrogate the comparative performance of lined and unlined canals and their impact on groundwater recharge. We measured seepage losses from 14 selected irrigation-lined canals in Punjab province under actual field conditions using ponding and/or inflow-outflow methods. Results have been compared with pre-lining seepage rates. The results indicate that canal lining has reduced seepage losses by 78%. The heavy pumping in fresh groundwater areas invites saline groundwater from adjacent saline zones. The canal flows at the head have reduced the recharge to groundwater by 9%. Nonetheless, saving from seepage losses by canal lining has increased available water for crops, but it has decreased groundwater recharge.

Strategies of water resource utilization and agricultural water management in the coastal saline zone of Bangladesh

Strategies of water resource utilization and agricultural water management in the coastal saline zone of Bangladesh

Ecology of the Copepods at a Low Salinity Zone of the Ganges Estuary, India

Ecology of the Copepods at a Low Salinity Zone of the Ganges Estuary, India

Options for Intensification of Cropping System in Coastal Saline Ecosystem: Inclusion of Grain Legumes in Rice-Based Cropping System

The coastal saline zone of West Bengal in India is the home to millions of the world’s poorest and most vulnerable people. Due to a gradual increase in salt accumulation on soils of the coastal saline zone of West Bengal in India from winter to summer days, cultivation of the second crop in the winter season becomes possible in a limited area. To address these issues, field experiments was conducted in rainy and winter seasons of 2016–17 and 2017–18 at the farmer’s field of the coastal saline zone (CSZ) of West Bengal, India. The experiment was carried out to study the system productivity, nutrient uptake, and profitability vis-à-vis salinity dynamics of the crops in rice-pulse-based cropping systems under different land elevations (medium upland and medium lowland). The experiment was conducted in a strip-split plot design having horizontal factors namely, Factor A: Six dates of sowing of rice at an interval of one week (2nd week of June to 3rd week of July), Factor B: Two land situations (medium upland and medium lowland) and Two Cropping Systems (Rice-Lathyrus and Rice-Lentil) as vertical factor, replicated four times. The results suggest that irrespective of land situation, early sown rice (15 June to 21 June) produces higher dry matter and grain yield compared to late sown crops. This early sowing of rice also facilitated the better performance of subsequent lathyrus and lentil, by avoiding the worst situation of the salinity build-up and drought stress later in the winter. Moreover, significantly higher productions were obtained from medium-lowland situations for both the cropping systems. Sowing date has also significantly influenced macro-nutrient uptake (NPK) by rice and pulse grains. It may be concluded that early sowing of rice may be a potential option for intensification of rice-pulse-based cropping systems under CSZ of West Bengal, India.

A trait-based approach to quantify ecosystem services delivery potentials in the Sundarbans mangrove forest of Bangladesh

An ecosystem’s potential to deliver goods and services, primarily determined by the functional traits of the species, is crucial for assessing its health and conservation value. This study uses a trait-based approach to evaluate the spatial and temporal variation in the ecosystem services delivery potentials (ESDPs) of the Sundarbans mangrove. We analyzed abundance data from 81 permanent sample plots (PSPs), spread over different saline zones, collected between 1986 and 2014, paired with species-specific plant functional traits. Seventeen traits were examined, with species-level data sourced from secondary literature, linking traits to ecosystem services. Study revealed that plant height, specific leaf area, and root length are key traits for assessing ESDPs. The Sundarbans’ ESDPs varied spatially across different saline zones but remained stable over time. Notably, ESDPs were significantly higher in low saline zones compared to ESDPs in high saline zones. This spatial variation is partly explained by differences in plant species composition among saline zones and the relatively stable species composition over time. Results from mixed effect models showed that PSP as a random factor for all four models is attributed to the nested effect. The study highlights the value of functional traits in ecosystem service assessments, providing a robust method to predict how changes in biodiversity and environmental conditions affect service delivery. The observed trend of decreasing ESDPs with increasing salinity offers insights into the potential impacts of sea-level rise on mangrove forest functions. These findings are essential for developing conservation strategies and policies to preserve the Sundarbans’ ecological integrity and support local livelihoods. This research advocates for a trait-based framework as a critical tool for the sustainable management of mangrove ecosystems globally.

Spatial difference of salt dynamics within burrows in high salinity zone (high intertidal zone & supratidal zone) of mudflat

Surface evaporation and seawater concentration in the tidal flat have been demonstrated as sources of salt for phreatic brine layers. However, the mechanisms of salt replenishment in such layers remain unclear. Here, the transport processes and factors influencing surface salt in mudflat under the influence of bioturbation were examined in the high salinity zone (high intertidal zone & supratidal zone) along the southern coast of Laizhou Bay, China. In this study, the laboratory model and numerical model of salt transport in the mudflat brine distribution area under the influence of crab burrows were constructed through in-situ investigation to simulate the salt transport process in the mudflat brine distribution area under different tidal amplitudes and different beach salinity, and the influence of the diameter and density of crab burrows on the water-salt exchange process was analyzed. The results indicate that in the salt-crust-free region, the salinity change in burrow-dense area (BDA) lags behind that of independent burrow (IB). At the same time, the salinity increase of IB is at most 17 ppt higher than that of BDA, and the salinity decrease of IB is at most 9 ppt lower than that of BDA, which made BDA promoting more salt accumulation and facilitating the salt transport to deeper layers. In the salt-crust-bearing region, due to the high salt content on the beach surface and the weak tidal amplitude, salt is continuously released and accumulated in the surface during multiple tidal cycles, but the ability to transport to the deep layer is weak. In this case, burrows characterized by larger diameters become the preferential paths for salt transport. In addition, crab burrows affect the evaporation of sediments. In this study, the surface salinity of sediments under the influence of burrows is around 10–45 ppt higher than that without burrows.

Climate and salinity together control above ground carbon accumulation in the Sundarbans mangrove ecosystem

Mangroves play a crucial role in mitigating climate change through carbon (C) accumulation. Developing robust methods for the estimation of long-term C accumulation is essential for monitoring and reporting carbon financing projects under different global mechanisms. Moreover, understanding the effects of climate and habitat on C accumulation is important for developing mangrove management plans and conservation options. In this study, long-term above ground C accumulation in the three most dominated mangrove tree species, Heritiera fomes, Excoecaria agallocha and Sonneratia apetala, was estimated using a dendrochronological approach in two contrasting (low vs high) salinity zones in the Bangladesh Sundarbans. In addition, how climate and habitat (e.g., regulators and soil resources) influence C accumulation was analyzed. For the first time, we estimated C accumulation in mangrove species over their entire lifetime using stem disc-derived tree-ring data. The C accumulation patterns exhibited similar patterns of variation, increasing from pith to bark in each case. C accumulation was significantly higher in the low salinity zone (LSZ) than in the high salinity zone (HSZ) (p < 0.05). The cumulative C accumulation increased with tree age in each case, and C accumulation was influenced mainly by seasonal precipitation, especially during monsoon. General additive modeling (GAM) revealed that soil salinity had a stronger effect on C accumulation variability in the studied tree species. Other regulatory variables (i.e., siltation and elevation) and soil resources (i.e., P and K) had species-specific influences on C accumulation. Tree-ring data show the potential to yield better biomass and C estimations, which could aid frequent and rapid reporting of C accumulation in mangroves. Moreover, the results also suggest that climate and regulatory variables, especially soil salinity, together impact the C cycles in the Sundarbans.

Freshwater input significantly reduces specific and functional diversity of small subarctic estuaries

Estuaries face mounting anthropic pressures. Climate change is projected to increase heavy precipitations, while agricultural and logging activities reduce land permeability, leading to more frequent flooding events that lower estuarine salinity. These hyposaline conditions pose physiological challenges for marine species, potentially reducing taxonomic diversity and biomass, and ultimately, ecosystem functions and services. Polar and subpolar estuaries, historically understudied, will undergo significant changes in freshwater discharge timing and variability due to predicted shifts from snow to rainfall regimes in these high latitudes. The investigation of biotic communities structure through functional traits is crucial for understanding biodiversity changes, evaluating ecosystem tolerance, and addressing the loss of function under stressful conditions. Therefore, our study aims to characterize the taxonomic and functional diversity of hard-substrate benthic communities along a salinity gradient in small subarctic estuaries. We hypothesize that communities closer to the riverine input will be less taxonomically diverse and abundant and with smaller individuals, leading to a loss of functions due to environmental filtering. To test our hypotheses, we measured traditional (biomass, taxonomic richness and diversity) and functional (functional richness and Rao's quadratic entropy) indices, as well as observed traits, of epibenthic hard-substrate macroalgae and macroinvertebrates along a four-level salinity gradient found in five small subarctic estuaries in Quebec (Canada). As predicted, freshwater inputs induced a decrease in biomass, taxonomic richness, functional richness, and Rao's quadratic entropy, with a fivefold reduction in total biomass of the epibenthic community from marine to low salinity zones. Freshwater inputs significantly shaped both taxonomic and functional trait structures, as the proportion of suspension feeders and long-distance dispersers decreased with rising freshwater inputs. While there was no clear trend for the size-class structure of invertebrate taxa, macroalgae were smaller at low salinities. Estuarine communities in subarctic regions may be vulnerable to future alterations in flooding patterns caused by global change, potentially affecting key ecosystem services such as suspended matter filtration. Functional trait indices were found to be efficient and serve as a valuable complement to taxonomic measures in detecting community shifts along salinity gradients, highlighting their usefulness in developing effective conservation and management strategies.

Exploring factors that affect Microcystis abundance in the sacramento san joaquin delta

Cyanobacteria harmful algal blooms (cHABs) are increasing in frequency, intensity and duration in estuaries worldwide. In the upper San Francisco Estuary, also known as the Sacramento San Joaquin Delta (Delta), cHABs have been a topic of concern over the past two decades. In response, managers are urgently working to understand the factors that drive cHABs and identify feasible management options to avert ecological and human health consequences. We used a six year data set to explore relationships between flow parameters, temperature, and Microcystis biovolume to determine the potential for managing large scale hydrodynamic conditions to address Delta cHABs. We also looked at the relationship between Microcystis biovolume and the low salinity zone to see if it could be used as a proxy for residence time, because residence time is positively related to cyanobacteria abundance. We found the low salinity zone is not a useful proxy for residence time in the area of the Delta that experiences the most severe cHABs. Our finding suggest that climatic conditions (i.e., temperature and water year type) have the greatest influence on Microcystis biovolume in the Delta, with higher biovolume during years with lower flow and higher temperatures. Further, there are interannual differences in Microcystis biovolume that cannot be fully explained by flow parameters or temperature, meaning other factors not included in our model may be involved. We conclude that management actions to increase flow may be ineffective at reducing Microcystis to desired levels if water temperatures remain high.

Qualitative Evaluation of Cropping System Intensification in Coastal Saline Zone of West Bengal, India

Qualitative Evaluation of Cropping System Intensification in Coastal Saline Zone of West Bengal, India

Research of fresh/salt water in Upper-middle Pleistocene (qp\(_{2–3}\)) in Bac Lieu Province

This study identified the relationship between Total Dissolved Solids (TDS) and Electrical Conductivity (EC) with the TDS = 0.6628*EC = 0.1706 correlation, in which the correlation coefficient is r = 0.984 and the Standard Error (±%) is 2.78%. The TDS content is determined by small relative error and a high correlation coefficient of the conductivity, which applies to calculate the TDS content of other groundwater samples in Bac Lieu Province. Also, the temperature difference in the electrical conductivity measurement affects the TDS content; thus, the study has determined that the temperature compensated constant between the electrical conductivity on the field and those at 25oC is 0.002953. According to the data and the analysis of the TDS content, the fresh-salt water distribution map of the qp2–3 aquifers has been established, and its boundary has been more detailed and standardized than in the previous study. In particular, TDS values range from 0.437 g/L to 2.0 g/L. The areas that contain brackish and salt water are distributed discontinuously, forming the saline zones in Loc Ninh, Phuoc Long communes in Hong Dan district; Phuoc Long, Hung Phu communes in Phuoc Long district; Vinh Hau, Vinh Binh communes in Hoa Binh district, and Thuan Hoa commune in Bac Lieu city. The saltwater area of the qp2–3 aquifers is about 398 km2, accounting for 16% of the study’s total area. The results fresh-salt water distributed mapping provide the environmental management institution with a comprehensive view of the distribution of fresh-salt water to propose effective groundwater exploitation policies.

Role of mangrove afforestation in the high saline zone of Bangladesh: Changes of carbon stock with stand age and species composition

The carbon storage in Bangladeshi mangrove plantations is influenced by site-specific variations and plantation management practices. This study was focused on evaluating the impact of stand age and above-ground factors on the carbon storage of the western-side high-saline mangrove plantations in Bangladesh. A comprehensive field study was conducted, encompassing mangrove plantations aged 3, 5, 7, 10, 12, 15, and 35 years, to assess their potential for ecosystem carbon storage. The biomass data was calculated by estimating the tree height and diameter at breast height (DBH) with common allometric equations and soil samples were collected from every plot for estimating the organic carbon in the soil of the mangrove plantations. Stand structure attributes showed significant differences among all age classes and Sonneratia apetala was found to be dominant among all other species (IV = 147.41). Biomass C storage upsurges with age, ranging from 51.5±3.2–259.9±103.5 Mg C ha−1, while soil C storage averages 33.1±3.1 Mg C ha−1, showing no significant variation on age. Ecosystem carbon storage increases significantly with age and 35-year-old mangrove plantations exhibit the highest storage potential (292.0±105.5 Mg C ha−1). An analysis of species composition highlighted that Sonneratia apetala accumulated 94.1±28.4 Mg C ha−1 of biomass carbon, a figure notably higher than other species. The comparison indicated that, while the ecosystem carbon storage in Bangladesh is higher in mature-aged mangrove plantations, it lags behind young-aged mangrove plantations in Myanmar. Bangladesh exhibits a higher biomass carbon accumulation rate than China and India but falls short by 56% when compared to Myanmar's biomass carbon accumulation. However, these insights from mangrove plantation studies can serve as valuable guidance for policymakers and researchers, aiding in the conservation and management of mangroves and developing a blue carbon framework to meet the required funding for sustainable development of mangrove plantations and local communities in Bangladesh.

Impacts of vegetation and tidal conditions on porewater and salt transport in coastal wetlands: Numerical simulations and field evidence

Coastal wetlands are crucial ecosystems with high fragility to environmental alterations. Despite early efforts, interactions of water and salt with vegetation in coastal wetlands have rarely been investigated especially with increasing anthropogenic disturbances. Here, field data and numerical simulations were utilized to understand the influences of vegetation, evapotranspiration (ETa), tide, and inundation events on porewater and salt transport in highly regulated tidal riparian wetlands. At the site, the presence of Phragmites australis significantly enhanced soil hydraulic conductivity (KS) and facilitated hydraulic connections between groundwater and surface soil, resulting in the rapid response of soil salinity to groundwater level fluctuations. Simulation results further demonstrated the combined effects of tides and ETa on the salinity distributions, revealing the formation of a high salinity zone in the middle region due to the inflow of water from the tides and ETa-driven upward movements of water and salt; however, as salinity levels increased within the high salinity zone, the growth of Phragmites australis was constrained, leading to reduced transpiration (Ta). Tidal parameters such as amplitudes and mean water levels could affect salt accumulation processes in soils and further the high-salinity zone. Finally, inundation events from extreme upstream discharge had a flushing and diluting effect on accumulated salts, alleviating salinity stress and promoting vegetation Ta and growth; however, further increases in ETa led to salt re-accumulation, potentially reinstating the salinity distribution pattern after inundation. These findings enhance our understanding of ecohydrological processes in coastal wetlands and have important implications for wetland management and restoration.

A reference list of fish species for a heavily modified transitional water: The Zeeschelde (Belgium)

A crucial step in the development of a fish-based index for the ecological assessment of water bodies as provided by the European Water Framework Directive is to define a reference list of fish species occurring in pristine rivers. The aim of this study was to elaborate such a list. The reference corresponds to an ecological status that is referred to as Good or Maximal Ecological Potential (GEP/MEP). Based on historically-reported fish survey data of the Zeeschelde estuary (Belgian part of the Schelde estuary) and its tributaries, i.e. an affluent system, under tidal influence, presence/absence reference lists were compiled for different salinity zones and adjusted using information from recent catches. In addition, an MEP list of fishes occurring in the Westerschelde (Dutch part of the Schelde estuary), developed by JAGER &amp; KRANENBARG (2004), is provided to present a complete overview of the Schelde estuary. Inclusion of fish species in the reference lists depended on their natural geographical distribution and ecological demands. These reference lists contain guild-specific information for the different zones within the estuary and its tidal tributaries.

Fish assemblages across a salinity gradient in the Zeeschelde estuary (Belgium)

Between 1991 and 2008 a total of 71 fish species was recorded in the brackish and fresh water zone of the Schelde estuary (Zeeschelde). The results were obtained from fish surveys from the cooling water filter screens of the power plant at Doel (between 1991 and 2008) and fyke net surveys along the length of the estuary between 1995 and 2008. Species abundance in the different salinity zones was analysed using the fyke net data only. The ten most abundant species represent 90.8% of the total number of individuals caught. In decreasing order of abundance: flounder (Platichthys flesus), roach (Rutilus rutilus), herring (Clupea harengus), eel (Anguilla anguilla), pike-perch (Sander lucioperca), sole (Solea solea), common goby (Pomatoschistus microps), seabass (Dicentrarchus labrax), three-spined stickleback (Gasterosteus aculeatus) and white bream (Blicca bjoerkna). With fyke nets 33 species were caught in the tidal freshwater zone, 43 species in the oligohaline zone and 59 species in the mesohaline zone. Each salinity zone is characterised by a typical fish assemblage, although some species are shared between all three salinity zones: e.g. three-spined stickleback (Gasterosteus aculeatus), Prussian carp (Carrasius gibelio), roach (Rutilus rutilus) and eel (Anguilla anguilla). Diadromous species occur in all zones and make up, on average up 22% of the species richness. Freshwater species comprise about 70% of the species in the tidal freshwater zone. In the oligohaline zone the contribution of the freshwater species to the species richness is less while marine migrants become more abundant. As expected, the contribution of marine migrants and estuarine species is higher in the mesohaline zone. The recent increase in species richness in the freshwater and oligohaline zone coincides with a remarkable increase in dissolved oxygen since 2007.

The significance of farmers’ climate change and salinity perceptions for on-farm adaptation strategies in the south-central coast of Bangladesh

Climate change contributes to a rise in salinity levels in the coastal regions of Bangladesh, notably impacting agricultural productivity. Therefore, crop-level adaptation strategies against salinity are crucial to increase productivity. In this study, our objective is to explore farm-level adaptation to climate change-induced salinity in the south-central coastal area of Bangladesh, considering the farmers' perception of climate change and salinity ingress as well as their adaptation strategies. Subsequently, we compare our findings with climatic and salinity data acquired from secondary sources. The study area was partitioned into three distinct zones delineated by proximity to the coastline, and primary data was collected from 475 households within these salinity zones using a multistage random sampling technique. Data collection was carried out using semi-structured questionnaires, which had been pretested on the respondents' perceptions for validity and reliability. The results indicate that while farmers possess an awareness of long-term alterations in climatic conditions, such as changes in temperature and precipitation, they often fail to attribute these changes to climate change explicitly. They could perceive changes in salinity over time but had difficulty perceiving cyclonic events. Farmers realize the risks posed by hydroclimatic variability and extreme weather events. Interestingly, while farmers may not be taking explicit measures to address perceived climatic changes, we discern that they are indeed modifying their agricultural and farming practices, such as fertilizer application, land leveling, and freshwater application. Traditional farming systems increase vulnerability and reduce persistence. In pursuit of enhanced resilience, households must implement various adaptation strategies for resilient farming practices. Moreover, our findings indicate that farmers are interested in adopting diverse adaptation strategies that require technical and financial support, particularly for the smallholders. In conclusion, this research provides valuable information for formulating climate change adaptation policies in the context of coastal agriculture in Bangladesh.

Spatial and seasonal variability of dissolved metals in a monsoonal estuarine environment

Distribution of metals in estuarine surface water reflects their respective sources and abundances in the fresh- and marine- water end-members, mixing processes and ability to take part in various geochemical reactions prevailing in the transitional zone. However, these elements tend to show complex geochemical behaviour, which varies both within and among different estuaries. This makes it difficult to delineate a universal distribution pattern. In this study, the behaviour of multiple dissolved metals (Sr, Ba, Fe, Mn, Mo, V, Co, Cr, Cu, Ni, Zn, U and Pb) have been evaluated in a tropical monsoonal estuary located on the east coast of India during summer (May, 2018), monsoon (September, 2018) and winter (February, 2019) season. Concentration of Sr, Mo, V, Cu and U increases towards the mouth of the estuary, whereas, Ba, Fe, Mn, Pb and Cr concentration shows a declining trend along increasing salinity gradient. All the elements show non-conservative behaviour in the estuary during at least one season within the studied period. Rapid removal of Fe and Mn takes place in the low salinity region, whereas, Mo, V, Pb and U get removed in the low- to mid- salinity zone of the estuary as a result of salinity-induced flocculation. A pronounced mid-estuarine maxima is exhibited by Ni and Zn. Barium shows peak concentrations in the low salinity region. Spatial variation in the occurrence of Ba maxima along the salinity gradient exists between the low and high discharge periods. Seasonally, highest concentration of Sr, Mo, V and U are found in the summer, followed by winter and monsoon. On the other hand, Ba, Fe, Mn, Co, Cr, Cu, Ni and Zn exhibit the highest concentrations during the monsoon period as a result of increased inputs from fluvial sources. The intrusion of seawater during the dry seasons and dilution during monsoon, which shows the highest freshwater discharge, are responsible for the seasonal variations. Thus, this study highlights the variability in distribution of elements as well as the differences in the behaviour of individual elements under contrasting environmental conditions within an estuary.