Study region: Sharavati River, Karnataka, India. Study focus: A small mountainous river system, Sharavati, was selected to study the impact of river damming on the hydrological cycle. Sharavati river flow is regulated by two dams, Linganamakki and Gersoppa. Despite the Western Ghats' global significance in controlling local and regional climates, the effects of damming on its hydrological cycles have received limited attention. A stable water isotopic approach was employed in the study. New hydrological insights for the region: The line-conditioned excess (lc-excess) was primarily negative across all seasons. Notably, the pre-monsoon season exhibited comparatively higher evaporation with high negative lc-excess, while the postmonsoon lc-excess values approached zero, indicating minimal evaporation. The sampling points from the dams exhibited very high evaporation signals, the evaporative loss during the pre-monsoon season from the Linganamakki reservoir was estimated as 10 %, and from the Gersoppa dam was 6 %. Consequently, groundwater sampled near the dams, plotted along the local evaporation line indicating recharge from the evaporated reservoir water. Damming has affected the hydrological cycle of the heavily regulated Sharavati River, transforming the entire catchment into a connected, narrow lake-like structure, especially during the pre-monsoon season. Since the Western Ghat river systems are regulated by many large and small dams, it is pertinent to study the impact of damming on the hydrological cycles of the entire system.

A study was conducted for the first time to understand the geochemical processes controlling the chemistry of the Kali River in tropical southwestern India and to determine its chemical denudation and net carbon dioxide consumption. The samples were collected from the source to the mouth of the river (15 stations) during monsoon (July 2018), post-monsoon (December 2018), and pre-monsoon (May 2019) seasons. The catchment experiences intense chemical weathering on account of heavy rainfall accompanied by runoff during the summer monsoon. Seasonal variations in silicate weathering processes are significantly controlled by runoff, and their rates are proportional to the discharge. The annual chemical denudation rate (CDR) calculated exclusively from the upstream catchment that are dominated by silicate rocks estimated to be 48.2 tons/km(2)/yr with a silicate weathering rate (SWR) of 41.3 tons/km(2)/yr and carbonate weathering rate (CWR) of 6.9 tons/km(2)/yr. The CDR is two times higher than the global mean average rate. The mean CO2 consumption rate (CCR) for silicate weathering is 2.9 x 10(5) mol. km(-2) y(-1), which is three-fold higher than the global average. Silicate rock weathering intensity (Re) values indicate the formation of the gibbsite. The Re does not show variations from upstream to downstream, implying the rapid transport of weathered material from the river catchment area. Intense rain, runoff, and temperature are the dominant climatic factors that accelerate weathering in the study area. This underscores the significance of small mountainous coastal rivers as drivers of intense chemical weathering in humid tropical environments, which removes the atmospheric CO2. This study adds to the existing database on chemical weathering and associated fluxes in granitic catchments across the globe.

The present study aims to assess the extent of trace metal pollution in the sediments of Sita-Swarna estuary, west coast of India, and investigate their possible ecological risk on the aquatic environment. The sediment cores were analyzed for sand, silt, clay, organic carbon, and trace metals (Al, Fe, Mn, As, Cd, Co, Zn, Pb, Ni, Cr, and Cu) at 2-cm intervals. The study revealed that sediments have deposited in relatively violent to very violent hydrodynamic energy conditions. Factor analysis indicated that the metal distribution is mainly controlled by Fe-Mn oxyhydroxides and organic carbon. Further, the geochemical approach, pollution indices, and statistical evaluation revealed moderate pollution in the catchment. From an ecotoxicological perspective, the estimated risk index (RI) value was found to less than 150, indicating low risk for aquatic life. Thus, this baseline study would help to adopt strategies in pollution control and protect the fragile marine environment.

Tropical, small mountainous rivers draining granite/granitic gneiss exhibit intense weathering rates and associated carbon dioxide sequestration, which has implications on the global CO2 budget. However, there is paucity of data from these catchments. This study aimed to understand silicate weathering rates (SWR) and CO2 sequestration rates (CCR) in a small tropical mountainous river, Sharavati in the southwestern India. Bicarbonates, Cl-, Na+, Ca2+ and silica, are predominant in the river, indicating their source from catchment rocks and atmosphere. Groundwater shows a similar abundance of major ions, indicating identical sources. Intense chemical weathering due to hot and humid climate, heavy monsoonal rains and associated river discharge are the main controlling factors of major ion chemistry in the Sharavati river. The presence of clay mineral kaolinite in the catchment corroborates with the above controlling factors. The calculated silicate weathering rate (SWR) is 27 t km(-2). y(-1) and associated carbon dioxide consumption rate (CCR) is 3.9 x 10(5) mol km(-2). y(-1). When compared to other small tropical river basins having similar climate and lithology, CCR of Sharavati is comparable to Jiuhua Mountain rivers, (South China) and twice that of Sorocaba River (Brazil). CCR is 3.9 times higher than the global average on account of the peculiarity of the terrain. It can be concluded from a comparative study of small tropical rivers, that rainfall and runoff are the main parameters controlling the weathering rates irrespective of catchment lithology.

Trace metals act as a limiting nutrient and prerequisite for primary productivity in marine environments. The distribution of metals in dissolved phase along the salinity gradients of Swarna, Sharavati and Kali estuaries in southwestern India, during post and pre-monsoon seasons, were studied. We have investigated the behaviour of trace metals in the estuarine environment and their extent of impact on human health and ecosystem. The study revealed, non-conservative behaviour of dissolved Mn, Fe, Ni, Cd and Co in the estuaries. Whereas Cu behaved non-conservatively in post-monsoon and conservatively in pre-monsoon seasons. Risk assessment studies revealed that higher chronic daily intake (CDI) in humans, through dermal pathway, in Swarna and Sharavati estuaries during post-monsoon, whereas it was during pre-monsoon season in the Kali estuary. Hazard Index values for the studied metals in adults and children are below risk thresholds, though children are more prone to health risk through the dermal pathway.

There is very little knowledge on microplastic pollution in the Western Ghats (WG), a heritage site in southwest India. To address this, we have studied the spatiotemporal variations of sedimentary microplastics (MPs) from the River Sharavathi, a pristine river in the Western Ghats (WG), southern India. The rich biodiversity in the region makes it relevant to analyse the distribution of this emerging pollutant that is causing harm to the biota and the ecosystem. We analysed the sedimentological and carbon content (organic and inorganic) of these sediments and explored their relationship with MPs. Finally, risk assessment indices such as the Pollution Load Index (PLI), the Polymer Hazard Index (PHI), and the Potential Ecological Risk Index (PERI) were calculated to detect the levels of plastic pollution. The concentration of MPs ranged from 2.5 to 57.5 pieces/kg and 0 to 15 pieces/kg during the pre-monsoon and post-monsoon seasons, respectively. The dip in the MPs’ abundance during the post-monsoon season was due to the extremely high rainfall in the river basin during July–August 2019, which would have entrained the sedimentary MPs and transported them to the coast/Arabian Sea. Smaller MPs (0.3–1 mm) were more abundant than the larger MPs (1–5 mm), mainly due to the breakdown of sedimentary plastics by physical processes. Fragments, films, foams, and fibres were the main categories of MPs, and the main polymers were polyethylene, polyethylene terephthalate, and polypropylene. No significant relationship was observed between the sedimentological properties and microplastics, which may be due to the different physical properties of sediments and microplastics. The PLI, PHI, and PERI indices suggest different contamination levels in the river basin. Based on the PLI scores, all the samples belong to the hazardous level I suggesting minor risk category, and the risk of microplastic pollution falls under the high to hazardous risk category based on the PHI values. The PERI value ranged from 160 to 440 and 40 to 2240 during the pre-monsoon and post-monsoon seasons, respectively. The risk assessment in a region known for its rich biodiversity is crucial, as the data can be used by the district administration to mitigate plastic pollution.

Spatio-temporal behaviour of dissolved organic carbon (DOC), inorganic carbon (DIC) and silica (DSi) along the salinity gradient of three south-west Indian monsoonal estuaries are presented. This study showed both conservative and non-conservative behaviour of DOC along the salinity gradient under varying physico-chemical conditions. The gross flux of DOC arriving at the estuary from the rivers was estimated as 0.9 x 109 g/yr for Sita-Swarna river, 4.2 x 109 g/yr for Sharavati river and 5.6 x 109 g/yr for Kali river. Similarly, the net fluxes of DOC estimated beyond the estuarine zone was 5.5 x 109 g/yr (Sita-Swarna), 8.0 x 109 g/yr (Sharavati) and 7.7 x 109 g/yr (Kali). This indicates that these estuaries are the sources of organic carbon to the ocean. The DIC linearly increased towards the higher salinity with net fluxes of 38 x 109 g/yr (Sita-Swarna), 75 x 109 g/yr (Sharavati) and 97 x 109 g/yr (Kali). The combined DIC flux of Sita-Swarna, Sharavati and Kali rivers is -8% of the total DIC fluxes received from the west flowing rivers of India, to the Arabian Sea. The DSi showed a biogenic removal of 80?85% in all the studied estuaries. From this study it is concluded that the west flowing river estuaries are net sources of DOC and DIC and net sink for DSi. Consideration of the role of west flowing rivers of peninsular India is important for the better understanding of the carbon dynamics in the river-estuary-ocean boundary.