Abstract
In this study, we demonstrate the impact of the construction of a mega-dam on the nutrient export regime of a large tropical river into the Arabian Sea. Long-term (11 years) fortnight nutrient parameters, upstream and downstream to Sardar Sarovar (SS) Dam, were examined to determine the periodical change in nutrient fluxes from the Narmada River, India. During this 11-year period, the average discharge of the Narmada River upstream to Rajghat (35.3 km3 year−1) was higher than that of downstream at Garudeshwar (33.9 km3 year−1). However, during the same period, the suspended sediment load was reduced by 21 million tons (MT) from 37.9 MT at Rajghat to 16.7 MT at Garudeshwar. Similarly, mean concentrations of dissolved silica (DSi) reduced from 470 (upstream) to 214 µM (downstream), dissolved inorganic phosphate (DIP) from 0.84 to 0.38 µM, and dissolved inorganic nitrogen (DIN) from 43 to 1.5 µM. It means that about 54%, 55%, and 96% flux of DSi, DIP, and DIN retained behind the dam, respectively. The estimated denitrification rate (80,000 kg N km−2 year−1) for the reservoir is significantly higher than N removal by lentic systems, globally. We hypothesize that processes such as biological uptake and denitrification under anoxic conditions could be a key reason for the significant loss of nutrients, particularly of DIN. Finally, we anticipated that a decline in DIN fluxes (by 1.13 × 109 mol year−1) from the Narmada River to the Arabian Sea might reduce the atmospheric CO2 fixation by 7.46 × 109 mol year−1.
Highlights
Water is a crucial requirement to meet surging demands and secure year-round water supplies for irrigation, industries, and human settlements
This study demonstrates the retention of dissolved inorganic nutrients in a large tropical reservoir located in Central India
No significant change in nutrient concentration was observed at the upstream monitoring station; at the station downstream to the dam, a significant decline in dissolved nitrate, nitrite, phosphate, and silica was observed
Summary
Water is a crucial requirement to meet surging demands and secure year-round water supplies for irrigation, industries, and human settlements. Downing et al [18] estimated that there are at least 0.5 million impoundments with a reservoir area of ≥ 0.01 km globally, covering > 0.25 million km of the earth’s land surface. It includes about 58,000 large dams with a height of ≥ 15 m [37]. 8000 km of water is stored behind large reservoirs [72], globally This transformation impacted rivers’ natural flow patterns, water exchange among various reservoirs, and downstream transport of riverine load. Reservoirs have substantially impacted the riverine transfer of dissolved and particulate loads. A plethora of studies have documented sediment trapping behind dams; dams’ influence on dissolved materials transport is poorly understood
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