Evolution of the physico-chemical water quality in the Nam Theun 2 Reservoir and downstream rivers for the first 5 years after impoundment

Abstract

-The physical (temperature, conductivity and turbidity) and chemical water quality (O 2 , pH, conductivity, nutrients, major anions and cations) characteristics have been monitored in the Nam Theun 2 system (hydroelectric reservoir and rivers downstream) for the first 5 years after impoundment. The results emphasize that the impoundment of the reservoir induced a substantial modification of the water quality in the whole aquatic system which is partially controlled by the hydrodynamics in the reservoir. During the warm seasons, the reservoir water column is thermally stratified with a warm oxic epilimnion and a colder anoxic hypolimnion. During the cold dry season or during floods, the reservoir water column over-turns which enhances oxygen penetration down to the bottom waters and contributes to a global improvement of the water quality. Downstream of the reservoir, the oxygen level was always above the minimum requirement for aquatic life due to the specific design of the water intakes at the Nakai Dam and upstream of the Power House which both favour the withdrawal of a large proportion of oxygenated epilimnic waters. Over the five years, the water quality in the reservoir itself globally improved. However, the diversion of the Nam Theun River into the Xe Bang Fai watershed via the reservoir has impacted the Xe Bang Fai River in terms of temperature, conductivity and total suspended solids.

Low methane (CH<sub>4</sub>) emissions downstream of a monomictic subtropical hydroelectric reservoir (Nam Theun 2, Lao PDR)

“…2). As already shown in Chanudet et al (2015) and Guérin et al (2015), the reservoir was thermally stratified with higher temperature at the surface than at the bottom during the WD (surface: 26.8 ± 2.7 • C and bottom: 18.9 ± 1.6 • C) and WW (surface: 28.0 ± 1.6 • C and bottom: 21.5 ± 1.7 • C) seasons and it overturns during the CD season (average = 23.2 ± 3.9 • C; Fig. 2).…”

Direct fishing and eDNA metabarcoding for biomonitoring during a 3-year survey significantly improves number of fish detected around a South East Asian reservoir

“…There, the river joins the reservoir impounded in 2008. Downstream of the dam, a minimum environmental flow is maintained to supply the NTH [6]. On the other hand, the XBF has a typical morphology of highland river without visible strong gradient or large rapids [35].…”

“…However, hydropower reservoirs, the artificial lakes associated with dams construction, are recognized to modify local ecosystems by disturbing the river flows. Reservoir impoundment leads to characteristics change of water, either locally [6] or further downstream [7]. The reservoir also affects water velocity and transforms lotic environments, characterized by flowing waters, to lentic habitats, characterized by relatively still water.…”

Soils Drowned in Water Impoundments: A New Frontier

“…The monitoring of the complete emptying of the Guerlédan Reservoir in 1985 indicated high levels of dissolved iron (0.1 to 3 mg.l −1 ) in the water collected at the end of the complete emptying (Electricité De France, 2014). The discharge of red-colored water containing soluble iron during the first years after reservoir filling as well as after complete reservoir emptying has already been described in tropical reservoirs (Chanudet et al, 2016).…”