Experimental-numerical simulation of soluble formations in reservoirs

https://doi.org/10.1016/j.advwatres.2021.104109Get rights and content

Highlights

  • Gypsum rocks in parsian reservoir enhances risk of water quality degradation.

  • Dissolution rate was estimated between 0.023 and 0.056 gr/cm2-day through experiments.

  • The reservoir salinization was modeled by CE-QUAL-W2 under different scenarios.

  • Dissolution could enhance the average TDS of the dam outflow from 1429 to 1722 mg/L.

  • Omitting a saline inflow river at upstream reduces TDS in outflow to 900 mg/L.

Abstract

Saline inflows and soluble geological formations outcropped in reservoirs contribute to reservoir water salinization (RWS). We set up 32 laboratory tests to better understand rock-water interaction and determination of dissolution rate of Gachsaran geological formation (GGF) in the Parsian dam that is currently under the study phase on the Fahlian river, Iran. Then, we used CE-QUAL-W2 (W2) to simulate GGF dissolution outcropped in some parts of the reservoir. Finally, possible solutions for mitigating the RWS were investigated using W2 driven under different scenarios: (1) having Hamze-beigi saline inflow (a main tributary of the Fahlian river) and GGF dissolution, (2) removing the saline inflow, (3) confining the GGF dissolution, and (4) driving W2 in severe floods. Although the dissolution rates (0.023–0.056 gr/cm2-day) salinize deep water, saline inflow degrades surface waters. Density gradient in the water depth formed the monimolimnion and mixolimnion separated by a distinct pycnocline in the reservoir. The pycnocline established in the early impoundment stage and lasted throughout the study period (2002–2012), leading to a crenogenic meromixis state in the reservoir. W2 driven under the 100–year and 1000–year inflows shows salinity up to 2700 mg/L in top layers from partial chemical mixing in the reservoir in cold months, although no conversion of the lake from the meromixis to holomictic state was observed in warm months. The findings suggest that, although diversion of the saline inflow satisfies the salinity requirement for hydropower generation, peak saline inflow can salinize the reservoir water and downstream river.

Introduction

Water storage in reservoirs usually increases the water residence time that can lead to negative impacts on water quality (Kadivar, 2007; Noori et al., 2018; Noori et al., 2019a; Aradpour et al., 2021). Reservoir water salinization (RWS) emerges when soluble geological formations outcrop within the reservoir area, a relevant threat in many regions of Iran (Naderkhanloo et al., 2017) that is poorly understood. A recent example of this problem is the salinization of the Gotvand dam reservoir located in the southwest of Iran due to the presence of halite in the reservoir location, which has caused serious concerns about the operation of this dam in the future (Naderkhanloo et al., 2017; Jalali et al., 2019).

The Parsian dam, as a hydropower dam, is currently under the study phase on the Fahlian river that flows through gypsum-type formation, called Gachsaran geological formation (GGF) in Iran. Limestone compounds have been expanded in some parts of the reservoir and their main component is gypsum (CaSO4.2H2O). Considering the high solubility of CaSO4.2H2O, there are serious concerns regarding the RWS when the Parsian dam reservoir (PDR) is impounded (Baghdardokht and Heidari, 2005). The problem arises when the GGF dissolution rate is not fully understood. On the other hand, it is not recommended to use the suggested dissolution rates for different gypsum-type geological formations due to the high range of uncertainty reported around the world (Raines and Dewers, 1997; Klimchouk and Aksem, 2002; Baghdardokht and Heidari, 2005; Aljubouri and Al-Kawaz, 2007; Mbogoro et al., 2011; Valor et al., 2011; Lebedev, 2015; Domínguez-Villar et al., 2017; Feng et al., 2017; Hong et al., 2018; Tang et al., 2018). Therefore, attempts to understand the rock-water interaction and the GGF dissolution rates in the PDR should be made through laboratory tests or field measurements. Saline inflow, mainly from the Hamzeh-beigi reach (a tributary of the Fahlian river), can also contribute to the RWS once the PDR is impounded. Therefore, the question raised here is how great the effect of GGF dissolution outcropped in the reservoir would be on the RWS. The next question centers around the impact of Hamze-beigi saline inflow and its influence on the RWS. Since there is a hydroelectric power plant in Parsian dam, its activation in the operation phase makes constant oscillation at the higher levels of the reservoir which leads to increasing the vertical velocity at the top layers. Therefore, it is important to address the effect of water oscillation on RWS in top layers, where the water is released for hydropower generation. Answering these questions can help decision-makers to choose the optimum operation policy and consider the possible change in dam location to avoid the severe RWS in practice.

To answer the questions raised above, the salinity distribution should be simulated across the reservoir under different initial and boundary conditions using mathematical models. In this study, we explored the GGF dissolution rates in the PDR through laboratory tests using a physical model, under different conditions of flow velocity that exactly take place in different depths of Parsian reservoir. Then, we connected the physical model's results of the specified dissolution rates to CE-Qual-W2 (W2) model to further explore the RWS under different scenarios of reservoir operation. Actually, the definition of dissolution rate in the W2 is not possible simply, and it needs to improve the model's ability using a creative technique. Here, we fed the GGF dissolution as a pollution source through virtual tributary inflows with low flows and high concentrations of total dissolved solids (TDS) to the W2 model. It should be noted that we selected the W2 model, a well-known reservoir hydrodynamic and water quality model (Cole and Wells, 2015), as it has been successfully used for water quality simulation in different reservoirs around the world (Kim and Kim, 2006; Ma et al., 2015; Noori et al., 2015; Zhang et al., 2015; Ji et al., 2017; Al-Zubaidi and Wells 2017; Sadeghian et al., 2018; Kim et al., 2019; Lindenschmidt et al., 2019; Mi et al., 2019, 2020; Al-Zubaidi and Wells, 2020; Morales-Marin et al., 2020). This work aims to contribute to future recommendation on dam construction to account for saline inflows in regions with soluble geological formations outcropped in the lake area to limit RWS.

Section snippets

Study area

The Parsian dam is planned to be constructed on the Fahlian river in the Fars province, south of Iran. The main purpose of the dam is hydropower generation as well as some secondary goals, including supplying water for agricultural, industrial and domestic purposes. The geographic location of the dam is 51º 40´ E and 29º 45´ N (Fig. 1 and Movie S1). According to a 43-year historical data, the average annual precipitation, air temperature, and potential evaporation in the study area,

TDS equilibrium

After pilot preparation, GSg and GSmg samples were placed in the pilot and water circulation was started. TDS was recorded every six hours from the test onset until the TDS variation was very low (i.e., equilibrium state). Results showed that 6–8 days of water circulation was enough to reach an equilibrium state. Due to the number of experiments (32 tests) and the two available pilots, conducting the tests lasted more than four months. The TDS changes in the experimental tanks under different

Conclusion

The Parsian dam is under the study phase in the south of Iran. Given Ghachsaran geological formation in the reservoir area and a saline tributary inflow to the dam, water salinization is a serious concern for reservoir impoundment. Due to the importance of this issue, a physical pilot was conducted and regular tests were performed to estimate the Gachsaran formation dissolution rate. In the next step, a hydrodynamic and water quality mathematical model (CE-QUAL-W2) was applied to simulate the

Data availability

The raw data used in this study are available via Data Archive of the Iran Water Resources Management Company http://wrs.wrm.ir/amar/login.asp.

CRediT authorship contribution statement

Negar Tavoosi: Data curation, Formal analysis, Software, Writing – original draft, Writing – review & editing. Farhad Hooshyaripor: Conceptualization, Methodology, Formal analysis, Writing – original draft, Writing – review & editing. Roohollah Noori: Conceptualization, Methodology, Supervision, Writing – original draft, Writing – review & editing. Ashkan Farokhnia: Methodology, Validation, Software, Visualization, Writing – review & editing. Mohsen Maghrebi: Formal analysis, Visualization,

Declaration of Competing Interest

None.

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