Elsevier

Applied Geochemistry

Volume 100, January 2019, Pages 64-76
Applied Geochemistry

Impact of seawater intrusion and disposal of desalinization brines on groundwater quality in El Gouna, Egypt, Red Sea Area. Process analyses by means of chemical and isotopic signatures

https://doi.org/10.1016/j.apgeochem.2018.11.001Get rights and content

Highlights

  • Influences of brine disposal from desalinization plants and seawater intrusion on groundwater quality in a coastal aquifer are quantified.

  • Br/Cl ratio, δ18O and δ2H in H2O are used as proxies for the primary sources of salinity.

  • Br/Cl ratio, δ18O and δ2H in H2O behave as conservative tracers in the reverse osmosis processes of desalinization.brine disposal leads to zones with increased salinity and saturation/supersaturation with respect to gypsum in the aquifer

  • Brine disposal leads to zones with increased salinity and saturation/supersaturation with respect to gypsum in the aquifer.

Abstract

The processes of seawater intrusion, groundwater abstraction and disposal of high concentrated effluents from desalinization plants (DSP) were investigated in a coastal aquifer on the Red Sea coast of El Gouna, Egypt. The feeding wells (depths from 20 to 150 m), the product water and the residual brines of three desalinization plants were sampled for chemical analysis and were partially sampled for stable isotopes analysis (δ18O and δ2H in H2O).

The wells in the aquifer abstract mixtures of brackish groundwater (total dissolved solids TDS 5–15 g/l) and seawater from the Red Sea (TDS 42–44 g/l) that intrudes into the coastal aquifers. Primary mixing rates were determined by means of the Br/Cl ratio and ranged from 5% to >90% seawater. The disposal of the highly concentrated residual brines from the DSPs via infiltration/injection into the used aquifer causes a locally strong increase in the salinization of the groundwater (up to 60 g/l TDS, significantly above seawater concentration) in wells that are actually feeding the DSPs (containing up to 60% brine). The influence of the brines on the groundwater could also be identified by the Br/Cl ratio in relation to salinization. Conversely, the primary source (the mixing ratio groundwater-seawater) of the brines from the DSP could also be identified by Br/Cl ratio and stable isotopes (δ18O and δ2H in H2O), which behave as conservative tracers in the reverse osmosis processes in the DSP.

The brackish groundwater shows partially gypsum saturation and supersaturation. Mixing of this groundwater with seawater additionally increases the sulphate concentration and cation exchange processes and especially the calcium concentration. The abstracted groundwater and groundwater-seawater mixtures therefore have a high potential for sulphate scaling. Calcite, as the second important phase for scaling, is generally supersaturated in groundwater, seawater and the mixtures. Consequently, the brines that are produced from these waters are highly supersaturated with respect to gypsum and calcite.

The injection/infiltration of these brines into the aquifers leads to extended zones of groundwater with increased salinity and saturation/supersaturation in respect to gypsum and probably intense gypsum precipitation.

Introduction

The growing population and economy in the Red Sea Area of Egypt increase the water demand in a region with an arid climate and a lack of fresh water resources. Beside freshwater pipelines from the Nile Valley to the Red Sea Coast, the desalinization of seawater and brackish water in coastal aquifers are the only common options to satisfy the increasing demand (El-Sadek, 2010; Hafez and El-Manharawy, 2002; Khalil, 2004). Due to the coastal outline and the dispersed locations of tourist, industrial and urban settlements, the sizes of the desalinization plants are relatively small, with capacities of approximately 100 to approximately 1000 m³/d (El-Sadek, 2010).

The treatment of seawater from the Red Sea has some particular problems:

  • salinity with a TDS of 42.5–44.5 g/l (20–25% higher than standard seawater ≈ 35.5 g/l);

  • high sulphate scaling potential due to high sulphate content and a high ratio of sulphate/alkalinity (Hafez and El-Manharawy, 2002); and

  • richness in biota and suspended matter, which often require a pretreatment (Hafez and El-Manharawy, 2002).

The use of brackish water in the coastal aquifers (TDS from >2 to 10 g/l) of the Red Sea area is related to other problems:

  • normally non-renewable, finite resources, often with a lack of systematic investigation;

  • Salt layers underneath the coastal aquifers can lead to a strong increase in water salinity with increasing depth, and in case of abstraction from these aquifers, the upconing of high saline groundwater may occur;

  • The coastal aquifers are often in hydraulic connection with the Red Sea, so seawater intrusion might be a problem in case of groundwater abstraction in the coastal aquifers (investigated in a similar environment, i.e., by Eissa et al., 2016 at the Gulf of Aqaba).

Beside the problems of the economic abstraction and treatment of the saline and/or brackish water, the disposal of the hypersaline effluent from the desalinization plants is a common problem for a sustainable use of desalinization. The long-term effect of brine discharge into the environment has not been investigated sufficiently yet. In general, some models have been developed to check the distribution of the plumes (Del Bene et al., 1994; Doneker and Jirka, 2001; Purnalna et al., 2003). Other investigations were undertaken to study the effect of the brine disposal on the marine environment (Chesher, 1975; Höpner and Windelberg, 1997; Mabrook, 1994; Morton et al., 1997). In case of some inland desalinization plants where the brine has been discharged either in open pits or directly on the surface (Mohamed et al., 2005; Someswara et al., 1990), the soil salinity increased as well as the salinity of the feeding water where the discharge points were not far from the intake locations. The salinity of the aquifers can also increase through leakage from pipelines conducting the brine (Sadhwani et al., 2005; Tularam and Ilahee, 2007) or through discharging the brine directly into the underground (Baalousha, 2006).

This study aimed to analyse and quantify the impact of the disposal of saline effluents from desalinization plants on the groundwater chemistry in an arid environment with different sources of salinity (seawater, dissolution of evaporites and disposal of brines from DSP) to show methods for differentiating the influence of those sources and to show the interactions of those sources in hydrochemical processes (brine disposal, seawater intrusion, dissolution/precipitation of minerals, and ion exchange) on different parameters.

Section snippets

Water supply in the investigation area

El Gouna town is located directly on the Red Sea 20 km north of Hurghada City (Fig. 1 a) and situated in the Eastern Desert of Egypt. It was founded as a tourist resort in 1989. The population number was estimated as 6400 in 2004, while the present population is estimated at approximately 15,000 inhabitants, depending on the tourist volume.

The climate is characterized by very hot dry summers and mild winters. The temperature can reach 40 °C between July and August and decreases to 20 °C in

Sampling and chemical analytics

Samples from groundwater, seawater and desalinization plants were analysed between 2014 and 2018 to investigate the hydrochemical situation and processes in the area. Information such as depth of boreholes, length of casings and position of screens was unavailable for many of the wells. In addition, the exact positions of the wells are not documented and were estimated using a GPS-handheld device during the sampling campaigns (with an accuracy of ± several metres). Additionally, the well heads

Hydrochemical groups and sources of groundwater in the investigation area

The groundwater in the investigation area shows a broad range of salinizations from brackish to saline, with approximately 5–60 g/l of total dissolved solids (TDS). In some wells, the salt content in the groundwater even exceeds the salinization of the Red Sea (with 37–44 g/l TDS). The groundwater analyses presented in Table S - 1 can be interpreted as mixtures of three hydrochemical groups with different source and chemical characteristics:

  • a)

    natural brackish groundwater in the south and west of

Conclusions

The groundwater in the coastal aquifers of the investigation area is a mixture of brackish waters and intruded seawater. The impact of seawater intrusion on the groundwater decreases with growing distance from the coast. Seawater intrusion is forced by the continuous abstraction of the non-renewable brackish groundwater, which is a problem for the region due to its arid climate and negligible actual groundwater recharge.

The mixing ratio of seawater and groundwater in the wells can be identified

Acknowledgements

The authors would like to thank Mr. Hilal from the water supply authority of El Gouna for enabling the access into the wells on the El Gouna farm and the desalination plants. They also thank Dr. N. Tsoupanos and Mr. Bollkemper from the chemical laboratory at the TU Berlin-Campus El Gouna, Department of Water Engineering.

Many thanks go to an anonymous reviewer for his thorough reading and critical and constructive comments.

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