**1. Introduction**

As the supply of fossil fuels diminishes, the opportunity of switching to renewable sources of energy will put an end to some of the negative environmental impacts seen since the first industrial revolution [1,2]. The oceans are a major source of renewable energies, such as marine and tidal currents, wave energy, thermal, and salinity gradients, which can all be harnessed [3,4].

Chemical energy known as salinity gradients (SGE) or saline gradient potential (SGP) is available in coastal zones where two water flows of different saline content coincide, e.g., where a river meets the sea [5,6]. By controlling this mixture and capturing the energy before it is released, electricity can be produced without greenhouse gas emissions. It is possible to use only naturally occurring water flows, but it is also possible to employ hybrid systems, which use effluents of anthropic origin, such as residual waters from desalination plants [7–9]. Similarly, the effluent from wastewater treatment plants, of low salinity, could be used as input for an SGP system [10,11].

The methods for producing energy from a saline gradient are varied, but the most advanced methods are reverse electrodialysis (RED) and pressure retarded osmosis (PRO), both of which have already been tested outside the laboratory. Regarding RED, the companies WETSUS and REDstack have developed a 50 kW device in the Netherlands [12,13], while for PRO, the company Statkraft developed a 10 kW plant in Norway [14].

**Citation:** Marin-Coria, E.; Silva, R.; Enriquez, C.; Martínez, M.L.; Mendoza, E. Environmental Assessment of the Impacts and Benefits of a Salinity Gradient Energy Pilot Plant. *Energies* **2021**, *14*, 3252. https://doi.org/10.3390/en14113252

Academic Editor: Eugen Rusu

Received: 24 April 2021 Accepted: 28 May 2021 Published: 3 June 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The International Energy Agency has reported that 15,102 TWh of electricity could be produced through salinity gradient in river mouths worldwide; that is 74% of global electricity consumption [15]. However, various physical and environmental limitations were not included in this estimation. Today, taking some of these restrictions into account, and counting only river mouths where this type of energy plant would be feasible, the estimation is 625 TWh, 3% of world consumption [5].

There have been many technological advances in PRO and RED around the world, but there is little information on the impacts the operation and maintenance of SGE plants could have on the functions of nearby ecosystems. The scientific literature surrounding the implementation of SGE at a given study site is scarce. Early works addressing environmental conditions to be monitored mention the amount of water to be extracted (defined as environmental flow, maximum extraction factor, extraction flow, design flow, the annual variation of flow, etc.), the physicochemical characteristics of the water, the physical and chemical characteristics of the input solutions (fresh, marine, treated) and other characteristics, such as salinity structure and temperature (temporal and annual variations) at the extraction and discharge sites [5,12,16–19]. Few works address the very important effects that the SGE implementation could cause on the sediment balance, care in the use of cleaning products (which when accidentally released pollute), and care and disposal of final effluents and membranes [16]. Other studies mention the importance of hydrodynamic studies and environmental forcings that may affect the thermohaline structure and therefore the amount of energy generated from the saline gradient [16]. Even so, there are very few case studies that mention potential environmental impacts. A study proposing a potential site for SGE at Lake Urmia, in Iran, (a Ramsar wetland with a Biosphere Reserve status with endemic species) only assessed in detail the economic implications of implementation [17]. One reason for this is that no operational devices exist.

Some papers mention that the impacts are similar to those of water treatment, desalination, or other renewable energy plants [1,18,20–22]. These works give an overview of potential impacts to habitat, local vegetation and associated fauna, water quality, sediment properties, and social issues related to fisheries and navigation rights and hydrodynamic modifications (changes in flows and their directions and mixing zones). All these impacts are caused by the location of the devices or their interactions with the environment. Specific work on the impact of saline gradient technology highlights potential impacts regarding water intake, final effluent disposal, and impacts associated with infrastructure [23]. However, the study in [22] summarises the overall potential impacts of SGE implementation using a three-phase scheme (construction, operation, and decommissioning).

This paper aims to present a scheme for an environmental impact assessment (EIA) that allows the identification of possible environmental impacts from the implementation of SGE in a coastal lagoon within an environmentally protected area. Through the description of stressors, receptors, and responses, an EIA is developed for the coastal system of La Carbonera, in the state of Yucatan, Mexico.

#### **2. Materials and Methods**
