Desalination-induced salinity increases are not a cause of concern in the Gulf

You have recently released a paper titled: “Long-term, basin-scale salinity impacts from desalination in the Arabian/Persian Gulf.” What drew you to investigate this topic?

As of today, nearly half of the global freshwater production from desalination comes from plants located along the coastline of the Arabian/Persian Gulf. The amount of water treated is staggering: the current production already exceeds 20 million cubic meters per day of freshwater and will double by 2030, with no end to the growing trend in sight. The Gulf is also a very shallow sea (the average depth is just about 30m) connected to the rest of the world's oceans only through the narrow Strait of Hormuz. Therefore, concerns have been raised that ever-increasing brine releases from desalination plants could in the long term lead to a substantial increase in salinity in the entire Gulf.

In spite of the widespread oil and gas extraction activities, the Gulf is still a mostly uncontaminated body of water, which hosts important and unique marine ecosystems: coral reefs, uniquely adapted to extreme seasonal swings of sea temperature; seagrass meadows, home to the world's second-largest population of dugongs; mangrove forests, that can sequester from the atmosphere as much CO2 per unit area as a tropical rain forest. These ecosystems support thriving economic activities (mostly fisheries and tourism) but would be endangered by significant, basin-wide increases in water salinity. A handful of previous studies have yielded conflicting results: some didn't find a significant increase of salinity, while others did, with one of them reporting truly alarming results. This called for a thorough investigation that could simulate a vast number of scenarios, including the effect of climate change, and elucidate the underlying physical mechanisms affecting the salinity of the Gulf.

What were the main conclusions of the study?

We first compared the yearly amount of desalinated freshwater production with the data on the yearly net evaporation occurring from the sea surface. This means comparing the amount of freshwater drawn by human activities and that drawn from natural processes. We found that the levels of desalinated water production projected for the second half of the century will amount to about 10% of the net evaporation. This is a level where the impact of human activities cannot be automatically discarded as insignificant with respect to natural ones. We thus formulated a physical model of the overturning circulation of the Gulf. Its simplicity allowed us to explore several different scenarios, and an extremely wide range of desalination fluxes.

The levels of desalinated water production projected for the second half of the century will amount to about 10% of the net evaporation

Most importantly, it allowed us to identify positive feedback between brine discharge and the flux through Hormuz. The slightly saltier, shallow coastal waters sink into the deepest part of the Gulf (a wide underwater valley running parallel to the coastline of Iran, with depths gradually declining from -60m to -100m) and then flow along the bottom out of the Strait of Hormuz into the Indian Ocean. These volumes of water are replaced by much less salty Indian Ocean water flowing into the Gulf at the surface of the Strait of Hormuz. Any increase in salinity of the coastal waters makes the bottom water denser and heavier, and thus increases the rapidity with which it is expelled through Hormuz. Thanks to this feedback, it is very difficult to achieve any substantial, large-scale increase of salinity by adding brines along the coastlines, because doing so increases the rapidity with which they are flushed out of the Gulf. As a result, even with extreme amounts of desalination, we find increases in salinity smaller than the size of the observed salinity fluctuations due to natural causes (e.g., the seasonal cycle). Thus we conclude that desalination-induced salinity increases are not a cause of concern in the Gulf, at the basin scale.

Your research model is also the first to consider the possible future effects of climate change. What were your findings in this respect?

Our model is extremely simple, and we are currently working on more sophisticated general-circulation models that will be able to look at this question in more detail. However, as a preliminary investigation, we also run our simple model in a way that mimics IPCC's SSP5-8.5 (Shared Socio-economic Pathway n.5, forcing of 8.5 w/m2). This is an extreme climate change scenario where the average air temperature over the Gulf would increase by about 5ºC by the end of the century. Our model showed that warmer (and thus lighter) waters may throw a wrench in the feedback mechanism that flushes heavy, salty waters out of the Gulf. In the most extreme cases, the salinity increase in the coastal waters may increase by more than 1 PSU (practical salinity units). Of course, the rise in the Gulf region of the average temperature by 5ºC  would be an event capable, alone, of severely upsetting, and likely destroying, most of the marine ecosystems of the region (e.g. even the very resilient coral reefs of the Gulf underwent massive bleaching during the warm summer of 2017, with ecologically important genera, such as Acropora, becoming functionally extinct in most of the Southern Gulf). Thus, in the case of extreme regional warming, desalination activities would lead to a small, but measurable salinity stress added to the immense temperature stress due to climate change.

In the paper, you concluded that hypoxia, low or depleted oxygen levels in water, posed a larger threat to marine life than desalination brine discharge. What are the main causes of hypoxia in water?

Good question! Hypoxia is the major bane of marine ecosystems. Not only it stresses, and kills, if intense and prolonged enough, any multicellular organism (and many single-celled ones, too) but it also triggers several changes in the biochemistry of seawater (e.g., it causes the removal of nitrates, an essential nutrient of phytoplankton) which further fluster and disrupt the ecosystems. Hypoxia is well-known to occur in some deep, poorly ventilated water masses of the world's oceans (e.g., several hundred meters below the surface in the Arabian Sea, the portion of the Indian Ocean sandwiched between Africa and India). For those cases, there is a growing understanding of the physical and biogeochemical causes of low oxygen water content. But those mechanisms cannot be at play in the Gulf: it is just too shallow. In shallow waters, hypoxia is generally due to an overabundance of organic material, which is decomposed by bacteria that use oxygen for their metabolism. The typical example is the aftermath of water eutrophication, where after the end of an algal bloom, decomposing bacteria feasting on dead algae use up nearly all of the oxygen dissolved in the water column, killing most fish in the process. 

We recently published two papers, one focusing on coastal hypoxia, and the other on hypoxia in the central Gulf. In both cases, the oxygen levels are depleted mostly at the base of the water column. Our observations and modeling suggest the hypothesis that decomposition occurs in or close to the benthos (the sandy and muddy substrate that constitutes the seafloor). However, we still don't know which is the source of organic material that feeds the decomposers, thus we can't draw firm conclusions.

In your opinion, what are the environmental impacts of desalination in the Persian Gulf, and how are they being addressed?

It is really important to distinguish between local-scale impacts (in the vicinity of a desalination plant) and basin-scale impacts (which would involve altering the physical, chemical or biological conditions on areas spanning tens of thousands of square kilometers). There is an immense literature on avoiding, mitigating and/or managing the local scale impacts. Those are always present, but engineers are becoming better and better at reducing them to very low levels. Brine must be sufficiently diluted before being released in the environment. Recent research even shows that the aquaculture of halophytes may be part of this dilution process. Substances other than salt may be released in the environment by desalination plants. These are mostly heavy metals (whose concentration may escalate to toxic levels along the food network) and descalants (substances that avoid limescale encrustations, and generally maintain pipes and membranes clean). The amount and type of these substances strongly depends upon the specific desalination technology being used. Old technologies based on distillation are generally the worst offenders. For those, a proper management strategy calls, in addition, for the cooling and re-oxygenation of the brine, which comes out of the distillation stages as a hot, oxygen-poor fluid. More modern technologies, such as those based on reverse osmosis, greatly reduce these problems. Care in the selection of the descalants also reduces the impacts.

The potential for basin-scale impacts, on the other hand, is probably unique to the Gulf, because of its enclosed nature. Concerns that desalination activities could upset the Gulf-wide salinity balance were justified by the enormous amount of desalinated water produced along the shores of a small, extremely shallow marginal sea, connected to the rest of the world's ocean by a very narrow strait. Those concerns could have been legitimate, if it weren't for the vigorous overturning circulation, that flushes the entire Gulf out into the Indian Ocean in less than 3 years. In our study we focused on salinity, because any technology that extracts freshwater from seawater must leave some salinity-enriched water behind. We neglected heavy metals and descalants because those are technology-dependent, and are best managed by management practices that avoid their release in the environment in harmful quantities. We did not conduct a specific study on the policies (that's best left to social scientists). But the industrial data that we used for our study reveal that Persian Gulf countries have always been early adopters of new technologies. The existing distillation plants are generally old, and new installations have embraced the emerging membrane technologies. Neither I, nor my colleagues who spend a lot more time in the field than me, have ever observed any obvious signs of mismanagement of a desalination plant. Thus, I'm inclined to conclude that desalination in the Gulf has not had unbearable environmental costs, even in the vicinity of desalination plants.

In 2019, the UN called for improved brine management strategies due to a dramatic rise in the number of desalination plants posing major risks to ocean life and marine ecosystems. Your research contradicts the UN findings, does it not?

The rich Gulf countries have shown the viability of desalination as the main source of freshwater for arid lands close to the sea

No, it does not. The UN-backed paper (Jones et al. Sci.Tot.Env. 2019) offers a review on desalination technologies and calls for sound management strategies for minimizing local-scale effects. It is not concerned in the least on basin-scale impacts. We all agree that the leftovers of the desalination process cannot be casually dumped in the water, or severe environmental impacts will occur in the vicinity of the plant. Proper management, filtration and dilution are necessary. No doubt about this. What should be stressed, is that even that paper concludes that "There is a need to make desalination technologies more affordable and extend them to low income and lower middle income countries, increasing the viability of desalination for addressing SDG 6 in areas where developments have previously been limited by high economic costs." (SDG 6 is UN's 6th sustainable development goal: clean water and sanitation.) The rich Gulf countries have shown the viability of desalination as the main source of freshwater for arid lands close to the sea. All the metropolises of the Gulf depend on desalinated water to the point that they simply could not exist without it. As early adopters, the Gulf countries have effectively funded the development of new, better and cheaper desalination technologies, which can now be made available to developing countries.
 

Could your research be extrapolated to other areas of the world?

The potential for large-scale impacts was probably unique to the Gulf: if all the desalination plants of this region were moved, say, along the Indian coastline, the concerns connected with local impacts would remain, but the idea that desalination activities could upset the salinity balance of the entire Indian Ocean would simply be preposterous: an ocean is just too big! Even the Mediterranean Sea, which is closed by a narrow strait just like the Gulf, is too large and deep to experience measurable salinity increases due to desalination activities far from the desalination plants.