Clark, GF, NA Knott, BM Miller, BP Kelaher, MA Coleman, S Ushiama and EL Johnston. “First large-scale ecological impact study of desalination outfall reveals trade-offs in effects of hypersalinity and hydrodynamics”. Water Research. 145: 757-768. https://doi.org/10.1016/j.watres.2018.08.071
As the global population increases in a warming world, access to clean drinking water is increasingly at issue for millions of people. The World Heath Organization predicts that by 2025, half of the world’s people will be living in water-stressed areas. Dams, a common way of collecting and storing drinking water for centuries, are being recognized as damaging to freshwater ecosystems and watersheds and are increasingly being removed where possible.  Desalination plants are a more recent solution to the problem. They are particularly popular in arid, coastal countries, but are not without controversy of their own.
Desalination plants take saltwater and remove the salts, leaving behind drinkable, salt-less water. Initial concerns were focused on the hyper-saline wastewater released by these plants, and plants soon amended their design to incorporate high-pressure diffuser outfalls. These release the super salty brine at high speeds pointed upwards from the bottom into the water column, where, the theory goes, the plume will mix and diffuse with surrounding water, preventing organisms from confronting a wave of super high salinity water. Not until recently, however, have studies been completed to sufficiently test this theory.
MBACI – The Ferrari of ecological impact study design
The gold standard design for work of this type is called MBACI, which stands for Multiple Before-After-Control-Impact. This type of study uses multiple locations, divided into control and impact sites, and monitors them both before and after an impact event. Sometimes this is happy coincidence, for example when a hurricane plows across a 20-year study site, and sometimes the design is the result of an intentional schedule. In this case, a group of researchers in Australia was in place before a desalination plant began operations and remained monitoring even after operations ceased 2 years later, allowing for a unique glimpse into recovery.
First and foremost, the plume diffusers did in fact lower the bottom-water salinity to within one ppt (part per thousand) of the surrounding seawater, a different that most marine invertebrates would have no problems tolerating, as the authors point out. Water temperatures also didn’t differ. So, if neither of these major abiotic factors are the cause of the impacts, what remains? The authors posit that the strength of the flow created by the outfall plume itself is restricting “settlement, growth and survivorship” of invertebrates. At a first glance this, admittedly, sounds a little odd. Ocean currents are known to be variable, and at times strong. Many readers may have heard of rip currents or have seen storm surge. But the authors make a strong case.
In their study, the authors monitored several sites 30m (near) and 100m (far) from the desalination plant outfall plumes for six years and found clear negative impacts on sessile (largely stationary) invertebrate taxa, including polychaetes, sponges, and bryozoans, and positive impacts on barnacles and bivalves. Before operation, the median bottom-water current was 0.05 m/s and reached more than twice that (0.1 m/s) 10% of the time. During operation, the current 30 meters away was 0.25 m/s and 100 meters away was 0.1 m/s, approximately double of the before-outfall median current. This speed increase is known to restrict marine invertebrate settlement and feeding ability, particularly in the groups that showed the strongest population decreases: polychaetes, sponges, and bryozoans.
Polychaetes have slow-swimming larvae and are known to avoid areas with strong currents, possibly due to their delicate ciliated-tentacle crowns. Bryozoans also withdraw a ciliated feeding organ in strong currents, and sponges have a slow-swimming larvae. Conversely, barnacles, a group that showed population increases during operation, have strong-swimming larvae and have been shown to switch feeding mechanisms under strong current regimes from active feeding to passive suspension-feeding. Barnacles can also reorient their feeding structure (cirral fan) in turbulent flow, which is likely to occur in brine diffusion scenarios.
Possible ecological shift?
This MBACI study continued for 12 months after the desalination plant was decommissioned, and yet minimal recovery was seen for many taxa. This indicates a potential ecological shift towards a newly-stable invertebrate community resulting from an unexpected physical factor. While the hypersalinity seen in earlier plants was certainly an issue, an issue which has been successfully mitigated, projects that do not incorporate ecologists in the planning risk belatedly discovering unexpected ecological impacts. MBACI studies are the best way to discover and quantify these impacts if they are suspected. It is still possible that desalination plants are the best of a suite of ecologically-imperfect options, but if that’s the case, environmental engineers need to keep these long-terms impacts in mind.
 Warrick et. al. 2019. “World’s largest dam removal reverses coastal erosion.” Scientific Reports, 9.1: 1-12.