**1. Introduction**

Freshwater demand is increasing worldwide due to a variety of factors, including population growth, agricultural expansion and environmental changes [1]. At the same time, natural freshwater

resources are declining in quantity and quality [2]. In California, increasing agricultural activity and population growth have diminished natural groundwater reservoirs, resulting in substantial land subsidence and seawater intrusion [3]. Recurrent droughts that limit recharge of water reservoirs are common and are expected to increase in frequency, duration and intensity [4,5] exacerbating the problem. Seawater desalination by reverse osmosis (SWRO) is increasingly being seen as a way to counter freshwater shortages, and several desalination plants are being built or proposed along the Southern and Central California coastline [6,7]. Ten small seawater reverse osmosis (SWRO) desalination facilities with individual capacities from 30 × 10<sup>3</sup> to 10 × 10<sup>6</sup> L day−<sup>1</sup> (combined capacity ~20 × 10<sup>6</sup> L day−1) [8] are currently operating along the California coast, and one large-scale plant with a capacity of 180 × 10<sup>6</sup> L day−1, started operation in Carlsbad (Southern California) in December 2015. Seven additional large-scale facilities have been proposed (one is under construction) with capacities of 40–500 × 10<sup>6</sup> L day−<sup>1</sup> [8] in response to reoccurring droughts and increasing demand for water resources.

SWRO facilities draw coastal water as feed and continuously produce high salinity brine effluent that is typically diluted and discharged back into coastal environments. The discharge occurs either directly at the shoreline through outfall channels, or further from shore through pipes or diffusor systems [9–11]. Californian regulations require that the brine discharged not exceed two salinity units above ambient levels within 100 m offshore from the discharge point [12]. The efficiency of water mixing and the footprint of the discharged brine depend on (1) dilution prior to discharge and hence the final density of the discharging brine, (2) local coastal conditions (waves, currents and bathymetry), and (3) the design of the discharge method (channel, pipe or diffusor). To comply with these regulations, SWRO facilities need to control the salinity of the brine (by dilution prior to discharge) and select a discharge design that ensures easy mixing of brine with the surrounding seawater under the specific oceanographic conditions at the discharge location.

Water mixing potential at the discharge zone is usually evaluated prior to operation by using hydrodynamic computer models to ensure compliance [12]. While the California Ocean Plan (2015 Amendment to Water Quality Control Plan) specifies a salinity impact zone extending no more than 100 m offshore, the Carlsbad Desalination Plant received an exception to policy to extend its salinity impact zone to 200 m offshore, due to its high capacity [12]. For the Carlsbad Plant, the hydrodynamic model assumed a starting salinity of 42 at the outfall, and predicted that salinity would decline to 35.5 (ambient 33.2) at a distance of 196 m offshore (i.e., ~2 salinity units above ambient within the 200 m permitted limit) [13]. The brine produced at the Carlsbad SWRO facility is diluted by mixing with seawater used for cooling at a co-located power plant, and hence decreasing the brine salinity and increasing water temperature, hence reducing the brine density to increase the mixing potential and prevent bottom ponding of a high density brine.

Despite increasing use of SWRO desalination worldwide as well as in California, impacts of brine effluent discharge on the living organisms and ecosystems in the coastal environments are ill-constrained [14,15]. Past research on pelagic phytoplankton and benthic microbes, seagrasses, polychaetes and corals demonstrate that salinity tolerances are highly variable among species and also dependent on the magnitude of the salinity increase and exposure time [14,16–21]. For instance, seagrasses have low thresholds with a detectable mortality at salinity of 5% above ambient levels, whereas coral growth is not impacted at salinity as high as 10% above ambient [22–24]. Relative abundances and growth rates of phytoplankton, zooplankton, and benthic bacteria also do not seem to be significantly impacted at salinities of 10% above ambient, but community structure often changes [16,17,25,26]. The Benthic Opportunistic Polychaetes and Amphipods index (BOPA-index) is commonly used as an indicator of the level of "disturbance" to benthic communities in areas impacted by pollution [27–30], or other stressors such as changes in salinity [31]. This index is based on an inverse relationship between the abundances of sensitive amphipods and opportunistic polychaetes [27,29,30,32]. The index value specifies an ecological status ranging from "good" to "bad", where good is defined as an area dominated by sensitive species, and bad an area dominated mainly by opportunistic species.

Coastal California is a highly productive zone supported by upwelling of nutrient-rich sub-surface water. This productivity supports large kelp beds, productive rocky reefs with high biodiversity, and rich plankton communities that serve as food for numerous fish, seabirds, whales and dolphins. However, the population density along the coast in California is high (26 million people living in coastal counties) and many costal settings have been impacted by a wide range of anthropogenic activities including dredging, shipping, sewage discharge, eutrophication, commercial and recreational fishing and more transitional waters [33]. Along the state coastline, 124 marine protected areas (MPA's) are providing refuge for the ecosystem (total coverage of 16% of state water) [34,35]. Toxicity testing with high-salinity seawater has been conducted on a few key Californian rocky-reef species (i.e., *Haliotis rufescens* (Red Abalone), *Strongylocentrotus purpuratus* (Purple Urchin) and *Dendraster excentricus* (Sand Dollar)) and all proposed SWRO desalination facilities are required to use hydrological modeling to estimate the impact area of discharging brine [12,36]. However, uncertainties and concerns persist regarding potential impacts of SWRO desalination brine on coastal environments, especially among coastal users and the general public [37,38].

This study characterizes the spatial footprint of the discharge plume from Carlsbad Desalination Plant, both chemically and biologically, with the ultimate goal of informing legislators, regulators, plant managers and the public about appropriate locations and discharge methods for future SWRO plants and illustrates the need for monitoring. The extend and impact of the discharge zone is characterized by using (1) water samples collected at and around the outfall channel of the Carlsbad Desalination Plant, before and after the plant became operational (in December 2015), (2) biological surveys of benthic epifauna, (3) a BOPA analysis around the discharge zone, and (4) a laboratory bioassay with brittle stars (*Ophiothrix spiculata*). The study finds the brine plume to extend beyond the 200 m impact zone allowed in the California Ocean Plan (2015 Amendment to Water Quality Control Plan) but finds no significant impact on the benthic ecology. Using a model of coastal wave energy at Carlsbad Beach and in Southern and Central California, possible impact of future desalination plants in these California coastal zones is assessed.
