Bringing Life Back to Salty Rivers

Chloride contamination in outdoor rivers—often caused by road salt runoff, industrial discharge, or agricultural drainage—poses a growing threat to freshwater ecosystems. Unlike many pollutants, chloride cannot be easily broken down or filtered out by conventional methods. Biological approaches, however, offer a promising and sustainable path forward.

Constructed wetlands are among the most effective biological solutions. A multi-zone constructed wetland built on the Sokolowka River in Poland, designed to purify urban stormwater, removed 64% of chloride alongside significant reductions in suspended solids and nutrients. Similarly, wetlands placed at stormwater culvert outlets can delay peak salinity arrival by up to 30 days and reduce peak concentrations by 20–60%. These systems work through a combination of sedimentation, plant uptake, and microbial processes.

Phytoremediation—using salt-tolerant plants to absorb chloride—is another key strategy. Common reed (Phragmites australis), sea rush (Juncus maritimus), and cattail (Typha latifolia) have demonstrated strong chloride accumulation in their aboveground biomass. In a surface-flow constructed wetland, Phragmites australis reduced chloride concentrations by up to 92% under certain conditions. Wetland plants like Carex riparia and Phalaris arundinacea can tolerate chloride levels of 50–350 mg/L and accumulate chloride in harvestable shoots. Regular harvesting of plant biomass is essential, as it permanently removes the absorbed chloride from the water body.

Microbial bioremediation complements these plant-based methods. Halophilic and halotolerant bacteria—such as Halomonas species—can thrive under high-salinity conditions and remain active in nitrogen removal, achieving over 90% ammonia removal even at 10% salinity. Salt-tolerant bacteria have been successfully acclimatized in pilot-scale treatment systems by gradually increasing salinity loads. These microorganisms help maintain overall water quality even when chloride levels are elevated.

Integrated approaches that combine plants and microorganisms offer enhanced performance. Floating treatment wetlands and ecological ditches with immobilized salt-tolerant bacteria have shown improved nutrient removal under saline stress. These systems can be deployed directly in rivers, bank zones, or constructed wetlands, making them suitable for in-situ remediation of outdoor water bodies.

In practice, the most effective strategy for chloride-contaminated rivers is rarely a single method. Combining constructed wetlands with salt-tolerant vegetation, supplemented by microbial inoculation and regular biomass harvesting, creates a resilient biological treatment train. Site-specific factors—salinity level, flow rate, climate, and available space—should guide the selection and configuration of these biological tools. While biological methods may not eliminate chloride entirely, they can significantly reduce concentrations and restore ecological balance to degraded rivers.