Tackling Chloride Pollution in Southeastern Wisconsin: Halophilic Bacterial Indicators and Policy Alternatives

Abstract

There are few biological indicators for freshwater systems subjected to high chloride levels. Freshwater systems receive many forms of chloride such as road salts (e.g., NaCl, CaCl2, MgCl2), fertilizers (e.g., KCl), and year-round water softener pollution. The goal our study was to investigate Halomonadaceae populations as prospective biological indicators of chloride-impacted freshwaters. The bacterial family Halomonadaceae are halophiles that generally require the presence of salt to survive, which makes them an attractive candidate in determining chloride impaired areas. Field sediment surveys assessed how salt tolerant and halophilic bacteria abundance corresponded to chloride and conductivity measurements. Colony forming unit (CFU) counts on modified M9 6% NaCl plates (w/v) at urbanized sites compared to the rural sites had highest counts during winter and spring when chloride concentrations were also highest. Select CFUs identified as Halomonadaceae through 16S rRNA sequencing were kept as active cultures to determine the NaCl concentration and temperature preference that resulted in the isolates optimal growth. Isolates tested under 5°C (cold) grew optimally in 2% NaCl (w/v), whereas under 18°C (warm), isolates showed optimal growth at 6% NaCl. The majority of isolates had maximum growth in the warmer temperature, however, select isolates grew better in the cold temperature. Culture-independent methods were used and identified Halomonadaceae were widespread and permeant members of the microbial community in a Lake Michigan drainage basin. Quantitative polymerase chain reaction (qPCR) specifically targeting Halomonadaceae genera demonstrated that abundance varied by site, but overall were present throughout the year. However, community sequencing revealed there were a large relative proportion of unique Halomonadaceae populations in winter versus summer. Methods targeting salt tolerant bacteria and specific members of Halomonadaceae appears to be a promising approach to assess chloride-impacted areas to better understand the long-term ecological impacts as we continue to salinize freshwater resources. Furthermore, to better raise awareness of our findings and chloride pollution, we created educational outreach materials in the forms of a video series and classroom activities. Lastly, we took an economical approach and proposed a framework in valuing reduction in chloride loading, which could, if implemented, help inform winter road maintenance and stormwater management decisions.