The phrase "elevated concentrations" means having higher amounts or levels of something compared to what is considered normal or expected. Full definition
Not only are elevated concentrations toxic to wildlife and even to people, but also even moderate concentrations can, if allowed to persist, fuel the explosive growth of algae. (sciencenews.org)
At the intermediate salt concentrations, populations previously exposed to elevated concentrations of salt had higher rates of survival than populations previously exposed to natural concentrations of salt. (news.rpi.edu)
Large parts of Boston remain at this baseline, but Phillips and Ackley recorded elevated concentrations in many other areas. (showcase.casw.org)