Heavy Metals in Freshwater Ecosystems: Occurrence, Risk, and Analytical Advances

Introduction: Freshwater systems accumulate arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc from four broad routes: rock weathering, industrial effluent, agricultural runoff and atmospheric fallout. Because these elements persist and move up through food webs, they continue to pose a food-safety hazard. Objectives: To synthesise published research on the occurrence, chemical speciation and distribution of these metals in water, sediment and biological tissue. A further aim was to build a single source-to-health-outcome pathway across five connected themes: analytical progress, ecological risk indices, isotope-based bioaccumulation tracing, dietary exposure calculation and the regulatory landscape. Methods: Thirty-three field studies were compared in a structured way and are summarised in six tables and six figures. The comparison was organised around the five themes above and used to assemble the source-to-health-outcome pathway. Results: Instrumentation has advanced substantially (ICP-MS/MS, diffusive gradients in thin films, synchrotron micro-XRF and omics-based biomonitoring). Even so, sediment screening in practice still relies mostly on pollution indices that are rarely cross-checked against biologically validated guidelines. Paired stable-isotope analysis can now separate methylmercury’s trophic magnification from the sediment-linked pathway followed by cadmium, arsenic and lead, although few studies apply it. Dietary exposure is driven overwhelmingly by fish consumption. A January 2025 US EPA revision cuts the arsenic reference dose five-fold and raises the cancer slope factor thirty-two-fold, which substantially reshapes existing risk estimates. The reviewed studies rarely instrument more than two or three stages of the pathway simultaneously. Conclusions: The choice of analytical step, rather than the addition of more field sites, offers the greatest return for strengthening the evidence chain linking metal sources to health outcomes.