The World Health Organization (WHO) does not offer a single unified Water Quality Index (WQI) specifically for toxic chemicals in wastewater and lake water. However, they provide detailed guidelines on various pollutants, including heavy metals and pesticides. For assessing chemical toxicity in water, the U.S. EPA has extensive criteria, such as the National Recommended Water Quality Criteria, which include threshold levels for metals like mercury and lead. These are applied to both freshwater and saltwater, depending on environmental conditions. For more in-depth information, you can consult the EPA’s guidelines on aquatic life criteria, and the WHO's water quality guidelines, which cover health-related limits for a range of toxic substances in water.