The ion concentrations in Lake Onondoga are 10x typical values for freshwater lakes. Nitrites, excessive coliform bacterial levels, NH 3 are also very high in the lake. These water pollutants come from people, not the manufacturing processes. Compounds of it are almost always insoluble. Even still lakes, including Lake Onondoga, can become poisoned with mercury.
Mercury acts to effect health by interfering with enzyme function.
Heavy metal ions replace the ions needed for proper enzyme function, when the enzyme stops so does the biochemical processes. The mercury pollution found in Lake Onondoga is a result of a manufacturing process called the Chlor-Alkali process. The Hg leaked into the lake over time. Political problems exist to providing a concerted effort for cleanup. No one agency was responsible for oversight until very recently.