The Colorado Department of Public Health and Environment, along with Colorado Parks and Wildlife, monitor fish mercury levels in Colorado waters.

CDPHE and CPW have collected fish tissue samples at Gross Reservoir since 2007.  In 2015, the levels of mercury in lake trout exceeded the threshold established by the CDPHE for issuance of a fish consumption advisory and in 2016 the reservoir was added to the list of other bodies of water in the state to have FCAs. There are currently 22 bodies of water across the state on the list.

Mercury tends to accumulate in fish, but not in the water itself, so recreation and consumption of drinking water from Gross Reservoir is safe.

Mercury is a natural-occurring metal that is found in soil, rock, air and water.  The source of mercury in the environment is primarily from anthropogenic sources such as burning fossil fuels, which is transported to the aquatic ecosystem through atmospheric deposition. Fish are exposed to mercury that is in the water and their food. Nearly all fish have at least traces of mercury in them. The mercury can build up in their tissues over their lifespan.

According to the project’s Final Environmental Impact Study, “There may be a temporary increase in (methylmercury) concentrations in fish tissue in response to the proposed (Gross Reservoir) enlargement. This increase is not expected to be a long-term increase, but instead a temporary, post-inundation phenomenon that peaks in the years following the expansion and subsides over subsequent years.” (Chapter 4, Page 186)

One reason Denver Water must remove approximately 400 acres of trees and vegetation between the current and future high-water marks is to limit the volume of organic material that will decay in an expanded reservoir. It is this decay that releases organic matter and nutrients to the reservoir and influences mercury methylation.