Chatfield Reservoir Reallocation Project
GEI has provided limnological and sediment consulting services to the Chatfield Reservoir Mitigation Company (CRMC) to assist with the monitoring requirements of the Adaptive Management Plan for the Environmental Impact Statement and to identify potential impacts to water quality by storing an additional 20,600 acre-feet of water in Chatfield Reservoir.
The reallocated storage in Chatfield Reservoir can seasonally increase the water elevation by approximately 12 feet above historical operating conditions depending on the water year-type and water availability. This increased water level raised concerns for a potential change in hypolimnetic oxygen and sediment redox conditions that may affect internal nutrient loading and increase algae production in the reservoir. The increase in water storage and expansion of the reservoir footprint also required setback of shoreline facilities, removal of trees in the inundation zone, and dredging of a new marina; all of which required different monitoring strategies to evaluate water quality pre/post construction.
Erosion from the Hayman wildfire burn scar in the upper watershed also resulted in the reservoir exceeding the mercury water quality standard. The increased sediment deposited in the reservoir, combined with the potential for an increase in reservoir anoxia has changed the redox sediment conditions which can facilitate the methylation of mercury and release of soluble nutrients, iron, and manganese from the sediments. In addition, the disturbance of sediment during dredge operations posed a concern for metals release, increasing the water quality risk to beneficial uses.
GEI developed and implemented water quality and dredge sampling and analysis plans to document total and methyl mercury concentrations in the sediment, water column, and walleye fish tissues to evaluate the potential for risk to human health. GEI also implemented studies to evaluate sediment oxygen demand, the soluble and recalcitrant fractions of sediment-bound phosphorus, and the anoxic release of soluble nutrients from sediment (internal nutrient loading) that facilitates algae growth.
GEI performs the monthly tributary input monitoring to document nutrients and suspended solids entering the reservoir during baseflow and stormflow conditions, maintains temperature data loggers, and performs clean sampling for reservoir total and methyl mercury analyses and fish tissue mercury analyses. Our analytical laboratory also provides low-level nutrient and chlorophyll-a analyses and subcontracts with laboratories for mercury, phytoplankton, and zooplankton analyses for the watershed and reservoir monitoring studies. GEI manages the CRMC database and compiles the annual water quality, phytoplankton, zooplankton, and sediment data for submittal to the U.S. Army Corp of Engineers to meet reporting requirements of the Adaptive Management Plan.