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Historic mining activities at the London Fault Mine in Park County, Colorado, generate uncontrolled mine drainage which impacts water quality in South Mosquito Creek and Mosquito Creek. Water quality impacts have lead to 303(d) listing for both streams based on exceedances of chronic
zinc table value water quality standards and the need for a total maximum daily load (TMDL). Historically, zinc concentrations in South Mosquito Creek zinc ranged from 110 µg/L to 749 µg/L and 95 µg/L to 750 µg/L in Mosquito Creek. The chronic zinc table value standards
for South Mosquito Creek and Mosquito Creek are 250 µg/L and 110 µg/L, respectively. Historically, two mine adits at the London Fault Mine, the Extension Tunnel and the Water Tunnel, have had uncontrolled discharges of metals laden water to South Mosquito Creek which impacts
the quality of Mosquito Creek, located downstream. Currently, only the Water Tunnel is permitted under the Colorado Discharge Permit System and the Extension Tunnel is unregulated, but provides a zinc load to South Mosquito Creek five times greater than the Water Tunnel. Since multiple years
of discharge permit negotiations for the Extension Tunnel have been unsuccessful, alternative approaches to regulate both discharges were investigated, including obtaining funding from the owner (THF Reality) of the water right at the Water Tunnel to support site-specific studies. Since
limited funding was available from the owner of the London Fault Mine, Brown and Caldwell, through support from THF Reality, developed a multi-pronged approach to support appropriate South Mosquito Creek and Mosquito Creek beneficial uses while cost effectively allowing the Water Tunnel discharge
to provide augmentation water for downstream water supply needs. The multi-pronged approach included: (1) evaluating treatment alternatives for both the Extension Tunnel and Water Tunnel to meet the proposed draft TMDL wasteload allocation for zinc; and (2) develop site-specific zinc water
quality standards to reflect South Mosquito Creek and Mosquito Creek actual physical, chemical and biological conditions. Through the multi-pronged process a technology-based approach was developed and approved by the Colorado Water Quality Control Division and Commission. The technology-based
approach focused on developing site-specific chronic zinc water quality standards using ambient (85th percentile) water quality as the standard, with treatment of the Extension Tunnel discharge. This approach provided substantial improvements to South Mosquito Creek and Mosquito
Creek water quality, substantial cost savings by treating the Extension Tunnel discharge versus the Water Tunnel discharge and TMDL compliance.
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