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<br />London Mountain Clean Up <br />July 2025 <br /> <br /> <br />G2-3 <br />parameters. Chronic aquatic life standards represent the most protective criteria for continuous <br />exposure scenarios and are the most relevant for assessment of ongoing mining impacts. <br />Cadmium concentrations at the DRMS site (0.0002-0.0003 mg/L) approach or exceed the <br />chronic standard (0.00025 mg/L), while lead concentrations occasionally exceed chronic criteria <br />by up to 400%. The DRMS on-site data shows the highest metal concentrations, particularly for <br />zinc, confirming the London Mine as a primary contamination source. <br />pH Compliance: All pH measurements (6.34-8.1) fall within Colorado stream standards (6.5- <br />9.0), indicating that while metals contamination is present, extreme acid conditions are not <br />occurring at these monitoring locations. <br />Station Comparison <br />The monitoring network reveals a clear spatial pattern of mining impacts. The USGS upstream <br />station shows no metals contamination, focusing on basic hydrologic monitoring. The DRMS <br />on-site station (midway through London Mine property) exhibits the highest metal <br />concentrations, confirming it as the primary source. The CDPHE downstream station <br />documents the transport of contamination beyond the immediate mine area. <br />The highest zinc concentrations at the DRMS on-site location (0.05-0.117 mg/L) compared to <br />downstream CDPHE measurements (0.012-0.049 mg/L) demonstrates dilution and attenuation <br />effects as contaminated water moves downstream, while still maintaining detectable <br />concentrations that contribute to the 303(d) impairment. <br /> <br />4. Project Relevance <br />Baseline Conditions <br />The historical monitoring data demonstrates existing water quality impairments in Mosquito <br />Creek directly attributable to legacy mining activities in the watershed, providing regulatory <br />justification for the remedial nature of the London Mountain Clean Up project. <br />Expected Benefits <br />Removal of pre-law mine waste piles is anticipated to: <br />• Reduce heavy metal loading to Mosquito Creek by eliminating primary contamination <br />sources <br />• Improve pH conditions by eliminating acid-generating materials <br />• Contribute to eventual attainment of water quality standards for zinc, cadmium, and lead