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that every winter there has been a brief spike in the manganese concentration at this <br />outfall. Chart 1 presents a time series plot of average monthly manganese concentrations <br />since July 2007. This chart shows that the concentration has increased from very low <br />levels in December of each year to elevated levels in January and February. The <br />concentration then returned to low or non-detectable levels by April or May. Numerous <br />possibilities exist for this seasonality, e.g., the Moffat Pond freeze/thaw cycle, the <br />decomposition of organic matter in the Moffat Pond and/or the seasonal ice cover of the <br />Moffat Pond all resulting in the potential decrease in dissolved oxygen and subsequent <br />increase in manganese solubility. These scenarios may account for the seasonal spike in <br />manganese. It is important to note, however, that during only one of the previous three <br />winters did the concentration exceed the monthly average permit limit. Furthermore, no <br />individual sample has ever exceeded the daily maximum limit. Please note that there <br />have been three reported values since the January 2010 anomaly. The first two were well <br />_below th_e Hermit limit and the third was ap roaching the very low level typical of spring, <br />summer and fall. Therefore, the relative impact to Trout Creek regarding this sole permit,- <br />limit exceedance is minimal. <br />Chevron Mining will continue to monitor manganese at Outfall 004A two days each <br />month per CDPS Permit CO-0032638 requirements and track monthly values including <br />performing seasonal variation trend analysis. The WQCD will be notified regarding all <br />violations per rule as previously performed. <br />We appreciate the assistance WQCD personnel have provided in completing this <br />submittal. Should the CDPHE have any comments or concerns regarding this submittal, <br />please contact me at your convenience. <br />Sincerely, <br />Troy Summers <br />Project Manager <br />TNS <br />Enclosures <br />File: 99-144 <br />Cc: Leach (Chevron) <br />Musick (CDRMS) <br />Weinman (Permit)