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4.0 RELATIONSHIPS BETWEEN PRECIPITATION, CARLTON TUNNEL FLOW RATES, AND ZINC CONCENTRATIONS This section discusses the relationships between higher than normal precipitation events, higher flows in the Carlton Tunnel, and elevated zinc concentrations in the Carlton Tunnel water. 4.1 Precipitation and Flow at the Carlton Tunnel Between the dates of April 21 and June 1, 1999 (41 days), flow from Carlton Tunnel rose from 1,239 gpm to a peak of 1,887 gpm (a change of 648 gpm) (Figures 3 and 4). This corresponded [o the period of record rainfall in spring and summer of 1999 that is discussed above and that contributed to the upset condition. There aze three storm events that could potentially have caused the large increase in flow from the Carlton Tunnel. A snowstorm occurred between April 1 through 4, 1999 which produced the equivalent of approximately 0.4 inches of water (Figure 4). A second snowstorm, that produced the equivalent of 1.16 inches of water, was recorded between April 21 and 25 at the Bateman • Creek weather station. A third snowstonn began on April 28 and ended on May 1, 1999, and produced anear-record accumulation of 42 inches. Rain gauges at FFB, Guffey, Bateman Creek, and Hunter show that the April 28 storm event contained the equivalent of between 1.5 and 3.4 inches of water depending on the sampling station. Because these precipitation events occurred as snow, much of their water content is expected to have infiltrated into the subsurface as the snow slowly melted. Shortly after the series of three snowstorms in April of 1999, flow rates from the Carlton Tunnel began to increase (Figures 3 and 4). Above average rainfall in May through August of 1999 (Figure 2) likely contributed to the increased flow rates from the Carlton Tunnel and maintaining flows at high levels through the summer. 4.2 Flow and Zinc Canceotrations in the Carlton Tunnel At about the same time that flow rates from the Carlton Tunnel increased after the 1999 spring storms, zinc concentrations in Ilse flow also increased (Figures 5 and 6). Between the dates of . April 21, 1999 and May 19, 1999 (28 days) potentially dissolved zinc concentrations in the Carlton Tunnel increased from 0.036 mg/L to a peak of 0.78 mg/L. The data show that there is a Cripple Crcek & Viemr Gold ,4lining Compnny Shepperd ,41i!lcr, lnc. ^D/AMONO~-ORIVE1100JbTReryrtLl~mrhmrnJ.AOC 6 Nm•emGer?1. /999