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Willow Creek water quality 23 Table 5.2 Numerical concentration vs. flow relationships at WGL Weir Low flow Low flow Mid flow Mid flow High flow High Flow Analyte range concentration range concentration range concentration (gpm) (mg/L) (gpm) (mg/L) (gpm) (mglL) Cadmium 0 - 650 0.017 650 - 4000 0.011 Zinc 0 - 20 2.9 - 7.0' 20 - 555 7.6 - 2.9• 555 - 4000 1.6 Low (LFC) and mid flow concentrations (MFC) based on linear relations of LFC =Flow' (-0.205) + 7 and MFC =Flow' (-0.00252) + 2.95, respectively. Given this empirical relationship, chemical loading was calculated for each 0.5 hour interval for which flow data was recorded at the WGL Weir in 1999 from April through early fall. Using such a short Bow interval was important because of the diurnal nature of flow in Windy Gulch discussed previously. A total estimated mass of each solute which passed through the WGL Weir in the given time span was then derived by a summation of each interval loading. This calculation method also compensates for the highly seasonal nature of the flow observed in Windy Gulch. For the period in which flow occurs at the WGL Weir (April -November), loading calculations were completed to estimate the total mass of both cadmium and zinc at the weir. Assuming that negligible flow occurs in the winter months between November and April, the calculated total is assumed to represent the total annual loading. The results of the loading calculations are presented in Table 5.3. Table 5.3 Annual loading of cadmium and zinc in Windy Gulch at WGL Weir Solute Average loading Annual load (Ibs/day) (Ibs) Cadmium 0.0265 9.67 Zinc 3.72 1360 5.2.2 Chemical loading in Willow Creek Because no recent flow data has been collected on Willow Creek, chemical loading was estimated using average stream flows and analyte concentrations from regional historic data. Flow data on Willow Creek was measured and archived by the USGS from 1951 through 1982. Because highly seasonal flows are evident in Willow Creek as well as Windy Gulch, average high (spring) and low (non-spring) flow rates were calculated from the flow data. The average high and low flows are presented in Table 5.4. As discussed above, water samples have been collected along reaches of Willow Creek since the early 1960s. The average high and low flow concentration values for cadmium and zinc were calculated for waters sampled in Willow Creek near the confluence with Windy Gulch. These average concentrations were used in subsequent loading estimates. The average historic concentrations used in the loading analysis are presented in Table 5.4.