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009~ Monthly variations in the specific conductance of Monument Creek at sites M10 and M28 are summarized in figure 22. Median specific conductance is smaller during April, May, and June. The largest median specific conductance Occurs during August and September at site M]O and in August at site M28; how- ever, the largest specific conductance may Occur during any season depending on the sources of flow. pH The pH of Monument Creek varies considerably at each of the four sites where measurements have been made; however, all measurements were within the acceptable range of 6.5 to 9.0 (fig. 23). The median pH increased from 7.5 at site M]O to 8.0 at site M34, which is located about 2 mi upstream from the confluence with Fountain Creek. The increase may result from ground-water discharge or drainage of alkaline materials that enter the stream from urban- ized areas. At the measured levels of pH, most trace elements are relatively insoluble in this well-oxygenated stream. However, manganese, selenium, and zinc are fairly soluble at the pH levels measured on Monument Creek. Five-Day Biochemical Oxygen Demand Except for site M35, the BODs measured on Monument Creek was less than 4 mg/L for more than 50 percent of the time samples were collected, indicating that small quantities of biochemically oxidizable material were present in the stream. The BODs measured on Monument Creek is summarized in figures 24 and 25. Between ]975 and ]983, BODs generally increased downstream (fig. 24) because more oxidizable material was present in the stream, probably because of discharge from wastewater-treatment plants and from nonpoint sources trans- ported to the stream by runoff. A slight increase in median BODs occurred from ]975 through 1979, between sites M28 and M34 and, from ]980 through ]983, between sites M28 and M35 (fig. 25). ln addition, the BODs measured from ]980 through ]983 was consid- erably larger than concentrations measured from 1975 through ]979, indicating there was more biochemically oxidizable material in the stream from ]980 through ]983 than from ]975 through ]979 (fig. 25). Dissolved Oxygen Dissolved-oxygen concentrations were measured in Monument Creek during the daylight hours. Dissolved-oxygen concentrations frequently exhibit diel variations, and the largest concentrations OCcur during daylight as a result of photosynthesis. Thus, the dissolved-oxygen concentrations measured in Monument Creek are likely to be greater than the minimum daily dissolved- oxygen concentration (fig. 26). Since ]975, all dissolved-oxygen measurements made on Monument Creek, except for one measurement at site M34, have been greater than the recommended standard of 5.0 mg/L. The large velocities and shallow depths keep the stream well oxygenated. 37