Hydrology of Area 61, Northern Great Plains and Rocky Mountain Coal Provinces, Colorado and New Mexico

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6.0 SURFACE -WATER QUALITY -- Continued 6.5 Biological Quality Aquatic Organisms Are Indicators of Stream Quality Stream quality is reflected by the diversity of aquatic organisms. In general, streams having a diversity index of 3 or 4 are likely to be relatively free of waste materials (Wilhm, 1970, p. R223). Diversity index is a method of relating the number of organisms to the number of species. Where the diversity index is less than 1, the stream or stream reach is likely to be polluted (Wilhm, 1970, p. R223). A conceptual model show- ing the relationship between stream quality and diversity index is illustrated in figure 6.5 -1. As the quality of water in streams deteriorates, fewer species of organisms are able to survive, and the population of the remaining organisms tends to increase because of less competition. Most aquatic organisms are somewhat immobile and, therefore, reflect the condi- tions in a stream over a long time. Temporal changes in stream velocity, temperature, dissolved oxygen, chemical quality, and sediment concentrations can affect the aquatic community structure. The taxonomic group which is more abundant than others in a particular community is referred to as the dominant taxon. The dominant taxon general- ly indicates which species is thriving during some given period and, with limitations, may be used for assessing water quality. Of those organisms found in Area 61, stoneflies, mayflies, and caddisflies are considered sensitive to instream changes such as increasing concentrations in organic waste and sediment and decreasing dis- 62 solved- oxygen concentrations. Scud, snails, and craneflies are somewhat less sensitive to these changes; midges are considered to be tolerant of such changes. Biological data have been collected at 45 sites in Area 61 (fig. 6.5 -2). Organisms were found to inhabit all streams sampled; however, the Shannon - Wiener diversity index varied greatly (0 to 3.25). The accura- cy of estimating diversity decreases when samples contain fewer than 100 individuals. The data in table 6.5 -1 were collected during three different periods and by three different methods. The 1971 -72 data were collected by Wentz (1974). The 1974 data were collected by URS /Ken R. White Co. (1975), and the 1980 data were collected by the Colorado Depart- ment of Health, Water Quality Control Division (J. D. Gillespie, CF &I Corp., written commun., 1981). Because the methods varied between sampling peri- ods and the level of taxonomic classification is incon- sistent, comparison between data sets was not made. In general, the data indicate the stream quality is such that aquatic organisms of a moderate diversity inhabit the streams in Area 61. Notable exceptions are in the stream reaches downstream from the outflow of wastewater- treatment plants and reaches upstream and downstream from Trinidad Lake, Colo. F a w A POOR STREAM QUALITY GOOD Figure 6.5 -1 Diagram of conceptual model showing stream - quality diversity relationship.