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WATER-QUALITY CHARACTERISTICS OF LAKE HENRY AND COMPARISON TO WATER-QUALITY STANDARDS Water-quality characteristics of Lake Henry were delennined by onsite measurements made at seven sites in the lake and by laboratory analysis for selected chemical and biological constituents in water samples collected at site HEW2 (fig. 2). Physical properties and results of analyses for chemical constituents were com- pared with water-quality standards established by the Colorado Department of Health. Onsite Measurements The water-temperature profiles shown in figure 4 are representative of the thennal characteristics that existed in Lake Henry on the sampling dates. The tem- peratures in the lake ranged from 14.20C at a depth of 5 ft at site HEW2.on October 6 to 26.40C at the water surface at site HNE on August 19 (table 15, "Supple- mental Data" section at the back of this report). Water- temperature measurements on May I indicate that the lake was thennally stratified at all sites because of spring warming of the upper water layer. Variations of surface temperatures between sites are attributed to time of measurement. Water temperatures at the sur- face were cooler earlier in the day than those measured later in the day due to heating from solar radiation. On June 29, water temperatures were unifonn vertically and areal.lY throughout the lake indicating the lake was not thermally stratified. On August 19, water tempera- tures decreased with depth: at sites HEW2, HNW, and HSW water temperatures measured near the bottom of the lake were cooler than watertemperatures measured near the bottom of the lake on June 29. This apparent anomaly may be because of the 4-ft decline in reservoir elevation between June 29 and August 19. This decline may have allowed cooler ground water to mix with the water near the bottom of the lake. The dissolved-oxygen and pH profiles shown in figure 5 varied similarly with depth, indicating that dissolved-oxygen concentrations and pH are affected by photosynthesis and respiration. During periods when photosynthesis predominates, oxygen is pro- duced by algae, which increases dissolved-oxygen con- centrations, and carbon dioxide is used by algae, which increases the pH. Conversely, during periods of respi- ratIon, the use of oxygen by algae decreases the dis- solved-oxygen concentration, and the release of carbon dioxide decreases the pH. Dissolved-oxygen concentrations in Lake Henry are affected by photosynthesis and respiration, reaera- lion. and water temperature. Dissolved.oxygen concentrations in the lake ranged from 4.7 mgIL at the lake bottom at site HNW on August 19 to 7.8 mg/L at the lake surface and at the 3-ft depth at site HEW2 on October 6 (table 15). On May I, Lake Henry was ther- mally stratified, and a density barrier prevented transfer of dissolved oxygen from the surface-water layer to the lake bottom resulting in a stratification of dissolved oxygen. On June 29, the lake was nearly isothennal, and only slight changes in dissol ved-oxygen concentra- tions with depth were measured. As a result of warmer water temperatures on June 29, concentrations of dissolved oxygen were smaller than on May I. Probably because of increased biological activity on August 19, the largest dissolved-oxygen concentra- tions were measured at the lake surface due to photo- synthesis, and dissolved-oxygen concentrations decreased with depth due to respiration. On October 6, increased dissolved-oxygen concentrations probably are a result of cooler water temperatures. A minimum allowable dissolved-oxygen con- centration of 5.0 mg/L has been established for Lake Henry by the Colorado Department of Health (1987). The 4.7 mgIL dissolved-oxygen concentration mea- sured on August 19 at site HNW at the lake bottom was the only measurement less than the State standard for class I warm-water aquatic life and agriculture. The pH of water in Lake Henry is affected by water that enters the lake from the Lake Henry inlet (table 3) and by photosynthesis and respiration. The pH of water in the lake ranged from 8.1 to 8.6 on the sampling dates (table 15). Generally, larger pH values were measured near the lake surface because of photo- synthesis, and smaller pH values were measured near the lake bottom where respiration predominates. On May I, the pH tended to increase from the lake surface to about the middle depths and then decrease to the lake bottom (fig. 5). This was possibly because of a concen- tration of phytoplankton at the top of the density barrier caused by thennal stratification. On June 29. the lake was not stratified, and pH varied slightly from the lake surface to the lake bottom. The pH on June 29 gener- ally was smaller than on the other sampling dates, which indicated that less biological activity occurred at this time. The allowable range of pH established for Lake Henry by the Colorado Department of Health (1987) is 6.5 to 9.0, and the measured pH values in the lake during the sampling period were within this range. Profiles of specific conductance indicate there was minimal vertical and areal variation in measured values (fig. 6). Variations in specific conductance rang- ing from 889 ~S/cm on June 29 to I ,010 ~S/cm on October 6 were measured during the sampling period (table 15). A decrease in specific conductance of about 10 Reconnaissance of Water Quality of Lake Henry and Lake Meredith Reservoir, CrOWley County, Southeastern Colorado, April-October 1987