BASIN: Information on Water Quality Parameters

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City of Boulder/USGS Water Quality Monitoring
Diurnal Variations of Water Quality Parameters in Boulder Creek
Sheila Murphy, Research Analyst, BASIN project, City of Boulder.

Diurnal Variations of Water Quality Parameters in Boulder Creek

In 1986 and 1987, Aquatic and Wetland Consultants, Inc. (AWC) conducted a diurnal (24-hour) sampling study of Boulder Creek and Coal Creek water quality. This study was performed to determine what was causing short-duration excursions of unionized ammonia concentration in Boulder Creek. Sampling of Boulder Creek took place immediately upstream of the confluence of Boulder Creek and Coal Creek. This site is located 8.5 miles downstream from the Boulder Wastewater Treatment Plant.

Once every two weeks, samples were collected each hour for 24 hours. This study lasted over a year. Graphs of air temperature, water temperature, pH, and dissolved oxygen (DO) for two representative sampling events, which occurred in September 1986 and February 1987, are shown in Figures 1, 2, 3, and 4.

Air and Water Temperatures

Diurnal air temperature variations averaged approximately 15 degrees C, with no major differences in the degree of variation from season to season. Air temperature was found to be the primary influence on stream water temperature. Because water takes longer to heat up and cool down than air, increases or decreases in water temperature lag behind those of air temperature.

Dissolved Oxygen

DO concentrations varied significantly over 24 hours, mostly due to photosynthetic activity of plants. During photosynthesis, which occurs during the day, plants release oxygen into the water. During respiration, which occurs at night, plants remove oxygen from the water.

Downstream of Boulder’s Wastewater Treatment Plant (WWTP), nutrients in the WWTP effluent and in non-point sources (which may include fertilizers) feed aquatic plants in the creek, allowing significant growth to occur. The plant growth is further encouraged by the high amount of sunlight that reaches Boulder Creek, due to little riparian vegetation shading this part of the creek. As a result, DO concentrations can vary significantly; in September 1986, DO concentrations varied from 3.4 to 17 mg/L, while in February 1987, DO concentrations varied from 8.2 to 12.6 mg/L. Diurnal variations in DO were greatest in late summer, when longer sunlight exposure and warmer temperatures increased photosynthetic and respiration rates. Summer minimum DO concentrations could get very low due to a high effluent to base flow ratio when streamflow was low. Higher minimum concentrations were observed in winter, because colder temperatures slow decomposition rates and increase dissolved oxygen saturation.


pH varied significantly over 24 hours. These diurnal fluctuations in pH are controlled principally by rate of photosynthesis, rate of respiration, and buffering capacity. Photosynthesis rates are controlled by nutrient concentrations and availability of light. Respiration rates are influenced by availability of organic waste and temperature. The quality and dilution of effluent from the WWTP greatly influences diurnal changes in downstream pH.

During photosynthesis, aquatic plants remove carbon dioxide from water, which causes a rise in pH. During decomposition of organic matter, carbon dioxide is released as an end product. At night, when plants respire, they release carbon dioxide to the water, causing a decrease in pH.

Summer pH values show greater variation than winter pH values because there is more light and warmer temperatures, producing both higher community photosynthetic and respiration rates.

Supporting Data and Plots


Windell, J.T.; Rink, L.P.; and Knud-Hansen, C.F. (Aquatic Wetland Consultants, Inc.) 1987. "A One Year, Biweekly, 24-Hour Sampling Study of Boulder Creek and Coal Creek Water Quality." Prepared for the City of Boulder Public Works Department. June.

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