In general, the pH of Boulder Creek increases as it moves downstream (for example July, 1999). The pH of mountain sampling sites is typically between 6.5 and 8; the pH through the City of Boulder is typically between 6.8 and 8.8; the pH in lower Boulder Creek is typically between 7.3 and 10. This increase is due in part to changes in geology. In the mountains, Boulder Creek passes over granitic and metamorphic rocks, which provide little to no buffering capacity to the water. As Boulder Creek moves into the plains, the geology changes to sedimentary rocks, including sandstones and calcareous (calcite-containing) shales. The calcite (CaCO₃) dissolves in the water, and hydrogen ions are consumed in this reaction. This loss of hydrogen ions from the water increases the pH of the stream.

The pH of Boulder Creek drops at 75th Street. This drop is due to the effluent from Boulder's Wastewater Treatment Plant (WWTP), which has a lower pH than the water in the creek.

pH levels can be very high at the sampling sites downstream of the WWTP. While lower Boulder Creek contains more alkalinity than sampling sites in the mountains, alkalinity levels remains relatively low. Therefore, the pH is not very well buffered and can be easily altered by other factors. The pH in lower Boulder Creek shows extreme diurnal (day to night) variation for the same reason as does dissolved oxygen (DO) concentration: photosynthetic activity. Nutrients, released from the WWTP and from fertilizer runoff, feed aquatic plants and algae in the creek and allow significant plant growth to occur. The lack of riparian vegetation along much of Lower Boulder Creek allows significant sunlight to reach the creek, which further increases plant growth. During the day, plants consume carbon dioxide (CO₂) while undergoing photosynthesis. The removal of CO₂ from the water causes bicarbonate and carbonate ions to react with hydrogen ions (H⁺) to form more CO₂. This loss of H⁺ from the water causes the pH to increase. At night time, when plants, bacteria, and animals are undergoing respiration, CO₂ is released and oxygen is consumed. This releases H⁺ back to the water. Therefore, pH values during the day can be much higher than during the night at these sites. Monthly water samples are collected during the day; therefore, the samples from lower Boulder Creek reflect the high pH values. As with DO levels, the magnitude of the difference from night and day is affected by the season. During the summer, plants are exposed to sunlight for a much longer time, so they are growing faster, and the pH levels show the greatest variation at this time. More information on the daily variation of pH downstream of the WWTP can be found here.

COMPARISON TO REGULATIONS

All source water samples collected in 1998 and 2001 from North Boulder Creek and Middle Boulder Creek (used for Boulder domestic water supply) had pH values between 5.0 and 9.0, in accordance with the Colorado Department of Public Health and Environment Water Quality Control Division (CDPHE-WQCD) requirement for waters to be used for domestic water supply.