PPT Slide
The pH of Boulder Creek drops at 75th Street. This drop is due to the effluent from the 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. The pH in lower Boulder Creek shows extreme diurnal (day to night) variation because of photosynthesis by plants. Nutrients, released from the WWTP and from fertilizer runoff, feed aquatic plants and algae in the creek and allow significant plant growth to occur.
pH in Boulder Creek is discussed further at www.basin.org/data/COBWQ/bc/pH.html
In general, the pH of Boulder Creek increases as it moves downstream. The pH of mountain sampling sites was usually between 6.5 and 7.7; the pH in lower Boulder Creek was 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 calcite-containing sedimentary rocks. The calcite dissolves in the water, and increases the pH of the stream.
Data Delivery/Interpretation, cont.
http://bcn.boulder.co.us/basin/data/COBWQ/2000/06/pH.html