pH and the Bronx River
Knowledge is like a virus in that once you learn one thing, it is hard to stop it from stretching into learning another. The simple act of reading or listening can cause a word or an idea to hang in your mind until another comes across and makes the connection permanent. It is as though we are all our own zone of proximal development. On April 23rd I was lucky enough to work with the Friends of the Bronx Zoo to, ostensibly train or re train 7 people in water quality monitoring. The zoo has a history of monitoring several FOZ were interested in getting back into the flow. As I spoke about pH and buffers, I had a fairly clear idea that I knew what I was talking about. As I added water to the pH 4 buffer, I wanted to show how to dilute it and raise the pH. Instead, it actually went down before hovering back around pH 4. “You have just proved that it is a buffer,” Dione, one of the FOZ said. “That is why it is a buffer, and not just a solution with a pH of 4. It is making sure that the pH stays stable.” I thanked my student and then moved on. Dione, as it turns out, has a PhD in chemistry and should have been the one speaking to the group. But, that little thing she pointed out to me sent me off to look for more information.
One of the parameters that the Bronx River Stewards measure is pH. Most people know something about pH and its relationship to acids and bases, but just what does that mean, and why is it important to our river? What does the pH level tell us?
The term pH is derived from the French puissance d’hydrogene, meaning “strength of hydrogen”, referring to the hydrogen ion that affects acidity. The pH scale runs from 0-14 with values less than 7 being acidic and values greater than 7 being basic. At 7, there is equilibrium between the two. The scale is logarithmic, with each level having a difference of a power of ten from the other. So pH 5 is ten times more acidic than pH 6.
pH values of natural water vary, but below 5 or above 9 are detrimental to organisms, and normal values range from 6.5 to 8.5. Most aquatic life, however, has adapted to specific pH and sustained change can cause damage to a population.
To counter these changes, natural water acts as a buffer. A buffer resists change, like the pillows that my wife and I pile up to keep my 8 month old away from of the stereo. She pushes against them, put they bounce back, and Bethania cannot change my radio station (NPR builds vocabulary). The Bronx River Stewards use buffers of set values, 4, 7 or 10, to make sure that the pH meter is working correctly. The pH values of these solutions can only be changed by adding a buffer of a differing value. Natural water should react in the same way; using the natural hydrology to create a buffer against change. The problems occur when either a stronger acid (acid rain, pH 5.6) or a stronger base (concrete washout water>12) enters the system and causes a rapid change to the pH level.
But a pH change can be a secondary effect as well. For example, photosynthesis uses up dissolved carbon dioxide (pH 6.3) effectively raising the pH level. On days when photosynthesis is most likely, such as sunny days during the growing season, the pH level of the river may rise. But what appears to be a natural effect may have very human causes. Fertilizers and human waste, both generally acidic compounds which enter the Bronx River on a regular basis through stormwater runoff (CSOs) and illegal sewage connections (Yonkers), increase plant growth and even the algae blooms recently noted on the river. So the introduction of an acid causes a basic reaction. Organic respiration at night produces CO2 thereby lowering pH levels and dissolved oxygen levels, as does the massive decomposition that follows mortality.
The level of pH in the Bronx needs to be monitored closely, not just for the answer the value itself gives us, but for the chain of possible causes of that value that need to be looked into.
Thanks, Dione.
references: http://waterontheweb.org/under/streamecology/09%5Fph%2Ddraft.html, http://www.seed.slb.com/en/scictr/journal/environment/river/is54nyc.htm, http://www.geocities.com/capecanaveral/hall/9111/DOC.HTML
DG
Add comment April 30th, 2008
