Rain falling on the Hudson tends to be acidic; fortunately, the river has a built-in antacid: carbonaceous rock.
The average rainfall in New York has a pH between 4 and 4.5, 30 times more acidic than unpolluted rainwater. The sources of this acidity, sulfur dioxide and nitrogen oxide air pollution, are products of fossil fuel combustion and are emitted by automobiles, industry, and coal burning power plants. When combined with moisture in the atmosphere, they form sulfuric and nitric acids.
Using the high frequency data set provided by HRECOS, we can sometimes see rises in the river's acidity after a heavy rain. The daily fluctuations are due to the influence of photosynthesis, respiration, and tidal currents (see HRECOS Stories: "Plant Breath in the Hudson River" and "The Breathing Tide"). Responses to rain events will occur over the course of one to three days. For example, 43 millimeters of rain fell on Schodack Island from July 16 to July 18 this summer resulting in a change in acidity of 0.12 pH units.
the decline we observe in pH is less than expected. Assuming the
rainfall had a pH of at least 4.5, a 43 millimeter rain should have
reduced the pH of the estuary by at least 0.98 instead of the 0.12 change actually observed (see insert:
"Conservative and Simple Estimate of the Impact of Acid Rain"). The fact that the change was not this
dramatic indicates the system is well buffered. This is where the river's built-in antacid comes into play.
The buffering process works like this: the Hudson River Estuary is rich in calcium carbonate due to the erosion of carbonaceous rock such as limestone in the watershed. Calcium carbonate is the primary ingredient in antacid tablets and it works just as well for a large water body like the Hudson River as it does for our stomachs. This results in the partial neutralization of acid rain as it is added to the river.
It may be for this reason that we don't see a rise in acidity after every rain event. The natural antacid protects the system so well that we are unable to observe the impact of most rainfalls. In addition, there may be a regional difference in the buffering capacity of the river, a factor we will be considering for our next HRECOS story.
To observe changes in acidity after storm events, you may click "Current Conditions" in the menu to the left to access the data yourself or you can download the data used for this story: Data for this HRECOS Story.