pH
pH
| pH is a measure of how acidic or basic a solution is, which depends on the different concentrations of acidic H+ ions versus basic OH- ions. The pH scale generally goes from 1 to 14, although in natural waters, it is rare to find pH values lower than 4 and higher than 9. Solutions with an equal concentration of H+ and OH- ions are considered neutral and have a pH of 7; solutions with pH lower than 7 are acidic and those with a pH higher than 7 are basic. |
| In the water cycle, rain water starts out as more acidic due to the CO2 in the atmosphere, and it tends to become even more acidic because, as it precipitates, it collects pollutants, while groundwater is more basic because it collects basic ions as it weathers the rocks underground. The above graph from Old Stage Road, a sampling site along Neuquasset Stream just above Nequasset Lake, demonstrates this correlation by measuring pH against stage (water depth). Sudden increases in stage indicate rain events, and these are mirrored by correlating drops in pH. Thus, as acidic rain falls and runs off into Nequasset stream, it makes the stream more acidic overall. This trend can also been seen in the periodic drops in the pH of all sampling sites in the below graph. |
| As the above graph indicates, there is a general correlation between pH and seasonal change: as temperatures drop, so does pH. This is not necessarily because of a direct correlation between temperature and pH, but rather because of an overall rise of water tables during the fall and into the winter that can be seen in the graph of pH and stage at Old Stage Road. At the onset of the fall rains in mid-October, water tables began to rise, expanding wetlands and increasing surface runoff by preventing water from infiltrating into the deeper groudwater systems. This creates a greater surface water signature than in the relatively dry period of early fall, making the streams more acidic. This trend of decreasing pH continues through winter, although irregularly, with periodic snow melts, which put more surface water into the streams. In mid-March, pH begins to rise again because most of the snow has already melted and the water table begins to lower, allowing more precipitation to inflitrate into the ground and creating more of a groudwater signature throughout the watershed. Additionally, as temperatures rise and ice on the water melts in the spring, photosynthesis begins again, consuming acidic CO2 and raising the pH of the water. |
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Wetlands north of Indian Road |
| Certain sampling sites tend to be more acidic than the rest, in particular, Nequasset Stream at Indian Road, which was consistently one of the most acidic sites. This is most likely because Indian Road’s sub-watershed contains extensive wetlands. The presence of wetlands decreases pH, both because they act as reservoirs for acidic rainwater before releasing the water into the streams and because they are places where organic matter decomposes and releases CO2, an acidic compound. In this example, Indian Road receives the more acidic surface water held in the wetlands to the north. These wetlands also deliver decomposed organic matter to areas such as Indian Road, increasing the color reading of the water there. |
