Acid-Neutralizing Capacity
Acid-Neutralizing Capacity
| Acid neutralizing capacity (ANC) refers to water’s ability to buffer changes in pH. Depending on how many and what kinds of ions are dissolved in water samples, the water changes pH at different rates. When there are more ions in water, it resists change more than water containing fewer ions. The higher the ANC, the more ions there are in the water, and the more resistant to change the water will be. ANC is calculated on unflitered samples by measuring how much acid must be added to the sample to lower its pH to 4.7. The equivalent measurement on filtered samples is termed alkalinity. Very low ANC puts fish and other aquatic organisms at risk from acid shock associated with the runoff of acidic rainwater into streams. |
| Water collects buffering ions from weathered rock. Thus, groundwater, which has been in the ground weathering away at rock, has a higher buffering capacity than rainwater, which leaves many of these ions behind when it evaporates. Specific conductance is also affected by the ions in stream water, which is why these two variables often show a strong correlation. All of this could explain why the above graph shows a wide range of ANC during the warmer periods and a very narrow range during the winter. ANC signatures in the fall reflect how each sampling site receives the majority of their water. For instance, Wright Lane is characterized by higher ANC in the fall and the spring because its sub-watershed has little standing water and only minimal wetlands, which means that Wright Lane receives mainly groundwater. However, the site at Kohler Road Pond is just below a pond, which acts to collect rainwater, which has a much lower ANC, and release it into the stream. The same situation applies to Nequasset Park and Old Stage Road. This explains the difference of ANC between these two sites during the fall. |
| Groundwater-fed stream at Wright Lane, left. Surface water collected at Kohler Road Pond, right. |
| However, as with pH, when the fall rains begin to raise the water table and when periodic snow melts contrubute to this trend throughout winter, ANC drops at all of the sampling sites. This rise in water table prevents precipitation from infiltrating deep into the groundwater system and instead encourages the water to runoff into the streams. Thus, during the winter, all of the sampling sites display a surface water signature with low ANC, which is reflected in the ANC graph above. |
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