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Parameters Measured

Surface Water pH

pH is a measurement of hydrogen ion concentration and is used to describe the acidity or alkalinity of a solution. Measurements range from 0 to 14 on an exponential scale. A pH value less than 7 is acidic, while a pH value greater than 7 is basic (alkaline). A pH value of 7 is neutral. The pH scale is negatively logarithmic which means that each whole number down the scale is 10 times more acidic than the number before it (Example: A pH of 6 is 100 times more acidic than a pH of 8).

The pH of surface waters in Texas is generally between 5 and 9.

The pH of an aquatic system is determined by a number of factors. Carbon dioxide (CO2) from the respiration of aquatic organisms and from the atmosphere reacts with water to form a weak acid. Because plants release CO2 during the night and take in CO2 during the day, pH levels can vary from day to night.

The rock and soil of the watershed also affect the pH of an aquatic system. A drainage basin composed primarily of limestone tends to be alkaline because the slightly acidic nature of rain rapidly weathers the soft rock releasing minerals that buffer the water and slightly raise the pH. Mineral-rich water like this is commonly called hard-water. Watersheds containing sandy soils from harder igneous bedrock weather more slowly and there is little chemical breakdown. These soft-water aquatic systems are low in minerals that buffer the slightly acidic rainfall and tend to have a pH less than 7.

Human activities also influence pH. Water flowing through mine tailings (waste rock from mining operations) can become acidic because of the presence of minerals containing sulfide (which may form sulfuric acid). Drainage from these areas into soft-water streams or streams with little buffering minerals lower the pH. Also, emissions from power plants and car exhausts containing nitrogen oxides and sulphur dioxide can react with moisture in the atmosphere to form nitric acid and sulphuric acid. This is more commonly known as acid rain.