pHStatus and TrendspH is a measure of the acidity or alkalinity of a water sample. The symbol pH stands for potential for hydrogen. The pH of water, on a scale of 0 to 14, is a measure of the free hydrogen ion concentration. Water contains both H+ ions and OH- ions. Pure distilled water contains equal number of H+ and OH- ions and is considered neutral (at pH 7), neither basic or acidic. If water contains more H+ than OH- ions the water is considered acidic with a pH less than 7. If water contains more OH- ions than H+ ions, the water is considered basic with a pH greater than 7. The state's water quality criteria ranges from 6.5 to 8.5. Stream water usually ranges from pH 6.5 (slightly acidic) to a pH of 8.5, an optimal range for most organisms. Rain water by contrast is naturally acidic at about 5.6. The pH of a stream affects the organisms living there. The optimum for most is between 7 and 8 and aquatic insects 4-5. |
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Natural waters are buffered by a bicarbonate system and pH may be affected when dissolved CO2 is removed or added from the water solution as through photosynthesis or respiration, respectively. Sulfate, when added to natural waters from coalmine seepage can shift the bicarbonate buffer system to create more acidic conditions. Total alkalinity (TALK) is an indicator of these reactions Media pH ranged from 6.8 at Cherry Creek to 8.2 at Conococheague Creek at MD 68 and on the Potomac at Whites Ferry. Highest median H+ concentration occurred at 52% of the stations. Stations with high concentrations of H+ ions were located in the Upper Potomac Basin, Middle Potomac Basin, Patapsco/Back River and a single station in the Upper Western Shore Basin. Lowest concentrations occurred at the Cherry Creek station, on the Youghiogheny downstream of the confluence with the Little Youghiogheny and on the Choptank River. Increasing trends were evident at 37% of the stations. Increasing trends were uniformly distributed among stations of high, medium or low concentrations. Trends in hydrogen ion concentration decreased at stations on Catoctin Creek at MD 464, the upper Patuxent River near Unity, MD, on the Gunpowder River between Prettyboy and Loch Raven Reservoirs and on the Choptank River. pH, at stations with decreasing trends, did not correlate with any other parameters. Increasing trends at stations above Hancock, MD were positively correlated with TALK. None of the trends were correlated with SO4. Increasing trends below Hancock, MD were positively correlated with TALK and/or CHLA. pH data for sampled locations are summarized in these plots. |
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