MadSci Network: Environment

Re: Can the pH level of a stream vary?

Date: Wed Dec 8 08:40:41 1999
Posted By: David Kopaska-Merkel, Staff Hydrogeology Division, Geological Survey of Alabama
Area of science: Environment
ID: 944583258.En


The pH of stream water varies both in space and time. You should always expect to find differences in the chemical characteristics of stream water when you sample at different times and different places. This is because many factors influence pH and other parameters, and these factors vary over time and in space. For instance, stream water is composed of a mixture of ground water that seeps into the stream bed or comes from springs, water from tributary streams, direct precipitation from the air, and runoff from precipitation that falls in the stream's watershed. All of these different waters have differing pH values, and the relative proportions of the different waters vary -- this alone guarantees that the stream water cannot have a constant pH. Ground water flow is relatively constant, but ground water is recharged by precipitation, so shallow sources of ground water diminish during drought. If you visit a spring regularly and measure the flow, you will find that the amount of water is different every day, and the flow some days can literally be thousands of times greater than on other days. Also, the pH of the ground water itself can change for various reasons, albeit rather slowly.

Rainfall has a different pH than ground water (and it varies from time to time depending on the kinds and amounts of natural and human-generated pollutants in the atmosphere). Right after a rain on the water the pH falls because rain water is more acid than most ground water. The more rain, the more the pH falls. If rain falls on tributaries, their pH values change and so do their flow rates. So depending on where rain falls in the watershed it will have a varying effect on the pH in a particular stream in a particular spot. If rain falls on the land, it picks up minerals from the soil, so when that water enters the stream, its pH has already changed. If rain falls on the land in different places, it finds different soils, so its pH may change by different amounts. I hope you understand now that pH inevitably varies in stream water.

There are many sources of information about stream water quality. The Geological Survey of Alabama has published more than 300 reports about water in our state, and our list of publications is on our web site ( The premier agency responsible for water research in the U.S. is the U.S. Geological Survey:

U.S. Geological Survey Water Resources Division. The USGS is the lead federal agency in all areas of water research. Telephone: (800) 426-9000. E-mail:; Web site:

The second most important federal agency for water research is the U.S. Environmental Protection Agency:

U. S. ENVIRONMENTAL PROTECTION AGENCY National Agriculture Compliance Assistance Center (Ag Center), toll-free, automatic "fax-back" system for fact sheets and other publications: 1-888-663-2155 (new telephone number). The list includes information on pesticides, underground storage tanks, animal feeding operations, and other subjects.

In addition, USEPAs Office of Water has recently published a new document entitled "Catalog of Federal Funding Sources for Watershed Protection," which is available by telephone at (513) 489-8190 or (800)490-9198 and fax (513)489-8695. Please include the document number (EPA841-B-97-008) in requests. A new service of the USEPA, the Index of Watershed Indicators, is available on the Web at: . The IWI summarizes available water-quality information and other environmental information for every watershed in the country. A related page is the watershed information network, Web site: Wellhead Protection Document List Web site:

Both of these agencies have extensive educational outreach programs that include information about surface-water quality.

Best of Luck with your project!

David Kopaska-Merkel
Geological Survey of Alabama
PO Box 869999
Tuscaloosa AL 35486-6999
(205) 349-2852
FAX (205) 349-2861

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