Practitioners need to understand the chemistry of the well water they work with.
By Christopher S. Johnson, PG, CHg
The absence of codified procedures and standards for well rehabilitation requires those who conduct such projects to engage in sound science and technology. One of the more important components of our work is the thoughtful use and application of chemicals, and more specifically acids, in the prosecution of our efforts.
Acids are effective chemicals against mineral encrustation and biological fouling when properly applied by type, concentration, and volume. It goes without saying competent practitioners who use acids in the course of their practice know they need to understand the chemistry of the well water they work with.
However, acids are dangerous if not given full respect, handled carefully, mixed carefully, and all in the correct safety equipment.
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Acids can inflict severe and painful injuries by themselves when mixed with water and even in the wistfully concocted “dilute solutions” we deal with during well rehabilitation. As such, we need a means of understanding just how dangerous, or strong, these acids are.
Basics of Measurement
Enter pH, which high school chemistry taught us ranges from 0 to 14, with acids on the low end, and bases on the high end. Each incremental increase is logarithmic, meaning each increase (or decrease) in pH is 10 times greater than the preceding one.
The acids we employ often require the constant maintenance of a lowered pH to fully extract the entire efficacy of the product. As such, we must be able to monitor the pH of the rehabilitation solution, both as a need of the process and for safety.
So, how to measure pH? There are two generally accepted approaches. The original choice was with pH strips, which change color to indicate the approximate pH. The newer choice is to use a pH meter that will give you a direct measurement and reading of the pH.
The pH strips are small, light, compact, and do not require batteries or calibration. The good ones, have an expiration date, which if you are dealing with concentrated acids you should pay attention to. The strips can be obtained many places, ranging from pool supply stores, garden stores, and even big box stores. However, the best pH strips can be purchased from a lab supplies store. The better strips come in ranges, such as 0 to 6, 5.5 to 8, 6 to 13, and 0 to 14. The ranged strips provide gradations of pH, often down to 0.5 differences, and the very best of them have only one reaction strip, as opposed to 3 or 4.
The multiple reaction strips make interpretations difficult, as you try to match three or four color patches, whereas the single reaction strips will have a single, unique color to interpret. This makes deployment in the field more effective, as reducing the need for subjective interpretations may mislead us during well rehabilitation.
The pH meters bring utility and convenience to measuring pH in the field and theoretically reduces the need for subjective interpretation of measured pH values. These meters range in cost, what they can do, and can go up to several hundred dollars each.
Some will only measure pH, but as the cost of the meter increased, often the number of parameters (e.g. conductivity, alkalinity, dissolved oxygen, etc.) will increase. Also, the meters will usually become sturdier, which is an important factor for well rehabilitation or outside work.
Keep in mind “field grade meter” is a subjective term. You should be well informed and research just what a manufacturer means by “field grade.” Shock, water, and dust-proof would be ideal, and while most are resistant to the elements, most are not tested inside a tool box in the back of the driller’s pickup truck.
There are two other factors with respect to using meters in the field and in particular with rehabilitation acids and other strong chemicals. First, fresh batteries are important for accurate measurements that remain accurate (more on this in a moment), and changing the batters frequently is important to prevent them from leaking into the battery compartment and ruining the meter.
Calibration is the bane of field meters. It is always the last thing on any one’s mind to calibrate a pH meter—that is until you get a negative reading, or one of an unbelievable value. Then calibration is vital!
Tips from the Field
In my experiences, pH meters need three things when being used for well rehabilitation projects involving acids, or other strong chemicals. First, they should be calibrated before each use, not every three months. Second, the batteries should be changed monthly, if not more often, and stored with the batteries out of the meter. Finally, if the meter probe can be replaced, it ought to be replaced before each job. If that is not possible, then it should be cleaned and stored per the manufacturer’s recommendations without fail.
Batteries are pretty straight forward and following the manufacturer’s directions for use, storage, and maintenance are as well. Calibration is another thing. Calibration fluids can expire, so they need to be monitored and replaced as needed. Temperature can affect calibration, so calibrating in the field needs to be monitored and possibly calibrate in the more climate-controlled environment of the office or shop.
Ideally, the meter will be calibrated using a three-point calibration (i.e., three different pH calibration solutions are used to improve the accuracy of the pH readings and reduce the risk of the calibration drifting away from an actual, accurate measurement). Here is information that is just the beginning of understanding a specific meter’s needs, but it is a good introduction to the process and science behind meter calibration.
In the final analysis, experience teaches us meters are simply less resilient to the harsh environment of a well rehabilitation project site and the normal operational climate present during these projects. Batteries, maintenance, and calibration are critical to effective and efficient function of meters, and that is more often not likely to occur.
The pH strips are durable, non-electric, do not require calibration, and are generally insensitive to the environment (i.e. heat and cold), and are vastly less expensive. The pH strips—the good ones—are easy to use and accurate and reliable enough if you have the correct ranges to meet all the needs of a rehabilitation project site.
Christopher S. Johnson, PG, CHg, is the president and principal hydrogeologist at Aegis Groundwater Consulting LLC in Fresno, California. Johnson works with well owners and operators on a variety groundwater-related projects, including locating new water resources, well design and construction management, aquifer testing, and well rehabilitation. He can be reached at firstname.lastname@example.org.[/restrict]