Well Rehabilitation, pH, and pH Meters.
Chris Johnson, PG, CHg
Originally written for the NGWA toolkit
The absence of codified procedures and standards for well rehabilitation, requires those who conduct such projects to engage is 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 that those competent practitioners that use acids in the course of their practice, know that they need to understand the chemistry of the well water that they will be working with.
However, acids are dangerous if not given full respect, handled carefully, mixed carefully, and all in the correct safety equipment.
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.
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 that each increase (or decrease) in pH is ten 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, but the good ones will have an expiration date, which if you are dealing with concentrated acids you should pay attention to. The strips can be obtained from many places, ranging from pool supply stores, garden stores and even “big box” stores will stock them. However, the best pH strips will need to be purchased from a lab supplies store. The better strips come in ranges, such as 0 to 6.0, 5.5 to 8.0, 6.0 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 three or four.
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 interpet. This makes deployment in the field more effective, by reducing the need for subjective interpretations that may mislead us during well rehabilitation. Please note, the pH strips are not ideal for those individuals that are entirely, or partially color-blind.
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 and features, from a few tens of dollars, to several hundred dollars. 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 any outside work.
Keep in mind that “field grade” meter is a subjective term as well, and the reader would be well informed if they took the time to 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. Two other factors with respect to using meters in the field, and in particular with rehabilitation acids and other strong chemicals. First, is batteries, and battery life. 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. The bane of field meters, in so many cases. It is the last, and I mean last thing on any one’s mind to calibrate the pH meter, that is until you get a negative reading, or a reading of 19, or some such unbelievable value. Then calibration is vital!
In the author’s experience, 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 at least monthly, if not more often, and stored with the batteries out of the meter. Third and finally, if the meter probe can be replaced, it ought to be replaced before each job, and if not each job, 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 effect 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. The following link 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 that meters are simply less resilient to the harsh environment of a well rehabilitation project site, and the normal operational climate that is 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. t