August, 2000

The Importance Of Water-Supply Well Performance Monitoring And Rehabilitation

Well owners know that the original performance of a supply well will decline over time. Monitoring the performance and condition of a water-supply well is important so that informed and timely maintenance decisions can be made. In many cases, waiting too long to maintain a well will make it difficult or impossible to restore the yield.

Why is it important to track well performance and rehabilitate wells? Several reasons, including: clogged wells may have lower yields, clogging results in lower pumping levels and potentially higher pumping costs, clogged wells may have water-quality problems and, as discussed above, the performance of excessively clogged wells are difficult to fully restore. In many parts of the country, it is difficult to get new wells permitted. This makes proper care of existing wells even more important.

Wells can clog for a variety of reasons. The primary causes are physical plugging by particulates, mineral incrustation due to the precipitation of iron, manganese and calcium carbonate, and biological clogging from biofilms and the oxidation of minerals by the bacteria. The treatment method depends on the cause of the clog. Observing only a hard incrustation in a well does not necessarily indicate there is no biofouling problem. Research has shown that most iron and manganese incrustation is actually caused by bacteria that have oxidized the metals, building a protective shell over themselves.

When should wells be rehabilitated? We see situations where rehabilitation is performed based on the calendar, not on performance data. This is not recommended; rehabilitating a well based on time only could result in unnecessary work, or it may be performed later than it should have been. Waiting for the yield of a well to decline before rehabilitating a well is also not recommended; it is possible there has been undetected significant clogging that, based on the type of pump and available drawdown, has not resulted in a decline in the yield. Therefore, rehabilitation decisions should be based on performance and water-quality data.

The easiest way to track well performance is by tracking specific capacity. This is the yield of the well for every foot of drawdown in the well. The specific capacity can be determined during controlled short-duration tests, or from operational data as long as both pumping and non-pumping water-level data are available. An accurate determination of specific capacity just once per year is sufficient for most monitoring programs. Tracking changes in specific capacity over time allows one to identify trends and determine when a well should be rehabilitated.

Tracking the rate of population increases of clogging-related bacteria is another important indicator. The degree of biofouling can be tested with commercially available test kits. Research has shown advantages to treating a well affected by biofouling when the population increases, prior to observing significant yield declines. As a biofouling problem worsens, the bacteria penetrate further into the formation which makes treatment more difficult.

There are three primary steps to well rehabilitation: pre-treatment, chemical treatment and development. Pre-treatment loosens material on the screen or borehole and near the outside of the screen to allow for better penetration of chemical treatments. Chemical treatments penetrate the gravel pack and formation to disrupt and detach the material causing the clogging. Disinfection is also a chemical treatment. Development is typically a surging action to remove the loosened material from the formation and gravel pack.

Unfortunately, biofouling is not only the most common cause of clogging in many parts of the country, but it is also the most difficult to remedy. Just cleaning the well screen and casing is likely to be an insufficient cure. While the clog usually starts at the well screen (or borehole of a bedrock well), the bacteria population will extend outward over time. Treatments need to penetrate into the gravel pack and formation to break down the biofilm and/or incrustation to be able to reach the bacteria itself.

Chemical treatments include a variety of acids, dispersants, disinfectants and several proprietary chemical blends. Some of the newer chemicals include non-phosphate based surfactants. There is growing concern that the use of phosphates leaves a nutrient source behind to feed clogging-related bacteria. The chemical blends include compounds to help penetrate and break down biofouling. In addition, there are treatments based on the injection of gas and liquid-phase carbon dioxide, as well as heated chemical treatments. There are no single technologies that solve all problems, so a combination of treatments is typical.

Well owners may ask why they should consider the help of a consultant, as opposed to just calling their local drilling contractor. LBG can provide very cost-effective well performance tracking services, making recommendations when a well should be treated. Once a well is identified, LBG will make recommendations of treatment techniques. We have experience with a variety of treatments that an individual contractor may not be familiar with. By making timely rehabilitation decisions and choosing appropriate treatment methods, the well owner will likely realize long-term savings in their operations.

For over 56 years, Leggette, Brashears & Graham, Inc. has provided state of the art consulting services to our clients. The bulk of our work is repeat business from customers who have benefited from the value added services we provide. If you would like more information on how LBG can add value to your ground-water or environmental projects, or if you believe that a Web site would benefit your project, please contact your local LBG office for assistance.