Chemical Encrustation

Chemical encrustation is the deposition of minerals on the well screen or gravel pack which act to restrict the movement of water into a well. Chemical encrustation is caused by the precipitation of minerals dissolved in the groundwater due to changes in flow and/or pressure conditions at the well. Well encrustation typically consists of iron and manganese oxides or of calcium and magnesium carbonates or sulfates.

Chemical encrustation can be reduced by designing the well with a minimum of head loss through the screen, by correct placement of well seals, by proportioning the flow vertically in the well, and by maintaining proper pumping rates.

Bacteriological Plugging

For the purpose of this paper, the term "nuisance bacteria" is used to describe a variety of microorganisms that can cause clogging problems in wells, pipelines, and treatment facilities. This term includes the types of iron-related bacteria which utilize dissolved iron as an energy source and other nuisance bacteria which cause iron precipitation in a secondary manner.

Nuisance bacteria are not believed to cause health concerns but are a nuisance in the production and transmission of groundwaters. Nuisance bacteria have been characterized by their unusual capacity for accumulating ferric (iron) hydrate around their cells. A relatively small number of bacteria are able to clog a well because they can accumulate many times more ferric hydrate than the actual bacterial cell material.

There are two methods by which nuisance bacteria infest a well. The bacteria may either be native to the aquifer or they may be introduced directly by man. Bacteria are known to exist in the ground as either active organisms or as inactive spores. It is also possible for the operations of man to introduce new or additional bacteria into an aquifer during drilling or when a pump or other equipment is serviced or operated.

Bacteriological plugging problems exist in wells throughout the United States and the world. For many years the approach to cleaning wells plugged by bacteria has been one of trial and error. There has been a limited understanding of the dynamics associated with bacteria kill in a well. Today, more and more research is being undertaken by the water well industry and by microbiologists. This expanding awareness has led to several innovations and other improvements to existing well cleaning procedures will certainly follow.

Well Rehabilitation Techniques

A decline in well performance can often be counteracted by appropriate well cleaning and rehabilitation procedures. Many techniques have been used to rehabilitate wells. Most techniques have provided some beneficial results and all have failed at some time to achieve substantial well improvement.

An important obstacle to successful water well treatment is a lack of understanding of the cause of a problem at a specific well. In order to develop a successful treatment program for a specific well, it is necessary to review detailed operating data collected throughout the well's life, and any data regarding similar successful and/or unsuccessful treatment procedures conducted previously.

Well treatment typically includes some use of chemicals to break down encrusting materials. Mechanical techniques such as wire brushing, swabbing, jetting, and surging can be used to assist the chemical treatment. The best well improvement results are typically achieved by the use of a properly selected combination of treatment methods. This section describes various mechanical and chemical rehabilitation techniques.