Static water levels should be monitored over time to detect trends, so that loss of yield caused by regional static water level declines are not mistaken for well maintenance problems. Pump problems can be identified by inspection and/or a properly conducted step-drawdown test.

Factors (3), (4), and (5) can be limited by proper well design, construction, and operation, and are the primary topics of this paper. In the past, loss of well yield and/or sand pumping was often tolerated until continued operation of the well was no longer economical, and the well was then redrilled. Redrilling of a problem well is sometimes the most cost-effective approach. However, the performance of many wells experiencing these problems can be improved by well rehabilitation techniques.

Whatever the solution, it is important to identify and understand the cause of well failure. If redrilling is chosen, the previous design and construction methods should be evaluated and improved upon, if possible. if well rehabilitation is attempted, the proper technique must be selected. The most frequently encountered problems are discussed below.

Mechanical Blockage

There are two types of mechanical blockage which can act to restrict the movement of Groundwater into a well. The first type involves the movement of fine grained soil materials from the natural formation to the borehole face or the face of the screen. The second type is caused by corrosion by-products of the metal portions of the well which act to cover the openings of a well screen.

The movement of fine particles is typically caused by improper well design or by overpumping a well. An improper well design may include the selection of a gravel pack which is too large to effectively filter fine formation materials, or selection of an inappropriate screen slot size. Also, a well screen could be placed opposite layers of sand that are significantly smaller or more graded in particle size than the other aquifer materials.

Overpumping a well will cause turbulent flow in the formation near the well screen and promote the movement of fine grained materials. These same materials may not migrate at pumping rates that maintain laminar flow throughout the formation. The migration of fine materials acts to reduce the effective porosity and restrict water flow, increases head loss in the immediate vicinity of the well, and often results in sand pumping.

The corrosion of well casings or screens can cause holes to develop in a casing and cause the screen slot size to increase, allowing sand and/or gravel pack to enter the well. Corrosion by-products can also cover portions of the screen and cause higher entrance velocities through the remaining open area, thus increasing head loss across the screen.