Sources for the collection of groundwater samples

A single monitoring well can be designed to accommodate all of these uses and wells are usually designed to collect only those data required by the groundwater monitoring plan. 

To assess the well construction materials compatibility versus the subsurface environment and the pesticide of interest, manufacturers can provide data about the various well construction materials or samples can be acquired for laboratory analysis. Also, QC samples of each material can be collected during installation and preserved for laboratory analysis for potential sample bias, if necessary. In addition to well construction materials, the potable water used to clean drilling equipment and to prepare the grout and hydrate bentonite should also be collected for laboratory analysis (see Section 3.2.6). 

The most common filter pack material is quartz (silica). Quartz is relatively inert, readily available, and workable therefore, it is preferred to replace formation materials removed from the borehole. The grain size of the filter material (i.e., sand or gravel) should be chosen based on the characteristics of the formation to be monitored and the slot size of the screen. Sand and gravel are available in various uniform sizes to accommodate different monitoring environments. 

Unconsolidated or weakly consolidated sediments sometimes collapse around the well screen before the filter pack can be installed. This phenomenon is called formation collapse . Formation collapse can occur as a result of the inherently unstable nature of certain sediments or the disruptive nature of the drilling process. Formation collapse is most common below the water table. Although steps can be taken to minimize the amount of collapse, it may not be entirely preventable. The groundwater monitoring plan may need to accept natural formation material as the filter pack for some or all of the screen section. Well development activities (see Section 2.1.6) can be designed to maximize the effectiveness of the formation collapse materials as a filter pack. 

Bentonite is the name for a hydrous aluminum silicate comprised principally of the clay mineral montmorillonite, notable for its ability to swell in water and to form a very low-permeability seal. It is available as powder, granule (chip), or pellets. Powder and granule sizes are produced by processing after mining. Bentonite powder 

---