Successfully functioning environmental wells are the result of careful planning. Properly designed wells should optimize flow into the well consistent with aquifer yield, introduce no contamination into groundwater, provide particulate-free water and maintain material integrity for the length of the project.
Well design takes into account several factors, including well function, geologic and hydrogeologic conditions, and screen size aspects like slot width, length and diameter.
What purpose does the well serve?
Environmental groundwater monitoring wells may be installed to serve different purposes. Piezometers are simple wells used to measure water levels at a specific depth. Monitor wells are used to measure water levels, sample groundwater for chemical analysis, and conduct simple aquifer tests like slug tests.
Remediation or recovery wells are used for more robust aquifer tests, sampling and installation of extraction pumps or other types of remediation or product recovery systems. Piezometer or monitor wells are typically shallow with well casings and screens at least 2 inches in diameter. Deeper monitor wells may require larger diameter casing and screens to accommodate larger pumps or equipment. Remediation or recovery wells may require larger diameter casing and screens to accommodate larger pumps, more than one pump, or other recovery equipment.
What kind of screen do I need?
Well screens are designed to prevent 90 percent of filter pack particles outside the well casing from entering the well and allow free flow of water into the well. The classic method for determining well screen slot size is based on a formation grain size gradation curve generated from a sample of the formation from the proposed well screen depth. Where local formation characteristics are well known common acceptable practice is to use standard screen slot sizes and associated standard filter pack sand.
For example, in some areas wells may be installed using a 0.01-inch slot width and a 20-40 mesh clean sand filter pack. Well screens are available in different types of materials, such as stainless steel or polyvinylchloride, and in different constructions, such as machine-slotted or wire wrapped.
How does lithology affect well screen size?
Geologic conditions will influence well design and screen size. Finer-grained lithology of fine sand, silt and clay would require a smaller slot width than a more coarse-grained lithology of sand and gravel. Slot sizes are available in widths of 0.005 to 0.5 inches. If a formation grain size curve would indicate a smaller screen width then an alternative may be to use 0.005 inch screen with 100 mesh sand. Multiple screens alternating with blank casing may be a good choice to monitor or remediate more than one depth in complex lithology, such as interbedded silts and sands.
Are contaminants present in the groundwater?
Well casings and well screens can be subject to many types of subsurface conditions, such as presence of inorganic or organic contaminants, non-aqueous phase liquids, or corrosive conditions that occur with soil or fresh or saltwater. Appropriate well casing and screen materials should not leach constituents into or sorb from groundwater which can affect results of chemical analyses. Materials used for remediation wells, in particular wells designed for NAPL or floating product removal through dual-phase extraction must be able to withstand imposed vacuum pressures.
Professionals at Talon/LPE have installed hundreds of environmental wells throughout Texas, Oklahoma, and New Mexico. Contact us today for information on well screen design and about our environmental drilling services.