Monitoring wells were first used mainly for water level measurement. These wells were often referred to as piezometers in reference to the "piezometric surface" of groundwater. In recent years, the term "piezometric surface" is often replaced by "potentiometric surface." However, the term "piezometer" is still sometimes used for monitoring wells installed only for water level measurement.
Many water level monitoring wells constructed in the past were relatively large in diameter in comparison to today's monitoring wells. Wells up to 10- inches in diameter were often constructed to accommodate various means of water level measurement, including floats for mechanically-operated, continuous water level recorders. Many inactive water wells that could accommodate mechanical water level recording equipment were used as monitoring wells.
Modern electronic water level measuring and recording devices now allow for small-diameter water-level monitoring wells. Some continuous water-level measurement devices can be used in wells less than 2-inches in inside diameter.
The use of monitoring wells for groundwater sampling for chemical analysis has increased significantly in the past two decades. The following factors have all served to increase the frequency and scope of groundwater quality investigations and the number of monitoring wells constructed:
Since the 1970s an entire industry has developed around groundwater quality monitoring and monitoring well construction. Numerous private firms are involved in providing technical services for the design and implementation of groundwater quality investigations. Many firm are involved in the manufacture, distribution, and marketing of materials and equipment used in constructing and operating monitoring wells.
Most monitoring wells constructed today are used to assess:
Some monitoring wells are designed to be multipurpose. Monitoring wells can sometimes be used as "extraction" or "injection" wells for mitigation of pollution or contamination.
Although a significant number of monitoring wells constructed today are for detection and assessment of groundwater quality impairment, many monitoring wells are constructed for evaluating groundwater supply conditions by allowing groundwater level measurement and/or aquifer testing. Still others are constructed for observing water levels associated with excavations and irrigated agriculture.
During 1989, approximately 20 percent of all well drilling in California was for monitoring wells, based on well driller's reports received by the Department of Water Resources. Monitoring wells have been constructed in nearly all California counties. The largest concentrations of water quality monitoring wells occur in metropolitan areas of the State. Large numbers of monitoring wells are installed for detection and assessment of leaks from underground storage tanks.
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