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MONITORING WELL STANDARDS INTRODUCTION i Ground water monitoring wells are principally used for observing ground water levels and flow conditions, obtaining samples for determining ground water quality, and for evaluating hydraulic properties of water- bearing strata. Monitoring wells are sometimes referred to as "observation wells." I The quality of water intercepted by a monitoring well can range from drinking water to highly polluted water. In contrast,production or"water wells"are usually designed to obtain water from productive zones containing good-quality water. The screen or perforated section of a monitoring well usually extends only a short length to obtain water from, or to monitor conditions within,an individual water-bearing unit or zone. Water wells are often designed to obtain water from multiple water-bearing strata. Although there are usually differences between the design and function of monitoring wells and water wells,water wells sometimes are used as monitoring wells,and vice 1 versa. Monitoring wells, along with other types of wells, can provide a pathway for the movement of poor-quality water, pollutants, and contaminants. Because monitoring wells are often purposely located in areas affected by pollutants and contaminants, they pose an especially significant threat to ground water quality if they are not properly constructed, altered, maintained, and destroyed. The California Legislature amended the California Water Code in 1986 specifically to include requirements _ for monitoring well standards. Monitoring wells were previously assumed by the Department to be covered by the collective term "well" in the law. History of Monitoring Wells 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 ground water. 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 I 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 ground water 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 ground water quality investigations and the number of monitoring wells constructed: -33-