Thomas Kwader explains that probably the most important factor in determining the depth of a monitoring well is the relative ground water level at the time of installation.
In many cases, the well construction regulations may dictate that the top of the screen be placed above the mean high water table to ensure capture of any possible lightweight (floating) compounds, i.e., motor fuels or lubricants. However, the natural fluctuation range of the water table may be greater than the length of the screen, making it nearly impossible to construct a well that would meet all of the necessary criteria.