Twenty years ago, three wells were about all you needed to monitor almost anything -- a gas station or a 20-acre landfill, one upgradient and two downgradient. There is no doubt that many sites still don't have enough monitoring wells to monitor properly for contamination. On the other hand, there are many sites in which too many wells are being monitored for too long and not much is being done with the data collected.
Before we can address how many wells are being monitored, we must know what the objective of the monitoring program is. Generally, we monitor ground water levels and chemical constituents to assess the movement or cleanup progress of a contaminant in the subsurface. The number of wells necessary to accomplish this is dictated by many factors, including the complexity of the site's hydrogeologic conditions; the rate, or ground water flow; how many source areas are present; the mobility of the site compound of concern, including the specific gravity or weight of the compounds, etc.
One of the most important factors that must be understood is the site's hydrogeologic framework. We must know what the subsurface looks like, how many aquifers and confining beds are present, the horizontal extent of these units, and the vertical connection between these zones. How does the ground water flow across the site? In preferred channels? Does the ground water "stair-step" progressively downward as it moves laterally? Information helpful in mapping the subsurface includes continuous split spoon samples, cores and borehole geophysical logs.
Initially, when a site is assessed, a large number of temporary wells are installed throughout the site to assess the number and shape of plumes beneath the site. The temporary wells most often are constructed by the hollow stem auger drilling method or direct-push technology, which may be accompanied by a mobile laboratory that can analyze site compounds of concern within an hour of sample collection. This real-time data then can be used to guide the location of future wells.
One problem is that these temporary wells don't stay temporary, and many wells are added to a long-term monitoring program, which provides little useful information regarding movement of the plume.
At many sites, monitor well sampling can be reduced and frequency of sampling reduced after years of data have been collected, based on the prevailing trends at the site.
Again, monitoring well data are only important if the wells are in the proper locations and the data are being analyzed to assess the progress toward the goals of the program.