Bedrock
Bedrock monitoring wells are often installed at the same locations as saprolite monitoring wells, as saprolite often overlies bedrock in the Piedmont and Mountains. While a saprolite well gives a good indication of how the groundwater system reacts immediately to climatic conditions, it is often necessary to have water level data from the bedrock aquifer to assess how those climatic conditions are affecting the groundwater supply for communities in the Piedmont and Mountains. This is due to the fact that almost all the people in the western part of the state that rely on groundwater for their potable water supply obtain it from bedrock wells. Due to the nature of how groundwater flows in fractured bedrock, and the fact that most recharge to the bedrock aquifer has to first go through saprolite, there can be large time delays between actual climatic events and their affects. Thus, having monitoring wells in both the saprolite and bedrock aquifers affords the best view of short and long term affects of climatic conditions on groundwater supplies.Contrary to popular belief, groundwater does not flow through the bedrock aquifer in underground streams or rivers. Rather, it flows through fractures, joints, and faults in the bedrock. Well-fractured bedrock often allows more rapid recharge to the bedrock aquifer. Additionally, the more fractures present a particular portion of the bedrock aquifer, the easier it is to complete a high yielding well in that area. In areas without well-developed and/or abundant fracture sets, it is still possible to complete high yielding wells. However, it is much less likely that any given location will produce a well with adequate yield.
DWR's bedrock monitoring wells are typically constructed with steel or PVC casing. The casing is installed from land surface down into approximately 10 feet of competent bedrock. We then emplace in the annular space from the bottom of the casing to land surface to ensure no water from the saprolite aquifer or the surface can migrate along the casing into the bedrock. We then drill an open borehole from the bottom of the casing into the bedrock until we hit sufficient fractures such that we are confident that water levels in the well will accurately reflect groundwater conditions in the bedrock aquifer at that point. We typically do not have more than 150 or 200 feet of open hole in our monitoring wells. However, several of our bedrock monitoring wells are old water supply wells that may have several hundred feet of open hole.
Finally, we install a locking cap or lockable recorder box at the well head to protect the well and any equipment in it from vandalism and the elements.