Monitoring wells provide groundwater samples that can be chemically analyzed and water level elevations that indicate directions of groundwater movement. They can also be tested to estimate hydraulic characteristics of aquifers (discussed in Chapter 5). A monitoring well is similar to a piezometer in that it consists of piping, called casing, open at the bottom. However, monitoring wells typically have longer open (screened) intervals for retrieving water samples or testing aquifers (Figure 3.1 ). The interval of casing open to an aquifer varies with application, but is generally less than 5 m, and commonly ranges from 1 to 3 m. Wells designed to sample groundwater near the water table employ longer screened intervals, in order to accommodate fluctuating water levels. Monitoring wells that target discrete aquifer intervals utilize shorter well screens. Long well screens provide vertically composite rather than discrete samples - groundwater from different elevations enters and mixes within the well casing.