Groundwater sampling can be performed in existing facilities (wells, piezometers, springs, etc.) or in new ones that need to be built. In this latter case, it is more frequent to install piezometers of a small diameter, just enough to be compatible with the monitoring equipment to be subsequently used. Most groundwater sampling installations must be located downgradient to the road discharges in terms of the local groundwater flow if they are to look for road-induced contamination. A few upstream locations may also be chosen to allow a reference water condition to be established. Their depth should be at least 2-3 m below the minimum annual groundwater level in order to avoid having a dry piezometer. The necessary number of piezometers depends on the dynamics of groundwater (i. e. varying condition with time and place) (Leitao, 2003). Higher permeability and hydraulic gradient implies monitoring in more locations and more frequently.
To obtain a sample that is representative of the water in the well or piezometer in question, there should be a purging operation until electrical conductivity, pH and temperature of the outflow water have stabilized (Aller et al., 1989). In low permeability soils the purging operation should try to minimize the water displaced during purging, otherwise recharge of the sampling location will take too long to allow realistic sampling. The water can then be sampled directly or using a device similar to Van Dorn’s water bottle, but with a size compatible with the well diameter (Fig. 7.6).
