Based on the functions and purposes, the monitoring wells can be divided into the following categories: Water quality monitoring： Monitor the water quality parameters of rivers, lakes, reservoirs, oceans and groundwater. The monitoring can be automatic or continuous. Groundwater monitoring system：This monitoring system aims to detect the changes in groundwater quality or water quantity and looks into the monitoring wells. For example, to find out whether groundwater beneath the landfill is polluted or not, the monitoring agency would drill a reference well in the hydraulic gradient line on the upstream side of the landfill, which represents the water not contaminated by leakage. On the downstream side, three monitoring wells are set up, and samples are collected from the three sites to inspect the level of groundwater contamination due to leakage at different depths. Source monitoring well：This type of well aims to monitor the changes in groundwater quality due to contamination by a specific or potential pollutant nearby, and the well serves two monitoring purposes: inspection and evaluation. The former is to enhance the early warning capability for detecting groundwater pollution, whereas the latter is to evaluate the concentration and scope of pollutants when the known groundwater pollution is reported. This type of monitoring well is mostly a site-specific monitoring well. Enforcement monitoring well：This type of well is set up to address the actual pollution incidents that have already happened, and the purpose is to collect evidence to confirm the sources of pollution, so as to resolve the dispute. This kind of well is subject to legal enforcement, so the requirements for quality over the well are higher.