What is a groundwater monitoring well?
A groundwater monitoring well — also called a standpipe or piezometer — is a slim plastic tube installed in a borehole to measure the groundwater level. The lower section, the filter, is slotted so groundwater can flow freely in and out, and the water level inside the tube matches the water table at that spot.
Construction
From bottom to top a monitoring well consists of:
- Filter section. A slotted length of tube, typically half a metre to a few metres long, wrapped in filter sock or surrounded by filter gravel so that no sand enters.
- Riser pipe. The solid section up to ground level, usually PVC or HDPE a few centimetres in diameter.
- Seal. A layer of swelling clay (bentonite) above the filter gravel, so rainwater cannot run down alongside the tube and corrupt the reading.
- Head detail. A lockable road box in paved areas, or an above-ground capped head in a garden.
Taking readings
The water level is measured with a dip meter: a tape with a sensor that signals as soon as it touches water. You read the depth from the top of the tube and convert it to a national datum, because only absolute levels can be compared with each other. For continuous recording, a data logger with a pressure transducer is suspended in the tube and reads hourly; that series must be corrected for barometric pressure.
A single reading tells you little. The water table fluctuates seasonally, after rain, and with water level management. To judge a foundation you need a record of at least a year — and what matters most is the lowest level relative to the pile heads.
Applications
- Foundation surveys. Establishing whether timber pile heads dry out, the cause of pile rot.
- Damp problems in cellars and crawl spaces.
- Excavations. Monitoring drawdown during dewatering, both inside the excavation and at neighbouring buildings.
- Infiltration. Checking whether there is enough room between ground level and the water table for a soakaway.
A monitoring well a few metres deep is relatively inexpensive; expect a few hundred euros or pounds per well for installation, depending on depth, drilling method and access. Many municipalities already operate monitoring networks with publicly available data — worth checking before you commission drilling of your own.
Related terms
- Groundwater level
- Water level management
- Foundation survey
- Dewatering
- Pile rot
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