Gas and groundwater monitoring wells explained

A monitoring well is a borehole completed so that ground gas or groundwater can be measured over time, rather than sampled once. It lets a consultant build a picture of what is present, at what level or concentration, and whether it is changing - which is central to assessing contaminated land and ground gas risk.

Why monitor over time?

A single visit is only a snapshot. Groundwater levels rise and fall, and ground gas concentrations and flows vary with the weather and barometric pressure. A monitoring installation lets readings be taken across a period, so decisions rest on a trend rather than one reading.

How a well is installed

The well is installed into the borehole once it has been drilled to the required depth. We install through the hollow stem augers, rotary casing or within non-cased open boreholes (where drilled through competent rock). In principle:

• A slotted response zone (well screen) is set at the depth to be monitored.
• A gravel or sand filter pack is placed around the screen so it draws freely.
• The rest of the hole is sealed with bentonite pellets or grout to prevent shallower or perched water from entering the screened section.
• The well is finished with a secure, sealed cover, flush with the ground or in a stand-up chamber.

We install single, dual and multi-level wells - for example Solinst CMT - across our window sampling, cable percussion and rotary work, so the right installation goes in whatever method suits the ground.

Gas vs groundwater wells

The principle is the same but the detail differs: groundwater wells are screened across the water level or a specific horizon to allow sampling and level readings, while ground gas wells are set and sealed to draw soil gas from a defined zone. The design follows the consultant's monitoring objectives, and the same holes can be completed as injection, abstraction or vapour extraction wells where remediation is the aim.

Development, maintenance and decommissioning

If specified, new wells can be developed to remove any fines from the groundwater, giving a clean sample. We can also repair existing monitoring wells where the headworks have become damaged, and we can rehabilitate wells which have become silted up over time so that they return to a serviceable condition.

Once a monitoring or remediation well is no longer required we can decommission it to remove any pathway for possible contamination. Wells can be pressure grouted or backfilled with material to mimic the surrounding geology whilst remaining in situ, or they can be removed by overdrilling before the borehole is backfilled.