Unconfined Aquifer

Published: May 7, 2017 | Last updated: July 5, 2023

What Does Unconfined Aquifer Mean?

An unconfined aquifer is defined as a body of water formed from groundwater, rain water runoff and streams with its water table, or the upper surface, open to the atmosphere. They can be problematic as they fluctuate under atmospheric pressure.

These aquifers also form at a faster rate than confined aquifers. In the context of trenchless technology, the presence of unconfined aquifers can challenge trenchless construction in several ways. Unconfined aquifers have clay, sand and gravel layers which can complicate the process of dewatering an area where drilling or microtunneling is to be carried out.

Trenchlesspedia Explains Unconfined Aquifer

Unconfined aquifers are formed when groundwater or run-off seeps through the pores in the permeable rock and soil and collects in impermeable rock strata. The upper water surface of the aquifer or the water table is at atmospheric pressures, so rises and falls and is more impacted by drought conditions than confined aquifers. The National Groundwater Association also notes unconfined aquifers are also subject to losses due to plant uptake and evaporation.

To be classified as an unconfined aquifer:

  • The aquifer should not be confined by impermeable material on all sides.
  • Its water table or upper surface should not be confined from the effects of atmospheric pressure.

Encountering Unconfined Aquifers During Drilling

The water from unconfined aquifers is mainly used for domestic and agricultural purposes. Because unconfined aquifers are open to the atmosphere they carry a greater risk of contamination. Also,the presence of aquifer systems, or any other water-bearing formations, near a construction zone is a concern for drillers.

Unconfined aquifers can be found below rivers and streams and consist of layers of sand, limestone, and gravel. This poses challenges during directional drilling or microtunneling due to the variation in soil density which can cause over-excavation or instability. Contractors work closely with hydro-geologists to take necessary precautions to protect freshwater aquifers from getting contaminated by construction activities.

Groundwater Contamination

Contamination of groundwater takes place when fluids from the trenchless operation containing chemicals and other additives seep into the aquifers and mix with the groundwater. Some of the most common types of contaminants that can be produced as a result of drilling and construction operations are gasoline, drilling fluids, and heavy metals.

Chemicals used in drilling fluids have come under the scanner of the Environmental Protection Agency (EPA) due to the presence of toxic chemicals and additives. Hydro-geologists are therefore required to assess groundwater sources and analyze their susceptibility to contamination from drilling fluids before approval of the project.

On the contrary, it is also possible for water from aquifers to flow into trenchless tunnels and contaminate, dilute or destabilize drilling fluids, causing stability or control problems. Therefore, aquifer locations and groundwater movements should be properly investigated to ensure that the drill site or the surrounding water bodies are not compromised during construction activities. Hydrogeological reports give essential information in this regard by providing data on groundwater fluctuations as well as advising the requirement for dewatering.

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