You may have read about environmental cleanups where a "pump and treat" operation is either planned or ongoing. No, it's not related to Halloween. Pump and treat is a remediation method for cleaning up contaminated groundwater.
Groundwater is the water underground that flows through the porous sand, gravel, and fractures in subsurface clay and bedrock. It only rarely is in caverns or streams, and that only in very specific circumstances (called "karst" geology), but is more like the water you might see seeping from a hillside spring: it has saturated the soil, is prevented from flowing deeper into the earth by an impervious layer of clay or bedrock below it, and since water always seeks a lower level, it has both a flow direction ("gradient") and a speed in that direction. The speed at which groundwater flows is limited by the grain size of the sand and gravel through which water moves. In simple terms, groundwater that has measurable direction and flow is termed an "aquifer". The more productive aquifers are used for drinking water, farm irrigation, and industrial and commercial uses, so they are valuable resources, and we spend considerable amounts of effort cleaning them up if they get contaminated. Since groundwater is fed by surface recharge (precipitation in the form of rain and snow), which replenishes the aquifer by soaking through the surface of the earth, aquifers can also be impacted by other things that can soak through the surface with the rain: chemical spills, road runoff, agricultural pesticides and fertilizers.
There are deeper aquifers as well, that were created by glacier melt or glacial overlay on an ancient river system, that are only minimally replenished by surface precipitation; and there are non-flowing groundwater systems, like surface ponds, that are often termed "perched" groundwater-where rainwater and snowmelt is caught underground in a bowl or a depression on top of an impervious layer under the surface.
The "pump" part of pump and treat is groundwater wells. Wells are installed with a slotted screened section (so water can come in) that is placed in the vertical vicinity of the contaminated groundwater. These slotted sections are surrounded by a gravel pack that limits sand and sediment from entering the well. Usually a submersible well pump is installed in the slotted section of the well, and the pump is operated at a high enough pumping rate that a "zone of influence" is established near the well that induces all water within that zone to be pulled into the well. A set of multiple wells, installed perpendicular to the direction of the groundwater flow, with overlapping zones of influence, ensures that no water in the vicinity escapes.
The water from these wells is pumped to a treatment system. Depending on the contamination, the treatment can consist of units that strip contaminants from the water using a flowing stream of air (an air stripper), or by filtering the water though a media, often granulated carbon, that has a great affinity for the contamination, so it binds to the carbon and lets the water pass, or by various other treatment methods (using chemicals like hydrogen peroxide, or ultra-violet light).
After treatment, the water is often discharged to surface waters, where it will once again meld with the earth's water, or it may be injected upstream (to the groundwater flow) from the wells to serve as a "rinsing" system that dissolves other contaminants from the soil.
For petroleum hydrocarbon cleanups, like from a spill of gasoline, this is quite effective. Not only does gasoline float on water, so the liquid part of the spill can be captured, assuming the screen is adjacent to the top of the groundwater, the dissolved portion of the gasoline, typically benzene, toluene, xylene, and similar constituents, is highly amenable to the simpler methods of treatment, like air stripping or carbon filtration.
Other chemicals of concern can be much harder to treat. MTBE, methyl tertiary butyl ether, was used as an octane enhancer beginning in the 1970's as a means to fight air pollution from gasoline-powered motor vehicles. Unfortunately, the cure was as bad as the problem, because when MTBE is spilled, it holds tightly to water molecules and is more difficult to remove. It is not as toxic as benzene, but it is stinkier, so groundwater with MTBE contamination at virtually any level is unpalatable even if not as hazardous to life and health.
Chlorinated substances, like dry cleaning fluid (perchloroethylene) or dichlorobenzene, generally do not float on water-they are as a group called "Dense Non-Aqueous Phase Liquids" (DNAPL), and because they sink, rather than float in water, they are harder to recover as both liquids and dissolved components.
But even in the most auspicious circumstances, a pump and treat system is an expensive and long-term fix. Since the cleanup standard for benzene is only 5 parts per billion in water, you will pump a lot of water for a long time to capture any significant amount if the contamination is at a low level. The systems, the facilities to house treatment systems, and the requirements to maintain and change out carbon and strippers, are also an expensive route to travel.
The best benefit of a pump and treat system is that if properly engineered, it will stop the migration of contamination beyond the zone of influence barrier. So residential and other wells, and surface waters, and basements, can be protected from being affected by the contamination while cleanup continues.
There are other methods of cleaning up contaminated groundwater that may make more sense than pumping and treating huge volumes of water, but each system is dependent on local conditions, contamination types and sources, and the time-frame demanded.
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