Groundwater pump and treat is a traditional method for cleaning groundwater from dissolved chemicals, industrial solvents, metals, and fuel oil.
It involves pumping groundwater from wells to an above-ground treatment system that removes the contaminants. It is applicable to contain the contaminated plume, as well.
The process begins by pumping the contaminated groundwater directly out of the surface. The treatment occurs above the ground. The pump and treat technique then works by discharging the cleaned groundwater into sewer systems or re-injects it into the subsurface.
Pump and treat is an effective way of cleaning up contaminated groundwater while at the same time keeping it from water supplies, wildlife habitats, or rivers and lakes.
Pump and treat may last from a few years to several decades. It is an appropriate cleanup method on the sites where natural attenuation is not feasible due to lack of land area or slow degradation rates.
Groundwater pumping is a component of many pump-and-treat processes, some of the most commonly used groundwater remediation technologies at contaminated sites.
Possible groundwater pumping objectives include removing dissolved contaminants from the subsurface and containment of contaminated groundwater to prevent migration.
The first step of any remediation project consists of defining the remedial action objectives to be accomplished at the site. This involves gathering enough background site information and field data to make assessments of remedial requirements and potential alternatives for achieving those requirements. The next step is determining how much time it will take to achieve the required objectives and possible limitations on access or equipment use during that period. In cases where there are constraints on available resources, a plan must be developed to meet immediate site challenges with available resources.
Once a remedial plan has been defined, it is necessary to estimate the costs of implementing that plan. The final step in any remediation project involves projecting estimated costs and expected benefits over time for various possible control strategies. Management can then use these projections to develop an implementation strategy that considers both cost and feasibility factors. For example, site constraints such as existing structures or contamination, which may limit treatment area options, must also be considered when developing a remediation strategy.
In most cases of groundwater containment, the first technique employed is setting up an impermeable barrier around the contaminant plume. This technique is generally effective at restricting lateral groundwater contamination; however, contaminants present in the vadose zone can be affected by pumping. Pumping will decrease the pressure gradient and reduce the potential for contaminant transport [in layman’s terms: The pumping action decreases the pressure differential at which water is migrating (either up or down), slowing (lessening) migration of contaminants.
Calculations to determine long-term containment efficiency must include parameters such as estimated well life, rate of drawdown, and contaminant plume shear strength. Initial placement should consider site-specific factors such as land use and geology and their effects on aquifer hydraulic properties. Where there are concerns about persistent low-level concentrations of organic compounds reaching shallow wells via advection, a high flow pump and treat alternative with sufficient treatment capacity may be the preferred option.
Groundwater containment is often used with other groundwater remediation techniques such as pump-and-treat or natural attenuation. Containment can be more effective by completing it before construction begins on nearby property.
The effect of pumping on contaminant transport should be considered during project design because there are many potential consequences to changing groundwater levels near the site, including possible changes in contaminant plume migration paths resulting from increased drawdown, temporary loss of containment due to well screen dewatering, and disruption of nearby wells that occur when placing low permeability barriers around large areas of land.
Pump-and-treat is another approach for groundwater remediation that relies on pumping contaminated groundwater from wells to surface waterways to be treated prior to discharge. Pumping rates are typically high, and this method is often used at sites with large contaminant loads or long distances to treatment facilities. However, suppose elevated pump rates become impracticable due to excessive drawdown of overlying potable aquifers. In that case, the site may be suitable for an alternative passive system such as natural attenuation, which requires only low flow conditions in the subsurface (e.g., rainfall recharge).
Passive systems rely upon naturally occurring chemical reactions between water and minerals in the subsurface to break down contaminants into non-toxic substances and occur at meager rates. The potential for natural attenuation as a treatment strategy depends on site hydrogeology, aquifer properties, contaminant source, groundwater residence time, and other factors that affect contaminant degradation rates. In many cases, passive systems are only effective over long periods (years to decades).
If achieving remedial goals is dependent upon maximizing degradation rates for difficult to degrade compounds such as chlorinated aliphatics or semivolatile organic compounds (SVOCs), then pump and treat may be preferred over natural attenuation because active pumping will increase mass transport of these contaminants through the vadose zone. In addition, passive systems may be better suited for sites with lower contaminant concentrations or where degradation rates can be controlled by controlling the source of contaminants to the site.
For contaminated groundwater containment, passive systems are typically less expensive than pump and treat alternatives because they require fewer wells and equipment. They also have a little operational impact on the site after construction ends, making them appropriate for long-term cleanup of persistent contaminants. If natural attenuation is not feasible due to lack of available land area or slow degradation rates, pump-and-treat may be used instead.
