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Groundwater remediation: Countering water contamination

Summary

An overview of commonly used groundwater remediation methods and their limitations
The removal or containment of pollutants is called remediation. Contaminants must often be removed from groundwater before it reaches wells used by agriculture and water systems. Remediation is required when the concentration of contaminants exceeds or is expected to exceed predetermined levels for the type of resource affected. Some common remediation methods are outlined below.

Pump and treat

This involves: removing contaminated groundwater from strategically placed wells; treating the extracted water after it has reached the surface to remove the contaminates using mechanical, chemical or biological methods; and discharging the treated water to the subsurface, surface or municipal sewer system. This method has several limitations:

• Its effectiveness depends on the geology of the aquifer and the type of contaminant;

• It is slow, taking decades to centuries to remove contaminated water, and often fails to remove it all;

• It is costly and doesn’t always work. Some contaminants stick to soil and rock (they are adsorbed) and cannot easily be desorbed. Non-aqueous phase liquids cannot be removed.

Hydraulic containment

Water can be pumped from wells in such a way that it changes the flow through an aquifer in order to keep contaminants away from cities and farms. The technique works if the water flow through the aquifer is relatively simple, so the plume of contaminated water does not divide into different paths. It is often used in combination with the pump-and-treat method and has the same limitations.

Air sparging/soil vapour extraction

Limitations include difficulty of flushing in low-permeability zones, operating below 9m and extracting multi-component phases. An example of air sparging is illustrated (above, left): this system was used to remove volatile trichloroethylene from the soil and an aquifer below the Blaine Naval Ammunition Depot east of Hastings in Nebraska. At one time during World War II, the 50,000 acre facility produced 40% of all US Navy munitions. Later in the remediation, air-containing natural gas and triethyl phosphate was pumped into the groundwater to improve bioremediation by soil bacteria.

In-situ oxidation

An oxidant such as hydrogen peroxide is injected into the contaminated aquifer. The contaminant is oxidised, primarily producing carbon dioxide and water. In the illustration below, a permeable treatment zone is created by reducing the ferric iron in the aquifer sediments to ferrous iron by injecting a reducing reagent and appropriate buffers, such as sodium dithionite and potassium carbonate.

Permeable reactive barriers

This method uses a trench backfilled with reactive material such as iron filings, activated carbon or peat, which absorb and transform the contaminant as the water from the aquifer flows through the barrier. This only works for relatively shallow aquifers. A Dewind trencher installing a permeable treatment barrier in a trench is illustrated.

Phytoremediation

Some plants accumulate heavy metals and metal-like elements such as arsenic, lead, uranium, selenium, cadmium and other toxins, including nutrients, hydrocarbons and chlorinated hydrocarbons. Chinese Ladder fern Pteris vittata, known as the brake fern, is a very efficient accumulator of arsenic. Genetically-altered cottonwood trees have sucked mercury from contaminated soil in Danbury, Connecticut, while transgenic Indian mustard plants have been used to soak up dangerously high selenium deposits in California.

Remediation consists of growing such plants so their roots tap the groundwater, and then they are harvested and disposed of. This method is limited to the remediation of groundwater that is close enough to the surface to be reached by roots. A normal plant can be poisoned with as little as 1,000ppm of zinc or 20-50ppm of cadmium in its shoots. But, Thlaspi caerulescens (alpine pennycress – a member of the broccoli and cabbage family) can accumulate up to 30,000ppm of zinc and 1,500ppm of cadmium in its shoots, while showing few or no toxicity symptoms.

Natural attenuation

Sometimes natural processes remove contaminants without human intervention. Removal may involve dilution, radioactive decay, sorption (attachment of compounds to geologic materials), volatilisation or natural chemical reactions that stabilise, destroy or transform contaminants.

Intrinsic & enhanced bioremediation Bacteria and archaea can metabolise hydrocarbons and other contaminants, converting them to less toxic products. Some live deep underground while others survive in the absence of oxygen. Specific organisms are injected into the groundwater and, in some cases, special nutrient are injected with the microbes. The method is especially useful for the remediation of hydrocarbons in groundwater.


Courtsey GDI &  Department of Geosciences, Texas A&M University for material used in this article
 
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