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Groundwater remediation

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Groundwater remediation is the scientific process used to treat polluted water located beneath the Earth's surface. Groundwater exists in the pore spaces of subsurface soil and rock. This resource is vital for human survival and global industry. Globally, between 25 percent and 40 percent of drinking water comes from dug wells and boreholes. Farmers also rely on it for crop irrigation. Many industries use it to manufacture everyday goods. While most groundwater is clean, it can become contaminated by natural conditions or human activities. Contamination can lead to serious public health hazards, including poisoning or the spread of disease. Remediation aims to remove these pollutants or convert them into harmless products.

Pollutants in groundwater are highly diverse in their nature. They can include physical particles, inorganic chemicals, organic chemicals, bacteria, or radioactive materials. Because these contaminants vary, scientists use different technologies to reach safe water standards. Most successful projects use a combination of biological, chemical, and physical technologies. The goal is to bring the water to a quality level appropriate for its intended use. This requires a deep understanding of how different substances move through the subsurface environment.

Biological treatment technologies utilize living organisms to break down contaminants. One method is bioaugmentation, which involves adding specific microbial strains to a site. This is used if a study shows that natural degradation is too slow. This process increases the concentration of reactive enzymes to speed up the breakdown of pollutants. Another method is bioventing, which stimulates indigenous bacteria in the unsaturated zone. It does this by inducing air or oxygen flow and sometimes adding nutrients. Biosparging is similar but focuses on the saturated zone. It injects air and nutrients to help microbes degrade petroleum constituents dissolved in the water.

Other biological methods use different natural mechanisms to clean the environment. Bioslurping combines bioventing with vacuum-enhanced pumping. It uses a "slurp" tube to draw up liquid and soil gas. This is effective for recovering light non-aqueous phase liquids (LNAPL), such as oil, from the water table. Phytoremediation uses plants and trees to absorb contaminants through their roots. For instance, the Chinese Ladder fern is an efficient accumulator of arsenic. Genetically altered cottonwood trees can absorb mercury, while transgenic Indian mustard plants soak up selenium.

Chemical treatment technologies use reactive substances to alter or remove pollutants. Chemical precipitation is often used to remove heavy metals by turning them into insoluble compounds. These precipitates are then removed through settling or filtering. Ion exchange involves passing water under pressure through a bed of granular media, such as zeolites or synthetic resins. This process displaces specific ions in the water with others from the media. Carbon adsorption uses activated carbon, often derived from bituminous coal, to catch volatile organic compounds on its surface.

More intense chemical processes include In Situ Chemical Oxidation, or ISCO. In this process, oxidants like ozone, oxygen, or hydrogen peroxide are delivered into the subsurface. These chemicals destroy organic molecules by converting them into water, carbon dioxide, or other nontoxic substances. Another technique is surfactant-enhanced recovery. Surfactants are surface-active agents, similar to ingredients in soap. They are injected to increase the mobility and solubility of contaminants. This is especially useful for dense non-aqueous phase liquids (DNAPL), such as trichloroethylene (TCE). These dense compounds sink in groundwater and can create plumes that stretch for miles.

Physical treatment technologies focus on the movement of water and vapors. The most common method is pump and treat. This involves pumping contaminated groundwater to the surface for subsequent treatment. Air sparging is another physical method that blows air directly into the groundwater. As bubbles rise, they strip contaminants from the water through physical contact. These contaminants are then carried into the soil as vapors. Finally, dual-phase vacuum extraction (DPVE) uses a high-vacuum system. It removes both contaminated groundwater and soil vapor simultaneously. This is highly effective because it cleans the capillary fringe, an area just above the water table that often holds high concentrations of undissolved chemicals.

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