Groundwater pollution

DEFINITION: Degradation, by chemicals and other substances, of the water found below the surface of the earth

Many public and private water supplies rely on wells that tap important groundwater reserves. Pollution of groundwater leads to changes in water quality that can affect groundwater use for a given purpose.

Humans require vast amounts of fresh water for use in homes, livestock operations, agriculture, and industrial processes. Groundwater is an important source of freshwater. The pollution of groundwater by human activity can contaminate water-supply wells, making the water they provide unacceptable for drinking and other purposes. This can lead to a need for new water supplies that may not be readily available or easily accessible. In some instances, polluted groundwater interacts with surface water, thus contaminating the surface-water environment as well.

Groundwater constitutes a small but significant portion of the world’s total water supply. Much of the earth’s surface is covered by water, but an estimated 97.2 percent of it exists as salt water. Since fresh surface water may account for as little as 0.009 percent of the earth’s water, groundwater is a significant source of readily available fresh water. Groundwater occurs in the saturated zone of the earth, which is the area below the surface where pores between particles—void spaces in the soil or rock—are filled with water. In some places groundwater may be encountered near the surface, but in other areas, such as arid regions, it can be quite deep below the surface. Groundwater flows from areas of high hydrostatic head to areas of low hydrostatic head. Shallow groundwater often mimics topography, flowing downhill toward streams and lakes.

The soil and rock through which groundwater flows consist of particles of varying size, which help determine the classification of the soil or rock and how well water will move through the material. Sand-sized particles are seen in unconsolidated sandy soils or sandstones. Smaller particles may form silty or clayey soils or their bedrock equivalents of siltstones and shales. In the saturated zone, groundwater saturates the pores and voids between the particles. The size of the pores and the degree to which they are interconnected affect hydraulic conductivity—a measure of the ability of water to move through the rock or soil.

Transmissivity is the measure of the ability of an aquifer to transmit water and is a measure of the hydraulic conductivity multiplied by the saturated thickness of the aquifer. Therefore, a thick aquifer with relatively poor hydraulic conductivity might be able to transmit as much water as a thinner aquifer composed of materials with greater hydraulic conductivity. Groundwater is recharged by rainwater percolating through the soil, snowmelt, and rivers and streams.

Threats to Groundwater

Humans produce a wide array of pollutants and combinations of pollutants. The degree and extent to which individual pollutants can affect groundwater quality is dependent on a large number of variables, which can include the amount of contaminant introduced into the environment, the time frame in which it is introduced, its toxicity, its mobility, whether it will readily degrade in the environment, and the chemical and physical characteristics of the soil or rock through which it will pass.

Even something as common as nitrogen can lead to pollution in groundwater. Nitrogen can be mobile in the environment in the form of dissolved nitrates and nitrites. Sources for pollution include septic tanks, leaks from sewage treatment plants and lagoons, and animal wastes. Nitrogen is also an important component of many fertilizers used in agriculture, and such fertilizers may become dissolved by rainwater and percolate down into groundwater. In high enough concentrations, nitrates can make water unacceptable for human consumption. At even higher concentrations, the water can become unacceptable for livestock and other animals.

Gasoline spills and leaks from underground storage tanks are relatively common sources of groundwater pollution. Some of the dissolved-phase components of gasoline are quite mobile in the environment; however, many are also susceptible to biological degradation. Gasoline and other substances less dense than water can float on the surface of groundwater, but seasonal fluctuations in the water table can smear such contaminants in the soil, potentially making them more difficult to remove. Other contaminants, such as chlorinated solvents, can be denser than water and have the capacity to sink into aquifers.

Although metals as a group are generally not considered very mobile and tend to be adsorbed onto soils, some are quite mobile, and contamination by heavy metals can be a relatively common form of groundwater contamination. Although less common, radiological contamination of groundwater can be a concern. Groundwater often moves slowly, but radioactive half-lives can be quite long.

By the 2020s, new threats to groundwater had emerged. The first was where conditions such as global climate change and overconsumption lead to groundwater scarcity. This was especially prevalent in the western United States. Persistent drought conditions, increased demand from agriculture, and growing populations were shown in depleted groundwater levels in reservoirs, lakes, and rivers. As the sea level continues to rise, in inland areas, it will lift groundwater into shallower soils. Scientists fear that the rising water could infiltrate hundreds of US Superfund sites. These are severely polluted areas identified by the US Environmental Protection Area (EPA) for future cleanup. These and many other polluted areas could release pesticides, industrial chemicals, heavy metals, and radioactive elements into groundwater.

Ironically, attempts to address the impacts of greenhouse gas emissions had the potential to increase groundwater pollution. In 2022, the US Congress passed the $369 billion Inflation Reduction Act. This legislation was intended to fund projects that could reverse climate change, such as in the generation of electricity. Several related proposals to replace fossil fuels such as coal with corn-fed ethanol refineries and manure-fired generation came under fire. Runoff from agricultural fertilizers and fecal contamination into rivers and streams were feared to have potential and catastrophic consequences for groundwater sources.

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