What is Natural Attenuation?

Earth is an amazing thing. It’s fragile, but at the same time it has an incredible ability to find its own solutions to the problems we create. Take the sunflower solution to a nuclear plant meltdown: sunflowers can bioaccumulate, or uptake, radiation, thereby sucking radioactive isotopes out of the water and soil. 

Natural attenuation is the metaphorical ‘sunflower’ of groundwater contamination.

First: what’s attenuation?

Attenuation is the reduction of a signal; a weakening of magnitude. It’s a general term that’s usually applied to an electrical signal strength or in the field of physics.

Second: what’s natural attenuation?

In the field of science which includes soil pollution, environmental cleanup, geochemistry, bioremediation, groundwater, environmental engineering and the like, natural attenuation applies to nature’s ability to reduce the toxicity, mobility, volume or concentration of contaminants in water and soil.   

The environmental cleanup or remediation industry uses the term ‘natural attenuation’ as a pseudo-euphemism for ‘let nature take its course’

Fine-grained media, such as clays and silts, have a large surface area to volume ratio and a chemical charge to which contaminants often adhere. This process is called adsorption. As water molecules travel in the ground from high pressure to low pressure areas, the contaminants they carry can adsorb to the grains, allowing more clean groundwater (though not ‘clean’ in the classical sense) to be found downgradient.

Water moving through clays and silts is not unlike the process used in water filters. They use activated carbon (usually derived from coconut shells) in small bits the size of silt or clay to trap contaminants. In a sense, it’s as if the ground itself can act as a huge water filter.

But not all soils and rocks can naturally attenuate contaminants

As mentioned, very fine-grained materials with the right chemical charge can be conducive to natural attenuation, especially if layers are spatially continuous and thick. However, not all geology is so forgiving. Soils that are acidic or oxidized will not be suitable for natural attenuation. Larger grained materials such as sandstone (think: tan beach sand cemented together) or granitic rock will not have this effect either.

Natural attenuation is a process that can only apply in certain environments and under specific conditions. You’ll likely need a hydrogeologist to tell you if it’s an option at a specific site. It’s certainly not a blanket solution, but it’s great to know that we can use mother earth to our advantage at times.

Third: what’s monitored natural attenuation?

Monitored natural attenuation is conducted by professionals and technicians, usually hydrogeologists.  It consists of collecting and analyzing groundwater samples at regular intervals to observe if concentrations of contaminants are decreasing - an indication that natural attenuation may be successfully occurring. It’s possible that the ground can appear to attenuate a contaminant, but then become saturated by it (it can no longer take up or adsorb any more contaminants). In this case, follow up and proper record keeping of the plume migration becomes key.

Setting groundwater target levels helps us define success in remediation efforts. Groundwater cleanup target levels are usually defined by the EPA, but each state can opt for more stringent cleanup goals. 

What are groundwater monitoring wells and why are they important to natural attenuation?

Monitoring wells are typically between 2 to 4 inches in diameter and made of PVC pipe. They will be placed at a depth that reaches the groundwater in question. At the bottom of the pipe is a section that is slotted so that water can enter the pipe, but mud and gravel stay out.

Monitoring wells provide quick access points to sample the groundwater at regular intervals. They’re typically installed in three’s: one upstream and two downstream of the groundwater where contamination is found or suspected.

We have to monitor the contaminant before and after attenuation or remediation to know if it worked. Some constituents that we think of as contaminants, such as arsenic, occur naturally in the environment. (Not unlike the way poison ivy is ‘natural.’) In other words, groundwater can be unusable even before we humans pollute it. Because of this, it’s important to know the starting quality of water to know if concentrations of contaminants have attenuated over a distance or over time.

Why should I care about how to clean up groundwater contamination?

Because you’re probably drinking groundwater. 

The water from your tap usually comes from multiple sources (unless you are on domestic well water, in which case you really care about and already know about groundwater).

Municipal water treatment facilities usually have a primary source of water from a surface body of water, like a river or reservoir, and secondary sources of water supplies. Some of the additional or backup water supplies can include groundwater from city wells, transmountain pipelines carrying water from other catchment basins, stormwater, and more.

Will groundwater run out?

Often, groundwater is a supply that can be easily depleted without us even knowing it (or caring, since it doesn’t have an immediate effect on our day to day lives). Nevertheless, it is a precious resource and one that we ought to care about more given its foundational role in water supply resilience.

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