13 Jul WHY THE TYPE OF REMEDIATION YOU CHOOSE MATTERS
Client properties with soil or groundwater impacts often require remediation. The form of remediation used can make a significant difference regarding the outcome — especially as it relates to project duration and cost — and the choice isn’t always obvious. This article provides a general overview of how ESA examines and evaluates environmental impacts to decide on the best course of action, and what that means to the client’s bottom line.
Identifying the Contaminant
The chemical nature of a contaminant often dictates the most likely remedial approaches.
| Contaminant Type | Chemistry | Toxicity | Remedial Method |
| Heavy metals (E.g., arsenic, mercury, lead, etc.) | Largely insoluble unless exposed to acidic conditions | Variable, depending upon which metal and its form | Excavation and disposal. Sometimes chemical stabilization or isolation |
| Gasoline, oil, diesel, and various non-chlorinated solvents | Volatile. Fairly miscible with water. Less dense than water; floaters | Variable. Heating oil and diesel are low toxicity. The others tend to be more toxic | Introduction of bacteria and chemical oxidation, soil vapor extraction, air sparging, carbon adsorption |
| Chlorinated solvents | Volatile. Barely miscible with water. Denser than water; sinkers | Most tend to be toxic | Specialized bacteria introduction, chemical introduction to promote oxidation, soil vapor extraction, air sparging, carbon adsorption |
| Semi-volatile compounds (e.g. coal tars and PCBs) | Largely insoluble in water. Resistant to weather; persistent | Mostly toxic. | Difficult to remediate. Often excavation and disposal is the best option or, Isolation |
| Pesticides | Highly variable | Toxic by design | Variable, depending upon which pesticide is present |
| Asbestos | An inert mineral | Toxic only if particles are inhaled. No dermal toxicity | Excavation and disposal |
The table above shows some of the most common contaminants and the standard remedial treatment for each. It is not intended to be an exhaustive list or to provide definitive treatment solutions. Each instance of site contamination is unique and requires a qualified environmental consultant to determine the best course of remedial action.
Active vs. Passive Remediation
A host of variables determine if ESA will ultimately recommend active or passive remediation on any given project. Active remediation, as the name indicates, requires a hands-on approach to the removal of contaminants, such as pumping groundwater up from the subsurface and then treating it or injecting air into the subsurface to facilitate volatilization. Passive remediation is largely an administrative exercise that requires time and commitment to long-term monitoring while allowing natural processes to reduce and/or degrade various chemical contaminants. When a passive remedial approach is used for soil, an engineering control, such as an impervious cap, must be installed and monitored. When a passive remedial approach involves groundwater, natural groundwater flow and a variety of geochemical interactions gradually decrease impacts. Active forms of remediation are typically more expensive than passive forms. Consequently, ESA generally seeks to employ passive remediation whenever conditions allow.
Examples of Active Remediation
If soil impacts are easily accessible, ESA often recommends that material be excavated, transported, and disposed of (T&D) at an appropriate disposal facility. T&D has several advantages, including speed of execution, cost (reasonable, but not always the cheapest), and, perhaps most importantly, finality — because “gone is good.”
However, T&D sometimes isn’t practical or possible. Thus, other forms of active remediation may be implemented, including soil vapor extraction, air sparging, bioremediation, pumping and treatment of groundwater, thermal desorption, incineration, solidification, stabilization, vitrification, and oxidation. These various remedial approaches all work, but their effectiveness varies depending on the target contaminants, their form and concentration, and the geology in which they lie. These other forms of active remediation are often expensive and time consuming. That’s why T&D is most often the recommended solution when active remediation is necessary.
In-Situ vs. Ex Situ Remediation
Active remediation can be performed in situ or ex situ, depending on the situation. “Situ” is a Latin word meaning “original, natural, or existing place or position.” Thus, “in situ” means “in the original place” and “ex situ” means “outside the original place.” So, when ESA conducts “in situ remediation,” contaminants are being remediated where they are currently located — usually underground. Various considerations dictate whether in situ or ex situ remediation makes more sense.
Most active forms of remediation are conducted in-situ, but not all. For example, contaminated soil can be excavated, put into rows or a pile, and remediated ex situ through soil vapor extraction or bioremediation. The greatest advantage of ex situ versus in situ bioremediation is that the bacteria are far more likely to come in contact with the contaminants (a requirement for bioremediation to work), and it is easier to aerate/oxygenate the soil, thus hastening the bioremedial process. Pump and treat is also an ex situ process where contaminated groundwater is pumped to the surface and treated above ground in air strippers, carbon units, sparging pools, etc.
Oftentimes, the most efficient and effective solution — whether active or passive, in situ or ex situ — isn’t as obvious as it might seem. The experts at ESA pride themselves in their ability to identify and employ remediation strategies other environmental consultants frequently miss, consistently saving our clients thousands of dollars and a great deal of time in the process.