Investigation Of MGP Sites
How will these sites be investigated?
NYSDEC will investigate these sites with a site characterization followed by a remedial investigation. During these investigations, samples of any waste encountered, as well as groundwater, soils, surface water, and sediments will be collected and analyzed for MGP-related compounds. Samples will be collected from subsurface soils using test pits and soil borings. Monitoring wells will be installed to allow sampling of groundwater. If occupied buildings are present on the MGP site or nearby, then soil gas or indoor air sampling may also be undertaken.
What is a Site Characterization?
A site characterization (SC) is the initial assessment of a site. Typically, the site characterization consists of two parts: a records search to identify potential areas of concern, and a field investigation to inspect and sample those areas. Data gathered during the site characterization is used to determine whether a more thorough Remedial Investigation is needed.
The records search is intended to gather information on past site uses and disposal activities. This aids in the formulation of field characterization activities. Site inspection, in-field testing, and sampling of environmental media (such as soil, groundwater, surface water, sediments, or air) are then conducted.
What is a Remedial Investigation?
The remedial investigation (RI) phase is a detailed and focused continuation of the site characterization. The remedial investigation will fully delineate the nature and extent of site contaminants, impacted site environmental media, source(s) of contamination, and pathways for contaminant migration. Data collected during the remedial investigation is used to evaluate threats to public health and the environment and, when necessary, to conduct a detailed ecological impact analysis and/or Human health exposure assessment. Sufficient data and site knowledge must be generated through the remedial investigation to allow for effective identification and evaluation of remedial action alternatives and to facilitate remedy selection.
What are Typical Sources of Contamination at Former MGP sites?
Contaminants may be found in a wide variety of locations at these sites, but experience has identified the following former MGP structures which are very frequently found to be significant sources of contamination.:
- Gas Holders:
Early MGPs typically stored their gas in expandable tanks called gasholders, or gasometers. These consisted of a large-diameter circular pit dug into the ground, in which a steel tank was allowed to rise and fall according to how much gas needed to be stored. The bottom of the pit was kept full of water to provide a seal and keep the gas from escaping. In some cases, the tank was built in multiple telescoping sections to provide more gas storage space.
Freshly manufactured gas was piped into the relief gas holder while still hot, and was allowed to cool in the holder. Any coal tar which condensed as the gas cooled, would settle to the bottom of the holder foundation and accumulate there. Slow leakage of tar through cracks and joints in the holder foundation was extremely common. NYSDEC has found evidence of tar leakage from these "pit" gas holders at virtually every MGP site investigated to date.
When gas plants were closed and demolished, it was common practice to leave accumulated tar in the gas holder foundations, and fill them with demolition debris from the plant buildings. This is how many holder foundations are found today: circular foundations, often 10-20 feet deep, lined with bricks or concrete, and full of tar-soaked demolition debris such as bricks and timbers. The soil surrounding the holder foundations, is also often found to be heavily contaminated with tar.
Not all gas holders accumulated large volumes of tar. More modern MGPs often included additional distribution gas holders which were built on the ground surface, with no foundation. Although these "at grade" holders often result in lesser impacts to the environment, they occasionally are found to be a source of contamination.
- Tar Separators, Wells and Tanks:
Tar/water mixtures would be piped to in-ground tar wells or tar separators where the tar would be allowed to settle out by gravity. Once separated from the water, tar which was destined for sale or on-site use as fuel would be stored in tanks (often above ground) prior to use. Tar which accumulated in the holder foundations was commonly piped and stored in these tanks as well.
All of these structures were susceptible to spills and leakage. In addition, many in-ground tar handling facilities were left full of tar at the time the plants were abandoned. It is fairly common to find these structures intact today, still containing significant volumes of tar. As with the gas holders, the soil surrounding these structures is often heavily contaminated.
The gas handling and tar handling structures in the MGP were interconnected by pipes, many of which were underground.. Most piping was left in place when the plants were demolished, and it is common to find significant quantities of tar inside these pipes today. Because of the large diameter of some of these pipes (up to three feet in some cases), the amount of tar contained in them can be significant.
Are there any other sources of contamination which may exist at MGP?
In addition to the former MGP structures which often remain at MGP sites, an investigation will also determine whether other sources of contamination may exist. These may include:
- Tar Plumes:
Beyond the immediate vicinity of the MGP structures, tar may migrate through the soils and rock beneath the MGP plant, and past the site boundaries. Tar migration over distances of a few hundred feet is common, and in some cases tar has been found to have moved over 1000 feet from the original source structures. This tar (not just the tar contained in the original structures) is considered as a source of contamination since groundwater which comes into contact with tar will become contaminated with BTEX compounds and PAHs. This contaminated groundwater can them move further thorough the subsurface and potentially impact other areas.
- Purifier Waste Deposits:
Purifier waste was sometimes used as fill material on site or in areas surrounding the MGP site. Groundwater which comes into contact with purifier waste often becomes contaminated with complexed cyanide compounds and may also become highly acidic (low pH).
Why Does Tar Move in the Subsurface?
Both carburetted water gas tars and coal carbonization tars are slightly more dense than water, and thus tend to sink through groundwater and may also be found at the bottom of water bodies. For this property, coal tar is often referred to as a dense non-aqueous phase liquid or DNAPL. However, the difference in density is small enough to allow tars to float for significant periods of time. When liquid tar is found on the bottom of water bodies, it is not uncommon to see sheens and balls of tar rise to the water surface when the liquid on the bottom is disturbed. Riding on the surface tension of the water, these floating tar layers can remain stable and be transported along with the ordinary water flow.
In many cases, the tar is not found today as a free-flowing liquid. More typically, the material encountered in an excavation will be soil or demolition debris that is saturated or partially saturated with tar. Tar may drain out of this material when it is disturbed, but the contaminated soil itself can be handled with ordinary excavating equipment.