“Gas holders are typically the most contaminated structures on former [manufactured gas plant] sites. Water seal holders often accumulated large quantities of tar, especially in relief holders, where freshly manufactured gas was held and cooled prior to purification. Much of the tar held in these holders eventually leaked out the bottom. Furthermore, when MGPs ceased operations, it was a common practice to bulldoze the debris from the MGP plant buildings into the circular foundation of the water-seal holders. It is common to find tar-soaked demolition debris in these holder foundations today, decades after the plants closed down.”
— New York State Department of Environmental Conservation
One of the biggest problems associated with carburetted water gas plants, such as the one that operated on the property where the Dushanbe Teahouse now stands, was the need for a relief holder, a second large round tank-like structure for storing large quantities of gas (see illustration).
First, according to a white paper titled “A profile of Water Gas Plants, their history, design, development, application and the type of contaminants which may be associated with them” by Dr. Russell Thomas, technical director of Parsons Brinckerhoff Ltd., relief holders were used to smooth out the uneven flow of the gas production process of water gas plants.
This meant that as soon as gas was created, it went very quickly into the relief holder before any cleaning of the gas took place. The gas in the holder tended to cool, which allowed many of the gas’s nastiest contaminants to settle to the bottom of the relief holder, before the relief holder was eventually used to push the gas out and through the various scrubbers and purifiers that removed the remaining contaminants prior to moving the clean gas on to the larger holder that was used to store and push gas through the pipeline system to the customer.
Some relief holders were built with subsurface water-seal pit tanks made of brick and often without bottoms. The tanks needed water in them to maintain their seal for sucking and pushing purposes, as the top of the tank went up and down like a piston. Many of the tanks extended 15 to 20 feet below the surface, well below groundwater levels, and because they were often brick and/or bottomless, they began contaminating the groundwater as soon as they went into service.
Boulder Weekly asked Allen Hatheway, one of the nation’s leading experts on town gas sites, to examine photos of the 13th Street site’s relief holder and offer his opinion on its condition and what we might expect to be found during remediation.
The following are some of the observations he sent via email:
We need to listen to the ghosts of the two gasholders ….
1) The smaller holder is the “Relief” holder. 2) It is a necessary requirement of a CWG (carburetted water gas) plant.
3) Coal-gas plants needed only one holder as a requirement, and got additional holders as they expanded their gas output, over time.
4) In type, design and function, the two gasholders are greatly different.
5) Holder no. 1 (smaller; relief) has a subsurface water-seal pit tank, and that tank likely was abandoned in place at the time of the pre-1956 gasworks demolition.
6) These tanks were designed and operated as collecting sumps for toxic residuals contained in the raw gas, before it was “clarified” and” purified.” Hence, at abandonment, the pit tanks generally were “full” of tar, light oils of tar, and contaminated gas liquor.
7) Most pit-type tanks were constructed of mortared brick, or masonry; relatively few at the time were made of concrete, and even less of steel.
8) In the absence of Xcel’s archival documentation, I’d estimate that the holder “bell” is about 20 feet high and that the pit tank, into which it sank and rose from, was about 20 feet deep.
Here is a generic image of what we have at the small gasholder; the holder bottom may also have been of brick; the raised “dimple” [dirt piled against the sides of tank] was not always present; the walls likely were of brick; the mound at Boulder was perhaps as much as eight feet high.
9) Most of the pit tanks leaked substantially, almost from the beginning of gas-manufacturing operations; likely within the first year of operation.
Now, here’s what the holder ghost wants to suggest ….
Why the unusually tall berm surrounding the pit tank? Likely because the site groundwater depth at the time of construction was closer than 20 feet below the existing ground surface of the gasyard. Therefore, the site presented cost and construction difficulties for placement of the pit tank of the relief gasholder. As a result, the bottom of the pit tank was “raised” artificially, some eight feet or so. So when the pit tank began to leak tar, light oils and contaminated gas liquor, such “got” into the site’s groundwater immediately. At the present time, I would estimate that the site itself is heavily impacted with [polynuclear aromatic hydrocarbons], both in the soil matrices (voids), beginning especially at the up gradient (west) edge of the relief holder, and continuing below and around the relieve holder location, in a teardrop-shaped imprint toward the east. The subsurface groundwater contamination “plume” from this one holder could extend for several hundred feet or more to the east; as influenced somewhat from whatever irrigation water leaked from the “ditch.”
Hatheway went on to describe the larger holder on the Boulder site. “The 1939-1943 and 1945 aerial photographs indicate that this holder was poorly maintained and that it had continually leaked contaminated gas liquor from the bottom of the tank, outward, at the top of the slab-on-grade foundation,” says Hatheway.
Joe Castro, who is heading up the clean-up project at the site as the city’s facilities and fleet manager, has told Boulder Weekly that consultant USA Environment investigated the relief holder itself in March. He declined to say what excavation was done at that tank or what was found, saying the consultant’s report will be released in late summer or fall.