Old Gasworks - Builth Wells, Powys
In
March 1992, Powys County Council asked Ward Jackson
Associates to investigate the old gasworks site in Builth Wells
and report, with recommendations on the reclamation of the site. This
brief was later extended to include investigation of adjacent gardens,
some of which were left with deposits of gasworks wastes following flooding
of a small stream through the site in January 1992.
The site had
been used for the production of town gas from about 1862 to 1962 as well
as coke, tar
and other products and wastes typical of gasworks of that period. Because
customers were mainly to be found in built-up urban areas, gasworks tended
to be built on cramped sites in the middle of towns, to limit the cost
of laying expensive pipework. They were often located on low-lying land,
only available because it was unsuitable for housing and next to rivers
or other watercourses. Excess tar not sold for road building was stored
in underground tanks on the site and often allowed to seep away into the
ground beneath. Cyanide based waste was usually spread on or adjacent
to the site, thus raising the site levels, as well as contaminating the
land.
Beneath
the Builth Wells gasworks site were disused underground storage tanks
filled to ground level with tars, machine oil and phenol-contaminated
water. The largest was 9 metres in diameter and over 4.5 metres depth
and the poorly supported concrete cover slab was hidden by tarmac. The
site had been used as a council highways depot for many years and heavy
salt-spreading lorries frequently manoeuvred above the tanks.
Because
of the very confined nature of the site and the high levels of contamination
found and
the requirement that the site be made suitable for housing or light commercial
development, there was no scope for on-site treatment of the polluted
wastes and soils. The chosen option was to remove all contaminated materials
to landfill sites licensed to receive hazardous waste and replace this
with suitable inert fill to allow reinstatement to ground profiles suitable
for development. Contaminated liquids were removed by road tanker and
solid wastes and soils, by sealed and covered lorries. Clearance sampling
and laboratory testing of the excavated "void" surfaces was
carried out on a systematic basis until remaining materials were shown
to have contaminant concentrations below acceptable "trigger"
levels.
A new enlarged culvert through the site removed the risk of further flooding.
Groundwater in the gravels and weathered bedrock beneath the level excavated, was shown to have unacceptable concentrations of phenols probably due to seepage from the base of the tar tanks for 130 years. A collection sump was constructed within the groundwater zone and a submersible pump installed. Over a period of 18 months, the sump was pumped out into the public sewer for treatment. During the period, sampling showed a dramatic fall in phenol concentrations, down to acceptable levels. When further monitoring after pumping had ceased, confirmed the improvement, the sump was filled with crushed rock and the surface made good.
The main contract works were carried out between September 1994 and April 1995 and on completion, Ward Jackson Associates prepared a detailed "validation" report for the decontaminated site for the approval of the Environment Agency and the Client, Powys County Council.
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