A slick solution to oil pollution

The potential impact of an oil spill that contaminates the water-bearing rock strata – aquifer – cannot be underestimated. Contamination of the aquifer can be the result of contamination over a long period, up to 50 years, or the more immediate result of a serious accidental spill.

Flexible, innovative methods of treatment and decontamination need to be used to allow cost-effective reuse or redevelopment of the site in the shortest possible time frame. Post-spill, the client and regulator need to be re-assured that treatment results are satisfactory.

Through the British Oil Spill Control Association’s (BOSCA) accreditation scheme, spills of any size or age are classified into three levels according to their severity. In broad terms, local spills to a depth of 4-5m, category one, spills that contaminate soil to a depth of say 6-10m, a category two spill, and in the most severe cases in excess of 20m, a category three spill.

Companies are accredited to assess their competency to respond to groundwater contamination at the three levels. Six companies are accredited to respond to level three groundwater contamination.

Once an emergency spill is reported, or responders are asked to look at a historical spill scenario, local constraints and variations are an important initial consideration when determining the action to be taken for remediation.

Consideration must be given to the size of the spill and the time interval between the spill occurring and being reported. Many accidental spills go undiscovered for a period of time. Ascertaining this is vital in determining the likely impact of the spill on groundwater.

Spill location

The water table level fluctuates considerably between seasons and local knowledge in this context is invaluable. The time of year and weather conditions, temperature, and ground state (waterlogged or frozen) should be assessed.

The protection of buildings will also need consideration in the response process. Oil-contaminated soil which dramatically changes its characteristics and fluid dynamics can threaten the foundation of buildings.

Once on-site in an emergency situation a range of techniques are applied.

Where time and size of spill warrant detailing, computer-based mapping of the site can be employed to forecast the extent and direction of the spill and direct resources and techniques most appropriate.

Action taken in an emergency response situation will include:

• Preventing the spill from migrating. Simple trenching around the spill will enable the first step to be taken to prevent the spill seeping further,

• the drilling of boreholes to ascertain the extent of the contamination in the soil,

• the digging of pits down to groundwater level. This technique will encourage the groundwater to gravitate to the pit, bringing the oil with

  it. This is further encouraged by pumps and the subsequent treatment to extract

  hydrocarbons.

These methods can be used where access to a site is easy and there is the potential to work in with machinery and equipment unrestricted.

Work restrictions

Spills do not always afford this luxury. Commonly, in the case of an accidental spill from an oil storage tank, the oil will invariably flow under buildings. This not only threatens the structure of the building but will make traditional techniques of extraction, removal and re-instatement costly, disruptive and time consuming. In this instance vacuum extraction should be considered. The vacuum pumps are connected to pipe work 30cm below the level of the water table, this will involve some disruption in the building. The pumps are then run for 12 hours/day, one vacuum head connected to two pipes. The pumps are then run until the level of hydrocarbon contamination is below agreed levels. Once the area has been decontaminated the pipes are removed and the voids filled.

This technique was used in a recent example involving the spillage of 1,500 litres of oil under the concrete floor of an industrial building. The oil, on the water table was up to 1m below the floor level. The cost of removing the concrete floor, hardcore and contaminated soil was estimated to be £50,000. The use of the vacuum pumps on the site not only reduced the time and disruption, but cost about one third of this value.

Historical contamination

BOSCA members also respond to requests to clear contamination that has been in site for many years. Flexible techniques need to be applied to a large, historical site. In this instance contamination may have occurred over a 40-50 year period with the potential of groundwater being impacted off-site and involving significant volumes of soil.

The solution is to recover the free product through a series of high and low vacuum systems, to break down the clay soil matrix through high-pressure injection, to set up in-situ bioremediation through the injection of bacteria and nutrients and finally hot-spots are excavated and bio-remediated off-site.

In a recent example some 64,000 litres of diesel oil was recovered from a site – 44,000 tonnes of contaminated soil was treated and pollution was reduced to less than 5,000 milligrammes per kilogramme.

Experience in the cost effective de-contamination of groundwater counts for a lot. The adoption of new techniques and equipment in this process can save thousands of pounds in relation to less sophisticated ‘remove and dispose’ options. BOSCA continues through its functions, to promote ideas and concept to the ultimate benefit of customers, whether the contamination they look to deal with is 50 years old or the result of a broken pipe earlier that day.