case study for on site analysis

The contaminants were confirmed as spilled heating oil from the tank, creosote in the middle work area and high concentrations of copper, chrome and arsenic in the third work area.

The spilled oil contamination was classified as non hazardous, but high levels of PAH and arsenic exceeded the hazardous waste limits for the local landfill. Top area polluted with heating oil, middle area polluted with creosote (PAH) and bottom area with cupro-arsenate preservative.

A simple remediation strategy could involve removal of the contaminated soil up to the next "clean" sample point. The clay layer is sufficiently thick to prevent migration of pollutants through to the underlying soil, so the maximum depth of soil removed would be 1.2m.

Following this strategy the following statistics are generated

·	Cost to remove and replace 1200m3 of heating oil contaminated soil £50,000.

·	Cost to remove and replace 800m3 of PAH contaminated soil £145,000.

·	Cost to remove 400m3 of arsenic contaminated soil £102,000

·	A total of 600 lorry movements to complete the remediation

·	19 days to complete remediation.

·	Total cost £297,000 including plant hire.

The following example shows how on site analysis can reduce the costs of a typical remediation. The figures used reflect the current high costs associated with dig and dump methods.

Conventional Approach
The site is a former fencing contractor with an area approximately 70m x 60 m that has been operating for 15 years. It has been selected for redevelopment with domestic housing. A desk study indicated heating oil, PAH and heavy metal wood preservatives (CCA) were the most likely contaminants.

A simple SI plan using a 10m grid around the areas most likely to be polluted was made and identified several hotspots and confirmed a clay layer under the site at approximately 1.2m depth.

Analysis of the 38 sampling points showed 10 were contaminated. The red spots in the figure show the hot spots.

The location of the contaminated samples are shown as red or yellow spots and the “clean” as green. The soil removal area is shown as red for hazardous and grey for non hazardous

This approach generates the following statistics

·	Cost to remove and replace 720m3 of heating oil contaminated soil £27,000.

·	Cost to remove and replace 656m3 of PAH contaminated soil £108,000.

·	Cost to remove 208m3 of arsenic contaminated soil £53,000

·	A total of 396 lorry movements to complete the remediation

·	14 days to complete remediation.

·	On site analysis cost including analysis and technicians £11,000

Total cost £199,000 including plant hire.

This gives a total saving of £98,000, reduces the time on site and reduces the number of lorry movements

On Site Analysis Approach

An alternative strategy involves the use of on site methods to more accurately delineate the pollutant plume. From each sampling point identified in the initial SI as being contaminated, a 4m sampling grid is constructed.

It is assumed that areas in between 2 points identified as polluted in the SI will also be polluted and so no additional sampling is made.

If the new sample point is contaminated, new samples are taken another 4m away until a “clean” result is obtained.

The 4m grid was selected as the volume of a 4m x 4m x 0.5m area corresponds to the amount the landfill operator lorry can carry.

The above diagrams show the actual pollutant plume and the volumes of soil removed by the 2 approaches to the remediation. The 10m grid method shows two areas of pollution that could easily be missed.

The heating oil pollution could be caused by the spillage of just 700 litres of fuel oil, the PAH pollution by spilling less than a litre of creosote per day during the time the fencing contractor was working and the arsenic by spilling a 50Kg container of the neat CCA wood preservative.