Hot spots are small areas where very high concentrations of contaminants are found. When Soil mixing is considered as remedial option, hot-spots are a mayor thread to the success of the remediation. This is due to the high concentrations of contaminants found in the hotspots combined with their volume. When the volume and concentration are very significant the effect on the remediation result can be devastating. In some cases all the (mixed) topsoil has to be removed and transported to a registered landfill, where it will be charged at hazardous waste rate ($ 66 – 140.-/ton). For a acre site on which to top 300 mm have been mixed this means 12,000 tonnes to soil to be moved and tipped at the cost of $ 0.8 – 1.7 million.

To avoid this, hotspot detection should be the focus of the soil assessment prior to soil mixing. The size of hotspot which may remain undetected is depending on the size of the grid on which samples are taken. Below is calculated the volume of soil with ‘background' contaminant levels required (in red) to dilute a hotspot undetected during an investigation based on the given grid size (green).

Note 1 'final concentration' is set at 25 instead of at guideline concentration of 30 mg/kg (for Arsenic) as in practice the aim of a mixing operation is to obtain soil in which the maximum concentration found is to be below the guideline concentration

Note 2 this assumes sampling layers of 100 mm, which give 150 mm 'layers' due to sampling variability.

- this is for near surface related contaminants, Arsenic / pesticides (DDT Dieldrin), usually 3 - 4 layers are samples

- for deeper contamination (fuel, central sheep-dip area, farm-tips) the sampling may extend to 5 - 10 m deep

and is very much site specific as well as to a large extend the final depth is unknow before sampling commencement

Note 3 assumed is a 'square' hotspot equal to one grid area, in practive well upto 1.5 times the area is possible for oval hotspots