ANNEX C

RECOVERY TRENCHES or WELLS

Description

Recovery trenches or wells are used for the removal of leaked oil from a site. A typical case is petroleum hydrocarbon leakage from an underground tank which results in a plume of oil floating on top of groundwater. The floating oil should always be removed before implementing remediation. The trenches or wells are normally constructed at a groundwater downgradient position of the leaked oil hot spot to capture the oil. Two types of pumps are available for oil removal:

i) Skimmer pumps

This type is usually installed down a well and removes oil only. A typical skimmer pump is shown in Figure C1.

ii) Dual Pump Systems

A dual pump system has a groundwater pump at the bottom of the well which creates a cone of depression to increase the gradient for flow of floating oil to the well. The oil is captured by a skimmer and pumped by a second pump to the ground surface. Figure C2 shows a typical dual pump system.

Generally, the dual pump systems are able to recover more floating oil than skimmer pumps in a similar application. However, the dual pump systems are more costly and labour-intensive to operate because the extracted groundwater will usually require treatment.

In cases where oil/groundwater mixture is extracted, the mixture should be treated in an oil/water separator, where oil is segregated from the water and removed. The groundwater may then be recirculated into the ground for further treatment cycles. If disposal of groundwater is necessary, the water should be further treated, usually by activated carbon, to remove any residual oil before discharge. The recovered oil should be reused as far as possible. If there is a need to dispose of the oil, it should be disposed of as chemical waste.

Figure C1 Typical Skimmer Pump

Figure C2 Typical Dual Pump System