Land Contamination: Technical Guidance on Special Sites: Acid Tar ...

5.2.2 Water environmentThere is little evidence in the literature of investigation or assessment of groundwatercontamination associated with acid tar lagoons and it appears that, in the past, the principalconcerns leading to remedial action have been those documented above.Acid tar lagoons clearly represent a risk to groundwater or surface water (depending on thesite-specific context) and the previous imbalance probably reflects the high cost of addressingacid tar lagoons. This has meant that, by and large, action in the past has been taken when ithas been forced as a result of a situation that is clearly unacceptable in public health terms andhas justified expenditure which, in today’s terms could be in the several million pounds range.However, the assessment of acid tar lagoons in the future can be expected to place a highimportance on assessment of risks to controlled waters. In most respects, the assessment ofsuch risks would follow a conventional approach as documented in the Model Procedures.Thus, there is the possibility of past or future off-site migration both of dissolvedcontaminants (e.g. benzene) or free phase liquid tars and oils. However, there are a number ofconsiderations specific to acid tar lagoons as follows:• whether other wastes are known to have been co-disposed with the acid tars. Thiscould include general demolition type waste for stability reasons, but can also includedomestic refuse (absorptive capacity) and other industrial and chemical wastes.Clearly, account needs to be taken of all that is known from previous investigationsand where these wastes are present in quantities and patterns likely to impact onremedial works, they will need to be characterised;• reduced percolation of precipitation where there is a crusted, low permeability surface;• effect of acidity on naturally occurring mineral deposits, leading to mobilisation ofmetals in strata or other wastes;• effect of acidity on certain natural strata such as limestone and chalk (increasingsolution features and enlarging fissures);• interaction between acid tars and other substances present are especially importantwhen estimating risks to the water environment from acid tar lagoons. These can actto reduce mobility, by preferentially binding the deposits, or enhancing it, due to cosolvencyeffects.R&D <strong>Technical</strong> Report P5-042/TR/04 29