Monday, February 23, 2009: 1:20 p.m.
In recent years MWH have undertaken eight detailed water source investigations and assessments at a further five sources to determine the origin and causes of sediment mobilisation in Chalk water supply boreholes in Southern England. These studies have provided significant advances in the understanding of the genetic and transport processes that control the origin and mobilisation of sediment in the Chalk; a limestone aquifer with both dual porosity and karst flow properties. Sediment content (turbidity) water quality standards have historically been driven by customer sensitivity and thus the need to provide clear water from consumer’s taps. However the results from this study clearly show that mobile sediment in aquifers provides an alternative transport pathway for adsorbed heavy metals, radioactive minerals, toxic organic compounds and microbiological species that are removed by turbidity management. Consequently turbidity issues are even more important than was previously perceived. As a result the potential for mobile sediment and the processes that control the transport of the sediment phase need to be considered in much greater detail in groundwater contamination risk assessment studies and groundwater supply solutions that involve aquifers with karst, dual porosity, or fractured hydraulic systems. Unfortunately mobile sediment in Chalk aquifers typically consists of relatively small particles that are difficult to remove reliably by coagulation and/or sand filtration. Alternative treatment processes are significantly more expensive and require a waste stream that entails a loss of water resource. However the advances in knowledge of sediment transport and mobilisation provide the possibility of alternative source management solutions that have been pioneered and proved by MWH to be an effective low cost alternative that also has many environmental advantages.