Turbidity of Well Waters in Chalk

In recent years the importance of turbidity in well waters has increased. Wells often yield waters with particle concentrations that are well above drinking water standards, but which vary in a complex manner with pumping rates and other hydrogeological conditions. In addition to the problem of water quality, the particulates often interfere with treatment plants (especially for nitrate), and this can be very costly. Particle removal at well head is possible, but often the costs are high in both monetary terms and carbon, and many wells have been used at much lower than capacity in order to try and alleviate the problem. MWH have been investigating these issues for a range of major water concerns in around 12 sites in southern England, including undertaking some major pumping tests with extensive analysis of turbidity, chemical, and microbiological monitoring. On the basis of this, MWH has been able to advise its clients in a semi-quantitative way on the management of specific well groups. The aim of the proposed project is to build on this experience to produce a quantitative and generalized representation of the movement of particles around pumping wells in chalk aquifers. The approach, which draws on recent experience obtained on the Birmingham University campus research borehole array, is as follows: Task1: literature survey of all engineering fields involving particle movement in porous and fractured media. Task2: examination of existing chalk data, and collection of data from sites in other parts of the sequence to establish empirical relationships and whether patterns observed by MWH are repeated elsewhere. Task3: examination and analysis of sandstone well data collected by Birmingham University to act as a comparator to indicate the sensitivity of turbidity to hydraulic mechanisms. Task4: sampling and characterization of the particles to provide evidence on provenance and pathway size. Task5: test the hypotheses concerning mechanisms emerging from Tasks 1-4 using lab methods. Task6: synthesize conclusions from Tasks 1-5 to update the MWH conceptual model. Task7: develop a quantitative model based on the conceptual model of Task 6, probably with an empirical rather than detailed process-basis. Task8: test the model against new data, and modify appropriately. Task9: use the model to investigate management issues and what turbidity data may be able to indicate about near-well hydraulics (e.g. pathway residence times). The intended product is a general model that could be used in the management of pumping wells, and an improved understanding of the mechanisms of particle movement around wells. From this basis, the important finding of MWH that turbidity is correlated with pollutant concentrations (microbes, metals, and PAH correlations have all been observed) could be investigated in a future project.