When wastewater infiltrates into the soil, groundwater may be contaminated. If the distance from source of pollution to point of groundwater abstraction is small, there is a real chance of abstracting pathogenic microorganisms. In this book, the transport of Escherichia coli in aquifers under saturated conditions is studied. Starting point is the well known colloid filtration theory. Straining or physical sieving, geochemical heterogeneity, variable deposition rate coefficients, and preferential flow mechanisms are then included in the theory. The case of Sana'a, the capital of Yemen, served to evaluate the importance of bacteria transport processes in a field situation. This book is useful for hydrogeologists concerned with microbiological contamination of aquifers.