Abstract Construction excavations and tunnels in chalk can encounter groundwater challenges, including high water flow rates, instability of excavations in weathered chalk and basal instability in overlying aquitards caused by high groundwater pressures in deeper chalk aquifers. In hydrogeological settings where the chalk has been exposed to periglacial weathering during the Quaternary Period the upper zones may be degraded to structureless chalk which can potentially be of very low hydraulic conductivity (putty chalk) or very high hydraulic conductivity (chalk bearings). In deeper, structured chalk groundwater flow tends to be concentrated along fissures associated with pre-existing geological structures such as bedding planes, flint beds or faults. A range of groundwater control strategies can be deployed, including open pumping, pre-drainage pumping, shallow and deep cut-off walls, ground treatment and, for tunnels and shafts, application of fluid counter pressures to exclude groundwater. The strategy appropriate to a given site must be selected based on a thorough understanding of the hydrogeological setting and chalk weathering profile. This requires a ground investigation of appropriate scope, using suitable techniques to characterize the chalk. Borehole geophysics can play a key role in identifying discrete zones of inflow.

Abstract Urbanization is increasing worldwide, and it has drastic effects on groundwater systems with ramifications for water management. Effects can include overexploitation, subsidence, water quality deterioration, destruction of environmental resources, increased runoff, alteration of the permeability and porosity fields, and changes in recharge. Commonly, it is assumed that recharge decreases, but data indicate the opposite: Groundwater recharge increases because of leaky utility (water and sewage) systems and urban irrigation. Urban areas are hydrologically similar to karst settings because they possess internal drainage (storm sewers), surface streams (paved drainage ways) that flow after heavy rains, and a shallow permeability structure dominated by fractures, conduits, and caves (buried utility trenches, abandoned pipes, etc.) that evolves very quickly. Secondary porosity from underground construction is similar in magnitude to karst secondary porosity. These structures and utility trenches increase permeability and make prediction of groundwater flow and transport difficult. Recharge is grouped into the following categories: direct (from precipitation), indirect (from surface water bodies and leaky utility systems), localized (through preferential pathways such as sinkholes), and artificial. Indirect recharge is commonly ignored in urban water budgets, but water main losses range from 5% to over 60%. Additional recharge comes from leaky sewers, leakage from beneath homes and industries, and irrigation return flow (e.g., lawn overwatering). A case study of Austin, Texas, demonstrates significant indirect recharge and the difficulties in its estimation. Nearly 8% of Austin water main flow is lost to become recharge. However, lawn irrigation may be a larger source.