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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.

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