16. Viet Nam has abundant groundwater resources and an estimated total renewable potential of 63 billion m3 per year; however, it is subject to countrywide variations. High-yielding aquifers are located near the Red River, the Mekong River, and the coastal plains. Some aquifer levels have shrunk by as much as 30 meters in Ho Chi Minh City and Ha Noi due to overuse, causing land subsidence and water shortages (ADB 2009d). In areas with naturally forming arsenic, such as in the Red River Delta, overextraction can contribute to increased contamination of shallow wells. Expansion of brackish aquaculture, particularly in the Mekong Delta, has led to contamination of groundwater from both seawater and pollution from aquaculture inputs.