Impacts of 25 years of groundwater extraction on subsidence in the Mekong delta, Vietnam

Abstract

Many major river deltas in the world are subsiding and consequently become increasingly vulnerable to flooding and storm surges, salinization and permanent inundation. For the Mekong Delta, annual subsidence rates up to several centimetres have been reported. Excessive groundwater extraction is suggested as the main driver. As groundwater levels drop, subsidence is induced through aquifer compaction. Over the past 25 years, groundwater exploitation has increased dramatically, transforming the delta from an almost undisturbed hydrogeological state to a situation with increasing aquifer depletion. Yet the exact contribution of groundwater exploitation to subsidence in the Mekong delta has remained unknown. In this study we deployed a delta-wide modelling approach, comprising a 3D hydrogeological model with an integrated subsidence module. This provides a quantitative spatially-explicit assessment of groundwater extraction-induced subsidence for the entire Mekong delta since the start of widespread overexploitation of the groundwater reserves. We find that subsidence related to groundwater extraction has gradually increased in the past decades with highest sinking rates at present. During the past 25 years, the delta sank on average ∼18 cm as a consequence of groundwater withdrawal. Current average subsidence rates due to groundwater extraction in our best estimate model amount to 1.1 cm yr−1, with areas subsiding over 2.5 cm yr−1, outpacing global sea level rise almost by an order of magnitude. Given the increasing trends in groundwater demand in the delta, the current rates are likely to increase in the near future.

A Review of the Integrated Effects of Changing Climate, Land Use, and Dams on Mekong River Hydrology

“…Widespread aquifer storage depletion, water quality deterioration, and saltwater intrusion are already emerging in parts of the delta as a result of the compounded effects of the groundwater overexploitation and sea level rise due to climate change [20,50,51,91,95]. The Mekong Delta covers 50,000 km 2 of fertile alluvial plain extending from Kratie in central Cambodia to Vietnam's East Sea.…”

“…The lowered water table in the coastal regions from irrigation pumping is exacerbating saltwater intrusion problems [50,95,99]. Pervasive pumping of groundwater has also resulted in large-scale land subsidence, which studies have linked to the release of arsenic in deep groundwater of Pliocene-Miocene age [91] through vertical migration.…”

“…The occurrence of high salinity (salt water intrusion due to sea level rise and groundwater use) and Arsenic content in groundwater could limit agricultural production and pose serious human health risks in the future [51,[100][101][102]. A recent study confirms that excessive groundwater abstraction is the primary driver of land subsidence [95], which has increased dramatically over the past 25 years.…”

Groundwater impacts on surface water quality and nutrient loads in lowland polder catchments: monitoring the greater Amsterdam area

“…Similar patterns are expected to exist in other lowland areas, which are highly manipulated by human activities. Typical delta areas where subsurface processes are expected to release nutrients from reactive organic matter and peat in the subsurface are the Mekong delta (Minderhoud et al, 2017), the Mississippi delta (Törnqvist et al, 2008), and the Sacramento-San Joaquin delta (Drexler et al, 2009). In many of these areas the water management shows resemblance to the Dutch situation.…”

“…The reclamation of swamps and lakes and the drainage of peat areas to enable urbanisation and agriculture severely changed the hydrological, chemical, and ecological environment of these areas (Ellis et al, 2005;Yan et al, 2017). Lowland delta areas are vulnerable for water quality deterioration by processes like salinisation and eutrophication, which can be amplified by climate change (Wu et al, 2015) and land subsidence (Minderhoud et al, 2017).…”