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.
Groundwater quality is a matter of concern in Ca Mau Peninsula. There have been many organizations, domestic and foreign scientific researchersworking on the issue, and they have produced various results regardinggroundwater pollution (arsenic, heavy metal, minerals, or saltwater intrusion…). In this paper, the authors assess groundwater quality by using the water quality index (WQI) to transform complex data into simple indicators that are easy to understand, and let people in the studied areas be aware of the pollution issue of the water resources they are using. Groundwater samples are collected from national monitoring wells and from exploitation works in the area. Indicators such as pH, TDS (total dissolved solids), total hardness (TH), total alkalinity (Na ++ K +), sulfate (SO42-), chloride (Cl), and nitrate ( NO3) are used to calculate WQI values.The WQI values of the middle Pleistocene aquifers of Ca Mau Peninsula range from 36.09 to 1,344. Based on these values, authors have classified groundwater samples into 5 groups of different qualities, from the very high quality to the unusable one. The very high quality accounts for 14% of the samples; good quality accounts for 49%, average quality accounts for 24%, poor quality accounts for 7%, and unusable accounts for 6% of the total samples.There is about 10% of the samples exceeding permissible limits of TDS, whereas the sulfate and chloride range from 20% to 21%. Samples of poor and unusable qualityare mainlyfrom Soc Trang province. Most of themhavethe TDS levels greater than 1, and some have an abnormally high level ofsulfatewhen compared with other samples in the area. The study hasyielded a more comprehensive assessment of groundwater quality, allowing regulators to plan water resources more reasonably and be able togive in time advices tothe people.
Groundwater is an important resource of provinces in Ca Mau Peninsula. The water is supplied to household, industrial, agricultural and aquacultural activities. More important, in this area, as the majority of surface water is contaminated and requires further treatment to become usable, the main source of water supply is the groundwater. Under the impacts of socialeconomic development, the increase of population and the urbanization rate in the region, groundwater resource is under a pressure of exploitation and utilization. Yearly exploited amount of groundwater is larger than the replenishment amount in most aquifers, so the groundwater level is lowered gradually year by year. According to statistics from 2000 to 2010, the groundwater level has been lowered from 0 to -14m (in some places, the water level is lowered to -28m below sea level) in qp2-3 and qp1 aquifers, with the highest drops in Bac Lieu and Soc Trang provinces. The amount of pumped groundwater in the region has been increasing from 159.914 to 931.944 m3 / day, whereas the replenishment amount has been decreasing from 526.121 to 185.004 m3 / day. Stated otherwise, groundwater in the region is declining in both volume and quality under the impact of climate change and exploitation activities. This paper uses the DPSIR framework to assess causal relationships of factors that impact to the groundwater environment in the region, and thereby propose appropriate solutions under the impacts. In combination with groundwater environmental indicators to quantify the degradation impacts to groundwater resource, the results of this paper indicate that water supply used mainly in Ca Mau Peninsula is groundwater ( 85.74%); the capacity of renewable water per capita in the region is still very low (80.06 l / day / person); the water loss from aquifer systems is much larger than the amount of replenishment (141.02%), however the amount of water extraction for domestic service was still in permissible limits of the aquifers (8.71%). The numbers show that groundwater in Ca Mau Peninsula is in decline but still within safe limits. Results from this paper can give authorities a more intuitive view about the current situation of groundwater when planning and using water resources.
In Ca Mau peninsula region, groundwater resource is the main source of water supply for agriculture, industry, domestic uses. In this paper, the authors study groundwater origin in middle - upper pleistocene (qp2-3) aquifer in Ca Mau peninsula and give some opinions about the groundwater origin formation for planning and managing reasonable, safe, and sustainable. There are many studies about groundwater origin in the Mekong delta plains and these studies showed different results. Up till now, there is not a consensus on an exact origin of groundwater in Mekong delta plains, especially in Ca Mau peninsula. Therefore, the authors use the Xulin approach to determine groundwater origin based on the concentration of ions Na+, Cl-, SO42-, Mg2+. Then we use Piper diagram, Gibbs plot, Mercado plot, and hydro-geochemical section plot to explain the origin of groundwater in Ca Mau peninsula region. The results show that there are 4 types of groundwater origin in the studied region: magnesium – chloride, marine origin; calcium – chloride, deep metamorphic environment; sodium – sulfate, the continental leaching origin; bicacbonnat-sodium, continental environment. The groundwater origins relate to the marine transgression and regression process, or weathering, leaching, and freshwash processes. The groundwater origin formation do not have participation of rainfall water and river water, which are two important components contribute to the groundwater recharge formation. Therefore, researching groundwater recharge of th e qp2-3 aquifer in the Ca Mau peninsula is according to other aspects.
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