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.

This paper reveals the capacity of supplement for groundwater by rainfall resource based on the monitoring and analyzing result of rainfall, evaporate, groundwater table of central-collecting well and monitoring wells as well as the geophysical test result in dormitory area of VNU-HCM from November 2007 to February 2009.