Groundwater Exploitation Zoning Aiming at Management of Sustainable Groundwater Exploitation and Use in Ca Mau Peninsula, Vietnam

The research is financed by KC08.08/16-20: Study of measures for mitigating and adapting to drought and salinity intrusion as natural hazards in Camau peninsula , Ministry of Science and Technology, Vietnam Abstract Groundwater system in Camau Peninsula has 6 main aquifers (not including very poorly productive qh aquifer), of which 4 aquifers are predominantly exploited, namely qp 2-3 , qp 1 , n 2 2 and n 2 1 ; 2 minor aquifers are qp 3 and n 1 3 . Although the aquifers are located over the area, due to complicated fresh/saline interfaces in sections, exploitation and protection of groundwater sources is dealing with many problems. In the paper, information of aquifers is systematized into a map of groundwater exploitation zoning on scale 1:200,000 for the purpose of supplying essential information of water sources management in each socio-economical zone. Keywords : Camau peninsula, potential exploitable groundwater reserve. DOI: 10.7176/JEES/10-4-04 Publication date: April 30 th 2020


Introduction
In traditional hydrogeological study, special information is synthesized and displayed with hydrogeological maps. Hydrogeological maps are composed in accordance with Vietnam technical regulations, so they are sophisticatedly and exploitation of information from them (reading and understanding) faces certain difficulties. Actually, managers and people with no sound special background have troubles in adequate perceiving of information displayed on hydrogeological maps. In previous projects of Department of Geology and Minerals of Vietnam, people have tried to simplify presentation of special information with maps of potential groundwater exploitation (Chan, 2010;Linh, 1992;Tuan &Chan, 1992). In these maps, contents of information have been simplified, but some other contents of sophisticated specialty were added. Therefore, their popularity is not so high.
In studying distribution of groundwater reserve of an area, people rely on maps that contain synthesized information of reserve. This paper assesses potential of groundwater use based on map of groundwater exploitation zoning. On this map, the area in question will be divided into regions having similarity of distribution laws of aquifers and its exploitation potential. Besides, for the purpose of management and reasonable use of water sources, ecological regions will be attached, aiming at supplying information for development planning of fields which have water demand.

Method of zoning
Map of groundwater exploitation zoning was composed in accordance with the principle: water demand for development and perspective of groundwater exploitation. After this principle, the study area is divided based on the following criteria: -Water demand for development (demand of water use in ecological regions).
-Satisfaction level of water demand for domestic and production purposes (potential of groundwater source and satisfaction level).

3.2-Basis of groundwater exploitation zoning mapping
a-Topographical base map: Topographical base map used for mapping is topographical map of system VN-2000 on scale 1:200,000. Information shown are: Locations and names of cities, provinces, districts, communes, hamlets, rivers, coordinate grid, provincial boundaries, elevation marks, transportation system (ground, asphalt routes), water network, lakes, ponds…. b-Hydrogeological base map: Hydrogeological base map of the same scale 1:200,000. Boundaries and special information were extracted from this base map.

Contents and presentation method 4.1 Contents
In the study area there are combinations of one or multiple aquifers that have potential to meet water demand for domestic and production purposes. May be a case that in this locality a definite aquifer plays utmost important role, but in other locality it has a minor role or does not exist. In the entire area there are 6 aquifers (aquifer qh is neglected), of which potential of fresh groundwater exploitation is focused on 4 main aquifers, being: qp2-3, qp1, n2 2 and n2 1 . Aquifers qp3 and n1 3 , although can be exploited for fresh water of good quality, but considered as minor due to: -Aquifer qp3: small thickness, can be exploited on household scale. -Aquifer n1 3 : Large thickness and productive, but because of deep distribution and large exploitation investment, they are considered as minor. Based on this principle the Camau peninsula is divided into 3 main units downwards as follows: Exploitation region: is divided usually after criterion of purpose of exploitation of water sources for social economical development. In 2007, Southern Institute of Water Resources Planning divided Camau peninsula into 8 ecological regions based on water resources and production pattern (symbolized as: A, B1, B2, B3, C1, C2, D and E), and this paper inherits this result (Figure 1a). Exploitation area: is divided after perspective of fresh groundwater exploitation (fresh groundwater can be exploited or fresh groundwater is not available), see details in Figure 1b. -Area of perspective fresh groundwater exploitation (at least 1 fresh aquifer), refered to as (I), symbol: I. -Area of no perspective for fresh groundwater exploitation (all aquifers are saline), refered to as (II), symbol: II. Exploitation plot: is divided after level of satisfaction for water demand for domestic and production purposes. Criteria are number of main aquifers and potential groundwater exploitable reserve.

Implementation method Contents
On the basis of the mentioned principle and study results of the research, we composed map of groundwater exploitation zoning after following procedure: Step 1: Determination of groundwater exploitation regions -Compilation of groundwater exploitation regions, determination of distribution range, boundaries, areas -Calculation of groundwater exploitable reserve for each region (potential groundwater exploitable reserve and secure exploitable groundwater reserve).
Step 2: Determination of groundwater exploitation areas and plots -Superimposition fresh-saline boundaries (TDS = 1g/l) of 4 main aquifers qp2-3, qp1, n2 2 and n2 1 . Determination of areas and plots: range and area, see details in Figure 2 -Calculation of groundwater reserve for each plot (potential groundwater exploitable reserve and secure exploitable groundwater reserve), Step 3: Determination of range of possible fresh water exploitation of minor aquifers -Determination of fresh water distribution based on fresh-saline boundaries (TDS = 1g/l) of aquifers, -In each exploitation plot, calculation of groundwater reserve for each aquifer (potential groundwater exploitable reserve and secure exploitable groundwater reserve).
Step 4: Composition of map of groundwater exploitation zoning -Draft of map legend, -Superimposition of maps as implemented in previous steps, -To bring special information into the map.

Results and discussion
From the entire Camau peninsula, area from which fresh groundwater can be exploited is 13,871.9km 2 (83.6%). Area of saline groundwater is 2,728.1km 2 (16.4%). Potential fresh groundwater exploitable reserve is 11,340,102m 3 /day (of which, fresh groundwater secure exploitable reserve is 741,534m 3 /day, approximately 6.5% of potential fresh groundwater exploitable reserve). Potential saline groundwater exploitable reserve is Coast line Boundary of groundwater exploitation areas River, canal, tributary

Groundwater exploitation areas
Areas of fresh water (exist at least 1 fresh aquifer) Area of salt water (without fresh aquifers) Journal of Environment and Earth Science www.iiste.org ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online) Vol.10, No.4, 2020 27 4,865,900m 3 /day. In area range of fresh water (area I), potential exploitable groundwater reserve of each plot is different: -Scarce plot: Fresh groundwater can be exploited only from aquifer qp2-3 in total range of 2,844.8 km 2 . -Medium plot: Fresh groundwater can be exploited from aquifers qp2-3 and qp1 in total range of 4,832.6 km 2 . -Rich plot: Fresh groundwater can be exploited from aquifers qp2-3, qp1, n2 2 and n2 1 in total range of 2,847.1km 2 . -Abundant plot: Fresh groundwater can be exploited from aquifers qp2-3, qp1, n2 2 and n2 1 in total range of 3,377.5km 2 . Specifications of exploitation regions are shown in details in Table 5 and Figure 3. No fresh GW Besides, in each region fresh groundwater in also can be exploited from minor aquifers as follows: -Aquifer qp3 has fresh groundwater area 2,756.5km 2 and potential exploitable reserve 655,611m 3 /day. -Aquifer n1 3 has fresh groundwater area 2,350.8km 2 and potential exploitable reserve 713,620m 3 /day. Journal of Environment and Earth Science www.iiste.org ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online) Vol.10, No.4, 2020 28 Specifications of minor aquifers in exploitation regions are listed in Table 6 and its distribution ranges are displayed in Figure 2.

Conclusion
Results of map of groundwater exploitation zoning on scale 1:200,000 of Camau peninsula help readers to easily know potential exploitable groundwater reserve in each ecological regions. Besides, concrete information regarding number of aquifers and secure exploitable reserve are presented in details. This is essential information for management of licensing groundwater exploitation, zoning prohibition and restriction of groundwater exploitation in accordance with Decree 167/2018/ND-CP; orientation of groundwater exploitation and use of groundwater sources in socio-economical development planning or allocation of water sources in water resources planning (according to Circular 42/2015/TT-BTNMT).
In groundwater exploitation zoning in this paper just considering water as fresh when having TDS <1.0g/l (QCVN09, 2015). Practically, in localities of scarce water sources such as coastal area in southern part of Ca Mau province and some areas in Kien Giang, Bac Lieu and Soc Trang provinces people also exploit and use groundwater having TDS = 1.0 ÷ 1.5g/l.