Journal of Geology, Series B, 292007, p. 59‐68 GROUNDWATER INVESTIGATION: APPROACH TOOL FORTHE INTEGRATION WATER RESOURCE MANAGEMENT IN NAM ĐỊNH PROVINCE LÊ THỊ LÀIInstitute of Geological Sciences, VAST, Hà Nội
Journal of Geology, Series B, 29/2007, p. 59‐68 GROUNDWATER INVESTIGATION: APPROACH TOOL FOR THE INTEGRATION WATER RESOURCE MANAGEMENT IN NAM ĐỊNH PROVINCE LÊ THỊ LÀI Institute of Geological Sciences, VAST, Hà Nội Abstract: In the Nam Định Province only two available aquifers play the economic role as groundwater sources: the Holocene aquifer in the Hải Hưng Formation (Q2 hh) and the Pleistocene aquifer in the Hà Nội Formation (Q12-3 hn) The depth of Holocene water level is about 3-4 m from surface The unconfined Holocene aquifer has a maximum specific capacity of 4.5 l/s [7, 9] The depth of the confined Pleistocene aquifer is increasing in direction toward the sea from 30 to 35 m in the North until 80-90 m in Hải Hậu District, in some places even to 100 m The potential exploitation reserve of Pleistocene fresh water lenses in coastal areas is estimated as 203,453 m³/day [4, 7] Forecasting exploitation reserves of well field distributed along the Ninh Cơ River is 145,000 m³/day and 72,000 m³/day within Nghĩa Hưng District distributed along the Đáy and Ninh Cơ Rivers Especially, in Vụ Bản, Mỹ Lộc and Ý Yên Districts, the Holocene aquifer contains mostly brackish water Groundwater of the Holocene aquifer in Nam Định Province is widely contaminated by COD and BOD, iron and ammonia Generally, the Holocene aquifer has only a small potential for water supply The northeastern part of Pleistocene aquifer is characterized mainly by exceeded NH4-values In Nam Định, Vụ Bản and Giao Thuỷ Districts have not any available natural groundwater reserves for a safe water supply in Holocene and Pleistocene aquifers These districts have to use surface water as the only source for their water supply The groundwater level change is a serious problem in Nam Định, and a sustainable water resource exploitation is highly recommended I INTRODUCTION Nam Định has a total area of 1,6413 km2 and 225 communes, with 74 handi craft settlements and ongoing industry zones, both in Nam Định City and in the rural area As a result of the last 15 years of strongly development of industrial and aquaculture production, the economical situation in the province has been significant changed Even the main income in the province is from agriculture, but the industry plays at present important role in the society and economy The population is of 1,935,000 inhabitants (2005); with an actual population growth rate of 0.9 % a population of 2,040,000 is expected for the year 2010 The relief of the province´s teritory is flat and even at 0.2 - m above the sea-level with only some areas in the northwestern part reaching 7-100 m above sea-level The periodical wet tropical climate of Nam Định Province is characterized by a total annual rainfall of 1,400 - 1,750 mm and mean monthly temperatures of 18 - 29°C As the province is situated in the Red River (Sông Hồng) delta the naturally drainage system is well developed and dominated by the Red River and its tributaries as Đào, Ninh Cơ and Đáy Rivers The sewerage is one of most serious problems, because it in fact does not exist The waste water is drained by canals and ditches to the on-site preflooder For the future development of Nam Định Province the technology and know-how demand is focused on water resource management and planning, water supply, water distribution, sewerage, and environmental monitoring [6] Based on the results of former ground water investigation works in the Red River Delta and especially in Nam Định, such as: Đoàn Văn Cánh, Bùi Học [2], Lê Thị Lài & Đoàn Văn Cánh [7] etc a summary of main important features of groundwater in this area is given as support tool for a further integrated water resource management in the province Additionally, some results on ground water quality investigation especially in Pleistocene aquifer as framework for the development of water quality monitoring system will be reported briefly II GROUNDWATER RESOURCES IN NAM ĐỊNH PROVINCE Main groundwater aquifers The Red River Delta, building up the Red River and Thái Bình River System, which extends from Việt Trì City to the East Sea, is one of important hydrogeological regions in Việt Nam [11, 12] It covers an area of 17,000 km2, that is a part of the Hà Nội neotectonic depression The tectonic movement of the Red River deep-seated fault system is one of the most dominant geological process in the area The basin is filled up mainly with deltaic sediments of the Red River System and marine transgressional sediments of from coarse- to fine-grained materials, such as pebble, gravel, sand and silty, clay, silty clay The relief is declined from northwest to southeast That means also the groundwater flow direction too The elevation varied between 2-10 m The Nam Định area is situated in the most Southeastern part of this basin and has a elevation of about m Groundwater in this hydrogeological area exists mostly in loose sediments There are five available aquifers in the study area [4], but only two of them play an economical role as groundwater sources: the Holocene aquifer in the Hải Hưng Formation (Q2 hh) and the Pleistocene aquifer in the Hà Nội Formation (Q12-3 hn) The main hydrogeological feature of the region is briefly summarized according to data published in [4, 7, 9] and shown in the Table The Holocene aquifer is widespread in the province, the upper part ocures in the Thái Bình Formation (Q2 tb), and the lower part in the Hải Hưng Formation (Q2 hh) The lithography is characteristic by sand, silty clay, clay containing fauna and plant remains and fine sand, silty sand, silty clay, interbedded with sand lenses The depth of water level is about 3-4 m from surface The Holocene aquifer is unconfined one with a maximum specific capacity of 4.5 l/s [7, 9] The Pleistocene aquifer is distributed all over the province, but is not exposed on the surface The lithological composition of the aquifer consists mainly of quartz, sand, gravel and pebbles with a little amount of polymictic pebbles The aquifer covers directly Neogene sediments in the major area of province; only in some localities in the west it covers limestone and Proterozoic metamorphic rocks According to the drilling profile, the depth of the aquifer is relatively stable Following a cross-section with a northwest-southeast direction, one can see that in the northwestern part (it means the southern part of Vụ Bản District) the depth is smallest (30-35 m), to the middle part of Hải Hậu District the aquifer deepens to 80-90 m, in some places even to 100 m (Fig 1) The thickness of the aquifer increases in approaching the sea, too In the northwestern part belonging to the south of Ý Yên and Vụ Bản Districts, the thickness of the aquifer is only 10-15 m Towards, the sea it gradually increases, locally reaching 30-40 m The average thickness of the aquifer is 45 m Table Hydrogeological character of water-bearing formations in the Nam Định area Age Formation Holocene Thái Bình (Q2 tb) Hải Hưng (Q2 hh) Pleistocene Hà Nội 2-3 (Q1 hn) Pliocene Vĩnh Bảo (N2 vb) Triassic Đồng Giao (T2a dg) Thickness Lithological composition Water Discharge (m) (l/s) table 2-28 Sand, silty clay, clay containing fauna and plant remains Water type NaMgHCO3Cl 0.5-3 0.05-1.45 0.53.4 0.5- NaHCO3Cl 10-50 Quartz sand, grit and pebbles containing polymictic pebbles 0-2.5 >5 NaHCO3Cl 35-85 Fine and medium sandstone contains pebbles, siltstone and clay shale 0.61.2 < 12 HCO3 100 Gray and dark gray massive and thickbedded limestone 0.2 5.78 Na-CaHCO3Cl 1.3-27.5 Fine sand, silty sand, silty clay, interbedded with sand lenses The Pleistocene aquifer is a confined one; its piezometric level raises 40 m to 60 m and the static level is about 0.0-2.5 m from the ground surface Formerly, in the rainy season, water of the wells in coastal areas reaching this aquifer overflows onto the surface [4,7,9] Groundwater potential and water level change The most significant of the hydrogeological situation in this area is the existence of freshwater lenses within the Pleistocene aquifer located in the coastal region These lenses occupy the whole area of Hải Hậu and Nghĩa Hưng Districts, the southern part of Nam Trực, Trực Ninh and some parts of Xuân Trường, Giao Thủy Districts The water of these lenses is of very good quality and a precious resource for exploitation [4, 7] Using Visual Modflow 2.8.2 the exploitation potential reserves of these fresh groundwater lenses in the coastal area has been evaluated The potential exploitation reserves is estimated as 203,453 m3/day They are formed from static elastic reserve of 177,770 m3/day (87.37%), with the leakage of 25,683 m3/day (12.63 %) [4, 7] Furthermore, a possibility of large scale exploitation by concentrated well-fields has also been forecasted Results of forecasting can be applied to establish concentrated groundwater exploitation master plan for Nam Định City, coastal resort water supply and fresh-water export Forecasting exploitation reserves of well field including 69 wells distributed along the Ninh Cơ River is 145.000 m3/day Forecasting exploitation reserves of well field including 40 wells within Nghĩa Hưng District distributed along the Đáy River and Ninh Cơ Rivers is72,000 m3/day Sources forming the exploitation reserves include mainly static elastic reserves occupying 80-90% of total production of the well field [4, 7] Figure A NW-SE cross section passing the national monitoring netwok wells from Vụ Bản to Hải Hậu The Holocene aquifer as mentioned has a specific capacity from very small to 4.5 l/s and hence a medium water potential is expected for this In some area of the province, where the water quality is as good as the Việt Nam standard VN-TCVN-1995 for drinking water, is may satisfy for water supply in small scale suburban region and districts centres or for small dug wells in rural area, but in many region in the province, especially in Vụ Bản, Mỹ Lộc and Ý Yên Districts, the Holocene aquifer contains mostly brackish water, so that the use of this water resource for fresh water supply is very limited The monitoring data from national monitoring well system in the Red River Delta in the last 14 years from the Division of Hydrogeology and Engineering, North Việt Nam show, that in the region, where ground water is exploited intensively, such as Hà Nội, Nam Định and Hải Phòng, the water level is changed significantly At the national monitoring well 109a, for example, in Hải Hậu, Nam Định, the average water level from Pleistocene aquifer decreased from ca -1.0 m (1996) to ca -8.0 m (2006) It is estimated for July 2007 of ca 8.92 m under the surface respectively, and this will be expected for July 2007 of ca 9.10 m under the surface That means 0.28 m deeper (Fig 2) III GROUNDWATER CHEMISTRY AND POLLUTION Long-term stational monitoring data from the National Monitoring Well Network and results of chemical analysis of groundwater from different ground water investigation projects implemented by the Hà Nội University of Mining and Geology and Institute of Geological Sciences show, that the chemical composition of this groundwater changes insignificantly by dry season and rainy season The chemical type of brackish groundwater in rainy season is as follow: Figure Water level change from 1994 - 2006 estimated at the national monitoring well No.109 a, Hải Hậu, Nam Định (data from the Division of Hydrogeology and Engineering, North Việt Nam) M1.08 In dry season: M1.23 The chemical type of fresh groundwater from lenses in rainy season: M0.39 In dry season: M0.38 Results from former investigation show that the groundwater of the Holocene aquifer in Nam Định is widely contaminated by COD and BOD, iron and ammonia, while the nitrite content is not a dangerous contamination However, the fact that a number of analyzed samples showing a nitrite concentration exceeding the WHO’s guideline is a real reason for finding a solution in the near future in order to avoid the implicit dangers for the human use of this “unsafe” water [7] Also the high iron and ammonia content in this shallow groundwater source is a serious problem for a safe water supply for domestic use Some estimated environmental parameters from Holocene and Pleistocene aquifer resulted from former investigations is given in the Tab It is possible to see that the NH4+-concentration in water of Pleistocene aquifer ranges from 0.01 to 75.7 mg/l with average of 8.65 mg/l It is recognized, that water samples originate from Northeastern part of the province has a higher NH4+concentration (Fig 3) From 147 analyzed samples 28.57 % have a NH4+-concentration, lower than the VN-standard, the other Table Average content of some environmental parameters of Holocene and Pleistocene aquifer in Nam Định Province Value Parameter (mg/l) BOD5 COD pH NO2 NO3 NH4 Fe Holocene aquifer Max 24.0 78.6 8.03 13.5 12.4 176.4 26.3 Min 2.6 5.1 6.55 0.01