Báo cáo Groundwater investigation approach tools for the integration water resource management in Nam Dinh province

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

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 (Q 2 hh) and the Pleistocene aquifer in the Hà Nội Formation (Q 1 2-3 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 NH 4 -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 3 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 - 3 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 frame-work for the development of water quality monitoring system will be reported briefly

II GROUNDWATER RESOURCES IN NAM ĐỊNH PROVINCE

1 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 6 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 2 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 1

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 1. Hydrogeological character of water-bearing formations in the Nam Định area

Age Formation Thickness

(m) Lithological composition Water table

Discharge (l/s)

Water type

Holocene Thái Bình (Q2 tb) 2-28

Sand, silty clay, clay containing fauna and plant remains 0.5-3 0.05-1.45

Na- Mg-HCO 3 -Cl Hải Hưng

(Q2 hh)

1.3-27.5 Fine sand, silty sand,

silty clay, interbedded with sand lenses 0.5-3.4 0.5- 5

Na-HCO 3 -Cl

Pleistocene (QHà Nội 12-3 hn) 10-50

Quartz sand, grit and pebbles containing polymictic pebbles 0-2.5 > 5

Na-HCO 3 -Cl

Pliocene Vĩnh Bảo (N2 vb) 35-85

Fine and medium sandstone contains pebbles, siltstone and clay shale

0.6-1.2

< 12 HCO 3

Triassic Đồng Giao

(T2a dg)

100

Gray and dark gray massive and thick-bedded limestone 0.2 5.78

Na-Ca-HCO3 -Cl

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]

2 Groundwater potential and water level change

The most significant of the hydrogeological situation in this area is the existence of fresh-water 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 1 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 2 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 2

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 2 Average content of some environmental parameters of Holocene

and Pleistocene aquifer in Nam Định Province

Holocene aquifer

Pleistocene aquifer

-VN-Standards

TCVN 1995*

* Viet namese guideline for groundwater

** World Health Organisation Guideline for drinking water

70% of samples have a NH4+concentration, exceeding the VN-Standards Most of polluted areas are in Giao Thuỷ, Nam Trực and Mỹ Lộc Districts

The nitrate concentration of groundwater in Pleistocene aquifer is quite low Among 147 studied samples, there are only a low percentage of samples, which have a nitrate concentration exceeding the VN-Standard

The nitrite concentration of water from Pleistocene aquifer is estimated from 0.01- 13.5 mg/l The variety ranges from 0.05-12.8 mg/l in the rainy season and 0.01-13.5 mg/l in the dry season According to VN- standards (TCVN- 1995) about 40% of the studied samples have a

NO2-values exceeding the VN- standards and the WHO- Guideline for drinking water These

samples with high NO2-values are mostly from Thịnh Long, Yên Định, Hải Anh, Hải Đường, Hải Giang Communes of Hải Hậu District; Giao Long Commune of Giao Thuỷ District; Trực Hưng, Trực Đạo Communes of Trực Ninh District; Nghĩa Tân, Rạng Đông, Nghĩa Sơn, Nghĩa Hùng Communes of Nghĩa Hưng District and Mỹ Hưng Commune in Mỹ Lộc District

It is shown that groundwater from Holocene and Pleistocene aquifers in Nam Định contains almost high value of NH4 and organic compound This value is in water from Holocene aquifer however much higher than this one in water from Pleistocene aquifer

The Holocene aquifer (qh) is in contact with the surface without confining layer It has a direct relationship with the surface and stays under interaction with human activities Therefore the water in this aquifer is strongly effected through different pathways

Up to now, exists neither in craft settlements nor in the industrial zones an effective waste-water treatment system Untreated wastewaste-water flows directly in to the nature Additionally, because of a very high population density in the craft settlements, many ponds were filled up with waste to make place for production These are some of the main pollution’s pathways for the groundwater

Figure 3 Distribution of NH 4 -values in Pleistocene aquifer of Nam Định Province

The Pleistocene aquifer, covered by confining layer (qhvp), is only in contact with the surface and upper aquifers via “hydrogeological windows”, e.g., region near Red River In both rainy and dry seasons, the Red River is the regular natural supplemental water source The risk of pollution spreading through this pathway is therefore very high However, the increase of ammonium concentration in Pleistocene aquifer at areas near the Red River can not be interpreted by this mechanism Since, on one hand, the ammonium concentration inRed River is very low (<1 mg/l), and on the other hand, there is a clear proportional relation be-tween the salinity and ammonium concentration in Pleistocene aquifer

The pollution infiltration at confined aquifer in areas having thick confining cover is small, since the water compression level in this aquifer is always higher than in the upper ones The intensive water exploitation however strongly effects on the water level decrease, which may creates the vertical gradient speeding up the convection for pollution infiltration In the four studied seasons, at some points in the western area of Nghĩa Hưng District, the ammonium concentration increases significantly with the time This area is also the place where the groundwater in Pleistocene aquifer was the most intensively exploited in the last five years This inappropriate water exploitation can be target as the reason for the ammonium pollution

in groundwater in this aquifer

Considering the sedimentary cross section in the east-west direction from Vụ Bản through Nghĩa Hưng to Hải Hậu (see Fig 1), the sedimentary thickness of confining covernear Vụ Bản is very thin with the value of only from 10 to 25 m, while near Hải Hậu that increases from 30 to 80 m This consideration is clearly the reason for the organic compound and ammonium pollutions in Holocene and Pleistocene aquifers in the northeastern and northern parts of the province

The Pleistocene aquifer along the seashore of Hải Hậu and Nghĩa Hưng Districts, at the moment, is still in good condition (Fig 3-5): free of ammonium and other nitrogenous com-pounds though ammonium concentration increasing with the time has been detected in Nghĩa Hưng yet It is in fact a natural potentiality for regional economic development and inhabitant

Figure 4 Distribution of salinity in Pleistocene aquifer of Nam Định Province

Figure 5 Distribution of Fe-values in Pleistocene aquifer of Nam Định Province

living In term of biochemial oxygen and chemical oxygen demand, this area however has already been in the early stage of pollution

IV CONCLUSIONS

The groundwater potential in Nam Định is really satisfied for water supply for all purpose However, because of its quality, the use of water from Holocene aquifer is very limited But,

it is very important not to outright reject this shallow groundwater concerning its high iron and nitrogen content as a fresh-water resource It is possible that this kind of water resources could be used effectively by a number of potential industries

Further more, to have a fully understanding for the problem of water quality in the province is very important and it needs to develop a cohesive strategy about ‘safe’ water supply in regional and local basins

The water level change is a serious problem in Nam Định, and a sustainable water resources exploitation is highly recommended

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