CHAPTER 2 DESCRIPTION OF THE STUDY AREA

Transcription

1 CHAPTER 2 DESCRIPTION OF THE STUDY AREA 2.1 INTRODUCTION The first step in the groundwater model development process is the hydrogeologic characterization of the study area as it is necessary to understand the relevant flow or solute-transport processes. This can be achieved by carrying out field work that includes inspection of monitoring wells data, groundwater samples and their analyses. Without proper site characterization, it is not possible to select an appropriate model or develop a reliably calibrated model. At a minimum, the following hydrogeological and geochemical information must be available for this characterization (Mandle, 2002). Regional geologic data depicting subsurface geology. Topographic data (including surface-water elevations) Presence of surface-water bodies and measured stream-discharge (base flow) data Geologic cross sections drawn from soil borings and well logs. Well construction diagrams and soil boring logs. Measured hydraulic-head data. Estimates of hydraulic conductivity derived from aquifer and/or slug test data. Location and estimated flow rate of groundwater sources and sinks. This chapter starts with an introduction on the study area providing its physical set-up. Further, the chapter describes the hydrogeologic framework of the study area which includes the geologic set-up, stratigraphy, hydrogeologic units, and hydraulic characteristics. Maps are prepared to delineate the spatial extent of the major aquifers beneath the AG area and the actual direction of groundwater flow in each of these aquifers have been defined and described. These maps also show areas of recharge and likely areas of lateral leakage into and out of each aquifer. Groundwater flow directions are determined by constructing potentiometric surface maps. The maps thus constructed have been used to calibrate flow models and in the identification of likely groundwater flow paths. The chapter also describes water-level data acquired from about 98 wells that were used to develop potentiometric contours. The aquifers and confining units associated are based on hydrostratigraphic 31

2 interpretations in three-dimensional framework models that detail the hydrogeology of the study area. 2.2 PHYSCIAL SET-UP OF THE STUDY AREA The study area is a narrow sandy southern Caspian coastal plain extending from Amol to Ghaemshahr along West-East in Mazandaran province of Northern Iran (Figure 1.1). The AGP lies between latitudes N to N and longitudes E to 53 0 E, covering an area of 2300 sq. km. AGP extends about 65 km along E W and 26 to 50 km (av. 40 km) along N S directions. It is bound in the north by Caspian Sea, in the south, by Alborz mountains, in the east, by Siahroud river, and in the west, by Alishroud river. The study region is a highly irrigated agricultural land with urban settlements of relatively dense population. The land use map (Figure 2.1) of the study area illustrates that nearly 87% of the total area constitutes irrigated and dry farming lands and about 5.7% of the total area is covered with urban settlements. Paddy is the major crop and significant area is covered with citrus orchards. Figure 2.1 Land use map of the study area 32

3 2.3 TOPOGRAPHY AND GEOMORPHOLOGY Topographically, the study area forms a plain land between Alborz Mountains and Caspian Sea. The plain, which is at an elevation of 200 m above m.s.l. near its southern boundary, gradually slopes downward towards Caspian Sea and touches 26 m below m.s.l. in the coastal areas of the Caspian Sea (Figure 2.2). Water table also gradually slopes from north (~ 106 m above m.s.l) to south 25 m below m.s.l. Geomorphologically, the study area forms a plain land between Alborz Mountains and the Caspian Sea (Figure 2.3). The Alborz with its restricted width of km forms a steep high arc of mountains, abruptly joining the plains. The plain land consists of detrital materials derived from mountains and piedmonts, which were transported and deposited by flood waters and rivers. Deposition of sediments has led to the development of different physiographic units in this region. These units comprise of plateaus, alluvial fans, alluvial plains, marine deposits and flood plains. Figure 2.2 Topographic map of the study area 33

4 a ALBORZ MOUNTAIN N N b ALBORZ MOUNTAIN Figure 2.3 Satellite images of the study area (a) view from north to south (b) view from south to north 34

5 2.4 CLIMATE Rainfall There are four meteorological stations in around the study area which are located at Babolsar, Gharakhel, Amol and Sari. The first three stations are located within the study area and the fourth station Sari, is located near the study area. The maximum and minimum rainfalls are reported, respectively of Babolsar and Amol. Average annual rainfall at Babolsar is 972 mm and at Amol is 646 mm. At Babolsar 73% of annual rainfall occurs in non-growing season and 27% in growing season whereas at Amol, 71% of annual rainfall occurs in non-growing season and 29% in growing season (table 2.1). Relative humidity in the study area is consistently high throughout the year. The highest (80.13%) and lowest (75.69%) relative humidity in the area is witnessed, respectively in May-June. The average annual relative humidity is 78.25%. The humidity is much higher than 20%, implying that climate falls under humid category. Based on the above factors, the climate of the area can be classified as subhumid to humid with average annual precipitation of about 781 mm, with 70% of the precipitation taking place during the months of non-growing period (from October to March ). Table 2.1 Average precipitation in the study area during Meteorological Station X Y Elevation Non- growing Growing Annually Amount Percent Amount Percent Amount Amol Babolsar Gharagheil Sari Average Temperature Temperatures are high in the months from May to October during which the temperature can rise upto 44 0 C. It is relatively cool between November and March with average temperature of about 6 C. The average annual maximum temperature is around 21.8 C, whereas the average annual minimum temperature is 11.8 C (Source: Iranian Meteorological Organization). 35

6 2.4.3 Evapotranspiration During the heavy rains occurring in October to March, precipitation is obviously greater than potential evapotranspiration. In dry months (April to September), evapotranspiration is greater than precipitation. Because of the low precipitation, evapotranspiration decreases soil moisture in this period. The monthly average of pan evaporation in non-growing period it is mm and for growing period is 125 mm with annual average of 1005 mm. 2.5 HYDROLOGY River systems A group of several large river systems such as Aleshroud, Haraz, Garmroud, Babolroud, Talar and Siahroud have built up AGP. Haraz is the biggest river in the study area with about 57% of annual inflow and 66% of growing period inflow. Aleshroud river has the minimum of inflow into the study area (table 2.2). The rivers, flowing from south to north, have their sources in the distant Alborz and empty their waters into the Caspian Sea. Drainage pattern in the western and eastern parts of the study area differs. In the western part deltaic drainage pattern is conspicuous, whereas in the eastern part, two rivers flowing straight from south to north into the Caspian Sea constitute the fresh water source (Figure 2.4). The rivers of the study area are perennial and their waters are used for irrigation of farmlands growing semisalt tolerant variety of paddy, locally known as Shaltook rice. Table 2.2 Discharge from the rivers of the study area Name Station Non-growing Growing Annual Volume Percent Volume Percent Volume Aleshroud Oskomahaleh Haraz karesang Garmroud Baliran Bobolroud Entrance to plain Talar Entrance to plain Siahroud Sarokela All volume units are mcm. 36

7 2.5.2 Drain Natural surface drains collect and dispose runoff quickly or lower water table where water logging conditions occur. From the technical report of the MRWA, it is known that there are 14 drains in the study area that convey water from aquifer to Caspian Sea (Figure 2.4). Figure 2.4 Major rivers and surface drains constructed across the coastal line in the study area Caspian Sea The Caspian Sea is the largest landlocked sea body of water separating Europe and Asia with no outlets. The Caspian Sea is bordered by five countries namely Russia, Republic of Azerbaijan, Turkmenistan, Kazakhstan and Iran. In Iran, it is referred to as Daryâ-ye Mazandaran, meaning "the Sea of Mazandaran" in Persian, and sometimes also as Daryâ-ye Khazar ( The sea has a surface area of 371,000 km 2 and a volume of 78,200 km 3. It extends 1,200 km from 37

8 north to south and contains more than 40% of the inland waters of the world. The Caspian Sea has characteristics common to both seas and lakes. The Caspian sea is commonly divided into three parts; the northern, middle and southern (Kaplin and Selivanov, 1995; Froehlich et al., 1999). The shores of the Caspian sea are mainly made of Quaternary deposits Evolution of the Caspian sea: The Caspian Sea is a remnant of the ancient Paratethys Sea. The Caspian Sea became landlocked about 5.5 million years ago due to tectonic uplift and a fall in sea level. During warm and dry climatic periods, the landlocked sea has all but dried up, depositing evaporitic sediments like halite that have become covered by wind-blown deposits and were sealed off as an evaporite sink. During cool and wet climate conditions the basin was refilled ( as a resulting current inflow of fresh water. The Caspian Sea is a freshwater lake in its northern portions Caspian Sea level changes: The considerable drop in the Caspian Sea level began in 1930 (Firoozfar et al., 2012). Prior to that, during the 1837 to 1930, water level had been more constant, fluctuating around -26 m msl. In 1929, the water level stood at 26.1 m below open oceanic level. This level dropped rapidly by around 1.6 m and reached m msl by After that the Caspian sea level continued to decline but at a slower rate, falling by around 1.4 m within 37 years and reaching m by After 1977, the sea level suddenly began to rise and in 1995 water level of about msl was recorded. This water level indicates an increase of 2.6 m over the period from 1977 to Since 1995, a slow rate of decline has occurred. The water level of the Caspian Sea is currently positioned approximately 27 m below the mean sea level (Stolberg et al., 2006). The last short-term sea-level cycle started with a sea-level fall of 3 m (9.84 ft) from 1929 to 1977, followed by a rise of 3 m (9.84 ft) from 1977 until Since then smaller oscillations of water level have taken place. The surface of the Caspian Sea has shown long-term oscillations, and fell about 3 m between 1930 and 1977, since then it has risen about 2.5 m, providing an important example of a modern marine transgression. The historical data of sea level oscillation of Caspian Sea at the Anzali Harbour during the last 70 years is shown in Figure 2.5). 38

9 Tide ranges are very small, but sea level can rise and fall by up to 2 m as the result of storm surge events, fluctuations Wave regime and currents: The North basin is marked by low energy waves and the South basin is characterized by moderate energy waves. In the Middle of Caspian basin waves have higher energy. Generally, the southern Caspian Sea coasts are exposed to waves which come from the north, northeast and the northwest. The waves coming from the north and northeast reach the western part of the coastline, namely West Guilan, Central Guilan, and West Mazandaran, while the waves reaching the eastern part including East Mazandaran and Golestan come from the North and Northwest (Lahijani et al., 2007). As a result of these waves long shore currents can be observed in a North-South direction along the western and eastern coasts of the south basin and along Iran's Caspian Sea coast prevailing eastward long shore current flows Figure 2.5 sea level changes of Caspian Sea from 1941 to 2011 recorded at Anzali harbor station Water quality of Caspian sea: As distinct from other lakes, the water of the Caspian is not fresh, but brackish. The water of the southern basin is consistently brackish and salinity varies between 10 to 13 (Aubrey, 1994; Kosarev and Yablonskaya, 1994), making this water unsuitable for drinking or irrigation (Aladin 39

10 and Plotnikov, 2004). Salinity varies around an average of about 12 parts per thousand, and water temperatures range from about 25 C in summer down to 8 11 C in winter. Determination of TDS of the Caspian seawater in parts of the study area by NRCCS (Personal communication) indicated the values to range from to mg/l with average of mg/l and density of Caspian sea water varies from to with average of gr/cm GEOLOGY OF THE AREA The coastal plain, which is 0 28 m below mean sea level consists of thick sequence of sediments which have mostly accumulated in a post-oligocene foreland basin settings (Jackson et al., 2002). Alborz Mountains form gently sinuous E-W range across N Iran, south of the Caspian Sea, constitute the southern boundary of the study area. Rapid upliftment of Alborz between 6 and 4 Ma is synchronous with the subsidence in the south Caspian (Axen et al., 2001). In the southern Caspian basin rapid basin subsidence and large sediments input, at the same of m/Ma resulting in the formation 30 to 39 Km thick (Aliyeva, 2004). AGP is covered with alluvial sediments of Tertiary and Quaternary age and consists of rhythmically bedded fluvial deltaic sediments of, Haraz, Garmroud, Babolroud, Talar and Siahroud river systems. Sediments in the coastal plain consist of interbedded sand and clay with lesser amounts of silt and calcareous soil. Geological map of the study area is shown in Figure 2.6. The study area is largely covered by loamy soil with varying proportions of sand, silt and clay. Recent alluvium is restricted along Talar and Babolroud river courses. Geology of the study area mainly consists of thick beds of Quaternary unconsolidated to semiconsolidated fluvial and deltaic detrital sediments, which constitute more or less horizontal horizons. Log data of exploratory drill holes drilled to a maximum depth of ~350 m and geophysical data provide reliable details on the subsurface geological features of the study area. Exploratory bore wells and geophysical data reveal that the Quaternary sediments extend to a depth of more than 500 m. In the vertical sections (Figure 2.7 & 2.8), the study area consists of 4 more or less horizontal horizons of detrital sediments. These horizons, along N S direction exhibit broad U shaped morphology with the concave surfaces facing up. Along the E W direction, the sedimentary horizons are more or less horizontally disposed. 40

11 Figure 2.6 Geological map of the study area Figure 2.7 Generalized geological cross section along the flow lines (N-S) in the AGP 41

12 Figure 2.8 Generalized geological cross section across the flow line (W E) in the AGP Horizon 1 (top horizon) ranging in thickness from 10 to 60 m (av. = 25 m), is highly permeable and consists of sand, gravel and calcareous silt. This horizon constitutes the shallow and unconfined aquifer of the study area. Horizon 2 is composed of clay, silty clay and sandy clay and constitutes the impervious layer separating the upper unconfined aquifer and lower semi-confined aquifer. Horizon 2 varies in thickness from 6 to 62 m (av. = 19 m) and extends in the E-W direction for a distance of about 65 km. It extends in width along N S direction from 14 to 24 km (av. = 20 km). Horizon 3 is composed of silt, gravel and fine to medium sized sands. It varies in thickness from 50 to 200 m (av. = 150 m). Horizon 3 constitutes the lower and large semi-confined aquifer of the study area. Horizon 4 is mostly made up of marine sand and silt with bivalves. It carries salt water, probably palaeobrine water. (Dewan and Famouri, 1964). According to Dewan and Famouri (1964) Mazandaran province constitutes the Caspian sea littoral and later developed as a result of general retreat of Caspian sea, which at one time probably extended southward, upto the foot hills of Alborz mountains. This aspect of evolution of the Mazandaran province suggests the possible presence of relic connate seawater component in the permeable sedimentary beds of the semi-confined and unconfined aquifers of the AGP. 2.7 HYDROGEOLOGY OF AGP AGP is underlain by unconsolidated to semiconsolidated detrital materials of Cenozoic age. The sediments of Quaternary and Tertiary age are undifferentiated and principally consists of clastics (quartz sand) with minor amounts of clay, shell, 42

13 carbonates (limestone and dolomites) and evaporites (gypsum and anhydrite). The sand varies in size from fine to coarse grained, nonindurated to poorly indurated, and nonclayey to slightly clayey Hydrogeologic Units Limited geologic data are available in AGP to determine the characteristics of these aquifers and aquitards. In the vertical sections (Fig. 2.7 & 2.8), the study area consists of 4 more or less horizontal horizons of detrital sediments. These horizons, along N S direction exhibit broad U shaped morphology with the concave surfaces facing up. Along the E W direction, the sedimentary horizons are more or less horizontally disposed. Horizon 1 (top horizon) ranging in thickness from 10 to 60 m (av. 25 m), is highly permeable and consists of sand, gravel and silt. This horizon constitutes the shallow and unconfined aquifer of the study area. Horizon 2 is composed of clay, silty clay and sandy clay and constitutes the impervious layer separating the upper unconfined aquifer and lower semi-confined aquifer. It extends in the E W direction for a distance of about 65 km and varies in width along N S direction from 14 to 24 km (av. 20 km). Horizon 3 is composed of silt, gravel and fine to medium sized sands. Horizon 3 varies in thickness from 50 to 200 m (av. 150 m) and constitutes the large semi-confined aquifer of the study area. Horizon 4 is mostly made up of marine sand and silt with bivalves. It carries salt water, probably palaeobrine water which represent remnants of unflushed old marine water. Limited hydrologic and geologic data are available in AGP to determine the characteristics of these aquifers and aquitards. The mapped geologic units in the AGP have been grouped as hydrostratigraphic units based on their similar geologic and hydrologic properties. As mentioned under hydrogeologic units, two groupings of the most permeable hydrostratigraphic units have been classified as aquifers (Figure 2.7 & 2.8). The aquifers consist of the thick sequence of Quaternary alluvium comprising dominantly of sand of various granularity that extend throughout much of the subsurface of AGP. Groundwater flow at the study area is obstructed or diverted by low-permeability fine-grained alluvial sediment. Tertiary alluvium forms the basement of the semi-confined unit which holds palaeobrine water. As the study area covers an area of 2300 sq. km, it is 43

14 possible that widespread interconnected aquifers make up regional flow systems in which groundwater moves, nearly unimpeded, over long distances Aquifer Properties Under this heading groundwater level, groundwater depth, alluvial thickness, aquifer thickness, and transmissivity are considered as these properties are required for model development Groundwater Level and Movement: MRWA is monitoring groundwater levels in the study area on monthly basis in 98 observation wells falling in the model area. Out of these 22 observation wells are termed as deep and 76 observation wells are called shallow. Location of shallow and deep groundwater monitoring points is shown in figure 2.9. Shallow observation wells were installed during year 1985; therefore, groundwater levels data is available for quite a long period. However, deep observation wells were installed later, accordingly data of these observation wells is available from October 2006 to present day. Hydrograph of the study area is shown in figure From the figure, it is clear that the groundwater level doesn t have any important increase or decrease in the last 20 years. To determine groundwater elevation above mean sea level, the following equation ( was used E W = E D where: E W = Elevation of water above mean sea level (m) or local datum E = Elevation above sea level or local datum at point of measurement (m) D = Depth to water (m) The water table contours for the month of August 2010 is depicted in Figure The figure shows that the study area is characterized by comparatively steep hydraulic gradients in the upper part as compared to the lower part of the study area. Whereas, hydraulic conductivity is higher in the upper part of the study area as compared to that in the lower part of the study area. Groundwater table, near the foot hills of the Alborz Mountains, is at an elevation of ~ 106 m above msl, which gradually slopes down towards the Caspian Sea. Near the shore line the water table is at shallow levels, generally at a depth of ~ 25 m below msl. In the study area, 44

15 groundwater flows from south to north and the hydraulic gradient of the water table decreases from south to north and west to east. Figure 2.9 Locations of observation wells Figure 2.10 Map showing hydrograph of the study area from 1990 to

16 Figure 2.11 Map showing spatial variation of groundwater levels in the study area Depth to groundwater: Water-table conditions occur where unconfined ground water is under atmospheric pressure and will not rise above the level at which it initially entered a well. Under artesian conditions the water-bearing material (aquifer) is overlain by a confining bed. Recharge entering the system from a higher elevation results in pressure being imposed on the ground water. In this case the water will rise above the level at which it first entered the well--that is, above the bottom of the confining bed. Groundwater in the alluvial deposits in AGP is generally under water-table conditions. In the central part of the alluvium, coarse sand and gravel is overlain by a thick section of less permeable silt or clay which acts as a confining bed, and hence artesian conditions result. Surrounding the semi-confined aquifer, the artesian conditions will change to water-table conditions. 46

17 Depth to groundwater were collected and stored as monthly time-series data from MRWA. In the unconfined aquifer, the depth to groundwater level ranges from 0.2 meter to 62 meter with a mean value of 7.5 meter (Figure 2.12). Figure 2.12 Map showing spatial variation of groundwater depth in the study area Alluvial Thickness: The AG coastal area is composed of thick-layered unconsolidated coarse-grained highly porous sediments. They make up the alluvial aquifer. These sediments are diverse. The alluvium is composed of sand, gravel and lesser amounts of silt and clay. In general, the impermeable clay units along the base of the alluvium tend to get thicker towards the central portion of the study area. Generally, the coarser material is found at the southern part of the study area, closer to the position of the pediments of the Alborz mountain. Thicknesses of the alluvium differ from south to north along the direction of the flow of major rivers in the study area. The thickness ranges from a minimum of 5 m to a maximum of 373 m with a mean value of 160 m (Figure 2.13). In grain size, the alluvial deposits range from clay to gravels, with all possible admixtures. Textural changes occur both laterally and vertically. The coarser sections are overlain by 47

18 clayey silt which ranges in thickness from a few cms to more than m. The admixture of clay and silt acts as an aquitard and confines the aquifer from the overlying unconfined aquifer in the central part of the study area. The alluvium in AGP was formed during the Tertiary period. Figure 2.13 Alluvial thickness map of the study area Aquifer Thickness: The alluvial aquifers are under water-table conditions except where the coarser material is overlain by a relatively thick section of impermeable silt or clay and in such place artesian conditions exist. The aquifer thickness was obtained by subtracting groundwater depth from the alluvial thickness. As can be seen from the Figure 2,14, the aquifer is thin with a thickness of less than 5 m in the northeastern part of the study area and becomes progressively thicker towards northwestern side reaching a maximum of 368 m. 48

19 Figure 2.14 Aquifer thickness map of the study area Figure 2.15 Map showing spatial variation of transmissivity in the study area 49

20 Transmissivity: Estimation of transmissivity was carried out accurately by comparing with field records data obtained from pumping tests. Pumping test on wells at the study area showed that the transmissivity values of the alluvium range from 5 to 2800 m2/day (Figure 2.15). 2.8 HYDROCHEMISTRY Apart from knowing the aquifer type, material and their hydrologic properties, it is essential to have the knowledge of the processes responsible for the mixing of seawater with that of freshwater. The knowledge gained from the hydrochemical investigations will pave way for the construction of hydrogeological model of the study area. Investigation carried out in this regard has been dealt in detail in the succeeding chapter. 50