ASSESSMENT OF UTILIZABLE GROUNDWATER RESOURCES IN A COASTAL SHALLOW AQUIFER

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1 ASSESSMENT OF UTILIZABLE GROUNDWATER RESOURCES IN A COASTAL SHALLOW AQUIFER V. S. SINGH AND V. K. SAXENA National Geophysical Research Institute, Uppal Rd, Hyderabad, , India In the recent years there has been overall development in the field of industry and agriculture and the growth in population which has increased demand of potable water resulting into indiscriminate exploitation of groundwater resources. An unplanned exploitation of groundwater resources is causing not only depletion in groundwater reserve but also deteriorate the quality. This phenomenon is more significant in case of coastal aquifer where sea water is direct threat to pollute the groundwater resources. Coastal aquifers are often prone to sea water ingress. The indiscriminate exploitation of groundwater, particularly from the shallow aquifer, may induce the sea water intrusion. It is, therefore, necessary to understand the behavior of the aquifer in the vicinity of sea and develop sustainable withdrawal scheme which does not attract the sea water. Geophysical, hydrogeological and chemical studies have been carried out in a shallow aquifer at beach side near Sadras to evaluate groundwater regime. The groundwater regime is simulated using numerical model. The effect of sea water ingress is explored considering various groundwater withdrawal schemes. A sustainable scheme is, thus, suggested. INTRODUCTION In the recent years there has been significant growth in the field of agriculture and industry. This has lead to an increased demand for water. The easiest way to meet this demand is to tap the available groundwater resources in case no surface water source is available. There is an indiscriminate exploitation of groundwater resources in the developing countries, particularly in the absence of any legislation. This has resulted into decline in groundwater potential as well as deterioration in groundwater quality. In case of coastal aquifer, the threat from the sea water intrusion is more. The reversal in the hydraulic gradient attracts sea water intrusion. In order to protect the fresh groundwater in the vicinity of sea, it is vital to understand the behavior of aquifer system, its characteristics and response to various stresses so that sustainable development plan can be prepared. In an effort, detailed study has been carried out in an area where shallow aquifer is in the vicinity of sea. The study area is situated near Mahabalipuram, about 60km south of Chennai at the eastern coast of India (Fig. 1). The area is bounded on the east by Bay of Bengal, in the north by back water, and in the west by Buckingham canal. The entire area is covered 1

2 LATITUDE (in degree) LONGITUDE (in degree) KP11 Latitude (in degree) B U C K I N G H A M KP9 KP8 KP1 KP KP13 KP KP16 KP Longitude (in degree) 0Km Figure 1. Location map of study area 2Km with sands or weathered charnokite underlain by charnokite rock. The area receives on an average about 1334 mm. of annual rainfall. The area receives significant rainfall towards the last quarter of the year. In general, the area is having low undulating topography. The ground elevation varies from few meters to about 8m above mean sea level (amsl). In order to understand the subsurface geology and select sites for drilling test holes, geophysical investigations were carried out. GEOPHYSICAL INVESTIGATION One of the widely used geophysical techniques for groundwater exploration is DC electrical resistivity method. Geo-electrical soundings employing Schlumberger Array with maximum half-current electrode separation of 100 meters were carried out in the area. Initially, the resistivity curves were interpreted by curve matching method for estimation of layer parameters. The interpretation was further refined by using these layer parameters as the initial model parameters for computerized RESIST software program

3 for VES interpretation. In almost all cases, observed resistivity curves could be interpreted satisfactorily (RMS error less than 3%). The interpreted aquifer thickness values indicate that the depth to bedrock varies between 12.9 m and 46 m. The bed- rock is shallow in the north and western side and deeper in the central region. The thickness of alluvium is more in the south and in the east. Based on above studies, ten sites were selected for test bore wells. Lithologs were obtained during drilling. Based on these lithologs, distribution of aquifer system in the study area has been prepared as shown in Fig. 2. It can be seen that the maximum thickness of aquifer is encountered around well no. KP-5 in the east and minimum towards well KP-9 in the west. These test wells were utilized for measurement of groundwater water level as well as carrying out pumping test on these wells to estimate aquifer parameters KP KP KP-9 KP KP-8 KP-1 KP16 KP13 KP7 0km 1km KP KP-13 KP-5 KP-16 KP17 KP5 FINE SAND WITH CLAY 10 m SAND AND WEATHERED CHARNOKITE 20 m 500 KP HARD CHARNOKITE Reference Vectors E Figure 2. Fence Diagram showing aquifer system Figure 3. Groundwater flow GROUNDWATER REGIME Groundwater levels observed in the wells have been utilized to prepare groundwater flow map in the area. In order to prepare regional groundwater flow map the water level in the Buckingham canal has also been taken into account.

4 Groundwater flow map for the month of Dec. 02 is shown in Fig. 3. The figure depicts not only the groundwater flow directions but the hydraulic gradient is also indicated by arrows, which varies from 8.9x10-5 to GROUNDWATER QUALITY Groundwater samples have been collected in the month of May, 2003 from nine bore wells. These water samples were analyzed for major cation and anion. It shows that these waters are nearly neutral to mildly alkaline in ph ( ) except one sample which have shown their mildly acidic character (6.7 ph). This acidic behavior and low EC (130 micro S/cm) as shown in the bore well No.16, may indicate the occurrence of fresh rain water. The electrical conductivity of water samples from these bore wells varies from130 to micro S/cm. This shows very clearly the two extreme chemical values of EC i.e. very low to very very high of saline type. High contents of Na, Cl, Mg and SO 4 have been found in general and this watershed has three different types of water types. i. Na-Ca-Cl-HCO 3, ii. Mixed, and iii. Na-Cl ESTIMATION OF AQUIFER PARAMETERS In order to assess groundwater potential in any area, and/or to evaluate the impact of pumpage on the groundwater regime, it is essential to know the aquifer parameters. These are chiefly Transmissivity (T) and Storativity (S). These parameters are also vital for the management of the groundwater resources through the use of groundwater flow model. In the study area the test wells were selected for carrying out pumping test. Pumping test on these wells were performed using submersible pump and making observation in the nearby observation well and/or in the pumping well. The pumping test data (both pumping and recovery phase) have been interpreted considering the field conditions to evaluate aquifer parameters. Numerical method as described by Singh (2000), was used to interpret pumping test data considering both pumping as well as recovery phase. The transmissivity values have been found to vary from 2.2 m 2 /d to 44 m 2 /d. GROUNDWATER MODELING In order to assess the groundwater regime and evaluate groundwater velocity under given hydrogeological boundary conditions, SUTRA (Voss, 1984) software from USGS has been used. The software can simulate the groundwater along with sea water. 4

5 Basic Principles The mathematical equations describing density dependent flow and solute transport may be expressed as (Voss, 1984) 5 v = -[k, (εµ) -1 ]. ( p-ρg), (ερ) / t = -. (ερv) +Q p, and (ερc) / t = -. (ερvc) +. [ερ (D m I+D). C] + Q p C * where, v = average fluid velocity, ε = porosity of rock matrix, k = permeability of the rock matrix, µ = fluid viscosity, p = fluid pressure, ρ = fluid density, g = gravitational acceleration, Q p = fluid mass sources, C * = solute concentration of fluid source, D = dispersion tensor, I = Identity tensor, and D m = apparent molecular diffusivity (1a) (1b) (1c) Equation (1b) and (1c) effectively take into account the fluid mass balance and solute mass balance, respectively. The computer code SUTRA (Voss, 1984) was used to solve the equations (1a, 1b and 1c). Details of modeling procedure is given by Voss (1984). Physical Framework The physical framework of the modeled aquifer broadly conformed to geological set-up as determined by geophysical as wells as hydrogeological investigations as described above. The domain of the aquifer has been taken as shown in Fig. 4. The aquifer parameters have been considered as obtained from pumping test data. Figure 4. Domain of aquifer

6 Numerical values of the characteristic parameters of the sandy aquifer system as adopted in the present study from various publications are given below. Aquifer compressibility 2.5x10-9 m 2 /N Water compressibility 4.4x10-10 m 2 /N Fluid viscosity 1x10-3 kg/m/s Fresh water density 1000 kg/m 3 Sea water density 1025 kg/m 3 Solute mass concentration kg salt/kg sea water in sea water 6 Boundary conditions The nodes on the eastern side representing the sea boundary (Bay of Bengal), were assigned a mean sea level (pressure head as zero). The solute concentration at all the nodes along sea boundary was taken as kg/kg, i.e. the same as in the sea water. The northern side of the domain and the western side are bounded by the back water and Buckingham canal. Since the back water and water level in the Buckingham canal are of same level, the water levels measured at different times were considered for these boundaries. Aquifer thickness distribution has been considered based on drilling results which varies from 3 to 12m. Similarly the permeability values were assigned as obtained from the pumping test data. The effective porosity of the aquifer material has been considered as The bottom of the aquifer was considered as no flow boundary which is an impermeable charnokite as concluded from the drilling data. Water table constituted the upper boundary of the model. GROUNDWATER WITHDRAWAL AND ITS IMPACT In order to assess the impact of further groundwater withdrawal on the groundwater regime, particularly on the quality of groundwater, the model has been utilized as a predictive tool. The observed groundwater quality in terms of its electrical conductivity (EC) is shown in Fig. 5. An additional groundwater pumpage of 25 m 3 /d from the distributed wells in area of 1.8 sq. km has been considered. It has been found that the EC of groundwater has further increased from Birmingham canal. This increase has been found quite prominent if the groundwater withdrawal further increases to 50 m 3 /d as shown in Fig. 6. However, measures such as artificial recharge can be suitably implemented to stop such deterioration in groundwater quality in case further groundwater withdrawal is required.

7 7 CONCLUSIONS Based on the geophysical and hydrogeological investigations the following conclusions can be drawn: The depth to bed rock (charnokite) in the study area varies from 12.9 to 46m deepest in the eastern part, The aquifer transmissivity varies from 2.2 to 44 m 2 /d and storage coefficient from to , There has been no sea water intrusion except around a small area in the close vicinity of Buckingham canal. The groundwater quality in the area deteriorates if groundwater withdrawal rate increases more than 25 m 3 /d. The groundwater quality can be improved through suitable artificial recharge Figure 5. Present EC vales Figure 6. Predicted EC with enhanced abstraction ACKNOWLEDGEMENTS The project has been initiated by Dr. I Radhakrishna (Scientist-Retd). The project has been financed by IGCAR, Kalpakkam. Dr. A. Natarajan, Dr. P. Sasidhar, Dr. Mohan Kumar, Dr. S. V. M. Satyanarayana and other staff of IGCAR have been helpful during the field investigations and suggestions made by them have been of grate help. Director, NGRI has given all the encouragements to carry out various investigations. Authors are thankful to all.

8 8 REFERENCES [1] Singh, V.S., Well storage effect during pumping test in an aquifer of low Permeability, Hydrological Sciences Journal, Vol. 45, No. 4, (2000), pp [2] Voss, C. V., A finite element simulation model for Saturated Unsaturated, fluiddensity-dependent groundwater flow with energy TRAnsport or chemically-reactive single species solute transport, USGS Water Resources Investigation Report , (1984), p. 409