CHAPTER 4 METHODOLOGY

Transcription

1 59 CHAPTER 4 METHODOLOGY 4.1 INTRODUCTION The general methodology adopted for the hydrogeological study in Gadilam lower sub basin includes the data collection from various departments on water level, water quality, rainfall, climatological, bore well lithological, geophysical, well logging, pumptest, yield data etc were collected from TWAD Board and PWD Groundwater Division, Cuddalore. These data were used in the study. The overall methodology adopted is presented in the flow chart in Figure 4.1. The following methodology have been adopted to get an in depth knowledge of the hydrogeological conditions of the study area. 4.2 DATA USED The following data is used in the studies which were collected from different state and central government organizations such as Geological survey of India, Chennai, Central groundwater board Chennai, PWD ground water division, and Institute for water studies Taramani and Tamilnadu water supply and drainage board, Chennai. In addition to this, some other data is also collected pertaining to this study from TWAD Board Cuddalore, PWD ground water division, Cuddalore, Department of Statistics and department of Soil at Cuddalore.

2 Data Collection Satellite data analysis (IRS IC & ID LISS III) Survey of India toposheets on 1:50000 scale Collateral Data from different organisation Drainage map Base map Analysis of pump test data Hydro geophysical analysis Hydrogeo Chemical analysis Groundwater modeling Preparation of Landuse maps 97 & 03 Preparation of Geomorphology map Rainfall Data Water Level Data Analysis of VES data Bore hole data Analysis of SP log Water quality analysis Simulation of result Landuse change detection 97& 03 Preparation of Geology map Generation of Rainfall & Water Level Contour Maps Preparation of litho log Model forecasting Integration for Hydro geological study using GIS Identification of Groundwater Prospects zones and favorable recharge zones. Suggesting appropriate recharge structures Figure 4.1 Flow chart of methodology for research 60

3 Physiographic data District Maps, Taluk maps, Survey of India topo sheets 58 M/5, 58 M/6, 58 M/9 and 58 M/10&14 on 1:50000 and Geological map of Cuddalore district are collected from State Ground and Surface water Data Centre, Taramani Satellite data used 1. IRS 1C, LISS III and PAN IRS 1D LISS III and PAN Hydrometeorological data Climatalogical data Rainfall data Land utilization data Crop details These data were collected from various organizations such as Lekkur weather station and PWD, Groundwater Division, Cuddalore to study the climatic conditons and rainfall influence in the study area Hydrogeological data 1. Well inventory data 2. Geophysical resistivity data

4 62 3. Bore hole data 4. Groundwater level data 5. Groundwater quality data 6. Well yield data etc. 4.3 PREPARATION OF RESOURCES MAPS The following thematic maps were prepared using the visual interpretation techniques. In this study ArcView (ESRI 1997) GIS software is used efficiently for the analysis of hydrogeological information acquired from the surface geophysical techniques as well as borehole lithologs, where GIS helps in analyzing data / information objectively using different logical approaches. Different themes interpreted from satellite and topographical data on 1:50,000 scale were converted into raster format using colour scanner. These raster maps were geometrically rectified using Ground Control Points (GCP) and converted into vector format by on screen digitization method using Arc GIS software. These vectorised themes were put into GIS environ using common projection coordinates so as to obtain better results in overlay analysis Base Map Base map is prepared from Survey of India toposheets58 M/5, 58 M/6, 58 M/9 and 58 M/10&14 on 1:50000 scale.

5 Sub Basin Map Sub basin map is prepared by delineating the drainage systems from Survey of India toposheets58 M/5, 58 M/6, 58 M/9 and 58 M/10&14 on 1:50000 scale and updating it with the suitable recent satellite imagery Soil Map Soil Classification have been carried out in 1996 by National Bureau of Soil Survey and Landuse Planning, Bangalore (NBSS) in co-operation with the Department of Agriculture, Tamil Nadu. Soil plays an important role in (a) encouraging or discouraging the recharge of groundwater and (b) determining the quality parameters of ground waters. Due to different stage of weathering of parent material, the above soil types are met with in combination. According to this classification, the major types of soil are found in the Gadilam river lower sub basin is Inceptisols, Alfisol, Entisol and Vertisol. They are very much useful with respect to the rate of runoff, potential and find infiltration rate Drainage Map This map is also prepared from Survey of India toposheets58 M/5, 58 M/6, 58 M/9 and 58 M/10&14 on 1:50000 scale. This map is useful to know the rainfall infiltration and the runoff and also to understand the groundwater flow direction and recharge mechanism.

6 Relief Map It is prepared by digitizing the contours from the survey of India toposheets toposheets58 M/5, 58 M/6, 58 M/9 and 58 M/10&14 on 1:50000 scales. The maximum relief of the study area is 80 m and the minimum relief is 20m.It is very much useful to understand the slope of the terrain Wasteland Map Wasteland map is prepared from IRS-1D LISS III + PAN merged satellite data. It shows different categories of wasteland Geology Map Geology map is prepared from GSI maps of 1:50,000 scale and it gives the details of lithology and structure in the field by their distinct texture, structure and colour. The Gadilam river lower sub basin shows tertiary uplands in the south and recent alluvium in the north. It is useful to study the aquifer characteristics like aquifer thickness, type of aquifer, porosity, permeability etc. and also for selecting site for construction of check dam, ponds etc Geomorphology Map The study of the land and landforms constitutes geomorphology. The landforms are the surface features of the earth, shaped by the various processes operate over the earth. In other words geomorphology deals with the origin and

7 65 morphological characteristics of the surface of the earth s crust. The geomorphological map was prepared using Satellite data coupled with topographic maps. Geomorphology map is prepared by using IRS-1D LISS III + PAN merged satellite data false colour composite and it demarcates the geomorphic units like younger flood plain, old river coarse, valley fills etc. are excellent groundwater potential zones. IRS-ID satellite data LISS III false colour composite have been used to demarcate the different geomorphic units. These different geomorphic units are giving vital information about the groundwater occurrence in the sedimentary regions Landuse Map Remote sensing helps in analyzing the land use study is important for planning, development and land management activities of agricultural sector. The high resolution satellite data of IRS 1C and 1D provide a new dimension to the mapping of land use or land cover details on various levels. The Panchromatic and LISS III type sensors provide very good resolution facility to classify data with better details. Land use map of Gadilam lower sub basin was prepared using IRS-1D LISS III + PAN merged satellite data on 1: 250, 000 scale by visual interpretation and also by digital image processing and classification methods.. The supervised classification is done using maximum likelihood estimation by

8 66 assigning suitable training sites in the ERDAS imagine software. The topo sheet is used as reference for this classification. 4.4 HYDROGEOPHYSICAL STUDIES Vertical Electrode Sounding has been conducted in the study area (i). To study the nature and extent of the promising potential aquifers of groundwater and (ii). To find out the thickness of saturated zones and depth to basement topography.an enormous number of electrode spreads is being used in resistivity method. The two widely used arrays are (1) the Wenner and (2) the Schlumberger configuration. In the present work most of the data generated were from Schlumberger configuration. Geophysical methods are also employed for the selection of appropriate site for artificial recharge studies is to assess the unknown subsurface hydrogeological conditions economically, adequately and unambiguously. Mostly it is employed to narrow down the target zone, pinpoint the probable site for artificial recharge structure and its proper design. The application of geophysical methods is to bring out a comparative picture of the sub-surface litho environment, surface manifestation of such structures and correlate them with the hydrogeological setting. Besides defining the sub-surface structure and lithology, it can identify the brackish/ fresh ground water interface, contaminated zone (saline) and the area prone to seawater intrusion.

9 HYDROGEOCHEMICAL ANALYSIS Groundwater samples are collected for 25 locations and it was analyzed. Based on the analysis contour maps are drawn for selected parameters such as EC, TDS, TH, ph and Cl by using GIS. The study shows positive result in almost all locations except Nellikuppam and Cuddalore in two different periods. 4.6 PUMP TEST ANALYSIS Evaluation of hydraulic properties of aquifers is an important aspect of any scheme of groundwater resources assessment. The performance characteristics of a well and the hydraulic head (draw down) of the aquifer can be determined by the pumping test. Pumping test is one of the most useful means of not only determining the aquifer hydraulic characteristics but also in the determination of yield, drawdown, specific capacity and design of wells. These data give a measure of the productive capacity of the well. Properly and correctly computed data of pump test data gives the most accurate information which can be used for groundwater development plan. In fact one of the primary objectives of most groundwater resources studies is the determination of maximum possible pumping rates that are compatible with hydraulic environment from which the water still be taken. The specific yield of the formations arrived by conducting a pumping test is also termed as aquifer performance test in which the water in the borehole has been dewatered by pumping and the recuperation of water from different lithological formation and its aquifer characteristics are being studied.

10 68 The water level during pumping test was measured with a water level indicator. The discharge of water was measured with a known volume of container and the time taken for filling has been noted. The draw down of water level and recovery were observed periodically and tabulated in the pumping test format. From the draw down and the recovery curves have been drawn for the observation wells which were drilled especially for the pump test performance and interference due to pumping in the main bore wells. The draw down and the recovery curves have been drawn using a semi log graph sheet. The S Value and the to values are being found out using the tangential lines drawn to the respective curves. The transmissivity, permeability, Specific capacity and the Yield have been calculated from the data using the revised Theis formula. 4.7 STUDY ON BORE WELL LITHOLOGY It gives an overall idea about the sub surface lithology with respect to depth. The aquifer thickness map is prepared based on the field survey and the details collected from PWD and TWAD board. This data is used to determine the depth of occurrence of different formations, to know the thickness of the top soil and nature of the sediment and to find out the thickness of weathered, fractured, jointed zones to determine the aquifer type, thickness, flow pattern etc. and to correlate the variation in lithology. 4.8 GROUND WATER MODELING Ground water modeling was developed using GMS software. The pump test data, water level data, rainfall data, discharge and recharge data, bore well data and relief are effectively utilized for simulating groundwater for estimating the flow direction and groundwater head for different periods.

11 69 This model shows the groundwater head for different periods for 1997 and 2003.This ground water model also predicts the future trend of groundwater head for GIS DATA BASE CREATION Spatial data The spatial data are in the form of maps like geology, soil, geomorphology, landuse, lineament, wasteland etc, which are stored as different layers in digital form in the computer Non-Spatial data The non-spatial data pertains to attribute information in the form of statistics, tables & list, which could be rainfall, water level, resistivity, etc. Integration of spatial and non - spatial data, querying, analysis, etc is done in GIS environ. Thus the new maps are generated precisely by easily integrating innumerable layers of data and manipulating to evaluate relationships among the chosen elements in different layers under consideration. All the themes generated by visual and digital methods were scanned for digitization. Digitization is the process of converting the spatial features of map viz. Point, line and area of a map into a digital format. (Point, line and area features are converted into x, y co-ordinates).

12 Overlay analysis through GIS This analysis is done by combining data from multiple data categories and performing analytical, statistical measurement and other operations on the GIS data sets to transform the data into information suitable for a given application. Intersection of various theme maps can be done by overlaying one theme over the other progressively and by adopting suitable decision criteria. The typical operations include overlaying different thematic maps, computing areas, preparing proximity searches buffer zone creations, performing logical operation etc. analysis. The following themes of study area have been taken for overlay 1. Base map 2. Relief map 3. Rainfall map 4. Drainage 5. Water level contour 6. Water Quality 7. Aquifer thickness 8. Soil map 9. Geology

13 Geomorphology 11. Landuse map 12. Waste land map In order to classify the area with reference to groundwater potential zonation the final grid cell values have been taken and scaled in accordance with the minimum and maximum grid cell values and classified as excellent, good, moderate, poor groundwater zones. The geology map, soil map, relief map and geomorphology map are brought into GIS platform to generate groundwater recharge zonation map by overlay analysis assigning appropriate weightages to the themes. This map shows various classes of recharge zones such as high, moderate and less Based on the influence of the themes in groundwater potential and recharge ranking and weightage are deployed in GIS overlay analysis to each theme, Zonation evolved. This gives the solution to obtain zonation maps like 1. Groundwater prospects zonation map. 2. Groundwater recharge zonation map GROUNDWATER PROSPECTS ZONATION The demand for water resources is always increasing year by year, due to exploitation of groundwater. In view of this, the depletion of water level and changes in water quality were observed especially during a year where the annual

14 72 rainfall is below normal. In this condition, it is therefore imperative whether the available water resources could be optimally utilized for various sectors. This situation necessitates assessment of groundwater potential qualitatively and to preserve the aquifer and planning for a sustainable development. Groundwater resources in this sub basin mainly occur in buried river coarse, alluvial and tertiary uplands. Remote sensing and GIS techniques play a vital role in identifying and evaluating ground water resources. The GIS software Arc-View, Arc-GIS, and Image processing software ERDAS Imagine were used for generation of different thematic maps like geomorphology, soil, aquifer thickness, wasteland and landuse. Very limited field verification was made in the field for confirmation, correctness and accuracy of the map. The corrected maps are loaded and analyzed under GIS environment for generating of final output maps for ground water potential zonation and artificial recharge zones. All the spatial and non-spatial parameters, were integrated and weightages were assigned based on the characteristics of the features and finally a qualitative analysis were made from the GIS environ for assessing the groundwater potential for this sub basin Integration Visual interpretation techniques were mainly employed in preparation of thematic maps like geomorphology, wasteland, soil, drainage, etc. Among all the thematic maps derived by interpretation and generated under GIS

15 73 environment, hydrogeomorphological stand significant role in the evaluation of groundwater potential. These maps have been superimposed one over the other and integrated with non-spatial parameters for evaluating the hydrogeological condition of the sub basin. Hydrogeological information like water level, rainfall distribution, water quality, irrigation details, etc was collected and they were incorporated to generate the final groundwater potential zonation Groundwater Potential evaluation under GIS environ GIS plays a more important role in the evaluation of groundwater potential. It is a prime component for the integration and analysis of the spatial and non-spatial information. Using the GIS package, the specific application like map projection, georeferencing the spatial data, merging, mosaicking, data clipping, data updating, data aggregation, queries, proximating analysis, attribute analysis, overlay analysis, and integration analysis work done for generation of groundwater potential zonation map. The Phenomenon of map layers and its uses are summarized in the Table 4.1. Arcinfo and ArcView GIS software were used to create database for spatial and non spatial data for the sub basin. GIS query system under spatial analysis module is used for raster overlay analysis and the final output map of groundwater potential zonation map is generated.

16 74 Table 4.1 The phenomenon and need for the thematic layers Sl.No. Map Layer Phenomenon Need 1. Geomorphology Physical processes on the earth s surface that produce different landforms A geomorphic unit is a composite unit that has specific characteristics 2. Rainfall Rainfall Major source of water 3. Pre-Monsoon Water Level Depth at which water occurs in the unconfined zone (top zone) below ground level Tells us the scarcity of easily accessible water 4. Soil Soil Result of physical surface processes and the lithology 5. Landuse Purpose for which land has been put to use Indicates the state of current use. 6. Aquifer thickness Thickness of the aquifer Indication of the thickness of the unconfined aquifer 7. Relief Relief Controls the movement of water (surface and ground) GIS Data Base Creation In this present study the database consists of the following themes like, geomorphology, rainfall distribution, water level contour, land use, soil were integrated and analyzed. The spatial data were inverted to vector format by digitization. Interpolation techniques were used to find out the geographic values of point data such as rainfall, water level, aquifer thickness and elevation. Soil layer was derived from the soil map prepared by the National Bureau of Soil Survey and Land Use Planning on 1:500,000 scale and vectorized by digitization. Slope and aspect information were directly obtained from the relief map. These theme layers were brought under GIS environment and analyzed for generation of final output maps.

17 75 Various thematic layers used for this project were brought into the GIS environment, which are given in the Table 4.2. Table 4.2 Thematic layers used under GIS Environ Sl. Source of Map Layer No. information 1. Geomorphology Survey of India Topographical base maps, Satellite Imagery and Ground Truth Verification 2. Rainfall Rain Gauge Stations 3. Pre-Monsoon Auto-Water level Water Level recorders and field study 4. Soil National Bureau of Soil Survey and Land Use Planning 5. Landuse Satellite imagery, Field Check 6. Water bodies Survey of India, topographical maps and Satellite imagery. 7. Aquifer thickness Field observation of dug wells, data on bore wells & geophysical data Process of Data Scale data creation Format Digitization Vector 1:50,000 Interpolation Interpolation Raster Raster Digitization Vector 1:500,000 Digital Image Processing - Supervised Classification Raster/ Vector 1:50,000 Digitization Vector 1:50,000 Interpolation Raster The data that was available for present study was in two forms vector data (derived from existing map sources) and raster data (interpolated from point data or classified from satellite images).

18 76 Four point data sources were available from which layers were created to represent the study area, viz. rainfall, aquifer thickness and water level. The Inverse Distance Weighted (IDW) method was used for creating rainfall, depth to aquifer thickness and water level map layers. IDW method with four nearest points was used in finding out the values for the unknown. In this technique, input data points surrounding a raster position are given a weight that is inversely proportional to the specified power of their distance from the pixel. Distance threshold could have been a better method but could not be used here, as the point data was sparse and distributed. The relief map was derived from the elevation contours from the Survey of India topographical maps of the study area Data Preparation for analysis Weighted overlay analysis was done in the raster form because of the excellent depth, speed and flexibility. Moreover, assigning scores for individual classes in the map layers and the weightings for the individual map layers is possible in raster form in this technique. The various thematic grid layers were organized into a GIS database. The layers having various classes were organized into manageable classes through reclassification. Reclassification re-coded the existing attribute value for each grid cell, more appropriate to the queries being asked when the decisions to be made.

19 77 Each grid cell was given a score that described its capability in terms of groundwater potential and groundwater recharge. Mathematical addition of the grid layers in an overlay is not directly possible because it is not logical to add rainfall in units of mm, depth to bed rock in units of meters and so on. Moreover the influence of each layer towards the potential and recharge varies. Hence, the Nominal Group Technique was used to assign the scores and weights for the classes and layers respectively. All data used for this study was converted into raster form. This was necessary as scores and weights assigned for each class in a map layer and a layer respectively, needs to be incorporated in the analysis Analysis In any study of natural resource management, Raster Overlay Analysis is a necessary component to bring together data representing the phenomenon. Using the Boolean logic rules to combine different layers is one of the easiest methods, through Yes or No rules. Most Boolean logical based overlay procedures in GIS do not allow for the fact that variables may not be equally important and the decisions about threshold values are often subjective. Hence to quantify the parameters in the analysis Mathematical Overlay method was adopted. The details of weights, classes and scores of the map layers are shown in Table 4.3.

20 78 Table 4.3 Weights classes and scores for map layers Layer Name Class (category) Buffer width (m) Score for Potential Geomorphology Buried River Course - 8 Wt. For potential: Alluvium - 9 Gullies - 2 Upland - 7 Geology Alluvium - 9 Wt. For potential: 9.17 Tertiary - 7 Soil Entisol 9 Wt. For potential: 9.59 Alfisol 4 Inceptisol 6 Vertisol 2 Aquifer Thickness Wt. For potential: Drainage River/Stream 500 m 9 Wt. For potential: Tanks 100 m 9 Landuse Dry crop 6 Wt. For potential: 4.66 Wet Crop 8 Land with or without scrub 1 Plantation 7 Winter Water Level ( ) Wt. For potential: Average Annual Rainfall (35 years) mm 9 Wt. For potential: mm mm mm 3 Relief 7 Wt. For potential: 7.54

21 79 Map Algebra based modeling (4), a language specially designed for geographic cell-based system, was used. The language orders the user s thoughts and provides the rules and syntax needed for the user to communicate with the computer. The final result is in the form of a raster layer, where each grid cell acquired a value through the additive overlay process. The higher the value of the grid cell, the more preferred the cell is Groundwater Potential Zone. To be able to interpret the result, the final grid cell values have to be scaled in accordance with the possible minimum and maximum values that a grid cell can contain Scaling the output grid cell values This has been done on the basis of the possible minimum and maximum grid cell values calculated for potential given in the Table 4.4. Map Layer Table 4.4 Grid Cell Values for Potential Min grid cell score Max grid cell score Weight for the layer Min Grid cell value Max grid cell value Geomorphology Geology Rainfall Pre-Monsoon Water Level Aquifer thickness Soil Landuse Relief

22 80 This exercise using the GIS environ helped and generated groundwater potential map. The artificial zonation map shows the area in the sub basin that could sustain for any recharge possibilities. This may be used for further planning to improve a storage condition for future optimal groundwater development. The village layers are overlaid with recharge zonation map and identified the villages having recharge zones of different categories ARTIFICIAL RECHARGE STRUCTURE With the increasing use of ground water for Agricultural, Municipal and Industrial needs, the annual extraction of ground water are far in excess of net average recharge from natural resources. Consequently ground water is being withdrawn from storage and water levels are declining resulting in crop failures, sea water intrusion in coastal aquifers, land subsidence etc., Vagaries of monsoon and indiscriminate development of ground water often result in declining trend of ground water levels. There is an urgent need for artificial recharge of ground water by augmenting the natural infiltration of precipitation into subsurface formation by some suitable method of recharge. Artificial recharge is one method of modifying the hydrological cycle and thereby providing ground water in excess of that available by process. Keeping this in mind to avoid fast decline in ground water levels in various parts of Gadilam river lower sub basin, a study has been undertaken to identify the favourable areas for artificial recharge and suggest suitable recharge structures to augment the aquifer system. The analysis was carried out in the Gadilam River lower sub basin using remote sensing data and GIS techniques.

23 81 In this study both satellite data as well as extensive field data were used for preparation of various thematic maps. As far as satellite data is concerned IRS-1C LISS III data has been used in the analysis. In addition to the above satellite data, the following collateral data namely rainfall, ground water levels, geophysical data were used in the analysis. In this analysis 12 layers of thematic information are used. Among the 12 the following seven basic maps namely (a) geomorphology (b) geology (c) soil) (d) relief (e) land use (f) drainage and (g) transport network are used. The above maps were prepared using both satellite data and topographic sheets. In addition to the above basic layers the following derived layers such as drainage density, maximum groundwater level variation, aquifer thickness and water quality were used. These maps were generated using ArcGIS software. The thematic information generated through visual analysis was digitized using Arc GIS software and projected to real world co-ordinates. There is more number of classes in each theme. In order to simplify the analysis the above themes were reclassified into three classes and assigned ranks 1 to 3 based upon the suitability for ground water recharge. The first rank class is considered as most favorable zone for recharge and the third rank is for less recharge. The criterion table is developed for this study and is shown in the Table 4.5. Using the overlay module of ArcGIS workstation based GIS software; the above thematic layers were overlaid with suitable ranks and weights. The final map contains numerous polygons having the characteristics of all the above 12 themes. In order to group them in to three classes a statistical analysis was made assuming the distribution is normal. Considering the 1 sigma criteria using the above criteria, the final suitability map has been delineated into three classes namely highly favorable, moderately favorable and less favorable.

24 Table 4.5 Criterion Table Sedimentary Environs Theme Rank 1(4 x weightage) Rank 2 (3 x weightage) Rank 3 (2 x weightage) Rank 4 (1 x weightage) Weightage Geology Alluvium Latrite, Cuddalore sandstone Geomorphology Flood plain, Paleo channels and Alluvium Gullies and Ravines - Bad land Topography 15 Hydrogeological soil group A B C D 20 Slope 0-1% 1-2% 2-3% > 3% 10 Land use Wet crop, plantation Dry crop Scrub, barren Rock out crops, forest & others 10 Run off High Moderate Less moderate Low 13 Drainage density Low Less moderate Moderate High 7 Total

25 83 The general output maps were taken for field verification and it is verified the zonation created using GIS and also the suggested type and location of the water harvesting structures. In almost all the cases the zonations generated as well as the type and location of water harvesting structures suggested are agreeable. After detailed analysis of the zonation map generated using GIS techniques, the various suitable recharge structures such us check dams, percolation ponds, recharge pits, subsurface dykes etc were suggested based on the field condition. At the present conditions the artificial recharge is to be resorted to sustainable development of ground water. Hence the results obtained from the present study fulfill some of the requirements of selecting suitable artificial recharge structures SALINITY IMPACT The bore well lithology and logging details were collected from Central Ground water Board to know the salinity impact of the study area. The depth of bore well drilled at Cuddalore was 300m and at Chavadi was m.The logging data of Chavadi indicates that the saline water ranges from 0-100m, fresh water from m, brackish water from m and saline water beyond 420m. This is confirmed by the water quality. The EC value was 296 and Cl was 215mgl. The TKS value is calculated as 7860, 76.3 and 9.583x10-4. Three zones were identified in the bore well at Cuddalore. The top zone upto the depth of 100m shows saline water. It is confirmed that EC value is 1200 and Cl is The middle zone starts from 100m to till 160m indicates brackish water and the EC value is 970 and the Chloride value is 113. The depth range from 130 to 140 shows the lignite seems. But according to the log the depth ranges from160m to 300m indicates saline water.