Mapping Salinity Ingress Using GALDIT Model. Dr.S.M.Yadav Associate Professor CED, SVNIT

Transcription

1 Mapping Salinity Ingress Using GALDIT Model Dr.S.M.Yadav Associate Professor CED, SVNIT SURAT, INDIA

2 Co-Authors Dr.B.K.Samtani Professor CED, SVNIT SURAT, INDIA H.D.Vasava Junior Engineer VMC VADODARA, INDIA

3 Introduction The development and management of coastal ground water aquifer is a serious issue. Monitoring of sea water intrusion involves determination and prediction of ground water deterioration and assesses other managerial activities in coastal aquifers.

4 Ground water aquifers are often exposed to heavy pumping and consequently risks of sea water intrusion in case of coastal aquifers Vasava et al (2010) studied the vulnerability of sea water intrusion in ground water as carried out by Chachadi (2001, 2002) in coastal aquifer of Goa, India.

5 Coastin (2001) assessed ground water vulnerability to pollution. Mi (2004) id i i Mitra (2004) carried out sea water intrusion study using remote sensing and GIS for the Gulf of Cambay, Gujarat

6 Kumar (2007) modelled sea water intrusion in north Goa. Sharma (1976) assessed quality of ground water in the coastal aquifer near Visakhapatnam, India.

7 Number of solute transport models suitable for the simulation of sea water intrusion and up coning of saline water beneath pumping sites is commercially available. These include SUTRA(Voss,1984), FEFLOW( Diersch,1998),HST3D( Kipp,1987) and SALTFLOW( Molson and Frind,1994).

8 The system presented here allows the user to determine a numeric value for any hydrogeophysical setting by using an additive model. This model is an open ended model allowing the user for addition and deletion of one or more indicators.

9 However, under normal circumstance, present set of indicators should not be deleted and any addition of the indicator would require re deriving of the weights and the classification table.

10 Suggested System of Vulnerability Evaluation and Ranking The system of vulnerability evaluation and ranking is developed by Chachadi (2001) is used in the present study.

11 The most important mappable factors that control the seawater intrusion are found to be (Chachadi, 2002): 1. Groundwater Occurrence (aquifer type; unconfined, confined and leaky confined). 2. Aquifer hydraulic conductivity.

12 3 Depth to ground water Level above sea 4 Distance of the point in question from the shore (distance inland perpendicular from Shore line)

13 5. Impact magnitude of the existing iti seawater intrusion i in the area,if any. 6. Thickness of the aquifer (which is being mapped).

14 The acronym GALDIT is formed from the highlighted and underlined letters of the factors for ease of reference. These factors, in combination, are determined to include the basic requirements needed ddto assess the general e seawater ate intrusion tuso potential t of each hydro geologic setting.

15 GALDIT factors represent measurable parameters for which data are generally available from a variety it of sources without t detailed reconnaissance. A numerical ranking system to assess seawater intrusion potential in hydro geologic settings has been devised using GALDIT factors.

16 The system contains three significant parts: weights, ranges and importance ratings. Each GALDIT factor has been evaluated with respect to the other to determine the relative importance of each factor.

17 The various parameters adopted in the evolution of the present indicator tool include: (i)identification of all the indicators influencing the seawater intrusion This task was through extensive discussions and consultations with the experts, academicians etc.

18 The basic assumption made in the development of the tool includes: the bottom of the aquifer(s) lies below msl so that the seawater can move inside the aquifer laterally.

19 Any event of seawater moving inland and causing seawater mixing in the freshwater aquifers such as storm surges and tsunamis, which normally move seawater inland dby vertical uplift, is notconsidered d in the present model. (ii)derivation of indicator weights Indicator weights depicts the relative importance of the indicator to the process of seawater intrusion.

20 After identifying the indicators, a group of people consisting of Geologists, hydro geologists, environmentalists, students, and in house experts was asked to weigh these indicators in order of importance to the process of seawater intrusion. The feedbacks from all such interactions were analyzed statistically and the final consensus list of indicators weights was prepared (Table 1).

21 Table 1 Indicator weights. Factors Weights 1. Groundwater occurrence 1 2. Aquifer hydraulic conductivity 3 3. Height of groundwater level above sea level 4. Distance from the shore 4 5. Impact of existing status of seawater 1 Intrusion 6.Thickness of aquifer being mapped 2 4

22 The most significant indicators have weights of 4 and the least a weight of 1 in a five point scale indicating parameter of less significance in the process of seawater intrusion. As the indicator weights are derived after elaborate discussions and deliberations among the experts, academicians, researchers, etc., they must be considered as constants and may not be changed under normal circumstances.

23 (iii)assigning importance rates to indicator variables using a scale of 2.5 to 10 Each of the indicators are subdivided into variables according to the specified attributes to determine the relative significance of the variable in question on the process of seawater intrusion.

24 The important ratings range between 2.5, 5, 7.5 and 10. Higher value of importance rating indicates higher vulnerability to seawater intrusion.

25 (iv) Decision criterion It is the total sum of the individual indicator scores obtained by multiplication of values of importance ratings with the corresponding indicator weights. Higher the values of importance ratings of the variable, more vulnerable are the aquifers to seawater intrusion.

26 Indicator Descriptions Indicator 1: Groundwater occurrence (Aquifer Type) In nature, groundwater generally occurs in the geological g layers and these layers may be confined, unconfined, and leaky confined or limited by oneormoreboundaries. The extentof seawater intrusion is dependent on this basic nature of groundwater occurrence.

27 Therefore, in assigning the relative weights to GALDIT parameter G one should carefully study the disposition and type of the aquifers in the study area.

28 The confined aquifer is more vulnerable due to larger cone of depression and instantaneous release of water to wells during pumping and hence scores the high rating compared to other types of aquifers. Incaseof multiple aquifer system in an area, thehighest rating may be adopted.

29 Table 2 Ratings for Ground Water Occurrence. Indicator Groundwater Occurence Aquifer type Indicator variables Importance Ratings Confined aquifer 10 Un confined aquifer 7.5 Leacky confined aquifer Bounded aquifer(recharge and/or im pervious boundary alined parallel l to the coast) 5 2.5

30 Indicator 2: Aquifer hydraulic conductivity The parameter aquifer hydraulic conductivity is used to measure the rate of flow of water in the aquifer and hence to and fro from the sea in the coastal area. By definition, the aquifer hydraulic conductivity is the ability of the aquifer to transmit water.

31 The hydraulic conductivity is the result of the interconnected pores (effective porosity) in the sediments and fractures in the consolidated rocks. The magnitude of seawater front movement is influenced by the hydraulic conductivity of the aquifer.

32 For a given hydraulic head, a high value of hydraulic conductivity of an aquifer leads to larger inland movement of the seawater front. The high hydraulic conductivity also results in wider cone of depression during pumping and may allow greater seawater intrusion if located close to the coast.

33 Length of seawater intrusion in the coastal aquifer

34 The ratings for the GALDIT parameter A, which are modified from Aller et al. (1987), are given in table 3.

35 Table 3 Ratings Aquifer Hydraulic Conductivity. Indicator (A) Aquifer hydraulic conductivity (m/day) Indicator variables Importance Class Range,m Ratings High < Medium Low V low >5 2.5

36 The aquifer hydraulic conductivity can be estimated from pumping test data as well as from litho logical logs. The well log data and porosity logs can also be used to compute the aquifer hydraulic conductivity.

37 Indicator 3: Height of groundwater level above sea elevation The level of groundwater with respect to mean sea elevation is the most important factor in the evaluation of seawater intrusion in an area, primarily because it determines the hydraulic pressure availability to push back the seawater front.

38 As seen from the famous Ghyben Herzberg ( ) 1901) relation, for every meter rise of fresh water above mean sea elevation, a freshwater column of 40 m is developed below it down to the interface and vice versa.

39 In assigning the ratings to the GALDIT parameter L, the long term spatial variations of the groundwater levels in the area need to be carefully studied. Generally, the values pertaining to minimum groundwater levels above sea (pre monsoon during May) may be considered, d as this would provide thehighest possible vulnerability risk. The ratings adopted for L are given in table 3

40 Groundwater level data with respect to msl can be obtained by establishing observation wells in the area and measuring the pre and postmonsoon water levels and reducing them with respect to the mean sea level.

41 Table 4 Ratings for height of ground water. Indicator (L) Indicator variables Importance Class Range,m Ratings High < Height of ground water Medium level above msl (m) Low Very low >

42 Indicator 4: Distance from the Shore The magnitude of the impact of seawater intrusion i generally decreases as one move inland at right angles to the shore and the creek. The maximum impact is witnessed close to the coast and creeks under favorable hydro geological conditions. i

43 Table 5 provides the general guidelines for rating of the GALDIT parameter D assuming the aquifer is under undisturbed conditions i.e. the groundwater development in the area has not been significant to offset the balance.

44 The value of importance ratings is assumed to change linearly withdistance D. Data for parameter D can be computed using the topographical/ cadastral or any surveyed dt data mapof the areawherein the highh tide line for the coast has been demarcated.

45 Table 5. Ratings adopted for distance of the point tfrom shore D. Indicator () (L) Indicator variables Distance of the point from Shore ( m) Class Range (m) Importance Ratings Very small < Small Medium Far >

46 Indicator 5: Impact of existing status of Seawater intrusion Some times the area under mapping is already under water stress conditions and this stress might have affected the natural hydraulic balance between seawater and fresh groundwater.

47 This existing imbalance in the seawater freshwater interface should be considered d while mapping the aquifer vulnerability to seawater intrusion.

48 Revelle (1941) recommended the ratio of Cl / [HCO+ CO3] as a criterion to identify the extent of seawater intrusion into the coastal aquifers. Chloride (Cl) is the dominant ion in the seawater and it is only available in small quantities in groundwater while bicarbonate (HCO 3 ), which is available in large quantities ii in groundwater, occurs only in very small quantities in seawater.

49 This ratio can be used while assigning the importance rating for the GALDIT parameter I, provided the chemical analysis data is available for the areaunder investigation.

50 In case such chemical data is not readily available, then information gathered from the field survey and inquiries from the water users in the area can be used in assigning the importance rating for I. The information required for the above rating can be gathered from historical i reports, inquiryi from the local people, farmers, and chemical analysis data.

51 Table 6 Ratings for impact status of existing sea water intrusion. Indicator (I) Indicator variables Class Range of CI/(HCO3+CO3)Ratio in lpm in groundwater Importance Ratings Impact High >2 10 status tt of Medium existing Low seawater intrusion Very low <1 2.5

52 Indicator 6: Thickness of Aquiferbeing mapped Aquifer thickness or saturated thickness of an unconfined aquifer plays an important role in determining the extent and magnitude of seawater intrusion in the coastal areas.

53 Keeping this as a guideline the ratings given in Tbl Table 7 are adopted dfor various ranges of aquifer thickness T. The aquifer thickness in a given area can be obtained from litho logical logs and can also be deduced from carefully conducted vertical electrical sounding data.

54 Table 7 Ratings for Aquifer thickness Indicator (T ) Indicator variables Class Range Importance Ratings ] Large >10 10 Aquifer thickness Medium (saturated ) in meters Small Very small < 5 2.5

55 Computing of the GALDIT Index Each of the six GALDIT indicators has a predetermined fixed weight that reflects its relative importance to seawater intrusion. Computing the individual indicator scores and summing them and dividing by the total weight as per the following expression gives the GALDIT Index:

56 where, w = Weight of the indicator R = Importance rating of the i th indicator

57 Thus, compute the indicator score from hydro geologic and geological information from the area of interest and variables to reflect specific conditions o within that ataea area. This system allows the user to determine a numerical value for any hydro geographical setting by using this additive model.

58 The maximum GALDIT Index is obtained by substituting the maximum importance ratings of the indicators as shown in next slide.

59 Max = {(W1) R1+ (W2) R2 + (W3) R3 + (W4) R4 + (W5) R5 + (W6) R6}/ w = {(1) 10+(3) 10+(4) 10+(4) 10+(1) 10 +(2) 10}/ 15 = 10

60 Similarly, the minimum GALDIT Index is obtained by substituting the minimum importance ratings of the indicators as shown below:

61 Min = {(W1) R1+(W2) R2+(W3) R3+(W4) R4+(W5) ( ) ( ) ( ) ( ) R5+(W6) R6}/ w = { (1) (3) (4) (4) 2.5+ (1) (2) 2.5 }/15 = 2.5

62 Therefore, the minimum and maximum GALDIT Index varies between 2.5 to 10. The vulnerability of the area to seawater intrusion is assessed based on the magnitude of the GALDIT Index. In a general way, lower the index less vulnerable is the area to the seawater intrusion.

63 Decision Criteria Once the GALDIT Index has been computed, it is therefore possible to classify the coastal areas into various categories of seawater intrusion vulnerability the range of minimum and maximum GALDIT Index scores (i.e. 2.5 to 10) is divided into 3 groups.

64 All the six indicates have four classes i.e. 2.5,5,7 and 10 as their importance ratings. Table 8 provides the detailed classifications of vulnerability classes of GALDIT Index.

65 Table 8 Vulnerability classes SrNo No. GALDIT Index Range Vulnerability Classes High Vulnerability 2 5 to Moderate Vulnrability 3 < 5 Low Vulnerability

66 Application of GALDIT Mapping The coastal tracts in India cover parts of Gujarat, Mh Maharashtra, ht Karnataka, Kerala, Tamil Nd Nadu, Andhra Pradesh, Orissa and West Bengal. Besides the inherent ground water salinity in the coastal tract, Sea water intrusion is also one of the majorproblems in thecoastal region.

67 In certain areas problem of up coning of saline water has also been reported due to overexploitation of ground water. Problem of salinity ingress has been noticed in Minjur area of Tamil Nadu and Mangrol Chorwad Porbander belt along the Saurashtra coast.

68 In Orissa, in an 8 10 km. wide belt of Subarnrekha, Sl Salandi, Brahamani out fll fall regions in the proximity of the coast, the upper aquifers contain saline horizons decreasing landwards.

69 In Pondicherry region east of Neyveli ligniteit minessalinity ingress has also been observed. The inland ground water salinity in India occurs mainly in the states of Maharashtra, Punjab, Rajasthan, Haryana, Gujarat, Karnataka, Uttar Pradesh, Delhi, Orissa and Bihar.

70 The occurrence of inland salinity may be due to over exploitation of ground water, use of surface water and ground water in complete isolation, characteristics of aquifer or some other reasons.

71 About 1.93 lakh sq.km area has been estimated to be affected by saline water of Electrical Conductivity 4000 μs/cm. There are several places in parts of Rajasthan and southern Haryana where EC values of ground water are greater than μs/cm making water non potable.

72 In some areas of Rajasthan and Gujarat ground water salinity is so high that the well water is directly used for salt manufacturing by solar evaporation.

73 The present study area coastal tract of Surat coast. forms part of the The Tapi river estuary in the South and the Chapora River in the North form the hydrological boundaries of the study area.

74 The map along with the location of watershed is also shown in thefig1.

75 Fig.2 Map of study area

76 Fig.3 Singanpore weir, Surat, India

77 Fig.4 Line Diagram

78 The tidal waves intrude in the river up to Singanpore weir. The Singanpore weir was constructed on River Tapi. The River Tapi meets Arabian Sea downstream of a Singanpore weir at a distance of 2 km, from Singanpore weir.

79 The tidal effect of sea reaches up to Singanpore weir. The weir was constructed in The area is mainly covered by laterites and river alluvium and at some places the sediments are exposed in the north. met

80 In the study area 10 groundwater monitoring wells have been established and monthly groundwater levels have been recorded in all these wells for last 15 years.

81 The aquifer is mainly shallow unconfined in nature occurring both in river alluvium as well as laterites exposed in the low lying areas. The plateau laterites generally lack water table and at some places the water table is very deep.

82 The river alluvium is highly drainable whereas the laterites are not. The GALDIT scores at each of the groundwater monitoring wells were computed for the study area. The map derived for this study area is shown in the fig.1. The table 10 presents the details of wells and their distances from sea shore and weir.

83 Following dt data werecollected from GWRDC: Groundwater data from Gujarat Water Resource Development tcorporation (GWRDC) Water quality yp parameters: Cl,TDS

84 Depth of the Well (TD) Static Water Level (SWL) For pre monsoon and post monsoon period of of the years

85 The data ranges used for present study are as shown in table 9 and location details of wells are presented in table 10.

86 Table.9 Data Range Name of well Year S.W.L T.D.S Pre-Monsoon (May) CI R.W.L S.W.L R.W.L Post-Monsoon (Oct) Rise/Fall Rise/Fall Rise/Fall Rain fall Hazira Well Dumas Well Piplod Well Rander Well Sachin Well Jahangirabad Well Olpad Well Amroli Kathor Well Vav Well

87 Table.10 Observation well details

88 Results and Discussion The GALDIT index is computed using following considerations and presented in table 11. Unconfined aquifer is observed throughout the study area hence, the weight 7.5 for G is considered for all wells. Depths above mean sea level were computed Depths above mean sea level were computed from the well data.

89 Distance from the coastline was calculated using the satellite images. Impact of existing ground water intrusion has been considered only in case of two observation wells which are close to sea shore. Thickness of aquifer was taken equal to the depth of the well as the wells penetrate full length of the aquifer.

90 The analysis of results suggest that the Dumas well and Hazira well which are on the down stream side of weir and near to seashore have high vulnerability to sea water intrusion.

91 Table 11. GALDIT index for wells Sr No Well Name Distance from the shore Computed GALDIT Index 1 Hazira well 1.2 km Dumas Well 0.5 km Piplod Well 7.6 km Rander Wll Well 57k 5.7 km Jahangirabad Well 8 km Amroli Well 18 km Olpad Well 20 km Sachin Well 10 km Vav Wll Well 21 km Kathor Well 25 km 4.38

92 Conclusion Once the GALDIT index has been computed, it is possible to identify areas that are more likely to be susceptible to sea water intrusion than other areas. In the present study the area down stream of the Singapore gp weir is more susceptible to sea water intrusion potential. It has been confirmed by field observation that thesalinity in this area has been increased.

93 To control further sea water intrusion and protect ground water being more saline Surat Municipal Corporation has proposed to construct Balloon dam (storage dam) on the down stream side of the weir about 4 5 kms away from Singapore weir.

94 Reference Aller, L., Bennett, T., Lehr, J.H. and Petty, R. J. (1987) DRASTIC: A standardized system for evaluating groundwater pollution potential using hydro geologic settings, U.S. EPA Report 600/2 85/018. Chachadi A.G. and Lobo Ferreira, J.P. (2001) Sea water intrusion vulnerability mapping of aquifers isung GALDIT method, In: Proc. Workshop on Modelling in Hydrogeology,Anna University,Chennai,pp

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103 Thank k you