HYDROGEOLOGICAL STUDIES IN THE SOUTHEASTERN COASTAL TRACTS OF KUTCH WITH SPECIAL REFERENCE TO QUALITY OF GROUNDWATER

Abstract

The low lands of coastal Saurashtra and Kutch in Gujarat (India) have remained a part of the arid belt since at least Pleistocene time. Kutch is extremely susceptible to droughts and water scarcity conditions. The deficit of water supply during scarcity periods goes upto 98 percent. The coastal area of Kutch is facing the problems of the groundwater contamination either by sea water intrusion into coastal aquifers or by inland sea water ingress. The average annual rainfall in the southeastern Kutch varies from 315 mm (in Anjar) to 375 mm (in Mandvi) with 350 mm overall average. Earlier hydrogeomorphological studies have indicated the presence of deposits such as salt flats and mud flats covering the larger parts of the area, indicating thereby its very poor to negligible groundwater potential, with high salinity especially, in the vicinity of the coast. The vegetation anomalies are found to be confined largely to the dried up stretch or river courses reflecting the presence of moderate quality water and good groundwater potential. Sandy plains, sedimentary pediment and dissected volcanic hills are also covering some parts of the area exhibiting good to medium groundwater potential. The lineaments in the area seem to be parallel to the water courses, probably indicating the structural control of the drainage. The availability of good quality groundwater is, however, not that promising. In southeastern part of Kutch the Deccan Trap of Cretaceous-Eocene age forms the basement for the Pliocene, the Pleistocene and the Recent (alluvium and morrum) formations. The data of the Vertical Electrical Sounding (VES) available from various field agencies (a total of 111 sets of resistivity soundings taken along 12 section lines laid normal to the sea coast) have been interpreted by using auxiliary point method and then by Zohdy's computerised technique. The layer parameters (true resistivities and layer thicknesses) of the soundings located near known boreholes were, then, utilised to assign resistivity ranges of different geological formations in the area. Wherever such data did not exist, the resistivity ranges were fixed by availing the geological field evidences and the groundwater quality data of the nearby wells. The resistivity ranges for different subsurface formations are : sand (8 to 94 ohm-m), alluvium (4 to 33 ohm-m), morrum (5 to 28 ohm-m), limestone (11 to 1090 ohm-m), conglomerate (2 to 62 ohm-m), siltstone-claystone (4 to 29 ohm-m), sandy shale (5 to 16 ohm-m) and basalt (19 to 227 ohm-m). Occurrence of highly contaminated groundwaters with resistivity less than 5 ohm-m is indicated along a few sections upto a depth interval of about 20 to 30 m and at a distance of 1 to 2 km from the coast. In addition, preparation of four electrical sections showing vertical variation of electrical resistivity contours, have also brought out the occurrence of highly saline groundwaters at deeper levels (60-80 m). In the Mandvi area, contamination is indicated at deeper levels, although there is no evidence of it at shallow levels in the geoelectrical sections. The general groundwater flow in the study area is southwards, towards the coast. In the western part, however, reversal in the flow direction is observed. The hydraulic conductivity of the surficial material is uniform barring few exceptions. An overall rise or shallowing of the watertable is also noticed in the year 1992 in comparison to the previous year. The quality zonation maps, prepared on the basis of total dissolved solids (TDS) data show that salinity is comparatively high in the vicinity of the coast, in pre- and postmonsoon seasons. In the year 1991, brackish groundwaters with TDS varying between 1000 to 10,000 mgl" are recorded except Kukadsar hydrograph station (W8) of Mundra taluka, where highly saline groundwater (TDS > 10,000 mgl"1) is recorded. A somewhat similar nature of salinity pattern is observed in the premonsoon season data. However, due to mixing by rains In the postmonsoon season of 1992, comparatively low TDS values of the groundwater are obtained reflecting either the effect of dilution or the impact of the operation of the Reverse Osmosis plants, wherever installed for desalination of water. Chemically, Na+ is the dominant cation with concentrations varying from 12 to 3780 mgl . The continuous evaporation of the groundwater, prolonged dry spells and/or processes like ion exchange are considered to cause such high concentration of Na+ ion. The K+ is the next dominant ion in the groundwater (8 to 420 mgl"1) especially in the postmonsoon season of 1992 and the premonsoon season of 1993, which could be attributed to the presence of some isolated pockets of old saline water or connate water. On the other hand, the anion composition in premonsoon season of 1993 is characterised by high concentration of CI" (up to to 5200 mgl"1) as a consequence to prolonged dry spells. However, with heavy precipitation in the following postmonsoon season of 1992, these concentrated groundwaters underwent the dilution to represent the water type enriched with HCO, (120 to 2520 mgl"1). Also, in premonsoon season of 1993, dominance of HCOj is noted. Hill-Piper diagram and Romani's triangular plots have revealed the presence of NaCl type of the groundwaters in the premonsoon season of 1992 which got modified to NaHC03 type in the following postmonsoon season of 1992 and premonsoon season of 1993. The use of chloride-bicarbonate ratio, a basic tool for the recognition of sea water contamination of the groundwater, has revealed that in the premonsoon season of the year 1992 almost all the hydrograph stations (shallow openwells) recorded high to very high (1 to 23) CI /HCO, ratios which were subsequently reduced due to the dilution effect caused by monsoon rains. It is found that a few of the hydrograph stations consistently recorded high to very high (>3) Cl'/HCO, ratios one of which (at Kukadsar), is in the vicinity of the coast suggesting the probability of occurrence of sea water contamination in aquifers. However, no mixing of fresh groundwater and sea water is observed from the Mg /Ca++ vs CI" relationship. Four different types of the groundwater are obtained in the present study using chloride ratio method : I groundwaters with all cation-chloride and anion-chloride ratios greater than that of sea water (i.e. Na+/Cl">87, Ca++/Cl">3.7, Mg+ +/Cl"> 19.4, HCO"3/Cf>0.4 and SO4"/Cl">10.4). About 90% of the samples belong to this category; II groundwaters with all ionic ratios greater than sea water, except Mg /CI ratio, which is lower; III one or two samples of groundwater with only (Na++ K+)/Cl"ratio lower than that for sea water, rest all being higher, and IV one or two samples of groundwater with both Ca++/CI" and Mg++/Cl" ratios lower than sea water ratios. The incidence of these groundwaters mainly belonging to class I & II types, precludes the possibility of significant sea water intrusion into aquifers of the area as both HCO"3 and SO4" to CI" ratios as well as Na+ to CI" ratio, are higher than sea water ratios. A straight line relationship between CI" and other ions, indicating evaporation, is found to hold good for SO^" and SO^/CI", Na+/Ca+ +, Na+/Mg++ and Mg++/Ca++ ratios of different periods, highlighting the effects of evaporation of the groundwaters in the study area. The expanded Durov's diagram show the occurrence of end type (NaCl type) waters in the premonsoon season of 1992. This clearly indicates the concentration by evaporation as expected because of the occurrence of droughts in the study area during the last two dec ades. A small fraction of samples does, however, indicate mixed type of waters, probably with brackish/old saline waters. In the postmonsoon season of 1992, most of the ground waters exhibit NAHC03 type of composition as a consequence to the dilution caused by the rainfall in this season. In the following premonsoon season of 1993, which also included groundwater of deeper aquifers, though the majority of the groundwaters are characterised by NAHCO3 type, a good number of samples also indicate mixed type of water or dissolution of aquifer material by water. Only two well hydrograph stations situated away from the coast (north side of the Anjar taluka) have shown the NaCl type of groundwaters in all the three seasons irrespective of seasonal changes suggesting the possibility of occurre nce of highly saline water probably due to poor subsurface drainage. Different types of salinity in the area were demarcated, considering the overall salinity of the groundwater (TDS greater than 1500 mgl"1), sodium concentration and chloride-bicarbonate ratio (1 and above) and also the location of the well hydrograph station. Three type of salinities were deciphered viz. inherent salinity, coastal salinity and sea water intrusion into aquifers, of which first one refers to the concentration by evaporation and/or salinity of the soil, in contact with the groundwaters whereas, the last two are manifestation of the tidal fluctuations (onshore) and subsurface ingress of sea water respectively. During postmonsoon season, the extent of salinity varies mainly due to the dilution effect in the shallow aquifers. Some of the stations indicate occurrence of freshwaters, whereas others show the presence of inherently saline groundwaters. In the premonsoon season of 1993, although none of the well hydrograph station is affected by coastal salinity, a large tract of the area still reflects sea water contamination of groundwaters at deeper levels as in Mandvi area. Apart from the major ions, the groundwater samples were also analysed for halides (I , F and Br") to ascertain their approximate residence periods and the origin of their salinity. Exceptionally high (14 mgl"1) to very low (0.01 mgl"1) concentrations of F" are -1 -1 reported from the 45% (>1.0 mgl ) and 34% (<1.0 mgl ) of the groundwater samples respectively. Only remaining 21% of the groundwater samples have the acceptable F" -l concentration (-1.0 mgl ) for consumption purposes. A few cases of health problems related to fluorosis have been noted in the study area and it is, therefore, feared that unless some corrective measures are adopted by improving the F" concentrations in the groundwater, the population of the area will face severe health problems in the long run. The I" vs CI" plots & 17CI" vs CI" plots show enrichment of I" in the groundwaters. This may be attributed to : i) long residence time of groundwaters in contact with aquifer matrix rich with organic matter; ii) a depositional environment related to marine transgression in the geological past, and iii) prolonged entrapment of groundwaters owing to poor subsurface drainage resulting from relatively low permeability of the aquifer material (at shallow depths) and low recharge. The Br /CI ratio, too, in general, indicates the groundwater salinity related to brine especially, oil field brine. However, the Br"/Cl" relationship in the present area does not support the occurrence of brine in the groundwaters of pre- and postmonsoon seasons. Several rehabilitation structures have been installed in the region to arrest the deterioration in quality of the groundwater resources. Although limited in number, they have started showing improvement in the quality on local scale as indicated by the gradual reduction of the total dissolved solids and CI7HCO3 ratios. Summarising the study, it is concluded that the groundwaters of the coastal belt of Kutch region were fresh groundwaters not in the too distant past but were later affected by saline conditions viz. evaporation, mixing, dissolution etc. Indication of this is available from the quality of groundwaters of the period of 1986 to 1991 (which were years of severe droughts) and 1991 to 1992 (good rainfall years). In some areas however, old saline/connate waters are present, which is also confirmed from the geophysical investigations. The deterioration in the quality is largely due to the evaporation of the groundwater and the role of sea water encroachment into aquifers is of local importance. The salinity of the groundwater is also not related to the present day sea water but can be correlated to connate water which occur as isolated brine at a few places.