Assessing the origin and processes controlling groundwater salinization in coastal aquifers through integrated hydrochemical, isotopic and hydrogeochemical modelling techniques

Figures & data

Figure 1. Location of the study area indicating sampling points

Figure 2. Spatial variation in electrical conductivity (EC) in the upper aquifer in (a) July 2014 and (b) December 2014, and in the lower aquifer in (c) July 2014 and (d) December 2014

Figure 3. Temporal variation in the concentration of ions in the upper aquifer: (a) chloride and (b) sodium; and in the lower aquifer: (c) chloride and (d) sodium

Figure 4. Plot of electrical conductivity vs. chloride showing normal groundwater conditions, saltwater intrusion and mixing for (a) July (pre-monsoon) and (b) December (post-monsoon)

Figure 5. Hydrochemical facies evolution showing aquifer freshening and salinization in the upper aquifer in (a) July 2014 and (b) December 2014, and in the lower aquifer in (c) July 2014 and (d) December 2014

Table 1. Extent of seawater intrusion indicated by various indices in different time periods

		Extent of seawater intrusion (km)
		Upper aquifer				Lower aquifer
Indicator	Months	2011	2012	2013	2014	2011	2012	2013	2014	Remarks
EC	July	14.5	15.0	14.0	12.0	15.5	17.0	16.2	16.8	EC > 3000 µS/cm (Kim et al. 2003)
	Dec	10.9	11.0	10	10.5	13.5	14.2	14.7	13.5
Na/Cl	July	15.0	14.0	14.0	13.8	16.6	17.0	16.9	16.8	Na/Cl = 0.86 is seawater, Na/Cl between 0.86 and 1 is mixed water, Na/Cl < 1 is freshwater (Vengosh and Pankratov 1998)
	Dec	9.0	9.5	10.1	10.3	15.9	13.6	14.1	14.1
Cl/Br	July	13.3	14.5	13.4	13.8	16.0	17. 5	16.4	16.8	Cl/Br = 297 is seawater, Cl/Br > 297 is hypersaline water (Davis et al. 1998)
	Dec	10.3	11.9	11.3	11.5	13.9	14.7	13.2	13.7
BEX	July	12.7	13.3	12.2	12.9	14.7	16.3	15.2	15.9	BEX < 0 is seawater (Stuyfzand 2008)
	Dec	10.8	12.0	11.1	11.4	12.4	12.5	12.7	13.1

BEX: base exchange index; EC: electrical conductivity.

Figure 6. Temporal variation in stable isotopes for (a) March and May 2012 and (b) July and December 2013, indicating the three groups of groundwater evolution: Group I – groundwater recharged by rainfall; Group II – mixed zone, i.e. mixture of rainfall derived from groundwater and seawater; and Group III – groundwater enriched due to seawater intrusion and evaporation. Canal: the Buckingham Canal; River: mouth of Arani River; GMWL: global meteoric water line; and IMWL: Indian meteoric water line

Table 2. Mineral phases and total dissolved solids (TDS) in the groundwater of the three zones. Zone 1: western part; Zone 2: northeast; Zone 3: southeast

Minerals/TDS	Zone 1	Zone 2	Zone 3
Albite	0.26	23.56	16.12
Calcite	2.59	-	-
CaX2	–5.07	–3.00	–2.15
CO2g	2.63	11.23	5.59
Gypsum	-	5.48	2.64
Halite	3.43	92.36	35.23
Illite	–0.09	–11.37	–6.93
Quartz	–0.24	–31.93	–23.57
NaX	8.14	-	-
KX	-	6.01	4.30
TDS measured (mg/L)	1300	3554	10 452
TDS computed (mg/L)	1360	3741	10 610

Table 3. Physical and chemical characteristics of seawater and groundwater used in forward modelling. Chemical composition in mg/L

		Groundwater composition in the western part
Parameter	Seawater (eastern part)	1969	1975	1987	2007	2014
pH	8.5	7.2	7	7.4	7	7
pe	8	4	4	4	4	4
Density (kg/m^3)	1.025	1	1	1	1	1
Calcium	400	60	58	79	65	129
Magnesium	1200	12	42.5	60	97.5	70
Sodium	11 200	50.2	135	186	221	414
Potassium	720	4.7	12.6	18.5	11.2	9.8
Bicarbonate	183	355	500	510	289	348
Chloride	19 990	170.9	270	298	265	268
Sulphate	3767	39.9	89	101	56	55.6

Figure 7. Simulated spatial variation in the concentration of major ions for (a) December 1969, (b) December 1987, (c) December 2007 and (d) December 2014. Simulated spatial variation in the concentration of (e) sodium and (f) chloride during different periods

Figure 8. (a) Conceptual diagram of the seawater–freshwater interface showing the extent of seawater intrusion at different times. (b) Conceptual west-to-east cross section showing the identified geochemical processes

Kim, Y., et al., 2003. Hydrogeochemical and isotopic evidence of groundwater salinization in a coastal aquifer: a case study in Jeju volcanic island, Korea. Journal of Hydrology, 270 (3), 282–294. doi:10.1016/S0022-1694(02)00307-4

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