Freshening of seawater in the Mahim Bay, Mumbai, India: Insight from an environmental isotope study

ABSTRACT

An environmental isotope study was conducted to assess the cause for the freshening of seawater observed in the Mahim Bay, Mumbai, India after a storm event during the southwest monsoon period. Water samples were collected from the various locations of the coastal water and the suspected inland water sources such as rain, river and groundwater and analysed for major ion species and stable isotopes (δ¹⁸O and δ²H). Dissolved radon (²²²Rn) in the coastal water was monitored in-situ. Field monitoring survey in the coastal water indicated lower electrical conductivity (1,730 μS/cm) near Mahim Mosque compared to the surrounding shelf waters. Relatively high excess ²²²Rn activities (up to 55 Bq/m³) were observed in the Mahim Bay, even after 13 days of seawater freshening event. Based on the hydrochemical and isotope results, various prevailing hypotheses on the occurrence of low salinity water in the Mahim Bay were tested. It is inferred that the low salinity coastal water was associated with groundwater discharge occurring in the Mahim Bay and in the Mithi River and were unlikely due to the overflow of Vihar and Powai Lakes in the catchment of Mithi River and surface runoff because of the rain/storm events. Temporal variations of electrical conductivity and stable isotopic composition of coastal water in the Mahim Bay showed that the groundwater inputs were decreasing after the storm event. ²²²Rn was found to be a useful tracer for distinguishing the subsurface flow of water to the coastal system.

KEYWORDS:

• Low salinity seawater
• coastal water
• groundwater discharge
• radon
• stable isotopes
• Mahim Bay

Introduction

Freshening of seawater is known to occur in the coastal regions of many parts of the World. It may be derived from the flood waters of rivers (Moore et al. 1986; Ortner et al. 1995; Simstich et al. 2005), large scale groundwater discharges after storms (Moore 1996; Gwak et al. 2014), or from heavy offshore rain events (Moore and Kjerfve 1998; Valiela et al. 2012). These hydrological sources can be an important pathways for nutrients and contaminants to many types of coastal ecosystems. However, understanding the freshening processes in the coastal water is extremely difficult because of the complexities inherent in identifying the source of freshwater.

A large freshwater plume was noticed in the coastal waters of Mahim Bay near a Mosque, Mumbai, India on 18 August 2006. Thousands of people visited the beach, many of them not only bathed in that water but also drank it with the faith that a miracle had happened. Analyses of water samples indicated that the low salinity coastal water was not potable as it was contaminated with high nitrate, phosphate and fecal coliforms (DNA 2006; NIO 2006). A similar freshening phenomenon of coastal water was reported in Teethal beach, Gujarat (about 200 km north of Mumbai) on the same day of occurrence in Mahim Bay (Wikipedia 2006). Not many scientific studies were carried out to understand this phenomenon except those of NIO (2006) and Singhal et al. (2007). An attempt was made in the present study to assess the cause for the freshening of seawater in the coastal areas of Mahim Bay using hydrochemical and environmental isotopes (δ¹⁸O, δ²H and ²²²Rn) techniques.

Environmental isotopes are useful tools for hydrological, coastal and oceanographic studies (Fritz and Fonts 1980). Radon (²²²Rn), a naturally occurring conservative radioactive noble gas in the U decay series (half-life: 3.83 days), is recognised as a reliable tracer to understand the surface water – groundwater interactions and identify areas of significant groundwater discharge in surface water bodies (Cable et al. 1996; Dulaiova et al. 2010). The natural ²²²Rn activity in groundwater is typically 2–3 orders of magnitude higher than surface waters, thus providing a means to quantify the relative contribution of surface flows and groundwater into the coastal zone. Stable isotopes such as ¹⁸O and ²H are widely used as tracers for studying groundwater – seawater interactions (Schiavo et al. 2009; Lin et al. 2011; Noble et al. 2015), as they are integral part of the water molecule.

Physical settings

Differential weathering of the inter-layered soft tuffs and resistant basaltic flows (Deccan Traps) by seawater has created several bays and creeks around Mumbai; Mahim Creek is one of the prominent among them. The semi-enclosed Mahim Bay (Area = 6 km²; average depth = 4.6 m) is open to the Arabian Sea on the west and connected to the Mahim Creek on the north-east (Figure 1). The northern shores of the Bay are predominantly rocky with mud patches in-between, while, the central and the southern zones are largely sandy with occasional rock outcrops. Mahim Creek, which is wide and shallow, is an inland extension of the Mahim Bay. The Mahim Creek receives overflow of the Vihar and the Powai Lakes, located 14 and 16 km, respectively, in the northeast direction, during monsoon (June-September) via the Mithi River. The runoff decreases considerably after the withdrawal of monsoon leading to stagnant conditions in the Mithi River and the Mahim Creek after September, and the system resembles an effluent drain (Zingde and Desai 1980; Sabnis 1984; Sabnis and Zingde 1989). The annual flushing caused by heavy rains during the monsoon period substantially improves the ecological quality of the Mithi River and the Mahim Creek. The average annual rainfall is about 2400 mm. The mean minimum temperature is 16.3°C and the mean maximum temperature is 32.2°C.

Figure 1. Location map of the Mahim Bay showing the sampling points.

The Mahim area and its surroundings are underlain by basaltic lava flows of upper Cretaceous to lower Eocene age. The rock consists of highly vesicular bottom layer having closely spaced horizontal joints but the thickness is generally less. The vesicular zones are weathered in most parts of the area, thus making them moderately permeable. But if, vesicles are not filled, they act as highly permeable aquifers. Thin soil cover overlies the fractured massif basalt, which forms the major geological unit and they are exposed in the coastal areas. The shallow alluvium formation of recent age also occurs as a narrow stretch along the river. The groundwater exists in fractures, joints, vesicles and in weathered zone of basalt. The occurrence and circulation of groundwater in this area is mainly through vesicular unit of lava flows and through secondary porosity and permeability of the basalt, which is developed due to weathering, jointing, fracturing etc. (CGWB 2009). Here, the groundwater generally occurs under phreatic condition.

Methods

Field monitoring surveys were carried out in the Mahim Bay after 1 day, 3 days and 13 days of appearance of low salinity water in the Bay and coastal water samples were collected for the measurement of hydrochemistry and stable isotopes (δ²H and δ¹⁸O). Mithi River, shallow groundwater (from tube wells of <10 m deep), seawater away from the coast and precipitation samples were also collected and analysed as a reference for comparison (Figure 1).

Physical parameters such as electrical conductivity (EC) and pH were measured in-situ using portable conductivity and pH meters, respectively. For anion analyses, the collected water samples were filtered and stored in pre-washed polythene bottles. Another set was collected and acidified to pH < 2 by adding concentrated HNO₃ for cation measurements. Alkalinity was measured by titrating 10 mL of water sample with 0.02 N H₂SO₄, using methyl orange as the indicator. Major ion chemistry (anions & cations) was measured using Ion Chromatograph (Dionex-500) following the standard procedures (Tirumalesh et al. 2010).

For stable isotope (²H and ¹⁸O) analysis, water samples were collected in 30 ml airtight polyethylene bottles and measured in the laboratory by isotope ratio mass spectrometer (Geo-2020, PDZ Europa) using the gas equilibration method. For δ²H analysis, 1 ml of the water sample was equilibrated with H₂ at 50°C for 1 h with Pt catalyst and for δ¹⁸O analysis, 1 ml of the water sample was equilibrated with CO₂ gas at 50°C for 8 h and the equilibrated gases were introduced into the mass spectrometer. The results are reported in δ-notation, as deviation relative to a standard, V-SMOW (Vienna – Standard Mean Ocean Water) and in units of parts per thousand (denoted as ‰). The δ values are calculated using (Coplen 1996):(1) $\delta \left(\text{‰}\right)=\left(\frac{R_x}{R_s}-1\right)\times 1000$(1) where $R$ denotes the ratio of heavy to light isotope (e.g. ${\displaystyle \frac{{}^{2}H}{{}^{1}H}}$ or ${\displaystyle \frac{{}^{18}O}{{}^{16}O}}$) and $R_x$ and $R_s$ are these ratios in the sample and standard, respectively. The precision of measurement for δ²H is ±0.5 ‰ and for δ¹⁸O is ±0.1 ‰ (2σ).

²²²Rn activity in the coastal waters of Mahim Bay was measured 13 days after the appearance of the low salinity water. Since the ²²²Rn concentrations in surface waters are generally very low, large volumes of water are required for the measurement and hence in-situ monitoring is highly essential. A continuous radon monitoring system similar to those developed by Burnett et al. (2001) using a radon-in-air monitor was used in the present study.

For the in-situ measurement of ²²²Rn in the Mahim Bay, seawater was continuously pumped from 1 m above the sea bed using a peristaltic pump placed in a boat and sprayed as a jet into an air–water exchanger (Figure 2). The radon thus stripped out from the water was passed through a closed air-loop via a desiccant tube into a ²²²Rn counting system (RAD-7; Durridge make). The equilibrium of ²²²Rn between the liquid and gaseous phase was established within 30 min (Lane-Smith et al. 2002). The radon monitor uses a high electric field above a silicon semiconductor detector at ground potential to attract the positively charged polonium daughters, ²¹⁸Po⁺ (t_1/2 = 3.1 min; alpha energy = 6.00 MeV) and ²¹⁴Po⁺ (t_1/2 = 164 µs; alpha energy = 7.67 MeV), which are counted as a measure of ²²²Rn activity in air. The ions are collected in energy-specific windows that eliminate interference and maintain low background levels. ²²²Rn activities are expressed in Bq/m³ with 2σ uncertainties.

Figure 2. Layout of a continuous ²²²Rn monitoring system for the measurement of ²²²Rn in surface water bodies.

In order to get acceptable precision, ²²²Rn activity at each location was counted for two hours (3 cycles of counting) after reaching equilibrium. At room temperature, as there is about four times more radon in the air than in water at equilibrium, the measured radon concentration in air was corrected accordingly (Weigel 1978). The precision of ²²²Rn measurement by this method is ±5 Bq/m³.

For ²²²Rn measurement in groundwater, the sample was collected in airtight 250 ml glass bottle. In the laboratory, the dissolved radon in water is stripped out by bubbling air and circulated through a closed air-loop via a desiccant tube into the ²²²Rn measuring system. The system usually reaches equilibrium within 5 min. After attaining the equilibrium, the ²²²Rn activity is counted for 40 minutes (4 cycles of 10 minutes counting). The measured radon activities were also corrected for radioactive decay.

Results and discussion

During the first sampling campaign (i.e. 1 day after the appearance of low salinity water in the Mahim Bay), the Electrical Conductivity (EC) of the coastal water on the eastern flank of the Bay during the low tide varied from 1,730 µS/cm near the Mahim Mosque to 6,720 μS/cm near Worli, whereas it was 17,450 μS/cm in the western part of the Bay near Bandra (Table 1). The electrical conductivities of the Mithi River and the shallow groundwater were 489 and 320 μS/cm, respectively. The electrical conductivity of the seawater away from the coast was about 57,000 μS/cm. A freshwater spring was visually observed on the Mahim Beach near the Mosque during the first sampling period. pH of the coastal water indicated that it was neutral to alkaline in nature (pH varied from 7.5 to 8.1) while Mithi River, groundwater and seawater were alkaline (pH = ∼8.0).

Table 1. Results of the isotope analysis for samples collected from the Mahim area.

Sl. No.	ID No.	Location	Date	Type	Electrical conductivity (μS/cm)	pH	δ^18O (‰) ± 0.1	δ^2H (‰) ± 1	Excess ^222Rn (Bq/m^3)
1	C2X	Mahim Mosque	19/08/06	Coastal water	1,730	7.65	−1.20	−3.8
2	C5X	Dadar Beach	19/08/06	Coastal water	6,420	8.06	−1.01	−3.1
3	C6X	Worli Fort	19/08/06	Coastal water	6,720	7.97	−0.75	−2.4
4	C7X	Bandra Fort	19/08/06	Coastal water	17,450	7.52	−0.80	−0.9
5	P1X	Mahim Fort	19/08/06	Rain water			−2.10	−7.2
6	R1X	Mithi River	19/08/06	River water	489	7.97	−1.29	−4.8
7	C1Y	Mahim Fort	21/08/06	Coastal water	6,350	7.40	−0.87	−3.5
8	C2Y	Mahim Mosque	21/08/06	Coastal water	5,500	7.38	−0.39	−3.0
9	C3Y	Between Hinduja Hospital and Mahim Mosque	21/08/06	Coastal water	6,490	7.26	−0.91	−4.0
10	C4Y	Hinduja Hospital	21/08/06	Coastal water	6,570	8.05	−0.55	−3.6
11	C2Z	Mahim Mosque	31/08/06	Coastal water	14,270	7.30	−0.55	−0.1	55 ± 5
12	C3Z	Between Hinduja Hospital and Mahim Mosque	31/08/06	Coastal water	15,050	7.61	−0.33	−0.9	46 ± 5
13	C4Z	Hinduja Hospital	31/08/06	Coastal water	16,360				48 ± 8
14	C5Z	Dadar Beach	31/08/06	Coastal water	21,000				41 ± 7
15	G1Z	Mahim Fort	31/08/06	Groundwater	290	8.1	−1.55	−4.6	2,100 ± 250
16	G2Z	Dadar Beach	31/08/06	Groundwater	350	8.2	−1.52	−4.9
17	S1Z		31/08/06	Seawater	57,000	8.2	0.15	1	5 ± 5

Notes: X,Y,Z in the sampling ID represent 1, 3 days and 13 days after the seawater freshening event, respectively.

During the second sampling period (i.e. 3 days after the appearance of low salinity water in the Mahim Bay), the electrical conductivity of the coastal water remained more or less similar (6,470 ± 110 μS/cm) to that of the first sampling period, except near the Mahim Mosque, where EC was increased to 5,500 μS/cm. pH also remained more or less similar to that of the first sampling period. EC of the coastal water was further increased (varied from 14,270 to 21,000 μS/cm), 13 days after the occurrence of low salinity water in the Mahim Bay (during third sampling period).

The lower electrical conductivity seen in the eastern part of the Mahim Bay compared to the seawater could be due to the dilution of seawater with the freshwater discharges into the Bay. The possible sources of freshwater discharges include the surface runoff discharging into the Bay through numerous storm water drains after a rain event, the flood water in the Mithi River due to the overflow of Vihar and Powai lakes, and the submarine groundwater discharge occurring at the coastal areas.

Researchers of National Institute of Oceanography, Mumbai measured the salinity, dissolved oxygen, nutrients (${\mathrm{N}\mathrm{O}}_3^{-}\text{-}\mathrm{N}$, ${\mathrm{N}\mathrm{H}}_4^{+}\text{-}\mathrm{N}$, ${\mathrm{P}\mathrm{O}}_4^{3-}$-P etc.) and phytoplankton in the coastal waters, 1 day after the appearance of low salinity water in the Mahim Bay, and hypothesised that the discharge of Mithi River because of the overflow of Vihar and Powai lakes situated on the upstream of the catchment and the surface runoff from the nearby areas, were the causes for the occurrence of low salinity water in the Bay (NIO 2006). Singhal et al. (2007) partially supported this hypothesis based on the measurement of radionuclides such as total uranium (U) and potassium-40 (⁴⁰K) in coastal water. These measurements gave values similar to the rainwater, indicating that the discharge of freshwater dominated by rainwater as the cause for the freshening of seawater in Mahim Bay.

Based on the local hydrogeology of the area, the geologists at Indian Institute of Technology – Bombay, Mumbai opined that this phenomenon could be because of the submarine groundwater discharge in the coastal area (The Hindu 2006). The authors were of the opinion that the continuous rainfall occurred a few days prior to the event, might have fully recharged the aquifer and the aquifer would be discharging the groundwater through the fractures in the basaltic rocks. The above hypotheses were evaluated in the present study using hydrochemical parameters and environmental isotopes.

Hydrochemistry

From the hydrochemical facies of the water samples collected from the various suspected sources such as: rain, Mithi River and groundwater and the coastal water (Table 2, Figure 3), it is observed that, except the groundwater, Mithi River and rainwater samples, all other samples show Na–Cl type of water and fall on the saline zone of the Piper’s plot (Piper 1994). Mithi River and rainwater samples show Na–Ca–Cl–HCO₃ type while the groundwater shows Na–Ca–HCO₃ type. A similar water facies for groundwater of this region was reported by Tirumalesh et al. (2010). Although, all the samples collected from the coastal region show lower ionic content, still they are Na–Cl dominant type, similar to the standard seawater.

Figure 3. Piper’s trilinear plot (Piper 1994) showing the water type of various sources suspected of contributing to the low salinity water in the Mahim Bay.

Table 2. Results of the hydrochemical analysis for samples collected from the Mahim area.

Sl. No.	ID No.	Location	Type	Na^+	K^+	Mg^2+	Ca^2+	Sr^2+	F^–	Cl^–	Br^–	${\mathrm{N}\mathrm{O}}_3^{-}$	${\mathrm{S}\mathrm{O}}_4^{2-}$	${\mathrm{P}\mathrm{O}}_4^{3-}$	${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$
1	C2X	Mahim Mosque	Coastal water	245	13	32	39		0.2	425	1	11.8	79	4.3	90
2	C6X	Worli Fort	Coastal water	1,130	50	144	93	0.7	0.4	2,055	7.1	9.9	294	1.3	100
3	C7X	Bandra Fort	Coastal water	3,020	135	395	170			5,929	16	10	785		10
8	P1X	Mahim Fort	Rain	1.1	0.3	0.2	0.4			1	0.01	0.1	0.3		1.7
5	R1X	Mithi River	River	46	7.6	8.5	31.4		0.3	58	0.2	63	34	2.7	70
4	C2Y	Mahim Mosque	Coastal water	2,500	93	301	140	1.3		4,575	14	7.7	595		50
6	C2Z	Mahim Mosque	Coastal water	4,300	180	515	350			7,623	26	0.5	241		30
7	G2Z	Dadar Beach	Groundwater	58.9	1	6	17.6		0.8	15.2	0.05	0.5	18.5		206
9			*Sea water	10,500	380	1,350	400	8	1.3	19,000	65		2,700		142

Notes: All parameters are in mg/L.

*Standard seawater composition (Goldberg 1963).

In order to evaluate the variation of ionic concentrations with respect to chloride, mixing plots are generally used. Chloride is a conservative ion and it does not react or enter into precipitation – dissolution processes except at brine concentrations and rarely enters into redox or adsorption reactions. Its conservative nature makes it suitable to compare with the relative loss or gain of other ions, which can constrain the possible seawater-freshwater mixing or water – rock interactions. It is observed that except in ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$ versus Cl^–and ${\mathrm{N}\mathrm{O}}_3^{-}$ versus Cl^– plots, major ion concentrations of low salinity coastal waters fall on the freshwater – seawater mixing line (Figure 4). This indicates that additional sources or sinks contribute to ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$ and ${\mathrm{N}\mathrm{O}}_3^{-}$ ions in coastal water. Samples from the coastal water and Mithi River fall above the mixing line in ${\mathrm{N}\mathrm{O}}_3^{-}$ versus Cl^– plot (Figure 4(h)) indicating anthropogenic contamination. High nitrate concentration is expected from different municipal wastes originating from various dumping grounds and untreated sewage in the city. Surface runoff from the rain dissolves nitrate ions from the municipal wastes and contributes to Mithi River. Nitrate concentration up to 160 mg/L was reported in Mithi River by Nagarsekar and Kakde (2014), which was attributed to the contribution from untreated sewage. High nitrate in these low salinity coastal waters could be due to mixing of river water with seawater and/or by the discharge of untreated sewage directly into the coastal water. However, low salinity coastal water sample (after 13 days) show similar value that of groundwater and fall below the mixing line in the NO₃^- versus Cl^– plot (Figure 4(h)). This shows that low salinity coastal water is a mixture of river water, groundwater and seawater. In ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$ versus Cl^– plot (Figure 4(g)), it is observed that groundwater, river water and low salinity coastal water fall above the freshwater – seawater mixing line with groundwater having the highest ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$ concentration. Groundwater in this region contains relatively high bicarbonate due to rock – water interactions (Tirumalesh et al. 2010). River water and low salinity coastal water samples fall in between the groundwater and seawater end members (Figure 4(g)). This indicates that the low salinity coastal waters have a significant component of groundwater. Even in this case, one low salinity coastal water sample (after 13 days) shows low bicarbonate concentration and fall below the mixing line, indicating comparatively lower contribution of groundwater to the coastal water after 13 days.

Figure 4. (a–h) Mixing plots showing the variation of different ions with respect to chloride concentration.

Characteristic ionic ratios are very useful in interpreting the relative contribution of marine and freshwater components (Table 3). These ratios were used to compare the chemical nature of the low salinity coastal water with other suspected sources such as groundwater, rain water and Mithi River water. From the ionic ratios, it is inferred that the low salinity coastal water observed near the Mahim Mosque, 1 day after the seawater freshening event (C2X), has all the characteristic ratios higher than that of seawater. Higher values of these characteristic ratios are typical of freshwater like groundwater. These ratios are not very high as in the case of groundwater; it probably means some contribution of river water to the low salinity coastal water near the Mahim Mosque. Hence, these ionic ratios of low salinity coastal waters indicate the contribution from river as well as groundwater components. It is also possible that the groundwater contributes to both river and coastal water. Coastal water samples collected after 3 and 13 days of the seawater freshening event show decreased values for characteristic ionic ratios, which could be due to decreased contribution of freshwater.

Table 3. Ionic ratios of samples collected from the Mahim area.

Sl. No.	ID No.	Site	Type	Na^+/Cl^–	${\mathrm{S}\mathrm{O}}_4^{2-}$/Cl^–	${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$/Cl^–	Br^–/Cl^–	Mg^2+/Ca^2+
1	C2X	Mahim Mosque	Coastal water	0.58	0.19	0.21	0.0024	0.82
2	C6X	Worli Fort	Coastal water	0.55	0.13	0.01	0.0031	2.15
3	C7X	Bandra Fort	Coastal water	0.56	0.16	0.00	0.0034	1.47
4	P1X	Mahim Fort	Rain	3.88	1.22	10.72	0.0033	0.34
5	R1X	Mithi River	River	0.79	0.59	1.21	0.0034	0.27
6	C2Y	Mahim Mosque	Coastal water	0.51	0.13	0.00	0.0027	2.32
7	C2Z	Mahim Mosque	Coastal water	0.55	0.14	0.05	0.0035	1.55
8	G2Z	Dadar Beach	Groundwater	1.10	0.30	1.70	0.0000	0.50
9			Sea water	0.55	0.14	0.01	0.0034	3.12

Stable isotopes (δ¹⁸O and δ²H)

The stable ¹⁸O and ²H isotopes of water are generally considered to behave conservatively in the absence of significant evaporation and mixing with other water of different isotopic signatures. However, physical processes, such as evaporation and condensation, during which the associated waters, depending on their geographical locations, get labelled naturally with distinct isotopic signatures. By tracing the naturally labelled isotopic signatures, it is possible to obtain several information related to surface water groundwater interactions and associated hydrological processes.

The δ¹⁸O and δ²H values of coastal water varied from −1.2 to −0.3 ‰ and −4.0 to −0.1 ‰, respectively, with more depleted values during the first sampling period (1 day after the appearance of low salinity water) (Figure 5). The δ¹⁸O and δ²H in Mithi River were −1.3 ‰ and −4.8 ‰ while in groundwater, the average values were −1.5 ‰ and −4.7 ‰, respectively. The rainwater had δ¹⁸O and δ²H values −2.1 ‰ and −7.2 ‰, respectively, which was isotopically different when compared to Mithi River or groundwater. The similarity in stable isotopic composition of Mithi River and groundwater shows that there was substantial contribution of groundwater to the Mithi River. Although, Mithi River and groundwater were derived from the local rainfall, their stable isotopic compositions were found to be slightly enriched compared to the rainwater.

Figure 5. δ¹⁸O versus δ²H plot for water samples collected from the Mahim area. The Global Meteoric Water Line (GMWL) is also plotted for comparison, using the equation of Craig (1961), δ²H = 8δ¹⁸O + 10.

The rainwater samples lies near the Global Meteoric Water Line (GMWL), while samples from Mithi River and groundwater fall slightly on the right-hand side of the line indicating evaporation effect (Figure 5). The low salinity coastal water lies in a mixing line between freshwater sources (Mithi River and groundwater) and seawater. This indicates that the coastal water received varying amounts of fresh groundwater through the coastal areas and the Mithi River. A similar trend was also seen in electrical conductivity versus δ¹⁸O plot (Figure 6). The freshwater sources such Mithi River and groundwater are characterised by comparatively lower electrical conductivities and depleted stable isotopic composition while seawater is characterised by higher electrical conductivity and enriched stable isotopic composition. The low salinity coastal water samples falls in a mixing line between freshwater sources and seawater signifying freshwater input through Mithi River and groundwater.

Figure 6. Plot of electrical conductivity versus δ¹⁸O for water samples collected from Mahim area.

The lowest electrical conductivity and depleted δ¹⁸O in coastal water was observed 1 day after the appearance of low salinity water (Figure 7). The electrical conductivity of coastal water near Mahim Mosque increased from 1,730 μS/cm on the first day after the appearance of low salinity water to 14,270 μS/cm, after 13 days. The δ¹⁸O of the coastal water got enriched from −1.2 ‰ on the first day to −0.6 ‰ after 13 days. The gradual increase in electrical conductivity and enrichment in δ¹⁸O with time implies that the freshwater discharge was decreasing with time elapsed since the event. It was seen that relatively high rainfall occurred a few days before the seawater freshening event (Figure 8). Since, it was towards the end of the monsoon season, the aquifers might be fully recharged and it would have released excess water available due to high rainfall, in the form of submarine groundwater discharge through the fractures in the coastal areas and the alluvium present along the Mithi River. In order to confirm this hypothesis, in-situ radon monitoring study was also conducted in the Mahim Bay.

Figure 7. Temporal variation of δ¹⁸O and electrical conductivity in coastal water near Mahim Mosque.

Figure 8. Distribution of daily rainfall at Trombay (∼15 km east of Mahim), a few days prior to the seawater freshening event in Mahim Bay (modified from Singhal et al. 2007).

Radon (²²²Rn)

Radon is recognised as a potential natural tracer for studying the hydraulic interaction between groundwater and surface water (Zhang et al. 2017; Yi et al. 2018), mapping the groundwater flow (Tallini et al. 2013), estimating the groundwater velocity and residence time (Schubert et al. 2011) etc. The processes such as: rock weathering, radioactive decay of ²²⁶Ra to ²²²Rn, and restricted atmospheric interaction, leads to high activities (3–4 orders of magnitude) of unsupported ²²²Rn in groundwater (Kraemer and Genereux 1998). Conversely, surface waters typically have low ²²²Rn activities due to the negligible ²²²Rn content in rain. After groundwater containing ²²²Rn discharges to surface water bodies, ²²²Rn activities tend to decrease because of dilution due to mixing with surface water, gas exchange with atmosphere and inherent radioactive decay. As a result, groundwater discharges into surface waters can often be easily detected by their characteristic ²²²Rn enrichment in surface waters and thus helps as an indirect tracer of groundwater discharge into lake, river, sea etc. (Luo et al. 2016; Zhang et al. 2018; Peterson et al. 2019).

In order to verify the groundwater discharge into the coastal area, it is essential to measure the ²²²Rn activities in the end members i.e. in the groundwater and seawater away from the coast (Jacob et al. 2009). The groundwater in the Mahim area had a ²²²Rn activity of 2100 ± 250 Bq/m³ (Table 1). The seawater away from the coast showed an average ²²²Rn activity of 5 ± 5 Bq/m³, which is taken as the background ²²²Rn activity in the coastal water. The background ²²²Rn activity in the seawater away from the coast can be assumed as the radon flux emanating as a result of the diffusion from the sediments (Burnett et al. 2001). Excess ²²²Rn activities were calculated by subtracting the background ²²²Rn activity from the measured activities at various locations in the coastal water. Excess ²²²Rn activities in the Mahim Bay, 13 days after the seawater freshening event varied from 41 to 55 Bq/m³. The relatively high ²²²Rn activities in the Mahim Bay could be due to the direct inputs of ²²²Rn rich groundwater into the Bay or groundwater inflow to the Mithi River, which was eventually discharged into the Bay. It also implies that groundwater discharge to the coastal areas existed even after 13 days of seawater freshening event. A negative linear correlation was observed, with areas of lower electrical conductivities in the Bay, which were associated with higher amount of excess ²²²Rn activities, while areas of higher electrical conductivities are associated with lower amount of excess ²²²Rn activities (Figure 9). Spatial distribution of ²²²Rn activities in the coastal water of Mahim Bay indicated that the area close to Mahim Mosque received relatively high groundwater discharge compared to other shelf waters.

Figure 9. Variation of excess ²²²Rn activity with electrical conductivity in the Mahim Bay, 13 days after the seawater freshening event. The regression line is given by: A = −0.002EC + 75 (r²= 0.61, n = 4), where A is the excess ²²²Rn activity and EC is the electrical conductivity. The error bars represent the precision of ²²²Rn measurements. The intersection of x- and y-axis is not at (0,0).

Like ²²²Rn, various other uranium series isotopes such as radium (eg. ²²⁶Ra), uranium (eg. ²³⁴U/²³⁸U ratio) could also be used as tracers of groundwater discharge (Moore 1996, 1997). Singhal et al. (2007) measured the total uranium and ⁴⁰K in the Mahim Bay following the seawater freshening event. However, they could not find a characteristic groundwater signature of these radionuclides in the coastal water. This could be due to a lower solubility of uranium and ⁴⁰K to the shallow groundwater from the aquifer matrix (i.e. less rock-water interaction) and because of the short residence time of groundwater (may be a few days) in the aquifer. In contrast, ²²²Rn being a soluble gas, readily dissolves in groundwater and show appreciable activity in groundwater.

Conclusion

Based on the hydrochemical and environmental isotope (δ¹⁸O, δ²H and ²²²Rn) studies, it is concluded that the subsurface groundwater discharge was most likely responsible for the occurrence of low salinity water in the Mahim Bay. It appears that, the overflow of Vihar and Powai Lakes in the catchment of Mithi River and surface runoff due to rain were unlikely to be the causes for the freshening event. It was also found that these groundwater inputs decreased after the storm event.

Thus, the study highlights the usefulness of ²²²Rn as a tracer to distinguish the subsurface flow of water to the coastal system.

Acknowledgements

This study was initiated with the help and support of Dr Rakesh Kumar, then Head, Regional Centre, National Environmental Engineering Research Institute, Mumbai. We are thankful to him. We extent our sincere thanks to our colleagues, G. N. Mendhekar, for his help during the field monitoring programme and Md. Arzoo Ansari, for preparing the maps. We are also thankful to Ms Archana Deodhar, Group Leader, Isotope Hydrology Section, IRAD and Dr P. K. Pujari, Director, RC & IG, BARC for their encouragement and support.

Disclosure statement

No potential conflict of interest was reported by the author(s).