**5. Effects of past climate change**

The last glaciation has implied large variations in sea level, in the order of 125 m. Several areas that were under the sea level in preglacial time were drained from the sea during the Last Glacial Maximum (LGM).

This could enhance the recharge with fresh water. A Tertiary aquifer on the Kerala coast in SW India has groundwater dates varying between 23 and 34 kA BP (kA BP = kilo-annum, thousands of years before present) [8] and should thus have been recharged just before LGM. This is reasonable as the head for recharge was large, 60–80 m. However, just before LGM, the recharge has obviously been interrupted. In this case, the paleoclimatic conditions seem to have promoted the recharge during a wet SW monsoon which just before LGM was interrupted by a dry monsoon (**Figure 6**).

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**Figure 6.** Sea water levels during the last glacial period. The SW monsoon had, as per the abundant paleoclimatological data published, a wet period before LGM followed by a dry period after the LGM.

**4. Measures to halt sea water intrusion**

water near the shoreline [16].

copper mine waste in the catchment.

82 Aquifers - Matrix and Fluids

Glacial Maximum (LGM).

**5. Effects of past climate change**

rupted by a dry monsoon (**Figure 6**).

Where sea water intrusion is already a fact, there are measures to halt it. First of all, groundwater extraction should be diminished. While there are no visible signs of sea water intrusion in the Kerala Tertiary aquifers, there is a gradual development of surface water utilization for the larger towns in the coastal plain, which is a good measure to protect the coastal aquifers. On the Indian east coast, this is more difficult as the river flows are lesser there. Ballykraya and Ravi [13] describe the creation of a barrier by artificial recharge near the coast. A similar measure has been taken in the Salalah plain by well recharge of treated waste water along the coast line [14, 15]. A new approach, which could be more efficient, is when fresh water recharge into the nonsaline part of the coastal aquifer is combined with pumping of saline

**Figure 5.** Staple diagram with δ34S ratios in groundwater indicating recharge from the river Mati with sulfate having sulfide origin from oxidation of waste rock from copper mines [12]. The high δ34S ratios in sea near wells are above the sea water ratio (21‰), which indicates sulfate reduction in intercalated clay layers. The black bars are samples from

The last glaciation has implied large variations in sea level, in the order of 125 m. Several areas that were under the sea level in preglacial time were drained from the sea during the Last

This could enhance the recharge with fresh water. A Tertiary aquifer on the Kerala coast in SW India has groundwater dates varying between 23 and 34 kA BP (kA BP = kilo-annum, thousands of years before present) [8] and should thus have been recharged just before LGM. This is reasonable as the head for recharge was large, 60–80 m. However, just before LGM, the recharge has obviously been interrupted. In this case, the paleoclimatic conditions seem to have promoted the recharge during a wet SW monsoon which just before LGM was interOther aquifers on the Indian east coast has groundwater of a similar age and the same period and mechanisms of recharge may apply [9, 17]. The paleoclimatic data from India are abundant from both marine sediments as well as local pockets on current land illustrating the variations in the strength of the SW monsoon [18–23].

The past climate change has, in several aquifers near sea, formed a sequence of Ca-HCO3 → Na-HCO3 → Na-Ca-Cl types of groundwater formed by flushing of an initially formed saline/ brackish aquifer by fresh water [24, 25]. This could be seen laterally in Kerala (**Figure 1**). The direction of the recharge flow is from SE toward NW directed by the topography of the underlying Precambrian, which is likely to be intersected by faults in the same direction. The same zonation is seen depth-wise for instance in the Mekong river delta and in the Red River delta in Vietnam [26].

A secondary effect of flushing is that the softening effect of the process creates Na-HCO<sup>3</sup> type of groundwater (**Figure 7**) which tends to mobilize fluoride [27–30]. Where the recharge is less of a "piston flow" process, there will be more mixed forms of groundwater types. This is often the case on the Indian SE coast where the sedimentology is more intricate [10, 11].

Another effect of the last glaciation is the common occurrence of arsenic in groundwater in Holocene sediment, for instance not only in the Bengal delta [31–36] but also in other coastal plains in S and SE Asia [37–39] like in the Mekong river delta, the Red River delta, and the Irrawaddy river delta. When the sea level was lowered before Last Glacial Maximum (LGM) at around 18 kA before present, the sediments were subject to erosion and redeposition and this lowered the organic matter content [39]. Contrarily, after LGM, the sea level rose and created abundant wetlands rich in organic matter [40]. These sediments become easily anoxic with reduction of ferric oxyhydroxides and mobilization of arsenic into groundwater [41].

**Figure 7.** Na/Cl ratios versus chloride, showing the mobilization of sodium during the late stage of fresh water flushing. When the fresh water stage is approached, there is mobilization of sodium creating a Na-HCO3 type of water often with elevated levels of fluoride.

appearance of specific water such as NaHCO<sup>3</sup>

portion of the aquifer with elevated chloride levels.

18O 18O/16O for oxygen isotopes

ka BP Thousands of years before present

ESLR Effective sea level rise

LGM Last Glacial Maximum

SW monsoon Southwest monsoon

SMOW Surface mean ocean water

δ34S 34S/32S ratio for sulfur isotopes

\*Address all correspondence to: gunnjack@kth.se

δD Isotopic ratio for hydrogen isotopes (<sup>2</sup>

\* and Satheesachandran Thambi2

1 Royal Institute of Technology (KTH), Stockholm, Sweden

2 Central Groundwater Board of India, Thiruvananthapuram, Kerala, India

**Abbreviations**

**Author details**

Gunnar Jacks1

formed by ion exchange. At the end of the flush-

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85

H/H or deuterium/common hydrogen)

ing period, there is a pronounced increase in the Na/Cl ratio formed by the uptake of calcium in the fresh water recharge and release of sodium at adsorption sites from the saline period.

**Figure 8.** Age of groundwater in the Mati plain aquifer related to the chloride levels [12]. The slow turnover rate has left

Recharge and Turnover of Groundwater in Coastal Aquifers with Emphasis on Hydrochemistry…
