**1. Introduction**

About 1 billion out of 7 billion people in the world live in coastal areas and rely on coastal aquifers for their water supply and irrigation [1]. A simple calculation of the water requirement can be done by assessing that the population density is 2000/km2 , a figure taken from the Kerala coast in SW India. With the proposed population density, each person has an area of 500 m2 to reside on. The water need is dominated by the water requirement for food production and varies from 2500 l/day and person in developing countries to 5000 l/day in industrialized countries [2]. The water use in households is in the order of less than 200 l/ day and person. If using the figure 2700 litres per day and person for total water demand, then this would be equal to a water requirement of around 2000 mm/year or of the same

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order as the yearly rainfall in the actual area. Thus, even in rather rainfall-rich areas, coastal aquifers are subject to a considerable stress.

**3. Groundwater recharged from rivers**

brackish mixed type of water. The Na-HCO3

of the chloride concentration (**Figure 3**) (**Table 1**).

These water types range from a typical fresh water of Ca-HCO3

ride that diffused into the surrounding coarser sediments (**Figure 2**).

aquifer.

are found [10, 11].

recharge in the same direction.

This is a common case. To trace the river water into the aquifer is of interest to assess the recharge. In this case, isotope investigations may be of good use. In some cases, the river water has a specific composition not only regarding water chemistry but also regarding isotopes of some elements. This could be traced in the aquifer mirroring where the river is recharging the

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

With a past history of saline groundwater being refreshed by recharge, there are cases with elevated chloride ratio that could be interpreted as salt water intrusion. If the recharge occurs in a located area giving kind of a piston flow, a sequence of defined water types could be observed.

aquifer material from the saline period, picks up calcium from the fresh recharged water and releases sodium into the groundwater. This is observed in a Tertiary aquifer on the Kerala coast in India [8] and in the Tertiary and Cretaceous aquifers in Tamil Nadu [9] (**Figure 1**).

If the recharge area is scattered, not creating piston flow of recharge, mixtures of water types

Elevated chloride levels in the local Kerala Tertiary aquifer [8] were in places caused by diffusion of chloride from intercalated clay beds. Drilling and sampling of sediments, sand, and intercalated clay showed that the pore water in the clay contained elevated salinity with chlo-

This could also be seen by plotting the δ<sup>18</sup>Ο ratio in the water versus chloride. There was no mixing line toward sea water composition, but the δ18O ratios remained constant irrespective

**Figure 1.** Water types in the Kerala Tertiary coastal aquifer resulting from flushing by recharge before the Last Glacial Maximum. The Precambrian, under-laying the Tertiary strata, are faulted in the SE-NW direction, which also guides the

type via a Na-HCO3

http://dx.doi.org/10.5772/intechopen.73301

type is formed by ion exchange, the Na-saturated

type to a

79

Coastal plains are often underlain by thick sediments, and they are complex intercalations of finer and coarser sediments formed under different climatic conditions. Looking back on the recent past climatic history, just a few thousands of years back, there have been large variations in sea level and locally, there have been considerable tectonic movements.

This review will be focused on using the hydrochemistry of the groundwater and isotopes to decipher the groundwater exchange in coastal aquifers. This approach is especially useful in coastal aquifers where salt water intrusion could complicate the picture. In view of the authors' experience, there will be a focus on South and Southeast Asia, however, with some examples from other regions such as Europe.
