**3. Groundwater recharged from rivers**

order as the yearly rainfall in the actual area. Thus, even in rather rainfall-rich areas, coastal

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 varia-

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

Local groundwater is in many places not sufficient to recharge the aquifers. There are many aquifers in semiarid areas that are overused. Examples are common in S and SE Asia. Megacities with groundwater supply have in several places salinity problems resulting from land subsidence like in Bangkok and Jakarta [3]. In view of the climate change foreseen, there will be increase in sea levels that will affect the balance between groundwater and seawater. Ericson et al. [4] have evaluated 40 deltas regarding the effective sea level rise (ESLR), which in their evaluation is a combined effect of sea level rise and subsidence of the deltas. The Bengal delta in Bangladesh and India is experiencing an ESLR rate as high as 25 mm/year, while the Mississippi delta in USA has a rate of around 10 mm/year. The sea level rise as such is in the order of 3.3 mm/year, and the rate is expected to increase in the coming decades [5]. Ericson et al. [4] estimate that by 2050, about 8.6 M people are at risk due to ESLR. Local conditions such as tectonic movements and land subsidence due to excessive groundwater extraction thus play a large role in the risk of saline intrusion into costal aquifers in coming years [3]. The shoreline displacements may have a variety of reasons such as neo-tectonic movements but also anthropogenic activities such as deforestation and increased erosion [6]. Fog water condensation is a rare mechanism of groundwater recharge, but it may play a limited role under specific conditions. An example is the Al-Qara Mountain behind the Salalah Plain in Oman [7]. The browsing of a large population of camels has caused the degradation of mountain forest and decrease in recharge. The local precipitation is 100 mm, and the fog

tions in sea level and locally, there have been considerable tectonic movements.

aquifers are subject to a considerable stress.

78 Aquifers - Matrix and Fluids

examples from other regions such as Europe.

Groundwater recharge can occur in different ways:

• being a mixture of intruded saltwater and freshwater

collection now adds just 10 mm to the recharge.

**2. Groundwater recharge**

• recharged locally by rainwater

• recharged from coastal plain rivers

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 aquifer.

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. These water types range from a typical fresh water of Ca-HCO3 type via a Na-HCO3 type to a brackish mixed type of water. The Na-HCO3 type is formed by ion exchange, the Na-saturated 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 are found [10, 11].

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

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

**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 recharge in the same direction.

**Figure 2.** Residual pore water chloride in a clay layer dating back from a previous saline period was found in the Kerala coastal aquifers. The pore water was extracted from the core in the center of the clay layer.

The groundwater in Mati coastal plain in the northern Albania serves as water source for about 400,000 people, and there was a concern that this pumping rate may cause salt water intrusion [12]. In the catchment, there are several abandoned and active copper mines and the sulfate in river has an isotopic ratio typical of sulfides. This could be traced over large areas of plain showing that the recharge from the river is good. Close to the seaside, there were elevated chloride and sulfate levels that could be of old sea water origin. The δ34S ratio was 24‰, thus well over the sea water ratio at 21‰. This can be explained by sulfate reduction in the intercalated clay layers (**Figures 4** and **5**). Thus, sea water intrusion is not a current threat.

**Figure 4.** Cross section of the Mati Plain aquifer in Albania with δ34S ratios in groundwater [12]. In the plain, the δ34S ratios are mirroring recharge from river Mati, which has a δ<sup>34</sup>S ratio mirroring sulfide oxidation in mining areas in the catchment. The left well, close to the seaside, has a δ34S ratio above the sea water, likely to be caused by sulfate reduction in clay layers.

**Table 1.** δD and δ18O in groundwater in Mati Plain in Albania have no relationship with chloride concentration showing

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

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

81

**Sample δD (‰) δ<sup>18</sup>O (‰) Chloride (mg/l)**

 −44.20 −7.46 4.9 −43.85 −7.14 10.0 −44.52 −7.36 146 −44.83 −7.83 995 −48.12 −7.04 76 −47.12 −7.75 60.1 −43.10 −9.30 1220 −46.15 −8.35 392 −44.82 −8.22 819 −44.32 −8.46 191 SMOW 0.00 0.00 19,345

that sea water intrusion is not the source of chloride.

**Figure 3.** δ<sup>18</sup>O ratios in groundwater plotted versus chloride. Data from Kerala coastal aquifers. There is no mixing in of sea water, but the increase of chloride above about 80–100 mg/l is likely to come from pore water in clay layers as is seen in **Figure 2**.

Recharge and Turnover of Groundwater in Coastal Aquifers with Emphasis on Hydrochemistry… http://dx.doi.org/10.5772/intechopen.73301 81


**Table 1.** δD and δ18O in groundwater in Mati Plain in Albania have no relationship with chloride concentration showing that sea water intrusion is not the source of chloride.

The groundwater in Mati coastal plain in the northern Albania serves as water source for about 400,000 people, and there was a concern that this pumping rate may cause salt water intrusion [12]. In the catchment, there are several abandoned and active copper mines and the sulfate in river has an isotopic ratio typical of sulfides. This could be traced over large areas of plain showing that the recharge from the river is good. Close to the seaside, there were elevated chloride and sulfate levels that could be of old sea water origin. The δ34S ratio was 24‰, thus well over the sea water ratio at 21‰. This can be explained by sulfate reduction in the intercalated clay layers (**Figures 4** and **5**). Thus, sea water intrusion is not a current threat.

**Figure 4.** Cross section of the Mati Plain aquifer in Albania with δ34S ratios in groundwater [12]. In the plain, the δ34S ratios are mirroring recharge from river Mati, which has a δ<sup>34</sup>S ratio mirroring sulfide oxidation in mining areas in the catchment. The left well, close to the seaside, has a δ34S ratio above the sea water, likely to be caused by sulfate reduction in clay layers.

**Figure 3.** δ<sup>18</sup>O ratios in groundwater plotted versus chloride. Data from Kerala coastal aquifers. There is no mixing in of sea water, but the increase of chloride above about 80–100 mg/l is likely to come from pore water in clay layers as is

**Figure 2.** Residual pore water chloride in a clay layer dating back from a previous saline period was found in the Kerala

coastal aquifers. The pore water was extracted from the core in the center of the clay layer.

seen in **Figure 2**.

80 Aquifers - Matrix and Fluids

**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 copper mine waste in the catchment.
