**6. Conclusions and summary**

Coastal aquifers and their recharge is a crucial issue for approximately 1 billion people who are living on coastal plains globally. Due to high population density and high water demand for domestic and more so for food production, there is a high stress on the water resources. There are modeling tools that can be used to assess the sustainability of the groundwater in coastal aquifers, but in addition, hydrochemistry and isotopes may be a substantial help in the assessment, especially in costal aquifers where a possible sea water intrusion adds to the complexity. Dating, mostly by 14C methodologies gives reliable results adding to the assessments of the groundwater turnover [42]. For aquifers that that have a past memory of salinity, there may be a good relationship between age and chloride levels, chloride which is retained in parts of the aquifer where groundwater turnover is slower (**Figure 8**). Chloride in this case, as is shown in **Figure 2**, is not derived from salt water intrusion. For shorter turnover rates, tritium is used especially after the bomb tests in the 1960s has faded out [43].

Elevated salt content in coastal aquifers could have many sources in addition to salt water intrusion. A common case is found in coastal aquifers that consist of sand and gravel intercalated with clay layers. As the groundwater turnover is slow, often in the range of thousands of years, there might still be more or less saline pore water in the clay layers that continue to diffuse into the coarser material (**Figure 8**). In some, there is sulfate reduction in the clay due to elevated organic matter content and this is mirrored as elevated δ34S ratios.

The often intricate mixtures of water and dissolved salt in coastal aquifers could be interpreted by suitable isotopes such as δD, δ<sup>18</sup>O, δ<sup>34</sup>S, if different sources of the isotope ratios can be traced back to the source of the water. A good example is δ34S on a coastal aquifer in Albania where there were two main sources, sulfide-related sulfate from mines and sulfate in the groundwater formed by sulfate reduction in intercalated clay layers in the aquifer. A common feature often formed by flushing in postglacial time of formerly saline aquifers is the Recharge and Turnover of Groundwater in Coastal Aquifers with Emphasis on Hydrochemistry… http://dx.doi.org/10.5772/intechopen.73301 85

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

appearance of specific water such as NaHCO<sup>3</sup> formed by ion exchange. At the end of the flushing 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.
