The interrelation between the sea and coastal aquifer deduced from analyses of radioactive isotopes

The present study used radioactive isotopes in order to get a direct estimate of the rate of seawater intrusion into the coastal aquifer of Israel. These isotopes were used also to determine the connection among the different sub-aquifers, and between them and the sea, by dating both fresh and saline groundwater. Groundwaters from the vicinity of the shoreline were analyzed for their 14C and tritium content, together with their chemical and stable isotope composition. The results indicate that there are several distinct sub-aquifers in the coastal aquifer near the shoreline, differing in their heads, water chemistry and ages. Stable isotopes were used in order to correct the ages for water-rock interaction. For example, oxidation of old organic matter in the aquifer results in relatively negative δ13C values of DIC (∼ -9 per mille, compare to 0 per mille in seawater), which should be taken into account for age estimation. The age of the intruding seawater is expected to be younger in the upper sub-aquifers, due to the rise of sea level since the last glacial period. Indeed, the saline groundwaters (no tritium and 14C <12 pMC) in the lower sub-aquifers are older than ca 10,000 years, while in the upper sub-aquifers they are younger than 50 years (contain tritium), indicating recent seawater intrusion. In most cases, groundwaters in the lower subaquifers are found to be fresh and old (older than ca 10,000 years - no tritium and 14C of <5 pMC). This implies that the flow to the sea in these sub-aquifers is blocked or restricted. (author)