Delineation of the coastal aquifer of Israel based on the analysis of carbon-14 and tritium isotopes

Twenty-nine wells were sampled along the Coastal Aquifer of Israel for major ions, 14C, tritium, and 13C in the 1970s and again in the 1990s. It is shown that double sampling can more accurately determine the age of the groundwater and its recharge sources. The spatial variation in radiocarbon concentration in the coastal aquifer exhibits lower concentrations in the western section of the aquifer (55–70 percent modern carbon or PMC) as compared to the eastern section (80–100 PMC). These results can be explained by the existence of two systems within the same coastal aquifer, whereby the eastern section of the aquifer is a more open system and the western section is a more closed system.; Based on the presence of detectable levels of tritium throughout most of the aquifer, the groundwater in the coastal aquifer was determined to contain a young component with a transit time of less than forty years. The 14C results of most groundwater were found to be lower than 100 PMC, probably due to water-rock interactions rather than decay due to old age. However, it is possible that some older waters exist in the lower sub-aquifers of the deeper wells (120–140m) where both lower 14C values and low levels of tritium were found. It is also possible that in some parts of the aquifer there is a mixture of several sources of water, some of which are young (containing tritium), and others which are older (having a relatively low 14C concentration).; Analysis of the tritium values from the two sampling periods using the tritium bomb spike curve allowed a better age estimation, overcoming the complication of tritium data in which the same value could yield two ages. Therefore, repetition of sampling over an extended time interval was determined to be effective in assisting in calculating groundwater ages through both the use of the tritium curve and analysis of the chemical and isotopic data over the twenty-year period. The correct age could be used to determine the real dilution factor (Q) in the aquifer and thus be utilized for age determination of older samples in the aquifer.