Delineation of the coastal aquifer of Israel based on repetitive analysis of ¹⁴C and tritium

Repetitive sampling for radioactive isotope analysis can be used to more accurately estimate the age of groundwater. Groundwater was sampled in wells along the Coastal Aquifer of Israel for tritium, ¹⁴C, and ¹³C analysis in the 1970s as well as the 1990s. Examination of the tritium values from two sampling periods on the tritium bomb curve enabled us to overcome the ambiguity of tritium data, in which a single value could yield two ages, and allowed a better age estimate. Therefore, repetitive sampling over an extended time interval was found to be effective in assisting calculations of groundwater ages through the use of the tritium curve over the 20-year period. The age, obtained by repetitive sampling can be used to determine the dilution factor (Q) of the initial ¹⁴C value in the aquifer. This corrected initial value can, thus, be utilized for better age determination of older samples in the deeper sub-aquifers. In most cases, 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. However, the ¹⁴C levels of most of the same groundwater samples were found to be lower than atmospheric levels (34-102 vs. >100 pMC), probably due to water-rock interactions rather than decay due to old age. Some older waters exist in the lower sub-aquifers of the deeper wells (120-140 m) where both lower ¹⁴C values (42-69 pMC) and very low levels of tritium were found (0.1-0.7 TU).