Carbon isotopes in pore water of the unsaturated zone and their relevance for initial 14C activity in groundwater in the coastal aquifer of Israel

Abstract

This paper presents a study of the isotopic composition of dissolved inorganic carbon (DIC), stable and radioactive, in pore water of the unsaturated zone (USZ) above the coastal aquifer of Israel. The carbon content and its isotopic composition in the gas and solid phases of the USZ are also presented. In the soil gas, large quantities of CO 2 with quite modern 14C activity were measured along the section (0.15 to 2.7% and 97 to 109 pMC respectively). In the inorganic fraction of the sediments, the 14C activity between 2.5 and 13 m was 33 to 1.5 pMC. In the organic fraction the 14C activity between 7 and 9 m was 40 to 33 pMC. In the pore water, the high values of tritium and 14C at depths of ~ 15–18 m have been attributed to the thermonuclear contamination of the 1960s. A significant decrease with depth in the DIC of pore water, from 23 to 4 mmol C/L, along with a decrease with depth in dissolved inorganic 14C, from 100 pMC near the surface to 72 pMC at depth of 20 m were observed. The δ 13C values in the DIC are similar to the values in the inorganic sediment (~ − 10‰). A first order reaction was applied to estimate the yearly rates of DIC loss by net precipitation (3.2%/year) and of 14C activity (dpm/L) loss from the DIC by gross precipitation from the DIC on the sediment, (4.4%/year). From the gradient of dissolved inorganic 14C, the initial level at the bottom of the USZ, which is the groundwater table of the coastal aquifer of Israel, was estimated to be 0.54 of contemporaneous atmospheric value of 14C when the rain fell on the ground. This value can be used for improved dating of groundwater with 14C in the coastal aquifer of Israel.