Radiocarbon age dating and quality evolution of seawater intruding beneath a freshwater lens

Radiocarbon has been used in a number of studies to establish the age of intruded seawater, yet there appears to be a paucity of investigations that has attempted to quantify the processes that influence radiocarbon along the flow path of actively intruding seawater. The present study contributes to filling this research gap using radiocarbon and hydrochemical data of intruding North Sea water along two shore-perpendicular transects across the coastal dune belt of the western Netherlands. The objective is to establish how well radiocarbon can be used to determine the intruding seawater's age, considering the corrections that are required to account for geochemical reactions and mixing.

The effect of geochemical reactions was quantified for each of the 18 samples of intruded seawater based on a chemical mass balance calculated with a new Excel based code (R + SWi). It considers 20 quality parameters (including δ¹³C_DIC and ¹⁴C_DIC) and 15 reactions and utilizes Excel's Solver routine to calibrate the model parameters. The reactions along the flow path are initially dominated by O₂ and NO₃ reduction by sediment organic matter in seafloor sediment, with little CaCO₃ dissolution. Next, during passage of a Holocene tidal aquitard, Fe(OH)₃ and some SO₄ are reduced, pyrite and CaCO₃ precipitate, and opaline SiO₂ dissolves. In deeper, Pleistocene aquitards and further downgradient in aquifers, cations are exchanged, some CaCO₃ precipitates due to Ca exchange, and siderite dissolves.

Correcting radiocarbon concentrations for the calculated sedimentary carbon contributions yields ages between 0.37 and 6 ka. Sensitivity analysis reveals that the corrected ¹⁴C age is most sensitive to the assumed ¹⁴C activity of the carbonate and organic matter of the Holocene seabed sediments. The intruded seawater's age appears to be bimodal: Old seawater (3–6 ka) intruded when the coastline was located much further to the east than at present. Younger seawater (<3 ka) started to intrude after the freshwater lens developed when the coastline had reached its present-day position. Groundwater extraction and especially the reclamation of the Haarlemmermeer lake accelerated intrusion rates massively.

The results of a 3.5 ky numerical model simulation of freshwater lens formation and seawater intrusion are consistent with the radiocarbon age pattern inferred from the samples. The spatial distribution of seawater ages indicates a higher vulnerability to salinization in the northern part of the study area, highlighting the added value of radiocarbon data of saline groundwater for water resource management purposes.