Principles and uncertainties of ¹⁴C age estimations for groundwater transport and resource evaluation

Radiocarbon (¹⁴C) is useful for estimating groundwater ages for transport and water resource exploitation assessment. If the ¹⁴C content of dissolved inorganic carbon (¹⁴C_DIC) is known, the age of groundwater can be estimated by applying a radiocarbon decay equation combined with an appropriate geochemical correction model. However, age determinations are subject to uncertainties caused by parameters which need to be estimated or assumed. Here, we discuss the principles of ¹⁴C-based groundwater age estimations and the corrections and errors that affect age determinations differently. Generally, the two factors that impact the results of ¹⁴C groundwater age are Type-1 and Type-2 errors. Type-1 errors are pulse-type changes on derived groundwater ages that are independent of the water age. Type-2 errors cause gradual changes on derived groundwater ¹⁴C ages that depend on the water age. The cumulative impact of these errors substantively reduces the accuracy and confidence of ¹⁴C age determinations and corrections. When using ¹⁴C for groundwater age, consideration of both error types along with the use of samples having a range of ¹⁴C_DIC contents helps practitioners recognize and minimize ¹⁴C age uncertainty, especially for groundwater ages of <1000 and >30,000 years B.P.