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 (14CDIC) 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 14CDIC contents helps practitioners recognize and minimize 14C age uncertainty, especially for groundwater ages of <1000 and >30,000 years B.P.