Reliability of recharge rates estimated from groundwater age with a simplified analytical approach

Abstract

Groundwater age is often used to estimate groundwater recharge through a simplified analytical approach. This estimated recharge is thought to be representative of the mean recharge between the point of entry and the sampling point. However, given the complexity in actual recharge, whether the mean recharge is reasonable is still unclear. This study examined the validity of the method to estimate long-term average groundwater recharge and the possibility of obtaining reasonable spatial recharge pattern. We first validated our model in producing reasonable age distributions using a constant flux boundary condition. We then generated different flow fields and age patterns by using various spatially varying flux boundary conditions with different magnitudes and wavelengths. Groundwater recharge was estimated and analysed afterwards using the method at the spatial scale. We illustrated the main findings with a field example in the end. Our results suggest that we can estimate long-term average groundwater recharge with 10% error in many parts of an aquifer. The size of these areas decreases with the increase in both the amplitude

Estimation of Groundwater and Spring Water Residence Times near the Coast of Fukushima, Japan

“…However, the calculation of age dates and residence time are obscured due to mixing of old and young water (Weissmann et al 2002). Various methods can be used to estimate the residence time of groundwater, including groundwater modeling, examination of soil physical conditions, and radioactive isotope dating (Chesnaux and Allen 2007; Post et al 2013; Chesnaux et al 2018; Dong et al 2021). Of these, radioisotope dating using 3 H, 14 C, or 36 Cl is one of the most effective techniques (Clark and Fritz 1997).…”

“…The Dupuit‐Forchheimer model, which is built upon it to develop the simplified forms for both the confined and unconfined aquifers flow, and is usable for studying groundwater flow into wells (Chesnaux et al 2018; Okuyade et al 2022). The Vogel method is simplified analytical model introduced by Vogel (1967), and it is available for estimating the groundwater recharge with age tracers (Dong et al 2021). Post et al (2013) used the three different approaches to compute the groundwater age in coastal aquifer, and which are referred to as the piston flow model (based on particle tracking), the direct age (based on the concept of age mass), and the tracer‐based age (based on a simulation of either 3 H and 3 He, or 14 C decay).…”