Conjoint use of hydraulic head and groundwater age data to detect hydrogeologic barriers

Abstract

Hydraulic head and groundwater age data are effective in building understanding of groundwater systems. Yet their joint role in detecting and characterising low-permeability geological structures, i.e. hydrogeologic barriers such as faults and dykes, has not been widely studied. Here, numerical flow and transport models, using MODFLOW-NWT and MT3D-USGS, were developed with different hydrogeologic barrier configurations in a hypothetical aquifer. Computed hydraulic head and groundwater age distributions were compared to those without a barrier. The conjoint use of these datasets helps in detecting vertically-oriented barriers. Two forms of recharge were compared: (1) applied across the entire aquifer surface (uniform), and (2) applied to the upstream part of the aquifer (upgradient). The hydraulic head distribution is significantly impacted by a barrier that penetrates the aquifer's full vertical thickness. This barrier also perturbs the groundwater age distribution when upgradient recharge prevails; however, with uniform recharge, groundwater age is not successful in detecting the barrier. When a barrier is buried, such as by younger sediment, hydraulic head data also do not clearly identify the barrier. Groundwater age data could, on the other hand, prove to be useful if sampled at depth-specific intervals. These results are important for the detection and characterisation of hydrogeologic barriers, which may play a significant role in the compartmentalisation of groundwater flow, spring dynamics, and drawdown and recovery associated with groundwater extraction.

An Approach to Include Hydrogeologic Barriers With Unknown Geometric Properties in Groundwater Model Inversions

“…Groundwater age increases gradually in the direction of groundwater flow except downstream of the barrier, where a tail of older groundwater extends from the barrier toward the downstream end of the aquifer (Figure 2b), these results are similar to those observed in Marshall et al. (2020).…”

“…First, the real model was controlled with fixed recharge and fixed aquifer hydraulic conductivity in areas without barriers. We know that recharge can influence hydraulic head and groundwater age distributions across barriers, as can the geometry of the barrier itself (Marshall et al., 2020). Future studies could include additional parameters in the model inversion, such as heterogeneous aquifer hydraulic conductivity, recharge, and more complex barrier geometries.…”

“…Results of running the model with parameters listed in Table 1 show that hydraulic heads gradually decrease in the direction of groundwater flow except across the barrier, where it sharply declines (Figure 2a). Groundwater age increases gradually in the direction of groundwater flow except downstream of the barrier, where a tail of older groundwater extends from the barrier toward the downstream end of the aquifer (Figure 2b), these results are similar to those observed in Marshall et al (2020).…”

“…These structures may also form conduits to groundwater flow or complex conduit‐barrier systems (Caine et al., 1996). Geologic structures that act as hydrogeologic barriers can form important controls on groundwater flow (Bense et al., 2003; Cilona et al., 2015), chemistry (Gumm et al., 2016; Mayer et al., 2007), groundwater age (Castro & Goblet, 2005; Marshall et al., 2020), discharge processes (Bense & Kooi, 2004; Gleeson & Novakowski, 2009), and drawdown (Hadley et al., 2019; Marshall et al., 2019). Locating and characterizing hydrogeologic barriers can be difficult and costly.…”