Aquifer Vulnerability Assessment Based on Sequence Stratigraphic and ³⁹Ar Transport Modeling

Abstract

A large-scale groundwater flow and transport model is developed for a deep-seated (100 to 300 m below ground surface) sedimentary aquifer system. The model is based on a three-dimensional (3D) hydrostratigraphic model, building on a sequence stratigraphic approach. The flow model is calibrated against observations of hydraulic head and stream discharge while the credibility of the transport model is evaluated against measurements of (39)Ar from deep wells using alternative parameterizations of dispersivity and effective porosity. The directly simulated 3D mean age distributions and vertical fluxes are used to visualize the two-dimensional (2D)/3D age and flux distribution along transects and at the top plane of individual aquifers. The simulation results are used to assess the vulnerability of the aquifer system that generally has been assumed to be protected by thick overlaying clayey units and therefore proposed as future reservoirs for drinking water supply. The results indicate that on a regional scale these deep-seated aquifers are not as protected from modern surface water contamination as expected because significant leakage to the deeper aquifers occurs. The complex distribution of local and intermediate groundwater flow systems controlled by the distribution of the river network as well as the topographical variation (Tóth 1963) provides the possibility for modern water to be found in even the deepest aquifers.

Estimation of effective porosity in large-scale groundwater models by combining particle tracking, auto-calibration and <sup>14</sup>C dating

“…The results of our study differ significantly from findings by Sonnenborg et al (2016) who investigated a regional aquifer 15 system with a similar geological setting located a few hundred kilometers north of the present study area. Their direct simulation of groundwater ages shows a pattern of much younger water than here, rarely exceeding 700 years even in the deepest aquifers, while in our study ages exceeding 10000 years occur.…”

“…Sonnenborg et al, 2016). The seven-porosities model is able to match all the observations reasonably.…”

“…Other regional-scale studies (e.g. Sonnenborg et al, 2016) dominates at this scale), a macrodispersivity of 0 m was used in the following simulations of direct age.…”

“…The number and position of the released particles contribute to the uncertainty especially in heterogeneous systems as pointed out by Konikow Sonnenborg et al, 2016). The reliability of the estimated effective porosities was assessed through the identifiability that depends on the observation density (see section 4.2) and is high for four out of the seven estimated porosities.…”

“…Manning et al 2005;Bethke and Johnson, 2008;Molson and Frind, 2012;Sonnenborg et al, 2016) and indicate the advance of modern contaminated groundwater (Hinsby et al 2001a;Gleeson et al 2015;Jasechko et al 2017) and groundwater quality in general (Hinsby et al 2007), and (iv) to contribute to the understanding of the flow system, e.g. in complex geological settings (Troldborg et al 2008;Eberts et al 2012).…”