The onset of low-temperature alteration identified at the base of the aquifer is thought to control aquifer thickness, and may itself be controlled by thermal flux from below (Morse and McCurry 2002).
Initially, CO2 is transported down gradient in the permeable units at the base of the aquifer similar to case for the undeveloped aquifer (Figure 6).
Under the new conditions caused by so much pumping, the main factors determining the hydrochemical changes are the mixing of waters and the subsequent reactions of dissolution precipitation between (1) the recharge coming from rainwater, (2) the hypersaline inputs from the clay-evaporite aquitards situated on the edges and at the base of the aquifer, and (3) the water stored in each aquifer.
In the absence of pumping, lateral transport of CO2-rich water within the sandstone unit at the base of the aquifer is dominant over vertical CO2 transport towards the top of the aquifer.
Numerical simulation is used to examine the relative velocities of DNAPL and aqueous phase plumes in sandy aquifers where lateral spreading of DNAPL has occurred at the base of the aquifer.
This zone encompasses all processes taking place from base of the aquifer to the top of the canopy.
Had the monitoring well been screened at greater depth towards the base of the aquifer unit, then the pH perturbation would have been large enough to detect.
The main aquifer unit dips gently with a slope equal to 0.3% and is locally confined by low permeable clay layers at the top and base of the aquifer.
The base of the aquifer has been penetrated in eight deep wells on the INL site, with a depth ranging from 200 to 550 m based on temperature inflections (Smith 2004).
Figure 4 and Movie 1 (Additional file 1) provide an example of the development of a pH plume in response to a CO2 leak equal to 105 t/yr at the base of an aquifer unit.
This well, which was cored from 12 m to total depth, is the only deep well in the central SRP to penetrate the base of the aquifer (Shervais et al. 2013, 2014).
