Developing a robust geochemical and reactive transport model to evaluate possible sources of arsenic at the CO2 sequestration natural analog site in Chimayo, New Mexico

Abstract

a b s t r a c tMigration of carbon dioxide (CO 2 ) from deep storage formations into shallow drinking water aquifers is a possible system failure related to geologic CO 2 sequestration. A CO 2 leak may cause mineral precipitation/dissolution reactions, changes in aqueous speciation, and alteration of pH and redox conditions leading to potential increases of trace metal concentrations above EPA National Primary Drinking Water Standards. In this study, the Chimayo site (NM) was examined for site-specific impacts of shallow groundwater interacting with CO 2 from deep storage formations. Major ion and trace element chemistry for the site have been previously studied. This work focuses on arsenic (As), which is regulated by the EPA under the Safe Drinking Water Act and for which some wells in the Chimayo area have concentrations higher than the maximum contaminant level (MCL). Statistical analysis of the existing Chimayo groundwater data indicates that As is strongly correlated with trace metals U and Pb indicating that their source may be from the same deep subsurface water. Batch experiments and materials characterization, such as: X-ray diffraction (XRD), scanning electron microscopy (SEM), and synchrotron micro X-ray fluorescence (-XRF), were used to identify As association with Fe-rich phases, such as clays or oxides, in the Chimayo sediments as the major factor controlling As fate in the subsurface. Batch laboratory experiments with Chimayo sediments and groundwater show that pH decreases as CO 2 is introduced into the system and buffered by calcite. The introduction of CO 2 causes an immediate increase in As solution concentration, which then decreases over time. A geochemical model was developed to simulate these batch experiments and successfully predicted the pH drop once CO 2 was introduced into the experiment. In the model, sorption of As to illite, kaolinite and smectite through surface complexation proved to be the key reactions in simulating the drop in As concentration as a function of time in the batch experiments. Based on modeling, kaolinite precipitation is anticipated to occur during the experiment, which allows for additional sorption sites to form with time resulting in the slow decrease in As concentration. This mechanism can be viewed as trace metal "scavenging" due to sorption caused secondary mineral precipitation. Since deep geologic transport of these trace metals to the shallow subsurface by brine or CO 2 intrusion is critical to assessing environmental impacts, the effective retardation of trace metal transport is an important parameter to estimate and it is dependent on multiple coupled reactions. At the field scale, As mobility is retarded due to the influence of sorption reactions, which can affect environmental performance assessment studies of a sequestration site.

Zn(II), Mn(II) and Sr(II) Behavior in a Natural Carbonate Reservoir System. Part II: Impact of Geological CO2Storage Conditions

“…The increase of TE concentration in solution is evidenced, and sorption does not balance this release in solution in most cases, despite an available SSA that is much higher than the SSA that will be available in a real storage site. Even in a real storage context, the dissolution is supposed to be lower than what we observe in those experiments, due to aquifer flow, non negligible amounts are thought to be mobilized (Viswanathan et al, 2012;Zheng et al, 2009;Wigand et al, 2008) when one considers the huge volume of injected CO 2 that is envisaged (Lu et al, 2012;Rempel et al, 2011). To fill this gap in our knowledge, more experimental studies must be carried out to understand more the real behavior of those TE in CO 2 storage conditions.…”

“…Another study (Viswanathan et al, 2012) points out that in the specific context of their study, As is mobilized in the aqueous phase and that, despite sorption and/or reprecipitation, an amount of this As remains in solution. What we observe in the experiments presented here demonstrates that this mobilization effect also exists for elements such as zinc, manganese and Mean diameter ( m) μ Figure 7 St-Emilion carbonate mercury injection data comparison for initial carbonate, and reacted powder at 30, 60, and 90 bar of CO 2 .…”

“…The modifications of the petrophysical properties evidence a probable increase in the transport properties of the formation. But to complete the study of CO 2 impacted zones by a leakage, as discussed in other papers Little and Jackson, 2010;Viswanathan et al, 2012), the subsurface environment and shallow aquifers are also of importance.…”

CO2 Leakage-Induced Contamination in Shallow Potable Aquifer and Associated Health Risk Assessment

“…Mobilization of arsenic in a shallow groundwater aquifer due to CO 2 leakage has been investigated at one of the natural analog sites, Chimayó, New Mexico, where CO 2 -saturated brackish water was leaked into the shallow aquifer along the fault [7,13,24,25]. At this site, decreased pH and the resulting mobilization of trace metals, including arsenic, were observed.…”

“…Even if their adverse effects had been alleviated due to the high buffering capacity of the local groundwater aquifer, Keating et al [7] reported significantly elevated trace metal concentrations at a number of local wells due to the influx of brackish waters. Later, both Keating et al [13] and Viswanathan et al [24] integrated the field dataset into a multiphase reactive transport model to understand the behavior of arsenic, since some wells in Chimayó exceeded the maximum contamination level (MCL). In addition to studies targeting natural CO 2 release sites, several experiments have been conducted at field-scale CO 2 injection sites to determine secondary contamination caused by the injected CO 2 [11,[26][27][28].…”