Evidence for Dedolomitization and Mixing in Paleozoic Carbonates Near Oak Ridge, Tennessee

Abstract

Multiport ground‐water sampling systems in five deep core holes near DOE's Y‐12 facility at Oak Ridge, Tennessee provide data on the three‐dimensional distribution of ground‐water types and chemical evolution processes. Interpretation of ground‐water chemistry, coupled with data on the primary and authigenic mineralogy from drill core, indicates the ground water evolves chemically by three main processes: (1) open system dissolution of calcite and dolomite at shallow depths to produce Ca‐Mg‐HCO3 waters; (2) irreversible dissolution of gypsum, which causes dedolomitization and leads to the formation of Ca‐Mg‐SO4 waters at deeper levels; and (3) mixing with deep brines to form saline Na‐Ca‐Cl waters. Evidence for dedolomitization included the precipitation of authigenic calcite in gypsum dissolution cavities, decrease in ground‐water pH, and increases in dissolved Ca, Mg, and SO4. In this study, we document the dedolomitization process along a relatively short (< 2000 ft) flow path in low permeability Paleozoic carbonates. Mixing of the shallower ground water with an Na‐Ca‐Cl brine thought to be present at depth can account for the progressive increase in salinity of ground water with depth and the precipitation of authigenic barite and celestite. The mixing process could be a combination of diffusion from below, diffusion from older matrix water into fractures, or advective mixing.

Sensitivity of groundwater systems to CO2: Application of a site-specific analysis of carbonate monitoring parameters at the SACROC CO2-enhanced oil field

“…This well-documented process (also known as incongruent dissolution) is widespread, occurring in aquifers around the world including in China (Ma et al, 2011), the Middle East (El-Naqa et al, 2007;Matter et al, 2005), the United States (Raines and Dewers, 1997;Saunders and Toran, 1994;Deike, 1990), Canada (Stotler et al, 2011;Fu et al, 2008), the Mediterranean (Lorite-Herrera et al, 2008;Capaccioni et al, 2001), and Europe (Zeeh et al, 2000). Dedolomitization occurs under a variety of sustained and episodic environmental conditions that produce mixing of dilute and concentrated Ca-rich waters in the presence of calcite and dolomite (Appelo and Postma, 2007;Back and Baedecker, 1989;Back et al, 1979Back et al, , 1983Bischoff et al, 1994;Hanshaw and Back, 1985;Pacheco and Szocs, 2006;Plummer and Back, 1980).…”

Geochemical study of Central-Eastern Lombard Prealps mineral and thermal springs

“…La sottosaturazione rispetto alla dolomite ed i contenuti di Mg 2+ superiori a quelli previsti dalla sua semplice dissoluzione (le concentrazioni massime di Mg 2+ attese per acque in equilibrio con dolomite sono di circa 45 mg/l come da figura 8 mentre in queste acque superano 100 mg/l) possono indicare l'occorrenza del processo di dedolomitizzazione che può avvenire in profondità nella formazione evaporitica della Carniola di Bovegno. Tale processo, descritto da vari autori (Plummer et al 1990;Saunders and Toran 1994;Appelo and Postma 1999;Capaccioni and Didero 2001;Eberts and George 2000;Pacheco and Szocsb 2006), produce acque a composizione solfato calcico-magnesiaca e può essere schematizzato dalla seguente reazione, nella quale si assume la saturazione rispetto a calcite, dolomite e gesso: La reazione implica che l'aggiunta degli ioni Ca 2+ derivanti dalla dissoluzione di gesso ad un'acqua in equilibrio con calcite e dolomite determina la precipitazione di calcite, di conseguenza la concentrazione di CO 3 diminuisce causando dissoluzione di dolomite con aumento della concentrazione di Mg 2+ assieme a Ca 2+ secondo il rapporto di equilibrio tra calcite e dolomite (Ca/Mg = 1.25 a 25°C). Il risultato della reazione è quindi la trasformazione della dolomite in calcite nella roccia costituente l'acquifero con consumo di HCO 3 -, rilascio di ioni Ca 2+ , Mg 2+ e SO 4 2in soluzione e precipitazione di calcite (Appelo and Postma 1999).…”

Modeling of radionuclide and heavy metal sorption around low and high pH waste disposal sites at Oak Ridge, Tennessee: Classification review package

“…The shallow groundwater adjacent to these waste areas is typically a Ca-HC03 or Mg-Ca-HC03 type due to dissolution of variable amounts of calcite and dolomite (e.g., Haase, 1991;Saunders and Toran;1994). This zone is underlain by Na-HC03-type water at depths ranging from 20 to 50 m (Solomon et al, 1992, Saunders andToran, 1994;Toran and Saunders, submitted).…”

“…This zone is underlain by Na-HC03-type water at depths ranging from 20 to 50 m (Solomon et al, 1992, Saunders andToran, 1994;Toran and Saunders, submitted).…”