“Dedolomitization reactions” driven by anthropogenic activity on loessy sediments, SW Hungary

Abstract

In the Szigetvár area, SW Hungary, shallow groundwaters draining upper Pleistocene loess and Holocene sediments are considerably contaminated by domestic effluents and leachates of farmland fertilizers. The loess contains calcite and dolomite, but gypsum was not recognized in these sediments. The anthropogenic inputs contain significant amounts of calcium and sulfate. The calcium from these anthropogenic inputs is promoting calcite growth, with concomitant consumption of carbonate alkalinity, undersaturation of the system with respect to dolomite, and dolomite dissolution; in brief, is driving "dedolomitization reactions". Geochemical arguments supporting the occurrence of "dedolomitization reactions" in the area are provided by the results of mass balance and thermodynamic analyses. The mass balances predicted the weather sequence dolomite > calcite > plagioclase > K-feldspar, at odds with widely accepted sequences of weatherability where calcite is the first mineral in the row. The exchange between calcite and dolomite can be a side effect of "dedolomitization reactions" because they cause precipitation of calcite. The thermodynamic prerequisites for "dedolomitization reactions" are satisfied by most local groundwaters (70%) since they are supersaturated (or in equilibrium) with respect to calcite, undersaturated (or in equilibrium) with respect to dolomite, and undersaturated with respect to gypsum. The calcium vs. sulfate and magnesium vs. sulfate trends are also compatible with homologous trends resulting from "dedolomitization reactions".

Research Project on CO2 Geological Storage and Groundwater Resources: Water Quality Effects Caused by CO2 Intrusion into Shallow Groundwater

“…Many dolomitic limestones are sources of drinking water in the eastern United States, and it is therefore of interest to examine the state of saturation of all groundwaters with respect to this carbonate. Figure 3.14 shows that almost SI(dolomite) peaks at significantly less than zero, which is consistent with reported occurrences of dedolomitization in shallow groundwaters (Edmunds et al, 1982;Back et al, 1983;Plummer et al, 1990;James et al, 1993;Aravena et al, 1995;sacks and Tihansky, 1996;Hopkins and Putnam, 2000;Smedley and Edmunds, 2002;Pacheco and Szocs, 2006). Therefore, it is to be expected that groundwaters will not come into thermodynamic equilibrium with those representative host-rock mineralogies containing dolomite, i.e., St Peter Sandstone and Dolomitic Limestone (Section 3.6.3) in TOUGHREACT simulations described in Section 4 of this report, unless complete dedolomitization occurs during the groundwater initialization step.…”

Groundwater salinization processes and reversibility of seawater intrusion in coastal carbonate aquifers

“…The dissolution of even very small amounts of gypsum may cause this process to occur in carbonate aquifers, usually characterized by near saturation in calcite (Plummer et al, 1990;López-Chicano et al, 2001;Moral et al, 2008;Szynkiewicz et al, 2012). Infiltration of meteoric waters or irrigation return flow contaminated by domestic effluents and fertilizers rich in Ca could be promoting the dedolomitization process (Pacheco and Szocs, 2006). Irrigation water forms a source of nutrients to the groundwater.…”

Groundwater Chemistry Changes as a Result of CO2 Injection at the ZERT Field Site in Bozeman, Montana

“…Table 5.5 also lists the saturation indices of some other minerals of passing interest. Dolomite is strongly undersaturated, as is characteristic of shallow groundwaters (Edmunds et al, 1982;Back et al, 1983;Hopkins and Putnam, 2000;James et al, 1993;Pacheco and Szocs, 2006), and therefore dolomite, if present, would tend to dissolve. As mentioned in Section 4, individual grains of detrital dolomite and occasional fragments of dolomitic limestone were identified in the coarse fractions of sediments from the ZERT site.…”