14.2.2: Middendorf aquifer

As an example, we consider trends in geochemistry in the Middendorf coastal plain aquifer in South Carolina, USA. Groundwater recharges the aquifer where its sediment outcrops along the Atlantic Seaboard Fall Line, and from there flows southeast toward the Atlantic Ocean (Chapelle and Lovley, 1992). Along its flow, the groundwater passes through a series of redox zones that are consistent with the general trends described previously in Fig. \(14.1\) of Section 14.2.1, and in Chapter 8 (Section 8.5). Near the recharge area, dissolved oxygen is present in the groundwater (Fig. \(14.2\)) and aerobic microorganisms are interpreted to dominate. As groundwater flows away from the recharge area, oxygen concentration decreases and then the groundwater passes through a zone with relatively high ferrous iron concentrations and then a zone with low iron concentrations (Fig. \(14.2\)). These zones are interpreted to host iron- and sulfate-reducing microorganisms, with changes in the proportions of their activities causing the observed variation in iron levels (Park et al., 2006). Specifically, iron reduction appears to decrease relative to sulfate reduction with distance along flow.

A major factor that helps cause changes in the proportions of iron reduction and sulfate reduction along flow is variation in ferric iron availability. Sediment in the upgradient portion of the aquifer was deposited in an upper delta plain environment and its ferric iron content is greater than that for the sediment in the downgradient portion, which consists of lower delta plain and marine sediment (Chapelle and Lovley, 1992). Thus, the capacity of the aquifer to support iron-reducing microorganisms decreases along flow.

In addition, increasing groundwater pH along flow may contribute to shifts in the proportions of iron reduction and sulfate reduction. Along the nearly 100 km-long flowpath Park et al. (2006) sampled, groundwater pH increased from 4.9 to 8.5 (Fig. \(14.2\)). Microbial reduction of ferric iron in (oxhydr)oxides consumes a large number of hydrogen ions, and as a result, this increase in pH can sharply decrease the amount of energy available for iron reduction (see Section 8.3). In contrast, sulfate reduction consumes relatively few hydrogen ions and energy available for the reaction is much less sensitive to pH (Bethke et al., 2011; Postma and Jakobsen, 1996). Reflecting these relationships, previous studies have demonstrated that increasing pH can allow greater amounts of sulfate reduction relative to iron reduction in aquifers (Bethke et al., 2011; Kirk et al., 2016, 2013; Paper et al., 2021; Postma and Jakobsen, 1996).

In contrast to iron, concentrations of sulfate and sulfide are fairly stable throughout much of the aquifer (Fig. \(14.2\)). No sulfate sources are known to exist within the aquifer or recharge area. As such, previous researchers have suggested that sulfate is added to the groundwater as it flows through the aquifer from the adjacent confining layers or by cross-formational flow (Chapelle and Lovley, 1990). This sulfate input helps to prevent sulfate depletion from the groundwater even though sulfate reducers are active. Lastly, sulfide concentrations remain stable and relatively low despite sulfate-reducing activity likely because the sulfide that is produced reacts with ferrous iron to precipitate as mackinawite rather than accumulate in the groundwater (Park et al., 2006).

Changes in pH and major ion concentrations along the groundwater flowpath sampled by Park et al. (2006) are also consistent with the general trends described in Section 14.2.1. Near the recharge area, the groundwater is acidic because it contains a relatively high amount of carbon dioxide \((\sim 0.6 \ \text{mM})\), which is almost entirely added from soil respiration (Park et al., 2006). Nearly as much carbon dioxide is also added to the groundwater by subsurface organic matter degradation, particularly in the adjacent confining layers (Fig. \(14.1\)) (Park et al., 2009). Despite these additions of acid, pH increases along flow because iron reduction, sulfate reduction, and mineral reactions consume hydrogen ions at a greater rate than they are added. Minerals reactions expected to contribute to pH buffering in the aquifer included weathering of albite, anorthite, and illite (Park et al., 2009). Their dissolution helped increase pH and also the concentrations of sodium, calcium, potassium, magnesium, and silica.

Taken as a whole, these findings illustrate the strong influence of microorganisms on the composition of groundwater in the Middendorf aquifer. Microorganisms are major contributors to acid production in the recharge area and subsurface sediments. This acid drives mineral weathering reactions that increase pH and concentrations of major ions. Groundwater pH and concentrations of oxygen and ferrous iron also evolve along flow as microorganisms work their way along the thermodynamic ladder, pushing the groundwater into a more reduced state.

Figure \(14.2\): Variation along flow in groundwater pH and concentrations of oxygen \(\left(\text{O}_{2}\right)\), ferrous iron \(\left(\text{Fe}^{2+}\right)\), sulfate \(\left(\text{SO}_{4}^{2-}\right)\), and sulfide (S(-II)) in the Middendorf aquifer as measured by Park et al. (2006). Concentrations were below their detection limit for ferrous iron (0.2 µM) and sulfide (0.1 µM) for three and two samples, respectively. For those samples, detection limit concentrations are plotted. In addition, sulfide concentration was not determined for two samples, for which no scatter points are shown.
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