GROUNDWATER pH, Eh, AND FERROUS IRON    

Corrosion of Fe⁰ in groundwater follows two general pathways. In the presence of dissolved oxygen, Fe⁰ corrodes according to the reaction

Fe⁰  +  H₂O  +  O₂  à  Fe²⁺  +  2OH^-  .                                           (1)

However, under anaerobic conditions (e.g., oxygen consumed by the above reaction or by anaerobic microorganisms), Fe⁰ can react with water according to:

Fe⁰  +  2H₂O  à  Fe²⁺  +  H₂  +  2OH^-  .                                             (2)

Both of these reactions result in a decreased redox potential but an increased solution pH as 2 moles of OH^- are formed per mole of Fe⁰ oxidized.  As shown in Figure 1, the site groundwater pH (upgradient) is generally stabilized at ~6.5.  However, upon reaction with Fe⁰ in the barrier, groundwater pH within the Fe⁰ increased and stabilized at from ~7.5 up to ~10 under field conditions.  On the other hand, groundwater pH remained at ~6.5 in downgradient wells (except those in the Fe⁰ barrier, DP-23s,m), and corrosion of Fe⁰ in the barrier appeared to have little impact on the downgradient soil based on observed pH values. This observation may be attributed to the relatively high pH-buffering capacity of clay minerals and organic matter in downgradient soil. The groundwater pH in monitoring well from TMW-7 appeared to be somewhat high (up to ~9.5) because this well is situated in the gravel trench downgradient of the Fe⁰ barrier, that is low in pH-buffering capacity.

Groundwater redox potential (Eh), however, decreased dramatically in those monitoring wells both within and downgradient of the Fe⁰ barrier (Figure 2).  Although the site groundwater is generally aerobic, with Eh values mostly positive (over +200 mV) (Figure 4 upgradient), a generally low Eh was observed within the Fe⁰ barrier (<-200 mV).  This decrease in Eh may directly result from the consumption of dissolved O₂ and the production of dissolved H₂ as groundwater reacted with Fe⁰ in the reactive zone.  The Eh values were also in the negative range in most of the downgradient monitoring wells.  Note that a low Eh was also observed in the upgradient TMW-11 and DP-22s monitoring wells because the DP-22 well is also located within the Fe⁰ barrier and the TMW-11 is located adjacent to the Fe⁰ barrier.  The use of Guar gum could also have resulted in an increased anaerobic microbial activity (Gu et al., 2001) and thus may have contributed to a low redox potential in the TMW-11 monitoring well. 

Ferrous iron (Fe²⁺), one of the major byproducts of Fe⁰ corrosion in groundwater, may have a significant impact on water quality and cause clogging of soil porous structure as it oxidizes and precipitates out in the downgradient barrier or the soil.  In fact, discharge of Fe²⁺ to Bear Creek had been one of the major concerns initially regarding the implementability of Fe⁰ barriers at the site.  However, we found that Fe²⁺ concentrations in groundwater were relatively low after a few months of operation of the Fe⁰ barrier in the field (Figure 3).  Initially, the Fe²⁺ concentration was found to be extremely high (up to ~150 mg/L) in the center well (TMW-9) of the Fe⁰ barrier, and total iron concentration reached levels as high as ~700 mg/L (data not shown).  Relatively high Fe²⁺ concentrations were also observed in the pea gravel section adjacent to the Fe⁰ barrier for TMW-7 and TMW-11.  However, Fe²⁺ concentrations decreased rapidly over the first few months after the Fe⁰ barrier was installed (such as in the TMW-9 and TMW-7 wells).  Only a few monitoring wells within the Fe⁰ barrier showed a slightly high Fe²⁺ concentration but was <15 mg/L in general (e.g., DP-22s,m; DP-20s,m; DP-23s) (Figure 3).  As will be discussed below, Fe²⁺ ions may be precipitated as FeS , FeCO₃, or be further oxidized as Fe³⁺, which forms relatively insoluble iron oxyhydroxides in the Fe⁰ barrier.  These results suggest that Fe²⁺ discharge as a result of Fe⁰ corrosion is not always a significant concern at this site.  However, the rate of Fe⁰ corrosion and the production of Fe²⁺ will depend on both groundwater pH and constituent concentrations, such as nitrate, sulfate, and bicarbonate, which accelerate the corrosion process (Agrawal and Tratnyek, 1996; Agrawal et al., 1995; Gu et al., 1999) .  Depending on the removal rates via precipitation and sorption, high concentrations of ferrous ion may persist under certain conditions.

The high initial Fe²⁺ and total iron concentrations observed in groundwater (Figure 3) may be attributed in part to the following factors: (1) a rapid initial oxidation of Fe⁰ filings (particularly some fine iron particles) when they were emplaced into the groundwater; 2) the use of Guar gum and, subsequently, the addition of enzyme (used to break up the Guar gum), that resulted in a decreased groundwater pH in a short time period; and (3) increased microbial activity (or respiration), which may also contribute to an increased corrosion rate of Fe⁰.  

Groundwater at the barrier site is contaminated with relatively high levels of nitrate  (NO₃^-) at ~20–150 mg/L at the Pathway 2 site; in some deep monitoring wells or piezometers, levels >1000 mg/L NO₃^- were observed as a result of the migration of deep contaminated groundwater and the upward vertical hydraulic gradients at the site.  Within and in the vicinity of the Fe⁰ barrier, however, the NO₃^- concentration were low to non-detectable (Figure 4), suggesting that NO₃^- was effectively degraded as the groundwater passed through the Fe⁰ barrier.  The nitrate concentrations were also found to be low or non-detectable downgradient of the Fe⁰ barrier, including monitoring wells or piezometers TMW-7, DP-11, DP-14s, DP-15s, and many others that are not shown in the figure.  Even in some upgradient monitoring wells or piezometers (adjacent to the Fe⁰ barrier), a low NO₃^- concentration was observed in monitoring wells such as TMW-11 and DP-12.  These observations suggest that NO₃^- is readily degraded in the reducing zone of influence by Fe⁰ corrosion.

The reduction of NO₃^- observed in the downgradient and some upgradient monitoring wells may be partially attributed to denitrification by microorganisms.  As reported previously (Gu et al., 2001) , an enhanced anaerobic microbial population was observed in soils both downgradient and upgradient of the Fe⁰ barrier. This was presumably related to a low Eh and an increased level of dissolved H₂ in groundwater (a byproduct of Fe corrosion), which served as electron donors for the microbial reduction of NO₃^-.  However, direct abiotic reduction of NO₃^- by Fe⁰ filings should not be ruled out within the Fe⁰ barrier (Gu et al., 1997; Huang et al., 1998) . 

In laboratory, the reduction of NO₃^- and its associated byproducts by Fe⁰ filings was evaluated in the presence or absence of peat materials and/or denitrifying bacteria.  Results indicated that nitrate was effectively reduced by Fe⁰, despite a relatively high initial NO₃^- concentration (6000 mg/L) used in these laboratory batch experiments (Figure 5). The degradation half-life by Fe⁰ alone was found to be on the order of ~1–2 weeks by assuming a pseudo-first-order reaction kinetics, and more than 60% of NO₃^- was degraded after about 2 weeks of reaction.  The addition of peat materials (from Wards Scientific) was found to enhance the reduction rate of NO₃^-, with a decreased reaction half-life on the order of ~2 days.  More than 95% of the NO₃^- was degraded in a 1-week period.  However, note that the reduction rates did not increase significantly with the addition of a toluene-degrading denitrifying bacterium, Azoarcus tolulyticus Tol-4, into the Fe⁰ and peat mixture (Chee-Sanford et al., 1996) .  In fact, addition of this denitrifying bacterium directly into Fe⁰ filings did not increase the denitrification either, probably because of a high NO₃^- concentration and a relatively high pH condition (up to ~10) in the reactant solutions or an unfavorable environment for microbial reduction of NO₃^-.  The presence of peat (with indigenous microbes in the peat), however, buffered the pH of the reactant solution (pH <8.5) so that a substantially enhanced NO₃^- reduction rate was observed under these conditions.  Nevertheless, results of these laboratory experiments are consistent with the field monitoring results and demonstrate that Fe⁰ is an effective reactive medium for removing NO₃^-, in addition to degrading chlorinated organics and sequestering some redox-sensitive metals or radionuclides, as reported previously (Blowes et al., 1997; Cantrell et al., 1995; Gillham et al., 1994; Gu et al., 1998; McMahon et al., 1999) . 

The reaction byproducts between NO₃^- and Fe⁰ and peat mixtures were also examined.  The results indicate that a portion of NO₃^- (~25%) was converted to ammonia (NH₄⁺) in the aqueous solution, and a large percentage of NO₃^- may have been degraded as N₂ or N₂O gases. A good mass balance was not obtained in these batch kinetic experiments, largely because of the loss of N₂ and N₂O gases to the headspace or atmosphere.   

A decreased concentration of sulfate (SO₄^2-) also was observed within the Fe⁰ barrier.  As illustrated in Figure 6, sulfate was the highest in the upgradient soil and pea gravel portion of the barrier.  Upon entering the Fe⁰ portion of the trench, sulfate was found to be substantially reduced at all levels.  For example, at the multi-level monitoring wells of DP-19 and DP-20, the SO₄^2- concentrations were significantly lower than those in the upgradient wells.  In particular, sulfate was largely removed or degraded in some of the downgradient monitoring wells in soil (e.g., DP-14s), in iron (DP-23s,m), and in gravel (TMW-7).  These observations provide evidence of sulfate reduction in the zone of Fe⁰ influence, although the mechanisms of sulfate reduction are still a subject of investigation. These observations are also consistent with previous studies that show that groundwater SO₄^2- concentrations decreased through the Fe⁰ barriers at the Moffett Field and Lowry AFB sites, at the Elizabeth City, U.S. Coast Guard site (Puls et al., 1999) , and in the laboratory-simulated column studies with a continuous input of SO₄^2- and HCO₃^- solutions (Gu et al., 1999) .

The reduction of SO₄^2- resulted in the formation of sulfide (S^2-), although much of the sulfide produced may have been rapidly precipitated as FeS because of its low solubility (K_sp on the order of 10^-18).  This explains a relatively low S^2- concentration observed in most of the monitoring wells (data not shown).  Nevertheless, sulfide concentrations were found to be somewhat higher in those monitoring wells adjacent to the iron (TMW-11 and TMW-7) and within the iron barrier (TMW-9) than in those monitoring wells upgradient of the Fe⁰ barrier (TMW-12 and DP-12).  The exact mechanism of SO₄^2- reduction to S^2- is not yet clear because there is no direct evidence showing an abiotic reduction of SO₄^2- by Fe⁰, although reduction of sulfonic acid to S^2- by Fe⁰ has been reported (Lipczynska-Kochany et al., 1994) .  However, a decreased SO₄^2- concentration in the barrier could be at least partially attributed to reduction by sulfate-reducing microorganisms (Gu et al., 2001) . 

Using phospholipid fatty acids (PLFA) and DNA analyses (Dowling et al., 1986; Guckert et al., 1986; Tunlid and White, 1991; Zhou et al., 1996) , an increased microbial population was observed within and in the vicinity of the Fe⁰ barrier.  The microbial population was found to be on the order of 10⁵ to 10⁶ cells/mL groundwater, which is substantially higher than that found in the background soil, located ~50 ft upgradient of the Fe⁰ barrier (Gu et al., 2001) .  More importantly, perhaps, diversified microbial communities were also detected in groundwater by examining the characteristic fatty acid profiles or lipid biomarkers although PLFA analysis is unable to identify the specific functional groups of microorganisms.  Many microbial species may have similar PLFA patterns.  Therefore, DNA analysis based on polymerase chain reactions (or PCR analysis) was used to further identify different functional groups of microorganisms.  As reported previously, sulfate-reducing bacteria appeared to be one of the most abundant microorganisms identified in both groundwater and core samples obtained within and in the vicinity of the Fe⁰ barrier.  Both sulfate-reducing and denitrifying bacteria were found to be the highest in TMW-11 (upgradient adjacent to the Fe⁰ barrier) and DP-11 (~3 ft downgradient of the Fe⁰ barrier).  These observations provide additional evidence that a decreased SO₄^2- concentration within the Fe⁰ portion of the trench could be a result of microbial reduction of SO₄^2- to S^2- under anaerobic conditions.  Hydrogen generated by the corrosion of Fe⁰ (and the initial use of Guar gum for trench excavation) could have played a significant role in stimulating the growth of these anaerobic microorganisms (Gu et al., 1999) .

INTERACTIONS WITH CALCIUM AND CARBONATES

The groundwater at the barrier site contains high concentrations of both Ca²⁺ and bicarbonate because of the presence of calcium-rich bedrock, the calcareous Nolichucky shale, and strong nitric acid leachate from the S-3 Ponds, and because of the neutralization of the acid wastes by limestone in 1984.  An analysis of groundwater carbonate/bicarbonate and Ca²⁺ indicates that these groundwater constituents were partially retained or precipitated within the Fe⁰ barrier (Figure 7).  The Ca²⁺ concentrations in the upgradient side of the Fe⁰ barrier (e.g., TMW-11, TMW-12, DP-12 and DP-13) appeared to be relatively constant (between ~120 and 200 mg/L).  However, the Ca²⁺ concentrations within the Fe⁰ barrier from TMW-9, DP-19, and DP-20 monitoring wells were about an order of magnitude lower than those found in the upgradient monitoring wells, suggesting that Ca²⁺ was retained by the Fe⁰ barrier.  A relatively low Ca²⁺ concentration also was observed in many of the downgradient monitoring wells (e.g., DP-23s,m, TMW-7, and DP-14s). 

Examination of carbonate/bicarbonate concentrations in groundwater revealed that these constituents also were partially removed, suggesting that calcium carbonate precipitation is probably one of the dominant mechanisms responsible for decreased concentrations of Ca²⁺ and bicarbonate in groundwater.  These results can be expected because of an increased groundwater pH as Fe⁰ corrodes in the barrier and a resulting shift from bicarbonate to carbonate species in the groundwater.  As has been reported previously, an increased pH and relatively high concentrations of Ca²⁺ and bicarbonate in the groundwater could have induced the chemical precipitation of Ca-carbonate and/or of a mixture of Fe- and Ca-carbonate and oxyhydroxide coprecipitates (Gu et al., 1999; Phillips et al., 2000) .  Similarly, by examining the concentration profiles of Mg²⁺ (data not shown), we found that Mg²⁺ concentration within the Fe⁰ barrier also decreased over time because it also could form carbonate precipitates or co-precipitates with iron oxyhydroxides (Phillips et al., 2000) .