NATURAL ARSENIC IN GROUNDWATER: OCCURRENCE, REMEDIATION AND MANAGEMENT - CHAPTER 18 pdf

Natural Arsenic in Groundwater: Occurrence, Remediation and Management – Bundschuh, Bhattacharya and Chandrasekharam eds © 2005, Taylor & Francis Group, London, ISBN 04 1536 700 X Geoche

Natural Arsenic in Groundwater: Occurrence, Remediation and Management –

Bundschuh, Bhattacharya and Chandrasekharam (eds)

© 2005, Taylor & Francis Group, London, ISBN 04 1536 700 X

Geochemistry and geomicrobiology of arsenic in Holocene

alluvial aquifers, USA

J.A Saunders, M.K Lee & S Mohammad

Department of Geology and Geography, Auburn University, Auburn, USA

ABSTRACT: Groundwaters in Holocene alluvial aquifers in the USA contain elevated dis-solved arsenic (As) have similar geochemical characteristics and microbiology These include: (1) near-neutral pH and moderately reducing redox state; (2) significant dissolved iron and man-ganese; (3) presence of both iron- and sulfate-reducing bacteria; and (4) evidence that precipitation

of authigenic carbonates and sulfides limits metal (loid) solubility Our research indicates that anaerobic bacteria directly mediate dissolution of iron and manganese minerals in the alluvial aquifers leading to release of As and other trace elements, and also indirectly cause the precipita-tion of authigenic minerals such as siderite and rhodochrosite Genetic sequencing indicated that the

genus Geobacter are responsible for As release from Fe-oxyhydroxide We propose a global-scale Geo-Bio-Hydro (GBH) arsenic cycle that directly invokes Pleistocene glaciers as important in physic-ally weathering rocks setting the stage for Holocene chemical weathering that initiated a major

release of As to the hydrosphere

There is now a general consensus that bacterial dissolution of arsenic-bearing iron oxyhydroxides (also known as hydrous ferric oxides, HFO) is the principal mechanism leading to As release in groundwaters hosted by Holocene river flood-plain deposits in such places as Bangladesh, India, Hungary, etc (e.g Chatterjee et al 1995, Bhattacharya et al 1997, Nickson et al 1998, Acharrya

et al, 2000, Nickson et al 2000, Welch et al 2000, McArthur et al 2001, Smedley & Kinniburgh

2002, Dowling et al 2002, Anawar et al 2003, van Geen et al 2003, Ahmed et al 2004, Akai et al

2004, Zheng et al 2004) Further it is clear that this process occurs under moderately reducing redox conditions caused by the presence of aqueous or solid organic matter in the sediments (Nickson et al 2000, McArthur et al 2001) We (Saunders et al 2003, Mohammad 2003) have proposed that the geochemical and microbiologic conditions leading to As-contamination of Holocene alluvial aquifers is universal in flood plains where river or stream sediments are deposited with organic matter In this paper, we review our research findings from two field areas in the USA which appear to be in every way analogous to the problematic areas of the world adversely impact-ing human health due to As contamination Further we show that one common type of iron reduc-ing bacteria (FeRB) causes the As problem at of our USA field sites, where we have identified the

“guilty” bacteria (present only in groundwaters with more than 50
g/L As) using DNA sequenc-ing techniques Moreover, we propose this bacterium is likely responsible for the As problem else-where in the world, although clearly confirmation outside of USA is necessary and is planned

2 GEOLOGY, GEOCHEMISTRY, AND GEOMICROBIOLOGY OF USA FIELD AREAS Korte (1991) was first to document natural As contamination at an alluvial floodplain in the USA, and

we chose his “discovery” area at the U.S Department of Energy’s (DOE) Kansas City Plant (KCP)

to conduct our research The KCP study area is located in floodplain deposits of the Blue River, a tributary of the Missouri River (Mohammad, 2003) The aquifer at the KCP site consists mainly of stream-valley alluvial deposits composed of Holocene alluvium which unconformably overlie Pennsylvanian strata of limestone and shale Alluvial deposits typically consist of sand and gravel deposits of locally intermittent lenses of sand, silt, and clay Groundwater at the KCP site is uncon-fined but locally can exist under conuncon-fined conditions where clay layers are present (Mohammad

2003, Saunders et al 1997) Water levels in wells at the KCP typically are 7 m Our second study area is located along Uphapee and Choctafaula creeks in an extensive Holocene floodplain deposits in central Alabama These alluvial deposits have C-14 ages 7000 years old and consist primarily of silty sand containing erratic but common macro wood fragments up 0.5 m in size The alluvium unconformably overlies the Cretaceous Tuscaloosa Group of the Alabama Coastal Plain

in the study area Groundwater is unconfined with depths to the water table typically 2 m Groundwater at the Kansas City Plant is highly variable in redox state, iron (and manganese) content, and has near-neutral pH conditions Arsenic was significantly elevated (e.g., 50
g/L in three of the eight monitoring wells, which also happened to have the lowest Eh values (Mohammad 2003) Dissolved Fe ranged from 0.2 to 17.1 mg/L, Mn from 0.5 to 7.9 mg/L, and alkalinity rang-ing from 175 to 350 mg/L (as CaCO3) Groundwater chemistry for the Alabama field site is gen-erally similar to that of the KC site It contains elevated Fe and Mn (up to 1 and 3 mg/L respectively, consistent with its moderately reducing nature), pH 6.6 to 6.8, 1–10
g/L each of As, Co, Ni, Zn, REEs, and 50–175
g/L of Ba Further, sulfate reducing bacteria (SRB) in this alluvial aquifer use detrital wood fragments as the ultimate electron donor which leads to precipitation of pyrite cont-aining up to 0.62 wt.% As and commonly, pyritization of the wood (Saunders et al 1997) Several researchers have proposed that Fe- and possibly Mn-reducing bacteria may have been important in producing As-contaminated groundwater in Holocene alluvial aquifers However, this has only been inferred from the high Fe (and Mn) contents of groundwaters with elevated As, its moderately reducing state, and a general correlation between dissolved Fe, As, and alkalinity (e.g., Nickson et al 2000, McArthur et al 2001, Ahmed et al 2004) To evaluate this hypothesis,

we conducted reconnaissance microbiologic investigations at our two field sites At the Kansas City site, we collected two different types of samples for microbiologic investigations: groundwa-ter for culturing of viable bacgroundwa-teria; and (2) bacgroundwa-teria filgroundwa-tered from several ligroundwa-ters of groundwagroundwa-ter in the field for molecular microbiology studies Only wells at the Kansas City site that had elevated iron,

Mn, and As could anaerobic bacteria be cultured, and both FeRB and SRB were found in these water samples These groundwaters also had the lowest field Eh values The microbiologic data

suggest that FeRB are present and available to catalyze Fe-reduction in the As-enriched wells and

thus this is the first direct indication from the field of their involvement in producing arsenic cont-amination in groundwater

Similar to the culturing experiments from the KCP site, cloning and sequencing of the 16S rDNA genes extracted from bacteria filtered from groundwater also indicated the presence of FeRB and

SRB from the As-rich samples Clones with sequences similar to known FeRB (e.g., Geobacter

sp.) were also abundant in the As-rich water samples Thus the molecular biological data corrob-orates the bacterial culturing results In contrast to the Kansas City site, we used a truck-mounted

auger to advance a borehole into the alluvial aquifer at the Alabama field site and collected solid

samples below the water table for microbiologic and chemical analyses Both FeRB and SRB were cultured from the aquifer, and enumeration using the most probable number (MPN) method indi-cated that FeRB were most abundant

3 GEOCHEMICAL MODELING: IMPLICATIONS FOR ARSENIC MOBILITY

Geochemical modeling with PHREEQC (Parkhurst 1999) indicates that groundwater samples have positive saturation indices (SI) for siderite in two of the three As-elevated wells and super-saturated SI values for rhodochrosite in all three elevated-As wells at the KCP site Similarly, all eight groundwater samples appear to be supersaturated with respect to goethite and understaurated

with respect to ferrihydrite, suggesting that an iron oxyhydroxide phase with a solubility intermed-iate between those two might have been the source of iron in groundwaters

Additional geochemical modeling was conducted with the Geochemist’s Workbench (Bethke 1996) to compare groundwaters from the KCP study area to groundwaters from As-contaminated areas of Bangladesh, using published data from the British Geological Survey’s (BGS) “Special Study” where Eh was also measured in the field (British Geological Survey 2004) The BGS data plot in the same general area in Eh-pH space (Fig 1) with a shift perhaps to slightly higher Eh val-ues when the much larger data set is plotted Similarly, Eh-pH plots were made for average geo-chemical conditions (with respect to Fe, Mn, carbonate, and S species) for both the KCP site groundwaters and those from Bangladesh (Fig 1) KCP groundwaters plot near the Fe2 -siderite and Mn2-rhodochrosite boundary suggesting they are approaching local equilibrium with both mineral phases (consistent with siderite SI values discussed previously) The most As-rich ground-water samples from the KC study area plot close to the stability field of pyrite (Fig 1) As before, KCP and Bangladesh groundwaters plot in the same approximate areas in Eh-pH space Similarly, groundwaters from Bangladesh appear to be close to equilibrium (or are supersaturated with) with respect to siderite and rhodochrosite The Eh-pH values for the KCP waters indicate that they are

in the stability fields of both arsenate (As-V, HAsO4 ) and arsenite (As-III, As(OH)3or H3AsO3), although the most As-enriched samples are in the arsenite stability field and even into the narrow stability fields of solid mineral phases realgar (AsS) and orpiment (As2S3)

Results of our study support the general concept that As occurs in moderately reducing and typ-ically Fe- and Mn-rich groundwaters Anaerobic heterotrophic bacteria mediate the reductive diss-olution of Fe and Mn minerals (and arsenic release) by the following chemical reaction:

(1)

where *As is sorbed As on iron oxyhydroxide and CH2O is generic organic carbon Figure 1 shows that Mn-reduction occurs under more oxidizing conditions, and Mn-reducing bacteria will out-compete Fe-reducers as long as reactive solid Mn phases are available (Chapelle & Lovley, 1992)

The presence and abundance of FeRB such as Geobacter in our two study areas provide the first

Figure 1 Redox-pH diagram for arsenic drawn at 25°C and fixed As and H2S activities of 10 2 Also shown

is the groundwater geochemistry data from Bangladesh (oval) and the Kansas City Plant (cross  As  10
g/L; circle  10–15
g/L; diamond  50
g/L).

field evidence supporting the hypothesized bacterial reduction of iron oxyhydroxide and As release to groundwater

Moderately reducing, As-enriched groundwaters from the Kansas City site are generally super-saturated with respect to rhodochrosite and siderite Saunders & Swann (1992) proposed that metabolism of Fe- and Mn-reducing bacteria could facilitate precipitation of both carbonate

phases in aquifers because the products of reaction 1 become the reactants for Fe- and Mn-carbonate

precipitation:

(2) Mukherjee et al (2001) and Pal et al (2002) have observed authigenic siderite and rhodochrosite

in alluvial aquifers in India, and documented textures very similar to those from Saunders & Swann (1992) Further, Saunders & Swann (1992) observed that authigenic carbonate phases often had inclusions of sulfide minerals such as pyrite and proposed that biogenic sulfate reduc-tion probably occurred at the site of carbonate deposireduc-tion Because biogenic sulfate reducreduc-tion raises pH and produces alkalinity as well as H2S (reaction 3a), this could explain both carbonate precipitation and iron sulfide formation (e.g., reaction 3b):

(3a) (3b) Thus we propose that if Fe- and Mn-reducing bacteria are producing the moderately reducing, metal- and As-rich waters observed in this study and elsewhere in river floodplain deposits, then siderite and rhodochrosite are important phases controlling the solubility of iron and manganese

in these waters Precipitation of siderite will preferentially remove Fe relative to As in groundwa-ter (Dowling et al 2002), which can explain the often-observed poor statistical correlation for Fe and

As in many similar groundwaters By the same token, precipitation of iron sulfides could prefer-entially remove As relative to iron (by coprecipitation) and also affect the remaining As/Fe ratios in groundwater Several researches have observed As-bearing pyrite as an authigenic phase in alluvial aquifers, which has led some researchers to propose that it was the source of As in alluvial aquifer groundwaters This is highly unlikely due to the redox state of the water and the general lack of dis-solved sulfate We propose that SRB have removed limited sulfate in solution in these terrestrial waters and removed a small amount of Fe and As in the process (e.g., reactions 3a and 3b along with

As coprecipitation in Fe-sulfide) Thus As-bearing pyrite is a sink for As rather than a source.

3 GEO-BIO-HYDRO (GBH) ARSENIC-CYCLING MODEL

We propose that a series of linked geochemical, biologic, and hydrologic processes operating over geologic time scales can lead to natural As contamination in Holocene fluvial and fluvial-glacial deposits around the world We suspect that the conditions for this type of As contamination are very common, but the situation becomes particularly problematic in developing nations where existing surface water supplies are contaminated by parasites, fecal bacteria, etc Further, our prop-osed GBH-As process requires neither As-rich (above crustal abundances
2 ppm) source rocks nor rare or specialized microorganisms The one “out-of-the-ordinary” aspect to the GBH process

is the recognition that recent Earth history of continental glaciation and retreat exacerbates the nat-ural As contamination process “Normal” chemical weathering of minerals in folded mountain belts or other crustal rocks will release As to the hydrosphere and may even locally contaminate groundwater supplies However, we propose that continental and/or alpine glaciation during the last
5 million years lead to the present-day widespread occurrence of As in Holocene ground-waters in river floodplains that are most problematic around the world Although clearly not all As-contaminated groundwaters have been impacted by chemical weathering of glacial deposits, the

proximity of extensive As-contaminated groundwaters in Holocene river floodplains (e.g., in Indian Sub-continent, China, North America, Hungary) adjacent to significant glacial deposits

“up-stream” argues for a genetic link in space and time Low hydraulic gradients in the Holocene river floodplains exacerbates the problem, as the As-contaminated waters are not rapidly flushed out of these aquifers Thus from the perspective of geologic time, the problem is ephemeral but unfortunately coincides with recent human history

The GBH process is initiated by continental glaciation and its associated physical weathering of

continental rocks in its path Glaciation sets the stage for accelerated chemical weathering of the reactive, high-surface-area minerals ground down by glaciers, which release As to the hydrosphere Ferromagnesian silicate minerals typically contain minor amounts of arsenic and other trace elem-ents For example, the common sheet silicate biotite has a crystal structure leading to very rapid weathering (Saunders et al 2000) and As release Thus we propose that, Fe bearing silicates along with As-bearing sulfide and clay minerals are likely the ultimate source of As to the GBH-As process Weathering of Fe bearing silicates and sulfides leads to the formation of iron oxyhydroxide, which are then physically transported by running water initially to streams and then through meand-ering stream channels Some of the HFO would be colloidal in size and transported in suspension, larger grains, including HFO-coated sand grains, reside in stream bed load Both suspended and bed-load HFO’s have a tremendous affinity to adsorb dissolved As However, suspended HFO tend

to move at the same average velocity as the transporting stream flow, and thus have a short residence time estimated to be
2 weeks for major river systems discharging into oceans (Freeze & Cherry, 1979) On the other hand, HFO in river bed load has a much longer residence time and may become the loci of iron precipitation by iron-oxidizing bacteria in streams (Saunders et al 1997); such bio-geochemical reactions effectively create new and reactive HFO surfaces The residence times of river bed-load sediment are difficult to quantify but small scale-studies suggest they may be in the order

of 0.1 to 1 year per km length of the river (Meade 1982, Kelsey et al 1987, Olley et al 1997) Thus

a major river system may have a water residence time of 2 weeks and a bed load residence time of

103to 104years So using a residence time of 5 3years for a major river system, stream bed load would have 5river volumes flowing over and through it during its transport in the river, and As is slowly adsorbed onto the HFO surfaces We propose that the long-term adsorption is the important As-concentration mechanism that ultimately leads to natural As contamination of ground-water in the lower reaches of the river systems where extensive floodplains develop In the flood-plains, reactive organic matters co-deposited with As-bearing HFO’s causes anaerobic conditions and FeRB metabolism This portion of the GBH cycle is essentially the “HFO-As” hypothesis that

we and others have proposed previously We propose that the HFO-As hypothesis is just a part of a much bigger continent-scale process Fluvial and fluvial-glacial deposits typically have low diss-olved sulfate as opposed to river deltas where seawater sulfate can mix with fresh groundwater, which has caused the formation “high sulfur” (and locally As?) Carboniferous coal deposits in the eastern USA and elsewhere Thus SRB cannot effectively remove a significant amount of As released by FeRB bacteria in the anaerobic flood-plain groundwater However SRB do cause some local pyrite precipitation and As removal (Saunders et al 1997, Nickson et al 2000, McArthur et al 2001); this has led to confusion about pyrite being a source of As in river flood-plain sediments

Pyrite is really an As sink that also causes the observed inverse correlation between dissolved As and

sulfate seen in Bangladesh and our Kansas City study area A major implication of the GBH-As cycle is that it leads to a prediction of potential problem areas for natural As contamination as world population increases and new water sources are required Currently, because groundwater in Holocene aquifers have elevated iron contents, it has led to the under usage of these groundwaters in the developed nations, and many of these same groundwaters may also happen to have elevated As

4 POSSIBILITY OF BIOREMDIATING ARSENIC USING SRB

We have successfully stimulated naturally occurring SRB to remediate metal-contaminated ground-water at a number of sites (Saunders et al 2001, Lee & Saunders 2003, Saunders et al 2004) SRB

metabolism causes certain toxic metals to precipitate out of groundwater as relatively insoluble sulfide minerals (e.g., Zn, Pb, Cd, Ag, Cu, Hg, etc.), some redox-sensitive metals to precipitate as oxy-hydroxide phases (e.g., Cr, U), and raises groundwater pH which can also lead to increased sorption of some elements However, raising pH can cause As to desorb This, coupled with the possibility of As-thio complexing (Wilkin et al 2003), makes the geochemical behavior of As under sulfate-reducing conditions not so straightforward even though it forms its own sulfide min-erals and coprecipitates in Fe-S phases However, our research suggests that these problems can be overcome if a strategy of optimizing the geochemical conditions for removal of As from solution

is employed It appears that high dissolved iron content favors arsenic coprecipitation in Fe-S sul-fide phases under sulfate-reducing conditions, and limits the As-thio complexes (Wilkin et al 2003) Thus, we propose that bioremediating As-contaminated groundwater is possible by adding

a solution of hydrous ferrous sulfate and a carbon electron donor (e.g., sucrose, molasses, methanol, ethanol, etc.) through injection wells The lack of ferrous iron can limit SRB metabolic efficiency because Fe(II) is present in many of the enzymes and compounds used in electron transfers by SRB In the past, researchers have assumed that H2S was toxic to SRB, but actually the “toxicity” effect of H2S is the removal of Fe(II) by Fe-S precipitation Thus, injection of dissolved Fe(II) into As-contaminated groundwater: (1) insures that iron will be available for SRB metabolism; (2) limits the buildup of potentially toxic levels of H2S; (3) keeps the Fe(II)/H2S ratio high enough to keep As-thio complexes from occurring to any significant extent, and (4) provides both the Fe and S needed for the “encapsulating” FeS phases Thus this approach can be effective in treating As-contaminated groundwater as the conditions for As-coprecipitation in FeS are optimized

4 CONCLUSIONS

Regulatory agencies around the world typically have a “secondary” drinking water standard (not health-based) for iron of 0.5 mg/L, and thus Fe-rich groundwater in the United States and other

“developed” nations has largely been used for irrigation of crops and not for human consumption

We believe the high iron content has led to the under-appreciation of the fact that elevated As is common in alluvial aquifers in the developed nations of the world as well as countries such as a Bangladesh, India, Pakistan, etc The major difference is that rivers in the developed nations are kept clean enough for drinking water purposes because of the investment in expensive infrastruct-ure, and rivers in developing nations are commonly polluted and not fit for drinking This has driven the use of the iron- and locally As-rich groundwaters of Holocene river floodplains Previous studies have noted that groundwaters with elevated arsenic in alluvial floodplain deposits also have elevated iron (typically 1–50 mg/L) Other specific conclusions:

(1) We have identified Geobacter using DNA analysis (and FeRB by culturing) in groundwaters

with elevated Fe, Mn, and As in our USA field areas and we propose that this type of anaero-bic FeRB bacteria cause the As problem in Holocene alluvial aquifers around the world, but verification is needed

(2) Authigenic carbonate and sulfide minerals appear to precipitate as a consequence of anaerobic bacteria metabolism These minerals will affect observed present-day groundwater As/Fe ratios and metal solubility

(3) Our GBH-As cycle proposes that Pleistocene glaciers played a major role in the weathering-induced release of As to the hydrosphere in the Holocene, and that the As-concentrating mech-anism is sorption by HFO in stream sediments which have much longer residence time than river waters The subsequent action of dissimilatory iron and sulfate anaerobic bacteria control the fate of As and other redox-sensitive elements in Holocene alluvial aquifers

(4) Bioremediation may be possible using SRB if the Fe/H2S ratio is kept elevated in As-bearing groundwater to prevent the formation of soluble thio-arsenite species However, this is not likely a panacea for remediating natural As contamination in the Bengal Basin and similar areas, but may prove useful for anthropogenic arsenic pollution

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