Natural occurrence of hexavalent chromium in a sedimentary aquifer in Urânia, State of São Paulo, Brazil

Anomalous concentrations of hexavalent chromium have been detected in groundwater of the Adamantina Aquifer in at least 54 municipalities located in the northwestern region of the State of São Paulo, southeast Brazil, occasionally exceeding the permitted limit for human consumption (0.05 mg.L –1 ). An investigation was conducted in the municipality of Urânia, where the highest concentrations of chromium were detected regionally. It was defined that the origin of this contamination is natural, since high concentrations of chromium were detected in aquifer sandstones (average of 221 ppm) and also in pyroxenes (6000 ppm), one of the main heavy minerals found in the sediments. Besides, no other possible diffuse or point sources of contamination were observed in the study area. Stratification of groundwater quality was observed and the highest concentrations of Cr 6+ were detected at the base of the aquifer (0.12 mg.L –1 ), where groundwater shows elevated values for redox potential (472.5 mV) and pH (8.61). The origin of Cr 6+ in water may be associated with the weathering of pyroxene (augite), followed by the oxidation of Cr 3+ by manganese oxides. The highest concentrations of Cr 6+ are probably related to desorption reactions, due to the anomalous alkaline pH found in groundwater at the base of the aquifer.


INTRODUCTION
Chromium has been identified as a public health problem due to its toxic effects even at low exposure levels (ATSDR 2000).In the environment, chromium exists in two main oxidation states, Cr 3+ and Cr 6+ , which develop different geochemical and biological activities, since Cr 3+ is an essential metal nutrient and Cr 6+ is carcinogenic.Cr 6+ is more mobile, labile and toxic than tions in natural waters.Under acidic and reducin ditions, Cr 3+ species will predominate in water, Cr 6+ species will prevail under alkaline and mild dizing conditions.
Although the natural presence of chromium in aquifers is scarce and little studied, concentrations of chromium higher than the recommended value for drinking water (0.05 mg.L -1 ) have been known since 1977 in the Adamantina Aquifer (Almodovar and Pacheco 1995), located in the northwestern portion of the State of São Paulo.Water supplies in this region are primarily obtained via deep groundwater wells, most of which are managed by public water supply companies.
The main objective of this paper is that of characterizing the distribution of chromium species in groundwater of the Adamantina Aquifer and present the results of an investigation performed in the municipality of Urânia (Fig. 1), where the highest concentrations were detected.The concentration of major ions and water quality parameters were also measured and their relationship with chromium occurrence was considered, as well as the relationship between chemical species and well depth.The results shown in this paper are part of a wider study and some interpretations of the occurrence and distribution of chromium are also discussed.

GEOLOGICAL SETTING
The study area is located in the Paraná Basin, which is a volcanic-sedimentary basin that covers virtually 1 million km 2 in Brazil.More specifically, the study site is located in the western part of the Paulista Plateau, which covers 50% of the State of São Paulo.The parent rock consists of sedimentary rocks of the Bauru Basin (Upper Cretaceous age) and overlays basalts of the Serra Geral Formation (Jurassic-Cretaceous age).
The Bauru Basin covers an area of 104,000 km 2 and 42% of the area of the State of São Paulo.Based on the conception of Fernandes and Coimbra (2000), the portions of the basin.The Caiuá Group is subdivided into the Rio Paraná, Goio-Erê and Santo Anastácio Formations, which are present in the western portion of the State of São Paulo.The Bauru Group, which is predominant in the State of São Paulo, outcrops in oriental domains of the Basin and is subdivided into the Vale do Rio do Peixe, Araçatuba, São José do Rio Preto, Presidente Prudente and Marília Formations (Fig. 1).
In this context, sedimentary rocks of the Vale do Rio do Peixe Formation or, in other words, the Adamantina Formation conceived by Soares et al. (1980), outcrop in the municipality of Urânia.This stratigraphic unit is constituted of well-sorted fine to very fine sandstones, interbedded with siltstones.Carbonate cementation is found locally.The sandstones are disposed in submetric tabular layers of massive aspect, presenting zones of coarse tabular bedding and zones with tabular and through cross-bedding.Fernandes and Coimbra (2000) suggest that deposition of the Vale do Rio do Peixe Formation occurred primarily by eolic action in extensive plain-like areas and, secondarily, by occasional torrents, forming wadis deposits.Finally, the authors suggest that the lower contact of the Vale do Rio do Peixe Formation is gradual with the Santo Anastácio Formation or, as occurs in the study area, discordant and directly on the basalts of the Serra Geral Formation.
Brandt Neto et al. (1985) indicate that the mineralogy of the sediments is mainly composed of quartz and, secondarily, feldspars, kaolinite, montmorillonite, opaque minerals and carbonates as cement.The origin of the sedimentary rocks is related to the deposition of sediments caused by the erosion and transport of other phanerozoic sediments, metamorphic rocks of the Araxá and Canastra Groups, basalts of the underlying Serra Geral Formation and alkaline rocks of the Triângulo Mineiro (A.M.Coimbra, unpublished data).
The average thickness of the Bauru Group is around 100 m.Greater thickness generally occurs on the crests of hills between the main rivers of the western portion of the State of São Paulo.In the Urânia region, the sed- gion, the Serra Geral Formation has a thickness of between 500 and 1500 m.In the Urânia region, deep supply wells exploiting groundwater from the underlying Guarani Aquifer show basaltic rocks with a thickness of around 900 m.

HYDROGEOLOGICAL SETTING
There are two main hydro-stratigraphic units in this region, namely: the Bauru Aquifer System, which constitutes the main source of water supply, and the Serra Geral Aquifer, represented by basalts of the underlying Serra Geral Formation.The municipality of Urânia is located in the context of the traditionally named Adamantina Aquifer, in concordance with designation of the Adamantina Formation by Soares et al. (1980).The Serra Geral Aquifer system is constitu flood basalts and associated intrusive rocks.It i acterized as a fractured, unconfined to semi-co discontinued, anisotropic and heterogeneous aqu The northwestern region of the State of São shows an average temperature of 20  (Almodovar and Pacheco 1995).
From June 1998 to June 1999, groundwater samples from 31 wells located in the municipality of Urânia were analyzed in order to assess variation of chromium concentrations with depth and pumping time.During sampling activities, water from public and private supply wells was collected, together with water from handdug shallow supply wells.In order to evaluate variation of chromium concentrations over a pumping period of 24 hours, a sampling program was performed during several pumping intervals at two deep supply wells (PP02 and PP04).
Samples collected between 1977 and 1993 were obtained and analyzed by a public water supply company.Little information is available regarding sampling techniques, sample preservation and analytical methods.
Samples collected between 1994 and 1999 were obtained at the nearest tap, located prior to the water tank.All wells were equipped with electric pumps.These samples were split and an aliquot for determination of cations and metals was filtered through a 0.45μm-pore filter and acidified with ultrapure HNO 3 to pH < 2.0.The aliquot for anion determination was just stored at a temperature of 4 • C until analysis.
Measurements of pH, Eh, EC, Cr 6+ and alkalinity of water samples were performed in the field.Alkalinity analysis was performed by titration with H 2 SO 4 using an end-point based on the Gran plot (Appelo and Postma 1993).Analytical methods for chemical species quantified in the sampled groundwater are summarized in Table II.

REGIONAL DISTRIBUTION OF TOTAL CHROMIUM AND CR 6+
A general assessment of total and hexavalent chromium concentrations in groundwater was conducted in the Northwestern region of the State of São Paulo based on existing data gathered between 1977and 1993(Almodovar and Pacheco 1995).Total chromium is distributed in various municipalities and districts of the region.In some localities, concentrations are lower than or equal to the potability limit (0.05 mg.L -1 ), whereas in many others concentrations vary from this limit up to 0.155 mg.L -1 .
Table III shows a summary of basic statistics regarding chromium concentrations detected in groundwater at various localities during this period.Figure 2 shows a regional map with indication of the concentrations of total and hexavalent chromium obtained in groundwater in the study region.
Around 39% of all available analyses showed concentrations of chromium higher than (or equal to) the potability limit.The values showed a great degree of variation between different localities in the region and a clear distribution pattern could not be obtained.However, comparing the concentrations found in wells installed in the Adamantina and Serra Geral aquifers, it was observed that the highest concentrations of total chromium occurred in groundwater from the Adamantina Aquifer.Concentrations varied between 0.005 and 0.155 mg.L -1 , with the highest concentrations in the municipality of Urânia, whereas concentrations varied from 0.006 to 0.018 mg.L -1 in basalts of the Serra Geral Aquifer.
NATURAL VERSUS ANTHROPIC SOURCES Anthropic processes and natural sources can be considered as possibilities for assessing the origin of chromium concentrations in groundwater.The Urânia region, a "main" -2009/5/6 -10:11 -page 231 -#5 NATURAL OCCURRENCE OF HEXAVALENT CHROMIUM    ential or agricultural areas.Agriculture is the main economic activity in the area, primarily fruit cultivation (grape and pineapple) and small-scale cattle farming.Thus, the application of fertilizers and pesticides could possibly be a significant diffuse source of the chromium found in the aquifer.The chemical composition of fertilizers and pesticides commonly used in the Urânia area was then checked and no significant concentration of chromium was present.
Point sources of contamination, like steelworks, electroplating, leather tanning and chemical manufacturing, were not found to be present in the study area.Only small industries are present, such as rice processing and furniture manufacturing, which do not show an associated contaminant load of chromium.Besides this, these types of point sources of contamination would gener-of chromium contamination in the study area, which reinforced the hypothesis of natural origin in the case of this contamination.This hypothesis was confirmed by Marcolan and Bertolo (2007), through chemical and mineralogical analysis using X-ray fluorescence, X-ray diffraction and scanning electron microscopy (SEM-EDS) on borehole samples collected from the top to the base of the aquifer in Urânia.The results of this investigation indicated that:

Conceptual model
The geological and hydrogeological features of the Adamantina Aquifer in Urânia are quite similar to those described for the aquifer regionally.The aquifer is unconfined to semi-confined, constituted of well-sorted fine to very fine sandstones with carbonate cementation, relatively homogeneous and isotropic in the scale of study, and with a thickness varying from 66 to 165 m.The urban area of Urânia is located on the crest of a smooth hill, which corresponds to the local aquifer groundwater divide.
Groundwater flow lines that originate in this area converge to the Comprido Creek, which is considered as the main discharge area of the aquifer locally (Figure 3).The hydraulic gradient varies from 0.011 to 0.025 and the advective flow velocity of groundwater ranges from 20 to 400 m/year.Pumping tests showed a hydraulic conductivity value of 8.54E-6 m/sec (F.A. Cagnon, unpublished data).
Three classes of supply wells were registered in Urânia, each representing conditions at a specific depth in the aquifer: private hand-dug wells ( 14), private 4" wells (10) and public 6" wells (7).In general, the private hand-dug wells (shallow wells) have a diameter of 1.2 m and reach an average depth of 11 m, being characteristic of the shallow part of the aquifer, where a local groundwater flow system with a shorter transit time predominates.The private 4" wells (intermediate wells) pump groundwater from intermediate depths in the aquifer, show an average depth of 57 m and are commonly cased with 4" tubes to a depth of 20 m.The public 6" wells (deep wells) extend to depths varying from 75 to 160 m and usually reach the top of basalts that comprise the Serra Geral Formation.These wells are continuously cased with 6" tubes and filters that are installed from the intermediate to deep intervals of the aquifer.This situation might result in mixture of water indicate the occurrence of downward flow poten the urban area and upward flow potential in the the Comprido Creek, which is confirmed by nearb PP05, where results showed conditions of artes controlled by topography.
Three hydrochemical zones were identified Adamantina Aquifer in Urânia that are in acco with the depths of the studied wells.The first chemical zone is the Shallow Zone (up to 30 m), ated with the private hand-dug wells (shallow wel exhibiting Na-Ca-Cl-NO 3 facies (Figure 3).The mediate Zone (30-70 m) is associated with the pri wells (intermediate wells) and shows Ca-HCO 3 The Deep Zone (> 70 m) is associated with the 6" wells (deep wells) and shows Na-Ca-HCO 3 fa Table IV shows the results of a statistical e tion performed on the analytical results of wate ples collected from shallow, intermediate and dee during four rounds of monitoring activities perf between June 1998 and June 1999.Elevated va standard deviation were advective obtained for Cr 6+ , Fe total , Mn 2+ and SO 2− 4 , indicating that the lated mean value is not representative for the a In general, an anomalous high concentration o parameters detected in a sample resulted in a ten for elevation of the mean value.
Table IV also shows the average chemical co tion of water from well PP04, which may better rep hydrochemical conditions existing in the deep a below intermediate depths, as it is solely screened base of the aquifer.As mentioned previously, th lic 6" wells are frequently screened from interm depths to the base of the aquifer.As a result, the c trations shown for water from these wells may in considered as representative of a mixture of wate these depths.
From shallow to deep groundwater zones, sults showed an increase in pH and HCO 3 − con tions and a decrease in electrical conductivity (E dicating a drop in salinity.Eh also decreased, but indicate oxidizing conditions for the aquifer.Co  centrations of HCO 3 − , Na + , F -, and Cr 6+ are higher in the deep aquifer. Comparing the mean values obtained for water samples collected in the different aquifer zones, NO 3 − and Cl -concentrations in shallow groundwater are 5 and 8 times higher than in deep groundwater, respectively.At shallow depths, nitrate concentrations exceed the drinking-water potability value of 45 mg.L -1 and gradually decrease with depth.The occurrence of nitrate in the proximity of the surface can be attributed to the organic charge of septic systems and/or fertilizer use.Its decrease, together with the decrease of EC (salinity), Cl -and Na + , can be attributed to contaminant dispersion and dilution processes.The sample collected from well PP04, which better represents hydrochemical conditions such parameters as pH, HCO 3 − , Na + , Ca 2+ and Mg 2+ .Increase in pH and HCO 3 − concentrations from the top to the base of the aquifer controls the solubility of calcite and dolomite minerals (reaction 1), which exhibit increasing saturation values, until their saturation at the base of the aquifer (Table IV).
In addition, there is substantial enrichment of Na + in the aquifer relative to Cl -.The shallow and intermediate depths of the aquifer show a Na/Cl molar ratio of 1.2, which increases to 46.3 in the deep aquifer, thus indicating that the rock is providing the water with Na + .The increase in Na + , followed by a decrease in Ca 2+ and Mg 2+ concentrations and Ca 2+ /HCO 3 − molar ratios, sug-"main" -2009/5/6 -10:11 -page 235 -#9 NATURAL OCCURRENCE OF HEXAVALENT CHROMIUM  Reaction 2 decreases Ca 2+ concentration and seems to control the dissolution of carbonate minerals, driving reaction 1 to the right, increasing the pH and HCO 3 − concentration.Table V summarizes the hydrogeochemical processes identified in the aquifer.

Behavior of chromium concentrations
Chromium concentrations also showed distinct behavior at different depths in the aquifer (Fig. 4), with an increase from the shallow to deep zones (Table IV).Ten at low concentrations, ranging from the detectio to 0.025 mg.L -1 .All deep wells showed the detec chromium at concentrations that were generally than those found at intermediate depths.Three o samples showed concentrations exceeding the po limit (0.05 mg.L -1 ).
Cr total and Cr 6+ concentrations are usually (Table IV), except in the case of the shallow we dicating that dissolved chromium in groundwate sentially present in its hexavalent form at the inte ate and deep aquifer levels.This is in accordanc "main" -2009/5/6 -10:11 -page 236 -#10

236
CHRISTINE BOUROTTE et al.Correlation coefficients between chromium and the other chemical components were obtained.Except for significant correlations of Cr with pH (R = 0.80) and NO 3 − (R = -0.61)(Fig. 5), no statistically linear or inverse relationships were found in comparing chromium with other chemical species or physical parameters.As suggested in Figure 4, pH, Cr 6+ and NO 3 − box plots showed different patterns between shallow, intermedi-a correlation between Cr 6+ and pH, NO 3 − , Na + and total alkalinity has been observed (Fig. 5).The highest concentrations of chromium were observed in three deep wells (PP04 included), where the highest values of pH, Na + and total alkalinity were found, as well as the lowest concentrations of NO 3 − .Cr 6+ and Mn 2+ concentrations showed contrasting tendencies, although no statistical correlation was found Sampling throughout different pumping intervals was performed over a 24-hours period with a constant flow rate in the public 6" wells (deep wells) PP02 and PP04.Samples were collected at 15 min, and then at 1, 6, 12, 18 and 24 h after the start of pumping.The results indicate that pH values and Cr 6+ and Na + concentrations increase in both deep wells over the pumping period, whereas NO 3 − concentrations decrease (Fig. 6).
Given that the characteristics of the aquifer are considered to be of the unconfined to semiconfined type, continuous pumping allows for the capture of deeper flows in the aquifer, represented by older waters that have been subject to a longer period of water-rock interaction.The observed relationships between Cr 6+ , pH, Na + and NO 3 − during this experiment are in accordance with the correlations observed in Figure 5.
According to Richard and Bourg (1991), Cr 6 erals are very scarce in nature and amorphous Cr 3+ ] hydroxide is probably the main phase cont chromium solubility in natural environments.Ho the chemical and mineralogical analyses condu this investigation indicated high concentrations o mium in sandstones (average of 221 ppm), and cially in pyroxene (augite) crystals (6000 ppm), w one of the main heavy minerals found in the san Pyroxene is one of the most reactive minerals in ering processes, according to the Goldich stability (Kehew 2001), thus, this mineral may be consid the main source of chromium found in water.Ho chromium occurs as Cr 3+ in pyroxenes and a red action must take place to make Cr 6+ available and in groundwater.
The redox process for transformation of Cr 6+ of Cr 3+ into Cr 6+ at pH values greater than 9 (Fendorf and Zasoski 1992), however, according to Richard and Bourg (1991), the rate of oxidation is very slow and enables Cr 3+ to be involved in faster concurrent reactions, like sorption or precipitation.Considering that the residence time of water in aquifers may reach thousands of years and pH values are quite elevated in the deep aquifer, the possibility cannot be neglected that this re-the oxidation of Cr 3+ to Cr 6+ in aquifers (Apte et al. 2006, Sedlak and Chan 1997, Richard and Bourg 1991, Eary and Rai 1987, Fendorf and Zazoski 1992, Fendorf 1995, Bartlett and James 1979).
This reaction is more effective under acidic conditions.the deep aquifer, where Cr 6+ concentrations are higher.However, any Mn 2+ formed during Cr 3+ oxidation will be converted back to MnO 2 in the presence of dissolved oxygen (Apte et al. 2006), which is observed in all depths in the aquifer in Urânia.In fact, chromium and manganese form a pair of chemical elements of opposite tendencies, since, under oxidizing conditions, Cr 6+ is soluble as CrO 2− 4 and Mn 4+ is present as MnO 2 ; under reducing conditions, Cr 3+ is removed from solution as Cr(OH) 3 and Mn 2+ is soluble (Richard and Bourg 1991).
In the shallow aquifer, more acidified wate tribute to more intensive weathering of pyroxene Cr 3+ rapidly precipitates as hydroxides or is ad by Fe and Mn hydroxides (Richard and Bourg Part of the possible Cr 6+ produced in the shallow might be reduced back to Cr 3+ by organic matter ( acids and humic or fulvic acids) or by Fe 2+ oxi which are fast reactions.Nitrification processes, convert ammonium to nitrate, acidify waters an also contribute to the immobilization of Cr 3+ (C "main" -2009/5/6 -10:11 -page 240 -#14
immobilized by adsorption to clay minerals (low pH zpc minerals), a process that increases with pH (Richard and Bourg 1991).
Due to its anionic nature, Cr 6+ is preferentially retained on positively charged surfaces, for example, Al and Fe hydroxides (high pH zpc minerals), principally in more acidic conditions.In addition, most of the Cr 6+ produced by Cr 3+ oxidation reactions may be adsorbed by these minerals in the intermediate and deep portions of the aquifer.
However, adsorption of Cr 6+ to minerals strongly decreases in the alkaline pH range and in the presence of other competing anions, increasing its mobility (Rai et al. 1989, Zachara et al. 1987).This may be the most probable phenomenon occurring in the deep aquifer, since the anomalous increase in pH, probably driven by cation exchange reactions involving Na + , Ca 2+ and Mg 2+ and by the dissolution of carbonate minerals up to their solubility limits (reactions 1 and 2), is likely causing the desorption of Cr 6+ anions to the aquifer.Table V summarizes possible geochemical processes involving chromium in the different parts of the aquifer.

CONCLUSIONS
Chromium is found in groundwater of the Adamantina Aquifer, at concentrations often exceeding the established potability limit (0.05 mg.L -1 ), in an extensive area covering at least 54 municipalities located in the northwestern region of the State of São Paulo.An investigation conducted in the municipality of Urânia, where the highest concentrations were found, indicated that the origin of this contamination is natural, since elevated concentrations of chromium were detected in sandstones (average of 221 ppm) and especially in the mineral augite (6000 ppm), which is the main heavy mineral found in the sandstone.Besides this, no possible diffuse or point sources of contamination were observed in the study area.The results obtained for Urânia are good indication that the chromium found in groundwater regionally may have the same origin.
The distribution of chromium concentrations in the and hydrochemical conditions allow chromium release into aqueous solution.In most of the groundwater samples collected from the shallow wells, chromium has not been detected or observed concentrations are very low.
The distribution pattern of these concentrations also reinforces the hypothesis that the origin of chromium is natural.Dissolved chromium is essentially present in its hexavalent form, especially at the intermediate and deep levels of the aquifer.The origin of Cr 6+ seems to be associated with the weathering of Cr-minerals, such as the Cr-augite identified, followed by the redox transformation of Cr 3+ into Cr 6+ by another redox couple, for example, H 2 O/O 2 (aq.) or Mn 2+ /Mn4+.According to many authors, manganese oxides are most likely responsible for Cr 3+ oxidation.The processes of adsorption/desorption of Cr 6+ probably represent important reactions, since the highest Cr 6+ concentrations are found in the deep aquifer, where groundwater pH is more alkaline, which is a condition necessary for the desorption of Cr 6+ .This phenomenon may be occurring in the deep aquifer, where cation exchange reactions are driving the dissolution of carbonate minerals, causing an anomalous increase in pH.Complementary field and laboratory studies are now being conducted in order to confirm the occurrence of such geochemical processes responsible for the presence of chromium in groundwater.

"
Fig. 1 -Location of the study area and stratigraphy of Bauru Basin (modified from Fernandes and Coimbra 2000) area, where groundwater divides coincide with the age basins and the local effluent rivers correspond discharge areas of the aquifer (Hirata et al. 1997 aquifer is a moderately permeable formation, w erage values of hydraulic conductivity and transm ity close to 1.0E-5 m/sec and 40 m 2 /day, respe (DAEE 1976).

( 1 )
N wells = number of wells; Std.: standard deviation; Min.: minimum; Max.: maximum; Sat.Ind.= saturation index.(2) Concentr italic and lower case highlight elevated values of standard deviation, indicating that the mean value is not a representative figure.

Fig. 4 -
Fig. 4 -Box plots showing a comparison of chromium, pH and nitrate distribution for groundwater samples from shallow, intermediate and deep wells.

Fig. 6 -
Fig. 6 -Variation of pH values and chromium, nitrate and sodium concentrations throughout pumping period.