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Radon in groundwater contaminated by dissolved hydrocarbons in Santa Bárbara d´Oeste, São Paulo State, Brazil
Applied Radiation and Isotopes 70 (2012) 2507–2515
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Radon in groundwater contaminated by dissolved hydrocarbons in Santa Ba´rbara dOeste, Sa~ o Paulo State, Brazil J.A. Galhardi, D.M. Bonotto n Departamento de Petrologia e Metalogenia, Universidade Estadual Paulista (UNESP), Campus de Rio Claro, Av. 24-A, No. 1515-CP 178, CEP 13506-900, Rio Claro, ~ Paulo, Brazil Sao
H I G H L I G H T S c c c
Integration of chemical and radiometric data acquired in a polluted area. Possible use of the information in other areas. Potential hazard to human health due to Rn and organic compounds.
a r t i c l e i n f o
abstract
Article history: Received 24 May 2012 Received in revised form 24 June 2012 Accepted 25 June 2012 Available online 14 July 2012
This investigation reported the 222Rn activity concentration and dissolved hydrocarbon content in ~ groundwater collected in three gas stations where occurred tanks leaks, in Santa Barbara d’Oeste, Sao Paulo State, Brazil. The results indicated a tendency of correlation between the radon and BTEX, suggesting that the presence of dissolved hydrocarbons increase the radon concentration in water, due to the preferential partition at this phase. The radiometric data are useful for the detection of residual contamination and dissolved hydrocarbon plumes in groundwater, reinforcing the findings of previous studies held elsewhere. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Radon Groundwater Contamination NAPL
1. Introduction The fuel leaks in storage tanks constitute one of the most common sources of contamination of groundwater and soil and can give rise to the formation of plumes of dissolved hydrocarbons in groundwater and residual phase stored in the pores of the aquifer. In this context, techniques involving the use of markers of partition have been used to the evaluation of the amount and distribution of organic phase in aquifers. The marker partition tends to accumulate at the interface between water and organic phase (also known as non-aqueous phase liquid, or NAPL). The magnitude of the decrease in tracer concentration during its movement through the aquifer is used to delineate the interface area and to estimate the degree of saturation of the aquifer pores by the specific hydrocarbon (Schubert et al., 2007a).
n
Corresponding author. Tel.: þ55 19 35269244; fax: þ 55 19 35249644. E-mail address: [email protected] (D.M. Bonotto).
0969-8043/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2012.06.029
The radionuclide 222Rn has been used in field tests and laboratory experiments by some authors as a marker (Hunkeler ¨ et al., 1997; Semprini et al., 2000; Hohener and Surbeck, 2004; Davis et al., 2005; Fan et al., 2007; Schubert et al., 2005, 2007a, 2007b), who demonstrated its efficiency in tracking contamination plumes and for quantification of the residual saturation of the aquifer by hydrocarbons. Its applicability as a marker is based on the fact that the 226Ra gives rise to a flow of gas 222Rn, after the radioactive decay, which occupies the empty spaces between mineral grains of the aquifer, dissolves in the groundwater or escapes into the atmosphere. The gas fraction escaping from the host materials and retained in the pores corresponds to the emanation coefficient or emanating efficiency (Andrews and Wood, 1972; Wanty et al., 1992), where, in general, the smaller the particle size, the larger the specific surface area and the greater the emanation power of materials containing 226Ra. Other factors influencing the radon activity in water include (a) the composition of the soil or rock storing the water, (b) the presence of minerals containing uranium or radium, (c) the extent of the materials surface in contact with water, (d) the speed that water moves through the aquifer,
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(e) the moisture in the pore space, (f) the pressure and water temperature, (g) the 222Rn recoil following alpha-decay of 226Ra atoms close to the materials surface (within the 222Rn recoil range of 0.036 mm), (h) 222Rn diffusion through the crystalline lattice from production sites below the materials surface and (i) 222Rn diffusion along crystal defects, grain boundaries or microfractures from greater depths below the rock surface. In the absence of free organic phase or residual hydrocarbons, concentrations of 222Rn in groundwater quickly reach an equilibrium determined mainly by the mineral composition of the aquifer solids. In the presence of free organic phase, the concentration of radon in groundwater can be substantially reduced due to preferential partitioning of radon in this phase. This reduction can be quantitatively related to the degree of saturation ¨ of the of the aquifer matrix (Semprini et al., 2000; Hohener and Surbeck, 2004). The advantage to use 222Rn as an indicator consists on the fact that it occurs as a natural component of groundwater, being continually produced by 226Ra decay in the 238U decay series (Bonotto, 2004). 222 Rn is an alpha emitter with an energy of 5.48 MeV, half-life of 3.8 days and decay constant of 0.0001258 min 1. Its solubility in the organic phase is illustrated by the gas partition coefficient in the organic phase and water (KNAPL/W), which depends on temperature and NAPL composition. In toluene, for example, KNAPL/W equals 46. It is estimated that for KNAPL/W equal to 50 (estimated value for chlorinated solvents (CHCl3)), the residual saturation of only 1% of NAPL in the aquifer results in 33% reduction in the concentration of 222 Rn in water (Semprini et al., 2000). In research conducted by Schubert et al. (2005), radon was investigated in a military area abandoned and contaminated by non-aqueous phase liquid. The data revealed a nonuniform distribution of radon gas in soil air sampled in the region, and some values were about 90% lower than the background area. This anomaly may be associated with the presence of NAPL in the subsurface, due to the fact that radon is associated with this phase, moving from the air soil or groundwater and being dissolved at the organic phase. This method is successful when the radon activity concentration in groundwater is compared to the presence of residual NAPL in the aquifer matrix, but not to the dissolved organics (typically benzene, toluene, ethylbenzene and xylene, or BTEX). This is because the dissolved products do not bring safety qualitative and quantitative information about the source of hydrocarbons because their presence and concentration depends on the mix of organic contaminants, the spill time, the flux and velocity of groundwater flow and the presence and rate of other factors of natural attenuation of hydrocarbons. According to Schubert et al. (2007a), the evaluation of the results of the radon
activity in groundwater and the concentration of dissolved BTEX can be used to distinguish areas contaminated with dissolved fuel and residual NAPL (in a case in which the radon activity in water is less than the background of the region) of those only contaminated with dissolved fuel (in the case in which the radon activity in water is about the same of the average background of the region). The greater the proportion of free and residual phase less radon is expected in the soil air and groundwater near the site of the fuel spill, due to the partition of radon in organic phase. But the more hydrocarbon dissolved in a sample, it should contain more radon, due to the preference of dissolved radon in this phase. Thus, it may be possible to perform a preliminary analysis of the presence of residual NAPL contamination and the presence of dissolved contaminant plumes, although the dissolved hydrocarbons do not provide quantitative information about the fraction of residual fuels in aquifers. Despite these factors, it is assumed that the radon concentration in groundwater correlates positively with the presence of dissolved hydrocarbons, due to the gas preference in it. This papers aims to focus these aspects in three gas stations located at Santa Ba´rbara d’Oeste city, Sa~ o Paulo State, Brazil.
2. General features of the studied area The region of Santa Ba´rbara d’Oeste is located in the northeast portion of the Parana´ Basin, consisting of a varied sequence of sediments deposited on the crystaline basement, that comprises a set of older igneous and metamorphic rocks in the Depressa~ o Perife´rica Paulista (IPT, 1981). The lithostratigraphy is composed by the Itarare´ Group and Tatuı´ Formation (PCJ, 1999). Cenozoic deposits occur in extensive areas, overlying the Itarare´ Group (WHB, 1999), which storage the water collected in this study. The location of the three gas stations (GS) investigated in this search can be viewed in Fig. 1. The samples SBO-01–SBO-06 were collected at the GS 1, the sample SBO-07 at the GS 2 and the samples SBO-09–SBO-11 at the GS 3. The sample SBO-08 was disregarded due to its high amount of impurities. The potentiometric map for the GS 1 in January 2011 is ilustrated in Fig. 2. The kerosene tank is located between the sampling points SBO-05 and SBO-06. The points SBO-06 and SBO-05 are located downstream and upstrearn from the kerosene spill area, respectively. The point SBO-03 is located upstream of the local of oil change, the point SBO-01 is situated in the area for washing vehicles and the point SBO-02 is located upstream of the fuel supply area.
Fig. 1. Location of the study area. Modified from Machado et al. (2005).
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Fig. 2. Water table altitude (m) of GS 1 (January 2011).
3. Sampling and analytical methods Samples were collected monthly in monitoring wells located at gas stations from August 2010 to January 2011. Sampling was carried out using a hand pump attached to glass bottles with a capacity of 1 l, avoiding the radon gas loss at the collection time. The bottles were completely filled by the water samples and sealed with rubber stoppers containing two 5 mm diameter nylon tubes (Lima and Bonotto, 1996). A 100 mL aliquot for radon analysis was separated and the flask was sealed again to preserve the sample for 226Ra analysis after it comes into equilibrium with the radon (about 25 day). Samples for analysis of anions and cations were collected in polyethylene bottles of 2 L capacity. Once collected, samples were sent to the laboratory for radon measurement. The time elapsed between the collection and analysis was used to correct the activity due to its radioactive decay. Dissolved oxygen (DO), temperature and pH were measured in field using portable devices. The concentration of aluminum, potassium, total iron, calcium, magnesium, barium, silica, nitrate, chloride, sulphate and total phosphate was determined at LABIDRO—Laboratory of Isotopes and Hydrochemistry of the Department of Petrology and Metallogeny of the Institute of Geosciences and Exact Sciences, UNESP, Rio Claro. The Hach spectrophotometer Model DR 2000 was used for such purpose (Hach, 1992). The total alkalinity was evaluated by titration (Hach, 1992), using a standard solution of sulfuric acid (0.020 N). The determination of 222Rn and 226Ra was also held at LABIDRO, whereas the analysis of BTEX was performed at ASL Laboratory, Rio Claro city, using gas chromatography based on EPA 8015-C method. The technique used to quantify 222Rn and 226Ra was alpha spectrometry. The device used was Alpha Guard PQ2000PRO (Genitron GmbH) equipped with an appropriate drive (Aquakit) (Genitron, 2000). The meter consists of an ionization chamber that measures radon in a system consisting of two glass containers where occurs the radon degassing contained in the sample. Indoors, the samples were monitored by Alpha Guard in five
cycles of 10 min. The determination of the radon concentration in water samples by Aquakit–Alpha Guard units was done by Eq. (1) 1000cw ¼ ca ½kþ ðV SY V SA Þ=V SA c0
ð1Þ 1
where: cw ¼radon concentration in water (BqL ); ca ¼measured value (Bq/m3) indicated by Alpha Guard; c0 ¼zero level radon concentration (0 Bq/m3); VSY ¼system volume (1122 mL); VSA ¼volume of water sample (100 mL); k¼distribution coefficient of radon between air and liquid phase (0.16). Eq. (2) was used for correcting the radon decay between collection and analysis. C w0 ¼ C w eDt
ð2Þ
where: Cw’ ¼corrected radon concentration (BqL 1); Cw ¼radon concentration measured in the water sample (BqL 1); D ¼radon decay constant; t¼time elapsed between sampling and analysis. A period of 25 days is needed for 222Rn to reach radioactive equilibrium with 226Ra. After elapsing it, the same procedure was used for 226Ra analysis by alpha counting. For cleaning purposes, the Alpha Guard system was coupled to an activated charcoal filter to remove the radon of the system, after processing each sample.
4. Results and discussion 4.1. Chemical and physical data Table 1 summarizes the mean values of the parameters characterized in the study area during the sampling period from August 2010 to January 2011, whereas Figs. 3 and 4 illustrate the whole data set. The values of pH, temperature and DO do not change with rainfall in the area. All samples are classified as acid, whose highest pH value was found in November 2010 (6.93, for SBO-09)
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Table 1 Mean values of the parameters characterized in GS 1, GS 2 and GS 3 during the sampling period (August 2010 to January 2011). GS 1
T pH DO Ca2 þ Mg2 þ Fetotal Kþ Ba2 þ SiO2 Cl NO3 SO24 HCO3 PO34 222 Rn 226 Ra
GS 2
GS 3
Unit
SBO-01
SBO-02
SBO-03
SBO-04
SBO-05
SBO-06
SBO-07
SBO-09
SBO-10
SBO-11
1C – mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 BqL 1 BqL 1
26.41 5.36 4.58 4.80 0.54 0.27 13.12 8.54 9.45 14.32 0.83 7.33 10.39 0.29 5.67 0.19
26.57 5.17 3.50 4.46 0.43 0.13 15,11 958 7.07 14.18 0.53 17.33 14.78 0.22 6.47 0.14
25.52 5.06 4.14 5.36 0.62 0.44 14.51 9.25 8.88 11.63 0.72 6.83 8.89 0.22 7.24 0.15
25.59 5.76 4.15 5.28 0.42 0.60 15.92 7.50 7.42 10.00 0.07 2.33 34.56 0.19 7.09 0.14
26.15 5.60 3.84 5.38 0.56 1.51 1566 6.83 5.48 8.62 0.08 6.67 30.28 0.31 7.40 0.13
27.59 5.96 3.71 5.10 0.41 3.53 16.42 9.04 7.05 11.92 0.27 6.33 22.61 0.26 9.05 0.15
24.94 6.20 3.86 6.08 0.48 0.46 6.34 8.25 7.42 2.10 0.32 17.67 35.94 0.25 1.11 0.11
26.29 5.72 3.23 5.11 0.21 0.44 0.48 8.67 8.32 2.92 0.80 4.83 15.56 0.24 9.27 0.10
26.13 5.60 4.04 4.25 0.23 0.33 0.44 7.46 8.80 2.62 0.63 5.67 25.83 0.49 9.06 0.12
26.03 5.59 4.06 4.18 0.32 5.87 0.80 11.38 13.23 4.53 0.25 6.17 8.89 0.22 5.02 0.14
Fig. 3. Chemical and physical–chemical parameters of groundwater in the sampling period.
and lowest in September 2010 (4.63, for SBO-01). The average pH of the samples ranged from 5.04 (SBO-02) to 6.2 (SBO-07). Potassium showed higher concentrations in monitoring wells located in the GS 1, ranging from 6.93 mgL 1 (SBO-02 in August 2010) to 19.72 mgL 1 (SBO-06 in August 2010). The concentration of this element was lower in wells of GS 3, ranging from 0.14 mgL 1 (SOB-10 in August 2010) to 1.42 mgL 1 (SBO-11 in October 2010). Silica showed concentrations ranging from 4 mgL 1 (SBO-06 in November 2010) to 17.6 mgL 1 (SBO-11 in December 2010).
A significant correlation was found between silica and magnesium in SBO-03 (0.82), indicating a possible increase of these constituents due to leaching processes. The variations in the concentrations of calcium were slight during the sampling months. Calcium was correlated with magnesium in SBO-02 (0.79), indicating an increase related to dissolution processes occurring in soils, rocks and sediments. In general, samples collected at GS 3 exhibited lower magnesium concentration than in others.
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Fig. 4. Chemical and radiological parameters of groundwater in the sampling period.
Table 2 Concentration of BTEX (mgL 1) for samples collected in January 2011. Point
Benzene
Toluene
m,p-xylene
O-xylene
Total xylene
Etilbenzene
Total BTEX
SBO-01 SBO-02 SBO-03 SBO-04 SBO-05 SBO-06 SBO-07 SBO-09 SBO-10 SBO-11
1.25 2.32 1.25 3.25 3.05 ND ND 2.02 33.40 22.63
ND 11.20 3.33 13.91 4.29 4.17 ND 32.13 144.94 203.81
3.26 3.25 10.25 2.90 2.43 1.59 ND 6.53 123.10 573.17
2.25 1.25 2.85 10.79 2.32 3.48 ND 8.53 133.94 380.63
5.51 4.50 13.10 13.69 4.75 5.06 ND 15.06 257.04 953.80
1.01 ND 3.28 7.53 1.18 1.80 ND 5.84 26.35 64.78
7.78 18.03 20.97 38.39 13.28 11.04 ND 55.06 461.73 1,245.03
ND: not detected.
Abnormalities were identified in the concentration of iron in some samples collected at SBO-06 and SBO-11. Chloride exhibited higher concentrations in the samples of GS 1, ranging from 2.2 mgL 1 (SBO-02 in November 2010) to 23.9 mgL 1 (SBO-02 in October 2010). The samples on GS 3 exhibited concentrations of chloride ranging from 0.4 mgL 1 (SBO-10 in August 2010) to 1.42 mgL 1 (SBO-11 in January 2011 ). The chloride values were between 0.4 mgL 1 (in August 2010) and 3.4 mgL 1 (in November 2010) at GS 2 (SBO-07). There was no presence of carbonate in the samples. Bicarbonate exhibited concentrations ranging from 4 mgL 1 (SBO-11 in August 2010) to 79 mgL 1 (SBO-04 in August 2010). The phosphate concentration in the samples generally ranged from 0.02 mgL 1 (SBO11 in August 2010) to 1.61 mgL 1 (SBO-10 in August 2010). For sulfate, low values were found at SBO-04, while the points SOB-02 and SBO-07 showed the highest mean values. The nitrate concentration in the sampled points ranged from 0.1 mgL 1 to 1.8 mgL 1. The nitrate concentration in SBO-01, SBO-02, SBO-03, SBO-07, SBO09 and SBO-10 tended to increase with the rainfall. In GS 1, the concentrations of the cations followed the order K þ 4Ca2 þ 4Mg2 þ in all months, while anions followed the order HCO3 4Cl 4SO24 in August, September, November and December 2010 and January 2011, but the order Cl 4 SO24 4HCO3 in October 2010. In GS 2, the cations followed the
order K þ 4Ca2 þ 4Mg2 þ in October 2010 and the order Ca2 þ 4K þ 4Mg2 þ in the remaining months. Anions exhibited the order HCO3 4SO24 4Cl in all months. In GS 3, the values of cations followed the order Ca2 þ 4K þ 4Mg2 þ in all months. Anions exhibited the order HCO3 4SO24 4Cl in August and September 2010, the order of SO2 in October 2010 4 4HCO3 4Cl and the order HCO3 4Cl 4SO2 in November and December 4 2010 and January 2011. 4.2. BTEX distribution in groundwater Dissolved hydrocarbons were detected in groundwater, in spite of the long time after spilling. This indicates that retained or adsorbed hydrocarbons in the soil represent a constant source of contamination, as kerosene was detected in soils at the GS 1 (AMGMA, 2009) and diesel at the GS 2 (WHB, 1999), after the contamination in 1994. The analyzed values of BTEX in January 2011 are showed in Table 2. The values found at GS 3 are higher than those in GS 1 and GS 2. Figs. 5 and 6 show the distribution of organic compounds. The data indicate a predominance of pollutants in SBO-03 and SBO04 points, in accordance with the groundwater flow direction (Fig. 2). In January 2011, the concentration of BTEX in SB0-05 and SBO04, near the site of the kerosene spill, tended to be higher than
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Fig. 5. Maps of distribution for benzene (left top), ethylbenzene (right top), m,p-xylene (left bottom) and o-xylene (right bottom).
that found in SBO-06. BTEX followed the groundwater flow direction, with a tendency of decreasing the concentration of hydrocarbons according to the amount of the contamination source area that moves from SBO-05 towards SBO-03 and SBO04. This decrease in the BTEX concentration in the leak source can be attributed to processes such as advection, dilution, dispersion, sorption, volatilization and biodegradation. It is possible verify in Fig. 6 that the highest concentration of total BTEX in SBO-04, near the gasoline tank, may indicate another spill. 4.3. 226Ra and 222Rn distribution in groundwater and its relationship with BTEX The activity concentration of radium ranged from 0.04 BqL 1 to 0.28 BqL 1 in the months sampled (Fig. 4). During the dry season, the average activity concentration is 0.16 BqL 1, while in the rainy season is 0.12 BqL 1. In point SBO-05 was identified a significant correlation (0.82) between radium and chloride. This can be explained by the ease of complexation between these compounds. According to Bonotto (2004), waters with high salinity, which typically occurs in producing basins of oil and gas, are carriers of radium and considerable amounts of radon. The mean radium activity concentration was higher (0.20 BqL 1) in September, whose rainfall was the lowest. The average radon activity concentration was also greater in September (9.00 BqL 1). This may be due to water infiltration in the previous months, which may have dissolved materials containing
radionuclides. On the other hand, the lowest average radium (0.12 BqL 1) was observed during the months of greatest rainfall, December 2010 and January 2011. This activity containing can be attributed to possible dilution of radium because of a greater volume of water infiltrated into the aquifer during this period. The activity concentration of radon in the sampling sites ranged from 1.53 BqL 1 (SBO-03 in December 2010) to 13.55 BqL 1 (SBO-03 in August 2010). The mean activity concentration of radon in the dry season is 7.24 BqL 1, while the average during the rainy season is 6.23 BqL 1. Fig. 7 illustrates the distribution for radium and radon at GS 1 in January 2011. It is possible to verify the highest radon concentrations near the point SBO-05, where the kerosene leak occurred in 1994 and whose BTEX concentrations were also high. The relationship between the average concentration of radium and radon during the sampling period was significant in the points SBO-01 (0.77), SBO-09 (0.68) and SBO-10 (0.65). The following correlations between radon and hydrocarbons were obtained for analysis performed in January 2011: toluene (0.58), benzene (0.56), ethylbenzene (0.54), total BTEX (0.53), o-xylene (0.52 ), total xylene (0.49), p-xylene (0.47). They indicate that the presence of dissolved hydrocarbons tends to control the concentration of radon in water, increasing it due to the preferential partition at this stage. Thus, radon can indicate the presence of a plume of dissolved fuel contamination, however, it does not provide information about the presence of residual contaminants or saturation of the aquifer pores.
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Fig. 6. Maps of distribution for total xylene (left top), toluene (right top) and total BTEX (center bottom).
The radon concentration in groundwater is directly proportional to the extent of the contact surface between water and mineral grains. When comparing the values of the average activity of radon (Table 1) from August 2010 to January 2011 with the particle size of each point, it is observed that the radon activity increases in materials whose particle size is smaller (clayey soil osilty soil osandy soil) and whose layer depth also increases. However, exceptions occur at the point SBO-06 (higher radon activity, silty soil and higher depths that are factors possibly contributing to a greater radon accumulation in groundwater) and point SBO-04 (clayey soil, intermediate radon activity and smaller layer depth, i.e. 6 m).
5. Conclusion The dissolved BTEX concentration has been used elsewhere to distinguish areas contaminated with dissolved fuel and residual NAPL of those only contaminated with dissolved fuel. For such purpose, it has been compared the radon activity in water with the average background value determined for the site studied. This investigation focused three gas stations located at Santa Ba´rbara d’Oeste city, Sa~ o Paulo State, Brazil. It is located in the northeast portion of the Parana´ Basin and comprises a varied sequence of sediments deposited on the crystaline basement, which consists on a suite of older igneous and metamorphic rocks. Groundwater samples were collected from August 2010 to
January 2011 in several monitoring wells drilled at the gas stations. The sampling was carried out using a hand pump attached to 1 L glass bottles with minimal radon gas loss at the collection time. The radon concentration was non-uniform in the aquifer systems investigated whose presence in groundwater depended on factors like the radium activity, the radon emanation coefficient, the dry soil density and the soil porosity, among others. In January 2011, the BTEX concentration near the kerosene spill site tended to be higher than that found in the area situated downstream. The BTEX concentration followed the groundwater flow direction, with a tendency of decreasing the hydrocarbons content in accordance with the extent of the contamination plume. This decrease in the BTEX concentration in the leak source could be attributed to processes such as advection, dilution, dispersion, sorption, volatilization and biodegradation. It was also possible to verify that the highest radon concentration occurred in a site in which (1) the kerosene leak occurred in 1994 and (2) the BTEX concentration was high too. The following correlations between radon and hydrocarbons were obtained for analysis performed in January 2011: toluene (0.58), benzene (0.56), ethylbenzene (0.54), total BTEX (0.53), o-xylene (0.52), total xylene (0.49), p-xylene (0.47). They indicate that the presence of dissolved hydrocarbons tends to control the radon concentration in water, increasing it due to the preferential partition at this stage. Thus, radon can indicate the presence of a plume of dissolved fuel contamination, despite it does not provide information about the presence of residual contaminants or saturation of
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Fig. 7. Maps of distribution for
226
Ra (top) and
222
the aquifer pores. However, for a more refined study, it would be necessary to evaluate the 226Ra presence in a larger scale than that focused in this paper as the radium concentration in the groundwater samples did not vary significantly. This shoud be performed with the aim of detecting possible anomalies in the 222 Rn concentration and hence reducing possible error sources, contributing to detection of residual saturation of hydrocarbons in the aquifer. Therefore, the monitoring of chemical and radiological parameters in the study area allowed to find significant results on the presence of dissolved hydrocarbons in groundwater. The findings don’t allow quantify the aquifer pore saturation by hydrocarbons, but are useful for detecting the contamination of dissolved hydrocarbons in groundwater and for evaluating possible areas in which occur residual contamination. This can help in a preliminary investigation of a site contaminated by fuel that is an event becoming relevant in present days as can cause environmental damage, affecting the human health.
Rn (bottom) in January 2011. All values are in BqL 1.
Acknowledgments This investigation was performed under a financial support from CNPq (National Council for Scientific and Technologic Development), Brazil.
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Genitron, 2000. Alpha Guard PQ2000/MC50—Multiparameter Radon Monitor. Genitron Instruments, Frankfurt, Germany. Hach, 1992. Water Analysis Handbook. Hach Company, Colorado, United States. ¨ Hohener, P., Surbeck, H., 2004. Radon-222 as a tracer for nonaqueous phase liquid in the vadose zone: experiments and analytical model. Vadose Zone J. 3, 1276–1285. ¨ Hunkeler, D., Hoehn, E., Hohener, P., Zeyer, J., 1997. Radon-222 as a partitioning tracer to detect diesel fuel contamination in aquifers: laboratory study and field observations. Environ. Sci. Technol. 31, 3180–3187. IPT (Instituto de Pesquisa Tecnolo´gica do Estado de Sa~ o Paulo), 1981. Mapa Geomorfolo´gico do Estado de Sa~ o Paulo. Scale 1:1.000.000. Sa~ o Paulo, Brasil. Lima, J.L.N., Bonotto, D.M., 1996. Etapas analı´ticas para mensurac- a~ o de radˆonio222 e uso na avaliac- a~ o da radioatividade das a´guas de A´guas de Prata (SP). Geochim. Bras. 10 (2), 283–295. Machado, F.B., Nardy, A.J.R., Melo, R.P., Oliveira, M.A.F., Squisato, E., 2005. As rochas intrusivas da Formac- a~ o Serra Geral na porc- a~ o leste da bacia do Parana´ no estado de Sa~ o Paulo: aspectos petrogra´ficos e geoquı´micos — resultados preliminares. Geociˆencias 24, 5–17. PCJ (Comitˆes das Bacias Hidrogra´ficas dos Rios Piracicaba, Capivari e Jundiaı´), 1999. Mapa geolo´gico das Bacias Hidrogra´ficas dos Rios Piracicaba, Capivari e Jundiaı´. Scale 1:250.000. Sa~ o Paulo, Brasil.
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Schubert, M., Penab, P., Balcazar, M., Meissner, R., Lopez, A., Flores, J.H., 2005. Determination of radon distribution patterns in the upper soil as a tool for the localization of subsurface NAPL contamination. Radiat. Meas. 40, 633–637. ¨ Schubert, M., Paschke, A., Lau, S., Geyer, W., Knoller, W.K., 2007a. Radon as a naturally occurring tracer for the assessment of residual NAPL contamination of aquifers. Environ. Pollut. 145, 920–927. Schubert, M., Lehmann, K., Paschke, A., 2007b. Determination of radon partition coefficients between water and organic liquids and their utilization for the assessment of subsurface NAPL contamination. Sci. Total Environ. 376, 306–316. Semprini, L., Hopkins, O.S., Tasker, B.R., 2000. Laboratory, Field and Modeling Studies of Radon-222 as a natural tracer for monitoring NAPL contamination. Transp. Porous Media 38, 223–240. Wanty, R.B., Lawrence, E.P., Gundersen, L.C.S., 1992. A Theoretical Model for the Flux of Radon from Rock to Groundwater. In: Gates, A.E., Gundersen, L.C.S. (Eds.), Geologic Controls on Radon. Geological Society of America, Boulder, Colorado. WHB (Waterloo Hydrogeologic Brasil), 1999. Caracterizac- a~ o Geolo´gica, Hidrogeolo´gica e Ana´lise Qualitativa da Contaminac- a~ o no Solo e na A´gua Subterrˆanea. WHB, Sa~ o Paulo, Brasil.
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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Radon in groundwater contaminated by dissolved hydrocarbons in Santa Ba´rbara dOeste, Sa~ o Paulo State, Brazil J.A. Galhardi, D.M. Bonotto n Departamento de Petrologia e Metalogenia, Universidade Estadual Paulista (UNESP), Campus de Rio Claro, Av. 24-A, No. 1515-CP 178, CEP 13506-900, Rio Claro, ~ Paulo, Brazil Sao
H I G H L I G H T S c c c
Integration of chemical and radiometric data acquired in a polluted area. Possible use of the information in other areas. Potential hazard to human health due to Rn and organic compounds.
a r t i c l e i n f o
abstract
Article history: Received 24 May 2012 Received in revised form 24 June 2012 Accepted 25 June 2012 Available online 14 July 2012
This investigation reported the 222Rn activity concentration and dissolved hydrocarbon content in ~ groundwater collected in three gas stations where occurred tanks leaks, in Santa Barbara d’Oeste, Sao Paulo State, Brazil. The results indicated a tendency of correlation between the radon and BTEX, suggesting that the presence of dissolved hydrocarbons increase the radon concentration in water, due to the preferential partition at this phase. The radiometric data are useful for the detection of residual contamination and dissolved hydrocarbon plumes in groundwater, reinforcing the findings of previous studies held elsewhere. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Radon Groundwater Contamination NAPL
1. Introduction The fuel leaks in storage tanks constitute one of the most common sources of contamination of groundwater and soil and can give rise to the formation of plumes of dissolved hydrocarbons in groundwater and residual phase stored in the pores of the aquifer. In this context, techniques involving the use of markers of partition have been used to the evaluation of the amount and distribution of organic phase in aquifers. The marker partition tends to accumulate at the interface between water and organic phase (also known as non-aqueous phase liquid, or NAPL). The magnitude of the decrease in tracer concentration during its movement through the aquifer is used to delineate the interface area and to estimate the degree of saturation of the aquifer pores by the specific hydrocarbon (Schubert et al., 2007a).
n
Corresponding author. Tel.: þ55 19 35269244; fax: þ 55 19 35249644. E-mail address: [email protected] (D.M. Bonotto).
0969-8043/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2012.06.029
The radionuclide 222Rn has been used in field tests and laboratory experiments by some authors as a marker (Hunkeler ¨ et al., 1997; Semprini et al., 2000; Hohener and Surbeck, 2004; Davis et al., 2005; Fan et al., 2007; Schubert et al., 2005, 2007a, 2007b), who demonstrated its efficiency in tracking contamination plumes and for quantification of the residual saturation of the aquifer by hydrocarbons. Its applicability as a marker is based on the fact that the 226Ra gives rise to a flow of gas 222Rn, after the radioactive decay, which occupies the empty spaces between mineral grains of the aquifer, dissolves in the groundwater or escapes into the atmosphere. The gas fraction escaping from the host materials and retained in the pores corresponds to the emanation coefficient or emanating efficiency (Andrews and Wood, 1972; Wanty et al., 1992), where, in general, the smaller the particle size, the larger the specific surface area and the greater the emanation power of materials containing 226Ra. Other factors influencing the radon activity in water include (a) the composition of the soil or rock storing the water, (b) the presence of minerals containing uranium or radium, (c) the extent of the materials surface in contact with water, (d) the speed that water moves through the aquifer,
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(e) the moisture in the pore space, (f) the pressure and water temperature, (g) the 222Rn recoil following alpha-decay of 226Ra atoms close to the materials surface (within the 222Rn recoil range of 0.036 mm), (h) 222Rn diffusion through the crystalline lattice from production sites below the materials surface and (i) 222Rn diffusion along crystal defects, grain boundaries or microfractures from greater depths below the rock surface. In the absence of free organic phase or residual hydrocarbons, concentrations of 222Rn in groundwater quickly reach an equilibrium determined mainly by the mineral composition of the aquifer solids. In the presence of free organic phase, the concentration of radon in groundwater can be substantially reduced due to preferential partitioning of radon in this phase. This reduction can be quantitatively related to the degree of saturation ¨ of the of the aquifer matrix (Semprini et al., 2000; Hohener and Surbeck, 2004). The advantage to use 222Rn as an indicator consists on the fact that it occurs as a natural component of groundwater, being continually produced by 226Ra decay in the 238U decay series (Bonotto, 2004). 222 Rn is an alpha emitter with an energy of 5.48 MeV, half-life of 3.8 days and decay constant of 0.0001258 min 1. Its solubility in the organic phase is illustrated by the gas partition coefficient in the organic phase and water (KNAPL/W), which depends on temperature and NAPL composition. In toluene, for example, KNAPL/W equals 46. It is estimated that for KNAPL/W equal to 50 (estimated value for chlorinated solvents (CHCl3)), the residual saturation of only 1% of NAPL in the aquifer results in 33% reduction in the concentration of 222 Rn in water (Semprini et al., 2000). In research conducted by Schubert et al. (2005), radon was investigated in a military area abandoned and contaminated by non-aqueous phase liquid. The data revealed a nonuniform distribution of radon gas in soil air sampled in the region, and some values were about 90% lower than the background area. This anomaly may be associated with the presence of NAPL in the subsurface, due to the fact that radon is associated with this phase, moving from the air soil or groundwater and being dissolved at the organic phase. This method is successful when the radon activity concentration in groundwater is compared to the presence of residual NAPL in the aquifer matrix, but not to the dissolved organics (typically benzene, toluene, ethylbenzene and xylene, or BTEX). This is because the dissolved products do not bring safety qualitative and quantitative information about the source of hydrocarbons because their presence and concentration depends on the mix of organic contaminants, the spill time, the flux and velocity of groundwater flow and the presence and rate of other factors of natural attenuation of hydrocarbons. According to Schubert et al. (2007a), the evaluation of the results of the radon
activity in groundwater and the concentration of dissolved BTEX can be used to distinguish areas contaminated with dissolved fuel and residual NAPL (in a case in which the radon activity in water is less than the background of the region) of those only contaminated with dissolved fuel (in the case in which the radon activity in water is about the same of the average background of the region). The greater the proportion of free and residual phase less radon is expected in the soil air and groundwater near the site of the fuel spill, due to the partition of radon in organic phase. But the more hydrocarbon dissolved in a sample, it should contain more radon, due to the preference of dissolved radon in this phase. Thus, it may be possible to perform a preliminary analysis of the presence of residual NAPL contamination and the presence of dissolved contaminant plumes, although the dissolved hydrocarbons do not provide quantitative information about the fraction of residual fuels in aquifers. Despite these factors, it is assumed that the radon concentration in groundwater correlates positively with the presence of dissolved hydrocarbons, due to the gas preference in it. This papers aims to focus these aspects in three gas stations located at Santa Ba´rbara d’Oeste city, Sa~ o Paulo State, Brazil.
2. General features of the studied area The region of Santa Ba´rbara d’Oeste is located in the northeast portion of the Parana´ Basin, consisting of a varied sequence of sediments deposited on the crystaline basement, that comprises a set of older igneous and metamorphic rocks in the Depressa~ o Perife´rica Paulista (IPT, 1981). The lithostratigraphy is composed by the Itarare´ Group and Tatuı´ Formation (PCJ, 1999). Cenozoic deposits occur in extensive areas, overlying the Itarare´ Group (WHB, 1999), which storage the water collected in this study. The location of the three gas stations (GS) investigated in this search can be viewed in Fig. 1. The samples SBO-01–SBO-06 were collected at the GS 1, the sample SBO-07 at the GS 2 and the samples SBO-09–SBO-11 at the GS 3. The sample SBO-08 was disregarded due to its high amount of impurities. The potentiometric map for the GS 1 in January 2011 is ilustrated in Fig. 2. The kerosene tank is located between the sampling points SBO-05 and SBO-06. The points SBO-06 and SBO-05 are located downstream and upstrearn from the kerosene spill area, respectively. The point SBO-03 is located upstream of the local of oil change, the point SBO-01 is situated in the area for washing vehicles and the point SBO-02 is located upstream of the fuel supply area.
Fig. 1. Location of the study area. Modified from Machado et al. (2005).
J.A. Galhardi, D.M. Bonotto / Applied Radiation and Isotopes 70 (2012) 2507–2515
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Fig. 2. Water table altitude (m) of GS 1 (January 2011).
3. Sampling and analytical methods Samples were collected monthly in monitoring wells located at gas stations from August 2010 to January 2011. Sampling was carried out using a hand pump attached to glass bottles with a capacity of 1 l, avoiding the radon gas loss at the collection time. The bottles were completely filled by the water samples and sealed with rubber stoppers containing two 5 mm diameter nylon tubes (Lima and Bonotto, 1996). A 100 mL aliquot for radon analysis was separated and the flask was sealed again to preserve the sample for 226Ra analysis after it comes into equilibrium with the radon (about 25 day). Samples for analysis of anions and cations were collected in polyethylene bottles of 2 L capacity. Once collected, samples were sent to the laboratory for radon measurement. The time elapsed between the collection and analysis was used to correct the activity due to its radioactive decay. Dissolved oxygen (DO), temperature and pH were measured in field using portable devices. The concentration of aluminum, potassium, total iron, calcium, magnesium, barium, silica, nitrate, chloride, sulphate and total phosphate was determined at LABIDRO—Laboratory of Isotopes and Hydrochemistry of the Department of Petrology and Metallogeny of the Institute of Geosciences and Exact Sciences, UNESP, Rio Claro. The Hach spectrophotometer Model DR 2000 was used for such purpose (Hach, 1992). The total alkalinity was evaluated by titration (Hach, 1992), using a standard solution of sulfuric acid (0.020 N). The determination of 222Rn and 226Ra was also held at LABIDRO, whereas the analysis of BTEX was performed at ASL Laboratory, Rio Claro city, using gas chromatography based on EPA 8015-C method. The technique used to quantify 222Rn and 226Ra was alpha spectrometry. The device used was Alpha Guard PQ2000PRO (Genitron GmbH) equipped with an appropriate drive (Aquakit) (Genitron, 2000). The meter consists of an ionization chamber that measures radon in a system consisting of two glass containers where occurs the radon degassing contained in the sample. Indoors, the samples were monitored by Alpha Guard in five
cycles of 10 min. The determination of the radon concentration in water samples by Aquakit–Alpha Guard units was done by Eq. (1) 1000cw ¼ ca ½kþ ðV SY V SA Þ=V SA c0
ð1Þ 1
where: cw ¼radon concentration in water (BqL ); ca ¼measured value (Bq/m3) indicated by Alpha Guard; c0 ¼zero level radon concentration (0 Bq/m3); VSY ¼system volume (1122 mL); VSA ¼volume of water sample (100 mL); k¼distribution coefficient of radon between air and liquid phase (0.16). Eq. (2) was used for correcting the radon decay between collection and analysis. C w0 ¼ C w eDt
ð2Þ
where: Cw’ ¼corrected radon concentration (BqL 1); Cw ¼radon concentration measured in the water sample (BqL 1); D ¼radon decay constant; t¼time elapsed between sampling and analysis. A period of 25 days is needed for 222Rn to reach radioactive equilibrium with 226Ra. After elapsing it, the same procedure was used for 226Ra analysis by alpha counting. For cleaning purposes, the Alpha Guard system was coupled to an activated charcoal filter to remove the radon of the system, after processing each sample.
4. Results and discussion 4.1. Chemical and physical data Table 1 summarizes the mean values of the parameters characterized in the study area during the sampling period from August 2010 to January 2011, whereas Figs. 3 and 4 illustrate the whole data set. The values of pH, temperature and DO do not change with rainfall in the area. All samples are classified as acid, whose highest pH value was found in November 2010 (6.93, for SBO-09)
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Table 1 Mean values of the parameters characterized in GS 1, GS 2 and GS 3 during the sampling period (August 2010 to January 2011). GS 1
T pH DO Ca2 þ Mg2 þ Fetotal Kþ Ba2 þ SiO2 Cl NO3 SO24 HCO3 PO34 222 Rn 226 Ra
GS 2
GS 3
Unit
SBO-01
SBO-02
SBO-03
SBO-04
SBO-05
SBO-06
SBO-07
SBO-09
SBO-10
SBO-11
1C – mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 mgL 1 BqL 1 BqL 1
26.41 5.36 4.58 4.80 0.54 0.27 13.12 8.54 9.45 14.32 0.83 7.33 10.39 0.29 5.67 0.19
26.57 5.17 3.50 4.46 0.43 0.13 15,11 958 7.07 14.18 0.53 17.33 14.78 0.22 6.47 0.14
25.52 5.06 4.14 5.36 0.62 0.44 14.51 9.25 8.88 11.63 0.72 6.83 8.89 0.22 7.24 0.15
25.59 5.76 4.15 5.28 0.42 0.60 15.92 7.50 7.42 10.00 0.07 2.33 34.56 0.19 7.09 0.14
26.15 5.60 3.84 5.38 0.56 1.51 1566 6.83 5.48 8.62 0.08 6.67 30.28 0.31 7.40 0.13
27.59 5.96 3.71 5.10 0.41 3.53 16.42 9.04 7.05 11.92 0.27 6.33 22.61 0.26 9.05 0.15
24.94 6.20 3.86 6.08 0.48 0.46 6.34 8.25 7.42 2.10 0.32 17.67 35.94 0.25 1.11 0.11
26.29 5.72 3.23 5.11 0.21 0.44 0.48 8.67 8.32 2.92 0.80 4.83 15.56 0.24 9.27 0.10
26.13 5.60 4.04 4.25 0.23 0.33 0.44 7.46 8.80 2.62 0.63 5.67 25.83 0.49 9.06 0.12
26.03 5.59 4.06 4.18 0.32 5.87 0.80 11.38 13.23 4.53 0.25 6.17 8.89 0.22 5.02 0.14
Fig. 3. Chemical and physical–chemical parameters of groundwater in the sampling period.
and lowest in September 2010 (4.63, for SBO-01). The average pH of the samples ranged from 5.04 (SBO-02) to 6.2 (SBO-07). Potassium showed higher concentrations in monitoring wells located in the GS 1, ranging from 6.93 mgL 1 (SBO-02 in August 2010) to 19.72 mgL 1 (SBO-06 in August 2010). The concentration of this element was lower in wells of GS 3, ranging from 0.14 mgL 1 (SOB-10 in August 2010) to 1.42 mgL 1 (SBO-11 in October 2010). Silica showed concentrations ranging from 4 mgL 1 (SBO-06 in November 2010) to 17.6 mgL 1 (SBO-11 in December 2010).
A significant correlation was found between silica and magnesium in SBO-03 (0.82), indicating a possible increase of these constituents due to leaching processes. The variations in the concentrations of calcium were slight during the sampling months. Calcium was correlated with magnesium in SBO-02 (0.79), indicating an increase related to dissolution processes occurring in soils, rocks and sediments. In general, samples collected at GS 3 exhibited lower magnesium concentration than in others.
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Fig. 4. Chemical and radiological parameters of groundwater in the sampling period.
Table 2 Concentration of BTEX (mgL 1) for samples collected in January 2011. Point
Benzene
Toluene
m,p-xylene
O-xylene
Total xylene
Etilbenzene
Total BTEX
SBO-01 SBO-02 SBO-03 SBO-04 SBO-05 SBO-06 SBO-07 SBO-09 SBO-10 SBO-11
1.25 2.32 1.25 3.25 3.05 ND ND 2.02 33.40 22.63
ND 11.20 3.33 13.91 4.29 4.17 ND 32.13 144.94 203.81
3.26 3.25 10.25 2.90 2.43 1.59 ND 6.53 123.10 573.17
2.25 1.25 2.85 10.79 2.32 3.48 ND 8.53 133.94 380.63
5.51 4.50 13.10 13.69 4.75 5.06 ND 15.06 257.04 953.80
1.01 ND 3.28 7.53 1.18 1.80 ND 5.84 26.35 64.78
7.78 18.03 20.97 38.39 13.28 11.04 ND 55.06 461.73 1,245.03
ND: not detected.
Abnormalities were identified in the concentration of iron in some samples collected at SBO-06 and SBO-11. Chloride exhibited higher concentrations in the samples of GS 1, ranging from 2.2 mgL 1 (SBO-02 in November 2010) to 23.9 mgL 1 (SBO-02 in October 2010). The samples on GS 3 exhibited concentrations of chloride ranging from 0.4 mgL 1 (SBO-10 in August 2010) to 1.42 mgL 1 (SBO-11 in January 2011 ). The chloride values were between 0.4 mgL 1 (in August 2010) and 3.4 mgL 1 (in November 2010) at GS 2 (SBO-07). There was no presence of carbonate in the samples. Bicarbonate exhibited concentrations ranging from 4 mgL 1 (SBO-11 in August 2010) to 79 mgL 1 (SBO-04 in August 2010). The phosphate concentration in the samples generally ranged from 0.02 mgL 1 (SBO11 in August 2010) to 1.61 mgL 1 (SBO-10 in August 2010). For sulfate, low values were found at SBO-04, while the points SOB-02 and SBO-07 showed the highest mean values. The nitrate concentration in the sampled points ranged from 0.1 mgL 1 to 1.8 mgL 1. The nitrate concentration in SBO-01, SBO-02, SBO-03, SBO-07, SBO09 and SBO-10 tended to increase with the rainfall. In GS 1, the concentrations of the cations followed the order K þ 4Ca2 þ 4Mg2 þ in all months, while anions followed the order HCO3 4Cl 4SO24 in August, September, November and December 2010 and January 2011, but the order Cl 4 SO24 4HCO3 in October 2010. In GS 2, the cations followed the
order K þ 4Ca2 þ 4Mg2 þ in October 2010 and the order Ca2 þ 4K þ 4Mg2 þ in the remaining months. Anions exhibited the order HCO3 4SO24 4Cl in all months. In GS 3, the values of cations followed the order Ca2 þ 4K þ 4Mg2 þ in all months. Anions exhibited the order HCO3 4SO24 4Cl in August and September 2010, the order of SO2 in October 2010 4 4HCO3 4Cl and the order HCO3 4Cl 4SO2 in November and December 4 2010 and January 2011. 4.2. BTEX distribution in groundwater Dissolved hydrocarbons were detected in groundwater, in spite of the long time after spilling. This indicates that retained or adsorbed hydrocarbons in the soil represent a constant source of contamination, as kerosene was detected in soils at the GS 1 (AMGMA, 2009) and diesel at the GS 2 (WHB, 1999), after the contamination in 1994. The analyzed values of BTEX in January 2011 are showed in Table 2. The values found at GS 3 are higher than those in GS 1 and GS 2. Figs. 5 and 6 show the distribution of organic compounds. The data indicate a predominance of pollutants in SBO-03 and SBO04 points, in accordance with the groundwater flow direction (Fig. 2). In January 2011, the concentration of BTEX in SB0-05 and SBO04, near the site of the kerosene spill, tended to be higher than
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Fig. 5. Maps of distribution for benzene (left top), ethylbenzene (right top), m,p-xylene (left bottom) and o-xylene (right bottom).
that found in SBO-06. BTEX followed the groundwater flow direction, with a tendency of decreasing the concentration of hydrocarbons according to the amount of the contamination source area that moves from SBO-05 towards SBO-03 and SBO04. This decrease in the BTEX concentration in the leak source can be attributed to processes such as advection, dilution, dispersion, sorption, volatilization and biodegradation. It is possible verify in Fig. 6 that the highest concentration of total BTEX in SBO-04, near the gasoline tank, may indicate another spill. 4.3. 226Ra and 222Rn distribution in groundwater and its relationship with BTEX The activity concentration of radium ranged from 0.04 BqL 1 to 0.28 BqL 1 in the months sampled (Fig. 4). During the dry season, the average activity concentration is 0.16 BqL 1, while in the rainy season is 0.12 BqL 1. In point SBO-05 was identified a significant correlation (0.82) between radium and chloride. This can be explained by the ease of complexation between these compounds. According to Bonotto (2004), waters with high salinity, which typically occurs in producing basins of oil and gas, are carriers of radium and considerable amounts of radon. The mean radium activity concentration was higher (0.20 BqL 1) in September, whose rainfall was the lowest. The average radon activity concentration was also greater in September (9.00 BqL 1). This may be due to water infiltration in the previous months, which may have dissolved materials containing
radionuclides. On the other hand, the lowest average radium (0.12 BqL 1) was observed during the months of greatest rainfall, December 2010 and January 2011. This activity containing can be attributed to possible dilution of radium because of a greater volume of water infiltrated into the aquifer during this period. The activity concentration of radon in the sampling sites ranged from 1.53 BqL 1 (SBO-03 in December 2010) to 13.55 BqL 1 (SBO-03 in August 2010). The mean activity concentration of radon in the dry season is 7.24 BqL 1, while the average during the rainy season is 6.23 BqL 1. Fig. 7 illustrates the distribution for radium and radon at GS 1 in January 2011. It is possible to verify the highest radon concentrations near the point SBO-05, where the kerosene leak occurred in 1994 and whose BTEX concentrations were also high. The relationship between the average concentration of radium and radon during the sampling period was significant in the points SBO-01 (0.77), SBO-09 (0.68) and SBO-10 (0.65). The following correlations between radon and hydrocarbons were obtained for analysis performed in January 2011: toluene (0.58), benzene (0.56), ethylbenzene (0.54), total BTEX (0.53), o-xylene (0.52 ), total xylene (0.49), p-xylene (0.47). They indicate that the presence of dissolved hydrocarbons tends to control the concentration of radon in water, increasing it due to the preferential partition at this stage. Thus, radon can indicate the presence of a plume of dissolved fuel contamination, however, it does not provide information about the presence of residual contaminants or saturation of the aquifer pores.
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Fig. 6. Maps of distribution for total xylene (left top), toluene (right top) and total BTEX (center bottom).
The radon concentration in groundwater is directly proportional to the extent of the contact surface between water and mineral grains. When comparing the values of the average activity of radon (Table 1) from August 2010 to January 2011 with the particle size of each point, it is observed that the radon activity increases in materials whose particle size is smaller (clayey soil osilty soil osandy soil) and whose layer depth also increases. However, exceptions occur at the point SBO-06 (higher radon activity, silty soil and higher depths that are factors possibly contributing to a greater radon accumulation in groundwater) and point SBO-04 (clayey soil, intermediate radon activity and smaller layer depth, i.e. 6 m).
5. Conclusion The dissolved BTEX concentration has been used elsewhere to distinguish areas contaminated with dissolved fuel and residual NAPL of those only contaminated with dissolved fuel. For such purpose, it has been compared the radon activity in water with the average background value determined for the site studied. This investigation focused three gas stations located at Santa Ba´rbara d’Oeste city, Sa~ o Paulo State, Brazil. It is located in the northeast portion of the Parana´ Basin and comprises a varied sequence of sediments deposited on the crystaline basement, which consists on a suite of older igneous and metamorphic rocks. Groundwater samples were collected from August 2010 to
January 2011 in several monitoring wells drilled at the gas stations. The sampling was carried out using a hand pump attached to 1 L glass bottles with minimal radon gas loss at the collection time. The radon concentration was non-uniform in the aquifer systems investigated whose presence in groundwater depended on factors like the radium activity, the radon emanation coefficient, the dry soil density and the soil porosity, among others. In January 2011, the BTEX concentration near the kerosene spill site tended to be higher than that found in the area situated downstream. The BTEX concentration followed the groundwater flow direction, with a tendency of decreasing the hydrocarbons content in accordance with the extent of the contamination plume. This decrease in the BTEX concentration in the leak source could be attributed to processes such as advection, dilution, dispersion, sorption, volatilization and biodegradation. It was also possible to verify that the highest radon concentration occurred in a site in which (1) the kerosene leak occurred in 1994 and (2) the BTEX concentration was high too. The following correlations between radon and hydrocarbons were obtained for analysis performed in January 2011: toluene (0.58), benzene (0.56), ethylbenzene (0.54), total BTEX (0.53), o-xylene (0.52), total xylene (0.49), p-xylene (0.47). They indicate that the presence of dissolved hydrocarbons tends to control the radon concentration in water, increasing it due to the preferential partition at this stage. Thus, radon can indicate the presence of a plume of dissolved fuel contamination, despite it does not provide information about the presence of residual contaminants or saturation of
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Fig. 7. Maps of distribution for
226
Ra (top) and
222
the aquifer pores. However, for a more refined study, it would be necessary to evaluate the 226Ra presence in a larger scale than that focused in this paper as the radium concentration in the groundwater samples did not vary significantly. This shoud be performed with the aim of detecting possible anomalies in the 222 Rn concentration and hence reducing possible error sources, contributing to detection of residual saturation of hydrocarbons in the aquifer. Therefore, the monitoring of chemical and radiological parameters in the study area allowed to find significant results on the presence of dissolved hydrocarbons in groundwater. The findings don’t allow quantify the aquifer pore saturation by hydrocarbons, but are useful for detecting the contamination of dissolved hydrocarbons in groundwater and for evaluating possible areas in which occur residual contamination. This can help in a preliminary investigation of a site contaminated by fuel that is an event becoming relevant in present days as can cause environmental damage, affecting the human health.
Rn (bottom) in January 2011. All values are in BqL 1.
Acknowledgments This investigation was performed under a financial support from CNPq (National Council for Scientific and Technologic Development), Brazil.
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