geophysical methods in radon risk studies
TRANSCRIPT
Journal of Environmental Radioactivity 82 (2005) 351e362
www.elsevier.com/locate/jenvrad
Geophysical methods in radon risk studies
Malgorzata Wysocka*, Andrzej Kotyrba,Stanislaw Chalupnik, Jan Skowronek
Central Mining Institute, Laboratory of Radiometry, Plac Gwarkow 1, 40-166 Katowice, Poland
Received 3 February 2004; received in revised form 4 October 2004; accepted 3 February 2005
Abstract
The results of the studies presented in the paper have shown that in the Upper SilesianRegion in Poland, radon indoor concentration levels depend first of all on the geologicalstructure of the subsurface layers. The essential factors influencing radon migration ability are
the mining-induced transformations of a rock mass. In some cases, significant variations ofradon potential have been found at sites featuring similar geological structures andexperiencing comparable mining effects. To find out the causes of these variations, studiesinvolving geophysical methods such as electrical resistivity profiling (PE) and electrical
resisitivity sounding (VES) were used. These studies have shown that the measurements madeusing the electrical resistivity method can be helpful in evaluating radon potential of both thetectonically disturbed areas and the mining-transformed ones.
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Keywords: Radon; Geology; Mining; Subsurface layer; Geophysical methods
1. Introduction
The most important source of indoor radon in the Upper Silesian Coal Basin areais the bedrock, over which the buildings are located (Chalupnik and Wysocka, 2003).Radon is carried to the indoor space of buildings through cracks and fissures in
* Corresponding author. Tel.: C48 32 2592814; fax: C48 32 2585979.
E-mail address: [email protected] (M. Wysocka).
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doi:10.1016/j.jenvrad.2005.02.009
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foundations, floors and walls. The results of previous studies have shown that radonconcentration levels depend, first of all, on the geological structure of the Earth’scrust subsurface layers (Ball et al., 1991; Gundersen, 1992; Neznal et al., 1994). Theessential factors influencing radon migration ability are the mining-inducedtransformations taking place in a rock mass. In this study, short- and long-termmeasurements of radon concentrations in buildings were taken. Moreover,concentrations of this gas in soil air and its exhalation were also studied. In somecases, significant variations in radon potential were found at sites featuring similargeological structures and experiencing comparable mining effects. To bring tolight causes of these variations, studies involving geophysical methods wereconducted.
2. State of knowledge
For the description of the hazard level resulting from the occurrence of increasedradon in soil air concentrations in a study area, a concept of radon potential orradon risk can be used. Radon potential is assumed to be the arithmetic mean ofradon concentrations in soil air with respect to a given area unit (Akerblom, 1986).Some researchers use the concept of radon risk and in their studies, besides radonconcentrations in soil air, the soil permeability can also be considered (Barnet, 1995).
From the studies carried out in Upper Silesia, it follows that for the majority ofsites, an average or low radon potential should be assigned because radonconcentrations in soil air can rarely exceed a value of 50 000 Bq m�3. However,the results of measurements taken in certain areas of northern and north-easternparts of the Upper Silesian Coal Basin show departures from the general trend. Inthe areas of outcropping of Triassic sediments, radon concentrations in soil air areusually higher than in other areas.
The distribution of radon concentrations in houses can also indicate therelationship between the site geological structure and the gas emission level. Theaverage value of radon concentrations in ground floor in houses in the UpperSilesian Coal Basin area amounts to 47 Bq m�3 (Wysocka and Chalupnik, 2003). Inthe areas of the impermeable Miocene clay layer, radon concentrations both in soilair and in houses can be ranked as low since radon concentrations in soil air and inhouses do not exceed values of 8000 Bq m�3 and 120 Bq m�3, respectively. In theareas of outcropping of the Middle Triassic carbonate deposits the indoor radonconcentrations have considerably exceeded the average value for the whole of theUpper Silesian Coal Basin and amounted to 490 Bq m�3, whereas radonconcentrations in soil air ranged from dozens of to more than 70 000 Bq m�3.
Analysing the results of these studies against a background of the geologicalstructure of the Upper Silesian Coal Basin, we have found that the areas with thehighest levels of radon concentration both in soil gas and in houses exhibit a specificgeological structure. The formations with increased gas and radon migrationefficiency are the Middle Triassic carbonate rocks. The diploporita and ore-bearing
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dolomites and the Gogolin limestones of the Lower and Middle Muschelkalk ofMiddle Triassic sediments are characterised by high fracturing with many fissures,small spatial density and the porosity markedly higher than that of the surroundingrocks (Paw1owska and Szuwarzynski, 1979; Sass-Gustkiewicz, 1985). These featuresmake easier both the accumulation and transportation of radon gas. The results ofmeasurements taken at sites densely located in the high radon potential areasindicate a significant differentiation between the measured parameters. Within theneighbouring test sites, the values of radon concentrations in soil air and in housesvaried in a wide range. Additional research conducted at selected test sites wasfocused on radon exhalation measurements. Exhalation rate is the amount of radonatoms leaving the soil or bedrock per surface unit and time interval. It has becomeevident that at outcrops of the Middle Triassic Muschelkalk carbonate sediments,the measured radon exhalation rates ranged widely from about 2 mBq m�2 s�1 toabout 80 mBq m�2 s�1, as compared with the Miocene clayey sediments where thecorresponding exhalation rates ranged from about 2 mBq m�2 s�1 to about 4 mBqm�2 s�1 (Cha1upnik and Wysocka, 2003).
3. Geological structure of the study area
To better illustrate a relationship between the area geological structure and radonemission level, geological cross-sections of the selected test sites based on availabledata have been made. The map of the location of the cross-sections and test sites ispresented in Fig. 1. The cross-section No. 1 (Fig. 2) is made through the test site,where radon concentrations in houses reach maximum values. As shown in Fig. 1,the Triassic rocks form a cover estimated as 120e140-m depth overlaying theCarboniferous sediments. Carbonate rocks are predominant. The Quaternarysediments are of inconsiderable thickness and, in some places, almost totally vanish.In such places, increased radon concentrations were measured.
Cross-section No. 2 (Fig. 3) is across a zone where buildings with high values ofradon indoor concentrations were located. The geological structure of the area doesnot depart from that of the above mentioned example.
Cross-section No. 3 (Fig. 4) includes the areas of both low and high radonconcentrations. In the SSE part of the section, the Triassic carbonate rocks with thinand, in some places, much reduced Quaternary cover are predominant. In this partof the test site, high values of radon concentrations were measured. Towards theNNW of the area, where radon concentrations have not exceeded average values, thethickness of the Triassic carbonate rocks is reduced and a rapid increase in thicknessof the Quaternary sediments can be found. Immediately underlying the Quaternaryare the Triassic clastic sediments, the thickness of which declines until they disappearand give place to the Carboniferous rocks.
The presented geological cross-sections clearly show a relationship between radonemission level and the bedrock geological structure. They also corroborate a generalscheme that the Middle Triassic carbonate rocks enable radon migration and itsexhalation. However, they fail to explain why, at closely spaced measuring points
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located within the Triassic formation outcrops, the measured values of radonconcentrations in soil air and its exhalation vary in a wide range.
The most important of all sources of radon indoors are the bedrock and soilformations from which, due to the diffusion and convection transfer phenomena,radon is transported through the natural cracks and fissures into the buildings. Inconsidering the geological material as a potential radon source, one has to take intoaccount the sediments subjacent or adjacent to the home. According to Nazaroff andNero (1988), the radon flow path cannot be longer than several to a dozen or sometres negotiated at a time of from several days to several weeks. According to thecalculation results obtained by Lebecka et al. (1988) from the measurements andobservations carried out in hard coal mine workings, the radon migration zonereached by the test borehole was about 10 m.
Fig. 1. The location of cross-sections and test sites.
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Fig. 2. The geological cross-section of test site No. 1.
Fig. 3. The geological cross-section of test site No. 2.
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Therefore, to better understand a relationship between radon emission levels andthe geological structure of bedrock stratum, geophysical methods such as electricalresistivity profiling (PE) and electrical resistivity sounding (VES) were used. Thesemethods together with microgravity and ground penetrating radar (GPR) are widelyapplied for the assessment of mining or karst induced alternation of rock strata.Measurements of radon concentrations in the surface layer related to miningsubsidence voids have been done by Kies et al. (2004). He applied the GPR methodto analyse underground inhomogeneities of the rocks.
Using electrical resistivity methods allowed us to analyse the geological conditionsto a depth up to 50 m (Kotyrba et al., 2001). The first method was focused on thelocation of the strata discontinuities. It has been assumed that the anomalies of therock electrical properties could be recorded using the electrical resistivity profilingmethod and could be attributed to the presence in bedrock of contact zones oflithologically different rocks, zones of cracks and fissures, as well as zones of voidspaces and caved waste. The presence of this type of strata discontinuities can bea factor conducive to an increase in effective cross-section of gas migration paths.The objective of the second method, electrical resistivity sounding, was to determinethe bedrock lithostratigraphy of test sites and to obtain discontinuity images onstrata vertical cross-sections. The test sites of geophysical investigations were locatedin zones exhibiting the highest values of indoor radon concentrations andexhalations.
Fig. 4. The geological cross-section of test site No. 3.
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4. Results of geophysical investigations
4.1. Results of resistivity profiling (PE)
The profiling measurements have been carried in symmetrical Schlumberger arraywith electrode layout C40P40C (C, current electrodes, P, potential electrodes, dis-tance in metres). Spacing between measurements points ranged between 5 and 10 m.
The values of the apparent electrical resistivity for the bedrock formationsobtained from the profiling method measurements taken at three test sites, andincluded in the data set, range from 2 ohmm to 200 ohmm. The number of datacollected at each site ranged from 25 to 30. The lowest and the highest values wereobtained from test site No. 2 and test site No. 1, respectively. In Table 1 arepresented the characteristics of a set of measured data.
The apparent electrical resistivity curves obtained from profiling (PE) measure-ments taken at test sites Nos. 1 and 3 were of similar shape. Also similar can be thebackground level or the values considered to be normal in given geologicalconditions. The resistivity variation dynamics in each configuration midpoint can beconsiderable, which clearly indicates the presence of discontinuities in the rockstructure. On each curve, anomalous regions can be distinguished, where theresistivity exhibits values higher than those of the background. In the uniform rockstrata, positive anomalies on PE curves indicate the location of dry, macroporouszones, whereas the negative anomalies indicate the water saturation zones orpresence of clayey intercalations. In real geological environments, such zones areusually due to the presence of fissures or voids in the bedrock.
Data obtained from all of the three test sites characterise the electrical propertiesof Triassic sediments. At site Nos. 1 and 3, the values of the resistivity are typical ofthose corresponding to limey dolomitic rocks. At site No. 2, the values of theresistivity are clearly lower. In the locations being considered as anomalous, thebedrock resistivity has increased to a value of around 40 ohmm. The increased valueof the resistivity may be a result of the presence, in the bedrock layers, of the fracturezones and/or individual fractures of significant opening that could drain the Triassicaquifers.
4.2. Geological interpretation of the electrical resistivity sounding results (VES)
In interpretation of VES curves, geological maps (Detailed Geological Map ofPoland, 1991), field boreholes and mining data have been used (Kotyrba et al., 1998).
Table 1
Data set characterization obtained using the electrical resistivity profiling method
No. Test site Range of electrical
resistivity [ohmm]
Average value of electrical
resistivity [ohmm]
Standard deviation
1 No. 1 60e200 110 38
2 No. 2 2e45 10 12
3 No. 3 35e150 85 29
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The results of the VES curve interpretation for each test site were summarised inthe form of geoelectrical cross-sections. The layers distinguished in the cross-sectionswere presented for the case of lithostratigraphic layers composing the bedrock. Theelements disturbing the stratified earth structure or generating the anomalies thatwere localised using the electrical resistivity profiling method are positioned on thecross-sections (Figs. 5e7).
The geoelectrical measurements at test site No. 1 were taken in the vicinity of thehouses where high radon concentrations were measured. Fig. 5 shows a geophysicalcross-section produced based on electrical resistivity soundings.
The houses where high radon concentrations were measured are located ona weathered soil layer 1-m thick overlying the marly dolomites of Tarnowickie bedsof Upper Muschelkalk (Upper Triassic sediments) that are underlain by thediploporita dolomites of Middle Muschelkalk (Middle Triassic sediments). The lastlayer comprises highly fractured dolomitic limestones. All the layers marked on thecross-section, reveal the presence of numerous voids, caverns and fissures thatfacilitate gas migration. The bedrock top strata can be, to a large extent, weathered,which facilitates gas flow.
The documentation shows that ore mining operations were conducted in thesubstratum of test site No. 1 during various periods of time. Also, in deeper lyingCarboniferous strata, the coal mining operations were conducted most likely in thisor a near-by area. The electrical resistivity anomalies can be attributed to the miningorigin. They can be generated in the places where the strata continuity has been
Fig. 5. The geoelectrical cross-section of test site No. 1.
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broken by mine workings located in the ore deposit, or where coal mining-inducedsubsidence effects accumulated.
Considerable fracturing and caverns of the substratum rocks make it possible tofacilitate gas migration from both the subsurface layers and from the deeper lyingstrata. On the electrical resistivity cross-section shown in Fig. 5 are marked probablelocations of post-mining voids.
In the case of test site No. 2, it has been found that, as follows from the VES curveinterpretation, the geoelectrical layers can hardly be attributed to the lithostratig-raphy of Triassic strata. The house in which radon concentrations were measured is
Fig. 6. The geoelectrical cross-section of test site No. 2.
Fig. 7. The geoelectrical cross-section of test site No. 3.
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located in the differentially weathered ore-bearing dolomites (weathered clays withlimestone fragments) about 50 m away from the outcrops of Middle Triassic Gogolinbeds. The layer affected by differential weathering is not thick, being in the order of1 m. The deeper lying strata form three marlyelimey rock complexes. Worthy ofnotice are the very low values of electrical resistivity of subsurface rocks, rangingfrom 8 to 16 ohmm, and their irregular deposition. Only at a depth of about 45 m,a layer that can be regarded as composed of limestone, is there a relatively high valueof electrical resistivity, in the order of 120 ohmm (Fig. 6). The low value of electricalresistivity of the subsurface strata can be regarded as being due to a considerablecontamination of Triassic waters. The contaminants appear to derive from out-of-control wastes emitted by the near-by housing estate.
No hard coal and ore mining operations have been conducted in the study area. Itfollows from the geophysical surveys that the fracture zone located under thebuildings being studied has formed as a result of tectonics-related interactions.However, it is important to note that three mines: one ore mine and two coal mines,have conducted operations in the vicinity of the settlement. Some authors seeindirect effects of the mining activity on the study area (Kaczmarski, 1997).
Fig. 7 shows a geoelectrical section through the bedrock strata at test site No. 3obtained from measurements taken in close proximity to the houses, in thebasements of which high radon concentrations were measured. In this case, like attest site No. 1, four layers of variable electrical properties were distinguished. TheTriassic sediments appearing in the form of well permeable limestone and dolomitesunderlie a Quaternary soil layer of 2-m thickness. The last of the layers can becomposed of siltstone and saturated sandstone of the Upper Carboniferous. Itfollows from the geophysical surveying results that the house, in the basement ofwhich the highest value of radon concentration was obtained (Fig. 7), is situated inthe fracture zone. Considerable openings of the fractures facilitate gas migration.
In the case of this test site, the geophysical surveying and the available dataanalysis have unequivocally shown that the shallow coal mining conducted in theneighbourhood had influenced the structure of the Triassic overburden strata. Thiscoal mining has induced fractures and discontinuous strains in the strata which canbe confirmed by the anomalies on the electrical resistivity profiling (PE) curve. In thesection of the profile, where the anomaly was detected, the bedrock contains zones offractures and fissures with considerable openings that might lead to the increasedpermeability of the rocks. The results of the study also indicate that the bedrockstrata have, most likely, been drained, making the gas migration towards the surfacemuch easier.
5. Conclusions
The results of the natural radioactivity study carried out in the Upper SilesianCoal Basin area reveal that, within the outcrops of certain geological formations, thevalues of radon concentrations obtained from the measurements taken in houses andsoil air and the values of radon exhalation vary in a wide range. To better understand
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the structure of a bedrock top stratum layer as the main source of radon emission,studies involving the following geophysical methods such as electrical resistivityprofiling (PE) and electrical resisitivity sounding (VES) were used.
The results of geophysical research supporting analysis of geological data suggeststhat at sites where the values of radon concentrations and exhalation vary in a widerange, the Triassic formations exhibiting tectonic and mining induced structuraldisturbances play a fundamental role in the structure of roof strata. The tectonic andmining factors, mentioned above, bring about considerable rock fragmentation ofthe strata, which can lead to a formation of the increased active surfaces of radonmigration pathways in rock masses and to radon exhalation to the atmosphere.
The studies have shown that the measurements made using the electrical resistivitymethod can be helpful in evaluating radon potential of both the tectonicallydisturbed areas and the mining-transformed ones. The electrical resistivity methodscan be particularly useful in conducting studies on rock mass structure because theyenable:
e recording of anomalies of electrical properties of rocks caused by the presence offractures and voids formed by caving resulting from mining, karst and otherphenomena;
e distinguishing variable geoelectrical layers, which allows their lithostratigraphicnature to be determined;
e scanning of heterogeneity of physical properties in a vertical profile.
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