Intrinsic and specic groundwater vulnerability using the GOD method as a pre-operational baseline assessment of an unconventional hydrocarbon industry

In the middle of the São Francisco basin, the sub-basins of Indaiá and Borrachudo river highlight the considerable potential for unconventional hydrocarbon exploration. The present study seeks to evaluate the groundwater resource's intrinsic and specic vulnerability in the Indaiá and Borrachudo basins as an environmental baseline study for further development in the shale gas industry. The overlay and index GOD method was applied to assess the aquifer's intrinsic susceptibility regarding the Groundwater connement (G), the Overlying Strata (O), and the Depth of the groundwater table (D). The specic vulnerability assessment considered the intrinsic vulnerability and the land cover/use of the area, concerning the diversity and the toxicity of pollutants inherent in each anthropogenic activity. The results indicate that 52.69%, 25.12%, and 17.57% of the aquifer area have Medium, High, and Extreme intrinsic vulnerability, respectively. The specic vulnerability assessment suggests an overall vulnerability reduction due to the low anthropogenic stress on the basin. Three gas research wells overlay moderate intrinsic vulnerability. One well is above the high vulnerability area, which concerns how the unconventional industry could develop regarding groundwater susceptibility in this ecosystem.


Introduction
Aquifers can be sensitive to surface anthropogenic activities; therefore, evaluating the vulnerability of groundwater resources is crucial in the implementation of new strategies on environmental monitoring and conservation (Oliveira et al. 2020). The need to delineate speci c policies towards the underground water supply regarding sustainable development is a world consensus. Nowadays, Brazil retains 12% of the planet's freshwater resources (ANA, 2020), requiring special attention to the conservation and protection of those waters.
The São Francisco hydrographic basin is one of Brazil's most relevant water resources, covering 8% of the national territory. It is responsible for the water supply of 521 municipalities in six federal units (Torres et al. 2019). Environmental studies focused on the vulnerability of water resources are fundamental to the country's strategic and sustainable development regarding this region's socioeconomic relevance Considering the São Francisco basin context, the Indaiá River and the Borrachudo River are signi cant drainage areas with extreme potential in many economic activities (Fig. 1). These subbasins are one of the main poles of sh farming in the country; it has a growing development of the agricultural activities and cattle ranching; and it has mineral resources applicable in stonemeal techniques. The Indaiá and Borrachudo basins also emerge as a strategic area for the potential exploitation of unconventional hydrocarbon reserves. Intensive prospective research for unconventional gas exploration was being conducted in this area until the Brazilian public prosecutor's o ce decided to stop those activities under the pretext of the lack of environmental background to guarantee safety and clarity concerning this economic practice (Camargo et al. 2014;FGV, 2018).
The controversial processes of unconventional gas exploration can lead to various in uxes of contaminants into local aquifers through the use of hydraulic fracturing techniques (fracking). These ows can be classi ed into two groups: Ascendant Flows -coming from the deep rock strata (unconventional gas reservoirs) towards the aquifers (e.g., failure to cement the gas wells; induction of connectivity between fractures) and Descendant Flows -the contamination comes from the surface and in ltrates through the soils and sur cial water bodies until it reaches the groundwater pools (e.g. spills, leaks, and/or the disposal of inadequately treated shale gas wastewater) (Vidic et al. 2013;Vengosh et al. 2014). The relationship between the unconventional hydrocarbon industry and water resource quality is con icting, thus triggering an intense public debate about how the environmental and regulatory policies should be conducted (Vengosh et al. 2014).
In this context, the R&D project, GASBRAS (Research and Development Network in Non Conventional Gas in Brazil), proposed the assessment of intrinsic and speci c vulnerability of groundwater resources in the Indaiá and Borrachudo basins. This research provides an environmental baseline of aquifers regarding the potential unconventional hydrocarbon industry development in the São Francisco basin. Therefore, the objectives of this work are: (1) to evaluate the intrinsic vulnerability of aquifers by applying the GOD method (Foster, 1987) -an overlay and index mapping technique adapted for large areas with low data background (Ghazavi and Ebrahimi 2015), (2) to establish the speci c vulnerability of aquifers regarding land use and soil management criteria, and (3) to elucidate the susceptibility of local aquifers to a potential unconventional hydrocarbon industry development.

Study area
The Indaiá and Borrachudo watershed cover 4,488 km² and are responsible for twelve cities' water supply, including an overall population of approximately 90,000 inhabitants. The climate in the study area is de ned as an Ay-type (Koppen classi cation), describing a typical rainy tropical environment with two well-de ned seasons: a wet summer from October to February and a dry winter from March to September (Trindade et al. 2018). There is a predominance of cambisol soils followed by latosols and litholic soils (Fig. 2). According to the Brazilian Agricultural Research Company (EMBRAPA), different climate conditions and protolith sources provide signi cant heterogeneity in the cambisol textures. The latosols are deep (> 2 m), non-hydromorphic soils, with B horizons higher than 50 cm, with a clay content between 15% and 80% (EMBRAPA, 2020). Litholic soils (neossols) show rocky surface strata overlaid directly on the rock or in the saprolite.

Geology and Hydrogeology
The study area consists of the San Francisco craton domain, in a polyhistoric environment with sedimentary Neoproteozoic formation (Bambuí Group) below the phanerozoic formations (Areado and Mata da Corda Group) (Dardene et al. 1997;Fragoso et al. 2011;Reis, 2018). The Bambuí Group sedimentation is formed in a marine paleoenvironment by the sequence of metapelites (Serra de Santa Helena Formation and Serra da Saudade Formation) and carbonates (Sete Lagoas Formation and Lagoa do Jacaré Formation), overlapped by siliciclastic units and arcosean sandstones (Três Marias Formation). A second depositional stage is represented by the Areado Group (conglomerates, clays, and sandstones) and by volcano-sedimentary and ultrama c alkaline intrusions of the Mata da Corda Group (Fig. 3A). This lithological diversity contributes to distinctive hydrodynamic behaviors in the study area, classi ed into different groundwater systems.
Groundwater systems include two or more lithologies and aquifers according to their similarities in water dynamics. These were classi ed according to the geology (Fig. 3B), following the classi cation criteria proposed by the Geological Survey of Brazil (CPRM, 2019). Five aquifer systems were de ned in the study area: I) Detritic and Alluvial Coverage Systemcorresponding to the tertiary-quaternary cover deposits whose speci c yield discharge and hydraulic transmissivity in the river courses are in the order of 10 − 2 m³/s/m and 10 − 6 m²/s, respectively. In higher altimetric zones, it can reach the order of 0.026 m³/h/m and 1.7 × 10 − 6 m²/s, respectively; II) Arenitic-system -associated with cretaceous sandy rocks of the Areado and Mata da Corda groups. The speci c yield discharge and the transmissivity in this system are close to 0.77 m³/h/m and 3.6 × 10 − 4 m²/s, respectively; III) Arcosean-Silt-Sandstone -the siltstones and ne sandstones of the Três Marias formation (Bambuí Group) with a speci c yield discharge and a transmissivity close to 0.19 m³/h/m and 4.5 × 10 − 5 m²/s, respectively; IV) Pelític-Carbonate System -The pelitic-carbonated units of the Serra da Saudade and Serra de Santa Helena formations of the Bambuí Group. The speci c yield discharge and transmissivity are 0.44 m³/h/m and 7.2 × 10 − 5 m²/s, respectively. V) Carbonate system -Limestones and dolomites from the Lagoa do Jacaré and Sete Lagoas formations, both from the Bambuí Group. In this system, the speci c yield discharge and transmissivity are around 4.4 m³/h/m and 8.7 × 10 − 4 m²/s, respectively.
The underground ow lines presented in Fig. 3B were inserted in the groundwater systems map based on the potentiometric levels registered in the well's data. These lines aim to highlight the regional underground water ows by following the topography. However, the authors consider that due to the amount of data, further studies will be necessary to assess the groundwater ow dynamics.

Land use/cover
The agropastoral activity and the natural vegetation areas (areas without direct anthropogenic modi cations) are predominant in the Indaiá and Borrachudo basins. Figure 1 describes nine kinds of land use/cover, where three categories are natural environments and six are anthropogenic activities. The Grassland, Savanna, and Forest formations compose the natural vegetation ground cover, totaling 1,881km² that are well distributed throughout the study area. The grazing areas and animal husbandry vary from subsistence to intensive cattle raising, covering 2,323km² with greater prominence in the center-south portions. Forestry occupies 86 km² of the study area and is primarily practiced in the Northwest, near the Três Marias reservoir. The perennial farming (mostly corn and sorgos) covers 36 km². Areas of urban infrastructure and small mines have also been reported in the region (emphasis on stonemeal exploitation for fertilizers). Table 1 detailed the characteristics of each local anthropic activity. 2. Methodology GOD method (Foster, 1987) consists of evaluating groundwater's intrinsic vulnerability regarding three conjugated criteria: Groundwater con nement (G) (ranging from free to con ned); O -Overlying strata, and D -the depth to water resources. Each parameter receives a score from 0 to 1 and then multiplied to yield a nal GOD index (Fig. 4).
The GOD method's choice considered the large extension of the study area and the lack of lithological and pedological data in high spatial resolution. In this context, GIS tools, data processing techniques, and geostatistical methods were applied to evaluate groundwater vulnerability. The GOD index de nition crossed the lithological, pedological, topographic, hydrographic, and tubular well data, both from different sources ( Table 2). All data procedures were managed using ArcGIS 10.2.2 from the Environmental Systems Research Institute (ESRI)®.  (2020) 2.1 G Index -Groundwater Con nement The "G" parameter contemplates the degree of con nement of each aquifer (Table 3). The "G" rating considered the groundwater systems based on the similarity of the hydrogeological behavior of the units (hydraulic transmissivity and speci c yield discharge), as well as the regional groundwater con nement de ned for each system by the Brazilian Geological Survey (CPRM, 2019).
The alluvial aquifer area was demarcated using a buffer of 300 m from the Indaiá and Borrachudo rivers and 150 meters for their respective tributaries, assuming both as uncon ned systems. It was possible to validate and adjust the G index scores by checking the registered wells' geological pro les. The "O" criterion values the type of lithology above the groundwater level regarding the layers' contaminant retention capacity. Bambuí, Mata da Corda, Areado groups, and Quaternary sediments were adopted with different O Index values.
In the surrounding Três Marias reservoir, the pedological map was used as the dominant overlying strata due to the water level's proximity to the surface. The association between the geological map and the pedological map resulted in ve classes with a different O index (Table 4 and Fig. 4).

D Index -Depth of the Groundwater Table
The D Index considers the distance between the surface and the groundwater table, quanti ed by a value between 0.6 and 1.0. It is assumed that the more profound the groundwater level, the lower the groundwater vulnerability (Fig. 4). The occurrences of sedimentary carbonate rocks bring out the speci c dynamics of karst ow; hence, it receives a maximum value (1.0).
The groundwater table was settled by interpolating tubular well data and the perennial drains inside the Borrachudo and Indaiá rivers using ArcGIS. The tubular well data also con rm where the semi-con ned groundwater systems are. In this case, the roof of the layer was used for the interpolation instead of the well's static water level.

Intrinsic vulnerability (GOD)
The GOD intrinsic vulnerability assessment multiplies three indexes (G -Groundwater con nement; O -Overlying strata; D -Depth of the groundwater table) to yield a nal GOD index. The GOD value interpolation results in an intrinsic groundwater vulnerability map.

Speci c vulnerability (GODs)
The intrinsic vulnerability represents the overall susceptibility of aquifers to be affected by contaminants from the surface, considering the geological, hydrological, and hydrogeological settings (Ghazavi and Ebrahimi 2015). Speci c vulnerability complements the intrinsic vulnerability by assessing contamination risks based on contaminants' nature and anthropic activities developed on the surface (Gogu and Dassargues 2000).
The different land use/cover was evaluated regarding their respective potential contaminants that might spread there (Fig. 1). The diversity and the toxicity of pollutants inherent to each type of anthropic activity de ned the risk assessment index (Table 5). According to their toxicity, the risk index involves ranking substances quanti ed by a range from 0.0 to 1.0. Therefore, following the overlay and index methodology, the intrinsic vulnerability map of aquifers (GOD) was combined with the risk assessment to generate a speci c vulnerability map (GODs). The risks index range varies from 0.0 to 1.0, representing the lowest and highest risk of contamination, respectively. The speci c vulnerability map (GODs) maintained the grade intervals of the intrinsic vulnerability, according to the classi cation in Table 6.

Intrinsic vulnerability assessment -GOD
The three maps corresponding to the three parameters of the GOD method are exposed in Fig. 5. The three maps show the propensity of the zones proximal to the dam to present higher vulnerability indexes.
The GOD Index assessment yields an intrinsic vulnerability map (Fig. 6A). Each GOD class and its respective area (%) is shown in Table 7. It may be observed that Medium, High, and Extreme classes are predominant in the Indaiá and Borrachudo basin. Low vulnerability corresponds to 4.62%. These areas correlate with a higher degree of con nement and the presence of the Três Marias formation (sandstones-siltstones) near the reservoir. Medium vulnerability represents 52.69%, covering most of the middle-south of the basins under analysis. High and extreme vulnerability areas majorly concentrate in the reservoir's surrounding areas and alluvial systems, corresponding to 25.12% and 17.57%, respectively. There are no areas of negligible intrinsic vulnerability. The Três Marias reservoir extension was excluded in the intrinsic vulnerability assessment. Fig. 5 The three index mapping used according to the GOD methodology: (a) G -Groundwater con nement; (b) O -Overlying strata, and (c) D -Depth to the groundwater table.
It is also essential to observe that due to the large extension of the basins, the classes with a relatively smaller coverage on the map still correspond to a signi cant territorial extension area (e.g., the low vulnerability class occupies 4.62% of the basin and represents 197.29 km² of the area).
The south-central portion of the GOD map shows a predominant variation between the medium and high degree of vulnerability. The relatively small depth of the water table and the overlying lithotypes conditioned a context of higher susceptibility. The northern portion of the basin has a mostly high and extreme intrinsic vulnerability, both fostered by layers of ne soil and the water table closest to the surface. There is a low vulnerability zone near the reservoir conditioned by the Latosols (Fig. 3). The alluvial aquifers are linked to the main drainages of the basin. These systems have an extreme vulnerability, justi ed by the composition of unconsolidated sediments and by the shallow groundwater water table.

Speci c vulnerability assessment -GODs
When the anthropic activities and the nature of the contaminants are taken into account, the speci c vulnerability map showed a general reduction in the vulnerability classes as compared to the GOD map (Fig. 6B). An expected result, once the natural vegetation and extensive small/medium-sized livestock are predominant in the basin, both activities with a relatively low impact on groundwater resources.
Due to their high susceptibility to the presence of contaminants, alluvial aquifers were outlined as areas of extreme vulnerability. Higher vulnerabilities are concentrated in the reservoir's vicinity, highlighting the expressive pastoral activity ( Fig. 1) in places of considerable intrinsic vulnerability in GOD assessment.
The mining areas and urban infrastructure might have a signi cant impact on groundwater systems. However, those activities have small spatial coverage in the basin. It overlays low intrinsic vulnerability zones, so it did not elevate the speci c vulnerability to those areas. Each GODs class and its respective area (%) is shown in Table 8.

Groundwater vulnerability and the unconventional hydrocarbon resources
Four research wells for unconventional hydrocarbons are currently installed in the Indaiá and Borrachudo basin (Fig. 1). The areas of exploration and production of unconventional gas are prone to generate in ows descending from harmful contaminants to aquifers, either by the production and processing operations of commodities or by the presence of intensive machinery and infrastructure elements of the exploration.
The GOD mapping demonstrates that the three southern gas wells overlap the Medium intrinsic vulnerability zones. The fourth gas well in the northeast, on the other hand, is in a high intrinsic vulnerability area, nearby the alluvial groundwater system. It alerts as to how the activity could be developed without resulting in environmental hazards to groundwater resources. The GODs mapping shows that the northernmost gas well is above a speci c medium-high vulnerability area, which implies a region that is more susceptible to human activities, which highlights the possibility of contamination coming from other anthropogenic sources.
The intrinsic and speci c vulnerability elaborated here only concerns the ows of pollutants originating from the surface that can in ltrate and reach the aquifers, thus not encompassing the other possible routes of upward or lateral contamination of the exploitation of unconventional gas.

Conclusions
Using the GOD overlay and index methodology, the intrinsic and speci c vulnerability of aquifers in the Indaiá and Borrachudo rivers basins, two major potential areas for the exploitation of natural unconventional gas, was evaluated. The area showed a predominance of classes of medium and high intrinsic vulnerability in all its extensions, with the most susceptible areas being concentrated near the Três Marias reservoir, where there is a relatively higher concentration of anthropic activities. The smaller groundwater table depth from the surface concomitantly with the structural-geological carbonate context leads to a higher susceptibility to water resources around the reservoir. Speci cally, the alluvial aquifers have an extreme intrinsic vulnerability in all their extension, highlighting the susceptibility of the surrounding areas of the Indaiá and Borrachudo rivers. On the other hand, the predominance of areas of natural vegetation and areas of incipient anthropic impacts predominantly appeared on a map with low and negligible speci c vulnerability, with the exception of the northern locations, where economic activities and the environmental context increase the risk of contamination.
Three gas wells for unconventional hydrocarbons are located in a medium intrinsic vulnerability area, and one gas well is above a high intrinsic vulnerability area. Both of them are relatively close to the alluvial aquifers (extreme vulnerability), highlighting the need for greater caution if economic activity expands there.
The Indaiá and Borrachudo basins are an area of strategic socioeconomic relevance, and regarding the low volume of geological-environmental background data available prior to this study, the GOD vulnerability assessment brings early insights about the spatial potentialities and susceptibilities concerning these groundwater resources. GOD and GODs provided crucial information for the monitoring and development of anthropic development, and it could represent the rst step toward yielding a preoperational environmental baseline for unconventional hydrocarbon exploration in those basins.

Figure 5
The three index mapping used according to the GOD methodology: (a) G -Groundwater con nement; (b) O -Overlying strata, and (c) D -Depth to the groundwater table.