Abstract

The decline in groundwater availability and quality has become a worldwide issue and has been the subject of several studies in recent decades. In this sense, the goal of this study is to assess the vulnerability of the Campeche Aquifer (Florianopolis, Brazil), identifying potential areas of possible contamination by the direct infiltration of runoff in drainage compensatory techniques. To achieve this goal, the following methodological steps were used: (1) data collection and preparation, (2) application of the DRASTIC model, (3) sensitivity analysis and (4) analysis of potential contamination by compensatory techniques. The results show that approximately 33% of the aquifer area presented moderate vulnerability to contamination. However, 29% of the remaining areas had high and extremely high vulnerability. Analysing the potential of contamination with drainage compensatory structures we verified that approximately 95% of them are located in areas of vulnerability classified as moderate and high. The other 5% were identified in areas with extremely high vulnerability. Sensitivity analyses indicated that the removal of topography, soil type and the impact of the vadose zone caused a large variation in vulnerability index. It is evident that there is a high potential of contamination of groundwater by direct infiltration of drainage compensatory structures.

INTRODUCTION

The decline in the availability and quality of drinking water has become a global issue. The process of urban growth, expansion of industrial and agricultural activities, operational failures in water distribution networks, natural disasters and extreme weather phenomena, among several other types of contamination, whether accidental or malicious, have negatively impacted the water quality of the springs in almost all of the cities in the world (Löbler & Silva 2015; Kanakoudis & Tsitsifli 2017).

Climate change scenarios project large spatial and temporal variations in the hydrological cycle contributing to environmental and socioeconomic impacts on urban water supply (UN WATER 2011). However, it is difficult to quantify these impacts. Studies by Kanakoudis et al. (2017a, 2017b) in the Adriatic Sea region showed that climate change is expected to impact negatively on water resources availability while at the same time, water demand is expected to increase. In addition, an intensification of water quality problems has been identified mainly due to changes in land use and salt water intrusion.

A significant increase in water demand is expected over the coming decades due mainly to population growth (Lathamani et al. 2015). According to the 2017 World Water Development Report, two-thirds of the world's population lives in areas experiencing water shortages for at least one month a year. Of these, about 500 million live in areas where water consumption exceeds the capacity of water resources renewal. It is estimated that in 2012, more than 800,000 deaths worldwide were caused by the lack of availability of fresh water (WWAP 2017). In this way, groundwater is of major importance as a source of supply, but worldwide it is also at risk of contamination (Eckhardt et al. 2009).

Contamination of groundwater is caused by anthropogenic activities, from major sources of pollutants, wastewater, leachate from controlled landfills, inadequate storage of petroleum products, and the use of pesticides and herbicides in gardens and parks (Takizawa 2008; Han et al. 2013). A source of contamination that has gained prominence in research in recent years is urban drainage runoff rich in heavy metals, motor oil, fuel, polycyclic aromatic hydrocarbons (PAHs), herbicides, and insecticides among many other toxic compounds (Jia et al. 2013). All of these compounds can be injected into the groundwater by the direct infiltration of auxiliary drainage structures, when these technologies are inadequately installed in vulnerable areas (Kemerich et al. 2011; Dearden et al. 2013; Dragon et al. 2016).

Infiltration contamination is probably the most common subterranean contamination mechanism, especially when groundwater levels are high, when the soil layer is thin and the soil permeability is high (Stigter & Dill 2000; Eckhardt et al. 2009; Kaliraj et al. 2015). The European Community has established important standards, such as the EU Water Framework Directive (WFD) and the Groundwater Directive (GD), which recognize groundwater as a valuable natural resource that should be protected from deterioration and pollution (Löbler & Silva 2015).

In Brazil, more than half of the public water supply comes from underground reserves, representing the main hydric source in many regions of the country. It is estimated that in the year 2000, approximately 61% of the Brazilian population was supplied by groundwater, 6% by shallow wells, 12% by springs and 43% by deep wells (IBGE 2003).

So, to ensure the quality of a shallow aquifer allowing it to be adopted as a source of water supply by the local inhabitants, it is necessary to determine the areas potentially vulnerable to contamination. Therefore, the goal of this study is to assess the vulnerability of the Campeche Aquifer, Florianópolis, SC – Brazil, identifying potential contamination areas by the direct infiltration of runoff in compensatory techniques constructed without planning.

STUDY AREA

The study area adopted in this study corresponds to the Campeche District, located in the southern portion of Florianópolis, SC, Brazil. With an area of 34.91 km², the Campeche District is considered the largest flooding area on the island, rich in marshy ecosystems such as lagoons, ponds, marshes, streams and mangroves. In addition to these characteristics, the area stands out due to the presence of a superficial free aquifer and numerous installed compensatory techniques.

DATA AND METHODS

The methodology of this study is organized in four large steps: (1) data collection and preparation, (2) application of the DRASTIC model, (3) sensitivity analysis and (4) analysis of potential contamination by compensatory techniques.

In the first step, data referring to seven hydrogeological parameters of the study area were collected from previous studies as shown in Table 1. Some information was generated using old software versions, requiring corrections for graphic quality, which did not interfere with the basic information. The reconstruction and correction of the information were performed using traditional geoprocessing software.

Table 1

Data used for creation of hydrogeological parameters for DRASTIC model

Data typeSourceDataProduced parameterWeight
Depth of unsaturated zone map CASAN 2002 Depth to Water (D) 
Net recharge map CASAN 2002 Net Recharge (R) 
Geology map Tomazzoli and Pellerin 2014 Aquifer Material (A) 
Aerial photography SDSa 2010 Soil Type (S) 
Topographical curves IPUFb 2000 Topography (T) 
Geology map Tomazzoli and Pellerin 2014 Impact of Vadose Zone (I) 
Hydraulic conductivity Pacheco 2015 Hydraulic Conductivity of the Aquifer (C) 
Data typeSourceDataProduced parameterWeight
Depth of unsaturated zone map CASAN 2002 Depth to Water (D) 
Net recharge map CASAN 2002 Net Recharge (R) 
Geology map Tomazzoli and Pellerin 2014 Aquifer Material (A) 
Aerial photography SDSa 2010 Soil Type (S) 
Topographical curves IPUFb 2000 Topography (T) 
Geology map Tomazzoli and Pellerin 2014 Impact of Vadose Zone (I) 
Hydraulic conductivity Pacheco 2015 Hydraulic Conductivity of the Aquifer (C) 

aSDS – Secretaria de Estado do Desenvolvimento Sustentável (State Secretary for Sustainable Development).

bIPUF – Instituto de Planejamento Urbano de Florianópolis (Urban Planning Institute of Florianópolis).

Also at this step, we collected information regarding the location of the compensatory structures present in the study area. The information was obtained from the drainage network projects of each street in the study area, provided by the Department of Public Works of the Municipality of Florianópolis, SC – Brazil. AutoCAD® 2014 software was used to integrate all compensatory structures.

In the second step, the DRASTIC model, developed by Aller et al. (1987), was used to map the index of vulnerability of the aquifer by the overlapping of the hydrogeological parameters. Each parameter is assigned a weight (w), varying between 1 and 5, representing the relative importance between them. These weights were pre-defined by Aller et al. (1987) and may not be changed. For the classes of each parameter, a rate (i) is assigned, which varies between 0 and 10. The pre-defined rates were adopted by CASAN (2002) for the classes of the parameters Depth to Water, Net Recharge, Aquifer Material and Impact of Vadose Zone. For the classes of the parameters Soil Type, Topography and Hydraulic Conductivity of the Aquifer, the rates predefined by Aller et al. (1987) were adopted. The DRASTIC vulnerability index is computed by applying a linear combination of all factors according to Equation (1): 
formula
(1)
After application of the DRASTIC model, the results were classified into five classes of vulnerability to groundwater contamination (Table 2). The intervals were determined based on the classification proposed by Aller et al. (1987) and adjusted uniformly by the Jenks natural breaks standard classification method.
Table 2

DRASTIC index intervals and their respective classifications

DRASTIC index intervalVulnerability classification
<95 Extremely low 
96–120 Low 
121–162 Moderate 
163–190 High 
>191 Extremely high 
DRASTIC index intervalVulnerability classification
<95 Extremely low 
96–120 Low 
121–162 Moderate 
163–190 High 
>191 Extremely high 

In the third step the sensitivity analysis of the DRASTIC parameters has been performed to evaluate the sensitivity of individual parameters in assessing aquifer vulnerability. It is believed that the high number of parameters used by the model limit the impact of errors or uncertainties in the final output, however, some authors e.g., Merchant (1994) and Napolitano & Fabbri (1996), argue that an equivalent result can be obtained using a lower number of input parameters.

Two sensitivity tests were performed, map removal and single parameter sensitivity analysis. The map removal sensitivity analysis represents the sensitivity associated with removing one or more layer at the time of model execution. In this analysis, the sensitivity analysis is performed by the following equation: 
formula
(2)
where S refers to the sensitivity measurement of a particular parameter, V and V′ are the unperturbed and perturbed vulnerability indices, and N and n are the numbers of parameters involved in the calculation of V and V′ respectively. The actual vulnerability index obtained from using all the seven parameters was considered as an unperturbed vulnerability.
Single parameter sensitivity analysis was developed to determine and assess the impact of the seven parameters of the DRASTIC method on the vulnerability index. It evaluates the degree of influence of an individual parameter on the vulnerability of groundwater. The effective weight of each feature class is found by the following equation: 
formula
(3)
where W refers to the effective weight of each parameter, Pr and Pw are the rating value and weight of each parameter, and V is the overall vulnerability index.

Finally, in the fourth step of the study, the potential of contamination of the aquifer by direct infiltration of compensatory techniques was analysed. For the execution of this step, the compensatory structures existing in the area and the vulnerability map of the aquifer overlapped.

RESULTS AND DISCUSSION

DRASTIC parameters

The graphical representation and the rates assigned for each class of parameters used to obtain the DRASTIC aquifer vulnerability index are presented in Figure 1.

Figure 1

Hydrogeological parameters and vulnerability rates.

Figure 1

Hydrogeological parameters and vulnerability rates.

Figure 2

Aquifer vulnerability and compensatory techniques in the Campeche District.

Figure 2

Aquifer vulnerability and compensatory techniques in the Campeche District.

The depth to water level in the Campeche District is shallow (average is 5 m), decreasing gradually from the center towards the Tavares River in the western portion and from the center towards the Atlantic Ocean in the eastern portion. This makes these regions more susceptible to contamination according to DRASTIC method assumptions.

Although the study area is characterized by high annual rainfall (average of 1,600 mm/year), the net recharge to the groundwater aquifer is controlled by soil-type association with altimetry. The lowest recharge rates (up to 300 mm/year) were associated with urbanized areas and elevation of the terrain over 80 m, which make rainwater penetration difficult. The Campeche Aquifer has a high net recharge (>300 mm/year) in the eastern portion associated with low altimetry (up to 40 m) and the presence of dunes, which was assigned a high rating score (9).

The Campeche Aquifer material is basically constituted by colluvial, lagoon and wind deposits. Among these, the wind deposits were assigned a high rating score (9) since they are directly related to the areas with high recharge rates and dune formation. Colluvial and lagoon deposits were assigned low rating scores, 1 and 5 respectively. The same classifications and rating scores were used to represent the Impact of Vadose Zone parameter.

The soil media are characterized by the presence of sandy and sandy loam texture, classifying almost 70% of these as Quartzarenic Neosol. The rest of the soils are classified as Red–Yellow Argisol characterized by mineral constitution. The different soil types were assigned rates according to their permeability (depending on the texture). A high score (10) was assigned to the sandy soil and a moderately low score (3) was assigned to the clay soil. Although clay soils have a high infiltration capacity when they are dry, after they receive moisture the soils are quickly saturated, becoming impermeable.

The topography layer showed a gentle slope (0%–8%) over most of the study area, which has been assigned the DRASTIC rating scores of 9 and 10. The slope percentage increases in the central and northern portions of the study area associated with hilltops. Areas with steep slopes (>20%) were assigned low rating scores (1, 2 and 3) indicating their minimal effect on aquifer vulnerability.

The Campeche Aquifer is characterized by a moderate hydraulic conductivity (5.2 × 10−6 m/s), and therefore was assigned the average rating score of 5.

Aquifer vulnerability and potential of contamination

The results obtained by the application of the DRASTIC model reveal that approximately 33% of the study area is under moderate vulnerability, being evidenced throughout the region of the Campeche Plain. In this region, there is an association between the low slope of the terrain (0%–3%) and the formation material of the saturated and unsaturated zones consisting of marine and eolian deposits. The formation material of these regions is characterized by a high quantity of sand and silt, which increases the risk of contaminant transport (Muhammad et al. 2015). Another factor which justifies the level of vulnerability is found in areas of thin soil layers, coupled with the superficial groundwater levels and high permeability of the soil, which increases the aquifer vulnerability (Stigter & Dill 2000).

High and extremely high vulnerability classes were observed in approximately 29% of the areas mostly located in the eastern region of the Campeche District. The following factors are exhibited in this region: low slope (0%–3%), low depth of unsaturated zone (<2 metres), formation material of the zones constituted by eolian deposits, Quartzarenic Neosol soil type, proximity to the Atlantic Ocean, and being directly influenced by marine tides.

The low slopes associated with extremely sandy soil increase the rate of infiltration, accelerating the transport process and risk of groundwater contamination (Kaliraj et al. 2015). Granular free-zone aquifers are considered highly vulnerable to potential contamination due to the relative and small capacity of self-purification of the formation material and the susceptibility to sea-level rise and salt intrusion (Kemerich et al. 2011; Kaliraj et al. 2015).

It should be noted that the existence of an aquifer with high vulnerability does not mean that it is contaminated, but that the area is at risk. The presence or absence of contamination of the aquifer depends basically on the activities carried out on it (Kemerich et al. 2011).

The extremely low and low vulnerability classes were evidenced in approximately 38% of the areas. High slope (>45%), absence of aquifer recharge, high depth of the unsaturated zone (>6 metres) and forming by material consisting of colluvial deposits were the most influencing factors for low vulnerability.

The high depth of the unsaturated zone, and consequent decline of the water table, associated with a consolidated rocky bed, hinders the mobility of the contaminants, reducing aquifer vulnerability (Kaliraj et al. 2015; Muhammad et al. 2015). Other factors that affect the decline of vulnerability are the steep slope of the terrain and the absence of aquifer recharge points, facilitating runoff and consequently reducing the infiltration rate (Lathamani et al. 2015; Kaliraj et al. 2015).

By analysing the Campeche District drainage network projects, we identified 1,017 compensatory structures, of which 1,016 were infiltration wells and one was an infiltration swale. All of them are infiltration based.

Overlapping the infiltration structures identified in the vulnerability map (Table 3 and Figure 2) we verified that approximately 95% of the structures are located in areas where the aquifer presents moderate to high contamination vulnerability. The other 5% are located in areas with extremely high vulnerability. Studies in the infiltration swale, located in an area were the aquifer presents high vulnerability, indicate high concentrations of heavy metals (Pb, Cr and Cu) in runoff and sample soil. In the groundwater, high concentrations of Pb, Cr, Fe and Mn were evidenced, pointing to the transfer of metals between the environments (Schuck et al. 2017) and consequently evidencing a high potential for contamination of groundwater by the infiltration of runoff.

Table 3

Campeche Aquifer vulnerability class and overlapping of compensatory techniques

ClassVulnerability
Compensatory techniques
Area (km²)%No. structures%
Extremely low 10.37 29.79 2.00 0.20 
Low 2.83 8.10 0.00 0.00 
Moderate 11.59 33.11 515.00 50.64 
High 7.47 21.41 447.00 43.95 
Extremely high 2.65 7.59 53.00 5.21 
Total 34.91 100.00 1,017.00 100.00 
ClassVulnerability
Compensatory techniques
Area (km²)%No. structures%
Extremely low 10.37 29.79 2.00 0.20 
Low 2.83 8.10 0.00 0.00 
Moderate 11.59 33.11 515.00 50.64 
High 7.47 21.41 447.00 43.95 
Extremely high 2.65 7.59 53.00 5.21 
Total 34.91 100.00 1,017.00 100.00 

Sensitivity of the DRASTIC model

The statistical summary of the seven rated parameter maps used to compute the DRASTIC index is provided in Table 4.

Table 4

A statistical summary of the DRASTIC parameter maps

DRASTIC
Minimum 
Maximum 10 10 10 
Mean 5.8 6.5 5.2 
SD 3.2 3.3 3.3 3.5 3.4 3.3 
CV (%) 54.5 65.4 65.4 53.9 66.3 65.4 – 
DRASTIC
Minimum 
Maximum 10 10 10 
Mean 5.8 6.5 5.2 
SD 3.2 3.3 3.3 3.5 3.4 3.3 
CV (%) 54.5 65.4 65.4 53.9 66.3 65.4 – 

SD stands for standard deviation and CV for coefficient of variation.

The soil type parameter induces the highest risk of contamination with a high mean value of 6.5. The net recharge, aquifer material, impact of vadose zone and hydraulic conductivity imply a low risk of contamination (5) while depth to water level and the topography imply moderate risks with mean values 5.8 and 5.2 respectively. Net recharge, aquifer material, topography and impact of vadose zone are highly variable (CV% are 65.4%, 65.4%, 66.3% and 65.4%, respectively), while depth to water level and soil type are moderate variables with CV% values 54.5% and 53.9% respectively. The hydraulic conductivity parameter did not present a coefficient of variation since its value is constant across the study area.

Map removal sensitivity analysis

Table 5 displays the variation of the vulnerability index as a result of removing only one layer at a time. It is stated that a high variation in vulnerability index is obtained upon the removal of the topography parameter (mean variation index is 12.3%). This could be attributed to the high contamination risk associated with the predominance of flat to gently undulating slopes.

Table 5

Statistics of the map removal sensitivity analysis

Parameter removedVariation index (%)
MeanMinimumMaximumSD
9.6 3.4 32.6 4.5 
10.6 3.5 34.1 4.2 
11.2 3.6 33.4 4.2 
11.5 3.3 34.1 4.5 
12.3 3.8 19.3 3.7 
10.1 3.4 31.6 3.9 
11.2 2.4 34.6 4.8 
Parameter removedVariation index (%)
MeanMinimumMaximumSD
9.6 3.4 32.6 4.5 
10.6 3.5 34.1 4.2 
11.2 3.6 33.4 4.2 
11.5 3.3 34.1 4.5 
12.3 3.8 19.3 3.7 
10.1 3.4 31.6 3.9 
11.2 2.4 34.6 4.8 

One parameter is removed at a time. SD refers to the standard deviation.

The vulnerability index seems to be moderately sensitive to the removal of aquifer material, soil type and hydraulic conductivity, although the three parameters are considered theoretically less important (weights 3, 2 and 3, respectively). For the aquifer material and soil type parameters this may be due to the high contamination risk associated with the predominant classes (wind deposits and Quartzarenic Neosol). For the hydraulic conductivity parameter, the moderate sensitivity may be related to the generalization of the same rate for all the study area.

The sensitivity variation upon the removal of one or more parameters at a time is shown in Table 6. The least mean variation index was calculated upon the removal of the soil type parameter (3.3%). The mean variation index decreased as more parameters were removed from the processing, which demonstrates a consistency in the trend of the variation. According to the statistics of the map removal a considerable variation in the vulnerability assessment is expected if a lower number of parameters has been used.

Table 6

Statistics of the map removal sensitivity analysis

Parameters usedVariation index (%)
MeanMinimumMaximumSD
R, A, S, T, I and C 9.6 3.4 32.6 4.5 
R, A, S, T and C 7.1 2.9 28.1 3.4 
A, S, T and C 5.5 2.5 26.1 3.9 
S, T and C 4.8 2.2 23.4 5.4 
S and T 3.4 0.7 21.9 5.4 
3.3 0.1 19.9 6.8 
Parameters usedVariation index (%)
MeanMinimumMaximumSD
R, A, S, T, I and C 9.6 3.4 32.6 4.5 
R, A, S, T and C 7.1 2.9 28.1 3.4 
A, S, T and C 5.5 2.5 26.1 3.9 
S, T and C 4.8 2.2 23.4 5.4 
S and T 3.4 0.7 21.9 5.4 
3.3 0.1 19.9 6.8 

One or more parameters are removed at a time. SD refers to the standard deviation.

Single parameter sensitivity analysis

The single parameter sensitivity analysis compares the theoretical weight assigned to a parameter by the DRASTIC model with its effective (or real) weight. The statistical summary of the single parameter sensitivity analysis is displayed in Table 7.

Table 7

Statistics of single parameter sensitivity analysis

ParameterTheoretical weightTheoretical weight (%)Effective weight (%)
MeanMinimumMaximumSD
21.7 20.5 3.9 39.3 14.0 
17.4 14.1 3.1 28.3 10.9 
13.0 12.9 2.4 21.2 8.1 
8.7 11.6 4.7 15.7 5.3 
4.3 6.0 0.8 56.5 2.5 
21.7 21.5 3.9 35.4 13.4 
13.0 11.8 11.8 11.8 0.0 
ParameterTheoretical weightTheoretical weight (%)Effective weight (%)
MeanMinimumMaximumSD
21.7 20.5 3.9 39.3 14.0 
17.4 14.1 3.1 28.3 10.9 
13.0 12.9 2.4 21.2 8.1 
8.7 11.6 4.7 15.7 5.3 
4.3 6.0 0.8 56.5 2.5 
21.7 21.5 3.9 35.4 13.4 
13.0 11.8 11.8 11.8 0.0 

SD refers to the standard deviation.

The research shows that the impact of vadose zone and depth to water level have a high degree of effective weight in assessing vulnerability, with a mean value of 21.5% and 20.5% respectively. The effective weight of soil type, together with the topography, exceeds the theoretical weight imposed by DRASTIC. This reflects the importance of these parameters together with the impact of vadose zone and depth to water level for the model and the need to get accurate and representative information about these factors.

CONCLUSIONS

The results presented in this paper aimed to map the vulnerability of the Campeche Aquifer, Florianópolis, SC – Brazil and to identify the potential contamination areas by the direct infiltration of the runoff in compensatory techniques constructed without the installation of a viability study.

Due to its location in a coastal region and under a relatively small thickness of Quartzarenic Neosols, the groundwater of the Campeche District has, in general, a high vulnerability to contamination. In addition, the spatial distribution and choice of types of compensatory structures installed in the study area indicate that the district presents high contamination potential by infiltration.

The results demonstrate that approximately 29% of the aquifer areas present a vulnerability classified as high to extremely high. However, these areas cover approximately 500 compensatory drainage structures, of the wells and swale infiltration type, evidencing the high potential of contamination of the groundwater by the direct infiltration of runoff. In order to prevent contamination by runoff infiltration in Campeche District, drainage structures should be carefully selected, privileging the ones that promote runoff treatment, such as bioretention systems.

The map removal sensitivity analysis indicated that the vulnerability index is highly sensitive to the removal of topography, soil type, hydraulic conductivity and impact of vadose zone, but is least sensitive to the removal of the depth to water level parameter. The single parameter sensitivity analysis showed that depth to water level together with topography, soil type and impact of vadose zone are the most significant environmental factors which determine the high vulnerability to the study area.

ACKNOWLEDGEMENT

The authors are very grateful to CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) for financing Master's and Doctoral scholarships, and CNPQ (Conselho Nacional de Pesquisa) for supporting research activities.

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