1. Introduction
Karst groundwater resources are at constant pollution risk due to their own natural features like dolines, fissures, and conduits that can allow a fast infiltration of pollutants into the aquifer. These fast-flowing pathways increase groundwater exposure to pollution since a contaminant would reach groundwater faster, experiencing little to no degradation process in comparison with unconsolidated aquifers. As karst aquifers are the source of water supply for almost 25% of the world population, as estimated by Ford and Williams [
1], their protection is urgent, principally in areas where groundwater is the only freshwater source. From the necessity to evaluate and characterize how endangered a given aquifer is, either consolidated or not, the vulnerability concept emerged as an essential part of groundwater management strategies. Vulnerability refers to the aquifer’s susceptibility to contamination by human activities [
2].
To date, several methods to estimate either source (spring or well) or resource (groundwater) vulnerability in karst areas have been proposed and tested on different areas of the world. An accurate overview of karst vulnerability methods developed during the last three decades can be found in the work of Iván and Mádl-Szònyi [
3]; nevertheless, more literature introducing regional adaptations or new methods have also been published, but in languages other than English [
4,
5].
The majority of current methodologies use a multi-parameter approach to represent the variables affecting a theoretical pollutant’s travel time, being discretized using score intervals according to a relative degree of protection [
6]. In general, development of groundwater vulnerability maps can be seen as the evaluation of multiple layers representing geology, pedology, meteorology, hydrology, and others for their influence on the vertical travel time from surface to the water table. Nevertheless, among the proposed methodologies, parameters are categorized and evaluated with different scores and weights, displaying contrasting outcomes when several methods are applied over the same area [
7,
8,
9,
10].
To work out vulnerability maps, it is necessary to deal first with some factors that complicate the process, such as data availability and the determination of an adequate method to be used for a given karst area. Some vulnerability methods need specific data, which in some cases is sparse or even null; for example, Malík and Svasta propose point classifications based on spring discharge data to estimate the degree of karstification as part of the REKS method [
11]. This data is not easy to obtain, such as in the case of coastal karst areas with underwater springs, like the Yucatan karst aquifer, thus not allowing the usage of such approach. Current methodologies have been validated via field tests, mostly tracer tests, on their respective areas of development; nevertheless, high subjectivity and personal interpretation regarding intrinsic features behavior also influence results. For example, the EPIK method [
12] considers epikarst as a feature which increase vulnerability due to probable infiltration though vertical shafts; this interpretation is opposite to that presented in the PaPRIKa method, where epikarst behaves as a protective feature for its hydrogeological function as perched aquifer; therefore, delaying infiltration [
13]. Despite the fact that vulnerability maps can be interpreted by stakeholders without a broad hydrogeological knowledge, outcomes must be taken as theoretical approximations and further analysis is always encouraged.
Global change elements (anthropogenic practices, extreme events) are important factors to be included into a vulnerability analysis. If we consider precipitation as an external stressor, tropical areas benefit from excessive rainfall events since substantial water volumes, infiltrating the subsurface, would promote dilution [
14]. Methodologies like the Slovene approach take into account precipitation analyzed in terms of volume, intensity and extreme events [
15]. Other methodologies like KARSTIC and DRISTPi consider recharge as a parameter derived from precipitation regimes [
16,
17]. Nevertheless, vulnerability assessment in tropical areas, with pronounced seasonal precipitation patterns, can be over/under estimated since precipitation ratings from the before mentioned methods are based on year averages. Additionally, dilution potential will depend on pollutant volumes which can be estimated in current scenarios (untreated wastewater, landfills leaching, etc.), while its residence time in the aquifer would be shorter depending on the degree of karstification. Despite effects of global change and its influence on real vulnerability scenarios, there is no method which evaluates such influence together in a vulnerability analysis. Inclusion of dilution and pollutant residence time must be considered carefully in a vulnerability analysis, as the evaluation of two or more independent criteria can lead to ambiguous situations [
18].
Combination of travel time, aquifer residence time, and pollutant concentration would resolve into a new, integrated groundwater vulnerability model, increasing the confidence when considering influence of global change factors on regional vulnerability. The IKAV project, affiliated to the INOWAS group (research unit under the Department of Hydrosciences of the Faculty of Environmental Sciences at Technische Universität Dresden, Germany), aims to develop such an integrated methodology to deal with and evaluate current and future vulnerability scenarios.
As a first step, analysis of selected intrinsic vulnerability methodologies, their congruence with regional features, and parameter sensitivity analysis are contemplated to reach an adaptive karst groundwater vulnerability methodology (
Figure 1). According to data availability, a total of eight methodologies were chosen and applied on the Yucatan State, Mexico. This paper presents the analysis of four of them (DRISTPi, KARSTIC, RISKE and the Slovene approach), the other being presented in a previous work [
10].
Outcomes from this work will serve as a basis to highlight critical parameters, weights adaption, and further considerations to be implemented in an adaptive karst vulnerability approach. Due to its unique characteristics and the increasing aquifer contamination, the chosen study area is Yucatan, Mexico. This work aims to highlight advantages, disadvantages, and the relationship between regional features and vulnerability rates from the four methods here applied. Additionally, this work extends the study and analyzes the outcomes and spatial match of vulnerability classes between the four methods, including a map layer-removal sensitivity analysis with “final vulnerability class” sensitivity, based on vulnerability class changes.
4. Discussion
Each methodology evaluated here display, to some extent, agreement with Yucatan characteristics (low slopes, shallow water table, and a regional diffuse infiltration). Nevertheless, they also show areas where the assigned vulnerability can be arguable. This questionable regional classification depends on the method and the values given to specific parameters, which are also dependent on the characteristic of the area where the original methods were developed and tested. The most significant vulnerability class in Yucatan according to this study is MV; this shows agreement with results obtained in previous studies for the EPIK, PI, and COP methods [
10]. Moderate values in the area could be explained by different factors, like a regional diffuse infiltration, lithology mostly composed by limestone, low slopes, and a shallow water table in most of the region. These intrinsic features are the most important according to sensitivity analysis and the overlap of final maps with individual parameters. The four methods agree that high sinkhole density areas and extensive fissuring indicate either VHV or HV, of course varying depending on the vulnerability rates given for each one of them. Having this as a base to adapt or generate a new intrinsic vulnerability map, analysis of MV and their relation with regional characteristics is necessary to further classify the area. To adapt or create a new intrinsic groundwater vulnerability approach, considerations presented further could help to develop a new adaptive methodology.
The first process for karst groundwater vulnerability evaluation must be the separation between areas promoting point infiltration at the surface (swallow holes and the catchment feeding it) and the areas with diffuse surface infiltration. This is highly recommended since categorizing diffuse areas as “rest of the area” will inherently decrease vulnerability classes for those areas without surface point infiltration. This will lead to a different evaluation of soils, slope, and other features affecting runoff generation (vegetation, precipitation intensity). For example, a clay-rich soil and high slopes are more likely to promote runoff, hence will increase vulnerability towards a swallow hole if this condition exists (
Figure 6a). However, having the same intrinsic characteristics in a diffuse infiltration area leads to an inverse analysis, since the scenario will theoretically minimize the possibility of vertical infiltration; therefore, vulnerability would decrease for the given grid map (
Figure 6b). This evaluation is not considered by multiple methodologies since they mostly focus on slope as the sole feature promoting surface runoff.
Estimation of MV in coastal areas, as shown by the four final maps, could be considered inaccurate. Despite considering a regional diffuse infiltration, which is clearly less vulnerable than surface-focused infiltration, the shallow water table in the coastal rim could be reached by pollution faster in comparison with southern areas, such as the Sierrita de Ticul hill. Therefore, a method must contemplate differences in unsaturated zone thickness based on these ranges. To achieve this classification, it is necessary to increase the importance (or weight) of the unsaturated zone thickness and re-classify rates according to regional groundwater table intervals. For example, a Jenks classification could be used to reduce the variance between values contained in a given interval and to increase the variance between different intervals, hence defining regional ranges statistically. The KARSTIC method presents a high weight for this parameter, classifying the hill area as LV; however, shallow water table regions remain with MV.
The area being mostly a flat terrain, categorized with a regional diffuse infiltration, an approach to classify topography (slope) must consider solely the influence on vertical infiltration. For example, the SA is the only method presented here where a high slope means an increase in vulnerability contemplating its influence on runoff generation. Not having significant surface streams and without surface point infiltration in Yucatan, high slopes must be considered as promoting low vulnerability, since vertical infiltration, at those given map grids, would be decreased. For this case, the RISKE method represents such behavior since areas with a strong change in elevation display LV, mainly in the hill area at South of Yucatan.
The Yucatan lithology is characterized as limestone without further discretization. Despite the Yucatan limestone corresponding to different ages, as displayed in
Figure 2, the majority of vulnerability methodologies consider solely the lithological material and its karstic development for a vulnerability evaluation. Most of the methods analyzed in this work consider several lithological materials (marls, breccia, shale, sandstone, etc.) for their influence on infiltration capacities (see DRISTPi, KARSTIC, and SA methods). However, when applying these methodologies in Yucatan, lithological materials analyzed by the methods but inexistent in the study area are also evaluated, thus affecting the final outcomes. To better adapt lithology influence on regional vulnerability, this could be coupled with fissuring degree to discretize areas from low to high possible infiltration. This could be similar to the PI method evaluation, where lithology and fracturing are directly related.
When methodologies focus on point infiltration, the parameters affecting runoff towards a swallow hole, generally the slope, are the most influential. For diffuse infiltration areas, soils are the first layer affecting pollutant transport and must be evaluated accordingly. The influence of soils on vulnerability is generally analyzed according to its texture and thickness; however, low weights are assigned to this parameter in most of the methods. Despite soils in Yucatan being considered as absent or for general purposes, due to their shallowness and karst outcrops, they must be given a more important role when diffuse infiltration areas are being studied. For example, the SA method displays the importance of soils since they are the most influential parameter to define MV and LV areas. This is supported by the sensitivity analysis as a consequence of the regional homogeneity of the P and C layers in the Yucatan case. If we analyze the hill area in
Figure 4d, we notice that LV and VLV match fine texture soils with a considerable thickness.
Regarding precipitation as an external stressor, or medium for pollutant transport, it is important to consider the fact of Yucatan being a tropical area. Either as recharge or precipitation volumes, this parameter must be also re-classified according to regional precipitation quantities. In this case, effect of precipitation on vulnerability must be contemplated regarding its seasonal influence due to the well-defined precipitation regimes and extreme events period. Additionally, this parameter must focus solely on terms of pollutant travel time and not for dilution potential (see COP method).