Assessing the Karst Groundwater Quality and Hydrogeochemical Characteristics of a Prominent Dolomite Aquifer in Guizhou, China

: Karst groundwater is one of the primary water resources in most provinces of Southwestern China where karst topography is strongly featured. In Guizhou Province, a prominent geologic sedimentary formation is the dolomite stratum which exists as the restricted platform facies and potentially provides a large reservoir for drinking water. A proper understanding and evaluation of its hydrogeochemical characteristics and water quality is the key to ensuring the drinking water safety. In the present study, groundwater samples were collected from 25 locations of the dolomite aquifer across Guizhou to determine their major chemical compounds, including the cations (K + , Na + , Ca 2 + , Mg 2 + ) and the anions (HCO 3 − , F − , Cl − , NO 3 − , SO 42 − ), as well as the pH, total hardness, and total dissolved solids. HCO 3 − and Ca 2 + were found to be the dominant anion and cation, respectively, which is characteristic of typical karst groundwater and supports the overall observation of a slightly weak acid to weak alkaline environment in the studied groundwater, as the pH measurements ranged from 6.80 to 8.37. Fuzzy comprehensive evaluation method is used to evaluate the groundwater quality based on typical drinking water safety standard. The results show that the groundwater in most of the studied aquifers is of reasonably good quality. However, in some aquifers, concentrations of NO 3 − and/or SO 42 − were found to be excessively high. Overall, the studied dolomite aquifer in its natural environment as investigated in the present study can be considered as a potential geological stratum for water resources exploitation in Guizhou. X.C.; validation, Z.W.; analysis, Z.W., M.T., L.H.; investigation, Z.W.; resources, Z.W.; data Z.W., P.P., and L.H.; visualization,


Introduction
Today, water scarcity remains one of the main challenges facing the modern world in the 21st century [1]; with continuous population growth and economic development, this threat is predicted to grow in the future [2]. Potential water shortage in karst mountainous areas is particularly significant due to its specific geochemical background and hydrogeological feature of karstic carbonate aquifers [3]. Karst topology occupies approximately 15% of the world's land area; one in every five persons depends on water from karst sources [4]. The evolution of geochemical compounds of karst groundwater is strongly affected by the karstification development [5][6][7][8]. Meanwhile, the hydrogeological structure of the karst formations may also considerably influence the geochemical processes in groundwater [6,[9][10][11][12][13]. Guizhou Province is located at the center of southwest China, and nearly three fourths of its area is formed by karst landscapes; the dolomite stratum that exists as dolomite stratum that manifests in the restricted platform facies, the limestone stratum in the open platform facies, and the limestone stratum in the reef facies. Of particular interest is the dolomite stratum widely distributed across Guizhou. It is concentrated primarily in the north and northeast, and sporadically in the central and south of Guizhou. As this karst stratum is characterized by a significant amount of dissolved pores and fissures, it is a major stratum targeted for karst groundwater exploitation in Guizhou. The present study is mainly motivated by the possibility of exploring this karst aquifer in this region. Typically, karst water evolves in a complicated manner shaped by the interplay between rock-water interaction, geological processes, and environmental changes as affected by human activities [40]. The present study aims to investigate the dolomite aquifer at its natural environment across Guizhou Province and offer a preliminary assessment of its quality and suitability for drinking water.

Groundwater Sampling and Testing in the Study Area
Groundwater samples were collected in 25 drilled boreholes in a field investigation that was performed across Guizhou between 2008 and 2012, as shown in Figure 1. The examined karst ground water were typically located in less-populated, rural areas under no evident influence of industrial or mining processes. The drilled boreholes were approximately 150 m-deep. Each groundwater sample was of 1.5 L and stored in a specialized container after the container was rinsed by the same water for 3∼5 times. Subsequently, it was sealed with a membrane and kept in a constant temperature of 3 • C before it was transported to laboratory for testing. All samples were tested and analyzed within one month of the date of collection. The concentrations of the cations (K + , Na + , Ca 2+ , Mg 2+ ) and the anions (HCO 3 − , F − , Cl − , NO 3 − , SO 4 2− ), the total hardness, the total dissolved solids (TDS), and the pH value of each sample were measured. The pH tests were conducted in the field using the handheld equipment (Hanna HI98129 pH tester, Hanna Instruments, Smithfield, Rhode Island, USA); all other tests were carried out in the laboratory based on the national standard of China on the technical specifications for environmental monitoring of groundwater.

Results
Concentrations of various ions in all groundwater samples collected from the dolomite aquifer are detailed in Table 1, which also includes the results of the pH, the total hardness, and the total dissolved solids (TDS). The minimum, maximum, and mean values of these concentrations are also reported in Table 2. It is evident that the calcium cation Ca 2+ , followed by significant concentration of magnesium Mg 2+ , is the predominant cation in the groundwater, which is characteristic of typical karst aquifers.  [41,42]. The SO 4 2− concentrations, while obviously lower than HCO 3 − , are still consistently higher than all other anions. In a few samples, relatively high concentrations of nitrate NO 3 − were found, indicating the possibility of natural nitrate salt deposit or agricultural use of fertilizers. Its presence may become a potential inhibitor for exploiting the studied karst groundwater as a drinking water resource. The high concentrations of HCO 3 − also support the overall observation of a slightly weak acid to weak alkaline environment in the groundwater, as reflected by the pH measurements which range from 6.80 to 8.37 with a mean value of 7.39. The acidity in the groundwater of karst aquifers is mainly dictated by the geochemical interaction between water and rock, and usually exhibits complex dynamics between various processes including chemical reactions, mineralogical evolution, and hydrological processes. The observed narrow pH range of a very large studied area seems to suggest a relatively stable existence of karst groundwater and aquifers in Guizhou, which may be potentially beneficial for long-term water resources exploitation. Ca 2+ , Mg 2+ , and HCO 3 − in karst groundwater are mainly derived from karst processes [5]; they form the dominant ions in karst groundwater. Correlations among these ions of all 25 collected groundwater samples are presented in Figure 2, where the molar concentrations (mmol/L) are used. It shows a fairly strong correlation between the concentrations of Ca 2+ and Mg 2+ , close to a ratio of 1:1, while the correlation between the concentrations of Ca 2+ or Mg 2+ and HCO 3 − is close to a 4:1 ratio. Therefore, it is noted that an electrical neutrality might be preserved, indicating these three ions might originate from the same source minerals and might be involved in the same geochemical and geohydrological processes. Similarly, the concentrations of Na + , K + , and Cl − are plotted in the correlation diagrams in Figure 3. The concentrations of the sodium and the chlorine are close to a 1:1 ratio and seem to suggest the rock salt (halite) as the main resource. The potassium concentration is far lower than the sodium concentration but still provides a fairly notable presence.  It is of interest to compare the total equivalent concentrations of anions and cations with previous research studies of other karst aquifers. In the present study of the dolomite aquifer, the total equivalent concentration of cations ranges from 3.73 to 15.07 meq/L with an average value of 6.39 meq/L. The total equivalent concentration range of anions was 3.65∼15.18 meq/L, and its average value is 6.27 meq/L. Evidently, an electrical neutrality among the measured concentrations has been reasonably preserved. Several relevant findings reported in literatures are summarized in Table 3. Guo et al. [43] reported a mean value of 4.05 and 3.99 for cations and anions, respectively, in the karst groundwater of Guangxi province, a neighboring province to the south of Guizhou; these results are very close to those found in the south of Spain [44] and the northwest of Florida in the United States [45]. Elliot et al. [46] reported much higher concentrations in the confined fissured chalk aquifer of the London Basin of southern England. The present study shows the studied groundwater contains an abundance of mineral ions and the dolomite aquifer has a moderately active water-rock geochemical interaction.
The experimental results are also analyzed in the well-known piper plot, i.e., trilinear piper diagram as presented in Figure 4. The trilinear piper diagram is commonly used in hydrogeology and groundwater analysis to show the percentage composition of different ions [47]. The water-chemical types of groundwater can simply reflect the main ions and their relative contents in water. The diagram consists of three components: a triangle in the lower left representing cations (Ca 2+ , Mg 2+ , and Na + +K + ), a triangle in the lower right representing anions (Cl − , SO 4 2− , and CO 3 2− + HCO 3 − ), and finally, a diamond plot in the middle which is a matrix transformation of the two triangles.
Each concentration in a sample is normalized to 100% with respect to the total concentration, and the relative concentrations are on a percentage basis. For each sample, the two points in the two lower triangles are projected into a point in the upper diamond, this final location indicates the type of groundwater. Figure 4 shows nearly all samples fall into the category of calcium bicarbonate water (Ca · Mg − HCO 3 − ); the only outlier is Sample #22 mentioned previously, which is a calcium sulfate water (Ca · Mg − SO 4 2− ). The results are remarkably consistent, considering the large area investigated in the present study.

Assessment of Groundwater Quality with Fuzzy Comprehensive Evaluation Method
It is of interest to assess the quality of karst groundwater based on comprehensive evaluation which attempts to produce an overall evaluation from a set of individual evaluations [48]. The fuzzy comprehensive evaluation method is adopted in the present study, and the 25 water samples studied in the previous section are assessed.

Evaluation Method
The concentrations of SO 4 2− , NO 3 − , Cl − ,F − , TDS, and total hardness are of pertinence to the safety of drinking water, and thus are chosen as the evaluation factors. Five classes from I to V are established for evaluation of each individual factor, as described in Table 4. The threshold values for each factor are selected based on the Quality Standard for Groundwater of China [49], where a higher threshold value, typically associated with higher class, indicates a lower water quality. Class I and II are typically associated with excellent and good water quality, respectively. Class III is considered to contain modest amounts of chemical content but of no harm to human health. Class IV can be used only for industrial or irrigational use but not suitable for drinking, Class V is considered obviously not potable and not even suitable for industrial or irrigational use. The membership grade of the individual factor at all levels (I, II, III, IV, V) is determined by the following membership functions which are generally of a reduced semitrapezoid shape.
, and µ i5 are the membership (a "fuzzy value" between [0, 1]) of each single factor i, associated with Class I, II, III, IV, and V, respectively. It is noted that µ i2 , µ i3 , and µ i4 are defined via Equation (2) for j = 2, 3, 4. Evidently, a 1 , a 2 , a 3 , a 4 , and a 5 are the threshold values under each class for each factor defined in Table 4. Consequently, the actual value x i for factor i is then converted to a value between [0, 1] for subsequent assessment. For each data point out of the 25 samples, there are a total of I = 6 factors, each of which is associated with J = 5 membership values for 5 classes. Therefore, a fuzzy relational matrix R of I × J rank is established for each sample.
Subsequently, the intermediate weight of each factor can be calculated via where w i is the weight of Factor i; c 0 is the average value of all threshold values for Factor i, equal to (∑ j a ij )/J = (a i1 + a i2 + a i3 + a i4 + a i5 )/5. Subsequently, all final weights need to be normalized viz Clearly, the sum of all weights ∑w i = 1, and these weights form a weight row vector (1 × 6). For all 25 samples, a weight fuzzy matrix (25 × 6) is established and hereafter denoted as A.
Finally, the evaluation for the entire samples set (25 samples) is rendered by the multiplication of the relationship matrix R and weight matrix A: The fuzzy evaluation matrix, B (25 × 5) then provides the degree of quality level for each membership (Class I∼V).

Evaluation Results
The evaluation results are presented in Table 5. Each "fuzzy" value in this table indicates the strength for the sample to be qualified as the prospective class. The final evaluation outcome is the one with the highest value. Table 5 show that the groundwater form the studied dolomite aquifer is mainly Class II; almost 60% of samples fall in this category, indicating good quality as a drinking water resource. The percentage chart is shown in Figure 5. 20% of samples are classified as Class I with excellent quality. Only two samples are classified as clearly not drinkable, Sample #7 (Class IV) and Sample #22 (Class V). Sample #7 is located at Xinzhou town of Huangping County in the south of the study area. Its nitrate concentration is very high, far exceeding normal drinking water standard which considers 20 mg/L as the maximum acceptable concentration (Table 4). Sample #22 was collected from Tanchang town of Renhuai County in the north of the study area. As discussed earlier, it contains excessively high concentrations of nitrate and sulfate, and its total hardness is also very high. While the possibility of natural nitrate salt deposit or agricultural use of fertilizers may explain the presence of nitrate, the cause for its high total hardness may be related to the geochemical background of this dolomite aquifer in this region and worthy of further investigations. It is worth mentioning that the presented results are based on the commonly existing ions in typical karst water, possible heavy metal concentrations and other sources of potential pollution are not considered.

Results summarized in
It should also be noted that although the present study considers only the Chinese national standard in the numerical model, the framework of the adopted method can be applied to other standards as well by using the specified threshold values. For example, WHO (World Health Organization) recommends 500 mg/L as the maximum acceptable concentration for total hardness, 600 mg/L for TDS, 50 mg/L for NO 3 − , and 1.5 mg/L for F − ; while the requirements for SO 4 2− and Cl − are the same as in the examined standard.   Table 4) of the quality of the collected groundwater samples.

Conclusions
The studied dolomite is a dominant geological karst stratum in Guizhou of Southwestern China and may potentially provide excellent water resources. The geochemical characteristics of this aquifer are investigated by studying collected groundwater samples collected from the natural environment in the field across Guizhou. HCO 3 − and Ca 2+ were found to be the dominant anion and cation, respectively. Overall, the groundwater can be considered as calcium bicarbonate water (Ca · Mg − HCO 3 − ) based on its trilinear piper diagram. There were notable exceptions, mainly influenced by the presence of excessively high concentrations of NO 3 − and/or SO 4 2− . The presence of excessive nitrate strongly indicates the possibility of the influence of agricultural or other human activities in certain regions of the province, which overall is still under rapid urbanization and industrial development. A quantitative approach based on the fuzzy comprehensive evaluation method is adopted in the present study to assess the quality of the groundwater. The concentrations of SO 4 2− , NO 3 − , Cl − , F − , TDS, and total hardness are the evaluation factors for the evaluation of the water quality. The numerical results show that most of the groundwater samples can be considered to be of good or excellent quality for drinking; however, it is noted that the presented results are based on the concentrations of commonly existing ions while concentrations of heavy metals or other harmful substances were not considered, as the examined samples were collected from less populated or industrialized areas in an attempt to study this type of aquifer in its natural environment. If sufficiently preserved to be immune from various sources of pollution, the studied dolomite aquifer has a great potential to be an important groundwater resource in Guizhou. Many studies have shown considerable water pollution as a consequence of various mining, urbanization, and industrialization processes, especially in the highly populated areas of Guizhou. Hence, further studies are still needed to better understand the dynamic of the evolution of this karst aquifer under the influence of anthropogenic activities in this rapidly developing region of southwest China.

Acknowledgments:
The authors wish to thank the Bureau of Geology and Mining of Guizhou Province for the assistance in the field investigation.

Conflicts of Interest:
The authors declare no conflict of interest.