3.2 Results from leaching experiment.
3.2.1 Variations in Electrical Conductivity (EC) and pH of leachates from the beginning of the experiment to the end.
The variations in EC obtained during the experiment, under all conditions, are reported in Fig. 5. This variable was measured to monitor the reaction progress with time. Rk 77 for experimental condition ‘A’ was lost along the way and was therefore not included in the graphs.
It is observed that EC steadily increases with time for all experiments except for experiment under acid conditions. The resulting EC was seen to be dependent on the experimental condition, the reaction or residence time, and the mineralogical composition of the rock.
Beginning with the effects of the mineralogical composition of the rocks; it is observed that, Rk 38 recorded the highest EC for all experimental conditions, which was expected because this rock sample had relatively lower percentage of quartz, a weathering resistant mineral (Clark, 2015). In addition, it contains more minerals with high resistance to weathering such as Biotite. It therefore leads to leaching of more ions, therefore resulting in the high EC. The decreasing order of EC in all rock types, is in the following order: Rk 38 > Rk 77 > Rk 91 > Rk 58 > Rk 90, which is especially observed in CO2 experiments. Using the CO2 experiment in Fig. 5 as an example, it is observed that Rk 90 resulted in EC of less than 200 µS/cm. This rock sample recorded the highest percentage of Quartz and other low temperature forming minerals such as Albite and K-Feldspar. High temperature forming minerals or ferromagnesian mineral have not been observed in this rock.
Another contributing factor to the resulting EC was the experimental condition. HNO3 + H2SO4 experiments resulted in the highest EC and as such was reported in mS/cm. All rock samples under this experimental condition were therefore recorded on one plot, i.e, Fig. 5a. The high EC values recorded is due to the presence of HNO3 and H2SO4. These acids speed up the rates of reactions (Tani et al., 2001). The H+ ions released when the acids react with minerals lead to the breaking of bonds in minerals which results in the high EC recorded.
Regarding the CO2 experiments, CO2 gas, when dissolved in water produces H2CO3, a weak acid which speeds up the rate of weathering, therefore, this experimental condition resulted in the next highest EC. HNO3 and H2SO4 are stronger acids than H2CO3 and therefore result in EC of several orders of magnitude than H2CO3. All samples under CO2 experimental condition, resulted in EC of about 6 orders of magnitude higher than room temperature and 50ºC experiments. These results highlight the importance of acidity of water on mineral weathering.
It was expected that 50ºC experiments would result in relatively higher EC compared to experiments at room temperature as observed in the results of the experiment conducted by Bucher & Stober (2002), however, this was not obtained. In their study, distinct crystalline rocks were experimentally reacted with water in a batch reactor, under series of different experimental conditions to better understand the composition and evolution of groundwater in crystalline basement rocks. Their 50°C experiment was exposed to atmospheric conditions, which causes their experiments at 50°C to yield higher EC than room temperature experiment. However, in our study, Results of EC for room temperature experiments were higher than 50ºC experiments because these experiments were placed in the GC oven and were not opened to atmospheric CO2. The mineralization of water therefore occurs in closed system. The amount of CO2 available for reaction is restricted. Moreover, solubility of CO2 required for high rate of mineral dissolution reduced with increasing temperatures (Hiscock and Bense, 2014). Room temperature experiments were opened to atmospheric CO2, the system is open and the amount of CO2 available for reaction is not restricted which led to the release of acids in water and resulted in the EC being higher than 50ºC experiments. However, since the partial pressure of CO2 in room temperature experiments was not as regulated and was lower than in the case of the experiment where CO2 gas was introduced into the leachates, the resulting EC was relatively lower. The pCO2 of room temperature experiments ranged from 0.00019 to 0.0030 atm, while that of experiments streamed with CO2 were within the range of 1 atm, thereby leading to the extremely higher values of EC from the CO2 saturated experiments than that of the room temperature experiments, since pCO2 plays a significant role in dissolution.
Regarding the effect of residence time, it is observed that, EC increases systematically with time because the longer the residence time, the longer rock-water interaction takes place to dissolve more minerals, which then results in the increasing EC with time. In addition, silicate rocks have the highest resistance to weathering, and the minerals are therefore seldom in equilibrium with the solution (Clark, 2015), which suggests another reason why EC is seen to be increasing even at the end of the experiment and would have continued to increase, if the experiment had continued until the equilibrium between water and silicate minerals is reached. This increasing trend was also observed in the study of Bucher and Stober (2002) where their experiment was only conducted for 100 mins on granites and gneisses.
An attempt was made to investigate the kinetics of Electrical Conductivity (EC) increase with time. Taking first CO2 experiments, the initial part of the graph could be modelled using the power law, because it is nonlinear and would therefore follow the equation Y = MXB, where Y is the derivative being sought for, X is the time (t), M is a constant which does not depend on time (t) and B is a constant relating to rate of reaction (Reed, 2023). Taking Rk 90 for instance, the equation of the power law would be (Eq. 1),
EC = ECºtn, Eq. 1
where EC is the derivative being sought for, while ECº is the initial Electrical Conductivity (EC), t, is the time and n is an exponent-which according to Bucher & Stober (2002) could be considered as 0.1 for CO2 saturated experiments, on granitic rocks such as those used in this study. This is valid for the initial part of the graph, from the first day the EC was recorded to about the next three measurements of EC, where a significant initial rise was recorded. Maintaining the example of Rk 90- Eq. 1, for the first part of the graph would be (Eq. 2)
EC = 52*t0.1. Eq. 2
The next part of the graph seemed to be at equilibrium, although there are slight increases. It could therefore be described using a linear function, with the following equation:
EC = ECº + kt Eq. 3
k in this equation could be defined as the slope. Eq. 2 may also be applied for plots under room temperature and 50ºC conditions since they almost mimic a linear graph. These equations can therefore be used to model the kinetics of resulting EC from the dissolution of minerals over time.
Regarding pH, its variations under various experimental conditions are presented in Fig. 6. The variations are seen to be largely dependent on the experimental conditions and reaction time. As expected, the least pH was observed in the experiments with HNO3 and H2SO4 with values ranging from 1.72 to 2.97, followed by experiments streamed with CO2 which ranged from 4.87 to 5.66.
Experiments at room temperature and 50ºC, resulted in similar variations with pH. This is because they were not mixed with acids and therefore the resulting pH would be a result of only the minerals.
First observing Eq. 1, it was observed that room temperature experiments for instance had a pH of around 9 at the beginning of the experiment, however, the pH reduced steadily with reaction time to about 7.1. Ideally pH should increase with reaction time because during mineral dissolution, the H2CO3 formed when CO2 reacts with water dissociates to form HCO3 and H+ (using Eq. 1 as an example). Therefore, as dissolution proceeds along the flow path, more H+ ions get produced, thereby increasing the pH with time. The reaction would take the form:
H2CO3 → H+ +HCO3 Eq. 4
However, the evolution from the experiment is unique and similar results were observed in the room temperature and CO2 experimental conditions from (Bucher & Stober, 2002). It was stated that their’s was a unique case and may be a new finding from the experimement.
3.2.2 TRENDS IN MAJOR ION CONCENTRATIONS OF LEACHATES FROM ALL ROCK SAMPLES FROM AUGUST 2021 TO THE END OF THE EXPERIMENT IN DECEMBER 2021.
Stiff diagrams present the various water types of leachates extracted for major ion chemistry under all experimental conditions to delineate the trends in major ion concentrations over time (Fig. 7). Only leachates from room temperature experiments were extracted in August and October 2021, because they were the only experiments which had duplicate samples. These duplicate samples were extracted for analyses of major ion chemistry, to preserve the weight of crushed rock and volume of water of the original experimental set-ups.
As mentioned in section 4.2.1, the room temperature set-up for Rk 77 was lost along the way. There was, therefore, no major ion concentration included for Rk 77 in October 2021, and therefore not included in the discussion.
The order of mineralization for rock samples was largely in the following order, Rk90 > Rk58 > Rk91 > Rk77 > RK38 as already highlighted by EC as well as the increasing of mineralization of water with the time. This trend was a result of the varying degree of weatherability of minerals contained in the rock and the length of water-rock interactions.
Beginning with only room temperature experiments; from August to December, it is observed that, leachates analyzed at the end of the experiment in December, resulted in higher concentrations of major ions than that of October while, October had higher concentrations than August 2021.The explanation for this could be due to similar factors resulting in the increasing EC with time as discussed in section 4.2.1.
The effect of natural environmental conditions on the resulting major ion concentration cannot be overlooked. These conditions were represented under acid environment (HNO3 and H2SO4 acids) in environment saturated with CO2 gas, at 50ºC and at room temperature experiments. The experiments with HNO3 and H2SO4 acids resulted in the highest concentration of ions, however due to the concentration of acids used, anions were not detected by the Ion Chromatograph ICS 5000 AS-DP DIONEX Themo Scientific analyzer, hence, the results of the experiment under acid environment were not presented on the stiff diagram. CO2 experiments resulted in the second highest concentration of major ion chemistry for all samples, and this is especially pronounced in sample Rk 38, the most mineralized leachate as observed in Fig. 7. Room temperature experiments resulted in higher concentrations of major ions than that of 50ºC experiments, due to the same reasons as that discussed in section 4.2.1.
Experiments at room temperature and at 50º C, yielded mostly Na-HCO3 and Na-K-HCO3 water types, even for the most mineralized water samples in contact with rocks which are composed of mostly high temperature forming minerals i.e., Rk 38 and Rk 77 (example: Ca-Amphibole and Epidote) (Figs. 7 and 8). However, it is observed that, only Rk 38 and Rk 77 under CO2 saturated experiments, produced Ca-HCO3 water types, which is likely a result of the following reason: Na-HCO3 or Na-K-HCO3 water types are largely a result of the solubility of Na or K feldspars (Equations 5 and 6), which are relatively low temperature forming minerals and therefore the solubility of these minerals is slow. If there was no CO2 induced to speed up the reaction rate, the low temperature forming minerals have enough time to react with the water, leading to a Na-HCO3 or Na-K-HCO3 facies. Ca-HCO3 water types, in this study are likely a result of the solubility of Ca-bearing minerals, which in this study is either Ca-Amphiboles or Epidotes. These minerals are high temperature forming minerals which weather faster than the low temperature forming minerals (Clark, 2015). Therefore, the experiments under CO2 conditions, which had higher reaction rates, may have caused preferentially the dissolution of the Ca-bearing minerals which may have led to the Ca-HCO3 waters observed in Fig. 7 (Eq. 7). This trend was also observed in the HNO3 and H2SO4 experimental conditions, although not present in the stiff diagram suggesting that strong or weak acids initially yield Ca-Mg-HCO3 water types in silicate terranes. This evolution was also observed in the study of Bucher and Stober (2002), where Na-K-HCO3 water types were obtained for room temperature and 50°C experiments, while their CO2 experiments yielded Ca-Mg-HCO3 water types.
The general implication from this section is that the dissolution of ions from minerals largely depends on the mineralogical composition, the experimental condition and residence time.
3.2.3 COMPARISON OF WATER CHEMISTRY EVOLUTION IN LEACHATES AND GROUNWATER CHEMISTRY
The chemistry of groundwater in the study area is observed to have Ca-Mg-HCO3 as the dominant water type, followed by the Ca-Mg- Cl, with Na-K-HCO3 and Na-Cl water types being the least dominant water types in the study area (Fig. 8). The evolution of major ions from the cation to anion triangles are in the following order: Ca2+ → No dominant ion field → Na+/K+ and HCO3− → No dominant anion field → NO3−/Cl− respectively- this could be termed as localized evolution of groundwater samples along a flow path.
The water types of the leachates evolved from Ca-Mg-HCO3 to Na- K-HCO3. However, the leachates had excess Na+ and low concentrations of NO3− or Cl– compared to samples. Only few leachate samples fall within the Ca-Mg-HCO3 and Na-K- HCO3 water types obtained from the groundwater chemistry.
It was expected that the evolution of leachate chemistry would closely simulate that of groundwater chemistry. However, only CO2 experiments closely simulated the water types of groundwater chemistry. Several factors may have accounted for this variation which includes:
The surface areas of the crushed rocks used for the leaching experiment are different from aquifers hosting groundwater in the study area. The diameter of crushed rocks used for the experiment ranged from 50 to 100µm, suggesting a bigger surface exchange area than that of natural aquifers. Minerals in the crushed rocks will be more exposed to rapid rates of reactions than that of the natural aquifers. Therefore, minerals with higher resistance to weathering such as Na and K Feldspars, which under natural conditions require a longer residence time for solubility to take place will be more quickly weathered in the leachates from the experiments since they have a larger surface area. The groundwater in the study area would however yield water types produced by the dissolution of minerals which are low resistance to weathering such as Ca-Amphibole, due to the relatively smaller surface area of mineral grains, even though they may have a longer residence time (Clark, 2015).
Another factor which could be responsible for the variations in water types could be the differences in residence time. Ca-Mg-HCO3 water types are typical of recharging areas; at the initial points of the flow path, because carbonic acid (H2CO3) is found at recharge areas in higher concentrations than at later points or discharge areas because meteoritic water dissolves CO2 from the atmosphere and soil zone before infiltrating into groundwater, however as groundwater travels along flow path, H2CO3 gets consumed and acids in the form of H+ ions are made available for mineral dissolution to take place (Eq. 4).
Partial pressure of CO2 (pCO2) is another important factor in determining the degree of mineral dissolution taking place. CO2 is found in higher concentrations at recharge areas than at discharge areas due to the fact that, CO2 gas dissolves in recharging waters from the atmosphere and through the soil, its solubity depends on the partial pressure of CO2 in gaseous phase in contact with water, the CO2 partial pressure at equilbium with water decreases with distance from the recharge in the case of closed system because CO2 is used to weather the rock, ( Clark, 2015). As a result, the influence of CO2 on rock-water interactions will yield varying concentrations of major ion chemistry along different points of the flow path. pCO2 of groundwater sampled from the study area varies from that of 50ºC and room temperature experiments because the partial pressure CO2 in soil in the study site is different than the partial pressure of CO2 in atmosphere (used in experiments at room temperature and at 50°C). Geochemical Modelling in PhreeqC (from WATEQ database) revealed that saturation indices of water with Amphiboles and Na/K Feldspars depend on partial pressure of CO2: the solubility of Amphiboles is favored under high pCO2 whereas Feldspars were more soluble under low pCO2.Taking the examples of groundwater samples for instance, GW 34 and GW 61 had water types of Na-K-HCO3 and the calculated pCO2 were 0.0018 and 0.0023 atm respectively. However, groundwater samples GW 75 and GW 26, with water types of Ca-Mg-HCO3 had pCO2 of 0.015 and 0.018 atm respectively. It is observed that samples with relatively higher pCO2 had Ca-Mg-HCO3 water types, which may be due to the dissolution of Ca-Amphibole as observed in Eq. 6, while samples GW 34 and GW61 with relatively lower pCO2 resulted in Na-K-HCO3 water types, which could be due to the dissolution of Na-K Feldspars- Thereby, validating results obtained from the calculation in phreeqC.
Section 3.2.5 discusses in detail the similarities between major ion chemistry of CO2 experiments and groundwater chemistry.
3.2.4 MASS BALANCE CALCULATIONS
Mass balance calculations were carried out on all leachates obtained from the five crushed rock samples used for the leaching experiment and mineral phases analyzed by XRD prior to the experiment. Residues were not used in this study because there were no significant changes observed between the original rocks and residues for each samples, only major cations were considered, because depending on their chemical composition, aluminosilicate weathering releases different cations into solutions such as leachates from the experiment in this study (Clark, 2015). This allows for the identification of the more dissolved mineral phases in each sample.
Quartz was not considered in the calculation because it is weathering resistant, with very low solubility at surface conditions of pressure and temperature. Clark (2015) noted that Quartz requires temperatures above 100ºC, for dissolution to begin. The expected chemical differences for quartz between the original rocks and that of the leachate would not be significant, for the temperatures applied in this study. The mineral phases considered for the calculations were Albite, K-feldspar, Ca-Amphibole, Epidote and Biotite. The data from the CO2 experiments were used for mass balance calculations because they closely simulated the chemistry of groundwater samples obtained from the North-Western part of the Volta River Basin of Ghana (Table 1).
Results from mass balance calculations revealed that Na+ ions identified in leachates were consistently due to the dissolution of Albite, because it is the only Na-bearing mineral identified in thin-sections and XRD. Taking Rk 90 for instance, the estimated Na+ dissolved from Albite was 1.28 *10− 4 mols, which represented 32.8% of Albite dissolved in this rock sample (Table 1). Regarding K+ ions, it was deduced that the K+ ions dissolved in leachates was either due to the dissolution of Biotite, K-Feldspar or both processes, which depended largely on the presence of each mineral phase and its abundance relative to the other. Taking the example of sample Rk 90 for instance, K+ ions dissolved from Biotite was 9.05*10− 6 mols, representing 52.3% of dissolved Biotite, while that dissolved from K-Feldspar was 9.29*10− 5 mols, representing 60.8% of ions dissolved from K-Feldspar (Table 1).
Mass balance calculations revealed that, for most samples, Ca2+ ion was mostly due to the dissolution of Ca-Amphibole, depending on the presence of this mineral in the rock sample crashed for the experiment (Table 1). However, in a sample such as Rk 90 for instance, there were Ca2+ ions present in solution, even though there was no Ca- bearing mineral present. This could have been because XRD analysis did not discriminate between pure Albite and intermediate plagioclase, bearing Ca2+. The presence of Ca2+ in solution indicates that there was an intermediate phase of this mineral, bearing Ca.
Finally, the Mg2+ ions were due to the dissolution of either Ca-Amphibole, Biotite or both minerals depending on the presence of the mineral phases and its abundance relative to the other (Table 1).
Generally, only dissolution of minerals was recorded by mass balance, probably because the reaction time was too short for precipitation to take place. This was confirmed by the Saturation Indices of Calcite modelled in phreeqC and Diagrammes software(Simler, 2014), which ranged from − 3.44 to 0.30, suggesting undersaturation with respect to Calcite.
Table 1
Results from Mass Balance Calculations
|
Moles of Na from Albite
|
Moles of K from K-Feldspar
|
Moles of K from Biotite
|
Moles of Ca from Ca-Amphibole
|
Moles of Ca from Epidote
|
Moles of Mg from Ca-Amphibole
|
Moles of Mg from Biotite
|
RK 90 (CO2)
|
1.28E-04
|
9.29E-05
|
9.05E-06
|
-
|
-
|
-
|
1.81E-05
|
RK 58 (CO2)
|
6.77E-05
|
1.06E-04
|
6.01E-05
|
1.37E-05
|
-
|
6.86E-06
|
1.20E-04
|
RK 77 (CO2)
|
2.09E-04
|
1.77E-04
|
3.15E-05
|
7.13E-05
|
-
|
3.57E-05
|
6.30E-05
|
RK 91 (CO2)
|
9.91E-05
|
-
|
8.40E-05
|
2.67E-05
|
-
|
1.33E-05
|
0.00E + 00
|
RK 38 (CO2)
|
1.64E-04
|
-
|
1.59E-04
|
-
|
5.10E-04
|
-
|
2.46E-04
|
Figure 9 presents results of a conceptual model showing estimates from mass balance calculations (for major cations) from data of Carbon dioxide experiments, because they closely simulated groundwater chemistry. The cross-section is from the North-West to South-East of the study area, indicating four boreholes along the flow path, from which groundwater was sampled and results of mass balance calculations of leachate samples which fell along the cross-sectional line. Two rock samples, obtained for the leaching experiments were sited far from the cross-sectional line and was therefore not included in the conceptual model. In addition, it shows the partial pressure of Carbon dioxide used for the leaching experiment. The flow paths were labeled 1 and 2, depending on the direction of groundwater flow, from a point of high hydraulic head to that of low hydraulic head.
Taking a careful look at only Albite and K-Feldsar, it is observed that Rk 58 had the least concentrations of Na+ and K+ ions dissolved from these minerals, while that of Rk 77 had the highest concentrations of Na+ and K+ dissolved from Albite and K-Feldspar. These minerals are low temperature forming minerals (Clark, 2015). Therefore, it is expected that only water with longer residence time should dissolve ions from the said minerals, hence the order of dissolution of the ions, along the flow path 1. Rk 38, on the other hand, had low concentrations of Na+ and K+, dissolved from Albite and K-Feldspar, and this followed flow path 2.
3.2.5 COMPARISON OF MAJOR CATION CHEMISTRY OF GROUNDWATER AND THAT OF LEACHATES.
Four major cations (Na2+, K+, Mg2+, and Ca2+) are the core of rock-forming minerals, including silicates which are the main minerals contained in rocks of the study area (Clark, 2015).
The major ion composition of leachates could be used as the pristine conditions of aquifer since the chemistry of leachates results only from water-rock interaction. The comparison between this chemistry and that of groundwater allow highlighting the contamination.
As a result, any excess major cation concentrations found in groundwaters in the study area, would be an indication of anthropogenic activities. Table 2 presents major cation compositions of leachates from experiments under CO2 conditions and that of groundwater samples obtained from sampling points in close proximity to rock samples. CO2 experiments were used because they closely simulated that of groundwater samples.
Results revealed no consistent trend between leachate chemistry and that of groundwater. However, there seemed to be close similarities between some major ion composition of groundwater and that of leachates of specific samples. An example is Rk 91; where the concentrations of., Na+, Mg2+, and Ca2+ in the leachate are similar to concentrations in groundwater (difference of -0.33 mg/L, -3.35 mg/L and 0.84 mg/L respectively, between groundwater and leachates). For this sample, the leachates (especially from the experiment under CO2 saturated conditions) is well representative of natural chemistry of groundwater where the concentrations of Na+, Mg2+, and Ca2+ come from the weathering of rock. The K+ concentration of this sample, is however, significantly lower than that of the leachates. The difference of surface area between natural rock and rock used in experiment could be an explanation. Indeed, in natural conditions, the extent of rock-water interactions depend on several factors, including the extent of fracturing of the basement aquifers, because fracturing controls groundwater flow in these aquifers, and surface exchanges between water and rock. Fracturing influences the degree of weathering of aquifer materials (Lachassagne et al., 2021), leading to varying distributions of minerals and therefore ions in groundwater. The rocks used for the leaching experiment were crushed to increase the rate of dissolution of minerals, which disturbed natural conditions of aquifers, increasing the surface area and exposing more minerals to dissolution, which may have led to the higher concentrations of the K+ ions in the leachates than in groundwater. All this suggests that the chemistry of groundwater GW 91 seems to be largely due to water-rock interactions.
There is an exception however for sample 90, where there are excess concentrations of Na+, Mg2+ and Ca2+ ions in groundwater samples than leachates. The differences between both groundwater samples and leachates are 75.61, 32.18 and 60.82 mg/L respectively. Since major ion composition of leachates mimic pristine conditions of groundwater, these significant differences may be largely due to other factors. A possible explanation for this, could be the soil type in the study area, which is classified as Vertisols. This soil type is characterized by high clay content mainly by a group of smectite phyllosilicate minerals, formed in the presence of Ca2+ and Mg2+ ions (Jordanova, 2017). The author mentioned that a major clay mineral is Montmorillonite (Na+ and Mg2+ bearing clay). The borehole where this sample was obtained had relatively shallow depth of 31m, therefore groundwater in this area may be largely influenced by leaching of clay-bearing minerals into groundwater over time.
Leachates from the CO2 experimental conditions with pCO2 of about 1atm, for the experiment with reaction time of six months, closely simulated the chemistry of groundwater in the North-Western part of the Volta River Basin of Ghana. The actual residence time of groundwater in the study area has not yet been estimated, however, GW 38, groundwater sampled in close proximity to Rk 38, had tritium (3H) activity of 1.45 TU. According to Clark & Fritz (1997), this tritium value implies groundwaters recharged after the year 1950 (indicating young groundwaters) and its simulated pCO2 was 0.02 bar, which is however lower than the pCO2 of leachates used for the current experiment. The implication is that leaching experiments under CO2 experimental conditions of pCO2, 1 bar, with residence time of six months may simulate groundwater chemistry from rock-water interactions in basement aquifers with tritium activity of 1.45 TU.
Other important indicators of contamination are NO3− and Cl−, because these were found in insignificant concentrations in the leachates, which is the pristine composition of natural groundwater from silicate aquifers in the study area. However, NO3− and Cl− have been found in high concentrations in groundwater sampled from the study area, some of which are beyond WHO background limit (WHO, 2012), indicating contamination. This further corroborates the importance of the experiments in delineating background conditions for investigating contamination in aquifers.
Table 2
Comparison between leachate chemistry and groundwater chemistry
LEACHATE
CHEMISTRY
|
|
|
|
|
GROUNDWATER CHEMISTRY
|
|
|
|
|
Sample ID
|
Na (mg/L)
|
K
(mg/L)
|
Mg
(mg/L)
|
Ca
(mg/L)
|
Sample ID
|
Na
(mg/L)
|
K
(mg/L)
|
Mg
(mg/L)
|
Ca
(mg/L)
|
Rk 58
|
6.23
|
25.96
|
12.35
|
10.99
|
GW 58
|
14.06
|
1.39
|
3.83
|
16.62
|
Rk 38
|
15.09
|
49.78
|
23.9
|
81.83
|
GW 38
|
16.69
|
12.63
|
11.57
|
18.84
|
Rk 77
|
19.19
|
32.65
|
9.59
|
57.19
|
GW 77
|
7.71
|
1.15
|
2.04
|
6.74
|
Rk 91
|
9.11
|
26.27
|
15.35
|
21.39
|
GW 91
|
8.78
|
6.94
|
12.00
|
22.23
|
Rk 90
|
11.77
|
15.94
|
1.76
|
11.36
|
GW 90
|
87.38
|
18.87
|
33.94
|
72.18
|