Determination of humic substances in deep groundwater from sedimentary formations by the carbon concentration-based DAX-8 resin isolation technique

ABSTRACT

Concentrations of humic substance (HS) in deep groundwater from sedimentary formations were determined by the carbon concentration-based DAX-8 resin isolation technique. The groundwater samples were collected from test galleries at depths of 140, 250 and 350 m at the Horonobe Underground Research Laboratory (URL) in Hokkaido, and two observation wells with different depths of 560 and 1050 m in Niigata, Japan. The analytical condition was optimized for the groundwater samples with a high salinity and a high concentration of dissolved organic matters (DOMs). The analytical results showed that the HS concentrations varied from 7.8 to 23.5 mg-C dm⁻³, depending on the depth and the area. The HS proportions to DOM (%HS) were evaluated to be 58 – 67% and slightly decreased in the deeper groundwater. The regression analysis showed that the HS concentrations are positively correlated with the DOM concentrations. In addition, the low deviation of the %HS values from the slope in the regression equation indicated that the slight variation of %HS can be trivial in the prediction of the concentration of HS. In previous studies, HS concentrations in the groundwater had been determined with a large uncertainty and potential usage of the regression equation for the prediction had not been elucidated. Therefore, the results in this study can provide useful information on the HS concentration and its prediction from the DOM concentration in deep groundwater from sedimentary formations.

KEYWORDS:

• Humic substances
• dissolved organic matter
• determination
• composition
• groundwater
• sedimentary rock

1. Introduction

In geological disposal system of high-level radioactive waste, effects of dissolved organic matters (DOMs) in groundwater on facilitated migration of radionuclides (RNs) are of concern, because DOMs have an ability to form a stable complex with RNs to suppress their sorption and diffusion to host rocks [1–4]. Among such DOMs, humic substance (HS) is a major fraction, and can be a factor determining the effect of DOMs [5,6]. Then, concentration of HS in groundwater as well as its metal-binding constant are essential. In previous studies, the concentration of HS in groundwater has been determined by a gravimetry after the conventional isolation using the hydrophobic resin such as XAD-8 [7–10] and DAX-8 [11–14], or in conjunction with preconcentration techniques such as reverse osmosis (RO) [15] and diethylaminoethyl (DEAE)-cellulose [16–18]. Because, however, these methods had not proved to be quantitative, the concentrations determined could include an uncertainty.

Recently, the determination method for aquatic HS, i.e. the carbon concentration-based DAX-8 resin isolation technique, has been developed by optimizing the extraction conditions for HS in terms of ratio of resin to water, salinity and structural feature [19,20]. In this method, the HS concentration is determined as the difference between carbon concentrations of DOM and non-HS, which are measured before and after the DAX-8 treatment, respectively. This method without an elution procedure has an advantage for the determination of HS at low concentration compared to that with an elution procedure [21], because the carbon contamination from the resin can be kept low. Furthermore, the analysis can be easy and simple, and then have an advantage in work in a grove-box of low-oxygen atmospheric condition. Based on such advantages, the method has widely applied into a variety of natural waters such as lake [22,23], bog [23] and river [24–26].

On the other hand, the analysis using the method may lead to the development of model to predict concentration of HS from those of DOM, because the HS concentrations obtained by the method can be linearly correlated with the DOM concentration [23,25–27]. Since there is a case that a volume of groundwater is insufficient to analyse both concentrations of HS and DOM and a lot of data on the DOM effect without an information on HS concentration has been presented, the development of the model can be useful. Because, however, the carbon concentration-based DAX-8 resin isolation technique has not been applied for the determination of HS in the groundwater, the linearity and predictability of the relationship between HS and DOM concentrations is still unclear.

In this study, the carbon concentration-based DAX-8 resin isolation technique is firstly applied to determine the concentration of HS in groundwater from sedimentary formations. The groundwater has relatively high concentration of DOM, and then has a larger impact for the effect of HS. Analytical condition is optimized for the dilution, which can be a factor changing the degree of the HS extraction. The HS concentrations or HS proportions to DOM in the groundwater were compared in terms of the geological conditions. The relationship between the concentrations of DOM and HS and its application into the prediction of HS concentration from DOM concentration are presented.

2. Materials and methods

2.1. Sampling sites

Groundwater samples were taken from boreholes at galleries in the Horonobe Underground Research Laboratory (URL), Hokkaido, Japan, in September 2016. The depths of the galleries are −140, −250 and −350 m, which are corresponding to the Koetoi Formation, the transition zone between the Koetoi and Wakkanai formations, and the Wakkanai Formation, respectively. The Koetoi and Wakkanai formations are composed of diatomaceous and siliceous mudstones, respectively. The detailed geology and geochemistry of the URL are described elsewhere [28]. Other groundwater samples in sedimentary formations were collected from observation wells for subsidence in Niigata city, Japan, on February and August 2017. The observation wells are located in Nuttari and Nishikambara areas, which are in the Niigata gas field [29]. The depths of the Nuttari and Nishikambara observation wells are −560 and −1010 m, which are at the Kanbara and Uonuma formations, respectively. These formations are found in alternate layer composed of sand gravel layer being natural gas bed and mud layer mainly including sand and silt. The details of the geology are described elsewhere [29–33]. The locations of the sampling sites are shown in Figure 1. The geological conditions of the sampling sites are summarized in Table 1.

Table 1. Geological conditions at the Horonobe boreholes and the Niigata observation wells.

Sampling sites	Latitude (N, decimal)	Longitude (E, decimal)	Depth (GL – m)	Geological formation	Composition
Horonobe, Borehole 07-V140-M03	45.0456	141.8593	142.3–150.1	Koetoi F.	Diatomaceous mudstone
Horonobe, Borehole 08-E140-C01	45.0446	141.8602	157.6–167.9	Koetoi F.	Diatomaceous mudstone
Horonobe, Borehole 09-V250-M02	45.0453	141.8596	248.9	Transition*	Diatomaceous/siliceous mudstone
Horonobe, Borehole 10-E250-M01	45.0446	141.8601	247 – 248.8	Transition*	Diatomaceous/siliceous mudstone
Horonobe, Borehole 13-350-C01	45.0459	141.8596	348 – 349.4	Wakkanai F.	Siliceous mudstone
Horonobe, Borehole 14-350-C04	45.0452	141.8586	348.6–353.7	Wakkanai F.	Siliceous mudstone
Niigata, Observation well Nuttari-560 m	37.9196	139.0782	553.0–558.0**	Kanbara F.	Gravel, sand, mud
Niigata, Observation well Nishikambara-1050 m	37.8504	138.9804	972.0–1043.0**	Uonuma F.	Gravel, sand, mud

*Tradition zone between Koetoi and Wakkanai formations

**Screen depth

Figure 1. Sampling sites of the sedimentary groundwater.

2.2. Groundwater sampling

A packer type sampling device was used for the collections of the groundwater from the boreholes in the Horonobe URL. The groundwater sample was taken into a brown glass bottle and then that was immersed into a larger bottle filled with same groundwater to prevent the oxidation during the transportation. The groundwater samples in the observation wells in the Niigata city were taken by means of water pumping after the water discharge followed by the stabilization of the water quality. The sample bottles were immediately brought into a grove box (<1 ppm O₂) and stored for the analysis. The pH and electronic conductivity (EC) in the groundwater samples were measured by a pH/EC meter (DKK-TOA, WM-32EP) after the calibration with the pH standard and EC cell check solutions (DKK-TOA). The groundwater qualities measured are summarized in Table 2. The details on the groundwater qualities are found elsewhere [34–36].

Table 2. Water qualities and concentrations of DOM and HS in the groundwater at the Horonobe boreholes and Niigata observation wells.

Sampling sites	Sampling date	Water type [34–36]	pH	EC (mS m^−1)	CDOM (mg-C dm^−3)	CHS (mg-C dm^−3)	%HS (%)
Horonobe, Borehole 07-V140-M03	Sep. 2016	Fossil seawater, Na-Cl (HCO3)*	7.4	1350	24.6 ± 0.6	15.9 ± 0.6	64.6 ± 1.7
Horonobe, Borehole 08-E140-C01	Sep. 2016	Fossil seawater, Na-Cl (HCO3)*	7.2	1240	20.0 ± 1.2	13.4 ± 1.6	67.0 ± 3.8
Horonobe, Borehole 09-V250-M02	Sep. 2016	Fossil seawater, Na-Cl (HCO3)*	7.3	860	12.5 ± 1.4	7.8 ± 0.4	62.8 ± 3.8
Horonobe, Borehole 10-E250-M01	Sep. 2016	Fossil seawater, Na-Cl (HCO3)*	7.3	800	15.3 ± 1.0	10.2 ± 1.3	66.3 ± 4.3
Horonobe, Borehole 13-350-C01	Sep. 2016	Fossil seawater, Na-Cl (HCO3)*	7.4	1990	17.3 ± 0.2	10.6 ± 0.4	60.9 ± 2.0
Horonobe, Borehole 14-350-C04	Sep. 2016	Fossil seawater, Na-Cl (HCO3)*	7.0	1660	20.7 ± 0.7	12.6 ± 0.6	61.1 ± 1.3
Niigata, Observation well Nuttari-560 m	Aug. 2017	Fossil seawater, Na-Cl	7.4	1970	19.3 ± 0.3	12.3 ± 0.1	63.6 ± 0.4
Niigata, Observation well Nishikambara-1050 m	Feb. 2017	Fossil seawater, Na-Cl	7.3	3540	40.5 ± 1.0	23.5 ± 1.1	58.0 ± 4.2

* HCO₃ is a measurable component [34]

2.3. Quantitative analysis of HS

Concentration of HS in the groundwater was determined by the carbon concentration-based DAX-8 isolation technique [19]. The groundwater samples were passed through a glass fiber filter with pore size of 0.3 μm (ADVANTEC, GF-75). A portion of the filtrate was transferred into a glass vial for a measurement of total organic carbon (TOC), and the rest was acidified to below pH 2 with 2.5 mol dm⁻³ H₂SO₄ aqueous solution for the DAX-8 treatment. The acidified samples were mixed with a purified DAX-8 resin in the water-resin volume ratio of 50:1. The mixtures were shaken below 4°C for 24 h in the dark. After the equilibration, the mixture was passed through the glass fiber filter to separate the sample solution from the DAX-8 resin. The carbon concentration in the filtrate (C_nonHS) was measured by a TOC analyser (Shimazu, TOC-Lcph). The procedure described here was conducted under a low-oxygen atmospheric condition (<1 ppm O₂), except for TOC measurement. The concentration of HS (C_HS) was calculated by the subtraction of C_nonHS from the total carbon concentration of DOM (C_DOM). The detection limit (3 μ) and the minimum limit of determination (10 μ) in this analysis were 0.7 and 2.4 mg-C dm⁻³ (n = 40), respectively. The analysis was conducted in duplicate or triplicate.

3. Results and discussion

3.1. Validations of the analysis

In the carbon concentration-based DAX-8 isolation technique, the sample with the C_DOM over 30 mg-C dm⁻³ needs to be diluted within the range 10–20 mg-C dm⁻³ [19]. Groundwater in sedimentary formations sometimes had a high C_DOM over the upper limit, then needed to dilute. However, such groundwater also have a high salinity (Table 2). The dilution may lead to a suppressed sorption of HS to DAX-8 resin due to the reduced salting-out effect [37,38]. On the other hand, Kida et al. show that the variation of salinity is negligible on the analysis, because no sorption of aromatic and aliphatic HS to DAX-8 resin were affected by the salinity [20]. In this study, however, only HSs isolated from the lake and river waters were used for the validation. It is not true that the finding is also available for the more aliphatic HS in groundwater from sedimentary formation [13].

To verify this, we investigated the HS proportion (%HS = 100 × C_HS/C_DOM) as a function of dilution factor (f_d = C_initial/C_diluted) in the groundwater. The relationships between %HS and f_d in the Horonobe groundwater at the different depths and the Nishikambara groundwater are shown in Figure 2. In the Horonobe groundwater, the %HS values seemed to decrease with increasing the f_d value but were within the error (e.g. %HS for the borehole 07-V140-M03 = 64.6 ± 1.7 at f_d = 1, 62.1 ± 2.3 at f_d = 4; %HS for the borehole 14-350-C04 = 61.1 ± 1.3 at f_d = 1, 56.6 ± 3.2 at f_d = 4). The Nishikambara groundwater showed the similar result (e.g. %HS = 60.8 ± 0.9 at f_d = 1, 59.3 ± 0.9 at f_d = 4). These results indicate that the dilution effect in the f_d range from 1 to 4 is negligible. Thus, this shows that the proposed method is also available for the HS in groundwater from sedimentary formations. Since the Nishikambara groundwater has a high concentration over 30 mg-C dm⁻³, this sample was diluted by a factor of 2 for the analysis.

Figure 2. Relationships between the HS proportion (%HS) and the dilution factor (f_d) in the different groundwater samples.

In the Horonobe groundwater at the borehole 09-V250-M02, the C_HS value was determined to be 7.8 mg-C dm⁻³ by the carbon concertation-based DAX-8 resin isolation technique (Table 2), while the value obtained by the gravimetry was 1.0 mg-C dm⁻³ [13]. This shows that the DAX-8 isolation technique provides higher C_HS. In the gravimetry, the groundwater sample was treated in the water-resin ratio of 1500 to 1, excessing the optimized ratio of 125 to 1 [7]. Because the excessed volume of water leads to the overcapacity of the DAX-8 resin, the gravimetry in the previous study could have resulted in the lower C_HS. Thus, the higher C_HS determined by the carbon concentration-based DAX-8 isolation technique can be reasonable.

3.2. Concentrations of DOM and HS

The C_DOM and C_HS values in the groundwater samples are summarized in Table 2. The C_DOM values varied within a range from 12.5 to 40.5 mg-C dm⁻³, depending on the depth and area. The C_DOM values in the Horonobe groundwater were comparable with that in the Nuttari groundwater, while the value in the Nishikambara groundwater is remarkably high. The observation wells in the Nuttari and Nishikambara are located in the Niigata gas field [29], where sedimentary organic carbon (SOC) can be widely distributed. Such SOC can have been subjected to the microbiological-mediated mineralization to enhance the C_DOM in groundwater [39–41]. Moreover, the Nishikambara groundwater can be less influenced by the dilution with the recharge of meteoric water, because it has the highest EC value (Table 2). Thus, the high C_DOM in the Nishikambara groundwater can be due to the high degree of the mineralization of SOM and/or the less influence of recharge water. On the other hand, the C_DOM values in the Horonobe groundwater varied with the depth: the C_DOM values are relatively high at the depth of 140 and 350 m, while low at the depth of 250 m. It is known that the transition zone at the depth of 250 m has a relatively high degree of permeability [42]. Thus, the relatively low C_DOM at the depth can be due to the dilution with the recharge of meteoric water.

The C_HS also fluctuated with changes in the C_DOM, depending on the depth and area (Table 2). The C_HS values in the groundwater of both areas ranged from 7.8 to 23.5 mg-C dm⁻³, and were accounted for 58–67% of C_DOM. This clearly shows that HS is a dominant fraction of DOM in the deep groundwater from sedimentary formations. When the %HS values were compared between different areas, it was found that the %HS values in the Niigata groundwater fall within the range of those in the Horonobe groundwater. On the other hand, the %HS values in both areas slightly decreased in the deeper groundwater. For example, the %HS values in the Horonobe groundwater ranged from 62.8% to 67.0% over the depths of 140 and 250 m, but from 60.9% to 61.1% at the depth of 350 m. The similar trend was also observed in the Niigata groundwater at the depth of 560 and 1050 m. Because it is suggested that the release of HS from SOC can be a dominant factor determining the C_HS in the groundwater [15], the variations of %HS among the different depths might be brought by the differences in characteristics of SOC and/or its mineralization process by a microbial activity at each depth.

3.3. Prediction of C_HS from C_DOM

Figure 3 shows the relationship between the C_HS and C_DOM in the groundwater together with the results of regression analysis. The C_HS increased with increasing the C_DOM, and the regression analysis for the relationship showed a good positive correlation (C_HS = 0.615 × C_DOM, r² = 0.971, P < 0.001). This suggests a possibility that the linear regression can be used to predict C_HS from C_DOM. On the other hand, it has been mentioned that the C_HS in surface waters cannot necessarily be predicted from the C_DOM by using a fixed regression equation [23], because the %HS may be variable even in a given aquatic environment. Because, however, the %HS could be altered by photodegradation and microbial processes highly activated in the surface of the water [22,23], the variations of %HS may occur only in the surface water such as lake and bog.

Figure 3. Relationships between C_DOM and C_HS in the groundwater at the Horonobe boreholes and Niigata observation wells and the result of the linear regression analysis.

To clarify this, the measured %HS were compared with the constant %HS defined as the slope of the linear regression equation. The degree of the deviation from the constant %HS was also compared with those in the surface waters found in the literatures, in which the carbon concertation-based DAX-8 resin isolation technique was used for the analysis. The results of the comparisons are shown in Figure 4. The measurements of the groundwater in this study agreed well with the constant value (Figure4(a)), while those in the lakes and bogs were widely distributed (Figure.4(c,d)). The measurements in the river waters well agreed with the constant value at higher C_DOM, but were largely deviated at lower C_DOM (Figure4(b)). The root mean square error (RMSE) for %HS measurements of the groundwater in this study was calculated to be 3.197, and was obviously small compared to those for the rivers (5.519), the lakes (8.612) and the bogs (12.86). This shows that, in the range of C_DOM from 12.5 to 40.5 mg-C dm⁻³, the %HS in the groundwater from sedimentary formations can be almost constant, while those in the surface waters are variable. The groundwater used in this study are a fossil seawater whose compositions are altered by diagenetic processes in marine sediments. In the fossil seawater, DOM may have similar characteristics in the composition and structure due to the similarities of origins, e.g. marine planktons and humification process [43]. In addition, such similar characteristics are likely to be maintained by the negligible percolation of organic carbon from surface to deep underground [44,45] and the low microbial activity induced by the consumption of available nutrients and energy sources [46]. Thus, the narrower deviation of the %HS supports the suggestion that the prediction of C_HS from C_DOM by the linear regression equation can be possible in the deep groundwater with higher C_DOM from 10 to 40 mg-C dm⁻³ from sedimentary formations. However, it should be noted that no the equation can be applied into other groundwater in different geological conditions, because %HS in the other groundwater may be variable due to different characteristics and fate of DOM. Therefore, for the prediction of C_HS from C_DOM in other groundwater, the equation needs to be verified by the determination of HS in a target groundwater and be modified if necessary.

Figure 4. Comparisons between the %HS measured by the carbon concentration-based DAX-8 resin isolation technique and the constant %HS defined as the slope of the regression analysis. The constant %HS from the literatures was calculated by the linear regression analysis as an intercept of zero. $\mathrm{R}\mathrm{M}\mathrm{S}\mathrm{E}=\sqrt{\frac{1}{n}\sum _i{\left(y_{\mathrm{o}\mathrm{b}\mathrm{s},\mathrm{i}}-y_{\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{d},\mathrm{i}}\right)}^2}$, where n is a number of sample. y_obs,i and y_pred,i arethe experimentally determined and predicted %HS (i.e. the slope of the linear regression), respectively.

4. Conclusion

In previous studies, the concentration of HS in groundwater from sedimentary formations had been estimated with a large uncertainty, because the gravimetry unestablished for a quantitative analysis was applied for the determination. In this study, the carbon concentration-based DAX-8 resin isolation technique, which has been well established for the analysis, was applied for the determination of HS in the groundwater. As a result, the concentrations of HS in the groundwater from sedimentary formations in Japan were determined to be 7.8–23.5 mg-C dm⁻³ and the HS proportions were then calculated to be 58 – 67%. Because the HS proportions in our previous studies were evaluated to be 1.0–9.4% [12–14], our result improves the knowledge on the proportion. Moreover, the linear relationship between the concentrations of HS and DOM and the low variability in %HS suggest that the linear regression equation can be useful for the prediction of C_HS from C_DOM in the groundwater in sedimentary formations. Although the application of the equation can be limited to the groundwater in the similar geological condition, it is expected that the applicability can be improved by the modification of the equation via the analysis of HS in groundwater in different geological conditions. Therefore, the findings obtained in this study can provide useful information on the HS concentration and its prediction from the DOM concentration in deep groundwater from sedimentary formations.

Acknowledgments

We thank Dr. Isao Machida at the Advanced Industrial Science and Technology (AIST) for groundwater sampling from the two observation wells in Niigata city. We also thank Prof. Nobuhide Fujitake at the Kobe University for the advice on the carbon concentration-based DAX-8 isolation technique and useful discussion on the analytical values. This study has been conducted as a part of the public offered project, a commission investigation of geological disposal technology (Development of enhancing the disposal system in the coastal region) under the contract with Agency for Natural Resources and Energy (ANRE), Ministry of Economy, Trade and Industry, Japan.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Agency for Natural Resources and Energy [a commission investigation of geological disposal].