Evaluation of carbon flux through groundwater and river water at the western foothills of Mt. Chokai

Abstract

Groundwater and river water are essential in geochemical transportation from land to sea. Freshwater discharge to the coastal sea as groundwater sometimes has large geochemical fluxes comparable to river water; however, it is more challenging to monitor groundwater than river water. In this study, we assessed the carbon cycle including an underground system in a highly porous coastal area along Mt. Chokai, northern Japan, where abundant submarine spring water emerges. Groundwater and river water chemistries are generally characterized by silicate weathering, reflecting the andesitic lava that occupies the basin. Groundwater discharge was determined using mass balance calculations, including precipitation, river water discharge, and evapotranspiration. Considering the underground dissolved bicarbonate flux from land to ocean, the CO₂ consumption by silicate weathering in the western foothills of Mt. Chokai is calculated to be 8.8 ± 0.15 t-C/km²/yr, which is significantly larger than the case where only river water flux was evaluated (4.7 ± 0.25 t-C/km²/yr). Therefore, considering underground flux when estimating the amount of chemical weathering in coastal watersheds is crucial.

Introduction

Groundwater accounts for approximately 30% of the world’s freshwater storage, considerably larger than river water, accounting for approximately 0.006% of the total freshwater (Fitts, 2002). Generally, the mean residence time of groundwater is hundreds or even thousands of years, which considerably exceeds that of river water (approximately two weeks) (Oki and Kanae, 2006). Therefore, groundwater tends to take up more dissolved solids through water-rock interactions and potentially plays an essential role in geochemical cycles (Zektser and Loaiciga, 1993; Taniguchi et al., 2002). Previous studies have reported that submarine spring water discharge accounts for only 6% of the total water discharge into the sea; however, it potentially contributes to approximately 50% of dissolved solids loading (Zektser and Loaiciga, 1993). In Japan, where the distance between mountains and the sea is short, and there is high precipitation, groundwater is directly supplied to the sea as submarine spring water (Taniguchi et al., 2002; Hosono et al., 2012; Katazakai and Zhang, 2021). Therefore geochemical fluxes via groundwater under the coastline need to be considered to accurately estimate total flux from land to sea.

Water-rock interactions promote chemical weathering, which consumes atmospheric CO₂ and generates bicarbonate. The bicarbonate flows into the sea and is released into the atmosphere as CO₂ by mineralizing marine carbonates. The difference in the molar ratios of CO₂ consumed by the silicate and carbonate weathering process suggests that only silicate weathering contributes to a net reduction in atmospheric CO₂ (Berner et al., 1983; Manaka et al., 2015). Therefore, evaluating the dissolved bicarbonate flux originating from terrestrial silicate weathering is essential for understanding the global carbon cycle.

Mt. Chokai, the highest mountain in northern Honshu Island, is located approximately 10 km from the coastline of the Sea of Japan. Its climate is characterized by high precipitation induced by the East Asian monsoon. The geology of the western foothill of Mt. Chokai is characterized by highly porous andesitic rock extending into the Sea of Japan. As a result, many springs emerge along the mountain slope, some of which occur on the seafloor as submarine spring waters (Taniguchi et al., 2002; Hosono et al., 2012). The aim of this study is to reveal the geochemical characteristics of river water and groundwater in the western foothills of Mt. Chokai bordering the Sea of Japan and estimate bicarbonate flux from the land to sea via the coastline.

Study Area

Geology and topography of Mt. Chokai

Mt. Chokai is 2236 m above sea level and the highest mountain in northern Honshu Island in Japan. The volcanic activity that formed Mt. Chokai can be divided into three stages (Hayashi, 1984a; Nakano et al., 2022). Lava flows in the first stage (600–160 ka) significantly shaped the mountains. The total volume of volcanic ejecta in the first stage was 47 km³, accounting for 64% of the total ejecta from Mt. Chokai. The total volume of the second stage (160–20 ka) ejecta was estimated to be 22 km³, accounting for 30% of the Chokai Volcano ejecta. The erupted lava in the third stage (20 ka in 1974) covers the eastern and western slopes of the volcano. Today, the andesite lava covers approximately 26 km from east to west and 14 km from north to south and extends along the coast of the Sea of Japan (Fig. 1).

Fig. 1.

Geological map in and around Mt. Chokai, where the circles indicate sampling points (red: rivers, blue: springs, numbers correspond to IDs in Table 1) (Geological Survey of Japan, AIST, 2022). The area enclosed by the dotted black line is the western foothills of Mt. Chokai, which is incorporated in the carbon flux estimation.

Abbreviations: A, Koyoshi River; B, Osawa River; C, Shirayuki River; D, Akaishi River; E, Naso River; F, Kawabukuro River; G, Gakko River; H, Nikko River.

Climate and hydrology in the western foothills of Mt. Chokai

Mt. Chokai is one of the highest precipitation areas in Japan, with annual precipitation exceeding 2,600 mm even in the coastal area (Akimichi, 2010) and approximately 12,000 mm around the summit (Tsuchiya, 1990). The western foothills of Mt. Chokai can be divided into several river catchment areas of the Koyoshi, Osawa, Shirayuki, Akaishi, Naso, Kawabukuro, Gakko, and Nikko Rivers (Fig. 1). Numerous springs are distributed along the mountain slope; some of them are submarine springs. The submarine spring water inflow is one of the largest worldwide (Hosono et al., 2012; Nakano et al., 2022) and contributes to oyster production in the western foothills of Mt. Chokai (Hosono et al., 2012). Some springs at relatively high altitudes on Mt. Chokai form wetlands such as the Ryugahara and Toshi Wetlands (Nakano et al., 2022).

Materials and Methods

Sampling protocols

In this study, we collected river and spring water samples from the western foothills of Mt. Chokai from July 22nd to 25th, 2022 (Fig. 1). River samples were collected from the center of the stream by dropping washed buckets from the bridge. Spring water samples were collected directly from the stream. The location and altitude of the sample sites were recorded using the handy GPS application (horizontal and vertical errors are generally within 10 m).

Atmospheric and water temperatures, pH, and electrical conductivity (EC) were measured in situ using a pH meter (HORIBA, LAQUAtwin pH-33B) and EC meters (HORIBA, LAQUAtwin EC-33B). The measurement accuracy of pH is ±0.01 and that of EC is ±2%. Samples were prefiltered using acetate membrane filters (pore size: 0.45 μm) and stored in combusted glass vials and washed polypropylene bottles for total alkalinity (TA) and inorganic element analysis. We added HgCl₂ to the glass vials for TA analysis to prevent biological activity. The samples were stored in a refrigerator until the laboratory analysis.

Laboratory analysis

Concentrations of major cations (Na⁺, K⁺, Mg²⁺, Ca²⁺, and NH₄⁺) and anions (F^–, Cl^–, NO₃^–, and SO₄^2–) were measured at Hirosaki University using ion chromatography (Metrohm, Eco IC). The samples were diluted to within the concentration range of the standard solutions (cations: Merck, 89316-50ML-F; anions: Merck, 89886-50ML-F). The standard deviation of 5 repeated measurements of the standard solution averaged 0.5% (1σ) of the mean value. The detection limit was 0.05 ppm.

Using an automatic metric analyzer, total alkalinity (TA) was determined by neutralization titration (Metrohm, 876 Dosimat). A 50 ml (±0.01 ml) sample was titrated to pH 4.5 using 0.001 mol/L sulfuric acid (Junsei Chemical Co., Ltd). Three replicate analyses were conducted for all samples, and the average analytical error was 1.05% (1σ). HCO₃^– concentration was calculated from TA with in situ temperature and pH using the carbonate equilibrium calculation program CO₂calc (Robbins et al., 2010). The calculation was based on the equilibrium constants K₁ and K₂ of Millero (1979).

Evaluation of river water and groundwater discharge

We estimated annual river water discharge ($Q_r$) for the three rivers flowing over the andesite lava distributed along the western foothills of Mt. Chokai (Gakko, Naso, Kawabukuro rivers). The $Q_r$ for Gakko River was observed by Yagami and Goto (2000) as 1.2 × 10⁸ (m³/yr). Unfortunately, reliable meteorological data for the Naso and Kawabukuro Rivers are lacking. Here, $Q_r$ for these rivers was calculated based on Equation (1) below, which represents an empirical relationship between $Q_r$ and river catchment area based on vast river observation data (Syvitski and Milliman, 2007):

  

$$\begin{array}{c}{Q}_r=0.075\times {\left(\text{Catchment area}\right)}^{0.8}\end{array}$$(1)

Although this equation was developed from a pan-global dataset, it should also be suitable for estimating river water discharge in this region because it can calculate $Q_r$ for the Gakko River with an error of 18%.

The annual groundwater discharge ($Q_g$) of each river catchment was calculated based on the following mass balance expressed by Equation (2) (Ward and Trimble, 2003):

  

$$\begin{array}{c}P-E-SS=Q_r+Q_g\end{array}$$(2)

where P (m³/yr) is the annual precipitation input, E (m³/yr) is the annual evapotranspiration, and SS (m³/yr) is the annual change in soil storage. Assuming negligible changes in soil storage over the course of a year, the water balance of a river basin can be calculated using Equation (3) (Ward and Trimble, 2003):

  

$$\begin{array}{c}\hfill P-E=Q_r+Q_g\end{array}$$(3)

where P (mm/month) was calculated using 1 km-resolution precipitation data available for 2000–2020, as reported by Hatono et al. (2022). Nash-Sutcliffe efficiency, which is frequently utilized for evaluating the predictive skill of hydrological models, is approximately 0.6 for this mode, which is sufficient for quantitative evaluation (Moriashi et al., 2007). The error of the estimates compared to the actual observations was confirmed to be 9% on average (Hatono et al., 2022). E (mm/month) was calculated using Equation (4), as reported by Takahashi (1979), which can more precisely estimate evapotranspiration compared to the conventionally used Thornthwaite method (Thornthwaite, 1948), which is not suitable for calculating evapotranspiration over a wide area:

  

$$\begin{array}{c}E=\frac{3100R}{3100+1.8R^{2}exp\left(-\frac{34.4T}{235+T}\right)}\end{array}$$(4)

where R and T are the average monthly precipitation (mm/month) and atmospheric temperature (°C), respectively, derived from the nearby AMeDAS station of the Japan Meteorological Agency, AMeDAS (2023). Although AMeDAS measurements show little error, the limited number of observation station complicated the comprehensive assessment of the error for evapotranspiration.

Results

The in situ-analyzed EC, TA, and pH were 67–360 μS/cm, 117–501 μeq/L, and 6.8–8.1 for the river and 7–260 μS/cm, 27–555 μeq/L, and 6.8–7.9 for the spring. The water type was classified based on Piper (1944), with most spring waters being predominantly Na–Cl type or Ca–Cl type (Fig. 2). All analyzed dissolved solid concentrations are presented in Table 1. The total cationic charge (TZ⁺ = Na + K + 2Ca + 2Mg) and anion charge (TZ^– = F + Cl + NO₃ + SO₄ + HCO₃) were 619–1081 μeq/L and 523–996 μeq/L for the river and 26–1993 μeq/L and 26–1912 μeq/L for the spring. The normalized inorganic charge balance (NICB = (TZ⁺ – TZ^–)/(TZ⁺ + TZ^–)) ranged from 1.5 to 18% (average: 7.8%).

Fig. 2.

Trilinear diagram showing the composition of the water samples (rivers and springs). Numbers correspond to IDs in Table 1.

Table 1.

Sample information and concentrations of dissolved major ions

ID	Place	Altitude m	Lat. °N	Long. °E	Date	Air Temp.	Water Temp.	EC μS/cm	TA μeq/L	pH	Na	K	Mg	Ca	F	Cl	NO3	NH4	SO4	HCO3	NICB
						℃					μmol/L
River water
1	Koyoshi River	37	39.2534	140.1205	2022/7/22	25.3	20.0	360	218	6.8	354	34	101	114	2.6	246	11	n.d.	115	217	7.2
2	Naso River	183	39.1740	139.9634	2022/7/22	24.1	19.3	91	322	7.0	247	37	89	79	2.3	175	7.2	n.d.	20	320	6.4
4	Shirayuki River	35	39.2520	139.9483	2022/7/23	29.2	19.8	121	117	7.3	310	46	161	201	4.6	283	19	n.d.	286	116	4.1
5	Kawabukuro River	57	39.1454	139.9031	2022/7/23	27.1	13.9	96	377	7.5	404	55	105	121	2.6	351	15	n.d.	27	374	6.8
8	Gakko River	38	39.0169	139.9388	2022/7/23	26.9	19.9	67	267	8.1	228	34	96	91	1.8	160	21	n.d.	44	253	10
10	Nikko River	78	38.9947	139.9727	2022/7/23	23.9	22.2	99	501	7.8	350	38	133	169	4.6	216	7.6	n.d.	57	489	8.9
Spring water
3	Mototaki Waterfall	202	39.0780	139.9524	2022/7/22	17.8	10.8	68	281	7.4	294	40	75	81	2.2	228	10	n.d.	21	279	7.0
6	Hukuden Spring	4	39.1523	139.8917	2022/7/23	26.3	14.7	132	462	7.5	608	57	162	143	3.0	528	33	n.d.	40	458	7.3
7	Kamiko Spring	19	39.1050	139.8790	2022/7/23	25.3	14.1	260	436	7.0	1002	50	185	130	1.1	1035	17	n.d.	59	435	2.3
9	Dohara Falls	226	39.0415	139.9737	2022/7/23	23.7	11.1	58	189	7.9	228	30	58	75	1.5	189	18	2.1	38	184	5.6
11	Kamaiso	0	39.0800	139.8720	2022/7/23	24.9	12.9	220	555	7.8	1111	89	240	156	3.6	1216	31	n.d.	59	544	2.1
12	Toshi Wetland	361	39.2174	140.0202	2022/7/24	23.6	19.7	66	149	7.9	288	25	80	71	2.7	206	7.6	n.d.	44	144	16
13	Toshi Wetland	301	39.2031	139.9856	2022/7/24	25.4	18.9	76	176	7.8	251	27	104	107	2.7	175	7.6	n.d.	106	172	10
14	Ryugahara Wetland	1189	39.1390	140.0690	2022/7/24	18.7	14.1	38	67	7.3	109	6.6	47	54	0.83	79	12	n.d.	46	66	12
15	Source water of Mt. Chokai	1636	39.1002	140.0199	2022/7/25	16.2	10.0	7	29	7.5	16	1.0	2.6	2.3	0.11	15	2.9	1.4	2.3	2.9	1.5
16	Source water of Mt. Chokai	1579	39.1020	140.0056	2022/7/25	17.6	16.0	8	27	6.8	23	0.16	6.3	4.3	0.11	16	0.69	0.58	5.4	2.7	18

“n.d.” means “below the detection limit.”

P and E values for each river catchment were calculated to be 1854–2256 mm/yr and 573–584 mm/yr, respectively (Table 2). $Q_r$ and $Q_g$ for the western foothill of the Mt. Chokai area were subsequently calculated to be 2.6 × 10⁷–1.2 × 10⁸ m³/yr and 6.7 × 10⁶–1.1 × 10⁸ m³/yr, respectively (Table 2).

Table 2.

Information on the water balance in the river catchment (Input, Evapotranspiration, River discharge, and Groundwater discharge)

River	Catchment area km2	Precipitation mm/yr	Evapotranspiration mm/yr	P m3/yr	E m3/yr	Qr m3/yr	Qg m3/yr
Naso River	43	1854	584	8.0 × 107	2.5 × 107	4.8 × 107	6.7 × 106
Kawabukuro River	20	2256	584	4.6 × 107	1.2 × 107	2.6 × 107	7.7 × 106
Gakko River	167	1929	573	3.2 × 108	9.6 × 107	1.2 × 108	1.1 × 108
Total	230			4.5 × 108	1.3 × 108	1.9 × 108	1.2 × 108

The catchment area and the precipitation were estimated based on Yamazaki et al. (2018) and Hatono et al. (2022), respectively. The evapotranspiration in the river catchment area was calculated based on Takahashi et al. (1979) using monthly precipitation and temperature data observed at the Nikaho Regional Meteorological Observatory and Sakata Special Regional Meteorological Observatory. P, E, $Q_r$, and $Q_g$ represent annual precipitation (m³/yr), annual evapotranspiration (m³/yr), annual river water discharge (m³/yr), and annual groundwater discharge (m³/yr), respectively.

Discussion

Source of dissolved solids of river water and spring water

Several researchers have attempted to determine the chemical composition of submarine spring water to determine the underground chemical flux to the sea. Still, it took a lot of effort to eliminate seawater contamination during sampling and the influence of recirculated seawater on the seafloor (Hosono et al., 2012). Therefore, in this study, the average chemical composition of the three springs along the coastline (Hukuden, Kamiko Springs, and Kamaiso) is considered to be the chemical composition of submarine springs in this coastal area.

To evaluate the dissolved carbon flux from land to the sea via river and underground water, the source of the dissolved solids needs to be investigated. First, we plotted Na-normalized HCO₃^– relative to Na-normalized Ca concentrations, and Na-normalized Mg relative to Na-normalized Ca concentrations, to estimate the source of dissolved solids (Fig. 3) (Gaillardet et al., 1999; Manaka et al., 2015; Kajita et al., 2020). Figure 3 shows that all our samples are highly influenced by silicate weathering and evaporate dissolution/input of sea salt and are less affected by carbonate weathering. This is consistent with the fact that the watershed of our sampling sites in the western side of Mt. Chokai is 99% composed of silicate rock (Hayashi, 1984b) with no distribution of carbonate or evaporite (Fig. 1) and is susceptible to sea salt carried by winds (Nakano et al., 2022). The water chemistry for the Shirayuki River (ID: 4) is exceptionally characterized by high SO₄^2–, which may be influenced by the acidic iron ore springs in the upper reaches of this river (Shiikawa, 1979).

Fig. 3.

(a) Molar ratios of HCO₃^–/Na and Ca/Na. (b) Molar ratios of Mg/Na and Ca/Na. End member compositions for evaporite, silicate, and carbonate are estimated using data from small rivers draining a single lithology (Gaillardet et al., 1999). End member composition for sea spray inputs is based on seawater composition (Grasshoff et al., 2009). Numbers correspond to IDs in Table 1.

Assuming that there are few anthropogenic inputs because the sample sites in this study were located away from nearby residential areas and industrial facilities, we propose that dissolved solids are primarily derived from silicate weathering except for atmospheric sea salt input. According to Keene et al. (1986), if we assume that all Cl in water samples originated from atmospheric sea salt inputs, dissolved cations derived from atmospheric sea salt (${\mathrm{X}}_{sample}$) can be calculated using the following Equation (5):

  

$$\begin{array}{c}\hfill {\mathrm{X}}_{\mathit{\text{sample}}}=({\mathrm{C}\mathrm{l}}_{\mathit{\text{sample}}}\times \frac{x_{sea}}{{\mathrm{C}\mathrm{l}}_{sea}})\end{array}$$(5)

where $x_{sea}$ and ${\mathrm{C}\mathrm{l}}_{sea}$represent the dissolved solids and Cl concentrations of seawater, respectively (Grasshoff et al., 2009), and ${\mathrm{C}\mathrm{l}}_{sample}$ represents the Cl concentration of the sample. Using this equation, we estimated that dissolved cation concentrations originated from silicate weathering ($[\mathrm{X}{]}_{sil}$). Then, dissolved HCO₃^– concentration originating from silicate weathering (${\left[{\mathrm{H}\mathrm{C}\mathrm{O}}_3\right]}_{sil}$) was calculated using Equation (6):

  

$$\begin{array}{c}\hfill {\left[{\mathrm{H}\mathrm{C}\mathrm{O}}_3\right]}_{sil}=[\mathrm{N}\mathrm{a}{]}_{sil}+[\mathrm{K}{]}_{sil}+2\times [\mathrm{M}\mathrm{g}{]}_{sil}+2\times [\mathrm{C}\mathrm{a}{]}_{sil}\end{array}$$(6)

where the analytical error of ${\left[{\mathrm{H}\mathrm{C}\mathrm{O}}_3\right]}_{sil}$ was estimated to be approximately 7.8%, as the average of NCBI.

Silicate weathering and CO₂ consumption

The relationships between ${\left[{\mathrm{H}\mathrm{C}\mathrm{O}}_3\right]}_{sil}$ in river and spring waters and altitude are shown in Fig. 4. ${\left[{\mathrm{H}\mathrm{C}\mathrm{O}}_3\right]}_{sil}$ for the spring waters had negative relationships with altitude. Conversely, there were no clear relationships between ${\left[{\mathrm{H}\mathrm{C}\mathrm{O}}_3\right]}_{sil}$ in river water and that of sampling altitude, as the river water is composed of precipitation, while spring water emerges at various altitudes (Fig. 4). These results indicate that spring water gushing from lower altitudes has a longer residence time and incorporates more dissolved solids through silicate weathering.

Fig. 4.

Relationship between the elevation of the water sampling point and [HCO₃]_sil. Numbers correspond to IDs in Table 1.

To determine the relative contributions of river water and groundwater to the total carbon flux from land to sea at the western foothills of Mt. Chokai, we focus on three rivers and three springs in the andesite ejected area on the western foothills of Mt. Chokai (rivers: Kawabukuro, Naso, and Gakko Rivers; springs: Hukuden, Kamiko, and Kamaiso) in the following discussion (Table 3). This area (enclosed by the dotted line in Fig. 1) is predicted to be particularly rich in submarine spring water (Hosono et al., 2012).

Table 3.

Concentrations of dissolved cations originated from silicate weathering (corrected using Equation (5)) and estimated bicarbonate dissolution accompanied by the weathering. The error reflects the analytical error of the ion balance

ID	Place	Nasil	Ksil	Mgsil	Casil	estimated [HCO3]sil
		μmol/L
River water
2	Naso River	97	33	72	79	431 ± 34
5	Kawabukuro River	103	49	71	121	534 ± 42
8	Gakko River	91	31	80	90	463 ± 36
Spring water
6	Hukuden Spring	154	47	111	142	708 ± 55
7	Kamiko Spring	113	31	85	128	570 ± 44
11	Kamaiso	66	66	123	154	685 ± 53
	Average of spring water	111	48	106	141	654 ± 29

The CO₂ consumption by chemical weathering ($\mathrm{\Phi }{\mathrm{C}\mathrm{O}}_2$) in the river and spring waters at the western foothills of Mt. Chokai was calculated using Equation (7):

  

$$\begin{array}{c}\hfill \mathrm{\Phi }{\mathrm{C}\mathrm{O}}_2=\left({\left[{\mathrm{H}\mathrm{C}\mathrm{O}}_3\right]}_{sil}\right)\times \text{discharge}\end{array}$$(7)

As a result, $\mathrm{\Phi }{\mathrm{C}\mathrm{O}}_2$ at the western foothills of Mt. Chokai was calculated to be 1083 ± 57.0 t-C/yr when considering only river water, with additional 949 ± 42.1 t-C/yr when groundwater flux is included, as estimated from the average chemical composition of the waters from the three springs (Table 3; Table 4). Finally, the CO₂ consumption rate, $\mathrm{Ø}$CO₂, at the western foothills of Mt. Chokai was calculated using Equation (8):

  

$$\begin{array}{c}\mathrm{Ø}{\mathrm{C}\mathrm{O}}_2=\mathrm{\Phi }{\mathrm{C}\mathrm{O}}_2÷\left(\text{area of the catchment}\right)\end{array}$$(8)

Table 4.

Calculated CO₂ consumption amount and rate inside the dotted line area in Fig. 1. The error reflects the measurement error of the ion balance and does not consider the uncertainty of the water discharge

ID	Place	Catchment Area km2	Discharge ×106 m3/yr	$\mathrm{\Phi }{\mathrm{C}\mathrm{O}}_2$ t-C/yr	ØCO2 t-C/km2/yr
River flux
2	Naso River	43	48	248 ± 19.6	5.8 ± 0.46
5	Kawabukuro River	20	26	169 ± 13.3	8.3 ± 0.65
8	Gakko River	167	120	666 ± 51.8	4.0 ± 0.31
	Total	230	194	1083 ± 57.0	4.7 ± 0.25
Underground flux
	Total	ca. 230	120	949 ± 42.1	4.1 ± 0.18

The relatively high CO₂ consumption rate in the Kawabukuro River basin (8.3 ± 0.65 t-C/km²/yr) compared to the other river basins may be attributed to the active weathering of the most recent Holocene fresh andesitic lava distributed throughout the area (Fig. 1). Considering both the river and underground HCO₃^– flux, the mean CO₂ consumption rate at the western foothills of Mt. Chokai is estimated to be 8.8 ± 0.15 t-C/km²/yr (Table 4). Compared to the estimation of Hartmann (2009), which analyzed chemical weathering of silicate in five or six rock types focusing on only river water, our calculated $\mathrm{Ø}$CO₂ at the western foothills of Mt. Chokai is approximately 1.8–3.5 times higher than that of the northern Honshu Island area (2.5–5 t-C/km²/yr) and 1.5 times higher than that of the Japanese Islands (6.05 t-C/km²/yr). The western foothills of Mt. Chokai exhibit highly permeable terrain owing to the extension of Mt. Chokai’s highly porous lava to the Sea of Japan. Additionally, Mt. Chokai is one of the highest precipitation areas in the world. As a result, these factors accelerate chemical weathering, including CO₂ consumption. Although we only focused on a limited area of Mt. Chokai and there may be significant uncertainty in river and groundwater discharge estimations, this study suggests that evaluating carbon fluxes, including submarine groundwater, is essential for achieving an accurate understanding of the carbon cycle. In subsequent research, accurately measuring river water discharge in the area to determine more precise underground HCO₃^– fluxes is essential.

Conclusion

To understand the significance of the underground carbon cycle in coastal areas, we studied the water chemistry of river water and groundwater along with their associated geochemical fluxes at the western foothills of Mt. Chokai, Japan. The HCO₃^– concentration in the spring waters along the mountain-side increased with decreasing altitude, suggesting that underground water-rock interactions influence the water chemistry. The dissolved solids balance suggests that most dissolved HCO₃^– in river water and groundwater was derived from silicate weathering. Considering that the underground HCO₃^– flux through the coastline is estimated to be nearly equivalent to that through the rivers, the CO₂ consumption rate occurring at the western foothills of Mt. Chokai was calculated to be 8.8 ± 0.15 t-C/km²/yr. Chemical weathering is active on both surface and underground in the western foothills of Mt. Chokai, where heavy precipitation and highly porous fresh lava are distributed.

Acknowledgments

We are grateful for the valuable suggestions of Dr. Keishiro Azami from Waseda University who helped improve this manuscript. This research was funded by the Kurita Water and Environment Foundation awarded to H. Kajita, and the JSPS Research Foundation KAKENHI (20H01981) awarded to H. Kawahata. The paper was edited in English by Editage, whose cost was covered by a grant from Hirosaki University.

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