Weathering process and carbon sink effect of carbonates in typical karst small basin

doi:10.13745/j.esf.sf.2024.2.13

Abstract

Abstract:

Carbonate weathering is a process of absorbing CO₂ from the atmosphere, exhibiting a carbon sink effect. Utilizing the hydrogeochemical characteristics of water in karst areas to reveal the weathering process of carbonate rocks and calculate karst carbon sink fluxes in basins is an important aspect of global change research. A case study of the Guancun Groundwater (GGW) in Liuzhou, Guangxi, with monitoring results from four seasons in 2021, showed that: (1) The main anion in the basin is $\mathrm{HCO}_{3}^{-}$, and the main cation is Ca²⁺. The equivalent concentrations and TDS of anions and cations reflect the intense weathering of carbonate rocks. (2) Based on the spatiotemporal distribution characteristics of its hydrochemistry, the GGW belongs to the HCO₃-Ca type. Besides the weathering of carbonate rocks by H₂CO₃, the weathering by H₂SO₄ and HNO₃ also occurs in water-rock interactions. (3) Considering the combined water-rock interaction process involving H₂CO₃, H₂SO₄ and HNO₃, the average CO₂ consumption by H₂CO₃ weathering of carbonate rocks is calculated using δ¹³C_DIC as 2.38 mmol/L. The net CO₂ consumption is expressed as [$\mathrm{HCO}_{3}^{-}$]_mol-[Ca²⁺+Mg²⁺]_mol, with an average net CO₂ consumption of 1.98 mmol/L. The inorganic carbon sink intensity (C_m) of the GGW is 86.37 tCO₂·km^-2·a^-1. (4) However, the inorganic carbon sink intensity (C_m) of this groundwater, calculated using the hydrochemical-runoff method, is 94.49 tCO₂·km^-2·a^-1. The net CO₂ consumption is 91.41% of the CO₂ sink calculated by the hydrochemical-runoff method. (5) The carbon sink intensity in 2021 was 1.91 times that of 2008, providing a reference for assessing the potential to increase the karst carbon sink through artificial intervention.

Key words: carbonate rocks weathering, Guancun Groundwater (GGW), exogenous acid, net CO₂ consumption, inorganic carbon sink intensity

CLC Number:

CHEN Fajia, XIAO Qiong, HU Xiangyun, GUO Yongli, SUN Ping’an, ZHANG Ning. Weathering process and carbon sink effect of carbonates in typical karst small basin[J]. Earth Science Frontiers, 2024, 31(5): 449-459.

Figures/Tables 10

Fig.1 Geological map of study area and locations of the sampling sites. Modified after [41].

Table 1 Hydrochemical characteristics of the GGW

采样时间	样点名称	T/℃	pH	Ec/ (μS·cm^-1)	离子浓度/(mg·L^-1)								δ¹³C_DIC/ ‰
采样时间	样点名称	T/℃	pH	Ec/ (μS·cm^-1)	Ca²⁺	Mg²⁺	K⁺	Na⁺	$HCO 3 -$	Cl^-	$NO 3 -$	$SO 4 2 -$	δ¹³C_DIC/ ‰
2021-04	GC1	18.69	7.01	460.9	88.53	14.17	0.17	0.59	319.50	0.83	2.23	8.26	-16.13
	GC2	16.95	6.88	534.4	89.13	23.70	0.05	0.57	373.50	0.96	0.12	10.70	-16.91
	GC3	20.03	6.89	494.9	98.06	8.95	0.88	2.09	306.00	3.96	19.32	13.59	-13.15
	GC4	19.83	7.06	429.2	83.06	10.95	0.88	1.53	283.50	2.06	9.90	9.96	-13.74
	GC5	20.03	7.07	431.4	83.20	10.96	0.86	1.44	281.25	2.04	9.88	9.78	-13.81
	GC6	19.92	6.93	531.8	84.19	27.14	0.08	0.77	363.38	1.86	12.01	10.52	-16.33
	GC7	—	—	—	—	—	—	—	—	—	—	—	—
	GC8	20.36	7.22	434.1	83.24	11.90	0.83	1.26	288.00	2.29	9.61	10.00	-13.26
2021-06	GC1	19.23	7.16	415.8	74.82	11.07	0.48	0.51	270.58	1.01	<0.05	13.43	-15.90
	GC2	22.63	7.03	546.7	86.05	21.6	0.21	0.70	353.84	1.58	7.66	13.86	-16.67
	GC3	20.13	6.79	519.6	92.48	11.01	0.55	1.71	295.56	6.34	21.91	15.60	-13.28
	GC4	19.96	6.86	439.6	78.36	10.32	0.56	1.09	266.42	5.04	12.4	14.68	-14.15
	GC5	20.08	7.09	438.4	78.68	10.10	0.52	1.05	278.91	5.00	<0.05	14.57	-14.27
	GC6	21.38	6.80	576.2	82.29	27.33	0.07	0.82	362.16	1.60	12.13	13.63	-16.57
	GC7	20.84	6.82	488.7	85.26	12.48	0.27	0.85	289.32	5.63	17.77	14.08	-14.90
	GC8	21.01	7.00	437.0	76.62	9.42	0.44	0.94	253.93	5.20	11.67	14.53	-13.75
2021-09	GC1	19.41	7.00	408.2	69.32	13.02	1.10	1.13	255.38	1.21	<0.05	14.32	-14.48
	GC2	23.31	6.47	605.1	98.09	23.51	0.04	0.82	400.02	0.99	6.30	13.41	-17.05
	GC3	20.38	6.59	531.1	95.85	11.22	0.68	2.20	306.23	6.43	20.94	17.44	-13.01
	GC4	20.20	6.68	456.2	82.13	10.28	0.67	1.48	271.20	5.08	9.28	16.16	-14.01
	GC5	20.05	6.80	456.2	77.90	8.82	0.93	1.24	248.60	5.16	12.22	16.38	-14.20
	GC6	—	—	—	—	—	—	—	—	—	—	—	—
	GC7	—	—	—	—	—	—	—	—	—	—	—	—
	GC8	21.82	6.89	463.1	83.42	10.05	0.55	1.07	271.20	5.36	13.82	15.14	-13.50
2021-12	GC1	18.98	7.05	376.0	73.24	8.82	0.14	0.60	258.69	2.62	5.08	8.74	-15.92
	GC2	18.15	6.92	511.1	88.89	20.36	0.09	0.21	367.82	1.16	1.06	8.23	-16.77
	GC3	19.95	6.99	451.8	93.08	10.69	0.46	1.43	307.19	4.47	20.17	14.86	-13.23
	GC4	19.26	7.09	409.7	79.69	8.71	0.65	1.09	262.73	3.39	14.77	13.14	-13.92
	GC5	19.41	7.12	409.6	80.04	8.62	0.64	1.22	260.71	3.46	13.91	12.76	-14.08
	GC6	20.69	6.94	562.8	88.11	28.46	0.12	0.78	383.99	2.85	17.70	9.48	-16.42
	GC7	20.43	7.05	465.9	89.58	10.75	0.21	0.57	297.09	4.23	22.28	9.07	-14.74
	GC8	20.11	7.30	411.7	80.15	8.98	0.52	1.15	265.76	3.29	12.88	12.12	-13.45

Table 1 Hydrochemical characteristics of the GGW

采样时间	样点名称	T/℃	pH	Ec/ (μS·cm^-1)	离子浓度/(mg·L^-1)								δ¹³C_DIC/ ‰
采样时间	样点名称	T/℃	pH	Ec/ (μS·cm^-1)	Ca²⁺	Mg²⁺	K⁺	Na⁺	$HCO 3 -$	Cl^-	$NO 3 -$	$SO 4 2 -$	δ¹³C_DIC/ ‰
2021-04	GC1	18.69	7.01	460.9	88.53	14.17	0.17	0.59	319.50	0.83	2.23	8.26	-16.13
	GC2	16.95	6.88	534.4	89.13	23.70	0.05	0.57	373.50	0.96	0.12	10.70	-16.91
	GC3	20.03	6.89	494.9	98.06	8.95	0.88	2.09	306.00	3.96	19.32	13.59	-13.15
	GC4	19.83	7.06	429.2	83.06	10.95	0.88	1.53	283.50	2.06	9.90	9.96	-13.74
	GC5	20.03	7.07	431.4	83.20	10.96	0.86	1.44	281.25	2.04	9.88	9.78	-13.81
	GC6	19.92	6.93	531.8	84.19	27.14	0.08	0.77	363.38	1.86	12.01	10.52	-16.33
	GC7	—	—	—	—	—	—	—	—	—	—	—	—
	GC8	20.36	7.22	434.1	83.24	11.90	0.83	1.26	288.00	2.29	9.61	10.00	-13.26
2021-06	GC1	19.23	7.16	415.8	74.82	11.07	0.48	0.51	270.58	1.01	<0.05	13.43	-15.90
	GC2	22.63	7.03	546.7	86.05	21.6	0.21	0.70	353.84	1.58	7.66	13.86	-16.67
	GC3	20.13	6.79	519.6	92.48	11.01	0.55	1.71	295.56	6.34	21.91	15.60	-13.28
	GC4	19.96	6.86	439.6	78.36	10.32	0.56	1.09	266.42	5.04	12.4	14.68	-14.15
	GC5	20.08	7.09	438.4	78.68	10.10	0.52	1.05	278.91	5.00	<0.05	14.57	-14.27
	GC6	21.38	6.80	576.2	82.29	27.33	0.07	0.82	362.16	1.60	12.13	13.63	-16.57
	GC7	20.84	6.82	488.7	85.26	12.48	0.27	0.85	289.32	5.63	17.77	14.08	-14.90
	GC8	21.01	7.00	437.0	76.62	9.42	0.44	0.94	253.93	5.20	11.67	14.53	-13.75
2021-09	GC1	19.41	7.00	408.2	69.32	13.02	1.10	1.13	255.38	1.21	<0.05	14.32	-14.48
	GC2	23.31	6.47	605.1	98.09	23.51	0.04	0.82	400.02	0.99	6.30	13.41	-17.05
	GC3	20.38	6.59	531.1	95.85	11.22	0.68	2.20	306.23	6.43	20.94	17.44	-13.01
	GC4	20.20	6.68	456.2	82.13	10.28	0.67	1.48	271.20	5.08	9.28	16.16	-14.01
	GC5	20.05	6.80	456.2	77.90	8.82	0.93	1.24	248.60	5.16	12.22	16.38	-14.20
	GC6	—	—	—	—	—	—	—	—	—	—	—	—
	GC7	—	—	—	—	—	—	—	—	—	—	—	—
	GC8	21.82	6.89	463.1	83.42	10.05	0.55	1.07	271.20	5.36	13.82	15.14	-13.50
2021-12	GC1	18.98	7.05	376.0	73.24	8.82	0.14	0.60	258.69	2.62	5.08	8.74	-15.92
	GC2	18.15	6.92	511.1	88.89	20.36	0.09	0.21	367.82	1.16	1.06	8.23	-16.77
	GC3	19.95	6.99	451.8	93.08	10.69	0.46	1.43	307.19	4.47	20.17	14.86	-13.23
	GC4	19.26	7.09	409.7	79.69	8.71	0.65	1.09	262.73	3.39	14.77	13.14	-13.92
	GC5	19.41	7.12	409.6	80.04	8.62	0.64	1.22	260.71	3.46	13.91	12.76	-14.08
	GC6	20.69	6.94	562.8	88.11	28.46	0.12	0.78	383.99	2.85	17.70	9.48	-16.42
	GC7	20.43	7.05	465.9	89.58	10.75	0.21	0.57	297.09	4.23	22.28	9.07	-14.74
	GC8	20.11	7.30	411.7	80.15	8.98	0.52	1.15	265.76	3.29	12.88	12.12	-13.45

Fig.2 Piper diagram of the GGW

Fig.3 Gibbs diagram of the GGW

Fig.4 Spatiotemporal variation hydrochemical characteristics of the GGW

Fig.5 Scatter diagrams showing the ionic equivalent relationship. (a) Equivalent rations of [Ca2++Mg2+] vs. [ HCO 3 -]. (b) Equivalent rations of [Ca2++Mg2+] vs. [ HCO 3 -+ NO 3 -+ SO 4 2 -].

Fig.6 Equivalent ratios of [Ca2++Mg2+]/[ HCO 3 -] vs. [ SO 4 2 -]/[ HCO 3 -] in the GGW

Table 2 Estimation of net CO2 consumption and inorganic carbon sinks of the GGW based on ion concentration

时间	$HCO 3 -$ 浓度/ (mg·L^-1)	Ca²⁺浓度/ (mg·L^-1)	Mg²⁺浓度/ (mg·L^-1)	$Q C O 2 - n e t$ / (mmol·L^-1)	Q/ (m³·s^-1)	F/ (g·s^-1)	CSF(以CO₂计)/ (t·a^-1)	C_m(以CO₂计)/ (t ·km^-2·a^-1)
春季	316.45	87.06	15.40	2.37	1.10	114.66	911.42	29.89
夏季	296.34	81.82	14.17	2.22	1.13	110.48	878.21	28.80
秋季	292.11	84.45	12.82	2.14	0.32	30.18	237.27	7.78
冬季	300.50	84.10	13.17	2.27	0.78	78.07	607.13	19.90
平均值						83.35	658.51	21.59
总量						333.40	2 634.03	86.37

Table 2 Estimation of net CO2 consumption and inorganic carbon sinks of the GGW based on ion concentration

时间	$HCO 3 -$ 浓度/ (mg·L^-1)	Ca²⁺浓度/ (mg·L^-1)	Mg²⁺浓度/ (mg·L^-1)	$Q C O 2 - n e t$ / (mmol·L^-1)	Q/ (m³·s^-1)	F/ (g·s^-1)	CSF(以CO₂计)/ (t·a^-1)	C_m(以CO₂计)/ (t ·km^-2·a^-1)
春季	316.45	87.06	15.40	2.37	1.10	114.66	911.42	29.89
夏季	296.34	81.82	14.17	2.22	1.13	110.48	878.21	28.80
秋季	292.11	84.45	12.82	2.14	0.32	30.18	237.27	7.78
冬季	300.50	84.10	13.17	2.27	0.78	78.07	607.13	19.90
平均值						83.35	658.51	21.59
总量						333.40	2 634.03	86.37

Table 3 Estimation of inorganic carbon sink in the GGW based on hydrochemical-runoff method

时间	[ $HCO 3 -$ ]/ (mg·L^-1)	[CO₂]/ (mg·L^-1)	Q/ (m³·s^-1)	CSF(以CO₂计)/ (t·a^-1)	C_m(以CO₂计)/ (t·km^-2·a^-1)
春季	316.45	114.13	1.10	998.71	32.74
夏季	296.34	106.88	1.13	957.62	31.40
秋季	292.11	105.35	0.32	265.93	8.72
冬季	300.50	108.38	0.78	659.68	21.63
平均值				720.49	23.62
总量				2 881.94	94.49

Table 3 Estimation of inorganic carbon sink in the GGW based on hydrochemical-runoff method

时间	[ $HCO 3 -$ ]/ (mg·L^-1)	[CO₂]/ (mg·L^-1)	Q/ (m³·s^-1)	CSF(以CO₂计)/ (t·a^-1)	C_m(以CO₂计)/ (t·km^-2·a^-1)
春季	316.45	114.13	1.10	998.71	32.74
夏季	296.34	106.88	1.13	957.62	31.40
秋季	292.11	105.35	0.32	265.93	8.72
冬季	300.50	108.38	0.78	659.68	21.63
平均值				720.49	23.62
总量				2 881.94	94.49

Table 4 Data comparison between 2008 and 2021

数据时间	$HCO 3 -$ 浓度/ (mg·L^-1)	年降水量/ mm	Q/ (m³·s^-1)	C_m(以CO₂计)/ (t·km^-2·a^-1)
2008年	256.37	1 913	0.45	45.25
2021年	301.35	2 222	0.83	86.37

Table 4 Data comparison between 2008 and 2021

数据时间	$HCO 3 -$ 浓度/ (mg·L^-1)	年降水量/ mm	Q/ (m³·s^-1)	C_m(以CO₂计)/ (t·km^-2·a^-1)
2008年	256.37	1 913	0.45	45.25
2021年	301.35	2 222	0.83	86.37

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