Distribution and formation processes of high fluoride groundwater in different types of aquifers in the Hualong-Xunhua Basin

doi:10.13745/j.esf.sf.2022.1.34

Abstract

Abstract:

High fluoride (F^-) groundwater of natural origin is an environmental and drinking water safety issue, which has attracted worldwide attention. Although many studies have focused on the formation process of high fluoride groundwater, the distribution and formation processes of high fluoride groundwater in different types of aquifers (Quaternary aquifer, bedrock fissure or karst aquifer, clastic aquifer) under complex hydrogeological conditions in the plateau basin are poorly understood. Phreatic groundwater, confined groundwater, and spring water (Quaternary aquifer spring water and bedrock fissure spring water) samples were obtained from the Hualong-Xunhua Basin for chemical and isotopic analyses to reveal the genesis of high fluoride groundwater. The fluoride concentrations in groundwater samples were found to range from 0.25 to 7.73 mg/L. High fluoride groundwater (F^->1.0 mg/L) was mainly distributed in spring water and phreatic water along the Yellow River Valley area and the Bayan low mountaineous and hilly areas, as well as in deep confined groundwater. However, the variation pattern of F^- concentration along the depth is complex. Phreatic water in the Yellow River Valley area that recharged the Yellow River generally has high F^- concentration, while phreatic water in the Yellow River Valley recharged by the Yellow River has low F^- concentration. Ca-poor and Na-rich alkaline soda water is beneficial to F^- enrichment in groundwater. Fluoride in spring water and phreatic water are mainly derived from the dissolution of fluorite. However, in confined groundwater, it is mainly derived from fluorite and other fluoride-containing minerals. For phreatic and Quaternary aquifer spring water, evaporative concentration processes promotes fluoride enrichment. In addition, competitive adsorption of coexisting anions and cation exchange are the causes of high fluoride in spring water (Quaternary aquifer spring water and bedrock fissure spring water) and phreatic water, while cation exchange contributes the most to fluoride enrichment in confined groundwater. This study provides a scientific basis for the exploration and development of safe (low fluoride) groundwater in the Hualong-Xunhua Basin.

Key words: fluoride, groundwater, the plateau basin, hydrochemical characteristics, groundwater-rock interaction

CLC Number:

X523
X131

XING Shiping, GUO Huaming, WU Ping, HU Xueda, ZHAO Zhen, YUAN Youjing. Distribution and formation processes of high fluoride groundwater in different types of aquifers in the Hualong-Xunhua Basin[J]. Earth Science Frontiers, 2022, 29(3): 115-128.

Figures/Tables 14

Fig.1 (a) Locations and distribution of sampling sites in the study area and (b-g) variation patters of groundwater (spring water, phreatic water and confined groundwater) fluoride concentrations in various sections of the study area

Fig.2 Hydrogeological sections of Gandu (I-II) (a) and Jiezi (III-IV) (b)

Table 1 Statistics of various physical and chemical indicators in groundwater samples

指标/样品编号		pH值	ORP/mV	EC/(μS·cm^-1)	ρ/(mg·L^-1)
指标/样品编号		pH值	ORP/mV	EC/(μS·cm^-1)	Na⁺	K⁺	Ca²⁺	Mg²⁺
泉水(n=47)	最小值	6.57	87.2	336	2.81	0.53	30.8	7.26
	中位数	7.45	130	531	14.0	1.86	69.2	20.8
	最大值	8.85	212	3 540	601	9.93	168	79.2
	平均值	7.53	133	738	53.5	2.37	74.9	25.0
潜水(n=22)	最小值	6.75	33.3	439	14.7	1.63	57.0	12.6
	中位数	7.28	114	1 190	93.4	4.16	119	47.2
	最大值	7.88	164	13 040	2 480	15.6	450	286
	平均值	7.34	107	1 950	232	4.49	161	59.2
承压水(n=2)	HL-CG-1	9.91	-217	7 040	1 150	11.9	419	1.33
承压水(n=2)	HL-CG-2	9.11	-288	3 090	576	10.6	82.9	0.12
指标/样品编号		ρ/(mg·L^-1)					δ¹⁸O/‰	δD/‰
指标/样品编号		Cl^-	$NO 3 -$	$SO 4 2 -$	$HCO 3 -$	F^-	δ¹⁸O/‰	δD/‰
泉水(n=47)	最小值	3.85	0.00	15.8	128	0.25	-10.7	-73.4
	中位数	12.8	14.1	50.0	245	0.50	-8.80	-60.8
	最大值	472	102	1 370	388	2.32	-6.46	-46.6
	平均值	44.3	18.9	128	258	0.69	-8.87	-60.5
潜水(n=22)	最小值	14.53	3.31	43.2	220	0.32	-10.9	-80.2
	中位数	73.5	32.0	229	389	1.11	-9.52	-70.4
	最大值	1 580	90.4	5 210	683	3.78	-8.13	-57.2
	平均值	168	33.2	590	403	1.25	-9.56	-68.7
承压水(n=2)	HL-CG-1	2 045	1.54	1 090	17.5	5.06	-9.44	-80.3
承压水(n=2)	HL-CG-2	599	1.51	679	52.5	7.73	-10.0	-87.0

Table 1 Statistics of various physical and chemical indicators in groundwater samples

指标/样品编号		pH值	ORP/mV	EC/(μS·cm^-1)	ρ/(mg·L^-1)
指标/样品编号		pH值	ORP/mV	EC/(μS·cm^-1)	Na⁺	K⁺	Ca²⁺	Mg²⁺
泉水(n=47)	最小值	6.57	87.2	336	2.81	0.53	30.8	7.26
	中位数	7.45	130	531	14.0	1.86	69.2	20.8
	最大值	8.85	212	3 540	601	9.93	168	79.2
	平均值	7.53	133	738	53.5	2.37	74.9	25.0
潜水(n=22)	最小值	6.75	33.3	439	14.7	1.63	57.0	12.6
	中位数	7.28	114	1 190	93.4	4.16	119	47.2
	最大值	7.88	164	13 040	2 480	15.6	450	286
	平均值	7.34	107	1 950	232	4.49	161	59.2
承压水(n=2)	HL-CG-1	9.91	-217	7 040	1 150	11.9	419	1.33
承压水(n=2)	HL-CG-2	9.11	-288	3 090	576	10.6	82.9	0.12
指标/样品编号		ρ/(mg·L^-1)					δ¹⁸O/‰	δD/‰
指标/样品编号		Cl^-	$NO 3 -$	$SO 4 2 -$	$HCO 3 -$	F^-	δ¹⁸O/‰	δD/‰
泉水(n=47)	最小值	3.85	0.00	15.8	128	0.25	-10.7	-73.4
	中位数	12.8	14.1	50.0	245	0.50	-8.80	-60.8
	最大值	472	102	1 370	388	2.32	-6.46	-46.6
	平均值	44.3	18.9	128	258	0.69	-8.87	-60.5
潜水(n=22)	最小值	14.53	3.31	43.2	220	0.32	-10.9	-80.2
	中位数	73.5	32.0	229	389	1.11	-9.52	-70.4
	最大值	1 580	90.4	5 210	683	3.78	-8.13	-57.2
	平均值	168	33.2	590	403	1.25	-9.56	-68.7
承压水(n=2)	HL-CG-1	2 045	1.54	1 090	17.5	5.06	-9.44	-80.3
承压水(n=2)	HL-CG-2	599	1.51	679	52.5	7.73	-10.0	-87.0

Fig.3 Piper diagram of spring water, phreatic water and confined groundwater samples with varying fluoride concentrations

Fig.4 Bivariate plots of δD and δ18O values in groundwater samples with varying floride concentrations for three water types in the Hualong-Xunhua Basin

Fig.5 Variations of F- concentration along the sampling depth in phreatic water and confined groundwater samples

Fig.6 Bivariate plots of fluoride concentration and mass percent of Ca2+ (a) or Na+ (b) in spring water and phreatic water samples

Fig.7 Molar ratio bivariate plots of (a) Na-normalized Ca+ and Mg+ and (b) Na-normalized Ca+ and HCO 3 - in spring water, phreatic water and confined groundwater samples, revealing the dissolution of silicate and carbonate as the main source of ions in groundwater

Fig.8 Bivariate plots of various hydogrochemical indicators in spring water, phreatic water and confined groundwater samples with varying fluoride concentrations

Fig.9 Bivariate plots of molar concentrations of the major ions in spring water, phreatic water and confined groundwater samples

Fig.10 Scatter plots showing the relationships between F- concentration and Na+-Cl- (a) or Na/Ca (mass percent ratio) (b) in spring water and phreatic water samples

Fig.11 Scatter plots showing the relationships between F- concentration and pH (a) or HCO 3 - (b) in spring water, phreatic water and confined groundwater samples

Fig.12 Gibbs diagram of groundwater in the Hualong-Xunhua Basin (Na+/(Na++Ca2+) is the mass percent ratio)

Fig.13 Bivariate plots of (a) F- concentration and F-/Cl- (mass percent ratio) in spring water and phreatic water samples showing the effects of evaporative concentration on F enrichment in groundwater, and (b) scatter plot showing the relationship between F- concentration and δ18O in phreatic water samples (same samples as in the green dashed circle in Fig.4)

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