Distribution and source of nitrate in high-chromium groundwater in Jingbian, northern Shaanxi

doi:10.13745/j.esf.sf.2024.1.24

Abstract

Abstract:

Natural high-chromium (Cr) groundwater usually contains high concentration of nitrate, but the source of nitrate and its relationship to Cr enrichment are unclear. In this study, water and sediment samples were collected at different depths from the high-Cr region of southwestern Jingbian, Loess Plateau, northern Shaanxi, and the dissolved Cr, soluble ionic species, δ¹⁸O, δD, δ¹⁸O-NO₃, δ¹⁵N-NO₃, and main and soluble sediment components were analyzed. Unconfined groundwater from Quaternary loess aquifers was mainly of HCO₃-Na and HCO₃-Ca-Mg types. Confined groundwater from sandstone aquifers of the Cretaceous Huanhe-Luohe Formations was of complex hydrochemical types, mainly HCO₃-SO₄-Cl-Na-Mg, HCO₃-SO₄-Na-Mg and SO₄-Cl-Na-Mg types. The groundwater environments were weakly alkaline and oxic, with high concentrations of dissolved salts. The hydrochemical components of unconfined groundwater mainly derived from silicate weathering, while dissolution of evaporites mainly controlled the chemistry of confined groundwater. The average concentration of nitrate in confined groundwater was higher compared to unconfined groundwater and surface water. Spatial distribution of nitrate concentrations in groundwater showed a high-low-high trend from northeast to southwest in the study area. Nitrate contents in sediment and groundwater samples had similar variation trends with depth, indicating that nitrate in groundwater mainly originated from aquifer sediments. δ¹⁸O-NO₃ and δ¹⁵N-NO₃ values indicated that nitrification was the main process of nitrogen cycling and transformation in groundwater systems. Under alkaline, oxic groundwater environments, the presence of dissolved oxygen, nitrate, and nitrification was conducive to chromium oxidation in aquifer solids and release of Cr(VI) into groundwater.

Key words: groundwater, nitrate, hexavalent chromium, hydrogeochemistry, nitrogen and oxygen isotopes

CLC Number:

GUO Huaming, YIN Jiahong, YAN Song, LIU Chao. Distribution and source of nitrate in high-chromium groundwater in Jingbian, northern Shaanxi[J]. Earth Science Frontiers, 2024, 31(1): 384-399.

Figures/Tables 17

Fig.1 Hydrogeological profile of the study area

Fig.2 Piper diagram of studied water samples. Blue dots: unconfined groundwater; red dots: confined ground water; green dots: surface water.

Table 1 Statistical table for main physical and chemical parameters for the studied water systems

样品类型	统计指标	pH	Eh/ mV	DO/ (mg·L^-1)	TDS/ (mg·L^-1)	ρ_A/(mg·L^-1)											ρ_B/(μg·L^-1)
样品类型	统计指标	pH	Eh/ mV	DO/ (mg·L^-1)	TDS/ (mg·L^-1)	$NO 2 -$	$NH 4 +$	Ca²⁺	Mg²⁺	Na⁺	K⁺	Cl^-	$SO 4 2 -$	$CO 3 2 -$	$HCO 3 -$	N $HCO 3 -$ -N	Cr	Mn	Fe
潜水	平均值	7.77	332	3.20	625	0.03	0.10	33.1	56.7	132	4.34	68.8	144	0.55	367	20.0	54.2	8.43	13.0
	中值	7.82	335	3.41	479	0.01	bdl	25.8	44.1	109	2.30	19.9	35.9	bdl	339	13.8	40.1	2.56	6.53
	最小值	7.25	297	1.40	287	bdl	bdl	11.3	7.79	21.0	1.25	5.21	4.66	bdl	219	0.35	6.37	0.21	0.45
	最大值	8.14	361	4.75	2 009	0.29	1.36	145	183	419	23.1	510	1 208	8.28	708	125	180	77.2	72.8
承压水	平均值	7.72	341	4.06	1 136	0.02	0.01	46.0	98.7	281	3.71	243	431	1.62	381	24.3	138	10.4	17.9
	中值	7.69	346	4.29	988	0.01	bdl	36.2	75.9	259	2.08	164	317	bdl	379	21.2	137	4.48	11.5
	最小值	7.03	298	0.32	382	bdl	bdl	4.37	8.85	106	0.77	12.6	58.2	bdl	147	4.90	4.69	0.23	2.19
	最大值	8.54	371	6.03	2 862	0.27	0.20	153	384	621	24.3	1 467	1 308	37.3	627	85.3	355	147	110
地表水	平均值	8.69	320	5.94	1 969	0.04	0.05	39.1	175	527	10.4	654	787	42.1	400	2.25	13.1	5.47	29.5
	中值	8.68	326	6.22	1 420	0.01	0.03	32.2	141	374	8.98	357	554	37.3	330	0.64	8.26	6.05	29.9
	最小值	7.95	262	3.91	440	bdl	bdl	19.9	56.0	205	3.78	192	243	16.6	202	bdl	2.74	0.46	3.09
	最大值	9.24	355	6.91	4 058	0.19	0.11	68.8	384	1 073	25.0	1 764	1 783	70.4	957	9.72	41.6	10.2	57.8
总计	平均值	7.83	336	4.05	1 102	0.02	0.04	42.1	96.8	272	4.58	245	400	6.22	380	20.9	105	9.41	17.8
	中值	7.79	342	4.22	945	0.01	bdl	33.1	67.5	252	2.20	151	268	bdl	375	18.5	102	4.04	11.0
	最小值	7.03	262	0.32	287	bdl	bdl	4.37	7.79	21.0	0.77	5.21	4.66	bdl	147	bdl	2.74	0.21	0.45
	最大值	9.24	371	6.91	4 058	0.29	1.36	153	384	1 073	25.0	1 764	1 783	70.4	957	125	355	147	110

Table 1 Statistical table for main physical and chemical parameters for the studied water systems

样品类型	统计指标	pH	Eh/ mV	DO/ (mg·L^-1)	TDS/ (mg·L^-1)	ρ_A/(mg·L^-1)											ρ_B/(μg·L^-1)
样品类型	统计指标	pH	Eh/ mV	DO/ (mg·L^-1)	TDS/ (mg·L^-1)	$NO 2 -$	$NH 4 +$	Ca²⁺	Mg²⁺	Na⁺	K⁺	Cl^-	$SO 4 2 -$	$CO 3 2 -$	$HCO 3 -$	N $HCO 3 -$ -N	Cr	Mn	Fe
潜水	平均值	7.77	332	3.20	625	0.03	0.10	33.1	56.7	132	4.34	68.8	144	0.55	367	20.0	54.2	8.43	13.0
	中值	7.82	335	3.41	479	0.01	bdl	25.8	44.1	109	2.30	19.9	35.9	bdl	339	13.8	40.1	2.56	6.53
	最小值	7.25	297	1.40	287	bdl	bdl	11.3	7.79	21.0	1.25	5.21	4.66	bdl	219	0.35	6.37	0.21	0.45
	最大值	8.14	361	4.75	2 009	0.29	1.36	145	183	419	23.1	510	1 208	8.28	708	125	180	77.2	72.8
承压水	平均值	7.72	341	4.06	1 136	0.02	0.01	46.0	98.7	281	3.71	243	431	1.62	381	24.3	138	10.4	17.9
	中值	7.69	346	4.29	988	0.01	bdl	36.2	75.9	259	2.08	164	317	bdl	379	21.2	137	4.48	11.5
	最小值	7.03	298	0.32	382	bdl	bdl	4.37	8.85	106	0.77	12.6	58.2	bdl	147	4.90	4.69	0.23	2.19
	最大值	8.54	371	6.03	2 862	0.27	0.20	153	384	621	24.3	1 467	1 308	37.3	627	85.3	355	147	110
地表水	平均值	8.69	320	5.94	1 969	0.04	0.05	39.1	175	527	10.4	654	787	42.1	400	2.25	13.1	5.47	29.5
	中值	8.68	326	6.22	1 420	0.01	0.03	32.2	141	374	8.98	357	554	37.3	330	0.64	8.26	6.05	29.9
	最小值	7.95	262	3.91	440	bdl	bdl	19.9	56.0	205	3.78	192	243	16.6	202	bdl	2.74	0.46	3.09
	最大值	9.24	355	6.91	4 058	0.19	0.11	68.8	384	1 073	25.0	1 764	1 783	70.4	957	9.72	41.6	10.2	57.8
总计	平均值	7.83	336	4.05	1 102	0.02	0.04	42.1	96.8	272	4.58	245	400	6.22	380	20.9	105	9.41	17.8
	中值	7.79	342	4.22	945	0.01	bdl	33.1	67.5	252	2.20	151	268	bdl	375	18.5	102	4.04	11.0
	最小值	7.03	262	0.32	287	bdl	bdl	4.37	7.79	21.0	0.77	5.21	4.66	bdl	147	bdl	2.74	0.21	0.45
	最大值	9.24	371	6.91	4 058	0.29	1.36	153	384	1 073	25.0	1 764	1 783	70.4	957	125	355	147	110

Fig.3 Vertical distribution of Cr (a) and Mn (b) concentrations in groundwater in the study area. Red dashed line represents the total dissolved Cr concentration limit of 50 μg/L by China drinking quality standard.

Fig.4 Water chemistry. (a) Vertical variation of NO 3 --N concentration with depth. (b) Box plot showing the range of NO 3 --N concentrations in the studied water systems. Red dashed line represents the NO 3 --N concentration limit (20 mg/L) by China drinking quality standard.

Fig.5 Identification of groundwater recharge source in the study area by bivariate analysis. Solid line: global atmospheric waterline (GMWL); black dashed line: local atmospheric precipitation line.

Fig.6 Gibbs plot for studied groundwater

Fig.7 Scatter diagram for identification of major factors controlling groundwater chemistry in the study area

Fig.8 Scatter diagram for characterization of clay mineral-water interactions in the study area

Table 2 Major element composition of and Cr, Mn, Fe average contents in sediments of different lithologies

岩性	个数	MgO 含量/%	Al₂O₃ 含量/%	SiO₂ 含量/%	K₂O 含量/%	CaO 含量/%	Cr含量/ (mg·kg^-1)	Mn含量/ (mg·kg^-1)	Fe含量/ (g·kg^-1)
浮土	5	1.5	6.1	45.3	1.9	7.7	84.6	503	24.6
黏土	5	2.1	8.2	42.0	2.6	8.7	178	619	37.8
泥岩	4	2.1	8.8	42.8	3.8	3.6	100	401	36.0
砂岩	12	2.3	7.3	51.1	2.3	5.0	113	460	24.3
粉砂岩	1	2.0	7.9	45.7	2.9	6.2	154	550	30.2
砂岩夹泥岩	6	3.2	9.4	43.7	3.3	2.6	109	459	49.9
泥岩夹砂岩	1	3.6	9.2	43.4	2.7	2.9	92.0	429	31.4
粉砂岩夹泥岩	1	3.6	10.6	47.9	4.1	2.8	127	444	55.9
粉砂质泥岩	1	4.0	11.3	43.1	3.8	1.2	103	401	57.1
泥质砂岩	2	2.9	8.2	43.4	3.0	6.4	43.0	911	26.7
泥质粉砂岩	2	2.2	7.3	42.5	3.1	8.6	117	755	29.1

Table 3 Statistical table for pH and EC values of sediments in unconfined and confined aquifers

样品类型	特征值	pH	EC/(μS·cm^-1)
全部样品	平均值	7.90	1 267
	最小值	7.40	750
	最大值	8.60	1 860
	中值	7.78	1 280
潜水含水层样品	平均值	7.52	1 014
	最小值	7.40	750
	最大值	7.65	1 470
	中值	7.51	1 005
承压含水层样品	平均值	8.04	1 362
	最小值	7.48	870
	最大值	8.60	1 860
	中值	8.07	1 385

Table 4 Statistical table for water-soluble ionic species in sediments in unconfined and confined aquifers

样品类型	统计指标	w_B/(mg·kg^-1)
样品类型	统计指标	K⁺	Mg²⁺	Ca²⁺	Na⁺	F^-	Cl^-	$NO 3 -$	$SO 4 2 -$	$HCO 3 -$	C $HCO 3 2 -$
所有样品	平均值	59.9	13.8	25.6	201	8.3	54.6	4.9	71.9	517	29.4
	最小值	16.1	2.5	10.9	62.4	3.2	14.3	bdl	25.5	305	bdl
	最大值	127	48.0	44.1	330	16.5	160	17.4	155	702	114
	中值	55.4	10.0	23.3	228	7.4	43.0	4.4	54.3	519	bdl
潜水含水层	平均值	70.7	26.5	38.0	94.1	7.4	41.0	4.2	39.8	502	bdl
	最小值	24.5	14.7	31.7	62.4	4.2	14.3	bdl	25.7	397	bdl
	最大值	116	48.0	44.1	125	10.8	111	17.4	56.9	610	bdl
	中值	66.8	26.1	37.2	98.4	7.4	22.0	3.9	35.2	503	bdl
承压含水层	平均值	55.7	8.9	20.8	242	8.6	59.8	5.2	84.1	523	40.6
	最小值	16.1	2.5	10.9	131	3.2	17.8	bdl	25.5	305	bdl
	最大值	127	27.4	34.1	330	16.5	160	12.0	155	702	114
	中值	52.1	6.3	21.9	243	8.1	45.4	4.4	89.8	519	48.0

Table 4 Statistical table for water-soluble ionic species in sediments in unconfined and confined aquifers

样品类型	统计指标	w_B/(mg·kg^-1)
样品类型	统计指标	K⁺	Mg²⁺	Ca²⁺	Na⁺	F^-	Cl^-	$NO 3 -$	$SO 4 2 -$	$HCO 3 -$	C $HCO 3 2 -$
所有样品	平均值	59.9	13.8	25.6	201	8.3	54.6	4.9	71.9	517	29.4
	最小值	16.1	2.5	10.9	62.4	3.2	14.3	bdl	25.5	305	bdl
	最大值	127	48.0	44.1	330	16.5	160	17.4	155	702	114
	中值	55.4	10.0	23.3	228	7.4	43.0	4.4	54.3	519	bdl
潜水含水层	平均值	70.7	26.5	38.0	94.1	7.4	41.0	4.2	39.8	502	bdl
	最小值	24.5	14.7	31.7	62.4	4.2	14.3	bdl	25.7	397	bdl
	最大值	116	48.0	44.1	125	10.8	111	17.4	56.9	610	bdl
	中值	66.8	26.1	37.2	98.4	7.4	22.0	3.9	35.2	503	bdl
承压含水层	平均值	55.7	8.9	20.8	242	8.6	59.8	5.2	84.1	523	40.6
	最小值	16.1	2.5	10.9	131	3.2	17.8	bdl	25.5	305	bdl
	最大值	127	27.4	34.1	330	16.5	160	12.0	155	702	114
	中值	52.1	6.3	21.9	243	8.1	45.4	4.4	89.8	519	48.0

Fig.9 Vertical variations of soluble NO 3 - in sediments (a) and dissolved NO 3 --N in groundwater (b)

Fig.10 Scatter diagrams for source identification of nitrogen in studied water systems

Fig.11 Scatterplot of NO 3 -/Cl- vs. Cl- for studied water samples

Fig.12 δ15N- NO 3 --δ18O- NO 3 - scatter diagrams revealing minimal nitrification in studied water systems

Fig.13 Correlation between Eh value and NO 3 --N concentration in groundwater samples with different Cr concentrations

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