陕北靖边高铬地下水中硝酸根分布及来源

doi:10.13745/j.esf.sf.2024.1.24

摘要/Abstract

摘要：

天然高铬地下水通常含有较高浓度的硝酸根,然而高铬地下水中硝酸根来源及其联系却并不清楚。本文以陕北黄土高原靖边西南地区的高铬地下水为研究对象,采集了不同深度的地下水和沉积物样品,并测试了地下水样品中的溶解Cr、主要阴阳离子、δ¹⁸O、δD、δ¹⁸O-NO₃、δ¹⁵N-NO₃等以及沉积物的主要组分和可溶性组分。研究结果表明,研究区地下水的水化学组分主要受水文地质条件的影响。第四系黄土潜水水化学类型主要为HCO₃-Na型和HCO₃-Ca-Mg型;白垩系环河组、洛河组砂岩承压水水化学类型复杂,主要为HCO₃-SO₄-Cl-Na-Mg型、HCO₃-SO₄-Na-Mg型、SO₄-Cl-Na-Mg型,地下水处于偏碱性、氧化的环境,具有较高的可溶盐含量。潜水的水化学组分主要来自含水层中硅酸盐风化;承压水水化学组分主要来源于蒸发盐的溶解。垂向上,承压水中硝酸根的平均浓度高于潜水和地表水;地下水硝酸根浓度超标率在研究区从东北到西南呈现高-低-高的趋势;沉积物中可溶性硝酸根与地下水样品在深度上具有相似的变化规律,表明地下水硝酸根主要来源于沉积物。δ¹⁸O-NO₃和δ¹⁵N-NO₃结果表明,硝化反应是研究区氮素循环转化的主要过程。在偏碱性氧化性地下水环境中,受溶解氧、硝酸根和硝化反应等多种因素的共同作用,铬趋于从岩石中氧化溶解,迁移进入地下水中。

关键词: 地下水, 硝酸根, 六价铬, 水文地球化学, 氮氧同位素

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

中图分类号:

郭华明, 尹嘉鸿, 严松, 刘超. 陕北靖边高铬地下水中硝酸根分布及来源[J]. 地学前缘, 2024, 31(1): 384-399.

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.

图/表 17

图1 研究区水文地质剖面图

Fig.1 Hydrogeological profile of the study area

图2 研究区Piper三线图

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

表1 研究区主要理化参数统计指数汇总表

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

表1 研究区主要理化参数统计指数汇总表

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

图3 研究区地下水中Cr (a)和Mn (b)沿垂向分布图垂向虚线为我国地下水饮用质量标准总溶解Cr浓度限值50 μg/L。

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.

图4 NO 3 --N随深度垂向变化图(a)及 NO 3 --N浓度箱型图(b) 红色虚线为我国地下水饮用质量标准NO3-N浓度限值20 mg/L。

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.

图5 不同水体中δD和δ18O散点图(全球大气降水线(GMWL)方程为δD=8×δ18O+10[24],研究区内当地大气降水线(LMWL)方程(西安)为δD=7.49×δ18O+6.13[23])

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.

图6 研究区地下水TDS与Cl/(Cl+HCO3)散点图

Fig.6 Gibbs plot for studied groundwater

图7 研究区地下水Na++K+-Cl-与Ca2++Mg2+-S HCO 4 2 --C HO 3 -的散点图

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

图8 研究区地下水n( HCO 3 -)/n(Na+)与n(Ca2+)/n(Na+)(a)和n(Mg2+)/n(Na+)与n(Ca2+)/n(Na+) (b)散点图

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

表2 不同岩性沉积物主要组分及Cr、Mn、Fe平均含量

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

表3 潜水含水层、承压含水层沉积物pH、EC值统计表

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

表4 潜水和承压含水层沉积物可溶性组分含量统计表

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

表4 潜水和承压含水层沉积物可溶性组分含量统计表

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

图9 沉积物可溶性 NO 3 -(a)与地下水 NO 3 --N浓度(b)沿深度变化图

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

图10 δ15N- NO 3 -、δ18O- NO 3 -值对氮源识别图(a)及δ15N- NO 3 -、δ18O- NO 3 -值特征位点对应 NO 3 --N浓度图(b)

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

图11 不同水体 NO 3 -/Cl-摩尔比与Cl-摩尔浓度散点图

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

图12 研究区硝化反应识别图 (a)及研究区δ15N- NO 3 -与δ18O- NO 3 -值相关性分析 (b)

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

图13 不同溶解Cr浓度地下水Eh值与 NO 3 --N浓度散点图

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

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