Variability in spatiotemporal groundwater nitrate concentrations in the northeast Ganfu Plain

doi:10.13745/j.esf.sf.2023.2.84

Abstract

Abstract:

Groundwater in the Ganfu Plain exhibits high NO₃^- concentrations, yet few studies have investigated the seasonal variations and influencing factors of groundwater chemistry, particularly NO₃^- concentrations. In this study, groundwater samples were collected in the northeast region of the Ganfu Plain during both dry and wet seasons. The study aimed to explore the spatiotemporal variations in groundwater chemistry, focusing on NO₃^- concentrations, and the sources of NO₃^- in groundwater using hydrochemical diagrams, Self-Organizing Map (SOM), spatial autocorrelation analysis, and an inverse geochemical model. The results indicate that the primary groundwater types in the study area are Cl·NO₃-Ca and HCO₃-Ca. Human activities emerge as the key factor driving spatial variations in groundwater chemistry. Regions with elevated NO₃^- concentrations and significant seasonal variations are predominantly located in the lower reaches of Nanchang. Conversely, areas with lower NO₃^- concentrations and seasonal variations are primarily situated in the western and southeastern mountainous regions and the lower reaches of the Ganjiang River Delta. The spatial distribution and seasonal variability of groundwater NO₃^- concentrations in the study area are influenced by groundwater runoff conditions, redox environments, and land use patterns. Industrial and domestic sewage are identified as the main sources of NO₃^- in groundwater, with the impact of fertilizers on NO₃^- concentrations also warranting consideration. The results from the inverse geochemical model provide quantitative insights into the effects of water-rock interactions and human activities on groundwater quality during groundwater movement processes.

Key words: groundwater, nitrate pollution, spatiotemporal variation, hydrochemical evolution

CLC Number:

P641.12
X523

HE Jiahui, MAO Hairu, XUE Yang, LIAO Fu, GAO Bai, RAO Zhi, YANG Yang, LIU Yuanyuan, WANG Guangcai. Variability in spatiotemporal groundwater nitrate concentrations in the northeast Ganfu Plain[J]. Earth Science Frontiers, 2024, 31(3): 360-370.

Figures/Tables 15

Fig.1 Geographic location and hydrogeological map of the study area

Fig.2 Distribution of the land use types in the study area

Table 1 The descriptive statistics of analytical data for groundwater

分类	数值类型	ρ_B/(mg·L^-1)										Eh/mV	pH
分类	数值类型	DO	TDS	Ca²⁺	Na⁺	K⁺	Mg²⁺	Cl^-	$S O 4 2 -$	$H C O 3 -$	$N O 3 -$	Eh/mV	pH
枯水期地下水	最小值	2.38	40	2.93	3.7	0.97	0.85	1.45	1.36	5.41	0.1	107	4.75
	最大值	12.5	1 520	98.21	52.72	30.54	57.38	70.88	147.89	333.49	206.31	534	6.89
	平均值	9.9	396.35	29.72	20.11	10.77	10.85	24.46	27.27	93.89	41.78	239.82	6.17
丰水期地下水	最小值	0.12	15.3	1.66	0.97	0.46	0.54	2.98	0.66	5.79	1.4	-6	5.06
	最大值	11.68	441.5	90.41	56.63	70.36	44.02	85	365.85	353.41	224.88	575	7.34
	平均值	4.75	270	38.82	18.88	10.29	11.54	21.18	45.62	111.51	46.72	347.84	6.05

Table 1 The descriptive statistics of analytical data for groundwater

分类	数值类型	ρ_B/(mg·L^-1)										Eh/mV	pH
分类	数值类型	DO	TDS	Ca²⁺	Na⁺	K⁺	Mg²⁺	Cl^-	$S O 4 2 -$	$H C O 3 -$	$N O 3 -$	Eh/mV	pH
枯水期地下水	最小值	2.38	40	2.93	3.7	0.97	0.85	1.45	1.36	5.41	0.1	107	4.75
	最大值	12.5	1 520	98.21	52.72	30.54	57.38	70.88	147.89	333.49	206.31	534	6.89
	平均值	9.9	396.35	29.72	20.11	10.77	10.85	24.46	27.27	93.89	41.78	239.82	6.17
丰水期地下水	最小值	0.12	15.3	1.66	0.97	0.46	0.54	2.98	0.66	5.79	1.4	-6	5.06
	最大值	11.68	441.5	90.41	56.63	70.36	44.02	85	365.85	353.41	224.88	575	7.34
	平均值	4.75	270	38.82	18.88	10.29	11.54	21.18	45.62	111.51	46.72	347.84	6.05

Table 2 Seasonal variation of main ions in groundwater

水量变化分类	平均离子浓度ρ_B/(mg·L^-1)
水量变化分类	Na⁺	Ca²⁺	Mg²⁺	K⁺	Cl^-	$SO 4 2 -$	$NO 3 -$	$HCO 3 -$
枯水期	20.11	29.72	10.85	10.77	24.46	36.49	41.78	93.89
丰水期	18.88	38.82	11.54	10.29	21.18	45.62	46.72	111.51
变化率/%	-6	31	6	-4	-13	25	12	19

Table 2 Seasonal variation of main ions in groundwater

水量变化分类	平均离子浓度ρ_B/(mg·L^-1)
水量变化分类	Na⁺	Ca²⁺	Mg²⁺	K⁺	Cl^-	$SO 4 2 -$	$NO 3 -$	$HCO 3 -$
枯水期	20.11	29.72	10.85	10.77	24.46	36.49	41.78	93.89
丰水期	18.88	38.82	11.54	10.29	21.18	45.62	46.72	111.51
变化率/%	-6	31	6	-4	-13	25	12	19

Table 3 Correlation analysis of groundwater chemical parameters in study area

离子	地下水各化学参数间的相关系数
离子	Cl^-	$N O 3 -$	$S O 4 2 -$	$H C O 3 -$	Na⁺	K⁺	Mg²⁺	Ca²⁺
Cl^-	1
$N O 3 -$	0.691^**	1
$S O 4 2 -$	0.491^**	0.414^**	1
$H C O 3 -$	0.334^*	0.038	0.366^**	1
Na⁺	0.863^**	0.692^**	0.691^**	0.422^**	1
K⁺	0.660^**	0.575^**	0.339^*	0.274^*	0.712^**	1
Mg²⁺	0.638^**	0.658^**	0.658^**	0.756^**	0.714^**	0.378^**	1
Ca²⁺	0.546^**	0.467^**	0.763^**	0.799^**	0.678^**	0.379^**	0.868^**	1

Table 3 Correlation analysis of groundwater chemical parameters in study area

离子	地下水各化学参数间的相关系数
离子	Cl^-	$N O 3 -$	$S O 4 2 -$	$H C O 3 -$	Na⁺	K⁺	Mg²⁺	Ca²⁺
Cl^-	1
$N O 3 -$	0.691^**	1
$S O 4 2 -$	0.491^**	0.414^**	1
$H C O 3 -$	0.334^*	0.038	0.366^**	1
Na⁺	0.863^**	0.692^**	0.691^**	0.422^**	1
K⁺	0.660^**	0.575^**	0.339^*	0.274^*	0.712^**	1
Mg²⁺	0.638^**	0.658^**	0.658^**	0.756^**	0.714^**	0.378^**	1
Ca²⁺	0.546^**	0.467^**	0.763^**	0.799^**	0.678^**	0.379^**	0.868^**	1

Fig.3 The component planes showing weight vector values of variables in wet and dry season. Yellow and blue represent the high and low values of variables, respectively.

Fig.4 The pattern of cluster formation of groundwater in wet and dry season. Arrow represents the change in cluster from dry to wet season.

Fig.5 The spatial distribution of hydrogeochemical clusters of groundwater in wet and dry season

Table 1 Global Moran’s I value of groundwater NO 3 - in wet and dry season

取样时间	Moran’s I	P-Value	Z-Value
枯水期	0.418	0.001	6.45
丰水期	0.561	0.001	7.18
变化量	0.361	0.001	2.34

Fig.6 Indicator of spatial association (a: N O 3 - in dry season; b: N O 3 - in wet season; c: the difference of N O 3 - between dry season and wet season)

Fig.7 The indicator of spatial autocorrelation maps of (a) N O 3 - in dry season, (b) N O 3 - in the wet season, (c) the difference of N O 3 - between dry season and wet seasonThe indicator of spatial autocorrelation maps of (a) N O 3 - in dry season, (b) N O 3 - in the wet season, (c) the difference of N O 3 - between dry season and wet season

Fig.8 Relationship between ions (a: NO 3 - vs. Cl-; b: NO 3 - vs. SO 4 2 -)

Fig.9 Diagram of the NO 3 -/Cl- molar fraction ratio vs. Cl-

Fig.10 The path of inverse geochemical model in the study area

Table 5 The results of mass transfers for selected inverse geochemical models

化学成分	模拟出的浓度变化值/(mmol·L^-1)
化学成分	枯水期	丰水期
白云石	1.366	1.286
方解石	—	—
石膏	0.685	0.639
石盐	—	—
钾盐	0.81	0.699
CaX₂	-0.191	-0.505
NaX	0.381	1.01
钠长石	—	—
钾长石	0.661	0.825
伊利石	-1.102	-1.375
高岭石	0.937	0.568
CO₂(g)	-6.44	—
O₂(g)	5.594	5.801
硝化作用	2.8	2.9

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