Submarine groundwater discharge in Longkou coastal zones under the influence of human activities

doi:10.13745/j.esf.sf.2022.2.71

Abstract

Abstract:

The radium mass balance model based on ²²³Ra and ²²⁴Ra tracers has been widely used in the study of submarine groundwater discharge (SGD) in coastal zones. However, few researches are currently done to study the influence of human activities on SGD. In this paper, the water residence time, SGD flux and nutrient flux carried by SGD were compared between the Longkou coastal zones A and B, under the influence of two different human activities, namely an impervious dam in Zone A and land reclamation in Zone B. Results showed that the average water residence time in Zone A was 14.26 days and in Zone B 10.64 days. In addition, radium activities in groundwater and seawater in Zone B were generally higher than that in Zone A; whereas salinities were lower in Zone B than in Zone A. The discharge rates for SGD in Zone A ranged from 1.26 to 1.60 cm/d and in Zone B from 1.43 to 1.82 cm/d. Therefore the discharge rates for SGD in these two zones were comparable within the margin of error. However, the discharge rates for SGD were relatively low in these two zones compared to other natural coastal areas in the country. Moreover, the nitrogen and phosphorus nutrient concentrations in Zone B were generally higher than that in Zone A. The nitrogen and phosphorus nutrient fluxes carried by SGD were different in the two zones. Such unbalanced nutrient inputs from SGD could easily change the nutrient structure of the Longkou coastal zones, which has an adverse impact on the marine ecological environment, further confirming that SGD is important for coastal ecological environment and water pollution control.

Key words: submarine groundwater discharge, residence time, radium isotopes, nitrogen and phosphorus nutrients, human activities

CLC Number:

GUO Qiaona, ZHAO Yue, ZHOU Zhifang, LIN Jin, DAI Yunfeng, LI Mengjun. Submarine groundwater discharge in Longkou coastal zones under the influence of human activities[J]. Earth Science Frontiers, 2022, 29(4): 468-479.

Figures/Tables 8

Fig.1 Geographical location of Longkou coastal area and distribution of sampling sites in the study area

Table 1 Radium activity and salinity data for seawater and groundwater samples

取样	经度/(°)	纬度/(°)	盐度w(NaCl)/‰	²²³Ra活度/(dpm·hL^-1)	²²⁴Ra活度/(dpm·hL^-1)	²²⁴Ra/²²³Ra AR
S1	120.47	37.83	31.30	1.39±0.17	18.11±1.27	13.03±1.56
S2	120.35	37.78	34.40	2.06±0.25	40.16±2.81	19.50±2.06
S3	120.27	37.74	33.70	2.2±0.26	29.74±2.08	13.52±1.58
S4	120.20	37.70	33.30	1.22±0.15	25.9±1.81	21.23±2.19
S5	120.12	37.66	32.10	3.22±0.40	-	-
S6	120.04	37.72	33.50	1.27±0.15	21.27±1.49	17.32±1.85
S7	120.14	37.77	33.40	-	-	-
S8	120.22	37.82	33.30	2.7±0.32	33.15±2.32	12.28±1.48
S9	120.32	37.86	34.90	1.4±0.17	18.44±1.29	13.17±1.56
S10	120.44	37.89	34.60	1.27±0.07	15.32±1.07	12.06±1.17
S11	120.42	37.96	36.50	1.14±0.14	16.9±1.18	14.82±1.7
S12	120.28	37.92	33.70	1.06±0.13	14.98±1.05	14.13±1.65
S13	120.17	37.88	33.60	1.96±0.23	31.89±2.23	16.27±1.79
S14	120.08	37.83	33.40	1.62±0.19	25.94±1.82	16.01±1.77
S15	119.99	37.77	32.50	1.48±0.18	20.66±1.45	13.96±1.63
S16	119.92	37.83	32.90	1.65±0.2	17.52±1.23	10.62±1.36
S17	120.03	37.89	32.10	0.78±0.09	22.64±1.59	29.03±2.74
S18	120.14	37.94	35.20	2.08±0.25	27.51±1.93	13.23±1.57
S19	120.25	37.99	34.30	0.83±0.1	16.06±1.12	19.35±2.04
S20	120.40	38.03	34.50	1.15±0.14	5.62±0.39	4.89±0.84
GW1	120.33	37.65	30.20	-	-
GW2	120.25	37.55	33.10	32.98±3.96	1 528.41±106.99
GW3	120.39	37.69	29.80	10.19±1.22	1 011.75±70.82
GW4	120.32	37.67	34.50	40.23±4.83	2 028.05±141.96
GW5	120.34	37.69	33.10	38.47±4.61	1 975.74±138.30
GW6	120.43	37.73	31.20	8.77±1.05	544.23±38.096

Fig.2 Contour maps of salinity (a) and 223Ra (b) and 224Ra (c) activities in the sea area of Longkou City

Fig.3 Relationships between radium activity and salinity in groundwater (a) and seawater in Zone A (b) and Zone B (c)

Table 2 Nitrogen and phosphorus nutrient contents in seawater and groundwater of the study area

营养盐	分区	地下水营养盐浓度/ (mg·L^-1)		海水营养盐浓度/ (mg·L^-1)
营养盐	分区	范围	均值	范围	均值
DIP	A区	0.10~0.12	0.11	0.04~0.09	0.06
DIP	B区	0.05~0.23	0.11	0.05~0.10	0.08
$NO 3 -$	A区	0.58~20.00	10.29	0.09~0.15	0.11
$NO 3 -$	B区	0.01~2.66	1.09	0.10~0.74	0.40
$NO 2 -$	A区	0.05~0.34	0.20	0.02~0.05	0.03
$NO 2 -$	B区	0.004~0.12	0.10	0.03~0.07	0.04

Table 2 Nitrogen and phosphorus nutrient contents in seawater and groundwater of the study area

营养盐	分区	地下水营养盐浓度/ (mg·L^-1)		海水营养盐浓度/ (mg·L^-1)
营养盐	分区	范围	均值	范围	均值
DIP	A区	0.10~0.12	0.11	0.04~0.09	0.06
DIP	B区	0.05~0.23	0.11	0.05~0.10	0.08
$NO 3 -$	A区	0.58~20.00	10.29	0.09~0.15	0.11
$NO 3 -$	B区	0.01~2.66	1.09	0.10~0.74	0.40
$NO 2 -$	A区	0.05~0.34	0.20	0.02~0.05	0.03
$NO 2 -$	B区	0.004~0.12	0.10	0.03~0.07	0.04

Fig.4 Nutrient concentrations in groundwater (a) and seawater (b) samples

Table 3 Parameters for SFGD and SGD calculations

参数	符号	A区取值	B区取值	单位
面积	A_bay	3.92×10⁸	1.05×10⁹	m²
体积	V_bay	3.22×10⁹	8.87×10⁹	m³
水体表现年龄	t	11.14~21.87	5.90~13.75	d
盐度总量	M_s	1.10×10¹¹	2.97×10¹¹
²²⁴Ra地下水端员值	²²⁴Ra_gw	544.23	1 635.99	dpm/ (100 L)
²²⁴Ra外海水背景值	²²⁴Ra_sea	10.84	16.79	dpm/ (100 L)
外海盐度值	S_s	34.40	33.60	‰
蒸发量	E_T	3.39	3.39	mm·d^-1
降雨量	P_T	1.75	1.75	mm·d^-1

Table 4 SGD rates in different study areas in China

研究区域	方法	SGD排泄速度/ (cm·d^-1)	文献
渤海湾	²²³Ra,²²⁸Ra	1.98~4.78	[50]
黄河	Ra,²²²Rn	4.50~13.90	[30]
大亚湾	²²³Ra,²²⁴Ra,²²⁶Ra,²²⁸Ra	4.83~6.01	[51]
本文	²²³Ra,²²⁴Ra	1.90~2.57 1.42~1.81

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