Features and factors of radium isotopes in Tianjin’s typical estuaries

1. Introduction

The estuarine and coastal zone is an essential economic, social and ecological region. With China’s rapid urbanization, the hydrogeochemical environment of the coastal area has been seriously affected. However, the increasing deterioration of the estuarine and coastal zone ecological environment, in turn, inhibits the development of society (Liu et al., 2007). Therefore, it is crucial to conduct studies on the impact of human activities on the coastal environment for regional environmental governance and management.

The behavior of radium isotopes in the hydrogeochemical environment can be a good indicator of the urbanization process. Therefore, the process of urbanization can be indirectly reflected by studying the features of radium isotopes in surface water and groundwater (Tang et al., 2015). The four natural radioisotopes of radium are ²²⁶Ra (half-life period T_1/2 = 1 600 a), ²²⁸Ra (T_1/2 = 5.75 a), ²²⁴Ra (T_1/2 = 3.66 d), and ²²³Ra (T_1/2 = 11.4 d), which are produced by the radioactive decay of uranium (²³⁸U and ²³⁵U) and thorium (²³²Th) isotopes. Their different half-lives and sources present different geochemical behaviors in aqueous environments (Garcia-Orellana et al., 2021).

Elsinger and Moore (1980) found that ionic strength altered the adsorption and desorption behaviors of radium isotopes in estuarine and coastal zone. The features of radium isotopes in the estuaries are closely related to the ionic strength, which was also confirmed in the studies of Plater et al. (1995) and Vinson et al. (2013). Redox may be an important factor regulating the features of radium isotopes in estuaries by constraining the presence of metal oxides (such as Fe-Mn oxides) (Charette and Sholkovitz, 2002; Tang et al., 2015). In groundwater, aquifer rock types, mineral weathering and co-precipitation play an important role in regulating the distribution of radium. For example, Shao et al. (2009) pointed out that montmorillonite and barite have different migration behaviors of radium isotopes in groundwater through numerical simulation. Therefore, the activity level of radium isotopes in groundwater will be jointly affected by the physical and chemical processes and hydrological environment, including adsorption-desorption, redox, hydrolysis-co-precipitation, and other aquatic environmental indicators (such as temperature, salinity, total dissolved solids (TDS), and pH, etc.). The behavior of radium isotopes in surface water and groundwater systems are influenced by physicochemical processes and hydrogeology.

On the other hand, based on the difference in radium isotope activities between surface water and groundwater, radium isotope has been widely used in studying this system under urbanization process (IAEA, 2014; Charette et al., 2015). For instance, the water retention time of estuarine and coastal zone directly affects the productivity, it is proposed that the application of radium isotope to consider water residence time in this area can help to evaluate the metabolic rate of the water ecosystem and indirectly reflect the pollution self-purification ability of the water (Moore, 2000a, b). In addition, studies estimating submarine groundwater discharge (SGD) fluxes in estuarine and coastal zone based on the mass balance of radium isotopes have been reported more frequently (Kelly and Moran, 2002; Luo et al., 2014). In summary, radium isotopes can also be a good indicator of hydrogeochemical processes under urbanization.

It is common to analyze the non-conservative behavior of radium isotopes in estuarine and coastal zones based on hydrogeological conditions. However, the impact of rapid urbanization is rarely considered. This study selected the typical estuaries of Tianjin affected by human activities as the research objects, including groundwater (Hangu (HG), Tanggu (TG), and Dagang (DG)) and surface water (Haihe River (HH) and Duliujian River (DLJ)). The horizontal and vertical distribution characteristics of radium isotope activities in surface water and groundwater were analyzed. Finally, we further explored the behavior of radium isotopes in the hydrogeochemical environment and urbanization process. Clarifying and skillfully utilizing the relationship between behavior of radium isotopes and urbanization will actively promote the healthy development of the Tianjin Binhai New Area.

2. Sampling and measurement

The investigation area is located in Binhai New Area, Tianjin, which is close to the Bohai Bay (Fig. 1). A total of 28 field sampling surveys were conducted. The samples of the groundwater (HG, TG, and DG) were collected from pumping wells at depths of 15 m and 30 m, respectively, in May and September 2017. The surface water (HH and DLJ) samples were collected at a depth of 1 m from March 2013 to November 2015. All samples were stored in 25 L plastic buckets (made of high density polyethylene), with a total volume of 50 L for water samples. They were brought back to the laboratory for determination within 24 h.

Figure  1.  The scope of the study area and sampling stations. HG: Hangu; TG: Tanggu; DG: Dagang; HH: Haihe River; DLJ: Duliujian River.

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In the laboratory, all samples went through the necessary filtration and enrichment. First, we used cellulose acetate membrane (pore size: 0.45 μm) to filter in order to exclude the influence of impurities such as suspended particles on the enrichment of MnO₂ fibers, and then place the filtered water samples on a high platform with samples containing 20 g of dry MnO₂ fibers. Secondly, based on the siphon principle, we controlled the water sample to pass through the sample column at a flow rate of about 1 L/min to complete the extraction of radium isotopes, and the extraction efficiency was as high as 98% (Moore and Arnold, 1996; Moore et al., 1985). Finally, the enriched MnO₂ fiber was cleaned with deionized water to remove the salt and solid particles attached to the surface of the MnO₂ fiber. The water content of MnO₂ fiber was adjusted by vacuum pump (75% of the dry MnO₂ fiber mass (20 g)), so that the total treated MnO₂ fiber was (35.0±0.1) g (Moore, 2008).

To avoid decay affecting the results, the ²²⁴Ra activity of samples was determined within 1−3 d (the first measurement). The activity of ²²³Ra was affected by the alpha signal produced by the radioactive decay of ²²⁴Ra and its progeny, so the measurement of ²²³Ra was performed within 7−9 d (the second measurement) (Moore, 2008; Moore and Arnold, 1996). Since ²²⁸Th in water samples was also adsorbed on the MnO₂ fibers while samples were being filtered, the ²²⁸Th activity was measured in the fourth week (the third measurement) after sampling to correct the ²²⁴Ra activity. At this point, ²²⁴Ra and ²²⁸Th have reached the decay equilibrium, and then we conducted the third measurement to obtain the contribution of ²²⁸Th to ²²⁴Ra in the sample. Finally, this study combined the results of the first measurement to obtain the corrected ²²⁴Ra activity. After three measurements, the samples were sealed for 10−12 months for the fourth measurement, the ²²⁸Th decay on the MnO₂ fibers was completed and the activity of ²²⁸Ra has been determined (Moore, 2008). The computational errors of ²²⁴Ra, ²²⁸Ra, and ²²³Ra are approximately 6%, 7.5%, and 10%, respectively (Moore and Arnold, 1996; Waska et al., 2008). In this study, the concentration of cations (Ca²⁺, Mg²⁺, Na⁺ and K⁺) was determined by inductively coupled plasma optical emission spectrometer (ICP-OES, Platinum Elmer Inc., USA), with an error of ±5%, while the concentration of anions (${{\rm {SO}}_4^{2-}} $, ${{\rm {NO}}_3^-} $ and ${\rm{Cl}}^- $) is determined by ion chromatographs (ICS-2100, Diane, USA) with an error of ±6%. The concentration of ${{\rm {HCO}_3^-}} $ was measured by titration with an error of ±5%.

3. Results

3.1 Radium in groundwater

Radium activities of HG, TG, and DG in spring and autumn are presented in Table 1. The results indicated that the variation range of radium activities in groundwater is extensive, reaching 2−3 orders of magnitude, mainly due to the different lithology of the aquifer in the groundwater environment (Moore et al., 1995; Moore, 1996). The spring’s average activities of ²²³Ra, ²²⁴Ra and ²²⁸Ra in the HG (15 m buried depth, the same as below) were 4.14 dpm/(100 L) (1 Bq=60 dpm), 286.91 dpm/(100 L), and 140.96 dpm/(100 L), respectively. The average activities of ²²³Ra, ²²⁴Ra and ²²⁸Ra in the TG were 18.40 dpm/(100 L), 267.74 dpm/(100 L), and 240.88 dpm/(100 L), respectively. The average activities of ²²³Ra, ²²⁴Ra and ²²⁸Ra in the DG were 142.05 dpm/(100 L), 2 110.91 dpm/(100 L), and 907.77 dpm/100 L, respectively. The activities of radium in spring were HG<TG<DG, while the autumn is different from spring. The average activities of ²²³Ra, ²²⁴Ra and ²²⁸Ra in the HG were 5.46 dpm/(100 L), 165.70 dpm/(100 L), and 161.18 dpm/(100 L) in autumn, respectively. The average activities of ²²³Ra, ²²⁴Ra and ²²⁸Ra were 3.02 dpm/(100 L), 103.20 dpm/(100 L), and 82.02 dpm/(100 L) in the TG, respectively. The average activities of ²²³Ra, ²²⁴Ra and ²²⁸Ra in the DG were 31.32 dpm/(100 L), 879.31 dpm/(100 L), and 889.40 dpm/(100 L), respectively. The activities of radium in water in autumn were TG<HG<DG. As a result, groundwater radium activities were affected by seasonal variation (Underwood et al., 2009).

Table  1.  Measurements of ²²³Ra, ²²⁴Ra and ²²⁸Ra in groundwater

Sample	Depth/m	Latitude	Longitude	223Ra/(dpm·(100 L)−1)			224Ra/(dpm·(100 L)−1)			228Ra/(dpm·(100 L)−1)
				Spring	Autumn		Spring	Autumn		Spring	Autumn
HG1	15	39.21°N	117.63°E	3.18	1.07		35.22	21.92		23.61	35.97
HG3	15	39.18°N	117.66°E	0.28	0.57		26.97	41.07		3.61	40.31
HG4	15	39.17°N	117.67°E	0.13	−		31.30	−		11.12	−
HG4	30	39.17°N	117.67°E	0.26	−		9.03	−		6.71	−
HG5	15	39.16°N	117.67°E	5.61	17.78		383.21	503.87		201.48	332.40
HG6	15	39.15°N	117.69°E	5.68	−		631.97	−		374.02	−
HG6	30	39.15°N	117.69°E	0.24	−		56.28	−		64.14	−
HG7	15	39.15°N	117.70°E	9.93	2.44		612.76	95.93		231.90	236.06
TG1	15	38.93°N	117.67°E	10.53	−		209.42	−		425.16	−
TG2	15	38.94°N	117.66°E	5.87	−		138.73	−		416.04	−
TG3	15	38.95°N	117.65°E	102.94	−		618.67	−		488.19	−
TG3	30	38.95°N	117.65°E	1.49	24.74		65.34	1936.98		36.57	32.61
TG4	15	38.95°N	117.63°E	22.99	−		885.81	−		310.46	−
TG5	15	38.95°N	117.61°E	1.34	9.94		80.45	317.81		72.22	169.45
TG5	30	38.95°N	117.61°E	2.31	0.89		3384.80	104.72		1099.93	23.24
TG7	15	38.97°N	117.56°E	0.66	0.52		63.47	17.42		93.21	33.73
TG8	15	38.99°N	117.53°E	1.20	1.49		40.92	52.20		55.61	55.11
TG8	30	38.99°N	117.53°E	7.49	−		289.43	−		340.15	−
TG9	15	39.00°N	117.47°E	1.68	0.11		104.48	25.38		66.16	69.78
DG1	15	38.64°N	117.38°E	38.41	12.80		976.90	448.77		809.54	236.07
DG1	30	38.64°N	117.38°E	3.51	0.01		132.69	11.68		100.23	13.61
DG2	15	38.64°N	117.40°E	113.35	40.24		2702.10	906.58		949.95	904.35
DG3	15	38.65°N	117.44°E	70.22	1.06		1383.59	71.15		623.95	456.40
DG4	15	38.66°N	117.47°E	64.15	66.05		1489.13	1563.82		471.53	928.50
DG4	30	38.66°N	117.47°E	3.12	2.79		86.04	111.23		65.22	318.54
DG5	15	38.66°N	117.49°E	321.64	61.40		3022.17	1814.50		1047.14	1063.86
DG5	30	38.66°N	117.49°E	2.74	4.45		58.27	126.65		63.36	573.35
DG6	15	38.66°N	117.51°E	356.12	61.55		3281.96	1846.62		1452.83	1810.29
DG7	30	38.66°N	117.52°E	357.64	1.59		3026.60	182.59		1100.73	198.41
DG7	15	38.66°N	117.52°E	2.48	15.92		198.73	947.32		332.17	1035.72
DG8	15	38.66°N	117.56°E	314.88	11.17		4276.96	214.62		2004.84	572.41
DG9	15	38.66°N	117.55°E	47.34	24.28		1671.79	552.71		639.33	825.89
DG9	30	38.66°N	117.55°E	10.32	4.51		314.65	173.82		415.58	495.18
DG10	15	38.67°N	117.54°E	91.89	18.79		2105.74	426.99		766.39	1060.53
Note: HG: Hangu; TG: Tanggu; DG: Dagang. − represents no data.

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3.2 Radium in surface water and the difference with groundwater

Table 2 shows the radium activities of HH and DLJ in different seasons. The results showed that the variations of activity range of radium in the surface water are negligible. The average activities of ²²³Ra-HH in spring, summer and autumn were 0.64 dpm/(100 L), 0.70 dpm/(100 L), and 0.74 dpm/(100 L), respectively. The average activities of ²²⁴Ra-HH were 14.04 dpm/(100 L), 17.73 dpm/(100 L), and 14.61 dpm/(100 L), respectively. The average activities of ²²⁸Ra-HH were 49.81 dpm/(100 L), 44.94 dpm/(100 L), and 42.04 dpm/(100 L), respectively. Therefore, the radium activity of HH is less sensitive to seasonal variations.

Table  2.  Measurements of ²²³Ra, ²²⁴Ra and ²²⁸Ra in surface water

Station	Latitude	Longitude	223Ra/(dpm·(100 L)−1)				224Ra/(dpm·(100 L)−1)				228Ra/(dpm·(100 L)−1)
			Spring	Summer	Autumn		Spring	Summer	Autumn		Spring	Summer	Autumn
HH1	39.13°N	117.20°E	0.24	0.31	0.39		7.06	10.16	7.64		11.39	15.15	7.41
HH2	39.10°N	117.24°E	0.34	0.36	0.38		4.55	8.76	6.25		8.95	13.56	7.53
HH3	39.08°N	117.29°E	0.38	0.29	0.43		6.92	9.04	8.57		10.75	24.95	9.59
HH4	39.05°N	117.51°E	0.11	0.29	0.41		5.20	10.43	8.04		14.07	10.45	9.12
HH5	38.99°N	117.51°E	0.46	0.73	0.70		17.05	17.35	12.20		50.68	35.65	34.67
HH6	38.98°N	117.58°E	0.79	0.54	0.84		17.10	18.30	14.56		59.69	58.07	37.85
HH7	38.99°N	117.71°E	2.13	2.34	2.03		40.43	50.07	45.02		193.13	156.77	188.08
DLJ1	38.84°N	117.30°E	2.02	2.49	1.13		51.75	50.62	23.09		52.11	53.61	41.17
DLJ2	38.88°N	117.21°E	2.79	1.95	1.35		56.38	46.00	31.80		53.48	41.84	39.58
DLJ3	38.91°N	117.18°E	2.08	2.14	1.34		60.20	49.91	25.85		50.85	51.42	37.98
DLJ4	38.99°N	117.09°E	1.59	1.46	1.22		42.58	30.92	27.64		32.28	20.32	27.61
DLJ5	39.00°N	117.06°E	1.95	0.93	0.98		43.54	45.48	22.39		33.34	38.84	22.57
DLJ6	39.03°N	116.98°E	2.25	1.54	0.72		56.04	42.10	20.46		44.32	31.06	19.46
DLJ7	39.05°N	116.92°E	1.08	1.55	0.66		33.07	31.75	15.81		33.85	29.92	18.99
Note: HH: Haihe River; DLJ: Duliujian River.

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The radium activity of DLJ varies in different seasons, unlike that of HH. The average activities of ²²³Ra-DLJ in spring, summer and autumn were 1.97 dpm/(100 L), 1.72 dpm/(100 L), and 1.06 dpm/(100 L), respectively. The average activities of ²²⁴Ra-DLJ were 49.08 dpm/(100 L), 42.40 dpm/(100 L), and 23.86 dpm/(100 L), respectively. The average activities of ²²⁸Ra-DLJ were 42.89 dpm/(100 L), 38.14 dpm/(100 L), and 29.62 dpm/(100 L), respectively. Therefore, the radium activities of DLJ were autumn < summer < spring. Seasonal variations will also regulate the radium activity of surface water, and different types of surface water have different responses to seasonal variations.

Radium isotopes are widely distributed in different water environments (Langmuir and Melchior, 1985; Charette et al., 2015). Significant differences in radium activity between groundwater and surface water have been found. Radium activity and spatial variability in groundwater were higher than in surface water. It is consistent with previous studies (Ni et al., 2013). Tang et al. (2015) indicated that the activities of radium in groundwater were generally higher than that in surface water in estuarine and coastal zone of Tianjin, this is consistent with our research results, possibly due to the abundant rocks and minerals flowing through the groundwater, and the decay and α recoil of the parent body release a large number of radium isotopes (Sherif et al., 2018). However, the salinity and TDS of surface water are low, and the parent nuclides are relatively few, so radium adsorbed on the surface of suspended particles cannot be released into the water body vastly and rapidly (Kiro et al., 2012; IAEA, 2014).

3.3 Distribution of physicochemical parameters

The hydrochemical parameters of groundwater in spring and autumn are presented in Tables 3 and 4. The results showed that the TDS of groundwater differs at different depths in different seasons. In spring, the TDS of HG, TG, and DG (15 m depth, the same as below) were 18 071 mg/L, 42 432 mg/L, and 49 168 mg/L, respectively, while in autumn, the TDS of HG, TG, and DG were 11 999 mg/L, 30 037 mg/L, and 57 196 mg/L, respectively. The high TDS mainly occurred in TG and DG, which may be related to the development and utilization of the salt field industry. The proportion of brines in TG and DG groundwater reaches about 50% and 30%, respectively, which is much higher than that in HG. Land salinization leads to differences in TDS (Xiao et al., 2012).

Table  3.  Measurements of chemical concentration parameters in groundwater in spring

Sample	Depth/ m	TDS/ (mg·L−1)	K+/ (mg·L−1)	Ca2+/ (mg·L−1)	Na+/ (mg·L−1)	Mg2+/ (mg·L−1)	${{\rm {HCO}_3^-}} $/ (mg·L−1)	${{\rm {SO}}_4^{2-}} $/ (mg·L−1)	$ {{\rm {NO}}_3^-} $/ (mg·L−1)	${\rm{Cl}}^- $/ (mg·L−1)
HG1	15	15678	67	60	11613	197	1360	1250	0.92	1130
HG3	15	8824	82	43	4305	133	930	760	1.29	2570
HG4	15	6731	116	103	5434	284	553	118	0.52	123
HG4	30	4257	21	26	2998	62	558	232	0.25	360
HG5	15	34702	232	187	7602	357	362	2860	1.21	23100
HG6	15	23180	267	199	3057	343	412	4700	0.74	14200
HG6	30	14994	161	70	6949	254	89	740	0.67	6730
HG7	15	19313	205	113	2166	314	585	3230	0.62	12700
TG1	15	79654	481	370	9682	382	533	14200	6.60	54000
TG2	15	52203	354	351	8791	399	56	6550	1.76	35700
TG3	15	104833	490	303	5048	397	293	14000	2.21	84300
TG3	30	121341	532	299	15089	398	322	13700	2.36	91000
TG4	15	22963	351	69	2017	354	661	4110	0.81	15400
TG5	15	58255	249	339	12475	389	−	3900	3.09	40900
TG5	30	46801	254	279	10395	382	10	2580	1.40	32900
TG7	15	10776	35	46	8523	103	484	870	0.25	714
TG8	15	4665	19	44	2849	68	723	520	8.37	433
TG8	30	22007	141	105	15683	232	551	412	3.50	4880
TG9	15	6103	35	48	3978	75	755	462	1.97	748
DG1	15	26912	192	97	6682	343	841	855	1.86	17900
DG1	30	16949	117	21	3116	339	300	855	0.93	12200
DG2	15	44082	299	181	8850	386	474	2490	1.29	31400
DG3	15	28403	254	112	4691	376	508	2460	1.06	20000
DG4	15	43380	274	213	8939	355	218	4080	0.90	29300
DG4	30	17297	65	53	4542	334	181	1420	1.00	10700
DG5	15	70040	343	313	14673	379	231	6100	1.26	48000
DG5	30	20207	75	134	6355	310	72	1460	0.87	11800
DG6	15	57951	344	311	12980	373	303	4640	1.23	39000
DG7	30	59115	314	301	13010	377	273	3340	1.30	41500
DG7	15	23947	156	92	10128	318	82	970	0.97	12200
DG8	15	80300	395	264	16159	374	456	4650	1.52	58000
DG9	15	72836	395	234	14911	373	122	6000	1.44	50800
DG9	30	20547	239	116	3800	320	62	1410	0.83	14600
DG10	15	43826	324	198	10930	367	384	4420	2.99	27200
Note: − represents no data. TDS: total dissolved solids.

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Table  4.  Measurements of chemical concentration parameters in groundwater in autumn

Sample	Depth/ m	TDS/ (mg·L−1)	K+/ (mg·L−1)	Ca2+/ (mg·L−1)	Na+/ (mg·L−1)	Mg2+/ (mg·L−1)	${{\rm {HCO}_3^-}} $/ (mg·L−1)	${{\rm {SO}}_4^{2-}} $/ (mg·L−1)	$ {{\rm {NO}}_3^-} $/ (mg·L−1)	${\rm{Cl} }^- $/ (mg·L−1)
HG1	15	4691	47	26	1274	228	837	1005	0.52	1274
HG3	15	6997	66	46	2103	172	645	834	1.26	3130
HG5	15	9657	76	6	2848	267	231	1627	1.15	4600
HG7	15	26654	181	50	7687	909	249	1698	0.56	15880
TG3	15	73568	1699	847	41400	8015	31	1814	2.36	19760
TG3	30	164712	1721	832	44220	8546	176	12034	2.46	97180
TG5	15	70196	268	670	19520	3749	70	3376	2.24	42540
TG5	30	80081	280	757	20710	4257	66	4870	1.40	49140
TG7	15	1935	24	27	549	80	308	333	1.03	612
TG8	15	2015	11	43	645	41	506	231	4.64	533
TG9	15	2470	24	26	730	82	376	422	1.88	808
DG1	15	8701	50	63	2378	365	181	272	1.76	5391
DG1	30	1512	39	32	376	30	37	431	0.83	568
DG2	15	64348	337	211	17110	2960	113	2836	1.19	40780
DG3	15	46939	256	67	12490	2066	149	2660	0.99	29250
DG4	15	57208	75	196	16340	1661	61	4074	0.85	34800
DG4	30	53374	173	49	14480	1848	42	2541	1.00	34240
DG5	15	86353	380	423	22250	3193	59	6718	1.16	53330
DG5	30	67421	309	371	17490	2447	40	4004	0.85	42760
DG6	15	84240	402	583	22260	2173	77	5094	1.22	53650
DG7	30	84254	430	553	22060	2996	74	3700	1.29	54440
DG7	15	30442	205	45	8433	1055	353	−	0.87	20350
DG8	15	33488	281	475	8621	818	48	3933	1.44	19310
DG9	15	94240	680	8	25690	3030	49	7012	1.50	57770
DG9	30	38044	232	145	10350	1107	29	1890	0.93	24290
DG10	15	66001	446	220	17170	2350	141	5510	3.01	40160
Note: − represents no data. TDS: total dissolved solids.

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The concentrations of K⁺, Ca²⁺, Na⁺ and Mg²⁺ in spring were 232 mg/L, 167 mg/L, 8 189 mg/L, and 307 mg/L, respectively, which were lower than those in autumn (334 mg/L, 260 mg/L, 13 815 mg/L, and 2 094 mg/L). The concentrations of ${{\rm {HCO}_3^-}} $, ${{\rm {SO}}_4^{2-}} $, ${{\rm {NO}}_3^-} $ and ${\rm{Cl}}^- $ showed the opposite results, which were higher in spring (416 mg/L, 3 540 mg/L, 2 mg/L, and 24 897 mg/L) than in autumn (190 mg/L, 3 157 mg/L, 1 mg/L, and 28 713 mg/L). It showed that seasonal variation regulates ion concentration to a certain extent. For example, the concentration of cations in spring (dry season) was lower than that in autumn (wet season), which may not be suitable season for crop growth in dry season, and the irrigation demand for groundwater extraction is reduced. The ability of soil salt to be transported by water flow is weakened (Sun et al., 2018). However, the concentration of anions in the dry season was higher than that in autumn, indicating that ion concentration was also regulated by other processes. For instance, the concentration of ${{\rm {HCO}_3^-}} $ is related to the dissolution of carbonate and silicates (Trainer and Heath, 1976); the process of denitrification in groundwater will change the concentration of ${{\rm {NO}}_3^-} $ (Adelana et al., 2020); under the influence of human activities, the shallow groundwater may be polluted by sulfate and chloride ion to a certain extent (Abboud, 2018). Therefore, ion concentrations in groundwater are regulated by complex hydrological and biogeochemical processes.

The cation concentration is Na⁺ > Mg²⁺ > K⁺ > Ca²⁺, and the anion concentration is ${\rm{Cl}}^- $ > ${{\rm {SO}}_4^{2-}} $ > ${{\rm {HCO}_3^-}} $ > ${{\rm {NO}}_3^-} $. A Piper three-line diagram was used to divide the water body based on the different chemical compositions of the water body (Giggenbach, 1988; Lu et al., 2022). The results showed that the chemical type of HG is mainly Cl-Na. Cl-HCO₃-Na type also existed. The chemical type of TG is mainly Cl-Na, and Cl-Mg-Na, Cl-SO₄-Na. Cl-HCO₃-SO₄-Na type also existed. In contrast, the chemical composition and type of DG are relatively concentrated, all of which are Cl-Na, indicating that its hydrodynamic conditions are relatively mild.

4. Discussion

4.1 Geographical distribution characteristics

To vividly illustrate the distribution characteristics of radium isotopes in the estuaries, Surfer 15 was used to obtain the bubble maps of ²²³Ra, ²²⁴Ra and ²²⁸Ra in HG, TG, and DG (15 m depth) and HH, DLJ in different seasons (Figs 2 and 3).

Figure  2.  Horizontal distribution of ²²³Ra (a, b), ²²⁴Ra (c, d) and ²²⁸Ra (e, f) activities in groundwater (HG: Hangu; TG: Tanggu; DG: Dagang).

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Figure  3.  Horizontal distribution of ²²³Ra (a−c), ²²⁴Ra (d−f) and ²²⁸Ra (g−i) activities in surface water (HH: Haihe River; DLJ: Duliujian River).

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The ²²³Ra, ²²⁴Ra and ²²⁸Ra of HG, TG, and DG in spring was consistent with that in autumn, that is, HG < TG < DG, and showed an increasing trend from north to south. Low TDS and specific water chemistry types (Cl-Na and HCO₃-Na) may have promoted the activity reduction of radium in HG. High TDS increases the desorption capacity of adsorbed radium on aquifer laminae, and the water chemistry type of DG is all Cl-Na, which leads to high activitites of radium in DG. From inland to estuaries, the increasing trend is reflected in the activity of radium in HG and TG, and the activities of radium near inland stations were lower than other stations (about 1.5 km from the coastline). Still, the activities variation of radium in DG was different from radium in HG and TG. The activity of radium in DG showed a trend of increasing and then decreasing from inland to the coastline, with high values occurring at DG5, DG6, and DG7, probably due to a large number of oil fields distributed nearby. Oil exploitation activities lead to the contamination of shallow groundwater, and a large amount of minerals in oil provides a rich parent source of radium in DG.

The distribution of ²²³Ra, ²²⁴Ra and ²²⁸Ra in HH and DLJ was consistent in different seasons, showing a trend from low to high from inland to the coastline, with high values occurring in the estuarine zone (HH5, HH6, and HH7; DLJ1, DLJ2, and DLJ3). Figure 4 showed the variation of radium in HH and DLJ with distance. The “distance” is linear distance from HH1 and DLJ7 to the rest of the stations, using the upstream river (HH1 and DLJ7) as the origin. It was found that the activities of ²²³Ra, ²²⁴Ra and ²²⁸Ra showed a gradual increase from inland to the estuary, although there were slight differences. This demonstrates that the variation of radium activities in estuarine waters is greatly influenced by salinity and TDS with unconservative additive behavior. Elsinger and Moore (1980) indicated that salinity mainly controls the adsorption and desorption of radium isotopes in the estuary (Elsinger and Moore, 1980; van der Loeff et al., 2003) and that the estuary is an essential site for the desorption of radium isotopes (Moore et al., 2006; Beck et al., 2007; Gonneea et al., 2008). HH and DLJ are natural freshwater ecosystems, and radium isotopes are adsorbed on suspended particles and exhibit semi-granular activity. The high TDS in HH and DLJ contributed to the desorption of radium from particulate matter, indicating that the salinity effect was a significant mechanism controlling the migration of radium isotopes. In addition, the hydrochemical environment constrained by human activities and hydrogeological conditions near the estuary also significantly impacts behavior of radium activities (Moore, 2010; Chen et al., 2022).

Figure  4.  Activity of ²²³Ra, ²²⁴Ra and ²²⁸Ra in surface water (Haihe River (a) and Duliujian River (b)) at each station.

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4.2 Vertical distribution characteristics

The vertical distribution characteristics of radium isotopes in different water bodies are of great significance for understanding the hydrodynamic process, vertical recharge flux, and migration time of the region (Liu et al., 2015; Liao et al., 2020; Lu et al., 2022). In spring and autumn, the ²²³Ra, ²²⁴Ra and ²²⁸Ra activities at 15 m depth were higher than that at 30 m depth (Fig. 5). The activities of ²²³Ra, ²²⁴Ra and ²²⁸Ra in the water at 30 m depth varied in the range of ±2.72 dpm/L, ±84.34 dpm/L, and ±198.02 dpm/(100 L), respectively, while the activities of ²²³Ra, ²²⁴Ra and ²²⁸Ra in the water at 15 m depth varied in the range of ±127.54 dpm/L, ±902.79 dpm/L and ±287.36 dpm/(100 L). This may be related to rock properties, aquifer hydrodynamic conditions, and chemical composition (Yi et al., 2019).

Figure  5.  Groundwater vertical distribution of ²²³Ra, ²²⁴Ra and ²²⁸Ra activities in spring (a−c) and autumn (d−f).

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4.3 Factors influencing radium distribution

In the estuarine and coastal systems, the source terms of radium isotopes mainly include the mineral weathering in the bedrock aquifer, the α recoil of parent nuclides, and the desorption of radium adsorbed on the bedrock surface, river input, and submarine groundwater discharge. The sinks of radium isotopes mainly include their radioactive decay, co-precipitation, and adsorption (Beck and Cochran, 2013). In addition to the source-sink term, the distribution characteristics of radium are also regulated by the geochemical environment of water bodies, especially ionic strength, redox, hydrogeological conditions, seasonal variations, and urbanization. Therefore, revealing the characteristics of radium isotopes under specific hydrogeological conditions not only helps to understand its migration and transformation patterns in groundwater but also is a prerequisite for using radium isotopes to quantitatively study the environment of groundwater oxidation, groundwater recharge and discharge, and groundwater pollution identification (Grundl and Cape, 2006).

4.3.1   Ionic strength

The ionic strength indicates the degree of electrical strength of ions in solution; the higher the number of charges carried by an ion, the higher the ionic strength (Charette and Sholkovitz, 2006). For most natural water bodies, radium in the water column exists mainly in the form of free state Ra²⁺ and is adsorbed by clays and metal oxides. Therefore, ionic strength is an important factor affecting the adsorption and desorption of aqueous radium isotopes at the water-rock interface (Krest and Harvey, 2003).

Figures 6 and 7 showed the relationship between radium isotopes (²²³Ra, ²²⁴Ra and ²²⁸Ra) and TDS in different seasons for three aquifers (HG, TG, and DG) and two rivers (HH and DLJ), respectively. In spring and autumn, the ²²³Ra, ²²⁴Ra and ²²⁸Ra activities of HG, TG, and DG showed an increasing trend with the increase of TDS, but the growth rate differed. This was consistent with the previous research results (Tang et al., 2015). TDS is an essential indicator for judging the ionic strength of natural water. It has been widely used in studying the source and distribution of radium isotopes in estuarine and coastal areas (Liu et al., 2019). In subsurface aquifers, high TDS promotes the desorption of radium adsorbed on the surface of particulate matter, enhancing the radium activities of HG, TG, and DG. Although there is a positive correlation between radium isotopes and TDS in HG, TG, and DG, the data distribution is scattered. It indicates that other mixing processes (e.g., suspended particulate matter and sediment) also lead to irregularities in the distribution of radium isotopes. The ²²³Ra, ²²⁴Ra and ²²⁸Ra activities of HH and DLJ showed a better positive correlation with TDS in spring, summer, and autumn due to the radium isotopes’ desorption adsorbed on the suspended particulate matter at the river-sea interface, but the desorption rates were different. ²²³Ra and ²²⁴Ra activities increased more than ²²⁸Ra, mainly because the suspended particulate matter of HH and DLJ had ²²³Ra and ²²⁴Ra more than ²²⁸Ra and faster regeneration of ²²³Ra and ²²⁴Ra than ²²⁸Ra, which is consistent with previous studies on water bodies in estuaries and coastal zones (Liu et al., 2013).

Figure  6.  Different kinds of plots depicting ²²³Ra (a, b), ²²⁴Ra (c, d) and ²²⁸Ra (e, f) activities of groundwater (HG: Hangu; TG: Tanggu; DG: Dagang) in relationship with total dissolved solid (TDS) in spring and autumn.

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Figure  7.  Different kinds of plots depicting ²²³Ra (a−c), ²²⁴Ra (d−f) and ²²⁸Ra (g−i) activities of surface water (HH: Haihe River; DLJ: Duliujian River) in relationship with total dissolved solid (TDS) in spring, summer and autumn.

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4.3.2   Redox effects

There are a lot of inorganic or organic oxidants and reductants in water. The order of oxidizing ability of common oxidants is O₂, ${{\rm {NO}}_3^-} $, ${{\rm {NO}}_2^-} $, ${{\rm {SO}}_4^{2-}} $, S. Vinson et al. (2013) proposed that the existence of ${{\rm {NO}}_3^-} $ and ${{\rm {SO}}_4^{2-}} $ can improve the stability of Fe-Mn oxides at the thermodynamic level (Vinson et al., 2013). Fe-Mn oxides are widely distributed in underground estuaries and have a strong affinity for radium in water, which can hinder the desorption of radium in sediments (Charette and Sholkovitz, 2002). Therefore, the redox environment will indirectly affect the activities of radium isotopes in water.

The radium isotope activity in spring and autumn was mainly distributed in ${{\rm {NO}}_3^-} $ from 0 mg/L to 3 mg/L, with no significant seasonal differences (Fig. 8). Precisely, the radium isotope activity increased at ${{\rm {NO}}_3^-} $ of 0 mg/L to 1 mg/L and decreased at ${{\rm {NO}}_3^-} $ of 1 mg/L to 3 mg/L. Once ${{\rm {NO}}_3^-} $ in the water exceeded 3 mg/L, a few radium isotope activities were desorbed into the water. It indicates that the ${{\rm {NO}}_3^-} $ influences the radium isotope activities and that the Fe-Mn oxides are most capable of desorbing radium isotopes at a ${{\rm {NO}}_3^-} $ of 1 mg/L, which also indicates that the oxidizing properties of oxidants in water can only be optimal under certain conditions (Stefánsson et al., 2005). On the other hand, the response of aqueous radium activity to ${{\rm {SO}}_4^{2-}} $ differed from that of ${{\rm {NO}}_3^-} $. In spring, the radium activities was mainly distributed in the ${{\rm {SO}}_4^{2-}} $ of 0−6 000 mg/L. The radium isotope activity increased when the ${{\rm {SO}}_4^{2-}} $ was 0−4 500 mg/L. A decreasing trend was reflected in the radium isotope activity once the ${{\rm {SO}}_4^{2-}} $ exceeded 4 500 mg/L. It indicates that the water column has the most substantial ability to desorb radium isotopes when the ${{\rm {SO}}_4^{2-}} $ is 4 500 mg/L in spring. It is different from the autumn situation. With the increase of ${{\rm {SO}}_4^{2-}} $, the radium isotope activity in autumn showed an increasing trend, which is an interesting phenomenon. The possible reason is that ${{\rm {SO}}_4^{2-}} $ exhibits a more ionic effect than oxidative due to the season and the acid-base environment of the water column. ${{\rm {SO}}_4^{2-}} $ complexed with Ra²⁺ to form RaSO₄ ⁰, which causes the increase of radium isotope activity in the groundwater.

Figure  8.  Different kinds of plots depicting ²²³Ra, ²²⁴Ra and ²²⁸Ra activities in relationship with ${{\rm {NO}}_3^{-}} $ (a, b) and ${{\rm {SO}}_4^{2-}} $ (c, d) in spring and autumn.

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4.3.3   Hydrogeological conditions

The HG, TG, and DG profiles on the west coast of Bohai Bay were formed in the Upper Holocene−Middle Holocene (buried depth of about 4−40 m), and Table 5 showed their sediment composition (Pei and Wang, 2016). HG and DG profiles are mainly composed of silty sand (upper layer) and clayey, silty sand (lower layer). In contrast, the TG profile mainly comprises clayey, silty sand (upper layer) and silty sand layer (lower layer).

Table  5.  Stratigraphic division of groundwater (Hangu (HG), Tanggu (TG) and Dagang (DG)) profiles

Study area	Layer bottom elevation/m	Composition of sediments
HG	−9.6	sand silty sand layer
HG	−18.0	silt layer
HG	−19.0	silt layer
HG	−21.0	clay silty sand layer
TG	−18.6	clay silty sand layer
TG	−19.2	shell sand layer
TG	−20.3	clay silty sand layer
TG	−23.0	silt layer
DG	−10.0	clay silty sand layer and fine sand layer
DG	−16.7	clay silty sand layer
DG	−30.2	clay silty sand layer

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Stratified and lenticular micro-confined aquifers dominate the shallow groundwater. According to the hydrogeological conditions of the study area, the HG profile (15 m burial depth) is dominated by clayey silt layers. Due to the low percolation rate, the exchange between the upper and lower water bodies is hindered. The TG profile (15 m buried depth) is dominated by clayey silt, while the shell sand layer is dominated at the buried depth of 19 m. A large number of saline substances are accumulated in the shell sand layer, and the high TDS of groundwater in TG is conducive to the desorption of radium isotopes on the surface of the aquifer medium, which may be the main reason for the radium activity of this layer is higher than the buried depth of 30 m. The DG profile (15 m burial depth) mainly comprises clayey silt. It has a low seepage rate and a strong ion exchange capacity between clay with a specific surface area and radium. This further explains why the radium activities of HG, TG, and DG profiles at 15 m depth were higher than that at 30 m depth. Therefore, different aquifers have different lithological characteristics, one of the crucial factors affecting the distribution of radium activity in groundwater (Underwood et al., 2009).

4.3.4   Seasonal effects

The average ²²⁴Ra/²²⁸Ra activity ratio of HG, TG, and DG was 1.12 in spring and autumn, while HH and DLJ were 0.94 in spring, summer, and autumn. It indicates that ²²⁴Ra and ²²⁸Ra in groundwater and surface water approximated a radiogenic equilibrium with ratios close to 1 (Moore, 2010). Radium activity can be affected by seasonal variation and rainfall processes (Yi et al., 2019). The ²²⁴Ra/²²⁸Ra activity ratio of groundwater bodies showed a decreasing trend with increasing rainfall from spring to autumn (Fig. 9), which may be due to the lagging effect of the rainfall process. The water with slow vertical velocity has a long residence time. Therefore, ²²⁴Ra decays much more than ²²⁸Ra simultaneously. In contrast, the radium activities of surface water decreased with rainfall (see Section 3.2).

Figure  9.  Relationship between ²²⁴Ra/²²⁸Ra isotope activity ratio and precipitation (mm) under different seasonal variations in groundwater (a, b) and surface water (c, d).

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4.3.5   Relationship between rapid urbanization and radium activity

The relationship between rapid urbanization of estuarine and coastal zone in Tianjin and radium isotope is shown in Fig. 10. With the acceleration of urbanization in Tianjin’s estuarine and coastal zone, especially since the 1960s, human activities have significantly affected the features of radium isotopes in groundwater and surface water. On the contrary, many scientific studies have shown that radium isotopes can be used as a novel tool to reflect the impact of urbanization on surface water-groundwater systems, for instance, water flushing times (Moore et al., 2006), horizontal and vertical diffusion processes (Moore, 2000a; Ku et al., 1980), exchange processes between surface water and groundwater (Baskaran et al., 2009), and the fluxes of submarine groundwater discharge (SGD) (Moore, 1996).

Figure  10.  Relationship between rapid urbanization and radium isotopes in Tianjin Binhai New Area. SGD: submarine groundwater discharge.

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Features of radium in the surface water (HH and DLJ) are affected by the combined effects of industry and agriculture. Affected by rapid industrial development, the flow of rivers has been at a low level, and the accumulation of sediments and suspended particles in the river is severe, which reduces the activities of radium in HH and DLJ (Lei et al., 2007). Additionally, the waterway dredging changed the topography of HH and DLJ, and the reduction of the shallow intertidal zone allowed the sediment to be resuspended, the variable environment complicates the diffusion of radium isotopes on suspended matter (Wu et al., 2005). The discharge of large amounts of nutrients (${\rm{NO}}_3^- $, ${\rm{NO}}^-_2 $, and ${\rm{SO}}_4^{2-} $) from coastal domestic and farming wastewater has led to severe eutrophication of HH and DLJ, which was consistent with the previous study (Cao and Corriveau, 2008). This phenomenon alters the redox environment of the water column, which in turn affects the distribution of radium isotopes.

Unlike surface water, groundwater in the three regions (HG, TG and DG) faces severe challenges. For example, the large-scale land reclamation has changed the groundwater level and seriously hindered the runoff and discharge of groundwater, which may affect the exchange of surface water and groundwater and the inaccuracy of SGD flux assessment by radium isotopes (Jiao et al., 2008; Nie and Tao, 2009). Tang et al. (2015) also indicated that the land use pattern was one of the major factors controlling the features of radium in this area. Due to the over-exploitation of groundwater, the groundwater tables of HG and TG decreased significantly, which would change the groundwater runoff and further affect the migration behavior of radium isotopes (Yi et al., 2011). The activities of radium in DG were much higher than that in the HG and TG regions, which may be related to the oil exploration and production in this area. Liu et al. (2007) indicated that over 50 years of oil exploration have caused severe pollution of shallow groundwater in DG, and radium isotope is one of the common radioactive elements in petroleum exploitation by-products (Parmaksız et al., 2015).

In addition, agriculture, animal husbandry, tourism and high-tech industries (marine bioengineering and electronic engineering) may also directly or indirectly affect the features of radium isotopes in the surface water-groundwater system (Pulido-Bosch et al., 2018; Silva and Mattos, 2020). Therefore, rapid urbanization will inevitably affect the source, transport, fate, and distribution of radium isotopes in surface water and groundwater in estuarine and coastal zone.

On the other hand, radium isotopes can be used to reflect the impact of urbanization on surface water-groundwater systems. For example, the “radium isotope (²²³Ra and ²²⁴Ra) apparent age model” proposed by Moore (1996) was used in this study to calculate the water residence time of groundwater (HG, TG and DG). The results showed that the average residence time of HG, TG and DG in spring is 11.50 d, 15.50 d and 18.80 d, respectively, which is higher than 11.36 d, 14.99 d and 15.73 d in autumn. It may be that the rainfall in spring is lower than that in autumn, and the rainfall process accelerates the flow of groundwater (Yi et al., 2019). In addition, the research on the application of radium isotopes to SGD is based on the principle of mass balance model, assuming that the region is in a stable state, and the calculation of SGD fluxes is completed through the source and sink terms of radium isotopes (Moore, 1996). In summary, this study suggests that correctly viewing and clarifying the relationship between radium isotopes and urbanization will help better understand the regional hydrogeochemical processes and the sweet development of the Tianjin’s estuarine and coastal zone.

5. Conclusions

The estuarine and coastal area is the crucial area of material exchange between land and ocean. The features and factors of radium isotope activity in groundwater and surface water in Tianjin Binhai New Area were analyzed. The main conclusions are as follows:

(1) Radium activity (HG, TG, and DG) and spatial variability in groundwater were higher than those in surface water (HH and DLJ). Radium activity in groundwater increased from north to south. The radium activity of HG and TG showed an increasing trend from inland to the coastline, but the radium activities of DG increased first and then decreased. The radium activity at 15 m depth was higher than at 30 m depth. Radium activity in surface water also increased from inland to the coastline.

(2) High TDS promoted the desorption of radium adsorbed on the surface of particulate matter, enhancing the radium activity of groundwater and surface water, but the desorption rates were different. Nitrate affects the activity of radium, and when its concentration was 1 mg/L, the Fe-Mn oxide had the most substantial desorption ability to the radium isotope. The desorption of radium isotopes by Fe-Mn oxides was strong in spring when SO₄ ²⁻ was 4 500 mg/L. There was a consistent trend of increasing radium activity in autumn, influenced by seasonal variation and acid-base environments, and SO₄ ²⁻ showed ionic effects rather than oxidative.

(3) The radium activity in spring was higher than in autumn, and the radium activity of surface water decreased with increasing rainfall. The ²²⁴Ra/²²⁸Ra activity ratio of groundwater showed a decreasing trend due to the lagging effect of the rainfall process.

(4) Rapid urbanization will inevitably affect the features of radium isotopes in surface water and groundwater in estuarine and coastal zone. Meanwhile, radium isotopes can be used to reflect the impact of urbanization on surface water-groundwater systems.