Hydrochemical Characteristics, Controlling Factors and Strontium Enrichment Sources of Groundwater in the Northwest Plain of Shandong Province, China

Abstract

To elucidate the hydrochemical characteristics, controlling factors, sources and mechanisms of strontium ion enrichment in groundwater in the northwest plain of Shandong Province, China, 88 groundwater samples were collected, including 51 shallow pore groundwater samples, 29 deep pore groundwater samples and 8 karst groundwater samples. The hydrochemical characteristics of the different types of groundwater were quite different. The karst groundwater samples were all fresh water with a single hydrochemical type, either HCO₃-Ca or HCO₃-Ca·Mg. The deep pore groundwater samples were mainly brackish water, and the shallow pore groundwater samples were brackish water–salt water, which has complex hydrochemical types. The hydrochemical characteristics of all the types of groundwater were controlled by mineral dissolution and active positive cation exchange. In shallow pore groundwater, deep pore groundwater and karst groundwater, the dissolution of silicate, evaporite and carbonate minerals dominated the hydrogeochemical process. The strontium in groundwater was derived from the dissolution of minerals with strontium isomorphism. The average contents of strontium in shallow, deep and karst groundwater were 1.59 mg/L, 0.58 mg/L and 0.50 mg/L, respectively. The strontium in shallow pore groundwater was mainly derived from the enrichment of groundwater runoff, and its sources are abundant, with silicic rock being the main source. The deep pore groundwater mainly derived from the evaporative minerals containing strontium, and the karst water mainly derived from carbonate rock dissolution with similar characteristics.

1. Introduction

Groundwater is an important resource supplying human industry, agricultural production and domestic water [1,2,3,4,5]. The hydrochemical characteristics of groundwater depend on its formation process, storage conditions, flow path and interaction with the surrounding environment and are influenced by human activities [6,7,8]. In its natural state, the hydrochemical composition of groundwater is mainly controlled by water–rock interaction, including rock dissolution, evaporation and concentration, cation exchange and other factors [9,10]. However, with the economic growth and social development of human beings, human activities are increasingly impacting the Earth’s natural environment. Emissions from human production and life have greatly changed the hydrochemistry of groundwater [11,12,13].

Strontium (Sr) is a necessary trace element for human health, maintaining the normal function of the human body [14,15]. Humans consume strontium through food and drinking water. The main source of strontium in groundwater is the Earth’s crust. Suitable hydrogeological conditions are the basic conditions required for strontium enrichment in groundwater [16]. Groundwater reacts with minerals in surrounding rock continuously and dissolves them via processes such as leaching, ion exchange, etc., while runoff and other conditions promote the increase in strontium content in groundwater. The abundance of strontium in the surrounding rock determines the content of strontium in groundwater [17]. There have been many studies on the formation mechanism and enrichment factors of strontium in groundwater [6,14]. By means of the hydrochemical ion ratio, statistics, hydrogeochemistry, geological tectonics and other methods, scholars have studied the causes of the enrichment of relevant groundwater chemical components [18,19,20].

The northwest plain of Shandong Province is a typical alluvial plain area located in the lower reaches of the Yellow River Basin. Pore groundwater is the most widely distributed type of groundwater in the region, including shallow pore groundwater and deep pore groundwater. In addition, karst groundwater is distributed in a small area. In this area, the quality of pore water is generally poor, the groundwater contains more hydrochemical components, and the distribution area of high-quality groundwater is small, thus leading to a water shortage due to quality in northern China. At the same time, the plain in the lower reaches of the Yellow River Basin has a large population and is a major human gathering place with prosperous social and economic activities. The shortage of water resources restricts the development of this region to a certain extent. In this study, the groundwater sources in the northwest plain of Shandong Province (NPSP) were selected to carry out research with a view to determining the origin of this groundwater and conserving it, and the phenomenon of strontium enrichment in groundwater was elucidated during this investigation.

There have been many studies on groundwater in this region but few on the cause of strontium enrichment in high-quality groundwater. We studied the hydrochemical characteristics, controlling factors and cause of strontium enrichment in groundwater on the basis of hydrogeological conditions, descriptive analysis, a Piper trilinear diagram, a Gibbs diagram, the ion ratio and correlation analysis, and suggestions for the development and utilization of high-quality strontium-rich groundwater are put forward.

2. Study Area

2.1. Natural Conditions

The northwest plain of Shandong Province (NPSP) is located in the north of the lower Yellow River plain (Figure 1), which is located in a warm temperate climate zone. The average annual temperature is 12 °C. The average annual rainfall is about 574 mm. The terrain in the NPSP is flat with a ground elevation of 2–30 m, and the overall terrain slopes downward from the southwest to the northeast coast. The rivers in the NPSP include the Yellow River, the Tuhai River and the Majia River. Rivers within the territory are rain-fed rivers. Their direction and terrain are largely consistent.

There are more than 17 million people living in this area. Dezhou and Liaocheng are major agricultural cities in Shandong Province, China. Binzhou and Dongying are rich in petroleum, with a well-developed refining industry chain and a relatively high level of industrial activities. These activities profoundly affect the groundwater environment.

2.2. Hydrological and Geological Conditions

The majority of the NPSP is covered by thick Quaternary loose rocks and mainly loose rock sediments, and Cambrian–Ordovician limestone occurs under the Quaternary layer (about 260 m) along the Yellow River near Donga-Qihe. The groundwater in the NPSP is mainly pore groundwater, and the shallow pore groundwater aquifer is mainly found in fine sand, medium-coarse sand and clay at a depth of 0–60 m. The deep pore groundwater occurs at a depth of 260–800 m, where the aquifer is dominated by fine sand and medium-fine sand. At 60–260 m, the middle-deep loose-rock pore groundwater is mainly brackish water, which is not utilized and is not discussed in this study. Only in some areas is there karst water, which occurs in Ordovician limestone at a depth of 250–520 m.

The samples in this study were collected from a mining water source, reserve water source and other places with relatively good water quality, including shallow, deep and karst groundwater sources.

3. Materials and Methods

3.1. Field Sampling

In this study, 88 groundwater samples were collected from the NPSP from June to August 2017, including 51 shallow pore groundwater samples, 29 deep pore groundwater samples and 8 karst groundwater samples. The sampling points were located at groundwater sources, or alternate, smaller sources, and 1~2 groundwater samples were collected from each source. The depth range of shallow pore wells is 15–60 m, that of deep pore wells is 120–400 m, that of individual wells is 600 m, and that of most karst wells is 350–520 m. Figure 1 shows the locations of the groundwater samples. Before sample collection, groundwater was pumped for more than 30 min to ensure that the groundwater was fresh from the aquifer. pH and TDS were tested on site using a portable water quality analyzer. The samples were stored at 4 °C during transportation.

3.2. Laboratory Sample Analysis

The laboratory of the Shandong Lubei Geological Engineering Investigation Institute was used for this test. Cations (Ca²⁺, Mg²⁺) were tested by titration, K⁺ and Na⁺ were tested by flame spectroscopy, anions (HCO₃⁻, Cl⁻, SO₄²⁻) were tested by titration, and NO₃⁻ was tested by ultraviolet spectrophotometry. The test was completed according to the ‘Methods for analysis of groundwater quality’ [21] and complied with the relevant specification requirements.

Table 1. The accuracy and precision of the chemical analysis.

Index	Detection Limit (mg/L)
pH	/
TH	3 mg/L (CaCO3)
TDS	/
K+	0.5
Na+	0.6
Ca2+	4
Mg2+	3
Cl−	3
SO42−	10
HCO3−	5
NO3−	0.2

shows the accuracy and detection limits for the chemical analysis of individual ions.

3.3. Data Analysis and Methods

The obtained water chemical data were statistically analyzed, and the main water chemical components were counted using Origin2022 SR1 software [22]. A Gibbs diagram and Piper three-line diagram were drawn based on the above methods. Descriptive analysis, the ion ratio and correlation analysis were used to analyze and study the hydrochemical characteristics of high-quality strontium-rich groundwater in the NPSP, China.

4. Results

4.1. Hydrochemical Characteristics

4.1.1. General Hydrochemistry of Groundwater

A hydrochemical statistical analysis method was used to gain a more intuitive understanding of groundwater hydrochemical characteristics in the region [9]. The main hydrochemical data of the samples were statistically analyzed (

Table A1. Complete chemical analysis of all study samples.

Sample Number	Type	K+	Na+	Ca2+	Mg2+	Cl−	SO42−	HCO3−	NO3−	TH	TDS	pH	Sr2+
S1	SPGW	1.60	86.00	90.18	46.17	99.26	110.47	390.53	0.52	415.33	639.83	7.4	1.1
S2	SPGW	1.30	132.00	112.22	78.98	141.80	211.33	549.18	21.55	605.48	981.27	7.4	1.7
S3	SPGW	3.60	196.00	210.42	113.00	382.86	218.54	744.44	0.55	990.79	1508.78	7.0	2.3
S4	KGW	0.80	24.00	76.15	18.23	24.82	38.42	280.69	9.68	265.21	339.94	7.8	0.5
S5	KGW	0.80	24.00	76.15	21.87	21.27	62.44	274.59	3.95	280.22	355.28	7.8	0.5
S6	KGW	1.00	20.00	80.16	24.30	24.82	81.65	274.59	3.52	300.24	380.89	8.0	0.6
S7	KGW	0.80	46.00	124.25	34.02	85.08	96.06	378.32	17.67	450.36	600.54	7.6	0.7
S8	SPGW	0.40	80.00	52.10	59.54	56.72	43.23	457.65	42.70	375.30	574.01	8.0	1.2
S9	SPGW	1.80	70.00	148.30	94.77	191.43	165.70	536.98	0.86	760.61	949.17	7.4	1.8
S10	SPGW	1.00	240.00	78.16	94.77	194.98	240.15	683.42	0.45	585.47	1200.58	8.1	2.1
S11	DPGW	0.75	440.00	20.04	13.37	315.51	341.01	274.59	<0.20	105.08	1288.69	8.3	0.8
S12	SPGW	2.20	200.00	148.30	92.34	177.25	302.59	738.34	0.75	750.60	1304.84	7.4	1.1
S13	SPGW	1.20	150.00	80.16	104.49	131.17	196.92	677.32	3.53	630.50	1013.63	7.7	1.6
S14	SPGW	2.80	450.00	116.23	134.87	407.68	487.50	884.79	12.73	845.68	2061.57	7.4	3.0
S15	SPGW	1.70	100.00	122.24	69.26	56.72	72.05	781.06	0.35	590.47	822.08	7.2	1.3
S16	SPGW	2.80	204.00	78.16	113.00	198.52	67.24	915.30	0.86	660.53	1135.85	7.4	2.3
S17	SPGW	1.20	196.00	70.14	92.34	170.16	28.82	842.08	1.61	555.44	993.40	7.8	2.4
S18	SPGW	0.80	228.00	72.14	109.35	180.80	206.53	768.85	0.55	630.50	1191.84	7.8	1.8
S19	SPGW	1.50	180.00	116.23	85.05	223.34	206.53	585.79	1.43	640.51	1120.71	7.5	1.8
S20	SPGW	0.80	240.00	120.24	102.06	134.71	292.98	713.93	153.00	720.58	1408.26	7.8	1.1
S21	SPGW	11.00	180.00	74.15	72.90	109.90	124.88	671.22	23.16	485.39	939.09	7.8	0.8
S22	SPGW	0.80	260.00	44.09	64.40	63.81	158.50	817.67	0.91	375.30	1010.42	8.0	0.8
S23	SPGW	0.60	390.00	38.08	55.89	233.97	235.35	683.42	0.20	325.26	1304.30	8.2	0.8
S24	SPGW	0.50	180.00	44.09	111.78	99.26	211.33	720.04	3.64	570.46	1019.80	8.2	1.5
S25	SPGW	1.00	480.00	90.18	153.09	425.40	557.15	848.18	0.44	855.68	2138.29	7.7	1.8
S26	SPGW	1.00	184.00	100.20	103.28	255.24	196.92	604.10	0.40	675.54	1150.59	7.8	1.5
S27	SPGW	1.40	340.00	180.36	134.87	425.40	569.16	628.51	14.88	1005.80	1988.49	7.4	1.6
S28	SPGW	1.00	420.00	110.22	157.95	475.03	600.38	634.61	15.33	925.74	2104.90	7.6	1.9
S29	SPGW	2.50	600.00	176.35	221.13	645.19	840.53	1037.34	1.11	1351.08	3018.28	7.5	2.5
S30	SPGW	1.80	192.00	98.20	66.83	81.54	148.89	817.67	0.28	520.42	1009.21	7.8	0.8
S31	SPGW	1.60	96.00	82.16	78.98	85.08	110.47	610.20	<0.20	530.42	768.59	7.8	1.3
S32	SPGW	1.00	74.00	86.17	37.67	31.91	52.83	524.77	<0.20	370.30	556.86	7.6	0.6
S33	KGW	0.50	26.00	74.15	15.80	21.27	48.03	256.28	14.33	250.20	335.72	8.1	0.3
S34	KGW	0.40	18.00	70.14	19.44	24.82	48.03	237.98	14.89	255.20	322.20	7.8	0.3
S35	KGW	0.40	22.00	66.13	18.23	21.27	48.03	237.98	11.96	240.19	314.51	8.1	0.3
S36	SPGW	0.10	64.00	98.20	65.61	53.18	76.85	604.10	0.25	515.41	669.73	7.2	0.8
S37	SPGW	0.30	100.00	52.10	91.13	152.44	86.45	488.16	8.80	505.40	742.89	7.9	1.2
S38	SPGW	0.40	180.00	46.09	47.39	106.35	134.48	518.67	0.26	310.25	782.13	8.1	0.6
S39	SPGW	40.00	240.00	66.13	207.77	340.32	403.45	829.87	0.53	1020.82	1721.66	7.4	2.1
S40	SPGW	0.40	288.00	230.46	185.90	514.03	626.79	689.53	56.75	1341.07	2254.58	7.3	2.1
S41	SPGW	2.50	690.00	42.08	336.56	620.38	1071.07	1263.11	0.52	1491.19	3403.03	7.4	2.4
S42	SPGW	1.00	200.00	162.32	91.13	290.69	365.03	543.08	7.64	780.62	1396.85	7.8	1.0
S43	SPGW	1.50	53.00	132.26	80.19	163.07	134.48	506.47	<0.20	660.53	831.04	7.4	1.6
S44	SPGW	1.50	370.00	90.18	193.19	276.51	787.69	738.34	0.70	1020.82	2097.56	7.8	1.7
S45	SPGW	0.60	256.00	88.18	125.15	191.43	341.01	774.95	0.38	735.59	1398.72	7.7	1.3
S46	SPGW	0.50	94.00	78.16	98.42	60.27	182.51	659.02	<0.20	600.48	853.86	8.2	1.3
S47	SPGW	0.50	100.00	72.14	85.05	31.91	115.27	713.93	<0.20	530.42	770.66	7.8	1.7
S48	SPGW	1.50	380.00	118.24	172.53	283.60	725.25	817.67	2.60	1005.80	2101.05	7.8	1.9
S49	SPGW	1.50	460.00	152.30	246.65	496.30	991.82	781.06	14.49	1396.12	2761.24	7.5	2.9
S50	DPGW	0.80	296.00	4.01	1.22	85.08	110.47	463.75	0.63	15.01	760.40	8.4	0.2
S51	DPGW	0.50	390.00	14.03	25.52	223.34	206.53	500.36	0.43	140.11	1151.73	8.3	1.4
S52	DPGW	0.50	296.00	8.02	9.72	56.72	110.47	622.40	0.57	60.05	803.78	8.2	0.4
S53	DPGW	1.60	180.00	68.14	86.27	109.90	177.71	689.53	<0.20	525.42	978.39	7.8	1.3
S54	DPGW	1.00	130.00	80.16	74.12	109.90	148.89	573.59	0.43	505.40	841.29	7.4	1.5
S55	DPGW	1.00	370.00	36.07	10.94	95.72	105.67	811.57	0.58	135.11	1036.19	7.2	0.6
S56	DPGW	0.50	420.00	20.04	41.31	276.51	326.60	500.36	0.42	220.18	1345.94	7.8	0.4
S57	DPGW	0.50	280.00	12.02	10.94	173.71	139.29	372.22	0.60	75.06	812.49	8.0	0.9
S58	SPGW	2.60	168.00	108.22	92.34	194.98	43.23	878.69	0.45	650.52	1062.70	7.9	1.8
S59	DPGW	0.25	240.00	8.02	3.65	46.09	81.65	414.94	0.29	35.03	638.68	8.4	0.2
S60	DPGW	0.50	288.00	14.03	18.23	138.26	158.50	384.43	0.43	110.09	869.89	8.3	0.4
S61	SPGW	0.70	168.00	50.10	76.55	191.43	120.08	494.26	<0.20	440.35	862.56	7.8	1.7
S62	DPGW	1.25	450.00	34.07	20.66	301.33	461.09	305.10	0.58	170.14	1430.34	7.8	0.9
S63	DPGW	1.25	440.00	44.09	19.44	311.96	437.07	311.20	0.33	190.15	1419.05	7.8	0.9
S64	SPGW	2.40	288.00	140.28	95.99	358.05	408.26	543.08	0.36	745.60	1577.36	7.8	1.5
S65	DPGW	0.50	260.00	10.02	4.86	70.90	100.86	433.24	0.67	45.04	711.60	8.3	0.3
S66	DPGW	0.50	288.00	12.02	6.08	53.18	105.67	482.06	0.80	55.04	762.21	8.4	0.2
S67	DPGW	1.50	370.00	14.03	12.15	230.43	254.56	378.32	0.86	85.07	1082.60	8.0	0.2
S68	DPGW	1.00	380.00	20.04	8.51	241.06	273.77	360.02	0.58	85.07	1113.71	8.1	0.2
S69	DPGW	1.00	380.00	18.04	15.80	248.15	278.57	329.51	0.49	110.09	1116.79	8.0	0.3
S70	DPGW	1.00	360.00	16.03	30.38	212.70	312.20	396.63	0.77	165.13	1142.48	8.2	0.6
S71	DPGW	0.70	160.00	6.01	10.94	53.18	52.83	280.69	0.30	60.05	461.80	8.4	0.2
S72	DPGW	2.00	670.00	26.05	52.25	510.48	653.21	347.81	2.52	280.22	2129.59	8.5	0.7
S73	DPGW	0.60	256.00	14.03	4.86	70.90	76.85	433.24	0.54	55.04	687.82	8.6	0.2
S74	DPGW	5.00	670.00	116.23	128.79	659.37	691.63	781.06	0.78	820.66	2671.04	7.4	1.4
S75	DPGW	1.50	470.00	24.05	14.58	248.15	432.27	463.75	1.87	120.10	1435.39	7.8	0.4
S76	DPGW	1.00	340.00	14.03	14.58	127.62	211.33	524.77	0.78	95.08	983.58	7.8	0.2
S77	DPGW	1.00	308.00	6.01	7.29	56.72	168.11	482.06	1.30	45.04	838.31	8.3	0.1
S78	DPGW	0.80	94.00	104.21	44.96	226.88	120.08	237.98	5.01	445.36	724.92	7.4	0.7
S79	DPGW	0.90	84.00	106.21	38.88	106.35	220.94	262.39	4.61	425.34	702.58	7.5	0.7
S80	KGW	0.70	40.00	50.10	52.25	49.63	91.26	329.51	15.29	340.27	471.48	7.4	0.8
S81	DPGW	1.00	340.00	12.02	29.16	184.34	321.80	366.12	<0.20	150.12	1080.73	8.2	0.6
S82	SPGW	0.80	88.00	10.02	95.99	74.45	120.08	457.65	15.11	420.34	642.01	8.0	1.3
S83	SPGW	0.80	276.00	38.08	44.96	46.09	105.67	872.59	0.27	280.22	957.77	7.8	0.9
S84	SPGW	1.70	104.00	208.42	38.88	159.53	100.86	659.02	2.03	680.54	954.04	7.0	1.5
S85	SPGW	0.60	228.00	68.14	120.29	116.99	422.66	695.63	0.87	665.53	1315.20	8.0	1.3
S86	SPGW	1.00	370.00	164.33	170.10	319.05	790.09	787.16	0.31	1110.89	2218.25	7.5	2.1
S87	SPGW	1.00	460.00	100.20	166.46	315.51	730.06	884.79	0.68	935.75	2228.15	7.3	1.7
S88	SPGW	1.50	610.00	104.21	234.50	425.40	1104.69	970.22	0.26	1225.98	2978.98	7.2	2.0

and

Table 2. Mass concentration statistics of main hydrochemical indexes in the northwest plain of Shandong Province.

Type	Index	K+	Na+	Ca2+	Mg2+	Cl−	SO42−	HCO3−	NO3−	TH	TDS	pH	Sr2+
SPGW	AV	2.24	242.80	101.18	113.95	224.73	326.37	706.64	8.14	721.95	1382.29	7.65	1.59
	CV	2.50	0.63	0.48	0.53	0.70	0.89	0.23	2.87	0.41	0.50	0.04	0.36
	Min	0.10	53.00	10.02	37.67	31.91	28.82	390.53	0.00	280.22	556.86	7.00	0.60
	MD	1.20	200.00	90.18	95.99	191.43	206.53	695.63	0.68	660.53	1135.85	7.70	1.60
	Max	40.00	690.00	230.46	336.56	645.19	1104.69	1263.11	153.00	1491.19	3403.03	8.20	3.00
DPGW	AV	1.05	332.76	30.41	26.19	191.19	244.13	440.82	0.94	183.77	1069.72	8.02	0.58
	CV	0.82	0.42	1.06	1.09	0.74	0.67	0.33	1.27	1.02	0.42	0.05	0.71
	Min	0.25	84.00	4.01	1.22	46.09	52.83	237.98	0.00	15.01	461.80	7.20	0.10
	MD	1.00	340.00	16.03	14.58	173.71	206.53	414.94	0.58	110.09	983.58	8.10	0.40
	Max	5.00	670.00	116.23	128.79	659.37	691.63	811.57	5.01	820.66	2671.04	8.60	1.50
KGW	AV	0.68	27.50	77.15	25.52	34.12	64.24	283.74	11.41	297.74	390.07	7.83	0.50
	CV	0.32	0.36	0.27	0.48	0.66	0.35	0.17	0.46	0.23	0.25	0.03	0.39
	Min	0.40	18.00	50.10	15.80	21.27	38.42	237.98	3.52	240.19	314.51	7.40	0.30
	MD	0.75	24.00	75.15	20.66	24.82	55.24	274.59	13.15	272.72	347.61	7.80	0.50
	Max	1.00	46.00	124.25	52.25	85.08	96.06	378.32	17.67	450.36	600.54	8.10	0.80

Note: MD (median), AV (average), CV (coefficient of variation). Unit: mg/L, except for pH and CV.

). The pH of the three groundwaters ranged between 7.00 and 8.60, all of which were weakly alkaline. The shallow pore groundwater (SPGW) was predominantly brackish, with a TDS ranging from 556.5 to 3403.03 mg/L, while the TDS of deep pore groundwater (DPGW) ranged from 461.80 to 2671.00 mg/L, with an average of 1069.72 mg/L, and the karst groundwater (KGW) was freshwater with a TDS ranging from 314.51 to 600.54 mg/L. Nitrate is mainly derived from anthropogenic emissions, including agricultural cultivation and domestic wastewater discharge [23]. Nearly all the groundwater samples were low in nitrate, indicating that the groundwater in the area is less affected by human nitrate emissions.

The cation found in the highest concentration in SPGW was Na⁺, followed by Mg²⁺ and then Ca²⁺; their average mass concentrations were 242.80 mg/L, 113.95 mg/L and 101.18 mg/L, respectively. The anion found in the highest concentration was HCO₃⁻, followed by SO₄²⁻ and then Cl⁻; their average mass concentrations were 706.64 mg/L, 326.37 mg/L and 224.73 mg/L, respectively.

The cation found in the highest concentration in DPGW was Na⁺, and the anion found in the highest concentration was HCO₃⁻.

Ca²⁺ and HCO₃⁻ were the anion and cation, respectively, with the highest concentrations in KGW. Their average concentrations were 77.25 mg/L and 283.74 mg/L, respectively. The content of other ions was relatively low.

The coefficient of variation and standard deviation can reflect the differences in each component to some extent. Table 2 shows that the coefficient of variation of cations in SPGW followed the order Na⁺ > K⁺ > Mg²⁺ > Ca²⁺, the coefficient of variation of anions followed the order NO₃⁻ > SO₄²⁻ > Cl⁻ > HCO₃⁻ and Mg²⁺, Ca²⁺ and HCO₃⁻ were relatively stable. The remaining ions had large spatial differences, following the order Mg²⁺ > Ca²⁺ > K⁺ > Na⁺, NO₃⁻ > Cl⁻ > SO₄²⁻ > HCO₃⁻ in DPGW. The coefficient of variation of KGW was small. The coefficient of variation of Cl⁻ was 0.66, which is larger than that found for other ions, indicating that the spatial distribution of each ion in KGW was relatively stable.

The main ion hydrochemical box line diagram (Figure 2) shows that the content of different hydrochemical components in SPGW, DPGW and KGW was quite different, indicating that the composition of DPGW and KGW was relatively stable. The source of SPGW was rich, and the three material sources were different.

There were some differences in Sr²⁺ content in the different types of groundwater (Table 2). The concentration of Sr²⁺ in SPGW fell between 0.60 and 3.00 mg/L, with an average concentration of 1.59 mg/L and a coefficient of variation of 0.36. The concentration of Sr²⁺ in DPGW fell between 0.10 and 1.50 mg/L, with an average of 0.58 mg/L and a coefficient of variation of 0.41. The concentration of Sr²⁺ in KGW fell between 0.30 and 0.80 mg/L, with an average of 0.50 mg/L and a coefficient of variation of 0.39. It can be seen that Sr²⁺ had obvious differences in different sources of groundwater. The content of Sr²⁺ in SPGW was significantly higher than that in DPGW and KGW, and the coefficient of variation was small. The spatial distribution of Sr²⁺ content across samples of the same type of groundwater was not obvious, and there was a certain continuity.

4.1.2. Hydrochemical Type

The Piper diagram is a simple and effective water type classification method in hydrochemical research [2,11,24]. As can be observed in Figure 3, the hydrochemical type of SPGW was complex, and the cation content was dominated by Na·Mg, Ca·Na and Ca·Mg, while the anion content was dominated by HCO₃⁻, followed by HCO₃·Cl and SO₄-HCO₃·Cl. The DPGW cations were dominated by the Na type, followed by the Ca·Na·Mg type, while the anion types were relatively complex, with the HCO₃·Cl, HCO₃ and HCO₃·Cl·SO₄ types dominating. KGW’s chemistry was relatively unique, with the cations being mainly Ca, Mg and Ca types and the anion type and HCO₃⁻ being absolutely dominant, with the HCO₃·Ca-Mg type and HCO₃·Ca type largely present. Moreover, Figure 3 shows that there was no significant correlation between Sr²⁺ content and groundwater chemistry.

4.2. Controlling Factors of Groundwater Hydrochemistry

The chemical components of groundwater are determined by physical and chemical changes under complex geological and climatic conditions, and the main factors controlling their formation include rock weathering, evaporation and concentration and atmospheric precipitation [25,26]. Gibbs established a semi-logarithmic graph with ordinate and abscissa representations of TDS (unit: mg·L⁻¹) and Na⁺/(Na⁺ + Ca²⁺) or Cl⁻/(Cl⁻ + HCO₃⁻), respectively. We used it to determine the main factors that control the chemical characteristics of water bodies.

The water sample points were in the rock weathering and evaporation–concentration control areas (Figure 4), and the KGW was derived from the rock weathering area, indicating that rock weathering heavily influences the formation of KGW’s chemical components. The SPGW was mainly collected from the rock weathering and evaporation–concentration areas. The DPGW was collected from the evaporation–concentration and rock weathering areas.

4.2.1. Dissolution

Groundwater dissolves rock minerals during runoff. Gaillardet constructed ion ratio diagrams of Ca²⁺, Mg²⁺, Na⁺ and HCO₃⁻ to determine the effects of water–rock interactions between three main rock types (silicate, carbonate and evaporate) on the hydrochemical properties of a region [27]. As shown in Figure 5a,b, SPGW was mainly found in the silicate area, DPGW in the evaporite area, and KGW in the carbonate and silicate areas, indicating that the formation of the three types of groundwater is heavily influenced by the weathering and dissolution of these three minerals.

4.2.2. Alternating Adsorption of Cations

The chemical characteristics of groundwater in a region are affected by cation exchange. The ratio of (Mg²⁺ + Ca²⁺-HCO₃⁻-SO₄²⁻) to (Na⁺ + K⁺-Cl⁻) is generally used to identify cation exchange processes [9,28]. Figure 6a shows that the groundwater samples taken from almost all of the study areas were affected by cation exchange. We further introduce the chlor-alkali index to indicate the direction of cation exchange. As can be seen in Figure 6b, the chlor-alkali index of most the groundwater samples was negative, indicating that positive cation exchange occurs in groundwater in this area, the concentration of Na⁺ and K⁺ increases, and the concentration of Ca²⁺ and Mg²⁺ decreases.

CAI-I = (Cl⁻-Na⁺-K⁺)/(Cl⁻),

(1)

CAI-II = (Cl⁻-Na⁺-K⁺)/(HCO₃⁻ + SO₄²⁻ + CO₃²⁻ + NO₃⁻)

(2)

4.2.3. Impact of Human Activities

Nitrate is an important indicator of human activities affecting the groundwater environment [2,9]. Table 2 shows the average content of nitrate in groundwater. The higher the ratios of Cl⁻/Na⁺ and NO₃⁻/Na⁺, the more obvious the influence of human activities on groundwater [29,30]. Since the water samples in the NPSP were taken near the Y = 0 line (Figure 7), far from agricultural activities, the groundwater was mainly affected by the dissolution of minerals, and the impact of anthropogenic nitrate emissions was weak.

4.2.4. Ion Ratio

The ratio of major ions in water can be used to study the source of major ions and the evolution of water chemistry [31]. In order to further explore the water–rock interaction process in groundwater, we adopted the ratio method to analyze the major ions. The concentration ratio of Na⁺ and Cl⁻ dissolved in salt was 1:1 (Equation (3)), while the ratio in seawater was 0.86 [32,33,34]. The ratio of silicate dissolution was greater than 1 [35]. It can be seen from Figure 8a that the water sample points above y = x mainly derive from the dissolution of silicate rocks, and the water sample points below y = x may be affected by cation exchange or human activities. The effect of the dissolution of silicate rock minerals existed in different types of groundwater. The content of Na⁺ and Cl⁻ ions in KGW was low (Figure 8b), and HCO₃⁻ was the main ion present, which indicates that carbonate rocks are the main source of its hydrochemical composition. The Na⁺ content in DPGW was greater than that of HCO₃⁻, indicating that the dissolution of evaporative salt rock (such as salt rock) in DPGW is increased, releasing more Na⁺ and Cl⁻ content. Other mineral components contributed less.

NaCl→Na⁺ + Cl⁻,

(3)

2NaAlSi₃O₈ + 2CO₂ + 11H₂O→Al₂Si₂O₅(OH)₄ + 2Na⁺ + 4H₄SiO₄ + 2HCO₃⁻,

(4)

Studies have shown that Ca²⁺ and Mg²⁺ in groundwater are mainly derived from evaporative rock and carbonate rock, and SO₄²⁻ is mainly derived from evaporative rock. The ratio relationship between (Ca²⁺ + Mg²⁺) and (HCO₃⁻ + SO₄²⁻) can be used to characterize the sources of Ca²⁺, Mg²⁺ and SO₄²⁻ [36].

All KGS samples were taken around the 1:1 straight line (Figure 9a). Both the Ca²⁺ and Mg²⁺ ions in KGW come from carbonate minerals. The SPGW samples were taken around or below the line. The Ca²⁺ and Mg²⁺ contents in groundwater are not enough to balance the HCO₃- and SO₄²⁻ ions in the water, and more Na⁺ and K⁺ ions are needed to balance it, indicating that SPGW mainly comes from carbonate and silicate minerals. The DPGW samples were taken far away from this line, and combined with the above, this indicates that the Ca²⁺ and Mg²⁺ contents in DPGW are mostly derived from evaporative minerals.

The concentration ratio relationship between (Ca²⁺ + Mg²⁺-HCO₃⁻) and (SO₄²⁻-Na⁺-Cl⁻) can further determine whether SO₄²⁻ is derived from the dissolution of gypsum. SO₄²⁻-(Na⁺-Cl⁻) represents the dissolution rate of Ca²⁺ in gypsum, while SO₄²⁻-(Na⁺+Cl⁻) represents the dissolution rate of SO₄²⁻ in gypsum. (Ca²⁺ + Mg²⁺-HCO₃⁻) represents the dissolution rate of Ca²⁺ in gypsum, and SO₄²⁻-(Na⁺ + Cl⁻) represents the dissolution rate of SO₄²⁻ in gypsum [37]. Figure 9b shows that all water sample points fell near the 1:1 line, indicating that SO₄²⁻ in different types of groundwater in the NPSP is mostly derived from the dissolution of gypsum, which is consistent with the presence of gypsum in the Neogene strata in the work area.

5. Discussion

5.1. Controlling Factors of Groundwater’s Hydrochemical Characteristics

The above findings show that the strontium content in the three groundwater types was 1.59 mg/L, 0.58 mg/L and 0.50 mg/L, respectively. The hydrochemical characteristics were mainly controlled by mineral dissolution and cation exchange in the NPSP. Silicate, evaporative minerals and carbonate mineral dissolution are the dominant factors influencing the hydrogeochemical processes in SPGW, DPGW and KGW, respectively, and the cation exchange was found to be a mainly positive exchange. The quality of KGW was relatively good, indicating it is suitable for use as drinking water, and SPGW is suitable for farmland irrigation.

5.2. Origin of Strontium Ions in Groundwater

Human activities have relatively little influence on the NPSP, which can be regarded as a natural study site for groundwater chemical evolution analysis. On the basis of the above research, the model of the regional water chemical field was established (Figure 10). In the process of groundwater formation, minerals are first dissolved in groundwater, and the evaporation and cation exchange occurring during runoff affect the hydrochemical components.

The above indicates that the composition of groundwater in the NPSP is mainly derived from rock weathering, with the strontium in rocks being the source of elements in groundwater. Strontium is an alkaline earth metal group with active chemical properties and a strong migration ability. It belongs to the same group as Ca and Mg and has similar chemical properties. It often coexists with Ca and Mg in minerals in the form of homomorphism and is abundant in granite, diorite, carbonate rocks and clay minerals [38].

The SPGW recharge area is relatively close, but shallow burial, loose strata and rock minerals such as silicate rock and groundwater contact areas are highly present, and so it is easily affected by evaporation, concentration, ion exchange and other factors. The mineral solubility is remarkable; the Sr²⁺ in SPGW mainly comes from strontium minerals in silicate rocks, such as anorthite (Equation (5)). The contact area between the minerals and water is sufficient, and the degree of dissolution is high, so the content of strontium is high in shallow groundwater.

(Ca.Sr)Al₂Si₂O₈ + 2CO₂ + 3H₂O→AlSi₂O₅(OH)₄ + 2HCO₃⁻ + Sr²⁺(Ca²⁺)

(5)

In contrast, DPGW has the characteristics of deep burial, a slow runoff, a far-off recharge area, long formation, evaporation and concentration times and an ion exchange effect; the Sr²⁺ in DPGW mainly comes from evaporative salt rocks, including gypsum (Equation (6)).

(Ca.Sr)SO₄·2H₂O→Ca²⁺Sr²⁺ + SO₄²⁻ + 2H₂O

(6)

KGW originates from atmospheric precipitation, and the recharge comes from the upstream mountainous area. Groundwater immediately enters the Ordovician limestone strata after rainfall, with a short runoff distance. Meanwhile, the rocks in the Ordovician limestone strata are hard and dominated by solution pores and caves, and there are few contact surfaces between the groundwater and bedrock, which do not fully come into contact. However, its dissolution is closely related to the content of CO₂, so the content of all the ions in karst water is low, and the upper layer is covered by a thick Quaternary layer, and the effect of evaporation and concentration is weak (Figure 11). The Sr²⁺ in KGW is mainly derived from strontium-containing minerals (such as calcite, dolomite, etc.) in carbonate rocks (Equations (7) and (8)) [39]. Its runoff distance is short, and the contact area between rock and water is small. Therefore, mineral dissolution is relatively small, and the content of each ion is low. Compared with DPGW, the dissolution of Sr²⁺ is relatively greater, indicating that the content of Sr²⁺ in carbonate minerals is higher than that in gypsum-based evaporite, and the solubility of strontium-containing minerals is greater than that of gypsum-based evaporite minerals.

(Ca.Sr)CO₃ + CO₂ + H₂O→Ca²⁺Sr²⁺ + 2HCO₃⁻,

(7)

(Ca.Sr)Mg(CO₃)₂ + CO₂ + H₂O→Ca²⁺Sr²⁺ + Mg²⁺ + 2HCO₃⁻ + CO₃²⁻

(8)

5.3. Suggestions for Groundwater Development and Utilization

According to the chemical characteristics of groundwater in this area, SPGW contains component concentrations that exceed the standard concentrations, and thus can be used for agricultural irrigation after treatment. All the ion targets of KGW are in line with the characteristics of natural strontium-rich groundwater. It is suggested that the exploitation of strontium-rich karst water be appropriately increased and that DPGW be rationally exploited within the scope permitted by law without causing other geological environmental problems.

6. Conclusions

Three types of groundwater samples were collected in the northwest plain of Shandong Province, China. including shallow pore groundwater, deep pore groundwater and karst-soluble groundwater. The hydrochemical characteristics and controlling factors of different groundwater types were studied, and the source and enrichment of strontium in groundwater were analyzed. Our conclusions are as follows:

(1)

There are significant differences in different groundwater types in the northwest plain of Shandong Province, China. Shallow groundwater is mainly brackish–saline water, deep groundwater is mainly brackish water, with some fresh water, and karst groundwater is fresh water. The shallow pore water is rich in sources and diverse in hydrochemistry. The cation content mainly comprises the Na·Mg type, Ca·Na type and Ca·Mg type and the anion content mainly comprises the HCO₃ type, followed by the HCO₃·Cl type and SO₄·HCO₃·Cl type. The cations in deep pore water are mainly of the Na type, followed by the Ca·Na·Mg type, and the anionic types are relatively complex, mainly including the HCO₃·Cl and HCO₃·Cl·SO₄ types. The hydrochemical types of karst are relatively simple, mainly including the HCO₃-Ca·Mg type and HCO₃-Ca type.

(2)

The main components of groundwater sources in the NPSP are derived from water–rock interaction, and rock mineral leaching plays a dominant role, which is more obvious in karst water, and the influencing factors of evaporation and concentration are highly present in shallow and deep pore water. Shallow pore water mainly comprises dissolved silicate minerals, deep pore water mainly contains dissolved evaporative minerals, largely gypsum, and karst water is mainly composed of dissolved carbonate minerals. Cation exchange exists in these three kinds of groundwater, and a positive exchange is the main effect. Natural geological conditions and good water source protection measures mean that human activities have relatively little influence on the water chemical formation process.

(3)

The distribution of strontium-rich groundwater is the highest in shallow pore water, where the average Sr²⁺ content reaches 1.59 mg/L, while the average Sr²⁺ content in deep pore water and karst water is 0.58 mg/L and 0.50 mg/L, respectively. The strontium in shallow pore groundwater mainly comes from the enrichment of groundwater runoff, and its sources are abundant, though it mainly derives from silicic rock. Deep pore groundwater mainly comprises evaporative minerals containing strontium, and karst water is mainly derived from carbonate rock dissolution with similar characteristics.

Finally, on the basis of this study, we suggest strengthening the development of strontium-rich karst groundwater as drinking water resources and doing a good job of water source protection, which can produce good economic and social benefits. Shallow groundwater can be used for farmland irrigation.