Distribution, characteristics and influencing factors of fresh groundwater resources in the Loess Plateau, China

1.   Introduction

The Loess Plateau is located in the north-central part of North China. It starts from the Wushao Mountain in Gansu Province in the west and ends in the Taihang Mountains in the east. It is bounded by the Mu Us Desert (also name Maowusu desert) in the north and extends to the northern slope of the Qinling Mountains in the south. It has a loess coverage area of 275600 km² and possesses the largest and thickest loess deposits in the world (Zhang ZH, 2003; Liu DS et al., 2001; Sun JZ, 2005). It falls in a semi-arid and semi-humid climate, is sensitive to climate change, and has a weak ecological environment (Wang YR et al., 2011). Owing to a low precipitation amount and high evaporation, the Loess Plateau is extremely lacking in regional water resources, while low-quality groundwater is widely distributed (Lin XY and Wang JS, 2006; Wu AM et al., 2020; Wang SH et al., 2009), leading to the water shortage in terms of both quantity and quality.

Fresh water is an extremely rare and valuable resource that supports human beings and also serves as one of the essential elements of production (Lu YR, 2014). Fresh groundwater is an extremely important component of fresh water, accounting for about 30% of global fresh water resources. It is traditionally believed that the fresh groundwater can satisfy the water use of human beings in a certain time range and can be restored year by year (Zhang RQ et al., 2011). As a product of interactions among lithosphere, hydrosphere, biosphere, and atmosphere, fresh groundwater is heavily affected by climate change and human activities in terms of formation and evolution. Meanwhile, it enjoys high water quality and adjustable temporal-spatial distribution and is an important factor restricting regional economic and social development (Fei J, 1996). Fresh groundwater is the second largest fresh water source followed by glaciers and thus has critical strategical significance for the safety of food and drinking water (Wada Y, 2015). The formation and evolution of fresh groundwater as well as the impacts of human activities and climate change on fresh groundwater have been a focus in the management and research of fresh water resources. Groundwater is an extremely critical or even the only water source for human livelihood and production in the Loess Plateau. Affected by natural strata and climate, saline groundwater is widely distributed in the western part of the Loess Plateau, which heavily restricts regional economic development. Since the 1980s, the regional underlying surface has varied with any changes in the climate, plantation structure, and land use types in the Loess Plateau, thus changing the entire water circulation process consisting of precipitation, vegetation interception and transpiration, water infiltration, and groundwater recharge and reducing groundwater recharge (Wang WK et al., 2018). During the urbanization development and industrial and agricultural production, long-term irrational groundwater recovery has induced ecological and geological problems such as a decrease in groundwater level and the deterioration of water quality (Pang ZH et al., 2018; Huang TM et al., 2017; Cheng LP et al., 2016; Zhou ZF, et al., 2014). Overall, owing to intense human activities coupled with a weak ecological environment in past decades, the groundwater in the Loess Plateau is facing dual threats of reduced quantity and deteriorated quality (Li Z et al., 2019a).

Given the integrity of the groundwater circulatory system and the continuity of loess deposits, this paper determines the scope of the Loess Plateau mainly according to drainage divides. Based on existing data and results fully collected and sorted as well as the authors’ understanding obtained in recent years, this paper analyses and summarizes the distribution pattern and characteristics of the fresh groundwater in the Loess Plateau. Meanwhile, it dissects the formation and evolution of the fresh groundwater in loess tablelands and the impacts of human activities and climate change on them as a priority. The purpose is to deepen the understanding of the distribution, hydrodynamic characteristics, circulation evolution, and influencing factors of the groundwater in the Loess Plateau and to provide a scientific basis for the groundwater management, ecological protection, and drinking water safety in the Loess Plateau.

2.   Hydrogeological overview of the study area

The Loess Plateau features various landforms and highly complex terrains. Specifically, alternate loess ridges, loess knolls, and bedrock mountainous areas are distributed from west to east, and bedrock mountainous areas, river valley basins, loess tablelands, and loess hills are distributed from south to north. Owing to the factors such as climate, lithology, and geological structures, the hydrogeological conditions in the Loess Plateau such as the occurrence modes, distribution pattern, and recharge/runoff/discharge conditions of groundwater show a regional or zoning pattern. In detail, the annual rainfall gradually increases and the recharge conditions, water quality, and resource quantity of groundwater gradually improve from north to south and from east to west.

The groundwater in the Loess Plateau can be divided into four major classes and five subclasses according to the types and porous characteristics of water-bearing media (Fig. 1). Specifically, it can be grouped into loose-rock pore water, clastic-rock pore-fissure water, karst fissure water, and bedrock fissure water (Hou GC et al., 2017; Han SB et al., 2021). As a major type of groundwater in the study area, the loose-rock pore water can be further divided into loess pore-fissure water and alluvial-diluvial sand - sandy-gravel pore water. The loess pore-fissure water is widely distributed in loess areas and is concentrated in landform areas such as loess tablelands, loess gullies and hilltops, loess knoll ridges, and the loess terraces on both sides of the Fenwei Basin. Its burial depth and the water yield property greatly vary in space, with the water yield property reducing and the water level increasing from the tableland centers to tableland margins. The alluvial-diluvial sand - sandy-gravel pore water is mainly distributed in the Fenwei Basin and Yiluoqin faulted basin. It enjoys favorable water yield property, with a single well water yield of 1000–5000 m³/d and a burial depth ranging from a few to more than ten meters. The clastic-rock pore-fissure water is mainly distributed in the loess coverage area in the southern part of the Ordos Basin and is superimposed with the overlying loess water. It suffers poor water quality and its hydrochemical characteristics show a clear zoning pattern. The karst fissure water mainly occurs on the east, south, and west sides of the Ordos Basin in a banded form, covering an area of about 40000 km² in total. Its water yield property greatly varies in space, with a water yield up to a maximum of 10000 m³/d. The bedrock fissure water can be divided into magmatic-rock fissure water and metamorphic-rock fissure water. It is mainly distributed in bedrock mountainous areas of the Wushao and Qinling mountains and has poor water yield property.

Figure  1.  Generalized hydrogeology of the Loess Plateau, China.

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3.   Characteristics of the groundwater resources in the Loess Plateau

Based on previous results as well as the survey data of the study area obtained in recent years, the authors summarized the distribution and major occurrence areas of the fresh groundwater, the evolution of the hydrochemical types of the groundwater, and the occurrence and recharge of the fresh groundwater in loess in the Loess Plateau, obtaining the following five pieces of knowledge.

3.1   Uneven temporal-spatial distribution of regional groundwater and the distribution of shallow fresh water closely relating to precipitation

The water yield property of groundwater is directly related to the occurrence conditions of aquifers and recharge characteristics, which depend on geological, structural, hydrological, and meteorological factors. The zoning patterns of the regional climate and precipitation result in uneven water yield property and distinct zoning phenomenon of groundwater resources. Temporally, the study area is located in a Southeast Asian monsoon climate and thus has distinct annually dry, wet, rainy, and drought characteristics and interannually alternate dry and wet seasons. As a result, the groundwater is unevenly distributed interannually and seasonally. Spatially, the annual precipitation in the study area increases from 150 mm to 700 mm from north to south (Fig. 2). Such zoning pattern of the precipitation leads to the zoning distribution pattern of the water yield property of the groundwater. Meanwhile, the modulus of natural resources of different types of groundwater tends to gradually increase from north to south.

Figure  2.  Shallow groundwater TDS distribution and average precipitation contour line between 2000 and 2020 of the Loess Plateau.

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According to Table 1, the average precipitation monitored in major meteorological stations was 220.3‒769.3 mm and the distribution of fresh groundwater was closely related to precipitation amount in the past 20 years. The annual precipitation was less than 300 mm in the loess ridges and knolls, where no water occurred and the TDS of phreatic water in gullies and valleys was 3–10 g/L in general (Fig. 2). The TDS of shallow groundwater was generally greater than 1 g/L in areas with annual precipitation of less than 400 mm, while fresh water was distributed in most areas with annual precipitation of greater than 400 mm. For instance, the annual rainfall was greater than 400 mm to the south of the Huajialing area, where the TDS of regional phreatic water was generally less than 1 g/L. The annual rainfall was 250–300 mm in the loess hills to the north of Dingxi, Haiyuan, and Hongde areas in the basins of Kurui, Qingshui, and Zuli rivers, where the TDS of regional phreatic water was 3‒10 g/L. These loess hills are typical distribution areas of brackish water in China. The annual precipitation was 350‒400 mm in the Longxi Loess Plateau and the northern Longdong Loess Plateau, where the TDS of groundwater was generally 7 g/L and showed an upward trend with an increase in precipitation amount. Additionally, the loess strata become thinner from the center to the northwest and southeast sides in the Loess Plateau and become the thinnest near the mountainous area and rivers. The regional annual precipitation is 400–600 mm in areas with the thickest loess strata (Zhu YJ et al., 2018), and the resource quantity of fresh groundwater in loess increases with an increase in the thickness of loess strata.

Table  1.  Precipitation of major meteorological station of the Loess Plateau in recent two decades (mm).

Station/Year	2000	2001	2002	2003	2004	2005	2006	2007	2008	2009	2010	2011	2012	2013	2014	2015	2016	2017	2018	2019	2020	AVG
Jingyuan	215.9	203.0	222.6	278.4	177.0	146.6	139.2	296.8	202.1	159.2	153.1	200.2	222.0	182.1	343.4	193.4	242.1	264.3	287.6	271.9	224.7	220.3
Yuzhong	414.8	301.1	313.7	387.5	311.8	431.4	304.9	555.5	386.9	299.8	332.3	262.8	393.3	404.7	410.4	275.2	309.6	409.9	641.5	493.9	428.4	384.3
Huajialing	443.9	418.4	352.4	629.9	425.9	551.8	417.9	489.9	441.1	360.7	413.6	465.8	476.8	597.4	525.4	468.8	415.6	583.7	616.6	587.1	671.8	493.1
Youyu	419.2	339.5	500.9	473.6	450.1	392.4	453.9	309.8	503.9	308.6	443.3	356.1	522.1	497.7	432.1	470.9	518.4	403.4	597.7	472.4	520.9	447.0
Hequ	225.0	321.5	239.0	542.0	411.7	327.4	337.9	559.9	500.2	286.9	370.8	286.4	646.9	402.9	440.6	365.5	658.2	535.0	454.0	435.1	420.8	417.5
Yulin	264.9	568.7	526.6	436.3	420.0	248.7	313.5	439.4	447.5	420.8	363.9	445.4	566.8	562.5	378.3	451.0	724.9	639.3	696.7	525.5	361.6	466.8
Wuzhai	480.9	348.9	423.5	612.2	435.7	368.8	315.8	610.2	537.4	388.3	502.8	495.9	488.3	682.2	569.4	474.3	556.8	546.3	509.7	516.5	391.0	488.3
Xingxian	543.7	372.4	359.6	480.6	456.4	408.6	291.4	689.4	524.0	566.2	425.0	465.3	543.4	791.9	455.4	555.2	638.9	528.3	605.2	470.4	463.3	506.4
Wuqi	398.6	538.6	533.1	590.8	356.9	347.0	412.6	581.6	355.2	449.8	374.5	437.0	501.2	631.3	520.0	371.3	455.1	582.9	521.5	512.0	490.1	474.3
Suide	277.6	457.5	353.7	439.0	320.0	306.6	434.9	470.1	325.2	565.7	355.0	562.3	462.7	733.7	534.5	420.1	492.5	656.7	513.0	452.4	493.4	458.4
Lishi	483.6	434.3	459.1	644.0	437.5	322.8	487.9	555.1	393.9	681.1	478.0	578.0	513.0	648.8	559.9	362.2	732.3	691.1	501.5	521.4	618.8	528.8
Taiyuan	419.3	298.0	420.1	526.3	377.2	274.7	424.8	535.4	355.3	625.1	376.6	496.6	427.8	487.3	428.7	403.6	528.4	521.2	365.0	312.6	553.6	436.1
Haiyuan	327.9	371.2	456.2	453.0	247.1	284.2	255.2	444.3	283.5	277.7	353.5	345.7	430.7	422.4	507.5	338.3	449.8	527.0	493.3	555.9	427.8	393.0
Tongxin	214.0	298.0	280.0	281.7	194.5	119.4	224.3	307.2	191.7	176.9	202.9	218.0	285.7	248.6	424.3	238.3	212.7	346.8	266.2	291.6	312.7	254.1
Guyuan	406.2	404.7	401.9	605.1	385.4	372.2	426.2	350.6	373.6	357.1	458.3	393.8	449.6	706.2	589.4	377.6	465.2	504.5	659.4	710.1	488.2	470.7
Huanxian	332.2	535.8	543.0	509.4	342.3	372.8	258.1	382.4	315.7	353.7	407.2	369.9	540.5	674.5	569.4	357.6	412.9	519.8	651.0	525.9	417.2	447.2
Xixian	415.3	481.1	503.5	725.9	357.4	524.1	468.6	520.4	396.2	548.4	394.0	629.0	488.0	671.2	653.6	377.5	566.1	550.3	482.8	363.0	663.7	513.3
Jiexiu	320.6	422.9	366.0	646.6	370.4	430.8	449.2	566.6	357.3	489.3	423.7	539.2	490.0	614.0	504.6	357.7	577.0	492.5	384.8	403.4	627.3	468.3
Linfen	471.9	323.1	400.7	762.0	478.2	491.3	515.1	377.4	364.5	436.2	449.8	514.6	472.0	533.1	613.8	356.7	400.8	543.0	442.7	414.2	500.2	469.6
Xiji	402.2	348.2	335.2	433.2	472.1	393.2	329.3	366.0	315.7	255.4	410.7	314.0	377.9	592.2	544.7	382.8	300.5	478.1	639.1	573.2	477.3	416.2
Kongtong	363.6	546.8	578.1	644.2	411.9	472.9	466.0	389.2	322.9	374.8	616.7	524.7	453.6	745.8	537.5	493.7	507.9	547.4	745.4	654.9	771.2	531.9
Xifeng	465.7	556.1	601.3	828.2	485.9	506.9	577.1	517.3	391.3	459.6	541.7	614.5	493.6	737.4	562.0	534.9	478.4	675.9	717.8	668.9	569.4	570.7
Changwu	557.1	509.4	562.1	954.3	498.7	509.0	539.3	607.6	539.2	503.1	657.5	720.3	457.9	585.8	701.3	693.4	477.3	734.6	715.2	778.3	592.6	614.0
Luochuan	691.3	620.2	541.6	929.4	465.8	576.1	687.7	606.3	549.7	568.5	656.5	778.5	522.9	661.2	722.0	488.1	510.3	660.0	580.3	596.6	655.3	622.3
Yuncheng	427.6	405.5	481.9	849.2	425.1	417.5	419.6	637.0	410.2	476.6	530.4	732.1	333.1	401.1	657.9	476.6	447.8	451.4	407.5	583.9	499.2	498.6
Houma	416.1	378.1	488.6	742.9	494.5	419.4	536.2	563.6	383.6	425.6	423.2	681.0	375.0	574.4	702.3	396.5	455.1	500.6	431.7	455.7	650.8	499.8
Yangcheng	596.5	414.3	572.6	896.2	679.0	591.9	574.2	519.0	533.9	417.8	456.0	726.7	334.5	763.1	588.6	493.6	676.1	497.7	618.3	554.3	584.5	575.7
Wugong	419.1	398.7	435.7	944.0	571.0	666.1	651.5	657.4	504.5	588.4	703.0	905.4	503.5	488.3	683.7	567.8	559.4	644.6	566.7	711.1	727.7	614.2
Yaoxian	482.8	415.6	492.8	811.1	401.8	454.6	634.2	635.3	462.4	586.3	591.5	751.9	429.7	487.3	660.8	578.9	296.2	464.0	464.1	470.0	611.0	532.5
Huashan	743.5	509.6	548.5	886.8	648.1	945.6	670.8	713.8	602.6	909.3	989.5	828.8	736.8	561.9	817.7	814.1	840.3	1086.1	736.4	661.6	903.1	769.3
Sanmenxia	431.0	332.6	440.4	899.4	588.3	455.1	414.3	533.7	438.0	552.7	587.4	739.1	551.4	338.5	605.2	722.8	590.2	646.8	525.9	568.2	622.7	551.6
Mengjin	794.0	509.6	511.7	1041.9	708.5	531.7	498.6	510.2	535.4	561.4	527.3	840.7	401.7	392.0	667.7	652.7	746.8	676.1	821.2	580.6	607.0	624.6
Dingbian		426.2	354.8	344.2	399.8	245.4	282.4	365.3	258.8	372.7	302.5	409.4	381.5	332.8	356.9	380.5	274.8	372.3	414.3	373.6		349.9
Wubao		355.6	426.5	611.8	400.2	327.9	459.5	339.5	465.4	669.3	433.3	616.0	469.8	468.9	366.9	337.5	645.0	688.5	470.3	351.8		468.6
Yanan		552.3	555.3	659.1	451.7	440.8	487.1	398.2	430.6	628.5	454.4	552.8	442.7	947.3	511.7	390.6	425.8	620.1	525.2	680.5		534.5
Fuxian		398.5	508.7	741.9	355.5	507.4	526.1	516.6	515.0	610.0	600.9	691.6	461.6	834.5	586.8	484.2	415.4	693.0	642.5	537.5		559.4
Tongchuan		444.1	511.6	878.3	625.9	516.7	684.9	461.5	477.2	508.9	660.5	719.8	385.5	495.9	592.6	512.3	427.6	614.2	513.9	510.6		554.8
Baojji		451.3	492.1	901.7	434.4	627.7	622.2	463.1	545.2	805.5	647.4	1032.4	623.4	581.6	559.8	615.3	484.0	676.5	597.2	791.6		629.1
Binxian		460.0	465.6	941.5	430.0	476.3	498.3	461.0	487.8	457.5	735.1	729.7	437.0	437.1	504.3	575.4	312.9	622.0	499.5	622.1		534.4
Liquan		412.1	410.5	827.8	491.8	560.8	489.4	678.1	493.8	585.1	584.2	757.3	366.5	373.5	576.9	522.3	351.7	531.9	450.8	567.8		528.0
Xian		386.7	406.3	884.0	436.6	568.7	577.9	607.1	543.6	694.5	503.1	742.6	362.9	488.4	660.3	551.6	456.0	649.0	459.1	593.0		556.4
Hancheng		350.1	452.5	897.8	355.6	484.5	605.2	412.6	408.8	502.7	571.1	776.5	399.8	480.7	709.4	509.2	430.9	479.1	410.6	503.1		512.6
Weinan		383.4	470.8	886.2	726.6	476.0	474.5	451.8	570.1	620.1	605.6	706.4	338.2	362.8	490.9	541.6	493.6	605.4	517.0	725.8		549.8
Tongguan		367.3	472.0	993.2	643.0	648.1	541.5	601.8	496.4	806.0	823.4	743.8	374.1	424.0	631.1	636.6	487.0	539.9	518.4	567.9		595.6
Data source: China meteorological data service center.

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3.2   Complex hydrochemical types of groundwater and distinct zoning pattern

According to water samples collected from the Haiyuan faulted basin in 2016, the water quality data from the national groundwater monitoring wells in the Loess Plateau in 2019, and the water quality data cited from other literature, the concentrations of major ions in groundwater tend to gradually increase from recharge areas to discharge areas in different landform units, as shown in Table 2. The hydrochemical types of groundwater were determined according to the order of anions with a concentration of greater than 25% (milligram equivalent) in the groundwater following the Shukalev classification. The results are as follows. The hydrochemical types are dominated by HCO₃ for shallow groundwater and are SO₄·Cl or Cl·SO₄ for the groundwater in the northern part of the Huajialing area in the western Liupan Mountains. The hydrochemical types are increasingly simple from north to south overall in the Longdong and Shanbei loess plateaus (Liu X et al., 2021a; Sun YB et al., 2016). For the same type of groundwater in the Fenwei Basin and the piedmont diluvial fan area in the loess hills, the major hydrochemical process is shifted from leaching into concentration and the hydrochemical types show the zoning evolutionary pattern of HCO₃ → HCO₃·SO₄ → Cl·SO₄ from recharge areas to discharge areas (Tao H et al., 2017; Sun YB et al., 2014).

Table  2.  Groundwater hydrochemical characteristics of typical landform in the Loess Plateau.

Landform (region)	Hydrogeologic units		TDS	Ca2+	Mg2+	Na+	K+	HCO3−	Cl−	SO42−
	(sample number)		/(mg/L)
Valley basin (Guanzhong basin)	Recharge area (n=65)	MIN	126.0	3.9	0.4	4.5	0.2	82.8	1.9	0.0
		MAX	895.0	181.0	55.1	285.0	5.7	885.0	340.5	163.5
		AVG	364.1	53.1	19.1	56.8	2.0	287.0	23.6	35.0
		MED	327.0	55.9	18.9	30.5	1.6	277.0	10.9	20.8
	Runoff area (n=204)	MIN	132.0	2.0	3.0	11.7	0.3	77.2	2.3	0.0
		MAX	6666.0	316.0	525.0	1193.0	51.0	792.4	1731.0	2414.0
		AVG	715.1	60.4	44.0	137.6	3.2	372.0	84.8	148.6
		MED	631.6	57.0	34.6	102.5	1.8	340.0	44.9	85.3
	Retention and discharge area (n=197)	MIN	709.1	2.2	9.3	114.6	0.8	0.0	20.0	33.1
		MAX	21141.0	676.6	926.0	6057.0	79.4	1160.2	6626.0	6727.0
		AVG	2859.0	76.6	143.8	737.7	5.0	519.7	579.6	931.1
		MED	2142.0	53.5	110.0	541.0	2.9	534.7	328.6	609.9
Mountain alluvial fan (Haiyuan basin)	Recharge area (n=10)	MIN	381.7	29.8	27.6	29.7	2.1	229.3	13.1	66.9
		MAX	598.1	68.3	56.0	67.1	3.9	353.5	97.7	165.5
		AVG	482.1	53.6	37.8	48.2	2.8	272.1	40.4	109.3
		MED	487.6	55.1	33.8	51.3	2.7	263.3	25.6	105.3
	Runoff area (n=21)	MIN	580.2	53.8	37.8	54.6	2.4	171.5	74.1	130.9
		MAX	3360.5	246.5	262.1	530.4	7.9	311.4	937.1	1251.0
		AVG	1852.7	141.5	130.9	278.8	5.3	234.8	435.3	658.5
		MED	1926.3	136.4	136.0	256.2	5.6	229.3	476.5	684.2
	Retention and discharge area (n=31)	MIN	865.5	44.8	26.7	123.5	1.5	194.6	90.8	264.3
		MAX	6436.4	401.1	398.9	1232.7	10.5	329.5	1765.2	2489.8
		AVG	2587.4	182.5	168.1	407.1	6.4	249.6	653.4	911.3
		MED	2484.9	158.5	169.2	355.7	6.9	248.6	595.7	806.1
Loess tableland (Dongzhi Tableland)	Central (n=34)	MIN	188.0	7.3	16.7	16.6	0.4	181.0	3.7	2.5
		MAX	308.0	59.4	31.5	44.8	2.8	338.0	23.6	17.1
		AVG	265.9	47.2	24.6	22.5	1.0	291.3	7.4	8.3
		MED	266.0	51.3	24.6	21.7	0.9	299.0	6.2	8.4
	Edge (n=9)	MIN	158.0	30.6	6.1	14.0	0.8	184.0	4.8	1.8
		MAX	338.0	62.1	28.0	37.9	2.3	364.0	10.0	15.8
		AVE	253.8	49.8	19.4	22.5	1.2	272.6	6.4	9.1
		MED	258.0	51.7	21.7	20.6	1.0	277.0	5.7	9.6
Karst region (Shaizhu cave spring area) (Ma ZY et al., 2014)	Recharge area	Sample1	249.3	47.9	18.4	19.2	0.6	263.0	6.2	5.7
	(n=2)	Sample2	281.5	27.7	24.4	44.6	1.1	289.8	6.2	5.7
	Runoff area (n=11)	MIN	310.5	30.3	16.8	30.5	0.8	307.5	5.2	11.3
		MAX	596.8	85.6	45.8	123.7	4.8	352.1	120.6	98.9
		AVG	442.0	58.7	28.9	60.0	2.6	325.2	52.8	36.3
		MED	444.4	54.9	27.3	56.9	2.7	331.3	42.3	35.5
	Discharge area (n=7)	MIN	600.1	50.9	15.2	46.2	1.4	325.2	60.0	35.5
		MAX	894.7	159.9	41.0	138.5	7.1	369.8	215.7	171.6
		AVG	767.5	88.0	30.6	83.5	4.1	340.7	143.3	73.6
		MED	764.1	75.0	27.3	65.1	3.5	337.4	137.9	68.8

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The hydrochemical types of confined water in the Loess Plateau are mainly affected by the sedimentary environment and geological conditions. They are complex overall and show distinct change laws in local areas. In areas such as the Ningnan and Longxi (Longzhong) loess plateaus, the confined aquifers have high content of soluble salts and gypsum and the hydrochemical types are dominated by SO₄·Cl. In the Longdong and Weibei loess plateaus, the hydrochemical types of the groundwater mainly include SO₄·Cl and HCO₃·SO₄·Cl in the underlying Cretaceous. They show the evolution pattern of HCO₃ → HCO₃·SO₄ → HCO₃·SO₄·Cl → SO₄ → SO₄·Cl along the groundwater flow direction in the medium-deep Huanhe and Luohe formations in western Ziwu Ridge and are dominated by HCO₃·SO₄ in the eastern part of the Ziwu Ridge. Meanwhile, the hydrochemical types of groundwater in these loess plateaus show a distinct layered pattern in the vertical direction. In detail, the hydrochemical types evolve from SO₄·HCO₃ in the shallow part into Cl·SO₄ in the deep part in recharge areas and evolve from Cl·SO₄ in the deep part into SO₄ in the shallow part in discharge areas. For the Carbonaceous - Jurassic strata in the Ordos Basin to the west of the Yellow River, the hydrochemical types of groundwater include HCO₃ and SO₄ (or Cl) in the southern and northern parts of the Yanan–Yanchuan area, respectively, without clear laws. The hydrochemical types of karst groundwater show the evolution pattern of HCO₃ → HCO₃·SO₄ (or HCO₃·Cl) → SO₄²⁻ (or Cl⁻) from recharge areas to discharge areas, featuring a distinct horizontal zoning pattern.

3.3   Groundwater dominated by fresh water in shallow part and salt water in the deep part, and fresh water of exploitation value distributed in a small scope

The fresh groundwater in the shallow part of the study area covers an area of about 410000 km², accounting for 77.4% of the total area of the study area. However, only 25% of it is of extraction value. They are regionally distributed in the southeast and sporadically distributed in other parts (Fig. 3). Their major distribution areas are as follows. (1) Aquifers of sands and sand gravels in the Fenwei and Yiluo faulted basins and the loess aquifers in loess terraces on both sides of the faulted basins. The sands and sand gravels in these aquifers generally feature a high water yield of 1000‒5000 m³/d, and the burial depth of groundwater in these aquifers is from few meters to more than ten meters in general. (2) Loess aquifers in typical loess tablelands in the Longdong and Weibei areas. They cover an area of around 8300 km² in total, accounting for about 5% of the loess coverage area. They show greatly varying burial depth of groundwater and water yield property in space and serve as the major and even the only water source for the livelihood of local residents. For instance, the Dongzhi Loess Tableland covers an area of 828 km² and is the largest loess tableland in China. The single well water yield decreases from 500 m³/d to 50 m³/d and the burial depth of groundwater increases from around 30 m to more than 90 m from the center to the margin in the Dongzhi Tableland. (3) Aquifers of gravels and sand gravels in the piedmont alluvial-diluvial fans in the bedrock mountainous area. They are generally small. For instance, the diluvial fan in the Haiyuan faulted basin in the northern piedmont of the Nanhua Mountain, Ningnan area, Sichuan Province is about 400 km². Fresh water is distributed throughout the fan except for marginal areas, with a water yield of up to a maximum of more than 3000 m³/d. (4) Karst fissures on the southern, eastern, and western margins of the Ordos Basin. They are distributed in a banded form and cover a total area of about 40000 km². The water yield of the karst fissures varies greatly in space and is up to a maximum of 10000 m³/d.

Figure  3.  Distribution of exploitable fresh groundwater resources.

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The deep groundwater is brackish-saline water in most areas in the Loess Plateau except for the Fenwei and Yiluo faulted basins and karst area, where fresh groundwater is distributed. Neogene lacustrine-facies red beds are widely distributed below loess in areas such as the Ningnan and Longxi loess plateaus. They bear high content of soluble salts and gypsum. Therefore, the water quality is poor in most areas, with a TDS of greater than 3 g/L in general. The older the strata, the poorer the water quality in the vertical direction. Most especially, the TDS of the groundwater in the Neogene Ganhegou Formation in the Xihaigu area is up to 12 g/L. The Cretaceous clastic rocks in the Ordos Basin are covered by hugely thick loess strata. Therefore, the groundwater suffers poor recharge conditions and the strata bear a high content of salts. Owing to long-term leaching, the salinity of the groundwater in the water-bearing formations in the Huanhe and Luohe formations in the basin (except the eastern and southwestern margins of the basin) is mostly 1–3 g/L and is up to a maximum of 5‒10 g/L. However, Sr-rich fresh water occurs in local areas in the basin (Li HX et al., 2021). The TDS of the deep groundwater is greater than 1 g/L in the Carbonaceous-Jurassic water-bearing formations in the eastern Ziwu Ridge except for weathering zones, where fresh water is distributed due to intense water exchange and circulation. Moreover, the fluorine content in groundwater is above the standard limit in the Ningnan hilly area and the eastern Guanzhong Basin.

3.4   Storage space and migration pathways of fresh groundwater in loess featuring dual voids, vertical multilayers, and variable structure

The voids in the loess aquifers can be divided into two types according to their hydraulic characteristics (Fig. 4), namely pores (including micro-fissures) and fissures (including holes). The pores are similar to aquifers of loose sands and sand gravels and feature large quantities, even development, poor water conductivity, and large storage space. In contrast, the fissures are similar to karst aquifers and are quite contrary to the pores. The pores and fissures mostly alternately occur in different layers. In detail, the pores mainly occur in coarse-grained loess layers with a loose structure, while the fissures roughly exist in fine-grained loess layers and paleosol layers. Owing to the special sedimentary environment, the loess aquifers consist of multiple superimposed soil layers with different water-bearing properties in the vertical direction. During the groundwater recovery using water wells, tiny particles in fine-grained loess layers will be extracted together, thus changing the structure and permeability of the loess surrounding the wells. The nearer to the wellholes, the more damage to the loess structure and the larger the changes in permeability coefficient and specific yield. The influencing scope is generally greater than 100 m around the wells (Yan TB et al., 1983).

Figure  4.  Conceptual model for the two-component recharge processes (modified from Huang YN et al., 2019). P–precipitation; E–evaporation; T–plant transpiration.

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3.5   Fresh groundwater in loess suffering poor renewability and complex recharge processes with distinct spatial differences

Loess tablelands mostly occur in the form of tableland groups due to the cutting of gullies and valleys. However, there is no hydraulic connection between adjacent loess tablelands and thus a separate loess tableland can be deemed to be a hydrogeological unit. Loess aquifers overlie dense and hard Lower Pleistocene loess and Neogene mudstone aquicludes with poor permeability, which cut off the hydraulic connections between loess aquifers and bedrock aquifers. Therefore, the leaking recharge from confined water in bedrocks is unavailable. In terms of water level, the water level of groundwater in loess is generally higher than that of the surrounding river valleys. In this case, it is impossible for river water to recharge the water in loess. In terms of the irrigation of farmland, agricultural irrigation only occurs in a very small scope in loess tablelands. Therefore, it can be inferred from the above analyses that the atmosphere precipitation is the major recharge source of fresh groundwater in the Loess Plateau and is even the only recharge source in most areas. The content of hydrogen and oxygen isotopes in the groundwater is basically distributed along the local meteoric water line (LMWL) but lower than that in modern regional average multiyear precipitation (Wan H and Liu WG, 2016). The corrected age range of ¹⁴C in the groundwater is 136–23412 years and the groundwater bears no tritium (Huang TM and Pang ZH, 2011), indicating that the groundwater in loess mainly originated from atmospheric precipitation hundreds of years ago. Furthermore, as estimated according to unsaturated zone thickness and soil moisture following the law of mass conservation of chlorine elements, the annual groundwater recharge from precipitation in the Dongzhi and Luochuan loess tablelands was calculated to be 55‒71 mm and 36‒67 mm, respectively, for which it takes at least 130 years to penetrate the hugely thick unsaturated zone to recharge the groundwater. This is another evidence that fresh groundwater in the loess originated from paleo-precipitation and early precipitation and suffers poor renewability.

The process that atmospheric precipitation recharges groundwater after penetrating a hugely thick unsaturated zone is affected by multiple factors such as precipitation amount and intensity, surface vegetation, grain size of loess particles, and voids (pores, fissures, and cavities). The recharge process involves water exchanges at soil-air, soil-root, and water-soil interfaces. It is extremely complex due to the multiple-layer and dual-void characteristics of the aquifers. Up to now, there are still different opinions on the recharge mechanism, such as piston flow and preferential flow (Xu XX et al., 2010; Li P et al., 2013; Zhang CL et al., 2014). With an increase in depth, the grain size, pore size, and pore and cavity number of the loess strata decrease while the density of the loess strata increases. Driven by both the hydro-mechanical coupling and historical sedimentary effects, primary vertical joints are repeatedly formed and degenerates and all the loess layers and paleosol layers have undergone the evolutionary process of vertical joints. All these have resulted in notable anisotropy of the groundwater recharge process (Li F et al., 2021).

4.   Distribution pattern and formation mechanisms of fresh groundwater in the Loess Plateau

The hydrochemical spatial distribution of groundwater is generally controlled by natural factors. In detail, various hydrochemical constituents in groundwater originate from the dissolution or ion exchange and adsorption of minerals in strata, the spatial distribution pattern of groundwater is controlled by hydrodynamic conditions, and the accumulation of groundwater is determined by intense evaporation. According to the spatial distribution pattern of the fresh water in the study area, the paper summarizes the factors controlling the formation of the fresh groundwater in loess tablelands from the aspects of lithofacies paleogeographical characteristics, salt sources, hydrodynamic conditions, and the evaporation and transpiration of precipitation, thus revealing the formation mechanisms of the fresh groundwater in the study area.

4.1   Types and distribution pattern of fresh groundwater in the Loess Plateau

The fresh groundwater in the Loess Plateau can be divided into five types according to landform units and the void media of aquifers, namely the fissure fresh water in valley basins, piedmont diluvial fans, and loess; the fissure fresh water in bedrocks, and the karst fissure fresh water in karst areas. The distribution of regional fresh groundwater is affected by factors such as precipitation, landform, soluble salt content in strata, and recharge/runoff/discharge conditions. For the same type of groundwater, it is fresh water in recharge areas and its TDS increases in general as it approaches discharge areas. Meanwhile, different types of fresh groundwater resources show distinct distribution patterns (Table 2 and Table 3).

Table  3.  Character of different groundwater types in the Loess Plateau.

Type	Aquifer lithology and character	Fresh groundwater distribution pattern	Mainly distributed region
Fissure fresh water in valley basin and piedmont diluvial fan	Sand and Sand gravel, spatial variation of aquifer thickness, lack of stable impermeable layer in region	Mainly affected by leaching and evaporative concentration, TDS distribution showed horizontal zoning pattern , TDS less than 0.5 g/L in recharge area, TDS is 0.5–1 g/L in runoff area generally, and locally near retention areas TDS is 1–3 g/L; in retention-Discharge area, TDS is more than 3–5 g/L and the maximum value may exceed 22g/L	Fresh groundwater of exploitation value distributed widely, mainly located in Fenwei Basin, Yiluoqin Basin
Loess fissure fresh water	Loess and fossil soil, poor water Abundance, thickness of unsaturated layer is more than 30m	Mainly affected by leaching, TDS generally less than 0.3 g/L, sporadic areas is 1~3g/L	Widely distributed loess area with annual rainfall over 400 mm, the area with exploitation value is small, mainly in loess tableland such as Dongzhi Tableland, Luochuan Tableland loess tableland on both side of Fenwei Basin
karst fissure fresh water	Limestone, spatial variation of aquifer water abundance	Mainly affected by leaching, TDS distribution showed horizontal zoning pattern, TDS less than 0.5 g/L in recharge area, TDS is mainly 0.5–1 g/L in runoff area, near discharge area is 1–1.5 g/L, in retention-discharge area, TDS is 2–5 g/L and the maximum value may be more than 10 g/L	Strip distribution, mainly distributed in the east, south and west edge of Ordos Basin, freshwater mainly distributed in limestone exposed and shallow buried area
Bedrock fissure fresh water	sandstone, water abundance is generally poor	Mainly affected by leaching and evaporative concentration, TDS is less than 0.5 g/L in recharge-runoff area and 0.5–1 g/L in discharge area	Structural uplift bedrock mountain, bare rock area, distribution area is small

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The fissure fresh water in valley basins and piedmont diluvial fans is widely distributed in Quaternary aquifers of sands and sand gravels in river valleys and piedmont diluvial-diluvial deposits. Owing to the absence of stable aquicludes, the distribution characteristics of TDS in phreatic water are consistent with those in confined water. The distribution of the two types of fresh groundwater is mainly affected by leaching and evaporative concentration. The TDS of the fissure fresh water gradually increases and shows a horizontal zoning pattern from piedmont recharge areas to retention and discharge areas as the circulation and exchange of groundwater weaken. As a result, the fissure fresh water is brackish and saline water in or near retention areas and is fresh water in other areas in general. Additionally, due to the periodic recharge and dilution of river water, banded fresh groundwater resources are distributed along rivers on the front margin of the first-order terraces of river valleys and washland areas.

The pore fresh water in loess is regionally distributed. Owing to the burial depth generally higher than 20 m, the pore phreatic water in loess is roughly not affected by evaporative concentration. The groundwater is generally high-quality fresh water in loess hills (tablelands, gullies, and knoll ridges) with annual precipitation of greater than 400 m and loess terraces on both sides of the Fenwei Basin. However, the fresh groundwater in loess of exploitation value is mainly distributed in loess tablelands, with TDS of less than 0.3 g/L generally. The TDS of the pore fresh water in loess slightly but irregularly varies along runoff routes.

The karst fissure fresh water shows a distinct zoning pattern from recharge areas to discharge areas. Its water quality is mainly related to rock leaching during groundwater migration, as well as the water quality of recharge sources. The TDS of it gradually increases along the runoff routes of groundwater and it is fresh water in both recharge areas and runoff areas in general.

The fissure fresh water in bedrocks is mainly distributed in uplifted bedrock mountainous areas, which feature exposed rocks, fissure developing, relatively sufficient annual rainfall, favorable recharge/runoff/ discharge conditions of groundwater, and active water circulation and exchange. The recharge areas are characterized by shallow groundwater and intense water exchange. As a result, various ions in the recharge areas have a low content and are liable to migrate and thus the groundwater in the areas is fresh water (TDS < 0.5 g/L). The groundwater in runoff areas is still fresh water although it is prone to be polluted and has slightly high TDS. As for the groundwater in discharge areas, various ions dissolve in the groundwater and TDS is 0.5–1 g/L generally and is greater than 1 g/L locally. Overall, all the groundwater in the uplifted bedrock mountainous areas is fresh water. The clastic-rock fissure water underlying loess suffers poor recharge/runoff/discharge conditions, inactive water circulation and exchange, and long-term leaching. Therefore, it is brackish and salt water. However, high-quality Sr-rich fresh water resources have been discovered in the desert-facies Luohe Formation through sampling using the layered pumping technique.

4.2   Influencing factors in the formation of fresh groundwater in the Loess Plateau

4.2.1   Lithofacies palaeogeographic characteristics of loess tablelands

The regional lithofacies palaeogeographic conditions are the most important factors controlling the hydrochemical composition of groundwater and the distribution and evolution of groundwater quality. Meanwhile, the lithofacies palaeogeographic conditions such as the formation and distribution of paleoclimate, paleoterrain, sedimentary facies, and palaeohydrology during the formation of aquifers have a great impact on the groundwater circulation, hydrochemical characteristics of deposited water bodies, and salt content in strata (Xie Y et al., 2004).

The landform morphology of loess tablelands, loess ridges, and loess knolls in the Loess Plateau reflects the morphology of bedrock paleaoterrain (Fig. 5). In detail, the bedrock paleaoterrain in loess tablelands is relatively flat, and that in loess ridge and knoll area is mostly in the shape of drainage divides (Liu DS, 1964; Xiong LY et al., 2016). The difference in paleaoterrain serves as basic terrain conditions for the fact the water yield property of loess tablelands is higher than that of loess ridge and knoll area. The Quaternary loose strata in loess tablelands were formed from the vertical superimposition of multiple alternate loess and paleosol layers. Loess was generally formed from accumulation in a cold and dry climate, while paleosol was formed in a warm and humid climate (Ji JF et al., 1997). The cyclic changes and fluctuations of the paleoclimate caused notable differences in the leaching and convergence of chemical elements and the distribution of voids between loess and paleosol. There are close hydraulic connections between loess and paleosol despite their different water-bearing characteristics. Loess layers mainly retain water in pores, while paleosol mainly conducts water through fissures. Loess-paleosol layers are a set of deposits of silty clay and clayey silty sands accumulated through eolian processes in a weak alkaline - alkaline environment and an arid - semi-arid climate. Some of them have experienced frequent actions of groundwater. The loess and paleosol are mainly composed of SiO₂, Al₂O₃, and CaO and are rich in carbonate dominated by calcite, with an average content of soluble salts of 0.25%‒0.65% (Wen QZ, 1989). Therefore, the comparatively flat paleoterrain, strata with a low content of soluble salts, relatively active groundwater circulation and exchange, as well as the alternate cold and dry climate and warm climate have created a favorable environment for the formation of regional fresh groundwater.

Figure  5.  Typical landforms in the Loess Plateau.

a–the loess tableland was strongly cut by running water, with a wide top surface and flat terrain. b–the loess ridge is a narrow flat-top mountain beam formed by the erosion of the Loess tableland. c–the loess knoll is steamed bread shaped loess hills formed by erosion.

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4.2.2   Salt sources of groundwater

The TDS is an indicator that reflects the overall salinity of groundwater and is used to indicate the saltiness of groundwater. Naturally, the formation mechanisms of hydrochemical constituents in groundwater include leaching (weathering of carbonates, silicates, and evaporites) and evaporative concentration (Wang YS et al., 2019). Salts in the groundwater in loess mainly originate from recharge water and the rock and soil masses in contact with water during water migration. The leaching in unsaturated zones, lixiviation in aquifers, and water-rock interactions can dissolve the soluble minerals and chemical constituents in rock and soil masses and thus change the chemical constituents in the groundwater. Atmospheric precipitation is the most important source of groundwater in loess. However, the content of all the ions in rainwater is lower than 10 mg/L and the salt content in rainwater can be neglected due to the very low TDS. This indicates that salts in the groundwater in loess mainly originate from the rock and soil masses in contact with water. Therefore, the content of soluble salts in unsaturated zones and aquifers is closely related to the TDS distribution of the groundwater in loess.

According to the water quality data of sampling points in the national groundwater monitoring project in 2019 of China, the salts in the groundwater in the Dongzhi and Luochuan loess tablelands are as follows. The cations mainly include Ca²⁺, followed by Mg²⁺ and Na⁺, the anions are dominated by HCO₃⁻, and the hydrochemical types mainly include HCO₃⁻ Ca·Mg. The water samples mainly fall in the weathering zones of rocks in the Gibbs diagram of water samples (Fig. 6). This indicates that the hydrochemical composition of the groundwater in the tablelands is mainly controlled by rock weathering and is slightly affected by precipitation, evaporative concentration, and human activities. Meanwhile, the water samples mainly fall in the middle-upper area in the correlation map of Mg²⁺/ Na⁺ vs. HCO₃⁻/ Na⁺ vs. Ca²⁺/Na⁺, indicates that the chemical constituents in the groundwater in the tablelands are mainly attributable to carbonates and silicates (Fig. 7).

Figure  6.  Gibbs graphs of loess tableland groundwater.

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Figure  7.  Relationship between Mg²⁺/Na⁺, Ca²⁺/Na⁺ and HCO₃^－/Na⁺ and Ca²⁺/Na⁺ of groundwater in loess tableland area.

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As for the unsaturated zones and aquifers in the Dongzhi Loess Tableland, the minerals mainly include quartz, potash feldspars, albites, and micas, all of which account for more than 80%, and the content of soluble salts is low and is 433–894 mg/kg. Under natural conditions, HCO₃⁻ in groundwater is sourced from the dissolution of carbonates, Cl⁻ and SO₄²⁻ from the dissolution of evaporites, Ca²⁺ and Mg²⁺ mainly from the dissolution of evaporites, carbonates, and silicates, and Na⁺ and K⁺ mainly from the weathering of evaporites and silicates. The c(Na⁺)/c(Cl⁻) ratio in the Dongzhi and Luochuan loess tablelands ranges from 1.5 to 11.5, with an average of 5.4, which is much higher than that of rainwater, indicating that the Na⁺ in groundwater in the tablelands also originated from the weathering of albites and silicates in addition to the dissolution of rock salts (Ling XY et al., 2021; Pan F et al., 2014). The high content of CaO in loess results in the enrichment of Ca²⁺ in the groundwater. In addition, the loess is relatively rich in montmorillonites, kaolinites, and illite clay minerals, which provides favorable conditions for cation exchange between Na⁺ and K⁺ in groundwater and Ca²⁺ and Mg²⁺ on the surface of aquifer particles, thus leading to an increase in the Ca²⁺ content in water (Li Z et al., 2019).

To sum up, the salts in the groundwater in the loess are mainly the result of the weathering of carbonates and silicates in unsaturated zones and aquifers during groundwater runoff and are slightly affected by human activities and atmospheric precipitation.

4.2.3   Hydrodynamic conditions

Hydrodynamic conditions play an important role in driving the spatial distribution of hydrochemical constituents in groundwater. Gullies have developed around the loess tablelands and most of them cut through loess layers. Meanwhile, the hard and dense Wucheng loess with poor permeability underlies the loess aquifers. Therefore, each loess tableland can be regarded as a single hydrogeological unit with independent recharge, runoff, and discharge conditions. Under natural conditions, the groundwater level in the central part of a tableland is higher than that around. It dispersedly flows outward in all directions in a planar shape and discharges into valleys in the form of springs (Fig. 8).

Figure  8.  Recharge, runoff and discharge conditions of loess groundwater in Dongzhi Tableland.

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In terms of the dynamic trend of groundwater level, the phreatic water in loess fluctuates in a small range annually and does not show distinct recharge and non-recharge periods. This indicates that the unsaturated zones can temporarily store and regulate rainfall infiltration, thus leading to roughly continuous and uniform groundwater recharge through precipitation. Therefore, the recharge amount is mainly affected by multiyear precipitation amount and is not closely related to the precipitation amount in a single year. In terms of runoff, the loess tablelands feature poor groundwater circulation conditions in general. The average horizontal permeability coefficient of loess aquifers is < 0.16 m/d, and the hydraulic gradient in the center and marginal areas of loess tablelands are 0.85%–3.3% and 4%–7%, respectively. However, the ratio of vertical permeability coefficient to horizontal permeability coefficient is quite different for the loess aquifers in different areas. For example, the horizontal permeability coefficient is 2‒5 times the vertical permeability coefficient in the Dongzhi Loess Tableland, while the vertical permeability coefficient is about six times the horizontal permeability coefficient in the Luochuan Loess Tableland. This indicates that the dominant migration direction of groundwater in loess is significantly different in different areas. In terms of discharge, the discharge springs are mainly in the form of planar flow and strand flow. Meanwhile, the spring flow rate is roughly stable and the positions of spring opening do not vary with seasons.

The unsaturated zone in loess can generally be divided into four zones from top to bottom according to the characteristics of groundwater migration in loess tablelands, namely the active exchange zone, slow exchange zone, moisture transfer zone, and capillary water zone. The active exchanges zone generally falls within the influencing scope of precipitation and evapotranspiration. It features loose soil masses, the development of plant root systems, strong water absorption capacity, and fast water migration. The slow exchange zone serves as the major regulation zones of groundwater recharge in loess. Owing to the effects of vegetation transpiration, it is 6‒7 m generally and 10–18 m in areas with deep-rooted vegetation in depth. The humidity of this zone increases by about 1%‒2% after precipitation, and the water in it moves downward in the form of gravitational water and capillary water. The moisture transfer zone features a roughly stable water content of 0.5% in general, and water in the zone moves downward in the form of hygroscopic water and gravitational water. The capillary water zone is 4–5 m higher than the phreatic water surface and its humidity basically remains unchanged. Through this zone, the water in unsaturated zones recharges the saturated zone in the form of stable and continuous runoff.

In general, the groundwater in loess suffers poor circulation and exchange conditions, and it is an extremely complex and slow process for precipitation to permeate unsaturated zones to recharge groundwater, during which salts gradually accumulate in water.

4.2.4   Precipitation infiltration and evapotranspiration

Huang TM et al. (2020) established the distribution map of the Cl⁻ content in soil moisture at different depths in the unsaturated zone discovered in boreholes A and B in the Luochuan Loess Tableland. In detail, the Cl⁻ content in soil moisture reaches its peak value at a burial depth of 4 m (Fig. 9), which is inferred to be affected by farmland fertilization in the 1990s (Yang S et al., 2016). It tends to increase at a burial depth of 10–18 m and decrease at a burial depth of 18–30 m. Then it is roughly stable near the phreatic surface and is roughly the same as the Cl⁻ content in groundwater. Atmospheric precipitation is the major source of groundwater in loess tablelands, with an infiltration coefficient of 0.06–0.11. Meanwhile, the groundwater level in loess tablelands is generally greater than 30 m. According to the vertical distribution pattern of the Cl⁻ content in soil moisture and groundwater as well as the recharge mode of the groundwater in loess, it can be determined that evaporation has a small effect on the enrichment of salts in the groundwater. In contrast, the transpiration in plants increases the salt content in soil moisture to a certain extent, and the recharge through preferential flow formed by vertical fissures plays a certain role in diluting the salts in the groundwater. In addition, human activities may increase the salt content in the groundwater.

Figure  9.  Chloride distribution in soil water and groundwater (data source: Huang TM, 2020).

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4.3   Formation and evolution mechanisms of fresh groundwater

As stated above, the loess tablelands have thick unsaturated zones, and the groundwater in the loess tablelands is mainly recharged through precipitation infiltration and discharges in the form of lateral spring runoff under natural conditions. Owing to the loose structure, developed pores, and high permeability of surface loess, precipitation can be quickly absorbed in the surface loess after dropping on the ground. However, hugely thick unsaturated zones slow down water infiltration and block the evaporative concentration of groundwater. In this case, the dynamic changes of the groundwater in the loess tablelands are mainly affected by multiyear precipitation amount. The precipitation is subject to the transpiration in plants during its migration in the unsaturated zones, leading to an increase in the groundwater salinity in a certain depth range. Meanwhile, the recharge from the preferential flow of precipitation dilutes the salts in groundwater. The low content of soluble salts in loess strata and the weak water-rock interactions result in a weak supply of chemical substances to groundwater. Meanwhile, the unsaturated zones suppress the evaporative concentration of groundwater and there is large water storage space in aquifers. All these provide favorable conditions for the formation of fresh groundwater in the loess tablelands.

5.   Impacts of climate change and human activities on fresh groundwater resources

Deeply affected by the East Asian monsoon, the Loess Plateau is featured by high rainfall variability and the precipitation within a year is temporally concentrated and is dominated by rainstorms. Moreover, the water resources in the Loess Plateau are extremely sensitive to climate change and human activities due to irrational human disturbance and loose loess. Over the past 40 years, the soil moisture in the Loess Plateau has been in a negative balance state, the soil drying has accelerated, and the distribution range of dry layers has continuously expanded due to the long-term joint effects of decreased precipitation, strong transpiration in plants, and intensive human disturbance (Wang Y et al., 2021). The impacts of climate change and human activities on fresh groundwater resources are mainly reflected in the changes in the dynamic field and resource quantity of groundwater.

5.1   Temporal-spatial evolution of the dynamic field of groundwater

The groundwater in the Loess Plateau has shown a downward trend overall and obvious spatial heterogeneity. In the Dongzhi Loess Tableland—a typical loess tableland in the Loess Plateau, Qingyang City suffered long-term overexploitation of groundwater (Fig. 10). As a result, a groundwater funnel area with the city as the center has been formed and the groundwater that originally flowed toward the tableland edges has started to flow toward the funnel center. In addition, the soil drying has accelerated and the slow exchange zone thickened due to the increase in deep-rooted fruit trees. In this case, the vertical fissures originally conducting water have started to block water, thus changing the recharge speed of groundwater from vertical infiltration. Compared to loess tablelands, the valleys and basins have been exposed to greater impacts of human activities and the changes in groundwater have shown more notable spatial heterogeneity. For example, the groundwater level in the Guanzhong Basin has shown a downward trend overall since the 1980s, which can be divided into the following stages. During 1982‒1989, the groundwater level showed an upward trend overall due to the increase in precipitation amount. During 1990–1995, the groundwater level showed a continuous and slow downward trend overall with a decrease in precipitation amount. During 1996‒2003, the groundwater level showed a continuous and sharp downward trend as the precipitation amount substantially decreased and groundwater exploitation quantity increased due to the population growth and the acceleration of urban development in the Guanzhong area. During 2004‒2008, the groundwater level showed a slow upward trend with a slight increase in the precipitation amount (Li Q, 2015). Furthermore, the decline range of groundwater differed in different areas in the Guanzhong Basin, exhibiting spatial heterogeneity. The decline rate of the groundwater level on the north bank of the Weihe River is in the order of loess tablelands > diluvial fan area > river terrace area, and the groundwater level on the south bank of the Weihe River has also shown a downward trend (Zheng YX, 2013). The groundwater level in the Guanzhong Plain has mainly declined in Zhouzhi, Huxian, and southern Chang’an areas, which was mainly caused by farmland irrigation. The groundwater level in the southwestern suburb of Xi’an City is declining currently due to excessive exploitation and reduced recharge sources. In contrast, the water level in the southern and western suburbs of Xi’an City and in Xianyang City is rising, which is just the remedy of the funnel area formed due to decades of groundwater exploitation (Tao H et al., 2013).

Figure  10.  Variance of groundwater table in Dongzhi Tableland between 1981 and 2020.

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5.2   Impacts of human activities on fresh water resources

The impacts of human activities on fresh groundwater resources in the Loess Plateau mainly include the influence of groundwater exploitation and the change in underlying surfaces on infiltration and evaporation.

Groundwater exploitation will significantly reduce the resource quantity of fresh groundwater, especially in loess tablelands, where the groundwater mainly consists of ancient water and early precipitation. The shallow groundwater in the Dongzhi Loess Tableland is mainly recharged from atmospheric precipitation. The groundwater exploitation quantity has been much higher than the resource quantity of renewable groundwater in the loess tableland in the past 30 years. Besides, a large area of farmland has been converted into orchards in the loess tableland, leading to the decline in groundwater recharge. As a result, the groundwater level in the loess tableland has substantially dropped, the springs on the edge of the tableland have successive dried up, and the groundwater in the urban area of Xifeng City has been almost exhausted. During 1981–2020, the groundwater level in the Dongzhi Loess Tableland decreased across the entire region. It increased by more than 15 m on average and even more than 25 m in major exploitation areas. It showed a quick downward trend during 2004–2014 and has been roughly stable under the control of groundwater exploitation since 2014 (Fig. 10). According to Fig. 11, the changes in the groundwater level in the Dongzhi Loess Tableland were significantly affected by the changes in exploitation quantity. The groundwater exploitation quantity was 4.156 ×10⁶ m³/a in 1976 and reached 30.79 ×10⁶ m³/a around 2000. Afterward, the groundwater level has roughly remained stable with a steady decrease in the groundwater exploitation quantity. During 1984‒2008, the phreatic surface in the Guanzhong Basin showed a downward trend overall, and the coefficient of correlation between phreatic surface and the exploitation quantity and that between phreatic surface and precipitation amount were both greater than 0.6 in Baoji, Xi’an, Weinan, and Xianyang cities. This indicates that precipitation and exploitation quantity are the major factors causing the drop in the groundwater level in the Guanzhong Basin. High groundwater exploitation quality is the major factor in the decline of the groundwater level and especially in the dynamic trends of the groundwater in Baoji, Weinan, and Xi’an areas. Therefore, the ultimate approach to suppressing the decline in the groundwater level in these areas is to reduce the groundwater exploitation quality (Zheng XY et al., 2013). Owing to long-term overexploitation of groundwater in karst aquifers, the regional groundwater level in the Weibei area has decreased by about 20 m, the Yuanjiapo Spring in the area has dried up, and the recharge-discharge relationships between the groundwater and lakes in the area have been changed.

Figure  11.  Variance of groundwater table, exploration and precipitation in Dongzhi Tableland.

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The changes in land use will change the underlying surface and then affect the recharge of groundwater. Li Z et al. (2019b) studied the changes in isotopic composition, runoff-groundwater connectivity, and groundwater sources in five major river basins in the Loess Plateau and confirmed that the groundwater in the Loess Plateau originated from precipitation recharge. Huang TM et al. (2013, 2020) assessed the effects of different types of vegetation on groundwater recharge according to the accumulation of chlorine element in unsaturated zones in loess and confirmed that groundwater in the Xifeng Loess Tableland was mainly recharged from thick unsaturated zones in a continuous and dispersed manner. Vegetation changes such as land reclamation, returning farmland to grass, plantation of fruit trees, and vegetation restoration will reduce groundwater recharge to varying degrees. The annual precipitation of 600 mm may be the critical precipitation amount for whether planting fruit trees reduce the groundwater recharge in the Loess Plateau. There are two groundwater recharge mechanisms in loess, namely uniform infiltration (piston flow) and the preferential flow of fissures. The infiltration rate in different underlying surfaces affects the resource quantity of water in deep soil and shallow groundwater. Gates JB et al. (2011) assessed the impacts of soil and water conservation on groundwater recharge in the Ansai area according to the mass balance of chloride ions. They found that the infiltration coefficient of precipitation in farmland with shallow-rooted vegetation was 0.11‒0.18 and the groundwater recharge in areas with deep-rooted vegetation such as arbors and shrubs was zero in the past 20–30 years. According to the research conducted by Wang R et al. (2014) in the Changwu Loess Tableland, Shaanxi Province, the average water content in 20 m deep soil layers is in the order of waterlogged pools > bare land > wheat field > 12-year-old orchards > 20-year-old orchards and the water content of them is 21.41%, 19.71%, 18.55%, 17.92%‒17.84%, and 15.42%‒14.07%, respectively.

In addition to water quantity, human activities may also affect water quality and the examples are as follows. In the Haiyuan faulted basin at the northern foot of the Huashan Mountain in the southern Ningnan area, the regional groundwater level has significantly dropped with the water exploitation in the water source field. This has resulted in the changes in the local flow field and the recharge by the deep groundwater with high salinity, thus narrowing down the distribution space of fresh water. In the Lanzhou faulted basin along the main stream of the Yellow River, the groundwater level has risen due to the irrigation using diverted water from the Yellow River, urban pipeline seepage, sewage discharge, and the reduction in groundwater exploitation quantity. This has further intensified the leaching of soluble salts in upper soil layers and evaporative concentration and led to a significant increase in the TDS of the groundwater (Zhu L et al., 2020). In the central part of the Loess Plateau, the exploitation of mineral resources has led to an increase in SO₄²⁻ content and TDS in water, thus inducing the deterioration of water quality (Wang ZH et al., 2020; Liu X et al., 2021). Overall, the changes in underlying surfaces, human activities, and climate change directly or indirectly affect the quantity and quality of groundwater by changing the recharge and discharge of groundwater.

5.3   Impacts of climate change on fresh groundwater resources

Although human activities have a huge impact on fresh groundwater resources, climate change has a greater impact on the resource quantity of fresh groundwater in the long run. The annual precipitation in the Loess Plateau has shown a non-significant upward trend in the past 20 years, especially in the northern part of the study area. Hu PF et al. (2019) studied the terrestrial water storage in the Loess Plateau using the data from gravity satellites and found that the terrestrial water storage in the Loess Plateau showed a downward trend during 2003–2015, with a decreasing rate of −5.16 mm/a. Spatially, the terrestrial water storage reduced from west to east and overall. The total water-soluble carbohydrates (TWSC) in the Loess Plateau is affected by the climate systems in the Pacific Ocean and the South China Sea, and the coefficient of the correlation between the TWSC and precipitation decreases from southwest to northeast, with a maximum of up to 0.48. The change in the water storage in the Loess Plateau in the southern parts of Gansu and Shaanxi provinces is significantly affected by precipitation. The coefficient of correlation between the TWSC in the Loess Plateau and the surface temperature decreases from west to east, and the changes in the water storage in the Loess Plateau in the southern part of Shanxi Province and Henan Province are greatly affected by the surface temperature. Human activities have a great impact on the TWSC in Shanxi Province and Shaanxi-Shanxi-Henan border areas in the eastern and southeastern parts of the Loess Plateau, respectively. According to Zheng XY (2013), the changes in the precipitation amount serve as a more important factor in the groundwater level in the Guanzhong Basin in the long run, and the water level can be restored as long as the precipitation amount tends to increase. As indicated by Hu W et al. (2019), the average annual groundwater recharge in the Loess Plateau decreases from southeast (18.8 cm, accounting for 26% of precipitation amount) to northwest (0 cm), with an average of 1.8 cm and accounting for 4.1% of annual precipitation on average. The spatial distribution of groundwater recharge is mainly affected by precipitation. In contrast, the temporal changes in groundwater recharge are affected by both potential evapotranspiration and leaf area index. That is, they are affected by the dual effects of climate change and human activities, but climate change has a greater impact. Owing to global warming and a rise in precipitation, the groundwater recharge in the Loess Plateau will decrease by 75%–80% by 2080 compared with 2010. This may accelerate the exhausting of the soil reservoirs and aquifers in the Loess Plateau and threaten the ecological and social stability of the Loess Plateau.

6.   Conclusions

(i) The groundwater resources in the Loess Plateau feature uneven temporal-spatial distribution. They show interannual and seasonal differences and their modulus gradually increases from north to south. Meanwhile, the shallow parts in loess hills with annual precipitation of less than 300 mm roughly bear no water. The fresh groundwater in the Loess Plateau can be divided into five types according to landforms and the void media of aquifers, namely the fissure fresh water in valley basins, piedmont diluvial fans, and loess in loess tablelands and loess terraces; the fissure fresh water in bedrocks, and karst fissure fresh water in karst areas.

(ii) The fresh groundwater in the Loess Plateau is widely distributed in the shallow parts in areas with annual precipitation of greater than 400 mm. However, the fresh groundwater of exploitation value is mainly distributed in valley basins, loess tablelands, piedmont diluvial fans, and karst areas, accounting for only 25% of all distribution areas of fresh groundwater in the plateau. The hydrochemical types are dominated by HCO₃. For the same type of groundwater, the hydrochemical types show the zoning evolutionary pattern of HCO₃ → HCO₃·SO₄ → Cl·SO₄ from recharge areas to discharge areas in most areas in the Loess Plateau.

(iii) The groundwater in the loess tablelands suffers slow groundwater circulation and exchange due to the hugely thick unsaturated zone and the multiple layers, dual void media, and variable structure of aquifers. However, the relatively flat paleoterrain, low content of soluble salts in strata, dual recharge consisting of piston flow and preferential flow, and little evaporative concentration provide a favorable environment for the formation of fresh groundwater. The groundwater in the loess tablelands mainly originates from ancient water or early precipitation.

(iv) The groundwater resources in the Loess Plateau are sensitive to climate change and human activities. Owing to the generally low precipitation amount, human exploitation, and changes in land use types, the total resource quantity of groundwater in the Loess Plateau tends to decrease, the soil moisture has been in a negative balance state in many areas, and dry layers tend to expand. Furthermore, groundwater funnel areas have formed in local areas due to overexploitation, leading to the changes in the recharge-discharge relationships between surface water and groundwater. Human activities have a significant impact on the resource quantity of groundwater but slightly affect the water quality. In the long run, in the Loess Plateau faces the threat of the exhausting of soil reservoirs and aquifers.

CRediT authorship contribution statement

Hai-xue Li conceived the presented idea and prepared the manuscript. Hai-xue Li, Xi Wu, Wei-po Liu, Tao Ma, Meng-nan Zhang, Yu-tao Wei, Fu-qiang Yuan, Lei Yuan, Fu-cheng Li, Bin Wu, Yu-shan Wang, Min-min Zhao, Han-wen Yang and Shi-bo Wei drew all the figures. Shuang-bao Han and Sai Wang supervised the findings of this work. All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgment

This work was funded by the project of China Geological Survey (DD20190333, DD20211563) and the National Youth Science Foundation (41702280).