ZHANG Guang-hui, FEI Yuhong , NIE Zhen-long, SHEN Jian-mei, WANG Jin-zhe

（1-Hehai University, Nanjing, 210098；2-Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang, 050061, China）

Abstract: The study area is a drought region where is located in the northwest inland of China. On the field investigation with the environmental isotopic way and with Tamres and IAEA models, it is expounded that the groundwater recharge and renewable rate are related to the paleoclimatic variation and that the main recharge to groundwater occurred in the rainy stage respectively from 8,000 a BP to 5,000 a BP, 3,500 a BP to 2,500 a BP and last 1,000 years in the Heihe River Basin of the northwest internal region. The age of phreatic groundwater is younger and its renewable rate is quicker than that of confined groundwater, and the renewable rate in the eastern area of the middle reaches is quicker than that in the western area, in the fine soil zone slower than that in the Gobi zone down the Qilian Mountain and near to the riverbed quicker than that far from the riverbed. The precipitation of the Qilian Mountain is the main recharge resource of groundwater in the eastern area of the middle reaches and snowmelt and the fissure groundwater from the mountain area is the main recharge resource in the western area. Therefore, it is beneficial to groundwater resources sustainable utilization to observe the principle of water cycle together with the optimal adjustment of surface water.

Key words: Heihe River Basin; groundwater; isotopic characteristics; paleoclimate; renewable rate.

In this paper, it is expounded that the regional climatic change is the main factor to drive the evolution of groundwater recharge and its renewable rate in the Heihe River Basin of Northwest China on the environmental isotopic data and paleoclimatic research results.

The plain area of the Heihe River Basin in the northwest inland of China is a drought region where the recharge of groundwater from local precipitation accounts for less than 10 % of the annual total recharge. The groundwater recharge of the plain mainly depends on surface runoff from Qilian Mountain area, including glacier melted water and snowmelt water, precipitation in the mountain areas and fissure water, which is closely related to the climatic change. The isotopes in groundwater have recorded the response to the paleoclimatic variation (Zhang Guanghi, 2004).

Ding Yongjian (1999), Zhang Jie (2004) clarified the spatial-temporal variation characteristics of the precipitation in the Heihe River Basin since 1962. Chen Rensheng (2003) and Lang Yongchao (1999) expounded the process of surface runoff from the Qilian Mountain in the middle reaches of the Heihe River Basin. Wang Genxu (1998) revealed the relationship between the hydrological change and the ecological environment in the Heihe River Basin over past 50 years. Zhang Guanghui (2002, 2004a and 2004b) studied the process of the water circulation and the model of groundwater evolution in the Heihe River Basin. Wu Xuanmin (2003) analyzed the groundwater recharge in the lower region of the Heihe River Basin with the environmental isotopic way. Shi Yafeng (1995) researched the climatic change influence on water resources in northwest China and rebuilt the evolution process of the paleoclimate over past 5,000 years. The response of groundwater to the paleoclimatic variation in the Heihe River Basin is still a hotspot question.

1 Natural Environment and Recharge Condition

1.1 Natural environment

The south boundary of the Heihe River Basin is the Qilian Mountain, the north boundary borders Monolia, the east boundary borders the Shiyanghe River Basin and the west boundary borders the Sulehe River Basin. Its geographic coordinate is the north latitude 37°45¢~ 42°40¢N and the east longitude 97°05¢~102°00¢E with an area of 14.3´10⁴ km²，including the plain area of 5´10⁴ km²（Fig.1）.

The Qilian Mountain, where is upwards south of Yinglouxia stream measuring station，is the upper reaches in the Heihe River Basin with the altitude from 1,700 m to 5,564 m，the annual precipitation from 300 mm to 600 mm and the snowmelt water quantity about 4´10⁸ m³/a. The Qilian Mountain area is the main region of the recharge water sources to the plain in the Heihe River Basin. The middle reaches of the Heihe River Basin consisting of the Zhangye Basin and the Jiuquan Basin, is located between the Yinglouxia stream measuring station and Zhengyixia stream measuring station, with the altitude from 1,352 m to 1,700 m，the annual precipitation from 50 mm to 200 mm and the annual evaporation more than 2,050 mm. The lower reaches of Heihe River Basin where consisting of the Jinta-Dingxin Basin and the Ejina Basin, is upwards north of the Zhengyixia stream measuring station, with the altitude from 912 m to 1,249 m，the annual precipitation less than 50 mm (17 mm/a at least) and the annual evaporation more than 3,700 mm.

1.2 Water Cycle Characteristics and Recharge Condition

The upper reaches, the middle reaches and lower reaches are in series of the main stream of the Heihe River and the Beida River（Fig.1），which constitute of “river-groundwater” cycle system of the Heihe River Basin. From the upper reaches to the lower reaches of the Heihe River Basin, there are three transformation processes of between runoff water and groundwater (Zhang Guanghui et al., 2002). The transformation is more than 60% of total runoff quantity (Zhang Guanghui, 2003②) .

Fig.1 Distribution of the study area

in the Heihe River Basin, China

The profile of groundwater system in the middle reaches of the Heihe River Basin is shown as Fig.2. From the upper strip-zone to the lower strip-zone of the Zhangye Basin, the grit Gobi zone in single layer structure, the mutual layer zone of sandy clay- clay sand-grit in double layer structure and the mutual layer zone of clay-sand in multi-layer structure are distributed in turn. The section of groundwater system in the lower reaches of the Heihe River Basin is similar to that shown as Fig.2, and the only difference is that the Quaternary layer becomes thin, less than 400 m.

                                              1. sand and gravel; 2. clayey sand; 3. sand; 4. granite; 5. sandstone; 6. sandy shale;

                                              7.faultage; 8. number and depth (m) of borehole; 9. groundwater level

There are 26 main streams from the Qilian Mountain into the plain in the Heihe River Basin, including the main streams of the Heihe River, the Liyuan River and the Beidahe River, with each runoff more than 1,000´10⁴ m³/a. There are small rivers which the runoff is 2.4´10⁸ m³/a. Total average runoff of all rivers is 37.9´10⁸ m³/a over past 50 years. 70%~80% of which infiltrates into the Gobi zone（Fig.2）to recharge groundwater. The mean recharge was 34.7´10⁸ m³/a during the period from 1950s to 1960s and reduced to 26.6´10⁸ m³ in 1990s.

2 Method and Data

From 2000 to 2003, the data of the precipitation, snow melt water, river water and reservoir water, spring water and groundwater with different depths have been systematically obtained（Fig.3） and the dynamic variation of above all water was observed by every month. There are 258 groups of hydrogen and oxygen isotopes samples ( ¹⁸O, ²H or D and ³H) and 44 groups of carbon isotopes samples（¹⁴C and ¹³C）collected.

The quantity of the hydrogen and oxygen isotopes sample for measuring analysis is 1 L/group into the airproof glass bottle. The quantity of the carbon isotopes samples is 120 L/group with the NaOH and the BaCl₂ into the bottle forming the deposition of the BaCO₃, which the air temperature, the water temperature, the water pH value and the alkalinity were all measured on the spot. The ¹⁸O/¹⁶O data are from the balance measurement with CO₂ and the D/H data are from zinc deoxidize measurement, then they were measured by the mass spectrometer. Its standard is in SMOW and the analysis precision of δ¹⁸O is ±0.1‰ and of δ¹⁸D is ±1.0‰. The ³H data are from the electroanalysis for collection and measured by the coruscate appearance of the lowness fundus liquid，the unit is TU and the analysis precision is ±3TU. The ¹⁴C data are from the sample transforming into the benzene, then by the coruscate appearance of the lowness fundus liquid, the unit is PMC, and the analysis precision is ±0.3 PMC. The unit of ¹³C data is PDB and the analysis precision is ±0.2‰.

Fig.3 Distribution of sample places in the Heihe River Basin, China

1.mountain area; 2.river; 3.desert area; 4.observing station of isotopes; 5.city or county; 6.sampling place of confined groundwater; 7.number of sampling place; 8.sampling place of shallow groundwater; 9.sampling place of river; 10.sampling place of surface water and groundwater; 11.sampling place of ice-snow water; 12.sampling place of groundwater in divide systems; 13.sampling place of reservoir; 14 sampling place of spring

3 Isotopic Characteristics of Ground-water

3.1 In the Zhangye Basin

In the Gobi zone of the piedmont plain down the Qilian Mountain in the Zhangye Basin, the tritium (³H) value of phreatic groundwater ranges from 51.2 TU to 69.7 TU with the mean value 62 TU, and the carbon 14(¹⁴C) content is within the range from 90.5 PMC to 111.2 PMC. In the lower zone of the Zhangye Basin from Zhangye city down to the Zhengyixia stream measuring station, the ³H value of phreatic groundwater is within the range of 16.3 TU to 165.2TU with the mean value 70 TU. The ³H value is more than the mean value of the region because of the effect of the thermonuclear tests since 1952, and the ¹⁴C content is from 71.5 PMC to 114.0 PMC. The ³H mean value of confined groundwater in the Basin is 29 TU and the ¹⁴C content is within the range of 30.4 PMC to 84.9 PMC. The oxygen 18(δ¹⁸O) and the deuterium (δD or ²H) values of phreatic groundwater are all more than those of confined groundwater in the Zhangye Basin, as shown in Table 1. Theδ¹⁸O value of phreatic groundwater is from -9.5‰ to -7.9‰ with the mean value -8.4‰, and theδD value is from -54‰ to -49 ‰ with the mean -53‰ in the lower zone of the Zhangye Basin. Theδ¹⁸O value of phreatic groundwater is from -8.7‰ to -7.5‰ with the mean value -7.9‰, and theδD value is within the range from -57‰ to -47 ‰ with the mean -52‰ in the Gobi zone of the Zhangye Basin. The δ¹⁸O value is from -10.0 ‰ to -9.1‰ with the mean value -9.6‰，theδD value is from -59‰ to -51 ‰ with the mean value -56‰ in confined groundwater of the Zhangye Basin.

Table 1. Isotopic characteristics of groundwater in different zones of the Heihe River Basin

Notes: PG= Phreatic Groundwater, CG= Confined Groundwater.

Table 2. Isotopic content of groundwater with different depths in the Zhangye Basin of the Heihe River Basin

In the hydrogological profile of the Zhangye Basin，there are three groups of water-bearing formation with different depths in the Quaternary where the ³H, the ¹⁴C , the δ¹⁸O and the δD values are all decrease as the depth increases（Table 2），but only theδ¹³C value increases.

3.2 in the Jiuquan Basin

In the Gobi zone of the piedmont plain down the Qilian Mountain in the Jiuquan Basin, the ³H value of phreatic groundwater ranges from 25 TU to 105 TU with the mean value 49TU. In the lower zone of the Jiuquan Baisn, the ³H value of phreatic groundwater ranges from 4 TU to 12 TU with the mean value 8 TU, and the ¹⁴C content is from 55 PMC to 110 PMC. In confined groundwater of this lower zone, the ³H value is within the range of 2 TU to 10 TU with the mean value 5TU, and the ¹⁴C content is from 26 PMC to 94 PMC.

In phreatic groundwater of the Gobi zone, the Jiuquan Basin, the δ¹⁸O value is from -9.6 ‰ to -8.6‰ with the mean value -9.1‰, and the δD value ranges from -57‰ to -45‰ with the mean value -50‰. In phreatic groundwater of the lower zone, theδ¹⁸O value is from -9.7‰ to -8.8‰ with the mean value -9.2‰, and the δD value is within the range of -58 ‰ to -52‰ with the mean -55‰. In confined groundwater of the lower zone, the δ¹⁸O value is from -10‰ to -9.6‰ with the mean value -9.8‰, and the δD value is from -63‰ to -60 ‰ with the mean -62‰.

In the Jiuquan Basin, theδ¹⁸O and the δD of phreatic groundwater are all near to those of confined groundwater. The ³H value are all less and the SiO₂ content more in phreatic groundwater and confined groundwater, which is quite different from that of the Zhangye Basin（Table 1） because the groundwater recharge source of the Jiuquan Basin is different from that of the Zhangye Basin. In the Jiuquan Basin, the main recharge sources of groundwater are glacier-melt water, snow-melt water and fissure water. The recharge of glacier melt water and snow melt water is 30% and the recharge of fissure water is 53% and the recharge of the precipitation is 17%. In the Zhangye Basin, the recharge of precipitation is 52.4 %, the recharge of fissure water is 37.8 % and the recharge of glacier-melt water and snow-melt water is 9.8% (Zhang Guanghui et al., 2004).

3.3 in the Lower Reaches

In the Jinta area of the lower reaches, the Heihe River Basin, the ³H value of phreatic groundwater ranges from 10 TU to 39 TU with the mean value 25TU, and the ¹⁴C content is 125 PMC. In the Dingxin area of the lower reaches, the ³H value of phreatic groundwater ranges from 8 TU to 45 TU with the mean value 28 TU, and the ¹⁴C content is 56 PMC. Theδ¹⁸O value and the δD value of phreatic groundwater in the Dingxin area are all higher than those in the Jinta area (Table 1), and theδ¹⁸O value is from -8.3 ‰ to-6.6‰, with the mean value -7.5‰, theδD value ranges from -49 ‰ to-32‰ with the mean value -42‰. Theδ¹⁸O value is within the range of -10.0 ‰ to-8.4‰ with the mean value -9.2‰ and theδD value is from –57 ‰ to-52‰ with the mean value -54‰ in the Jinta area. The isotopic characteristics of phreatic groundwater mentioned above in the Jinta area is closely related with the recharge from the Jiuquan Basin.

In the Ejina Basin where is the lower zone of the lower reaches of the Heihe River Basin, the ³H value of phreatic groundwater ranges from 11 TU to 41 TU with the mean value 28TU. The change of the ³H value is less in the direction of the groundwater flow. The ¹⁴C content is more than 50 PMC; theδ¹⁸O value is within the range from -8.9 ‰ to -1.7‰ with the mean value -5.8‰, theδD value is from -56 ‰ to -41‰ with the mean value -46‰, higher than those of confined groundwater because of the effect of the intense evaporation. The ³H value of confined groundwater ranges from 2 TU to 8 TU with the mean value 6 TU, the ¹⁴C content is less than 50 PMC, theδ¹⁸O value is within the range of -10.3 ‰ to -6.1‰ with the mean value -7.6‰, and theδD value is from -66 ‰ to -53‰ with the mean value -57‰（Table 1）.

4 Recharge and Renewable Charact- eristics of Groundwater

The groundwater is the product forming in the geological history period and its isotopes record the past climatic status, which shows the property and the intensity of groundwater recharge sources. The Fig.3 from 48 groups of the groundwater isotopic data by the statistical frequency distribution of the ¹⁴C content- shows that the period with greater frequency was the main recharge period of groundwater and the precipitation is more considerable. The period with less frequency is less was the non-main recharge period of groundwater. In the non-main recharge period, the precipitation and the recharge were all less and the climate was dry.

Fig.3 Distribution of the statistical frequency on ¹⁴C of groundwater by 5 pmc in the Heihe River Basin

The recharge rate is related with the statistical frequency of the groundwater ¹⁴C samples and is inverse ratio with the square of the departure value of the deuterium from atmospheric precipitation line because the information of the groundwater recharge has been recorded in the groundwater isotopes occurring in the geological historical period (Fig.3). Therefore, Fig.4 has been given by above relationship from Allison et al.（1978）and it shows the evolution process of the groundwater recharge rate in the plain of the Heihe River Basin.

The results of the statistics frequency of the groundwater ¹⁴C samples in the Fig.3 are that the high frequency of the groundwater ¹⁴C content appears from 25% to 55%, from 60 % to 75% and from 90% to 95%, corresponding to the water-abundant periods respectively from 8,000 a BP to 4,500 a BP, from 3,500 a BP to 2,500 a BP and last 1,000 years (Zhang Guanghui, 2003).

Fig.5 Relationship between ³H and ¹⁴C content

in the Heihe River Basin

M-age: mixing water; P-water: present water; O-water: water of older age; Y-water: water of younger age

During last 10,000 years, there are three main phases of the recharge from the results of the Fig.3 and Fig.4, which are corresponding to the water-abundant period and the warm period of that time.

Groundwater in the plain of the Heihe River Basin may be divided into four groups according to its age on the basis of relationship between the ³H content and the ¹⁴C content in groundwater (Fig.5) and in the light of the Tamers model on the groundwater isotope (Zhang Guanghui, 2003). The water of the first group, referred to as the “present water”, is less than 50 years in age with the ³H value more than 20 TU and the ¹⁴C content more than 55 PMC. The water of the second group, referred to as the “ water of younger age”, is more than 55 years in age with the ³H value less than 10 TU and the ¹⁴C content from 55 PMC to 85 PMC, and is formed before the nuclear explosions. The water of the third group, referred to as the “water of older age”, is more than 10,000 years in age with the ³H value less 10 TU and the ¹⁴C content less than 55 PMC. The water of the fourth group, referred to as the “mixing water”, is from a few decades to a few thousand years in age with the ³H value within the range of 10 TU to 40 TU and the ¹⁴C content less than 55 PMC, and it is a mixture of the recharge from older water and younger water (Fig.3).

In the plain of the Heihe River Basin, the age of phreatic groundwater is generally less than 50 years and the age of confined groundwater is more than 50 years. The age of groundwater is a few thousand years in the zone with the poor recharge and cycle condition. The recharge of younger-age water is distributed in the Gobi zone down the Qilian Mountain and the riverbeds of the main streams such as the Heihe River and Beida River etc., its isotopic characteristics of the modern river water to recharge groundwater are evident. The age of phreatic groundwater in the Jiuquan Basin is older than that in the Zhangye Basin. In the Lianhua -Minghai-Yanchi area of the Jiuquan Basin, the age of phreatic groundwater is older, more than 50 years.

The age of confined groundwater is more than 1,000 years in the Jiuquan Basin and is less in the Zhangye Basin. Confined groundwater of older age is mainly distributed in the Lianhua-Minghai zone between the Zhangye Basin and the Jiuquan Basin and its calibrated ^{14C age is from 5,760 a BP to 2,490 a BP. In the Gaotai area of the Heihe River Basin, the calibrated 14C age of confined groundwater is 2,050 a BP. In the area between Gaotai to Zhangye City, the age is less than 1,000 years. In the southern area of the lower reaches, the Heihe River basin, the age of confined groundwater is less than 5,000 years. In the northern area of the lower reaches, the age of confined groundwater is within the range from 5,468 a BP to 8,779 a BP. Confined groundwater of older age is distributed in the Shaihantaolai-Subozuoer zone where is the northern area of the Ejina Basin and its age is within the range of 10,710 a BP to 14,020 a BP.}

5 Response of Groundwater Recharge to Paleoclimatic Change

5.1 Evolution of paleoclimate and paleohydrology

The development and the disappearance of the turves were the results of climate change from wetness to aridity and were closely related with the precipitation, the river runoff and the groundwater level. Cao Xingshan (1996) held that there were five stages of forming the turf layers in Gansu Province since the Quaternary Period. The last three stages are respectively from 8,000 a BP to 7,000 a BP, from 5,800 a BP to 4,500 a BP and from 3,500 a BP to 2,500 a BP. The climate changed from cold-aridity to warm-wetness, the precipitation increased and the lakes of the lower reaches outspread in the Heihe River Basin during the early stage of the Holocene from 8,000 a BP to 7,000 a BP. The climate in the middle stage of the Holocene from 5,800 a BP to 4,500 a BP was the best, the precipitation increased obviously, the lakes were in full growth and the grass was in the flourish. The period from 3,500 a BP to 2,500 a BP was relatively warmer-wetter from the middle stage to the late stage of the Holocene. In the late stage, the precipitation increased relatively and the turf layer was formed by 0.5 m in thickness.

It has been proved by the ice core recording the change of the paleoclimate in the study area that the stable warm-wet stage occurred from 7,200 a BP to 6,000 a BP (Shi Yafeng et al., 1995), at that time the precipitation was plentiful, the surface runoff was abundant, the lakes in the lower reaches outspread continuously and the recharge to groundwater was surfficent. In the Ejina Basin of the lower reaches of the Heihe River Basin, the total dissolved solids (TDS) of confined groundwater which was formed during the period from 8,000 a BP to 5,000 a BP is low and the water quality is better.

Since the late stage of the Holocene, the climate has been continuously dry so that the area of the lakes and the marshland has reduced in large extent, the pasture vegetation has rapidly degraded and the land has become the desert in the Heihe River Basin. It have been recorded that the area of the Juyanhai lakes in the Ejina Basin was the largest in the Northwest China with the area up to 2,600 km² during the Dayu period from 4,260 a BP to 4,150 a BP. In the period of the Qin Dynasty and Han Dynasty, the lake area was still 726 km² (Wang Hongcang, 2000). The period around 2,800 a BP was wetter period for last 3,000 years in the record of the Dunde ice-core (Wei Keqin et al., 1994).

In the past 500 years, the air temperature rose by 1.0℃ or 1.2℃ in the Qilian mountain, as a result, the glacier area reduced by 33% to 46%, the precipitation reduced by 50 mm to 80 mm，the glacier melt water reduced by 35% to 46% and the land evaporation increased by 7% (Shi Yafeng et al., 1995). The rainy stages were respectively from 1428 to 1532 AD, from 1622 to 1740 AD, from 1797 to 1865 AD and from 1924 to 1944 AD on the basis of the data of the tree growth ring in the Qilian Mountain (Cao Xingshan, 1996). The total quantity of water resources in the Heihe River Basin reduced by 20% , the river runoff reduced by 14.6% and the groundwater recharge reduced by 4.38´10⁸ m³/a to 7.61´10⁸ m³/a relatively in the rainy period on a century scale. The ratio of the precipitation and the glacier-melt water is 1.2% and 2.0%. The rate of recharge from the Qilian mountain increased by 3.2% due to the decrease of the total water resources quantity.

5.2 Relationship between groundwater recharge and climatic change

It has indicated by the correlative analysis that the correlation coefficient between the groundwater recharge in the plain and the surface runoff from the Qilian Mountain is 0.82, the correlation coefficient of the groundwater recharge in the plain with the precipitation is 0.97 and with the air temperature is 0.79 at the Qilian meterological station. And with the precipitation is 0.43 and with the air temperature is 0.60 at the Zhangye meterological station in the plain of the Heihe River Basin.

The impact ratio of the precipitation from the Qilian Mountain on the groundwater recharge in the plain is about 91% and the impact ratio of the air temperature is 9% in the Heihe River Basin. The variation of the precipitation of the Qilian Mountain is the main factor for the change of the groundwater recharge. The surface runoff will increase by 9.16% and the groundwater recharge in the plain will increase by 7.38% when the annual mean air temperature rises by 0.5℃ in the Heihe River Basin. The surface runoff will increase by 4.39% and the groundwater recharge in the plain will increase by 3.54% when the annual mean air temperature falls by 0.5℃ and the rainfall adds by 10% in the Heihe River Basin. If the precipitation increases by 10% or 20% but the air temperature remains unchanged, the surface runoff will increase by 4.56% and 13.45% and the groundwater recharge in the plain will increase by 3.61% and 10.38% in the Heihe River Basin.

Calculated in a century scale, glacier and snowmelt water to recharge the surface runoff will reduce by 10.9% and the surface runoff will reduce by 25.4%, and the groundwater recharge will reduce by 20.5% when the precipitation decreases by 11.6% in an extremely dry year. The glacier-melt water to recharge the surface runoff will rise by 13.6%, the surface runoff will rise by 34.6% and the groundwater recharge will rise by 27.8% when the precipitation increases by 29.5% in an extremely water abundant year (Table 3).

Table 3 Groundwater recharge and corresponding factors variation in the Heihe River Basin under different hydrological conditions

Notes: Data after Kang Xingcheng et al.，2002; “1,320 years” is the calculation period in Table 3;

6.     Conclusion

It has indicated from above researches that the recharge and the renewal groundwater in the plain are closely related to the change of the regional climate in the Heihe River Basin of Northwest China. The age of shallow groundwater is younger and its renewable rate is greater than that of confined groundwater. The renewable rate in the eastern area of the middle reaches is greater than that in the western area and in the fine soil zone is less than that in the Gobi zone down the Qilian Mountain and near to the riverbed is greater than that far from the riverbed. The main recharge resource of groundwater in the eastern area of the middle reaches is the precipitation in the Qilianshan Mountain area and the main recharge of the western area is snow-melt water and fissure groundwater in the mountain area. Therefore, it is beneficial to groundwater resources sustainable utilization to observe the principle of water cycle together with the optimal adjustment of surface water.