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Geology; March 2009; v. 37; no. 3; p. 243-246; DOI: 10.1130/G25472A.1
© 2009 Geological Society of America
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Rising springs along the Silk Road

Jian Sheng Chen1 and Chi-yuen Wang2

1State Key Laboratory of Hydrology, Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, China
2Department of Earth and Planetary Science, University of California–Berkeley, Berkeley, California 94720, USA


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 DATA AND ANALYSIS
 DISCUSSION
 REFERENCES CITED
 
Despite its extreme aridity, the Hexi Corridor in northwestern China, part of the ancient Silk Road, has recently been repeatedly flooded by rising springs, forcing ~1000 families to abandon homes. Here we use new isotopic and chemical data for waters collected from the corridor and the Qilian Mountains to investigate the cause of the rising springs. The data show that the springs may have originated from the mountain slopes where glacier melt mixes with the precipitation from a local convective system between the extensively irrigated Hexi Corridor and the Qilian Mountains. Accelerated glacier melting may have increased recharge of groundwater in the Qilian Mountains that was subsequently released by recent earthquakes from the mountains to the valley to raise the local water table. The result has potential implications for the impact of climate change on water resources and management in arid regions.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
DATA AND ANALYSIS
 DISCUSSION
 REFERENCES CITED
 
The Hexi Corridor, part of the ancient Silk Road, extends north-northwest for ~900 km in northwest China and is bounded by the Qilian Mountains on the southwest and Longshou Mountain and Heli Mountain on the northeast (Fig. 1). The corridor is filled with Mesozoic and Cenozoic sediments covering an early Paleozoic and Precambrian basement (Hou et al., 1999). The Qilian Mountains are composed of tightly folded Paleozoic strata and igneous rocks thrust over the southwestern border of the Hexi Corridor. Many earthquakes and faults in the region (Fig. 2) reveal an active tectonic setting.


Figure 01
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  		Figure 1. Location map of study area showing sampling sites, major rivers, and towns. Boundaries of Hexi Corridor are marked by dashed curves. Contours show elevation (in m); arrows show direction of groundwater flow. Rectangular box shows area enlarged in Figure 2.

 


Figure 02
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  		Figure 2. Area enlarged from Figure 1. Zhangye Basin is filled with Mesozoic and Cenozoic sediments, while surrounding mountains consist of folded and faulted rocks of various ages. Bounding faults, marked by active seismicity and revealed by geophysical surveys, extend to depth of many kilometers. Numbers by epicenters refer to recent earthquakes: (1) 2003 M6.1, (2) 2003 M5.9, (3) 2004 M3.2, (4) 2004 M4.5, (5) 2004 M3.5, (6) 2006 M3.2. Contours show area and magnitude of water-table rise between 2003 and 2006 (courtesy of Ding Hongwei).

 
The valley is extremely arid; precipitation is 50–200 mm/yr and the potential evaporation rate is 2000 mm/yr (Wang and Zhang, 2007). The northern slopes of the Qilian Mountains receive much greater precipitation (300–600 mm/yr) and are roofed by numerous glaciers. Precipitation on the mountain slopes is highly seasonal (Fig. 3A), i.e., most precipitation occurs during summer and fall, and there is little during winter and spring. Discharge in the Heihe River shows a similar seasonal pattern (Fig. 3B), indicating that discharge derives mostly from the summer and fall precipitation. Groundwater beneath the mountain slopes discharges to the corridor and appears as springs. Several towns of historical importance, including Zhangye and Jiuquan (Fig. 1), were established around springs and oases in the corridor more than 2000 yr ago.


Figure 03
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  		Figure 3. A: Time history of monthly and annual precipitation at Qilian Station (site 5, Fig. 1) on northern slope of Qilian Mountains. B: Time history of discharge of Heihe River at Yingluo Gorge (site 2, Fig. 1), the exit of Heihe River from Qilian Mountains (Han et al., 2007). C: Time history of groundwater level at monitoring well near Yingluo Gorge. Note close similarity in seasonal changes between monthly precipitation and Heihe River discharge.

 
Overexploitation of groundwater in the Hexi Corridor since the last century caused the water table to decline. Since 2003, however, the water table unexpectedly started to rise (Fig. 3C) and, since 2005, rising springs have repeatedly flooded both Zhangye (http://news.hainan.net/newshtml07/2007w12r14/293706f0.htm) and Jiuquan (http://geo-hydro.blogspot.com/2008/04/blog-post_1976.html), most severely during winter when irrigation stops; houses collapsed and ~1000 families were forced to evacuate. Zhangye and Jiuquan are separated by more than 100 km; the fact that flooding is reported in both places suggests that it may also be occurring in other places throughout the Hexi Corridor.

The amount of increase in groundwater supply from 2003 to 2006 in the Zhangye Basin, estimated by integrating the volume enclosed by the contours of the water table rise in Figure 2, is ~1 km3. Where did all this extra water come from? The answer to this question may bear on the sustainability of the region and other arid regions. Here we use new isotopic and chemical tracers in waters collected from the Hexi Corridor and Qilian Mountains to attempt to answer this question.


   DATA AND ANALYSIS
TOP
 ABSTRACT
 INTRODUCTION
 DATA AND ANALYSIS
DISCUSSION
 REFERENCES CITED
 
We collected water samples from springs and artesian wells in and around the Zhangye Basin and from the Heihe River over the mountain slopes (Fig. 1) and measured their isotopic and chemical compositions (Table 1; Fig. 4). The water samples from the river in fall show {delta}D and {delta}18O values that plot above the global meteoric water line (GMWL). The weighted average values of the precipitation in the Zhangye Basin over the past 20 yr ({delta}D, –40.7{per thousand}; {delta}18O, –6.22{per thousand}), as determined by the International Atomic Energy Agency, plot adjacent to the GMWL. A least square fit to the Heihe water data in the fall intersects the GMWL at {delta}D = –76{per thousand} and {delta}18O = –11{per thousand}. During the winter, irrigation in the region stops. We collected water samples from the Heihe River and measured their isotopic compositions; the weighted averages of the {delta}D and {delta}18O for the river waters during the winter plot close to the GMWL (Fig. 4), suggesting that this water, as well as the average precipitation in the Zhangye Basin, may originate from precipitation derived from marine water vapor.


Figure 04
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  		Figure 4. {delta}D plotted against {delta}18O for water samples from springs, Heihe River, and surface water in Zhangye Basin. Diamond with cross shows weighted average of precipitation in Zhangye; solid square with cross shows weighted average of Heihe River waters collected in winter. Both are adjacent to global meteoric water line (GMWL). Sizes of crosses show standard errors in {delta}D and {delta}18O measurements. Crosses show data for Heihe River waters collected in fall, all of which are above GMWL. Least square fit to river-water data (dashed line) intersects GMWL at point marked by open circle. Data for springs (open squares) are close to least square fit to river-water data. Data for surface irrigation waters (open circles; from Chen et al., 2006) are on opposite side of GMWL.

 


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  		TABLE 1. ISOTOPIC CHEMISTRY AND SALINITY OF WATER SAMPLES COLLECTED IN FALL 2007 FROM SPRINGS, HEIHE RIVER, AND TRIBUTARIES IN AND AROUND ZHANGYE BASIN

 
Most meteoric and groundwater processes shift the {delta}D versus {delta}18O of waters to a position below the GMWL (Clark and Fritz, 1997). For example, the {delta}D and {delta}18O of the surface irrigation waters in the Zhangye Basin all fall below the GMWL (Fig. 4; Chen et al., 2006), as would be expected for waters altered by evaporation. Two processes are currently known to cause exceptions. First, preferential fractionation of hydrogen isotopes between layered water in secondary clay minerals and brines in crystalline terrains leads to enrichment of deuterium in the brines (Frape and Fritz, 1982; Frape and Reardon, 1982; Clark and Fritz, 1997). When actively circulating groundwater mixes with such brines, it acquires an isotope signature with {delta}D and {delta}18O values that plot above the GMWL. In such cases, the isotope shifts correlate closely with the salinity of the groundwater (Clark and Fritz, 1997). However, the isotope shifts for the waters from the Hexi Corridor show no such correlation (Table 1), refuting this hypothesis in the present case.

In the second process, kinetic effects during evaporation of surface waters in arid regions produce vapor with {delta}D and {delta}18O that plot above the GMWL (Ingraham and Matthews, 1988; Lauriol and Clark, 1993); the clouds that form by equilibrium condensation from the vapor, and the precipitation from the cloud, also have {delta}D and {delta}18O that plot above the GMWL. The annual amount of water used in irrigation in the Hexi Corridor is ~6 km3/yr, about half of which evaporates (Huang et al., 2007). The evaporated water may feed a local climate system during summer and fall when irrigation is extensive. The vapor rises orographically along the northern slopes of the Qilian Mountains, condenses, and precipitates in a mesoscale circulation, as reported in other extensively irrigated arid regions (e.g., Pielke, 2001; Sato et al., 2007). The precipitated water mixes with glacier melt runoff on the northern slopes of the Qilian Mountains and recharges groundwater and the Heihe River. In this scenario, the least square fit to the Heihe water data in the fall (Fig. 4) represents a mixing line between the evaporated-precipitated water and glacier melt, the latter being characterized by {delta}D and {delta}18O values given by the intersection between the mixing line and the GMWL (Fig. 4). Assuming a {delta}18O gradient of –2{per thousand}/km of elevation (e.g., Siegenthaler et al., 1983) and an average elevation of ~1.5 km for the Zhangye Basin, we estimate that the glacier meltwater originated from an elevation of ~4 km.

During the winter, irrigation in the valley stops, and so does the evaporation from the valley; thus there is no convective rainfall over the northern slopes of the Qilian Mountains (Fig. 3A) and the precipitation and the Heihe River waters are entirely derived from a global circulation system, as reflected by their {delta}D and {delta}18O values (Fig. 4).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 DATA AND ANALYSIS
 DISCUSSION
REFERENCES CITED
 
Extensive irrigation in arid areas has been known to affect the local atmospheric circulation (Pielke, 2001). Sato et al. (2007) noted frequent cloud formation along the boundary of the extensively irrigated Hetao area in China and attributed it to a local circulation induced by the thermal contrast between the irrigated and surrounding areas. Numerical experiments (e.g., Lee and Kimura, 2001; Kawase et al., 2008) confirmed this concept. Thus it is reasonable to suggest that during summers and falls, when irrigation in the Hexi Corridor is active, a mesoscale circulation is set up between the extensively irrigated Hexi Corridor and the Qilian Mountains, while, during the winter, the local circulation stops but a much weaker global circulation remains.

The surface irrigation waters in the Zhangye Basin cannot be the source of the rising springs because, as noted earlier, the former are characterized by {delta}D and {delta}18O that plot below the GMWL, while the latter are characterized by {delta}D and {delta}18O that plot above the GMWL (Fig. 4). On the other hand, the {delta}D and {delta}18O values of the springs are similar to those of the Heihe River in the fall as it exits from the mountains (Fig. 4), suggesting that the spring waters may also originate from a mixture of glacier melt and the evaporated-precipitated water on the northern slopes of the Qilian Mountains. Accelerated glacier retreat in the Qilian Mountains, as much as 7 m/yr, associated with increased temperature at an average rate of 0.04 °C/yr, has been documented since the 1980s (Du et al., 2008). Increased temperature also reduces the area of the permafrost, which promotes infiltration of glacier melt to recharge groundwater beneath mountain slopes. Neither precipitation over the mountain slopes nor Heihe River discharge shows similar increases (Figs. 3A and 3B). Thus the increased glacier melt and recharge in the Qilian Mountains may be the major source for the extra water in the rising springs in the Hexi Corridor.

Numerous faults, marked by intense seismicity, crisscross the Hexi Corridor (Fig. 2). In 2003 two earthquakes with magnitudes of 6.1 and 5.9 occurred ~75 km southeast of Zhangye (Fig. 2). Earthquakes are known to release groundwater from mountains (Rojstaczer et al., 1995; Wang et al., 2004) and, since 47 B.C., as many as nine events of earthquake-induced groundwater discharge have been documented in the Hexi Corridor (Institute of Geophysics–CAS, 1976). For earthquakes with a magnitude of 6.1, such an effect can occur as far as 80 km from the earthquake source (Manga and Wang, 2007). Thus the 2003 M6.1 earthquake in the Hexi Corridor is a possible mechanism for releasing the groundwater from the Qilian Mountains and discharging it in the Hexi Corridor.

The current flooding caused by rising springs, however, is unique in the unusually large amount of discharge and long duration. The discharge may be due to the increased recharge from glacier melt in the Qilian Mountains, as discussed earlier; the duration may be due to active faulting in the region, as revealed by recent earthquakes (Fig. 2), which keeps the fractures open to allow continued release of groundwater from the mountains.

At the current rate of melting, most glaciers in the Qilian Mountains may disappear by A.D. 2050 and the rest may shrink significantly in size (Ding and Pan, 2005). By then, the recharge of groundwater in the mountains will diminish sharply and the water table in the valley would have dropped significantly. Decline in the water resources would upset the regional irrigation pattern, which in turn would disrupt the local atmospheric circulation in the summer and fall. Thus the precipitation over the mountain slopes and the discharge in the Heihe River would decrease, which in turn would further degrade the regional water resources, i.e., a runaway process. Innovative management of the regional water resources is needed to avert the above scenario and to preserve the local culture and livelihood.

The disappearance of mountain glaciers due to a warming climate is occurring worldwide, so similar hydrologic scenarios and the need for innovative water management may happen in other arid regions that also depend upon glacier melt as a main water resource.


   ACKNOWLEDGMENTS
 
We thank Michael Manga, Douglas Dreger, Steve Ingebritsen, and two anonymous reviewers for helpful comments. Chen acknowledges support from the Chinese National Science Foundation (grant 50579017).


   REFERENCES CITED
TOP
 ABSTRACT
 INTRODUCTION
 DATA AND ANALYSIS
 DISCUSSION
 REFERENCES CITED
 

Chen, Z.-Y., Wan, L., Shen, Z.-L., and Chen, J.-S. 2006, Identification of groundwater recharge in the Heihe Basin using environmental isotopes: Engineering Geology and Hydrogeology Quarterly Journal, v. 6, p. 9– 14.

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Ding, Y.-J., and Pan, J.-H. 2005, Positive and negative impacts of climate and environmental changes on the ecological, social and economic systems, in Qin D.-H. ed., Climate and environment changes in China: Beijing Science Publishing Co v. 2, p. 114– 194 (in Chinese).

Du, W.-T., Qing, X., Liu, Y.-S., and Wang, X.-F. 2008, Characteristic variations of the Tiger Ditch #12 glacier in the Qilian Mountains between 1958 and 2005: Journal of Glaciology and Geocryology, v. 30, p. 373– 378.

Frape, S.K., and Fritz, P. 1982, The chemical and isotopic composition of saline groundwaters from the Sudbury basin: Canadian Journal of Earth Sciences, v. 19, p. 645– 661.[GeoRef]

Frape, P., and Reardon, E.J. 1982, Isotopic and chemical characteristics of mine water in the Sudbury area: Chalk River, Ontario Atomic Energy of Canada Limited Technical Report 35 37 p.

Han, Y.-L., Cheng, X.-X., and Wen, Q.-S. 2007, Groundwater variations in the piedmont near Heihe: Yellow River, v. 29, p. 38– 39.

Hou, K.-M., Zhang, X., and Liu, X.-L. 1999, The neotectonic deformation mechanism of Qilianshan Structure Belt: South China Journal of Seismology, v. 19, no. 2, p. 50– 56.

Huang, H.-S., Bai, F., and Zhang, C.-J. 2007, The water resources and characteristic changes in present Hexi Corridor: Journal of Gansu Science, v. 19, p. 75– 79.

Ingraham, N.L., and Matthews, R.A. 1988, Fog drip as a source of groundwater recharge in northern Kenya: Water Resources Research, v. 24, p. 1406– 1410, doi: 10.1029/WR024i008p01406.[CrossRef][Web of Science][GeoRef]

Institute of Geophysics–CAS (China Earthquake Administration) 1976, China earthquake catalog: Washington, D.C Center for Chinese Research Materials 500 p. (in Chinese).

Kawase, H., Yoshikane, T., Hara, M., Kimura, F., and Sato, T. 2008, Impact of extensive irrigation on the formation of cumulus clouds: Geophysical Research Letters, v. 35, L01806, doi: 10.1029/2007GL032435.[CrossRef]

Lauriol, B., and Clark, I.D. 1993, An approach to determine the origin and age of massive ice blockage in two Arctic caves: Permofrost Periglacial Processes, v. 4, p. 77– 85, doi: 10.1002/ppp.3430040107.[CrossRef]

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Manga, M., and Wang, C.-Y. 2007, Earthquake hydrology, in Schubert G. ed., Treatise on geophysics, Volume 4: Pasadena California Institute of Technology p. 293– 320.

Pielke, R.A. 2001, Influence of the spatial distribution of vegetation and soils on the prediction of cumulus convective rainfall: Reviews of Geophysics, v. 39, p. 151– 177, doi: 10.1029/1999RG000072.[CrossRef][Web of Science][GeoRef]

Rojstaczer, S., Wolf, S., and Michel, R. 1995, Permeability enhancement in the shallow crust as a cause of earthquake-induced hydrological changes: Nature, v. 373, p. 237– 239, doi: 10.1038/373237a0.[CrossRef][GeoRef]

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Received for publication 22 August 2008

Revised manuscript received 27 October 2008

Manuscript accepted 28 October 2008





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JOURNAL HOME HELP CONTACT PUBLISHER SUBSCRIBE ARCHIVE SEARCH TABLE OF CONTENTS
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