Study On Groundwater Circulation And Renewability In The Middle Reaches Of Heihe River Valley, Northwest China

Groundwater circulation is not only an important part of continental hydrologic cycle, but also the main force to drive the groundwater system to carry out its functions. And groundwater circulation and renewability is essential for improving water resource management. So it is necessary to understand groundwater circulation and renewability.As an opening system, groundwater system is characterized by its complicacy and variability. Groundwater system contacts with its neighbor water systems by mass and energy transfer, which is influenced by many factors, such as the condition of weather, hydrology, geology, tectonics, hydrogeology and vegetation. So it is difficult to study groundwater circulation by using traditional hydrogeological methods, while the chemical and isotopic components in groundwater contain much information of hydrologic cycle, which provides an effective method to trace the process of groundwater circulation.The purpose of present work is to reveal groundwater circulation and renewability in the inland basins of Northwest China, Which is vital to groundwater sustainable development. Taken the basin of the middle reaches of Heihe River as the main study area, the present work focus on three problems: (1) the chemical and isotopic characters of different water bodies; (2) the process of groundwater circulation, which includes the patterns of groundwater recharge, groundwater flow and groundwater discharge; (3) estimation of groundwater renewal rate.Taken the system theory as guide and the chemical and isotopic component as information of hydrologic cycle, the present work carries out the research by using hydrogeology, geohydrochemistry, isotope hydrology and relevant mathematics methods. Firstly, based on understanding of the research background in study area, the present work analyzed the physical frame of groundwater system and the groundwater flow character。Secondly, the groundwater recharge, flow and discharge patterns are identified by analysis of the interaction of groundwater system and its neighbor environment. Thirdly, the groundwater renewal rate is estimated by using two isotopic models. At last, based on the understanding of groundwater circulation and renewability in study area, this paper gives a few advices on groundwater sustainable development.The following results are obtained from present study.1. Illustrated the characters of hydrologic cycle in mountain area of Heihe River.In the alpine zone, some precipitation and meltwater infiltrate in subsurface through fissures, and the remainder water forms the surface water flow. There are two kinds of groundwater flow patterns in mountain area: local groundwater flow and regional groundwater flow. The local groundwater flow system gets recharge from meltwater and precipitation at local spot with the proportion of 24% and 76% respectively. The local groundwater discharges to the near stream, with a short flow path. While the regional groundwater system gets recharge from meltwater and precipitation at the high elevation (>3600m) area and discharges to surface as springs at the interface of mountain area and basins, where the fracture belt exists. The surface water is composed of precipitation, meltwater and groundwater. The fractions of various waters in surface water are calculated by using three-component approach. The results show that the proportion of regional groundwater in the surface water is about 5～7%, and the proportion of precipitation, meltwater and local groundwater is about 93～95%.2. Validated the boundary condition of groundwater system in the basin of the Heihe River middle reaches.The water transfer presents between the groundwater system in basin and the water system in mountain area mainly by the river flow systems. The bedrock blowups at Gaotai is the natural boundary of the Zhangye basin and the Jiuquan basin and there is no water exchange between these two basins.3. Revealed the characters of groundwater recharge, flow and discharge in the basin of the Heihe River middle reaches.Based on the groundwater chemical and isotopic characters and groundwater age in basins, the subsurface flow system is classified into local groundwater subsystem and regional groundwater subsystem. And the local groundwater flow subsystem includes the piedmont local groundwater subsystem and the alluvial plain local groundwater subsystem. The local groundwater moves mainly in the vertical direction, while the regional groundwater moves in the horizontal direction.There are tree recharge mechanisms of groundwater system in the basin of the Heihe River middle reaches, namely, piedmont vertical infiltration, riverbed leakage and irrigative water infiltration. The vertical infiltration presents at the piedmont pluvial fan, where about 75～90% of groundwater recharged from river water infiltration and 10～25% of groundwater from precipitation infiltration. The vertical infiltration is the main recharge type of the piedmont groundwater subsystem and the regional groundwater subsystem in basin. The riverbed leakage and irrigative water infiltration exist mainly in the alluvial plan.The main discharge modes of groundwater in basin include springs, riparian seepage, evapotranspiration and groundwater pumping. The springs present at the edge of the piedmont pluvial fan, where about 20～40% of surface water from springs. The riparian seepage mainly takes place at the edge of basin, where about 19% of surface water from deep groundwater discharger and 59% from the shallow groundwater discharge. The evapotranspiration and groundwater pumping exist in the alluvial plan.4. Illuminated the groundwater circulation patterns of basin groundwater system in the middle reaches of Heihe River.At the piedmont pluvial fan, the basin groundwater gets recharge from precipitation and river. Along the path of groundwater flow, it forms two types of groundwater flow subsystem: local flow subsystem and regional flow subsystem. The regional groundwater moves toward the alluvial plan and discharges though upward leakage or pumping wells. Controlled by topography, lithology, and tectonics, the local groundwater subsystem in the piedmont pluvial fan discharges as springs at the edge of pluvial fan, and the local groundwater flow subsystem in the alluvial plan gets recharge from river and irrigative water infiltration and discharges by riverbed seepage, pumping wells and evapotranspiration.5. Estimated the mean annual rates of groundwater renewal in the basin at the middle reaches of Heihe River.The shallow groundwater renewal rates range from 0.01% to 2.5% of the aquifer volume. In different zones of the basin, the groundwater renewal rates are variable due to the different groundwater circulation condition. The groundwater renewal rates decrease from the piedmont to the north edge. And the groundwater renewability is better in the riparian zone than that in the zone far from river.In the vertical dimension, the groundwater renewal rates decrease with depth. In the unconfined aquifer with the buried depth not more than 40 meters, the mean annual rates of groundwater renewal is more than 2% of the aquifer volume, while in the deep confined aquifer buried more than 100 meters, the mean annual rates of groundwater is less than 0.8% of the aquifer volume. In the semi-confined aquifer that lies between the unconfined aquifer and the deep confined aquifer, the groundwater renewability is variable in a wide range with the mean annual renewal rates being of 0.5～2.5%.6. Discussed the groundwater sustainable development modes.Based on the understanding of groundwater circulation and the estimation of groundwater renewal rates, the groundwater system is divided into four subareas for groundwater development: the Minle subarea, the Zhangye-Linze subarea, the Gaotai subarea and the Gaotai-Zhengyixia subarea. Further more, the Minle subarea and Zhangye-Linze subarea are divided into six zones, namely, the groundwater conservational zone at the piedmont of Minle, the restricting development zone at the downstream of Minle, the groundwater conservational zone at the piedmont of Zhangye-Linze, the groundwater development zone at the piedmont of Zhangye-Linze, the groundwater development zone in the alluvial plan of Zhangye-Linze, and the restricting development zone in the alluvial plan of Zhangye-Linze. The present work gives the advices on groundwater development according to the groundwater renewability in different subareas and zones.