Feng Quanzhou Wangping Yue Chaojun

(China University of Geosciences)

Abstract: By reviewing the evolving history of the groundwater system in a city area, it is discovered that the advent of a new system structure is achieved through the process of mutation, and the whole process is irreversible; it is further believed that, under the influence of continuous artificial exploitation, the groundwater system makes necessary structural adjustments to meet the demands imposed on the performance of the whole system; furthermore, through evolvement, the whole system degenerates into a thermo-dynamic system, leading to the increase of its entropy. The general model for the evolvement of groundwater system goes as such: natural structure-evolving structure- degenerative structure.

Key wards: the evolvement of groundwater system, irreversible process, system structure, model of evolvement

1 Introduction

adjustment phase, rechargeable drainage phase and over-exploitation phase. At the result, the groundwater resources is in red deficit, and the groundwater ,as well as surface water, which containing different kinds of dissolved agents, converged towards the concentrating extraction areas, leading to the degeneration of geo-environment quality. The above phenomenon are pervasive in those cities in Northern China.

2 The Evolution History of Ground- water Flow system

In light of the hydrograph of ground water, it is noticeable that, due to the ever increasing artificial exploitation, the groundwater system has experienced three dynamic response phases (Fig.2). the first phase started well before June,1978;the second stage started in June 1978,and lasted until the end of 1985; the third phase has started well after 1986.The procedure of the three phases produces abundant insight into the characters of the groundwater system..

During the first phase, because the groundwater exploitation was slight, the groundwater system maintained a natural balance between recharge and discharge. The framework of groundwater system consisted of three natural sink systems (Cuiqiao sink system, An Yang sink system and Dongzhengsi sink system), which were located in order from N to S in the study area (Fig.3) . In Cuiqiao sink system , groundwater flowed towards Cuiqiao, the southern border of flow lines was located to the south of Hanling; Within An Yang sink system, the east running section of Anyang River absorbed flow lines, hence it discharged groundwater, in the region between the two borders for flow lines, which were situated in the north and south of the region separately, the western part was the linear recharge border from river, thus the groundwater system was recharged by Anyang River; Dongzhengsi sink system was situated in the south（Ⅲ）, in which groundwater flowed eastwards and converged towards Dongzhengsi.

made up the water divide between Cuiqiao sink system and Dongzhengsi sink system, the two systems received the free percolation recharge from Anyang River (Fig.5,c). Within the whole artificial sink system, groundwater received the free percolation recharge from Anyang River, but the up-conning of groundwater s was leveled off (Fig.5,d).

The 2^nd phase (June 1978-end of 1985):In light of the general pattern of groundwater system in the 1st and 3rd phase , and taking into account the groundwater hydrograph curves, we can make some predictions on the evolvement of groundwater system in the 2nd phase(Fig.6) : the 2^nd phase is the transitional phase between 1^st and 3^rd phase ; in the section to the east of Anyanghe bridge , where it had discharged groundwater during the first phase, Anyang River replenished groundwater; Anyang River constituted into the divide for groundwater, the framework of the groundwater system include: artificial sink system（Ⅵ）,Dongzhengsi sink system（Ⅲ）,Anyang River sink system（Ⅱ）and Cuiqiao sink system（Ⅰ）.

3 Some Discussions about Irreversible Process in Groundwater System

During the evolution process from phase 1 to phase3, the groundwater balance and regime, the hydraulic links between groundwater and surface water, the constituents of groundwater resources, the groundwater flowing field and chemical field, all underwent deep changes;these changes were achieved through ups and downs in the structure of groundwater system in the process of realizing its water supply purpose. When the abnormal ups and downs break through a critical point, a catastrophic shift occurs, resulting in the formation of a new structure, hens the evolution of the system embarks on a new procedure. Here, we refer to the system structure, which can not only meet the water supply demand , but also don not inflict too heavy burden on environment , as an evolving structure; however, we terms a structure ,which can only (or can not either)meet water supply demand at the expense of environment ,as a degenerative structure. Under the influence of continuous artificial exploitation, the evolution sequence of groundwater system structure goes like this: natural structure-degenerative structure, and the evolution process is irreversible.

3.1 The natural structure of groundwater system

During the first phase, thanks to the slight exploitation, the groundwater system maintained a natural balance between recharge and discharge. Groundwater mainly received recharges from precipitation and horizontal percolation from Anyang River, and was discharged by evaporation and river flow. The three sink systems were coupled together through various hydraulic borders, there was a unity potential field inside a convergence system ,among convergence systems ,between groundwater and river, therefore, the groundwater system exhibits holistic functions to the outside surroundings.; this is kind of an orderly structure in conformity with the landforms, water-bearing media, and replenish-discharge relationship.

The spatial orderliness of a system result in its stability over time (fig.2, phase 1).The uneventful curves of groundwater hydrograph observed in the five monitoring boreholes show that the above sink systems were closely interrelated. As a whole, it exhibits high stability; the fact that its yearly fluctuation was around 1m, the peak of hydrograph curves seemed similar to smooth hillock tops, indicate that the system had strong wave filtering function., the effects of rainfalls are offset by evaporation and discharge through river, and the transmission of potential energy between sink systems.

An interesting chemical field of ground water is also derived from the spatial orderliness and steadiness, the aqueous chemical features ,which are dominant in the region, formed in the phase .In the area situated to the east of Anyang City , from source to convergence, which is constituted by parts of Cuiqiao and Dongzhengsi sink systems, the transitional series of chemical composition types is H_H.S_H.L, water quality in resource area is usually good, while saline water occurs in the sink points which are accompanied by large patches of saline-alkali land; the general pattern of groundwater flow shows that the chemical tracing flow lines tended to be more mature towards the sink points, where all kinds of dissolved agents built up. In the western part of Dongzhengsi sink system ,which was situated to the west of Anyang City, in the course of flow, the transitional series of chemical composition types is H.S.L—H.L—H.S—H ,in the course of flow. The explanation for the above phenomenon goes like this: as aquifers became thinner in the vicinity of Nanliusi Village, water flow became less fluent owing to cementation of aquifers, leading to the midway building up of dissolved agents and the occurrence of water composition types in the range of H.S.L -H.L between Qugou Village and Dapo Village. The improvement of water transmissivity near Dapo Village resulted in the dispersion of dissolved agents during their build up, thus a reverse water quality sequence turned up between Dapo Village and urban areas.

3.2 The evolution of system structure

In the period between June 1978 and 1985, the system deviated from the normal scope of ups and downs, resulting in abnormal ups and downs, thus, the macro-steady state was broken up(Fig.2,the curve in 2^nd phase),till another macro-steady state was produced through the self organizing mechanism in the 2nd phase(Fig.2,the curve in 2^nd phase ). Though the groundwater level kept lowering, it was still above the Anyang River(fig.2), hence the system remained to be a natural one; owning to the increase of the exploitation of groundwater in urban area for industrial purposes, the level of groundwater had been lowered below Anyang River by 1981, attributing to the formation of a new system structure by mutation; the eastern part of Anyang River beyond Dazheng Village, where the river had discharged groundwater, turned out to be a divide of groundwater, making up a replenishment boundary for groundwater; Anyang River sink system retreated westward till the vicinity of Xiahan Village(fig.4); Cuiqiaohui sink system was situated to the north of Anyang River（Ⅰ）,what lied to the south of Anyang river ,including urban area and the area beyond the west of urban area ,was an artificial sink system. The original Dongzhengsi sink system（Ⅲ）（fig.2）was cut off by Anyang River, resulting in the formation of a divide of groundwater in the eastern part of urban area., leaving the remaining of Dongzhengsi sink system at the east side of the divide. Realization of function led to the evolution of system structure, and the evolved structure could maximize its capture from Anyang River, thus helped fulfill its function.

In the period from July 1982 through 1985,under the influence of the new structure, a new feedback relationship was set up between the artificial exploitation, side percolation from Anyang River, and replenishment by precipitation, attributing to the relative stability of the system state （fig.2，the curves of boreholes C4,C5 in the second phase）. There was a wave bottom of long endurance in C4, which was far off the urban area; meanwhile, there was only a relatively stable platform period inC5. The above phenomena can be explained as such: due to the great amount of exploitation for industrial purposes in the urban area , long lasting wave crests of groundwater hydrograph were inhibited, and the uneventful replenishment from Anyang River checked the emergence of too large dropdown in groundwater levels. In this phase, even if there was a unity of potential field between surface water and groundwater, but thanks to the transformation from low energy trough to high energy ridge in the east running section of Anyang River, the adjustment within the whole field in the region was no longer pronounced, thus leading to wide discrepancy of dynamic characters between different sink systems（fig.2，2^nd phase）：Anyang River sink system inherited those dynamic characters which it kept during the first phase, indicating a stable replenishment-discharge relationship; the zigzag type hydrograph curve in Cuiqiao sink system denoted its dynamic variance due to agricultural irrigation and seasonal precipitation changes; in Dongzhengsi sink system, the descending hydrograph curve shows the insufficient replenishment from run-off due west; artificial sink system exhibited the characters in conformity with concentrated exploitation. With the out-coming of evolved structure, run-off conditions improved greatly, aqueous chemical types beyond the west of Dapo Village were transformed from types of H .S、H. S. L、H .L to H, thus water quality was better off. Because of the descending of underground water table and the improved run-off conditions, the areas of saline soil in the eastern and northern parts shrunk, with their degree of saline dropped down.

3.3 The emergence of degenerative structure

During the period from 1986 to 1990, with the increasing rate of exploitation for industrial purpose, the evolved structure could no longer meet the demands of its function, at the center of the cone of depression, underground water table dropped almost vertically(fig.2，hydrograph curve of C6 in 2^nd phase),and in the mean time, the whole water system experienced two big events, made the system evolvement reach its critical point, leading to the cropping –up of degenerative structure.

When the underground water level dropped down near the critical position, the feeding model via Anyang river changed from side-feeding to vertical feeding in the east running section of Anyang river. The feeding capacity of Anyang River reached its maximum, the horizontal hydraulic links between groundwater and surface water were broken, and the potential field became discontinued, and there was an up-conning formed by high groundwater level, but the evolving structure remained. When Anyang rivers feeding rate reached its full capacity, ground water supply couldn’t meet the demand imposed by industrial extraction, so the groundwater level dropped continuously. When underground water level reached another critical point, the up-conning formed by high groundwater level was leveled off, the groundwater in the north of Anyang River flowed below Anyang River towards the center of urban area, until a sudden change happened and a new structure appeared（fig.4）.The artificial sink system（Ⅳ）,which doubled its area through expanding northwards across Anyang River, pushed back the Cuiqiao Sink System northeastward; the groundwater on the north side of Anyang River flowed towards the center of the cone of depression across Anyang River; In the period between 1988 and 1989,under the influence of new structure, the system rebuilt another macro-steady state. The rebuilding of the new macro-steady state was achieved by forming larger drop down to capture the groundwater resources in neighboring areas, after the system failed to meet the ever increasing requirements for groundwater through capturing evaporation ,river discharge, and generating recharge from rivers; The behavior of the system would definitely lead to the lowering of regional groundwater level, and exacerbating the existing contradiction between groundwater supply and requirements At that moment, the evolution of system structures reached its limit, and degenerating into a thermo-dynamic system in general sense.

Because the degenerative structure couldn’t generate any new feeding, the dropping-down of underground water level at the center of cone of depression became dramatic. At the west end of the region, due to over extraction of cobble and gravel aquifers, the drawdown caused by extraction neared to half of the thickness of phreatic aquifer until June 1990 ;at the same time, due to the descending of water table in large areas , large numbers of suspending pumps appeared in wells for agricultural irrigation. Because the artificial sink systems had no natural outlets ,all kinds of dissolved agents gathered up near the center of cone of depression, the groundwater quality was in grave state. The groundwater of H.S type has migrated to the center of the cone of depression, and replaced the original H –water there,and the high hardness water would follow up; several heavily polluted groundwater bodies around the cone of depression kept on flowing towards the center of urban areas. When Anyang River became the major feeding resource, the polluted river also became the major source of pollutants. Presently, it is impossible for the system to return fully back to its original state and structure well before 1985.

4 Conclusions

A groundwater system is characterized by self –organizing criticality .Around the critical point, the ups and downs of the system often touches off new structure. Under the influence of ever increasing artificial exploitation the definite evolvement model for a groundwater system is natural structure-evolving structure- degenerative structure, and the process is irreversible. The evolvement from natural structure to evolved structure is a process in which the system absorb negative entropy influx in order to achieve its function for water supply. When the groundwater level drops to a critical point, a catastrophic shift occurs, and a new orderly structure appears; the newly formed structure is able to generate recharge from rivers and capture run-offs and evaporation from neighboring regions, and the system evolves towards the decrease of entropy to better meet the requirements by its function. When groundwater level keeps dropping down, and the system cannot meet the requirement for water supply, a resultant change in the state of system follow up definitely. When the groundwater level drops to another critical point, the above changes of state will bring about massive ups and downs, leading to the catastrophic shift in its structure; the relationship between replenishment and discharge is broken up, resulting in the formation of a degenerative structure; however, the degenerative structure can not generate new negative influx, the system can only evolve towards the increasing of its entropy.

Some thoughts in this paper were inspired by Professor Zhang Renquan and Proferssor Xu Hengli during my study in China University of Geosciences. I sincerely acknowledge them for their valuable guidance.