Analysis of effect of water construction in different phases on groundwater environment

HE Hong, GUO Hong-bin, LIU Hong-yun

No. 904 Hydrogeology and Engineering Geology Prospecting Institute of Heilongjiang Province, Harbin 150027, China.

Abstract: This paper analyses the effect of some water construction in different phases on groundwater environment from several aspects, puts forward reasonable evaluation of groundwater development, and at the same time lists the problems that may arise in environmental hydrogeology. The above analysis and recommendations will have certain reference function to the reasonable development and utilization of groundwater in water construction.

Keywords: Water source; Implementation phase; Groundwater environment; Analysis

In order to evaluate the effect of some water source on the groundwater environment during the implementation phase process, we can analyse the current situation and quality situation of the groundwater environment, analyze and predict the influence of the water source project on the groundwater environment, and come up with relevant measures to reduce or remit the harmful environmental influence and protective measures for the groundwater environment, thus providing scientific basis for the reasonable development and utilization of groundwater resources and the protection of the groundwater environment.

This project's evaluating area is 140 km2, and proposed mining resources after the completion of water source is 30 000 m3/d.

Background

The production water supply and domestic water supply in the evaluating area have always relied on exploitation of groundwater in shallow part. In recent years, due to economic development and population growth, the water demand quantity in the city has increased year after year, and the original water supply source can't satisfy the exploitation needs. Finding a new water source with abundant water and high water quality becomes an issue which must be addressed during the development of this city.

According to the project's feasibility study report, the groundwater environment assessment period is set to the project construction period and the production operation period respectively (LIU Ze-qiang, 2006; Bureau of Geology and Mineral Resources of Heilongjiang Province, 1993).

New water source supply project mainly consists of water source project, water treatment plant project and water supply network project,and therefore, it can be determined respectively that the situations of pollutant sources are: (1) The pollutant sources generated from the water source project: the pollutant sources of the water source project are mainly residents' household waste,liquid waste and excrement of human and animals distributed around the water source area; pesticides and chemical fertilizers used in fields; upstream area of the groundwater supplying may be likely to run by polluted area; the distribution river of the earth surface river source and its upstream will affect the river pollution of water quality in this area; the contamination produced during the construction process of the water supply tube well and so on. (2) Contamination comes from the project of water treatment plant; if the purification plant project is located within the distribution area of the water source or the upstream area of underground water supply, then the pollution source will affect the underground water of this water source, and such pollutants are mainly liquid pollutants and solid contaminants formed after water treatment; life garbage and pollutants produced by residents of water treatment plant in their life; other pollutants resulted from production of water treatment plant, such as the use of disinfectant. (3) The polluters of water supply network engineering: due to the fact that water source project-water purification plant - city water supply are connected by the pipe network, the project of water supply network may produce polluters to some extent, which will pollute groundwater, mainly: the entrance of pollutants because of the incorrect installment of water pipe networks, the pollution source that may be caused in the water pipe network materials, and so on(ZHANG Zhong-xiang and QIAN Yi, 1998;TANG Liang and ZUO Yu-hui, 1998; CHEN Hong-ying, 2003).

1 Regional hydrogeological condition

This region formed the current overall pattern after experiencing crustal movements in a long geological history and relevant denudation and accumulation of outside forces. The fracture and porosity which form Cretaceous rocks and the porosity and fracture of Quaternary loose sand bed in this area provide storage space for the formation and migration of groundwater, thus forming the quaternary loose rocks void water and Cretaceous bedrock fracture pore water (YANG Xiang-kui et al. 2002; LUO Huan-yan and CHEN Yu-sun,1988).

This region is located on the border between Lesser Khingan Mountains and Songnen Plain,which determines its hydrogeological characteristics of a valley plain area. The annual precipitation is 559.8 mm, which recharges groundwater in this region; there are two rivers in this region; in wet seasons, the rivers recharge certain quantity of water to groundwater as well due to the water level elevation; underground runoff is a rather important replenishment source of groundwater in this area. Based on the above analysis, the replenishment sources of groundwater in this area mainly come from precipitation infiltration, underground runoff and surface water (SUN Feng-gen,1995; Javandel I and Withespoon P, 1968; SHAO Jing-li et al. 1994; TU Zhao-lin, 1997).

There are thicker Cretaceous strata in the Quaternary underlying, whose lithology is mainly gray, gray green sandstone-brown gray siltstone,and sandy conglomerate. Sandstone fissures provide migration channels and conditions for clastic rock cranny pore water, which is the mining target stratum of the water source; since the Quaternary period, the river valley plain in this region has settled slowly and deposited about 10-30 m sand and sandy gravel layer of Pleistocene series. The Quaternary unconsolidated rock pore water is very rich and it is also the main target stratum to exploit groundwater in this region(SHEN Guang-fan, 2000).

Different geomorphic units and lithological compositions determine the different occurrence conditions of groundwater in this region. Water bearing formation is widely spread in the region and the main aquifers include Quaternary unconsolidated rock pore water aquifer and Cretaceous sandstone pore water aquifer. Quaternary loose rock pore aquifer in the region is mainly composed of the upper Pleistocene and Holocene alluvial sand and gravel. The depth of water level is 7-20 m and water is more abundant. Bedrock fissure pore aquifer in this region mainly exists in the Mesozoic Cretaceous strata, with block or strip distribution in this region, and its lithology is mainly white sandstone and sandy conglomerate. This aquifer has groundwater with better quality and it is not easy to be polluted, which is an important watersupply aquifer (LI Shu-guang and McLaughlin D A, 2003; Mehl S and Hill M C, 2002; LU Wen-xi,2003).

Aquifer generalization: According to the geological and hydrological conditions of the evaluation area, the groundwater aquifer in this area is mainly Quaternary loose sand and Cretaceous sandstones, which are both as a target stratum for this simulation. Between the two is continuous impermeable mudstone. Since the Quaternary pore aquifer is well connected, the entire Quaternary pore aquifer has been generalized into one, namely Quaternary phreatic layer.Cretaceous sandstone aquifer has also been generalized into one, namely Cretaceous porous fractured aquifer. The lithologic characters and thickness of these two layers of aquifers have different degrees of changes in the region, so these two are generalized as heterogeneous isotropic aquifers (LIANG Xiu-juan et al. 2005; ZHANG Hong-ren and LI Jun-ting, 1979).

2 The effect on the groundwater in different implementation phases

2.1 Analysis of construction period's impact on groundwater environment

The project will occupy an area of about 50 000 m2, which is divided into three parts. The first part is for water treatment plant; the second part is used for distribution pipes network construction; and the third part is water resources site. The water treatment plant belongs to occupied farmland, with an area of 20 800 m2. Pipeline construction needs to temporarily occupy the land.Qing'an Town water pipe distribution network reconstruction workload is larger, so the pipeline is laid in green area and sidewalks as much as possible. Pipe-jacking construction should be employed when crossing the main streets. Temporary relevant formalities should be handled when local area needs to be broken. Water land occupies the farmland, and the total occupying acreage is 29 450 m2, among which the water source well occupies 16 250 m2, and water pipe occupies 13 200 m2.

The landforms the whole pipeline passes through are mainly plain zones. Through comprehensive analysis of geological and hydrogeological conditions along the pipeline, the groundwater type of the areas along the pipeline is mainly Quaternary pore water. The depth of ground water is generally 12-17 m; most of the pipes are constructed and built on the ground, thus having minimal influence on groundwater environment.

The pipe passing the plain zone influences groundwater mainly during construction period.The lixivium produced by leaching of the accessories, waste and filtered water, etc. during the construction enters into ground aquifer, which will have different effects on groundwater. The extent of effect is determined by the infiltration volume, thickness, and lithology, obstruction of pollution, absorbing and resolving ability in the unsaturated zone. From the surface soil of plain area alongside the pipe, it has the ability of purring naturally and has little effect on the groundwater.There is no camp during the constructing period,for the accommodation will be supplied by the folk houses and hotels. Wastewater and daily rubbish are dealt by the present facilities and obey the following environment protecting measures;therefore, the pipe section has little effect on groundwater.

2.2 Analysis of impact on groundwater environment during operation period

Because large areas are exploited for groundwater during the operation period, 11 Quaternary production wells and 14 Quaternary and Cretaceous system comprehensive production wells are totally designed to be laid. The intended initial exploitation quantity is 33 114.25 m3/d, and the current exploitation quantity in the study area is 15 000 m3/d, and then the total exploitation quantity is 48.11 thousand m3/d. The induced recharge of groundwater will be brought about by the exploitation at large flow in the study area,which may be caused by the surface water recharge resulting from the cone of groundwater depression in the exploitation area; leakage recharge from adjacent aquifers due to drawdown funnels from falling groundwater; recharge increased by the moving of underground watershed due to groundwater exploitation and so on.

Under normal working conditions, because the water-transmission line is an all-closed system,conveying water won't come in contact with the outside world. As we use the external anticorrosion coating combined with the impressed current cathodic protection, if the leakage is prevented, the underground water will not be affected, and minor influence will be exerted on the surrounding water environment. Only in the case of the leakage, the underground will be affected, resulting mainly in the decline of water delivery efficiency, which causes economic loss in a certain amount.

Though the leakage of water transmitting pipe has little influence on groundwater environment along the way, it will cause a certain influence on groundwater environment when it comes to pipe repairing and maintenance. To avoid any un-necessary loss, relevant protective measures shall be formulated to minimize influence on the water resource protection zone.

Protective measures shall follow the principles of “prevention first, combination of prevention and treatment, timely treatment, suiting measures to local conditions”, and highlight key points. Contingency measures shall be target-relevant and effective. The following protective measures are put forward by the evaluation:

(1) Water source accidents that may happen will be brought into the maintenance and repair system of project operation. By taking advantage of the closest repairing and emergency teams,equip a full set of pollution control equipment and water leak rapid detection and analysis equipment,and take advantage of the complete repairing and emergency system to implement accident prevention. React in first time and control the leakage quickly.

(2) Set up linkage mechanism with water conservancy and water supply departments,minimise the loss of water resource, and lessen the damage caused by pipe leakage to surrounding living and production environment.

3 Reasonable evaluation of groundwater development

Regarding the aspect of the quantity able to be exploited from groundwater, the quantity of groundwater is exploited as planned. The groundwater level is constantly declining within the extent of pumping well group, and the depression cone's range enlarges constantly. It is expected that in 10 years the groundwater levels in Quaternary sandstone bed and Cretaceous sandstone bed will drop by 8 m and 10 m respectively.For this project, the Quaternary phreatic drawdown is permitted to be 2 m, and the maximum drawdown of the Cretachous sandstone is permitted to be 12 m. Therefore, the exploitation quantity in the Quaternary exploited well would be larger and the water intaking quantity in the Cretachous sandstone would be relatively reasonable. We suggest that the water source area should be enlarged and the exploitation of the water of the Quaternary should be done separately to prevent the large-area depression cone of the groundwater.

From the perspective of water quality, ferromanganese content in groundwater within the region exceeds the standard, which belongs to the original ecological overproof of groundwater. The region is mainly farmland area, without industrial pollution, but there are threats from many aspects such as agricultural pesticide and fertilizer to groundwater quality. To build a water source in the area, the groundwater must be treated. The area outside of the water source area contains excessive iron and manganese. According to the calculation result, after five years since the construction of the water source area, the groundwater containing excessive highly concentrated iron and manganese will flow into the pumping wells slowly. But because the hydraulic gradient resulting from the exploitation of the groundwater in the water source area intensifies groundwater runoff, and then the content of iron and manganese will be reduced to a certain extent.

4 Possible environmental hydrogeological problems

From comprehensive analysis, we find that the construction of water supply system has caused the change of natural hydrodynamic field and chemical field in the groundwater system of this region, and then the geological phenomena that can deteriorate environment may occur, so water supply system construction may cause environmental hydrogeology problems, which mainly are the several aspects as follows.

4.1 Dropping of the underground water level and falling funnel of regional groundwater

After the water supply project runs, groundwater in water resources distribution area will be exploited intensively, and the groundwater daily output will be 3×104m3/d. The depression cone of shallow groundwater will form in water resources distribution area, and if over-exploitation occurs or exceeds the service life, it will cause the regional groundwater level to decline. By then, it will influence the ecological environment in the zone.

4.2 Ground subsidence

With the water-supply project completed and applied, if groundwater is not exploited according to designed water withdrawal, the groundwater level will be lowered after several years and will possibly cause ground subsidence. Therefore,during the construction period of water-supply project, quantity of water to be exploited needs argumentation, and exploit appropriately in an orderly way according to strictly designed exploiting method; at the same time, relevant monitoring and preventing measures should be made.

4.3 Groundwater pollution

The construction of water supply project will make shallow groundwater and deep groundwater of this area form channels, and if environmental protection on the distribution zone of surface water source is not in place, it will produce the danger of groundwater pollution. Therefore, Attention should be paid to groundwater pollution as an environmental hydrogeological problem of water supply project.

4.4 Impact on agriculture

As this region is mainly farmland with large areas, after the project of water supply is established, along with a large area of exploitation of groundwater resources, the groundwater level in the region will decline, which would lead to the reduction in the moisture content of the vadose zone, and would affect the growth of crops to a certain extent.

5 Conclusions

To sum up, during the construction of the water source, reasonable and effective measures for protecting groundwater environment should be taken according to the influence on the groundwater environment. At the same time, long-term supervision and planning should be strengthened and thus the water source can be used properly in an orderly way.