Groundwater monitoring in Beijing plain

Since 2003, China Geological Environment Monitoring Institute has organized and implemented the "Monitoring and Prediction of Groundwater in Typical Areas" project, which is attended by Beijing Geological Environment Monitoring Station, Shandong Geological Environment Monitoring Station and Xinjiang Uygur Autonomous Region Geological Environment Monitoring Institute. Three demonstration areas, Beijing Plain, Xinjiang Urumqi River Basin and Shandong Jinan Karst Spring Area, were selected to carry out groundwater monitoring and demonstration. This section takes groundwater monitoring in Beijing Plain as an example.

First, the historical status and existing problems of groundwater level monitoring in Beijing

In the early stage of groundwater monitoring, only 33 monitoring holes were set up in the suburbs of the city. By the end of 1950s, there were 558 dynamic monitoring holes of groundwater level in China. During the "Cultural Revolution", the number of monitoring holes decreased year by year, and the monitoring work in remote suburban counties was even interrupted. 1979, the number of groundwater monitoring holes in the city recovered to 624; During the period of 1983, the number of monitoring holes in the whole city reached 752, and the Beijing Groundwater Dynamic Yearbook was edited and published. The groundwater monitoring network covers the whole plain area of Beijing (including Yanqing Basin). After 1990s, the rapid development of urban and rural construction led to the destruction of some observation wells. By 2005, there were about 650 groundwater observation wells in China, including 150 special holes and 500 monitoring holes.

Over the years, groundwater level monitoring in Beijing has provided a lot of groundwater information for urban construction and industrial and agricultural development. Due to the rapid development of economy and society in recent years, the demand for groundwater information is increasing. Under such a development trend, there are some problems in groundwater level monitoring in Beijing: unreasonable distribution of monitoring points; The monitoring frequency is unscientific; Backward monitoring methods; The risk of data loss is high.

2. Optimization of groundwater level monitoring network in Beijing Plain.

In order to solve the problems of groundwater monitoring network in Beijing Plain, it is necessary to optimize the monitoring network to make the spatial distribution of monitoring points more reasonable, so as to comprehensively monitor the regional dynamic changes of groundwater. The method adopted this time is a multi-factor comprehensive zoning map method that affects groundwater dynamics.

According to the actual hydrogeological data, four maps of influencing factors of groundwater dynamics are drawn, namely, hydrogeological zoning map, vadose zone characteristic zoning map, groundwater recharge zoning map and groundwater local influence zoning map, and then the four maps are superimposed to obtain a multi-factor comprehensive zoning map affecting groundwater dynamics. Then, groundwater monitoring points are arranged on the multi-factor comprehensive zoning map that affects groundwater dynamics, so that each groundwater dynamic zoning is controlled by monitoring points. According to these four maps of influencing factors, a multi-factor comprehensive zoning map affecting groundwater dynamics is obtained. A total of 260 dynamic type areas were identified, each representing the synthesis of four different factors, which may have unique temporal and spatial variation characteristics of groundwater level. A dynamically typed region is named by a combination of element names. For example, a dynamic type area located at the top of the alluvial fan of Yongding River is Yongding River subsystem-single sand pebble-strong recharge area-Yongding River influence zone. Due to the thick aquifer, high permeability of vadose zone, large rainfall recharge and the recharge of Yongding River, the main characteristics of groundwater dynamics are large seasonal variation, strong horizontal runoff and short retention time. The other area is located in the alluvial plain in the lower reaches of Yongding River, and its dynamic type is Yongding River subsystem-multi-layer sand with a small amount of gravel-medium recharge area. Its main dynamic characteristics are small seasonal variation, obvious vertical seepage, slow horizontal runoff and long retention time.

Third, the layout of underground water level monitoring holes

Groundwater dynamic zoning map is the main basis for monitoring network design. Only by using monitoring wells to control every multi-factor comprehensive zoning that affects groundwater dynamics can we really monitor the regional changes of groundwater dynamics. At the same time, in the design process, it is necessary to focus on Beijing urban area, water level drop funnel, large water source area and main valley recharge area.

According to the investigation results of groundwater monitoring network, the well-monitored and available monitoring points are projected on the groundwater dynamic zoning map, and then new monitoring points are added in the dynamic zoning without monitoring points distribution. There are 153 phreatic monitoring points (dark blue points) in Beijing Plain. On the multi-factor comprehensive zoning map that affects groundwater dynamics, 108 phreatic monitoring points are added, among which 36 monitoring wells (light blue triangles) monitor piedmont recharge; 38 monitoring wells (light blue spots) monitor the interconnection between rivers and groundwater; 34 monitoring wells (light blue diamond) monitor the groundwater level in the blank area. The groundwater aquifer in Beijing Plain consists of 26 1 monitoring wells and a regional groundwater monitoring network (Figure 9- 1).

Figure 9- 1 Distribution Map of Groundwater Monitoring Wells in Beijing Plain

Four. Selection of automatic monitoring instruments for groundwater

The acquisition of water level and water temperature monitoring information at groundwater monitoring points will mainly adopt automatic instrument monitoring, supplemented by a small amount of manual monitoring. Using modern pressure and temperature sensor technology and digital storage technology, the basic principles of automatic monitoring instruments for groundwater level and water temperature produced at home and abroad are the same. The manufacturing technology and power consumption design of domestic products are far behind those of foreign products. The main difference is that foreign products are aimed at the characteristics of large changes in outdoor environment temperature. In order to improve the use efficiency of sensor and battery, the sensor, memory and its power supply battery are designed together and placed in groundwater with constant temperature environment, which successfully solves the problem that the efficacy and life of field power supply battery are greatly reduced due to temperature change. Coupled with the precise design of low-power integrated circuits, batteries powered by sensors and memories can generally be used for 8 ~ 10 years, which is almost the same as the life of displays.

At present, domestic products are in the stage of combining research with small batch production. Strictly speaking, they have not reached the formal product stage, and generally have large volume, relatively rough technology and high power consumption. Most sensors are placed in underground water, and storage and power supply batteries are placed in the wellhead protective cover; Although some sensors are combined with storage, the power battery is still placed in the wellhead protective cover. Because the temperature change in the protective cover is greater than the temperature change, the battery efficiency decreases or even fails, and it cannot be monitored normally. The service life of batteries is generally less than half a year, and the failure rate of equipment is high.

Diver of the Netherlands, Keller of Switzerland, J-WW- 1 of Hydrogeology and Environmental Geology Survey Center of China Geological Survey, XY-III of Xi 'an Xinyuan High-tech Company, Level of Canada and Insitu of the United States were installed in Beijing Demonstration Zone, and their performances were compared (Table 9- 1).

Table 9- 1 List of Monitors Used in Demonstration Zone

Verb (abbreviation of verb) Selection of data wireless transmitter

At present, there are two ways to transmit groundwater monitoring data: short message and data stream. China Mobile's GSM short message mode signal covers a wide range, but its GPRS data flow mode is not the whole network coverage, so the coverage is small. China Unicom's CDMA short message mode has a wide signal coverage, while the CDMA data stream mode is the whole network coverage-as long as there is a mobile phone signal, data stream transmission can be carried out, and the coverage is also wide. See Table 9-2 for the comparison of data stream transmission performance between them.

Table 9-2 Comparison Table of GPRS and CDMA Data Stream Transmission

To sum up, considering the characteristics of the two wireless communication networks and the wide demand of groundwater monitoring points, CDMA data flow mode is chosen in the areas where urban monitoring wells are concentrated, which has wide signal coverage, low operating price and low transmission power consumption, and is not easy to cause data blockage; In remote areas with sparse monitoring wells, GSM short message mode is selected, which has wide signal coverage, but high operating cost and high transmission power consumption, so data blocking caused by too many terminals should be avoided. Schematic diagrams of wireless transmission systems in these two modes are shown in Figure 9-2 and Figure 9-3.

Figure 9-2 Schematic diagram of wireless transmission system of groundwater monitoring data based on GSM short message.

Figure 9-3 Schematic diagram of wireless transmission system of groundwater monitoring data based on CDMA/GPRS data flow mode.

VI. Protection Scheme for Monitoring Hole of Groundwater Level

In order to protect the whole monitoring system, a special monitoring wellhead protection device is specially designed, which is durable and suitable for wireless signal transmission. The device includes a reinforced concrete base and an orifice cover made of thick steel plate (Figure 9-4). The orifice cover is designed with a firm locking device, and the wellhead cover can only be opened with special tools. The protective cover of the monitoring well is made of steel drums with a diameter of not less than 34 cm, and the signal transmitter is placed in the protective cover. For wireless signal communication, a 20 cm hole was opened on the top of the protective cover, and then it was sealed again with engineering plastic. This method can not only keep the strength of the protective sleeve basically, but also meet the needs of wireless communication.

Figure 9-4 Monitoring well orifice protection device