Hangzhou xx & Dyeing Co., Ltd is located in Tangqi Town, Yuhang District, Hangzhou City, Zhejiang Province. The factory is divided into two parts, east and west. The production workshop and sewage treatment station are mainly arranged on the west side, and the warehouse and office building are mainly arranged on the east side. See Fig.
Working purpose and task
The underground pollution of the site is entrusted to be investigated, and the purposes of the geophysical exploration task are:
1）To detect the underground soil pollution area of the factory area;
2）To identify the thickness and range of the polluted soil of the site.
High resolution resistivity method (i.e. ERT electrical method) is used for the soil pollution detection.
According to site survey and interview content, non-destructive geophysical method is selected for investigating areas with high pollution potential and serious pollution diffusion, which are located inside production area ② and near sewage treatment tank respectively. Construction is relatively difficult, due to the limited time and cement hardened pavement. Three measuring lines are arranged for analysis as shown in the red area of Fig. 1.3.
FIG . 1 . 3 survey line arrangement region
A horizontal measuring line is arranged in the production area ② printing and dyeing workshop. Lawn in the workshop is badly damaged, and cement hardening integrity is very poor, which cause the printing and dyeing materials directly contact the foundation soil. The first location selected of the original measuring line is the inside of the groove which is always used for wastewater or material transportation or is a barrier area. If there are structural cracks, the wastewater and other possible pollutants will permeate into the soil, polluting soil at the bottom. However, the groove hasn’t been used for a long time, many material wastes, sludge and water are accumulated inside the groove (Fig. 1.5), and there is a hardened base at the bottom of the groove, which is not suitable for electric drill construction and electrode arrangement. Therefore, a transverse resistivity measuring line (Fig. 1.4 and Fig. 1.6) is arranged in the area where serious damages to buildings can be seen nearby to investigate whether there is any pollutant intrusion. The resistivity measuring line has 32 electrodes in total, and the electrode interval is 1m, so the total length is 31m, which is named as line 1 with the gate side as the starting point.t
Figure 1.6 resistivity line 1 actual layout
At the same time, resistivity measuring line 2 (Fig. 1.8) and resistivity measuring line 3 (Fig. 1.9) are arranged in the area around the wastewater treatment tank in a crossed mode, as shown in Fig. 2.5. The reason of the measuring line is mainly for the wastewater treatment tank to collect all waste water and residues of printing and dyeing. However, as a larger pollution source, if the discharge is irregular or the reaction tank itself has leakage, the wastewater will immerse into the soil and cause pollution. Within a limited period of time, focus should be put on the areas which have the most possibility of pollution leakage. For example, two resistivity measuring lines are arranged in the middle and the outer part of the two waste water tanks to investigate the pollution situation, as shown in Fig. 1.7. The resistivity line 2 in the figure extends to the inside of the left pool with a unknown function, which is presumably the rainwater accumulation formed by the buildings. The measuring line has 35 electrodes in total with the electrode interval of 1 m, starting from the end of the water tank. The resistivity measuring line 3 extends to the inside of the production area ⑥, and it has 29 electrodes in total with the electrode spacing of 1m, starting from area nearby the production area ②. The two measuring lines meet at the 11th electrode of the resistivity line 1 and the 6th electrode of the resistivity measuring line 2.
Figure 1.7 resistivity line 2 , 3 aerial photo
Figure 1.8 Survey line 2 layout
Figure 1.9 Survey line 3 layoutc
Geophysical exploration technology is a non-invasive and non-destructive testing technology (NDT), which does not cause invasive damage to the detected object, and has the advantages of large detection range (horizontal and perpendicular range), high speed, high accuracy and etc. The different formation lithology and adhesive state results in different conductive properties which is generally represented by resistivity. Therefore, the resistivity corresponding to different formation lithology and moisture content is indirectly known by detecting the change of formation resistivity in vertical and horizontal directions.
This investigation uses ERT section method to conduct large-area hydrogeological investigation, so as to judge the overall geological structure characteristics in the industrial zone and the abnormal area of formation materials, distinguish the pollution accumulation area and diffusion, and reduce the penetration of the water barrier layer during subsequent well setting and soil sampling operation, to avoid the pollution diffusion to different aquifers.
1. Survey network arrangement
The principle of the geophysical exploration measuring line arrangement is to be according to the actual situation of the factory, the orientation should be determined by a compass, the distance of the point position should be measured by a tape.
Data acquisition with ERT resistivity method: connect the measuring instrument –arrange the cables and electrodes according to the location of the measuring line-connect the cables and converters – input parameters and measure ground resistance – enter data acquisition mode after grounding well – collect data. Other field data collection is carried out in accordance with the relevant regulations or specifications issued by the state.
(A) equipment requirements
ERT resistivity equipment can be divided into:
1）ERT resistance detector (control, display and store resistivity signals )
2）Automatic switch cable groups
3）Stainless steel electrodes
4）Batteries and chargers
5）Data processing software
6）Transport box(for assembling instrument)
7）Tape, electric drill, generator, etc.
8）Attachments (record paper, hammer, sickle, kettle, inclinometer, sun umbrella, etc.)
9）Satellite positioning system or measuring instrument (for large-area measurement or precision positioning)
This geophysical method of this method is based on the site topography, ground features and the target to be measured. At present, pole – pole array, wenner array, Schlumberger array and other various array and other methods are commonly used in domestic, and the array methods should be decided according to the actual situation.
(B) Operation Procedure
The factory operation procedure with ERT resistivity method is as follows:
1）Collect and judge the basic information of the measuring area.
2）Configure the appropriate measuring line according to the measurement purpose and determine the electrode arrangement and electrode bar internal.
3）Record the working date and climate, etc., and make appropriate description of the terrain and ground features near the measuring line.
4）Measuring the control point
5）Traverse measuring. When the measuring line is not long, the distance between measuring points should be measured and controlled by a tape. And the position of measuring points should be determined by measuring instruments in mountainous areas and undulating areas.
6）Connecting the cable leader, ERT cable and the mainframe
7）Measuring the ground resistance of the electrodes to ensure that electricity flows into the ground and, if necessary, filling saline water at grounding position of the electrode bar or replacing the original electrode bar with several electrodes used in parallel.
8） Connecting the battery with relevant joints.
9）The electrical current is used to measure the
2 .Array technology
The instrument used in this site investigation is the ERT system independently developed by Shenzhen Gomative Technology Co., Ltd: GD-10 SUPREME 3D PLUS Multi-electrode Res/IP Imaging System Distributed System.Test using the electrical resistivity method
Figure 2. 2 GD-10 electrical analyzer system
The electrode devices of ERT in this site investigation are wenner – Schlumberger array and edge – gradienter array. The minimum interval coefficient is 1, and the electrode point distance is 1 m. The total number of electrodes array is determined according to the site, and the detection depth is within 10m. Before measuring, the first thing is to detect the electrode grounding resistance. If the grounding resistance is normal, data collection and measurement can be carried out. The schlumberger resistivity method is one of the common methods in resistivity sounding.
1.Result Analysis of Measuring line 1
Resistivity results of measuring line 1 of wenner-sclumberger and edge-gradient arrays
There is no geological soil type data on site. It is known that the main soil composition of Hangzhou is clay, normally the resistivity of which is about 60 Ω–m, or as low as 30 Ω–m if wet. Therefore, as shown in the above figure, the whole strata substances present a ultra-low resistivity, which can be inferred that there is a serious pollution intruded into the field; the measuring results of both arrays show high resistivity between electrodes 6-8 where are located at 5-7m of the whole measuring line, and it can be inferred that there might be holes or pipelines in the building structure; while the resistivity below is as low as a fewΩ–m, so we can figure out that the accumulated pollutants of the upper pipelines diffuses into subsoil and causes the low resistivity phenomenon in depth of 1m-2.6m in this area. From the ground surface, it can be seen in the there is hardening cement and pavement distress, and the black soil components and some production waste can be seen on the surface, which may cause materials or waste materials pollution to permeate through the damaged pavement to the strata.