Groundwater pollution monitoring

Due to industrial wastewater, domestic sewage or other pollution, such as waste residue, slag, pesticides, fertilizers, sewage irrigation and other reasons, groundwater is polluted by surface water.

Fig. 6-2- 1 1α track measurement and magnetic field measurement results along the landslide direction

(According to Li Shuyi 1987)

1-α orbit measurement curve; 2— Magnetic field measurement curve; ρ represents the alpha track generation rate, and 30 days is the cup burying time.

The exploration of groundwater pollution is mainly to determine the location and scope of pollution sources. In most cases, pollutants and their displacement are directly monitored through inspection holes. But sampling drilling is time-consuming and expensive. Under certain conditions, the location of polluted area can be determined quickly and economically by geoelectric method.

The research shows that the change of resistivity before and after groundwater pollution is only related to the ion concentration in the water when the pore size and connectivity of the aquifer are basically unchanged. If the difference of groundwater resistivity before and after pollution is obvious, the polluted body has a certain thickness, its buried depth is not deep, and the surface electricity is relatively uniform, then the location, scope and pollution degree of pollution source can be effectively determined by resistivity method.

Here are two examples of monitoring groundwater pollution by resistivity method.

(1) resistivity method for monitoring groundwater pollution in a petrochemical plant.

The industrial wastewater and domestic sewage discharged into Liuhe by a petrochemical plant exceeded the standard for a long time, causing groundwater pollution. In order to find out the distribution of pollution zone, resistivity method has achieved good results.

The survey area is covered by Quaternary system, and the lithology is sandy pebble, clayey soil and sandy pebble interbedded, with a total thickness of 80 ~ 100 m generally ... The bedrock is limestone of Middle Ordovician. In the dry season, the Liuhe River has been cut off before it flows through the factory. At this time, the dry river becomes a channel for collecting and dispersing pollution, and the infiltration and lateral infiltration of sewage form a sliding river pollution zone, which causes deep groundwater pollution. Through physical property determination, the resistivity of sewage is 8 Ω m, and that of pure water is 90 Ω m, with a difference of 10 times. This provides a sufficient geophysical premise for the resistivity method to divide the polluted area. The input mode is symmetrical quadrupole electrical sounding, and the direction of the survey line is perpendicular to the river direction. The survey line density depends on the specific situation. In polluted areas, the density of survey lines is dense, while in unpolluted areas, the density of survey lines is sparse. The distance between points is 100 m, and the distance between electrodes is AB/2 = 4 ~ 150 m ... Because the surface resistivity of riverbed and floodplain is very high, reaching N×102 ~ N×103 Ω m, in order to eliminate the influence of high surface resistivity, the anomalies are highlighted. The results are shown in Figure 6-2- 12 to Figure 6-2- 18, from which the following points can be seen.

1)ρS and ρz curves show different characteristics in unpolluted area and polluted area. On the plane, the contour lines of ρS and ρz in the unpolluted area are distributed in strips, with narrow anomaly width and high ρSρz value, ρS is generally greater than 300 ρ m and ρz is generally greater than 200 ρ m ... In the polluted area, the contour closed circle is irregular in shape, with wide anomaly width, low ρS and ρz values, Ps is generally between 50 and 200 Ω m, and Pz is generally less than 200 961 On the cross section, the ρS and ρz isolines in the unpolluted area are smooth and continuous near the river bed, and there is no low-resistance anomaly (Figure 6-2- 14 and Figure 6-2- 15). In the polluted area, due to the downward and lateral infiltration of sewage, a triangular pollution circle is formed below the ground. Near the river bed, the contours of Ps and Pz are distorted, and the low resistance anomaly of herringbone distribution appears. The location of herringbone anomaly is the location of river bed runoff (Figure 6-2- 16 and Figure 6-2- 17).

Figure 6-2- 12 AB/2=35m ρS contour map

(According to Ji Zhenglian 1988)

Fig. 6-2- 13 AB/2=35m ρz contour map

(According to Ji Zhenglian 1988)

Fig. 6-2- 14 equal ρS profile of electric sounding in unpolluted area

(According to Ji Zhenglian 1988)

Fig. 6-2- 15 equal ρz profile of electric sounding in unpolluted area

(According to Ji Zhenglian 1988)

Fig. 6-2- 16 electrical sounding profile of polluted area.

(According to Ji Zhenglian 1988)

Fig. 6-2- 17 electrical sounding profile of polluted area.

(According to Ji Zhenglian 1988)

Fig. 6-2- 18 Distribution Map of Contaminated Area

(According to Ji Zhenglian 1988)

2) The gradient zone of ρ s and ρz isolines reflects the boundary of pollution circle. The pollution range can be defined by the connecting line of inflection point of herringbone gradient zone on equal ρS and ρz sections. Fig. 6-2- 18 is the distribution map of pollution zone enclosed by each section. Compared with the results of environmental hydrogeological investigation, the range of pollution zone delineated by geophysical exploration is basically the same as that of heavy pollution zone delineated by hydrochemical investigation. The results of water quality analysis of exploration wells in polluted zone show that there are many pollution items, which obviously exceed the standard. However, in the water quality analysis results of exploration wells outside the pollution zone, there are not many pollution items or obviously exceed the standard.

3) Because of the different strata on both sides of the river bed and the different pollution depths, the low-resistance herringbone anomaly is often asymmetric. The pollution depth can be roughly estimated by the polar distance (AB/2) corresponding to the inflection point of herringbone gradient belt. As shown in Figure 6-2- 16, the depth of herringbone abnormal pollution is estimated to be 30m on the left and 70m on the right [6].

(2) Monitoring the environmental pollution caused by industrial and mining wastewater

The wastewater from the factory is discharged into the ground, which not only pollutes the water source, but also accelerates the development of underground karst in some areas. For example, acid wastewater from a sulfuric acid plant seeps into the ground, dissolving gypsum rocks, expanding the original caves, creating new caves and forming new underground passages. Along these channels, dissolved substances flow into nearby rivers.

Through the resistivity measurement between the ground and the river, the channel position of karst water can be delineated and the development of karst with time can be studied. From t 1 and t 19 and t 19 in Figure 6-2, it can be seen that the low range of apparent resistivity on the t 1 curve observed at t2 is widened, which is the result of widening the karst cave area due to the dissolution of acidic wastewater. The range of low resistivity observed at t2 in Figure 6-2- 19 is larger than that at t 1, indicating that the dissolved substances flowing into the river are obviously increased due to the action of acidic wastewater.

Fig. 6-2- 19 monitoring the influence of factory wastewater on karst process by electrical method

(According to Ji Zhenglian 1988)

(a) low resistivity range at t1and t2; (b) Apparent resistivity curves measured at t 1 and t2 on section II.

1-observation profile; Karst water channel bidirectional