JP3369111B2 - Evaluation method of purification effect of contaminated ground - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for quantitatively evaluating a cleaning effect when cleaning soil of contaminated ground by a soil gas suction method.

[0002]

2. Description of the Related Art In recent years, a soil gas suction method has been used as a method for cleaning soil contaminated with volatile organic compounds. This method is to suck the soil gas in the ground to the surface of the ground from a suction well formed in the ground, thereby entraining the volatile organic compound in the soil gas. For example, as shown in FIG. Be implemented. That is, the soil gas is sucked from the suction well 1 formed in the ground by the vacuum pump 2, and the volatile organic compounds entrained in the soil gas are adsorbed by the activated carbon layer of the adsorption tower 3 and then released into the atmosphere. At that time, a gas-liquid separator 5 is also installed to separate moisture accompanying the sucked soil gas. Volatile organic compounds dissolved in water are separated by the gas-liquid separator 5.

[0003]

In carrying out such a soil gas suction method, even if some preliminary research is conducted on the properties of the target ground and the concentration of volatile organic compounds in the ground. However, the suction effect of volatile organic compounds from the suction well is often unknown unless actually observed. In other words, since it is difficult to quantitatively grasp the air permeability characteristics of the ground to be treated in advance, it is almost dependent on experience to determine the position and scale of the suction wells and the number of suction wells. However, the effect is often found only after the soil gas suction method is implemented.
Therefore, it was difficult to obtain purification results efficiently.

Therefore, the present invention has developed a simple method for quantitatively evaluating the air permeation characteristics of unsaturated soil, and designed a soil gas suction method (determination of the position, number, distribution, etc. of suction wells). It is intended to be useful.

[0005]

According to the present invention, when the soil gas suction method is used to clean the contaminated soil, a soil gas suction hole is provided in the ground to be treated and a predetermined distance is provided from the suction hole. and soil gas observation hole provided in the ground, the CO ₂ concentration at the observation hole while sucking soil gas from the suction hole was measured, change in soil CO ₂ concentration at the observation hole positions measured Provides a method for evaluating the effect of cleaning contaminated ground, which is characterized by evaluating air permeability characteristics such as permeability of the ground.

[0006]

DETAILED DESCRIPTION OF THE INVENTION The present inventors pay attention to the fact that CO ₂ gas having a higher concentration than that in the atmosphere is present in the ground due to biological activity, and the CO ₂ gas in the ground is absorbed. An attempt was made to use it as a tracer gas when experimentally examining the air permeability characteristics (permeability and radius of influence) when sucking the gas in the ground through the holes.

That is, first, a soil gas suction hole is provided in the ground to be treated, and a soil gas observation hole is provided at a predetermined distance from the suction hole. For both the suction hole and the observation hole, a PVC pipe or steel pipe with a specified inner diameter (for example, 25 to 50 mmφ) is buried almost vertically in the ground, and a strainer about the thickness of the layer to be purified is attached to the end Can be formed by.

At the suction hole, connect this tube to a vacuum pump,
By operating the vacuum pump, soil gas is drawn from the strainer. On the other hand, in the observation hole, the upper end of the tube is closed and a device for measuring the CO ₂ concentration and pressure in the tube is connected. As a result, the CO ₂ concentration and pressure in the ground are balanced with those in the pipe through the strainer in the observation hole, so that the CO ₂ concentration and pressure in the ground where the strainer is located can be measured.

It is preferable that a plurality of the observation holes are provided at different distances from the suction holes, and when the suction operation is performed in the suction holes, the change in CO ₂ concentration and the pressure change in each of the observation holes are simultaneously measured. .

The depths of the suction holes and the observation holes may be appropriately selected depending on the type of the target ground. In the present invention, since the air permeability characteristic of the unsaturated ground can be measured, the strainer is installed in the unsaturated ground to be cleaned. It is preferable that the suction hole and the observation hole are provided so that

The effects of the present invention will be specifically shown below with reference to an example of an in-situ air-permeation experiment conducted by the present inventors.

As shown in FIG. 2, there is a loam layer of about 2.5 m below the surface soil of about 2.5 m (black ground), the lower layer is a silty fine sand layer, and the groundwater level is 8.5 m. Targeting the ground. The ROHM layer has Wn = 72.3%, Sn = 8
7.5%, n = 68.3%, CO ₂ concentration is 9000
~ 21000 ppm. By the way, the CO ₂ concentration in the atmosphere is about 400 ppm. The silty fine sand layer is W
n = 34.5%, Sn = 77.2%, and n = 55.3%.

A suction hole R-1 was provided in this ground. This suction hole R-1 has a length of 25 mm at the lower end of a PVC pipe with an inner diameter of 50 mm.
A strainer of cm was attached, and the burial depth from the surface to the end of the strainer was 3.6 m. The upper end of this steel pipe was connected to a vacuum pump (not shown).

From the suction hole R-1, 1.25, 2.5, 5.0, 7.5,
Observation holes R-2, R-3, R-4, R at 10.0 m
-5 and R-6 were provided. Each of these observation holes was equipped with a strainer of the same length as the steel pipe with the same diameter as the suction hole of R-1, and the burial depth of the strainer was 3.
Although the length was 6 m, the upper end of the steel pipe was closed, and a device (not shown) for measuring CO ₂ concentration and pressure inside the pipe was connected. Also,
An observation hole S-6 having the same structure with a strainer burial depth of about 6 m was provided at another position at the same distance as R-6 so as to communicate with the silty fine sand layer.

A negative pressure was forcibly generated in the loam layer from the suction hole R-1 by a vacuum pump (suction air volume ≈1.4).
m ³ / min, initial suction negative pressure ≈ 75 kPa). The changes over time of the suction air volume Q and the suction negative pressure P ₀ are shown in FIG. Figure 3
According to the figure, the suction air volume in the suction hole R-1 increases with time, while the suction negative pressure decreases with time.

FIG. 4 shows a change with time of the generated negative pressure P _i observed in each observation hole when the suction is performed through the suction hole R-1 under the above conditions. According to FIG. 5, it can be seen that the negative pressure generated at each observation position also increases with time as the suction air volume increases with time.

FIG. 5 shows the change with time of the CO ₂ concentration observed in each observation hole when suctioned from the suction hole R-1 under the above conditions. The distance r from the center of the suction hole on the horizontal axis is a logarithmic scale. According to FIG. 5, C of the observation hole near the suction port
It can be seen that the O ₂ concentration decreases rapidly with the start of suction, and the CO ₂ concentration in the observation hole far from the suction hole begins to gradually decrease after a while from the start of suction. That is, the behavior of how the CO ₂ concentration in the soil at each observation hole position decreases (how it changes with time until it approaches the atmospheric CO ₂ concentration) is quantitative. Can be evaluated.

Therefore, from the results of FIGS. 4 and 5,
It is possible to grasp the air permeability characteristics when the gas is sucked in the unsaturated ground, that is, the permeability and the influence radius, and in particular, the gas suction effect at each position can be quantitatively evaluated from the results of FIG.

[0019]

As described above, according to the present invention,
When soil gas is sucked, the suction behavior of gas in the surrounding soil can be quantitatively grasped beforehand. Therefore, based on this measurement result, the size, number, and arrangement of suction wells can be determined, and the soil gas suction method, which has relied on conventional experience, can be modified to a method with a reliable result. Therefore, soil polluted with volatile organic compounds can be efficiently purified by the soil gas suction method.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing an embodiment of a soil gas suction method.

FIG. 2 is a diagram showing an arrangement of suction holes and soil gas observation holes showing an example in which the method of the present invention is performed on the ground of a loam layer.

FIG. 3 is a diagram showing changes over time of suction air volume and suction negative pressure in the suction holes of FIG.

FIG. 4 is a diagram showing a time-dependent change in generated negative pressure measured in each observation hole when suction is performed as in FIG. 3 from the suction hole in FIG.

5 is a diagram showing a change with time in CO ₂ concentration measured in each observation hole when sucked through the suction hole in FIG. 2 as in FIG.

[Explanation of symbols]

1 suction well 2 vacuum pump 3 adsorption tower 4 activated carbon 5 gas-liquid separator

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01N 33/24 G01N 1/24

Claims (4)

(57) [Claims] 1. When performing soil purification of contaminated soil by the soil gas suction method, a soil gas suction hole is provided in the ground to be treated, and a soil gas observation hole is provided in the ground at a predetermined distance from the suction hole. The CO ₂ concentration in the observation hole is measured while suctioning the soil gas from the suction hole, and the air permeability such as the permeability of the ground is measured from the change in the CO ₂ concentration in the soil at the measured observation hole position. A method for evaluating the effect of cleaning contaminated soil, characterized by evaluating its characteristics. 2. The evaluation method according to claim 1, wherein the negative pressure generated in the soil gas observation hole is simultaneously measured. 3. The evaluation method according to claim 1, wherein a plurality of soil gas observation holes are installed at different distances from the suction holes. 4. The evaluation method according to claim 1, wherein the ground to be purified is soil contaminated with a volatile organic compound.