A Study on a Method of Using Plants to Purify Leachate Containing Heavy Metals
Figure-1 Illustration of
phytoremediation (Sakakibara, 2015)
Photo-1 Purification experiment in
water storage tanks
Figure-2 Change in selenium concentration
(Click to Enlarge)
Background and aim of the study
In civil engineering, the excavation of ground generates rock and soil. That rock and soil can contain native heavy metals such as arsenic and selenium. When such excavated rock and soil are stored temporarily at excavation sites and exposed to precipitation, leachate containing heavy metals flows out. As a technique for purifying such leachate, phytoremediation has attracted attention. It uses plants for environmental remediation. The technique uses a plant's ability to break down harmful substances, such as heavy metals, that the plant absorbs from the soil and groundwater when it takes in water and nutrients through its stomata and roots (Figure-1). However, few civil engineering projects have used plants to purify leachate contaminated by heavy metals, and practical techniques have not been established. This study examines a method of using Eleocharis acicularis (E. acicularis), a plant in the Cyperaceae family, to purify leachate from construction-generated rock and soil that contains native heavy metals.
Outline of the research results
The Cyperaceae E. acicularis is a perennial that grows in clumps at ponds, reservoirs and paddy fields, and it is distributed throughout Japan, from Hokkaido to Okinawa. E. acicularis propagates with runners that branch off from the root area. E. acicularis is tolerant of a wide variety of heavy metals, and it absorbs these. E. acicularis was placed on water in storage tanks for purification experiments.
The E. acicularis was cultivated in plastic containers that were mounted on empty plastic bottles such that they floated in the water storage tanks (Photo-1). The selenium concentration in the leachate from construction-generated rock and soil was found to be reduced (Figure-2). In addition, an experiment using an artificial channel determined that E. acicularis reduced the arsenic concentration in leachate and absorbed arsenic.
In the future, research and studies will be conducted on the purification effectiveness and applicability of E. acicularis under different conditions, such as different weather conditions and leachate supply conditions.
(Contact: Geological Hazards Research Team, Civil Engineering Research Institute for Cold Region)
An Effective method for Controlling Invasion by Riparian Trees in the Satsunai River
In recent years, many rivers have faced serious invasions by riparian trees in their water channels, and this has posed serious problems to flood control and environmental protection. Frequent disturbances to bars in low-water channels cause vegetation to flow out as seedlings and young trees and consequently prevent the development of trees. However, once a bar is covered with forest, the water flow concentrates in a narrow watercourse and avoids the bar, and a watercourse is established. The location of the bar is also established, and the invasion of channels by riparian trees further progresses.
The Satsunai River, a tributary of the Tokachi River, has also been experiencing the invasion of channels by riparian trees that cause dry gravel riverbeds to decrease (Photo-1), and there have been concerns over the deterioration of habitats for animals and plants that live on dry gravel riverbeds, such as Chosenia arbutifolia, plovers and Japanese wagtails.
Photo-1 Thick, extensive tree growth and established watercourses on the Satsunai River
To solve such problems on the Satsunai River, our institute has been conducting a study on an effective method for controlling the establishment of watercourses on the Satsunai River. Major research results and developments of the study on Satsunai River are described here.
1.Artificial flood flushing from Satsunai River Dam has been conducted to restore dry gravel riverbeds since FY 2012. This study has been examining the effects of such artificial flood by understanding the characteristics of the flow regime through onsite investigations at the time of such discharge.
Photo-2 Observation of the flow regime at the time of artificial flood flushing from the dam
2.Investigations of channel morphology on the Satsunai River revealed that many of the braided channels (former watercourses) that had existed on bars were blocked, that the bars had been established because channels had not been disturbed, and that the establishment of the bars was a factor in the expansion of the forest area. Therefore, a method has been proposed for selecting an excavation location that maximizes the effectiveness of excavation to restore the blocked bifurcation leading to the former watercourses based on an understanding of the characteristics of channel morphology and the characteristics of the flow regime at the time of the artificial flood. The effectiveness of the method has been verified through onsite tests.
Photo-3 Restoration of the former watercourses by excavation at the former bifurcations
Technical proposals and advice reflecting the results of this study have been made to the Advisory Committee on the Satsunai River (the committee secretariat is part of the Flood Control Division of the Obihiro Development and Construction Department, Hokkaido Regional Development Bureau). The committee examines techniques for the restoration of dry gravel riverbeds. The results of our study have been utilized by the advisory committee in the project for the regeneration of nature on the Satsunai River (restoration of the dry gravel riverbed).
(Contact: River Engineering Research Team, Civil Engineering Research Institute for Cold Region)
Development of a New Type of Snow Fence That Can be Installed at the Immediate Roadside
At present, "blower snow fences" that can be installed at the immediate roadside are frequently used as blowing-snow control measures in cold, snowy regions. However, it was clarified that the snow-control performance of the blower snow fence decreases when the incident wind is oblique or when the bottom clearance of the fence is blocked by snow accumulated below the fence (Figure-1).
To solve such problems, the Civil Engineering Research Institute for Cold Region has developed a snow fence that is designed to be installed at the immediate roadside in a way similar to that of conventional blower snow fences but whose snow control performance is retained even when the incident winds are oblique or when the bottom clearance is blocked. (Patent Number: 5610251)
Figure-1 Problems with conventional blower snow fences when the bottom clearance is blocked
The snow fence that can be installed at the immediate roadside is a fence made of long, vertical snow control plates with a v-shaped cross-section butted close together (Photo-1). The fence mitigates visibility hindrance by using slanted snow control panels to divert the wind downward. Strong winds blowing through the bottom clearance of about 1m in height blow snow from the road.
Photo-1 A new type of snow fence that can be installed at the immediate roadside
The new type of snow fence that can be installed at the immediate roadside has the following advantages (Figure-2).
(1) Snowdrifts forming on the road are controlled because the fence traps the snow particles on its windward side when the bottom clearance is blocked.
(2) The v-shaped snow control plates divert oblique incident winds downward and retain the snow blowing function.
(3) Mechanized removal of snow at the bottom clearance can be conducted efficiently because the snow control plates are installed at a certain distance from the supporting pillars.
Figure-2 Advantages of the new type of snow fence that can be installed at the immediate roadside
Field experiments using a full-scale fence at the Ishikari Blowing Snow Test Field clarified that the new type of snow fence for installation at the immediate roadside has greater snow control effectiveness than that of the conventional blower snow fences. The transfer of technical knowledge to private companies is being carried out.
(Contact: Snow and Ice Research Team, Civil Engineering Research Institute for Cold Region)