Program October 11th, :00-5:00 pm Van Horne Ballroom A 2:00-8:00 pm Conference Centre Lobby, Upper Level

Transcription

1 Phytoremediation as a Technology for Both Risk Management and Remediation at a Former Herbicides Production Site Catherine Creber 1, Edward Gatliff 2, Frank Manale 3, Joanne West 4, Dave Wandor 5 A former herbicides manufacturing plant, part of a larger chemical complex, ceased operations and was demolished in the mid 1980 s. Risk management systems were instituted to minimize the potential for adverse impact to human health and the environment. While the application of these systems will ensure that there is no impact to the environment and human health in the short term, there is the desire to apply remedial technologies that will ultimately reduce residual shallow soil and groundwater contamination and remove the need for institutional or physical controls. Phytoremediation has the potential to be the lowest cost solution to control groundwater movement, while also reducing contaminant concentrations in groundwater and ultimately creating in-situ conditions that will result in degradation of contaminants in the soil. Growing plants on a site with a short growing season, harsh winters, high concentrations of herbicide compounds and poor soil conditions was rightly viewed as an interesting challenge that would require a special approach to maximize the chances to succeed. Phytoremediation on this site is being evaluated in a large scale pilot study to determine its suitability as the technology that can be employed as both a short term risk management technique, and the long-term remedial technology, consistent in all aspects with the policy for risk management of contaminated sites in Alberta. The manufacturing plant had operated for over 20 years producing various herbicides, primarily 2,4- dichlorophenoxyacetic acid ( 2,4-D) as well as dichlorophenol. Other chlorinated phenols were produced as byproducts. The historical operations have resulted in concentrations of 2,4-D and chlorophenols exceeding CCME generic values in both soil and groundwater at depths of.3 to 6 metres below grade. Upon ceasing production, the equipment was demolished and an asphalt cover placed over most of the former operating area of the site to reduce surface water infiltration and mitigate the potential for soil transport. As the geology of the area consists of fairly low permeability clays, resulting in low flux of contaminants to any potential receptor, the immediate need to manage the primary risks to human health and the environment was met with the placement of the asphalt cover. The soil and groundwater management plan defines the process for characterization, fate and transport modeling and risk assessment that will lead to the development of any further site specific risk management requirements. Although the risk assessment is still in progress, it is expected that the remedial program for this site will require long-term risk management controls, such as mitigation of lateral and vertical groundwater transport. However, Alberta policy 1 of 6

2 defines an ideal remedial program as one that can achieve the desired environmental conditions without the need for risk management controls. The search is on for the remedial solution for this site. Hydraulic control is traditionally the most expensive portion of remedial programs, not only because of the capital expenditure, but due to the annual operating, maintenance, treatment and monitoring costs, which can continue for decades. On this site, many conventional water removal and treatment technologies have been evaluated, however, none were satisfactory on the basis of cost or long-term effectiveness in removing residual concentrations remaining in the shallow soil and groundwater. Phytoremediation as a technology holds the promise to be an effective and cost efficient means to control groundwater movement and contain contaminated plumes. Phytoremediation can also remove contaminants from the soil and groundwater through combined biodegradation by plants and their associated rhizosphere (root-zone) microbial populations. Plant roots supply nutrients and important food substrates in the form of exudates and organic carbon to the surrounding microbial populations. These diverse microbial communities are essential for effective degradation of herbicides such as 2,4-D and chlorophenols. The planting of vegetation in affected soils can stimulate the development and growth of a more dynamic microbiological community. Even in cases of plant death, the dead plant s root tissue continues to contribute organic carbon to the rhizosphere, which helps sustain microbiobial populations and more importantly the degradative strains and their ability to continue breakdown of contaminants. With continued degradation of contaminants, mortality can be reduced in successive plantings. On this site, the asphalt cover limits oxygen and plant roots from entering site soils. The absence of oxygen and plant roots renders these soils a poor media for the growth and development of diverse and dynamic microbial communities. However, applying phytoremediation while leaving the asphalt cover in place greatly limits surficial water recharge and eliminates the expense of removal. Employing a phytoremediation system precludes the need to remove the asphalt.. The TreeMediation system is a patented planting methodology that helps to create a chemical sink at each planting location, hydraulically and diffusionally drawing in contaminants and contaminated groundwater. The system is designed to employ bacteria assisted degradation to act as a remedial system to buffer the plant from the full effects of the contaminants. A limited trial of the TreeMediation system was conducted in 2002 on a small section of the site contaminated with high levels of 2,4-D to determine if specific trees and grasses could remain viable in the harsh winter climate, and hostile soil conditions. Only a few of the 16 trees survived. Those that did survive were able to tolerate an unusually harsh winter the year of planting and basal feeding by rabbits that girdled the trees or caused severe damage. This continued survival and growth of the remaining trees and promising groundwater data led to an extended large scale pilot project that was implemented in the early summer of of 6

3 The large scale pilot project was designed to provide information in three distinct areas of interest. Several research studies were planned to monitor the plant survival, hydraulic control, and contaminant reduction. A Treatment Effectiveness Study area was setup to evaluate the survival of the trees using varying planting designs; a Species Viability Study was established to evaluate the ability of different tree species to survive; and an Area of Influence Study was designed to monitor the hydraulic control within a four tree network. Treatment Effectiveness Study Species Study Area of Influence Study The monitoring program was designed to collect information on plant survival and vigor, water drawdown, contaminant concentrations in groundwater taken from both monitoring wells within the tree plantings and monitoring wells placed between tree plantings. Four hundred and seventy-five (475) trees were planted over a ½ hectare area in the early summer of 2005, using the TreeMediation technology. A jackhammer was used to break up the asphalt and 61 cm holes were augered into the soil on a grid defined by 3 of 6

4 the design of the research studies, up to a depth of 2.4 metres below grade. Organic matter and fertilizer were incorporated into the planting technology as well as a system to promote oxygen enrichment at the root zone. Prior to planting, plastic guards were wrapped around the trunk of each tree to prevent rabbits from girdling or otherwise damaging the base of the tree. Monitoring wells were installed within many of the augered holes concurrent with the planting of the trees. Monitoring wells were also planted between the tree rows at various locations. Trees were planted on 4.5 metre centres, except in the Treatment Effectiveness Study where the tree spacing was 3 metres apart, with 4.5 metres between rows. The Treatment Effectiveness Study incorporated variations on this planting technology in a randomized complete block design to study the value and interaction of different fill materials, oxygen enhancement and degree of isolation of the roots from the native material. This study is concentrated in the middle of the site and studies the three variables with sixty-four (64) laurel leaf willows. To determine the most viable, effective trees capable of growing on this site six different species were planted in randomized complete block design at the four corners of the site in the Species Viability Study. The species used in this test were: Populus nigra (Theves Poplar), Populus tremuloides (Quaking Aspen), Salix pentandra (Laurel Leaf Willow), Fraxinus pennsylvanica (Green Ash), Celtis occidentalis (Hackberry); and Betula papyrifera (Paper Birch). These trees were selected for their phreatophytic (deep-rooted and water-seeking) properties and/or known hardiness in this climate. High transpiration rates make phreatophytic trees ideal for hydraulic control of contaminated groundwater, reducing contaminant migration while simultaneously remediating groundwater. Once mature, the extensive root systems of adult trees should promote widespread contaminant degradation in the rhizosphere. Vigor or mortality of these trees is being evaluated through visual examination early and late in the growing season. A monitoring program was designed to collect information on plant survival, water drawdown, contaminant concentrations in groundwater taken from both monitoring wells within the tree plantings and monitoring wells placed between tree plantings. The Area of Influence Study utilizes 4 willow trees and incorporates an in-tree monitoring well with 3 additional progressively and spirally spaced monitoring wells 4 of 6

5 surrounding each tree. This configuration enables the monitoring of drawdown capability by the tree and the area of influence of the tree. These data will ultimately help determine the optimal number and spacing of trees required to maintain hydraulic control on the site. In addition to hydraulic influence this study will also permit the evaluation of the area of remedial influence by the trees which may also influence future decisions on system design. The plantation has now seen two growing seasons. Monitoring of water table levels, tree vigor and mortality and groundwater concentrations is ongoing. At this early stage of the study, it is not expected that any conclusive results would be available, however, some observations made so far will result in changes to the program for future years. With respect to water drawdown, current data only reflect an artifact of the site conditions. Since efforts were made to isolate the area from the offsite drainage system the site has experienced a shallower than normal water table over the past 4 years. Precipitation has built up under the asphalt cap to the point that the water table is very near the surface. While steps have been taken to reduce the artificially high water table mechanically, to allow the trees to grow to the point where the root uptake of water matches or exceeds precipitation, these efforts have not yet eliminated the overburden of water. It is reasonable to assume that the shallow water table may be having a confounding effect on the data, discussed further herein. There has been high mortality among the willows and poplars and to a lesser extent among all the species. As noted before, the TreeMediation system is designed to attenuate high contaminant concentrations. It does this by forcing the groundwater to travel through oxygen and microbial rich media before contacting the roots. A shallow water table interferes with this design. Therefore the working hypothesis is that the root zone is being exposed to higher levels of contaminants. With only preliminary data this remains a hypothesis, however there is support for this argument in the evidence of chemical uptake and damage to the plants. The other possible consequence of the shallow water table is that some of trees could be drowning where the tree roots are deprived of oxygen in areas where the water table is almost at the surface. There is correspondingly high mortality in these areas in spite of apparently low contaminant levels. Those trees that have survived continue to improve which is demonstrated by the increase in vigor ratings throughout this past growing season. The installation of rabbit guards around the surviving trees planted in the initial 2002 study have significantly improved their condition. Results from the 2002 study were very intriguing. Data from 2002 showed the in-tree monitoring wells rapidly degrading the contaminants in the groundwater. Results from 2005 and 2006 for the in-tree and between tree monitoring wells show concentrations of 2,4-D in the study area have reduced from 300 mg/l to less than 0.1 mg/l. Byproducts of aerobic degradation were evident. 5 of 6

6 Between-Tree Well #1 Between-Tree Well #2 07/24/ /14/ /06/ /24/ /14/ /06/2006 mg/l mg/l mg/l mg/l mg/l mg/l 2,4 D & 4 - Chlorophenol While it would be imprudent to draw conclusions at this early stage, these data do support the potential for successfully deploying this technology to degrade the primary contaminants on this site. It has yet to be seen if the selected tree species can survive and grow to be robust enough to be a viable method for control the migration of groundwater. However steps to reduce the groundwater table to historic levels have been taken, which should remove one of the confounding factors in this study. Once the water table has been lowered, the trees that have died will be replanted and the study will continue. The policy for Risk management of contamination in Alberta provides the context for this work to define a long term site specific risk based soil and groundwater management plan to prevent exposures to receptors. At the same time, there is a desire to ultimately remove contaminants such that risk based protection systems will ultimately no longer be needed. Tantalizing results in a very limited area provide encouragement to proceed. If successful, over the longterm, substantial savings would be realized if operating, maintenance and monitoring costs for the site were reduced or eliminated. Phytoremediation has the potential to be both a risk management system, by mitigating both lateral and vertical migration and the remedial technology that ultimately eliminates the need for controls on this area of the site. The pilot study has been designed to provide the information that, if successful, will lead to the most effective phytoremediation system to accomplish both of these goals. Our challenge will be to manage precipitation in the short term and keep the rabbits at bay. 1. Catherine Creber, Dow Chemical Canada Inc. 2. Dr. Edward Gatliff, Applied Natural Sciences, Inc. 3. Frank Manale, Toxicological & Environmental Associates, Inc. 4. Joanne West, Dow Chemical Canada, Inc. 5. Dave Wandor, The Dow Chemical Company 6 of 6