PASSIVE AND NATURE-ASSISTED REMEDIAL TECHNOLOGIES TO DRIVE ENHANCED NATURAL ATTENUATION AT REMOTE SITES

Transcription

1 PASSIVE AND NATURE-ASSISTED REMEDIAL TECHNOLOGIES TO DRIVE ENHANCED NATURAL ATTENUATION AT REMOTE SITES Wayne McPhee ERM Toronto, Ontario Jamie Natusch ERM Calgary, Alberta Matthew Pullen ERM Toronto, Ontario INTRODUCTION Remote contaminated sites that are without power and are difficult to access create a unique set of design challenges for site management and remediation. These sites require a different set of technologies and design methods that provide site managers with simple, robust, inexpensive and low-maintenance approaches to enhancing natural attenuation processes that are inherently occurring at the site. Historical impacts, spills and chemical releases to the environment have a demonstrated ability to self-remediate or clean themselves up as nature acts via biological, chemical and physical processes to break down and attenuate contaminants. These processes are often effective in terms contaminant degradation and migration control over sufficient periods of time. However, limiting factors can include the degree of intervention required to manage migration pathways and the time required for remediation, which may be too slow to satisfy regulatory and marketplace demands. Typically passive remediation technologies, such as bioremediation, focus on enhancing biological processes in soil and groundwater to accelerate natural remedial processes. Many of these technologies were developed traditionally as part of active site operations where access to services, utilities and man-power was available. Adaptation of technologies for application at remote sites therefore requires specialized, site and climate-specific designs. A successful remediation design balances the capital investment required and the speed of cleanup. For remote sites it is often most economical to evaluate passive systems that will remediate the site over a longer period of time rather than large pieces of equipment or heavy earth works that will provide short-term results at a higher price. This paper evaluates the development of passive, low-cost technologies and underlying processes for remediation at remote sites, including phyto-remediation, photo-chemical processes, solar-thermal processes, solar-powered, wind-powered and forced air systems. Selected case study examples are provided for sites currently being treated using hybrid poplar trees combined with passive and wind-powered sparge/aeration systems using farm windmills developed specifically for application at cattle dugouts on the Canadian prairies.

2 THE DESERT ISLAND ANALOGY The challenges of remediation at remote sites is analogous to a classic desert island story where the hero has to survive with only the resources that are available. Our challenge as designers of remediation systems for remote sites is to evaluate and implement an effective remedial intervention program within the inherent limitations of the remote site. Specifically, the goal is to facilitate site restoration using, as much as possible, the natural resources and remedial processes available to us to overcome the design limitations associated with available remedial technologies, equipment, services, utilities, manpower and site access. The good news is that the abundance of natural resources and naturally occurring processes on most remote sites provides a wide range of opportunities for remedial intervention. The first step in remedial planning is to define and assess the range of nature-assisted remedial approaches that can be implemented; that is, to check the toolkit available to us on the remote site. THE TOOLKIT Naturally-occurring processes inherent at most sites provide a basis for passive remedial activity that can be targeted and enhanced. Natural attenuation, wind, solar, hydro and phyto activity represent powerful tools when applied to soil and groundwater remediation. A summary of the toolkit available for the DIA site remediation is summarised below. Wind Wind power enables the capture and conversion of wind energy into mechanical energy or electricity, using wind turbines. Electricity generation is achieved by converting the rotation of turbine blades into electrical current via an electrical generator, or into mechanical energy to do physical work, such as pumping water and driving forced air remediation systems. Solar Solar power enables the conversion of sunlight into heat, chemical reactions, electricity or mechanical power. Solar cells (or photovoltaic cells) comprise a light-absorbing material which absorbs photons and generate electrons via the photovoltaic effect. The generation of electrons is then directly transferred into electricity or mechanical energy that can be used to drive pumps or control systems at remote sites. Photo-Chemical Reactions use the synergistic effects of combining photo-chemical energy sources with chemical destruction reactions in controlled environments such as ponds or wetlands where UV light helps to breakdown some contaminants.

3 Water Hydro Power, or hydraulic power, enables the transmission of moving water into mechanical energy and electricity. Run of river power generation captures part of the natural flow of a stream/river for the purposes of capturing hydroelectric power that can be used to drive remediation equipment. Hydraulic Transport of contaminants via dissolved and separate phase fluid flow, dispersion and dilution can support remediation in terms of natural attenuation and/or controlled migration of contaminants away from source areas and into remedial treatment zones. Engineered contaminant transport mechanisms include the use of water head for hydraulic gradient control and pumping via pipe and channel flow. Wetlands, either naturally occurring or constructed on-site, can be used to treat contaminants on-site. The wetland leverages the reactions present from the microorganisms in the water and plant root zones as well as the direct solar reactions from the sunlight. Soil Natural attenuation of contaminants in soil and groundwater progressively reduces contaminant concentrations via a combination of chemical, physical and biological processes. Specifically, natural attenuation of contaminants at the site occurs by sorption, volatilisation, biodegradation and dispersion processes within the soil matrix. Vegetation Phyto-remediation, or phyto extraction, uses plants to extract contaminants from soils, sediments and water into harvestable plant biomass. Contaminants are absorbed through plant root systems and stored in root, stem and leave biomass, which can be harvested for disposal. Phyto-remediation is typically used for extracting heavy metals, but can also be applied to some organic contaminants and salt removal. Phyto-transformation can be applied to certain organic pollutants, such as hydrocarbons, pesticides, explosives and solvents, where specific plants can destroy or render these substances non-toxic via their metabolism. Similarly, micro-organisms living in association with plant root systems can also metabolize certain contaminants in soil and water. Soil Amendment, using the available vegetation on the site, can also be used to enhance the performance of subsurface bioremediation, bio-piles or landfarms to reduce the need for supplied fertilizers.

4 Referring back to the desert island approach to site remediation, the above toolkit of naturally occurring processes, albeit with varying degrees of engineered control and enhancement, represent a comprehensive range of mechanisms by which soil and groundwater remediation can be achieved. Many of these remedial approaches can be implemented as passive systems which will continue unassisted, with limited operations and maintenance requirements, for long periods of time. Furthermore, the requirements for extensive remedial technology, including engineered systems/equipment and infrastructure, is minimal in many cases. The following project case studies outline examples of remedial approaches implemented by ERM at Canadian sites where certain criteria associated with the remote site desert island analogy have applied. The examples provided present a unique set of challenges and opportunities for developing and testing remedial technologies that can add to the toolkit of available technologies at remote sites. CASE STUDY 1 - Control of Chlorinated Solvents at Property Boundary Remote site challenges are often encountered at decommissioned facilities where the infrastructure and site staff are not available to support remediation activities. At one abandoned site, the primary technical challenge was the off-site migration of chlorinated solvents, primarily vinyl chloride, from two separate plumes that required perimeter boundary or barrier control over 300 m of property boundary. The geology at the site consists of variable surfaces of concrete, asphalt, and grass. Underlying the ground surface is a brown sand and silty sand unit that extends to approximately 2.5 metres below grade. The sand unit is underlain by a brown to grey clayey silt unit with trace sand lenses that become a homogenous clay unit with depth. Shallow groundwater at the site is located within the bottom of the sand unit and the top of the clayey silt unit. Groundwater had previously been impacted with chlorinated solvents, which had naturally degraded to vinyl chloride, from a former degreasing operation. Vinyl chloride plumes were moving off-site at two locations. Primary design challenges at the site included: 1) Control of off-site migration within site boundaries; 2) Restoration of on-site soil and groundwater quality for selected future landuse; 3) Completion of remediation within a reasonable time frame; and 4) Use of the most cost effective remedial strategy available. After evaluating tradition infrastructure-intensive technologies including: pump & treat, in-situ chemical oxidation, physical barriers, reactive barriers and forced air sparge/vent technologies, the possibility of using a phyto-remediation barrier consisting of trees to provide a reactive root zone that could degrade the vinyl chloride in the shallow groundwater was investigated.

5 The initial design of the phyto-remediation barrier involved hydrogeological evaluation of groundwater flow across the site boundary, estimating the groundwater uptake from the trees and evaluating the nutrient requirements of the soil. These preliminary design parameters were used to install pilot system that provided the necessary detailed design data to design a full scale installation. The first stage of the design was to determine the movement of groundwater across the site boundary in order to ensure that the impact of winter conditions would not make the phyto-barrier impractical. Since tree growth and transpiration becomes relatively inactive during winter, while groundwater continues to move through the soil, the design must ensure that either: 1) No contaminants move past a single row of trees; or 2) Multiple rows of tress are installed to ensure seasonal treatment of the groundwater occurs. Although groundwater flow at the site was only 1m per year and the effective root zone of the trees was expected to be greater than 1m, the design called for two rows of tree to be installed to ensure that any winter migration if impacts get past the first row of trees is captured and treated in the second row. The trees selected for the phyto-barrier were deep-rooting poplar trees that typically develop maximum water removal rates by plant transpiration in 3-4 years. Healthy growth of the trees above ground ensures that hair-root growth and root exudation will stimulate microbial transformation of the chlorinated solvents. The root water removal will also modify soil aeration, which stimulates aerobic degradation mechanisms. Case Study 1 Excavating trench to plant hybrid poplar trees.

6 The distance between the trees was determined in part by estimated root zones and by the uptake of groundwater by the trees, allowing each tree to act like a mini-pumping well. Design was based on a simple Theis model that provided draw-down estimates based on the pumping rate of the trees provided by the supplier. The model assumed that the trees uptake approximately 150m 3 per year of groundwater, combined with the known 1m height of aquifer to provide the estimate that the drawdown extends 1 to 2 m from the tree after 1 year of growth, and that the drawdown increases slowly with additional years of growth. The phyto-barrier was designed with trees spaced approximately 2m apart within two rows spaced approximately 3m apart. The design required trench excavation to 2m below grade, at the depth of the water table, and for the trees to be installed in the trench which provided an opportunity to amend the soil as the trench was back filled. Soil studies were carried out that indicated additional carbon content and nutrients would assist with tree growth. As a result, the design incorporated the addition of compost from a local municipal works yard to the soil to enhance tree growth. In order to increase the probability of success of the pilot test, a slotted PVC pipe was placed in the trench to allow for variations in the selected technology, such as the addition of nutrients, and/or, introduction of air flow to stimulate aerobic bacteria in the root zone. Case Study 1 Tree growth after several months. Smaller trees in front row.

7 The pilot test was initiated in the Spring of 2005 with the installation of 90 hybrid poplar cuttings and 10 hybrid poplar trees that were planted along the property boundary. The hybrid poplar cuttings were essentially 3m long sticks with no branches, leaves or roots that are very inexpensive to purchase and the hybrid poplar trees were 4m to 5 m long trees with a few branches and a root ball that are more expensive but are expected to grow more rapidly. The pilot test was used to determine if the extra cost of purchasing partially grown trees was warranted on this site. The trees were installed at a depth of 2m below ground surface which meant that only 1m of the cutting showed above ground. The trench was competed as designed with a slotted pipe and compost added as a soil amendment. The trees planted in the pilot test grew aggressively and in only a few months, the small cuttings caught up with the larger trees, which indicated that there was no need to install the more expensive and mature trees. Given the initial success of the pilot test, approximately 200 cuttings were installed in the Fall of 2005 to provide treatment of the vinyl chloride plume across the entire site boundary. Case Study 1 Tree growth one year after planting.

8 After one year of operations, only three of the cuttings had died since the planting and the remaining trees had flourished. Many of the cuttings that were only 1m above ground initially reached heights of 15 to 20m after a one year period and have now reached heights of 30m tall two years after the installation. Two years into remedial operations, chlorinated solvent concentrations in the source area remain relatively unaltered. At the same time, additional growth and root zone development of the phyto-remedial barrier has significantly reduced chlorinated solvent concentrations in perimeter wells adjacent to the phtyo zones. Concentrations of vinyl chloride in monitoring wells at the center of the plume show a drop across the phytobarrier from 3400 ug/l upgradient of the phyto-barrier to < 1 ug/l in monitoring wells in between the two rows of trees and in down-gradient wells. Important lessons learned were that the smaller, less expensive hybrid cuttings proved to be as effective as the larger trees within a few months of installation and that patience is needed to give the root zone time to develop before treatment will occur even though the above-ground tree growth may be significant. CASE STUDY 2 Decommissioned Gas Station In another recent example, the remediation of hydrocarbons at a former gas station was accelerated using a combination of phyto-remediation and an inexpensive wind powered aeration system. The site is a former independent gas station in a primarily agricultural area. A leaking underground storage tank had created a petroleum hydrocarbon plume that was moving towards the site boundary. Primary design requirements established with the landowner included 1) An inexpensive remedial approach that would prevent the plume from moving off-site, 2) Clean up onsite contamination; and 3) Minimal ongoing operations and maintenance costs since the site was unused and not generating revenue. The first step was to decommission and remove the disused underground storage facilities and impacted soils in the source area to prevent the ongoing release of hydrocarbons. The second priority was to prevent off-site movement of hydrocarbons towards a neighbouring farm at which groundwater recovery wells were used for farm operations and drinking water for cattle. The remedial design, involved the installation of hybrid poplar trees into the shallow groundwater along the property boundary. The trees were installed by digging a narrow trench and planting dormant poplar cuttings in the Fall of The cuttings were approximately 3m long and were planted with 2.5m buried to a depth just above the shallow groundwater table, and only 0.5m of cutting above-grade. In Spring 2007 the cuttings grew roots near the water table, sprouted leaves and created a wall of trees along the property boundary. This created an active biological zone within the root system and acted as an abstraction pump to control groundwater movement. The tree cuttings were planted in two rows with space between the rows to ensure that the groundwater would not move past the phyto-barrier during dormant winter months.

9 Case Study 2: Planting hybrid poplar trees along site boundary. By the Fall of 2007 positive results have already been seen. The concentration of aromatic hydrocarbons, predominantly ethyl benzene, in groundwater downgradient of the phyto-barrier have dropped by over 90%. The growth of the trees has been over 10 ft in just one year with only a 5% tree loss over the first winter, mostly from snowplow damage. The majority of the source zone was removed during tank removal operations, but a significant area of impacted soil and groundwater remained in the middle of the site. Potential future landuses limited the use of trees in this area, so alternative approaches were evaluated. The installation of traditional remedial technologies like pump & treat, multi-phase phase extraction and forced air systems were limited by the lack of utilities to the site which had been disconnected when the site was decommissioned. Similarly, the lack available local personnel to monitor, operate and maintain such systems was also a limiting factor.

10 In order to accelerate the remediation of on-site soil and groundwater with minimal infrastructure, a modified farm windmill was installed as a power-free, air-sparging system to aerate the groundwater and accelerate the biodegradation of hydrocarbons. Inexpensive farm windmills are available where the shaft of the windmill is connected directly to a diaphragm pump that can generate air flows with pressures up to 30 psi when the wind is blowing. Case Study 2: Windmill and Air-Sparge system installation. The system design included 3 sparge wells installed below the water table that would allow air from the windmill to bubble up through the groundwater and provide oxygen for the native bacteria. One year after installation the sparge wells continue to provide air to the subsurface around the former tank nest and hydrocarbon levels have dropped by over 75%.

11 DESIGNING WITH WIND POWER The farm windmill powers a direct drive air pump that generates air flows and pressures up to 30psi. The air pump is an inexpensive and efficient means of producing air flow. The air can be pumped directly into the ground or it can be accumulated in a pulse tank until sufficient volume and pressure has been developed to provide a strong pulse to the subsurface. Pulse control can be performed using either a solar powered control panel or a specialized pressure release valve depending on site requirements. A simple schematic for windmill sparging is shown below. The air flow rate that can be generated by a farm windmill depends on the design of the windmill and the back pressure applied to the air line. For sparge wells the back pressure can be calculated from the height of the water column being sparged (1 psi = 2.3 ft of water column) plus roughly 1 psi during initial break-out. The graph below was adapted from the Alberta Renewable Energy Test Site for a back-pressure of 1.5m of water 1. 1 Summary of Wind and Solar Powered Pumping Units ( Test Season), Alberta Renewable Energy Test Site, Summary Report 737, September 1998, Alberta Farm Machinery Research Centre.

12 The windmill sparging systems perform in a similar manor to the traditional electrically powered air sparging applications. When the wind is blowing the effective radius of influence from each well will be the same as if a blower was being used. Wind is an inconsistent power source and changes in the supply of air must be incorporated into the design. For sparging systems the changes in wind create a natural pulsing pattern that can be incorporated into the design and is useful for bio-sparging applications where short pulses of air are traditionally used instead of the steady air flows used in combined sparge/vent systems. The required bioremediation rate for a typical contaminant can be used to determine the mass of oxygen and volume of air required for each sparge well over time. Based on the expected average wind flow at the site, the average injection flow rate per windmill system can be determined. Based on the required air flow for bioremediation and the output of the each windmill, the number of wells per windmill can be calculated. Typically a single windmill can provide sufficient air flow for 3 to 6 sparge wells. If additional air flow is require, additional windmills can be installed. Sparging Performance versus Wind Speed Air Injection Rate stp) Wind Speed (km/h) The graph above illustrates the range air-injection flow-rates achieved (at standard temperature and pressure) at different wind speeds using a Koenders-supplied 20-inch diameter windmill. Average wind speeds of km/h generate sparge air flow-rates in the range of 5-20 L/min (or CFM). Maximum air flow-rates of L/min (or 1-2 CFM) can be achieved with wind higher speeds of 40 km/h or greater.

13 SUMMARY In reviewing the challenges identified at the outset of this paper, remote sites inherently present a different set of design requirements to those typically encountered in developed communities and industrialised zones. The desert island analogy was introduced to promote evaluation of both: 1) The limitations of remote sites, in terms of available remedial technologies, equipment, services, utilities, manpower and site access; and 2) The opportunities, in terms of an abundance of natural resources and naturally occurring processes available to support remedial intervention. The toolkit of nature-assisted remedial approaches, including wind, solar and hydro power, photo-chemical reactions, hydraulic transport mechanisms, wetlands, natural attenuation, phyto-remediation, phyto-transformation and soil amendment processes, presents a wide range of opportunities to promote natural remediation processes. Case Study 1 outlined the successful use of poplar trees for the phyto-remediation of a chlorinated solvent plume in groundwater. A perimeter phyto-barrier was established as a boundary control mechanism and after 2-years a 99% concentration reduction was established across the two rows of trees. Case Study 2 outlined the application of a phyto-barrier for boundary control of a petroleum hydrocarbon plume, and the use of a windmill for power-free air sparging of the central plume area. After one year, the phyto-barrier achieved a 90% concentration reduction at the perimeter and the bio-sparge system promoted aeration and bioremediation of the central plume area to achieve a 75% concentration reduction. These case study examples are illustrations of the type of simple, and highly effective, nature-assisted remedial solutions that can be applied at relatively low cost. A successful remediation design balances the capital investment required and the speed of cleanup. When considering remedial options criteria for remote sites, it is often most economical to evaluate passive systems that will implement remediation over longer time periods, as opposed to importing equipment and technology that will provide short-term results at a higher price. However, these criteria can also be applied to non-remote sites, in cases where remedial timelines are not limited. Typical examples may include operating facilities where property divestment is not foreseen and long-term on-site remediation and/or boundary control of contaminants is required. This paper is intended as a discussion document, to promote information sharing and further application of the wide range of potential opportunities associated with natureassisted remediation technologies. REFERENCES Summary of Wind and Solar Powered Pumping Units ( Test Season), Alberta Renewable Energy Test Site, Summary Report 737, September 1998, Alberta Farm Machinery Research Centre.