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Use of an Evolving Conceptual Site Model and Collaborative High-Density Sampling Methods to Expedite Site Characterization and Remediation at a Former Landfill, Underground Storage Tank, and Manufactured Gas Plant, Poudre River Site, Fort Collins, CO

Triad Elements Used

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Table of Contents

  1. Site Information
  2. Project Information
  3. Triad Approach Information
  4. Supporting Information

At-a-Glance

Summary 

The 19-acre Poudre River Site contains a 12-acre former municipal burn landfill that operated from the late 1930's to the early 1960's. Through several historical site investigations, including a Brownfields assessment grant in 2001, the City of Fort Collins began to devise a long-term redevelopment strategy for the Site. However, fuel-related groundwater contamination and the presence of coal tar in the adjacent Cache La Poudre River remained issues of concern. The proximity of a former manufactured gas plant (MGP) and a gasoline supply station to the landfill prompted Environmental Protection Agency (EPA) Region 8 to conduct a Targeted Brownfields Assessment (TBA).

At the request of EPA Region 8, EPA's Brownfields and Land Revitalization Technology Support Center (BTSC) became involved with this project and assisted with the development of field work plans using the principles of the Triad Approach. Conventional methods of site characterization were augmented using real-time measurement technologies and dynamic, "learn as you go" work strategies (DWS), to compress the timeframe for the investigation process. Through the cooperative efforts of the stakeholders and BTSC, innovative approaches were used to progressively refine the CSM, select appropriate investigative technologies, and sequence data collection efforts to improve project efficiency. By assessing data gaps and decision endpoints using the evolving CSM, the project team and stakeholders reached consensus on the nature and extent of contaminants and pathways in approximately one year.

Two mobilizations were required for initial site characterization purposes under the TBA due to funding constraints. Additional investigative work was performed by both EPA and the potentially responsible party (PRP) based on the TBA results, followed by construction of a remedy for the coal tar contamination. This remedy was implemented by the PRP under a Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) consent order less than 2 years after the start of the TBA.

This Triad Profile focuses on site investigation activities performed by EPA and a PRP for the Site, which are of primary interest from a Triad perspective. These activities led into and facilitated the design and implementation of mitigation measures by the PRP to capture the coal tar and eliminate its flow to the river.

Highlights of the project from a Triad perspective include:

  1. Systematic project planning (SPP) facilitated design of a high-resolution site characterization (HRSC) sampling and analysis approach using an evolving CSM to increase data density, limit decision uncertainties, and understand the configuration of contaminant source areas and pathways.
  2. Use of polycyclic aromatic hydrocarbon (PAH) fingerprinting to identify the potential sources of contamination present in the river when no clear pathway for contaminant migration appeared was obvious.
  3. Use of DWS for sampling and development of decision logic diagrams to clearly identify limits of the non-aqueous phase liquid (NAPL) plume.
  4. Use of direct push technology (DPT) groundwater grab samples, coupled with gas chromatography/mass spectrometry (GC/MS) analyses for volatile organics by the EPA Region 8 Mobile Laboratory, to support real-time decisions in the field.
  5. Use of soil gas, passive diffusion bag (PDB) samplers, and non-intrusive geophysical methods to probe the landfill for the presence or absence of potential source materials without creating additional conduits for contaminant migration.
  6. Use of demonstration of methods applicability (DMA) studies on the reliability of the field-based GC/MS methods and the utility of passive soil gas methods for the site.

1. Site Information

Site Name Poudre River Site
Location Fort Collins, CO
Site Types 
  • Manufactured Gas Plant (MGP)
  • Municipal Landfills
Site Regulatory ID  CON000802289
Project Lead Organization EPA Region 8, State of Colorado, and Xcel Energy (PRP)
Project Lead Type  EPA Lead
Regulatory Lead Program  Targeted Brownfields Assessment
Triad Project Status Field Program Ongoing
Reuse Objective Identified  Yes
Proposed Reuse:  Recreational, Commercial/Industrial

Background Information About Site 

The area of concern includes a former MGP that has been demolished located west of the property, a gas station and supply terminal to the west/northwest of the property, and a municipal landfill that operated on the property from approximately 1940 to the mid 1960's. The MGP operated from approximately 1900 to 1930 and manufactured heating oil from coal and other petroleum products using a carbureted water gasification method. The former MGP was then purchased by a gasoline supply company (the Poudre Valley Gas Company). Releases of fuel were recorded several times between the closing of the MGP and the present. Releases from the gas station have also been recorded. Contaminated groundwater and dense non-aqueous phase liquids (DNAPLs) associated with the historical MGP and fuel handling operations were identified as extending downgradient beneath the landfill and into the nearby Cache La Poudre River.

In May 2003, EPA Region 8 initiated a TBA to further evaluate environmental issues related to the site. Unique site geology and hydrogeology presented a challenge during the site characterization and source identification process.

The City of Fort Collins currently owns the property and has constructed a Leadership in Energy and Environmental Design (LEED) Gold certified community center which opened its doors in November 2007. The community center was the first LEED Gold certified community center in the United States. Restoration of the site and the nearby Cache La Poudre River were completed in 2006 and have provided public access to river resources, protected recreational users and fisheries, and has protected wildlife habitat associated with the Cache La Poudre watershed.

Contaminant(s) of Concern 

Contaminated Media 

Project Results and Outcomes 

The drilling activities implemented under the DWS work plan produced a high-resolution understanding of site geology. The investigation indicated that coal tar DNAPL reached the river through bedrock fractures and bedding planes that acted as contaminant conduits.

The drilling activities also revealed that groundwater movement in the unsaturated zone was influenced by the high variability of aquifer materials (ranging from tight clays to sands, gravels, and landfill debris), and probably influenced by bedrock topography in the saturated zone. This was also confirmed by several geophysical surveys conducted at the site.

Preferential pathways for contaminant migration were not found in the subsurface at the site during the geophysical surveys. Conventional drilling activities and innovative geophysical technologies were used to determine that migration pathways were influenced heavily by bedrock fracture patterns that did not follow local topographic or hydrogeologic features. Coal tar-related contaminants were discovered in groundwater, subsurface alluvium, bedrock fractures, and river sediments during the site investigation. PAH fingerprinting techniques linked upgradient MGP-related wastes to DNAPL found in the river.

Numerous innovative technologies provided a high-density of information to help direct conventional sampling and analysis efforts. PDB samplers were used to optimize where more permanent wells were installed. Geophysical methods and soil gas were used to direct conventional drilling efforts and trenching was performed by the PRP to further refine the CSM in terms of expected geologic and contaminant transport conditions.

The PRP is currently implementing mitigation efforts to eliminate the continuing source of coal tar to the Cache La Poudre River. Information gathered during the TBA and the subsequent Non-Aqueous Phase Source Investigation, Site Assessment, and Removal Action was used by the city to close the landfill under Colorado’s voluntary cleanup program. The CSM was continually refined in support of the design and implementation of the NAPL interception system.

The DNAPL mitigation/removal action consisted of a 700 feet long sealed polyvinyl chloride (PVC) sheet pile barrier with supplemental hydraulic controls and NAPL extraction system. The system was located adjacent to the south bank of the river and extended 30 feet up from the bedrock to the soil surface. The barrier blocked the pathway between source contaminants and the river. Groundwater control pumps and DNAPL collection pumps were located on the upgradient side of the barrier to maintain a negative hydraulic gradient from the river toward the barrier. The pumps transport groundwater and NAPL to an onsite wastewater treatment facility. This facility will operate for several years.

2. Project Information

Project Objectives/Decisions 

The City of Fort Collins, in conjunction with other stakeholders, identified the primary objective as establishing a connection between potential source areas at the site and coal tar contamination found in the adjacent Cache La Poudre River. A second objective was to assess a former municipal landfill at the site for closure in accordance with State of Colorado requirements. Data collection efforts to support these objectives were also designed in anticipation of future redevelopment at the Site. Project objectives incorporated use of a Triad-based, HRSC to increase data density, limit decision uncertainty, and minimize field mobilizations in characterizing and cleaning up the site.

Remedial Phase

Site Investigation (Includes RI/FS or Similar Detailed Investigation Work)

Triad Project Benefits 

This project illustrates how contaminant distribution and migration patterns can often be deceiving and less than obvious at sites that involve NAPLs. Coal tar (which is a dense NAPL, or "DNAPL" that sinks in water) mixed with fuels (which are light NAPLs, or "LNAPLs" that float on water) was found to be moving in the alluvium and in fractured bedrock from the former MGP site to the Cache La Poudre River with little or no apparent connection to the dissolved phase plume. This was a source of confusion between stakeholders until sufficient site knowledge and high-resolution data density could confirm that the lack of a continuous dissolved phase plume to the river did not necessarily rule out the presence of a deeper NAPL plume. Through the continual revision of the CSM and the collaborative use of traditional and innovative data, the project team discovered that the dissolved phase plume related to the NAPL was absent because of low solubility of the coal tar combined with limited communication between the complex fractured bedrock system and the overlying alluvium. The DNAPL plume, which was found at depth in the bedrock fractures, had separated from the dissolved phase plume in the alluvial aquifer system directly beneath the former landfill and was following these dipping fractures toward the river.

Application of the Triad Approach at the site produced a combination of both conventional and innovative means for data collection that assured the reliability of site decisions. The weight-of-evidence approach along with intensive stakeholder involvement throughout the investigation aided in the efficient development of a consent order and implementation of mitigation/removal measures for coal tar.

Cost and Time Savings 

The project team estimated that use of the Triad Approach for site characterization resulted in a 30 percent cost savings compared with an equivalent conventional characterization approach. In this estimate, the conventional approach is assumed to involve additional mobilizations and fixed-base laboratory analytical methods. In addition to saving costs, use of the Triad Approach and HRSC increased the density and quality of the data set used to make decisions about the site. A conventional approach would not, for example, have been cost-effective for collecting a density of data comparable to that obtained using the PDB samplers, which allowed evaluation of groundwater discharge to surface water every 25 feet over a quarter mile stretch of the river.

3. Triad Approach Information

Systematic Project Planning 

Systematic project planning (SPP) activities focused on the development of a preliminary CSM and DWS with an associated sampling plan to collect data that specifically addressed project decisions. The preliminary CSM was revised as more data were collected and was used to focus when and where additional data were needed. The first step in the process was to redefine project objectives under the TBA versus those of the earlier Brownfields assessment program performed by the City. Discharge of DNAPL to the river was identified as the primary issue along with landfill closure.

The TBA program involved groundwater grab sampling and direct push technology (DPT) to delineate where contamination was entering and exiting the site. Electromagnetic geophysical surveys were implemented with an existing well sampling program to further increase the project team's initial site knowledge. The geophysical program did not identify any apparent sources within the landfill (such as buried drums) and confirmed the apparently flat-lying nature of bedrock shale beneath the site. DPT groundwater grab sampling results were analyzed for VOCs on-site using the EPA Region 8 mobile laboratory equipped with a GC/MS. DPT soil samples were also collected to identify where product was present and to direct which samples were to be sent to a fixed-base laboratory for PAH fingerprinting.

The PAH fingerprinting analyses showed a potential connection between the former MGP and the contamination in the river. The geophysical results further suggested that the landfill was not the source. Little or no dissolved phase contamination was identified leaving the landfill as a result of the TBA effort.

Based on these collaborative results from the TBA that provided a weight of evidence for the source of the coal tar, along with the need to stop the ongoing release of coal tar into the river, the PRP implemented a river channel investigation. This investigation used a DWS work plan provided by the EPA's contractor (Tetra Tech) that outlined a drilling program to identify the magnitude of the NAPL contamination in the river. Conventional drilling and trenching methods verified an ongoing source of DNAPL to the river and no apparent landfill or unidentified source area other than the former MGP site.

At this point in the investigation, the EPA implemented an extended site assessment investigation wherein soil gas, additional geophysical surveys (ground penetrating radar, conductivity, and resistivity), PDB samplers, and traditional drilling methods where used to delineate the flow path of the NAPL to the river. This information was collected sequentially to provide the greatest benefit to the project and ultimately provided attribution for the PRP to implement the design for the interim mitigation measures.

In addition to the activities conducted in support of the NAPL investigation and cleanup, a vapor emissions survey was conducted on the surface of the landfill. This survey employed an ambient air monitoring approach using Open Path Fourier Transform Infrared Reflectance spectroscopy (OP-FTIR) and other remote sensing methods after completion of the TBA. The OP-FTIR found no hotspots of organic vapor emissions on the landfill surface, but attempts to estimate low-level emission rates across the landfill were complicated by other potential vapor sources surrounding the Site. OP-FTIR is an emerging technology for which method and data interpretation protocols are still being established.

Project Team Description 

The principal decision makers for the site were from the State of Colorado and EPA Region 8. Other stakeholders included an EPA Region 8 support contractor (Tetra Tech), the City of Fort Collins, the city's consultant (Walsh Environmental), the PRP for the MGP (Xcel Energy), the PRP's consultant (Retec Group, Inc.), and the BTSC.

The BTSC's support was provided during the TBA in the areas of chemistry, statistics, geoscience, and engineering. URS Operating Services, Inc. (UOS) (the Region 8 EPA primary technical support contractor on the Superfund Technical Assistance and Response Team [START] contract) and Walsh Environmental provided much of the pre-TBA investigations.

Tetra Tech provided the field investigation team, overall project management and coordination, a conceptual mitigation design, and review of mitigation efforts by Retec. Retec provided additional field investigation, mitigation design, and implementation of mitigation/removal measures. Additional technical support for in-field GC/MS, the electromagnetic (EM) terrain conductivity meters, and the Supersting Resistivity Control Unit was provided by the Region 8 Mobile Lab; Geonics, Limited; and Advanced Geosciences, Inc.; respectively.

Dynamic Work Strategies 

Data was collected using low cost approaches and EPA's Region 8 equipment (mobile laboratory and DPT drill rig), including initial EM geophysical surveying and DPT grab groundwater sampling to identify dissolved phase contaminants. Information provided by these activities was used to refine the CSM and build consensus on the direction of the investigation and to convince the PRP to move forward with a river channel investigation. Once it was confirmed that the contamination in the riverbed was not the result of a dumping scenario (in the landfill), an additional Site Assessment was performed to confirm the migration pathway of the contaminants to the river and also provide attribution to the former MGP site.

Soil gas data, boring logs, and geophysical data were then used as part of the Site Assessment to place borings into the bedrock where NAPL had been discovered during the NAPL Source Investigation effort. Based on the data collected throughout the project, the PRP installed the vertical engineered barrier, hydraulic controls and a NAPL collection system to intercept the NAPL before entering the river.

Decision Logic 

Decision logic was developed for the progressive collection of grab groundwater samples during the TBA. Field screening using a PID/FID was used to trigger when and if deeper samples should be collected after encountering the top of the groundwater surface. This practice was intended to limit the potential for preferred pathways to go undetected.

During the subsequent Site Assessment performed by EPA, data from soil gas, previous drilling and trenching activities, geophysical surveys, PDB samplers, groundwater, NAPL, and soil samples were used in conjunction with a field screening and physical soil sample description process to optimize where conventional drilling was needed to confirm the pathway for the NAPL from the former MGP to the river.

Real-Time Measurement Technologies 

Several real-time measurement technologies were used to characterize the Poudre River Site. Activities performed using field-based methods included:

Other innovative analytical technologies applied at the Site (though not real-time) included PDB samplers and passive soil gas samplers.

The GC/MS analytical method used by the Region 8 mobile laboratory was based on EPA Method 8260 and was optimized through an initial DMA. Based on the DMA, the project team was able to determine the applicable detection and reporting limits for field-based GC/MS results, design an initial calibration and appropriate QC protocol, and evaluate the types and concentrations of contaminants expected in groundwater at the site. The DMA also provided site-specific information concerning method accuracy and precision.

DPT grab groundwater sampling methods met with limited success due to site geology and refusal experienced in the field. As mentioned previously, geophysical methods provided inexpensive methods for confirming the project team's presumptions about the distribution of geologic features, such as preferred pathways at the site and the presence or absence of buried metallic objects. Soil gas methods and PDB samplers were demonstrated to be very important alternative characterization methods for evaluating VOCs in the vadose zone.

Ambient air monitoring using OP-FTIR did not identify significant generalized VOC or methane emissions, or any hot spots, from the surface of the landfill. However, uncertainties in contributions from background sources and in monitoring system performance at low concentrations made the OP-FTIR data inconclusive in assessing the presence or absence of low-level emissions.

In addition to the field-based methods listed below, groundwater and soil samples from select sampling locations were sent to an offsite laboratory for analysis for Contract Laboratory Program (CLP) semivolatile organic compounds (SVOCs), CLP pesticides/polychlorinated biphenyls (PCBs), volatile organic compounds (VOCs) by SW-846 method 8260, total petroleum hydrocarbon (TPH) purgeables and extractables. Fingerprinting analysis for PAHs was also performed in a fixed-base laboratory.

Technology Category(s) for Sampling and Analysis 

Specific Technology(s)/Model Used (e.g., SCAPs) 

Technology Vendor(s)

Attached Technology Quick Reference Sheet (TQRS) Form 

TQRS being prepared


Data Quality Assessment 

BTSC provided support in developing detection and reporting limits for the modified SW-846 Method 8260 using the field-based GC/MS and the EPA Region 8 Mobile Laboratory. It was determined that the modified method was suitable for decision making at the site. Ten percent of the samples analyzed by the mobile laboratory were sent to the Region 8 fixed-base laboratory for confirmation. Duplicates, off-site splits, and other forms of quality assurance/quality control (QA/QC) sampling were used during field activities to ensure analytical quality for field and offsite laboratory analysis.

Data Management Approach and Tools 

Field data management was implemented during the geophysical survey and during soil and groundwater sampling activities. Data was collected and stored electronically during the geophysical survey.

BTSC used the statistical software package STATISTICATM and Excel spreadsheets to manage and process data for fingerprinting and source material identification. Using STATISTICATM, correlation coefficients and scatter plots were developed between river contaminants and potential source materials. The statistical correlations of individual PAH levels developed between river contaminants and potential source materials provided strong evidence of the contaminant source, which in turn helped to determine the PRP for the site. Excel spreadsheets were used to develop bar charts to visually exhibit similarities and differences in the chemical constitution of contaminants in the river and in potential source materials.

4. Supporting Information

Key Triad Project Milestones 

Dates of Operation - Field Work

September 2003 to 2006 (major characterization efforts completed).

2006 to Present (ongoing remedial activities).

Source(s) of Information

Electronic Documentation of Supporting/Related Information

PDF After Action Report Non-Aqueous Phase Source Investigation. Poudre River Site, Fort Collins, Larimer County, Colorado. Prepared for U.S. EPA by Tetra Tech EM Inc. October 25, 2005. (33.4 MB)
PDF Evaluation of a Former Landfill Site in Fort Collins, Colorado Using Ground-Based Optical Remote Sensing Technology. U.S. EPA Air Pollution Prevention and Control Division. April 2005. (3.1 MB)
PDF Poudre River Removal Site Assessment, Field Sampling Plan. Fort Collins, Larimer County, Colorado. Prepared for U.S. EPA by Tetra Tech EM Inc. (10 MB)
PDF Targeted Brownfields Assessment Field Activities Report. Ft. Collins Aztlan Center, Fort Collins, Larimer County, Colorado. Prepared for U.S. EPA by Tetra Tech EM Inc. (8.1 MB)

Point(s) of Contact 

Robert Howe
Technical Lead
Tetra Tech EM Inc.
4940 Pearl East Circle, Suite 100
Boulder, Colorado  80301
Telephone icon 303-441-7911
E-mail icon robert.howe@ttemi.com

Karen A. Reed
Brownfields Project Manager
EPA Region 8
999 18th Street, Suite 300
Denver, Colorado  80202
Telephone icon 303-312-6019
E-mail icon reed.karen@epa.gov

Stephen Dyment
U.S. EPA, Office of Superfund Remediation and Technology Innovation Technology Integration and Information Branch
1200 Pennsylvania Ave., NW (5203P)
Telephone icon 703-603-9903
Fax icon 703-603-9135
E-mail icon dyment.stephen@epa.gov


Last Update:  05/15/2012

To update this profile, contact Cheryl T. Johnson at Johnson.Cheryl@epa.gov or (703) 603-9045.