Canadian Consulting Engineer

Award of Excellence Groundwater Remediation wiht a Permeable Reactive Barrier

October 1, 2014
By PINTER & Associates

The trigger for the project was a groundwater nitrate plume caused by the long-term handling and storage of fertilizers at a Viterra fertilizer distribution facility in Northern Alberta.

The trigger for the project was a groundwater nitrate plume caused by the long-term handling and storage of fertilizers at a Viterra fertilizer distribution facility in Northern Alberta.

A nitrate plume was shown to be migrating off-site to the south towards sensitive receptors, including potable water wells. Conventional treatments or full-scale remediation proved too costly and disruptive to the site operations and nearby residents. Therefore, as prime consultant, PINTER developed and designed an innovative and cost-effective permeable reactive barrier, or PRB, where nitrate impacts would be removed in situ with minimal disturbance to the surrounding environment. The PRB was designed to remove the majority of nitrate from the plume through biological denitrification.

Viterra required a solution that would reduce short and long term

liabilities, would be cost effective and would minimize disruption to the site operations. At a larger scale than was reported in any literature, the PRB has been able to cost-effectively reduce the concentrations of nitrates in the groundwater to levels below the criteria. The PRB has prevented the expansion of the nitrate plume onto adjacent lands.

Scaling up to apply to fertilizers

One of the first tasks was to do a detailed cost benefit analysis of six potential solutions. Once the PRB was selected, Pinter gathered more detailed soil and groundwater data from the area and completed an extensive literature review. In-house expertise was applied in the disciplines of hydrogeology, bioremediation, contaminant transport, groundwater modelling, biochemistry and project management.

The task of scaling up the technology and applying it to fertilizers rather than sewage was one of the complex challenges of the project. The geochemistry of a fertilizer nitrate plume is significantly different than that of sewage related plumes shown in literature, including the presence of sulphates and ammonia in excess of 1000 mg/L. With a volume of 960 m3, this PRB is approximately six times larger than any shown in literature and is remediating nitrate concentrations more than 10 times higher than any previous work.

Located on an adjacent property

Another challenge overcome was the optimum placement of the PRB. The decision to locate it on the adjoining property was driven by the desire to allow the maximum amount of nitrification (conversion of ammonia into nitrate) upstream of the PRB, and to minimize disruption to the site. Making use of the natural drainage to help keep the PRB saturated, therefore maintaining anaerobic conditions was also a benefit. Constructing the PRB on the adjoining property required some clear discussions with the adjacent landowner regarding what the project was, why it was being done, and what the benefits to him would be.

The concentrations of nitrate present at the site were well beyond anything remediated with a PRB in the past. This PRB needed to be larger and had to contain a higher proportion of woodchips than anything seen in literature to ensure its long-term effectiveness. Laboratory scale testing and computer modelling were both used to aid in the design. Results of the testing and modelling suggested that the PRB could achieve nitrate reductions greater than 90%. This allowed for the design to be finalized with a high degree of confidence.

The PRB was constructed by removing native soils, mixing them with pine shavings on an approximate 2:1 volumetric basis, and returning the mixture to the excavated area. The amount of pine shavings used was determined by calculating the expected nitrate mass flux and using a mass balance approach to provide enough carbon to remove nitrate for a period of at least 30 years. In total, 960 m3 of native soils were mixed with 480 m3 pine shavings and placed into the 120 m long x 4 m deep x 2 m wide PRB.

Cost benefits of 50% and reduced

potential liability

The approach provided Viterra with a cost reduction of more than 50% compared to conventional solutions. Additional cost savings will be realized annually as there will be no ongoing maintenance or operational costs associated with the PRB. Also, site operations were never disrupted. Finally, Viterra’s potential for future liabilities has been significantly reduced.

Monitoring results from the early months following installation show a steady decline of nitrate concentrations within the PRB. Nitrate removal expressed as a percentage was greater than 95% as of the latest monitoring event.

The success with this project means the technique is now a proven tool and opens the door to other similar remediation projects worldwide where cost may be prohibitive and/or site specific considerations exclude more traditional approaches.

Pinter received tax credits for portions of this work under the federal Scientific Research and Experimental Development (SR&ED) Tax Incentive Program. cce

Project name: Groundwater Denitrification Using a Permeable Reactive Barrier

Award-winning firm (prime consultant): PINTER & Associates, Saskatoon (Lawrence Pinter, P.Eng., Ryan Riess, P.Eng.)

Owner/client:  Viterra


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