In-situ Remediation of Nitrate in Groundwater: Phase 1: Site Characterization

Executive summary

Report authors: Twyla Legault (E.I.T.), Bob Stewart (P. Geos.) and John Lebedin (M.Sc., P.Eng.)

Elevated concentrations of nitrate in drinking water is a concern to rural Canadians as nitrate in excess amounts can cause environmental and health problems. Rural areas, where livestock and drinking water supplies are found in a common location, are particularly at risk as animal manure contains high levels of nitrogen. When nitrogen in the form of nitrate reaches groundwater it becomes very mobile and can migrate long distances from the area of input, leading to the contamination of groundwater supplies.

To address the issue of nitrate contamination of groundwater, the Prairie Farm Rehabilitation Administration (PFRA) of Agriculture and Agri-Food Canada is conducting a study of nitrate plumes in groundwater originating from livestock waste. The study will provide information on the properties of groundwater environments that impact the natural attenuation and reduction of nitrate. This information will contribute to the development of preventative strategies and mitigative measures to protect groundwater from nitrate contamination and will help to improve knowledge on the siting of water wells and the management of groundwater resources in proximity to agricultural production systems.

An evaluation of in-situ groundwater nitrate remediation technologies and processes will be performed during the second phase of the study. Evaluations will focus on enhanced in-situ bioremediation, a treatment process by which microbiological activities are used to reduce the concentration or toxicity of a pollutant. Electrokinetics will also be evaluated as a means to concentrate contaminants in the aquifer within a defined treatment zone.

The PFRA Caledonia-Elmsthorpe Pasture was chosen as the research site to begin this study based on previous testing of the residence well which showed nitrate levels exceeded the Canadian Drinking Water Quality Guidelines. Livestock pens, located close to the residence well, were suspected to be the source of nitrate contamination

As part of this Phase 1 study, PFRA completed a site investigation at the Caledonia-Elmsthorpe Pasture to characterize the site hydrogeology and groundwater quality. Test holes were drilled at the site to define the geology and the source and configuration of nitrate contamination. Twenty piezometers were installed at the research site to act as monitoring wells. Water levels were monitored in each of the piezometers, and water samples were collected from select wells. These samples were tested for nitrate concentrations to: 1) determine the source and areal extent of nitrate contamination in groundwater at the research site, 2) benchmark the change in nitrate concentrations with respect to time, 3) identify vertical variations in nitrate concentrations with respect to the different geologic units, and 4) identify events that may increase or decrease the concentration of nitrates in the aquifer. A series of diagnostic tests were also performed on groundwater samples from the site, to compare initial site conditions to a known set of conditions required for successful remediation. These included tests for chloride (Cl), dissolved oxygen (DO), dissolved organic carbon (DOC) and biological activity. Information from this Phase 1 investigation will be used during the Phase 2 study to: 1) identify the best location for remediation efforts, 2) guide further sampling and testing procedures, and 3) design and implement the remediation process.

The aquifer at the research site contains high concentrations of nitrate. The source of nitrate is animal waste from livestock pens which are located south of the residence well. A plume of nitrate is moving to the east and north of the livestock pens which is the direction of groundwater flow. The nitrate plume extends to the residence well, resulting in unacceptable concentrations in the well. In general, nitrate concentrations are highest in the winter months when groundwater elevations are low. Nitrate is also present in the bedrock which underlies the aquifer at the site. However, nitrate levels in this bedrock unit are below maximum acceptable levels and are believed to be natural background levels in this unit.

Nitrate (NO3-)/chloride (Cl-) ratios were used to identify areas where biochemical reduction of nitrate (i.e. denitrification) may be occurring at the site. These ratios show that denitrification may be occurring in the aquifer north of the livestock pens, at monitoring wells C-17, C-14, C-18 and C-19. Diagnostic tests performed at the site show low levels of DO in these areas, which may contribute to the denitrification process. In addition, some of the wells in these areas contain active populations of denitrifying bacteria.

If measures are taken to prevent nitrate leaching into the groundwater, the effects of dilution along with the process of denitrification will cause nitrate concentrations in the aquifer to decrease over time. Dilution has a significant impact on nitrate concentrations at this site. As the aquifer is recharged the contaminated water is diluted, leading to a reduction in nitrate concentrations. As a result, nitrate concentrations will decrease in the aquifer if the source of contamination is removed.

Due to the low transmissivity and the discontinuous nature of the aquifer at the Caledonia-Elmsthrope site, it is unlikely that the whole area of contamination can be remediated. It is therefore recommended that any evaluation of in-situ nitrate remediation technologies be limited to a small area of the aquifer. The most favourable conditions for in-situ nitrate remediation, are found in the aquifer north of the pens in an area that encompasses monitoring wells C-14, C-16, C-17 and C

Evaluations of in-situ bioremediation may require a carbon amendment. In most areas of the aquifer, the level of dissolved organic carbon (DOC) is too low and the concentration of dissolved oxygen (DO) is generally too high for bioremediation to proceed. Adding a carbon source to the aquifer in the area chosen for remediation will provide the substrate necessary to increase biomass and will provide the electron donor necessary for oxygen and nitrate reduction.

Once the area for the in-situ remediation evaluation has been established, further tests will be required to ensure that all other geochemical parameters are adequate for successful remediation to occur. Regular monitoring of groundwater nitrate concentrations will continue during phase 2 of this study to provide a measure of nitrate reduction resulting from mitigation or remediation efforts.