Miroslav Nastev

Gas production and migration in landfills and geological materials

Landfill gas, originating from the anaerobic biodegradation of the organic content of waste, consists mainly of methane and carbon dioxide, with traces of volatile organic compounds. Pressure, concentration and temperature gradients that develop within the landfill result in gas emissions to the atmosphere and in lateral migration through the surrounding soils. Environmental and safety issues associated with the landfill gas require control of off-site gas migration. The numerical model TOUGH2-LGM (Transport of Unsaturated Groundwater and Heat-Landfill Gas Migration) has been developed to simulate landfill gas production and migration processes within and beyond landfill boundaries. The model is derived from the general non-isothermal multiphase flow simulator TOUGH2, to which a new equation of state module is added. It simulates the migration of five components in partially saturated media: four fluid components (water, atmospheric air, methane and carbon dioxide) and one energy component (heat). The four fluid components are present in both the gas and liquid phases. The model incorporates gas-liquid partitioning of all fluid components by means of dissolution and volatilization. In addition to advection in the gas and liquid phase, multi-component diffusion is simulated in the gas phase. The landfill gas production rate is proportional to the organic substrate and is modeled as an exponentially decreasing function of time. The model is applied to the Montreals CESM landfill site, which is located in a former limestone rock quarry. Existing data were used to characterize hydraulic properties of the waste and the limestone. Gas recovery data at the site were used to define the gas production model. Simulations in one and two dimensions are presented to investigate gas production and migration in the landfill, and in the surrounding limestone. The effects of a gas recovery well and landfill cover on gas migration are also discussed.

Groundwater Recharge Assessment in the Chateauguay River Watershed

The objective of this study was to evaluate groundwater recharge in the Chateauguay River watershed. The Hydrologic Evaluation of Landfill Performance (HELP) model was used to assess daily values of recharge, evapotranspiration and runoff. The study area was divided into a regular grid, 250 m × 250 m, for a total of 47,616 grid elements. The input parameters included soil physical properties, land use, vegetation and climate data. Calibration of HELP was carried out against runoff and baseflow estimates obtained from separation of five river hydrographs. Over a 39 year period, the mean annual recharge rate was estimated at 86 mm, or 9% of the total precipitation. Areas characterized by high water level elevations and unconfined flow conditions were identified as the main recharge areas. Daily estimates show that recharge takes place mainly in spring and fall. Over the observed period, the annual variations of evapotranspiration and runoff were directly related to changes in precipitation, whereas the annual recharge response was subdued, with much lower variations. HELP was also used to assess potential climate change scenarios using data for the driest and most humid years. The mean annual recharge was 51 mm for the driest year and 99 mm for the most humid year. Differences in the spatial distribution of recharge for the predictive scenarios indicate that the areas most sensitive to climate change correspond to the preferential recharge areas.

Numerical simulation of the radius of influence for landfill gas wells

In North America, most domestic waste produced is disposed in landfills. These sites generate leachate and gas, mainly CH 4 and CO 2 , which are harmful for the environment if not properly controlled. The design of active landfill gas recovery systems is based in large part on the radius of influence of vertical pumping wells. This parameter is commonly estimated empirically. This study presents results of numerical simulations of the radius of influence of gas recovery wells for different site conditions. The simulations were performed with the TOUGH2-LGM simulator. In the simulation scenarios, the radius of influence was defined in relation to several factors: the waste thickness, the generation rate of CH 4 gas in the waste, and CH 4 concentration in the recovered landfill gas. The results are presented in the form of general graphs that are not site-specific. The adequacy of the results still needs to be validated against field measurements. On the basis of simulation results, a systematic approach is proposed for the design of landfill gas recovery systems, and this approach is illustrated with a hypothetic example. This approach should guide landfill managers and engineers in the design of landfill gas recovery systems. The simulations only considered cases where landfills are open to the atmosphere, which are representative of most operating conditions. The results thus do not apply to post-closure conditions usually involving an impermeable cover built on top of the waste.

Numerical Simulation of Groundwater Flow in the Chateauguay River Aquifers

The Chateauguay River watershed extends over northeastern New York State (USA) and southwestern Quebec (Canada). Fractured sedimentary rocks of the St. Lawrence Platform host the regional aquifers. Quaternary sediments of variable thickness of up to 45 m overlie the bedrock. The geometric mean hydraulic conductivity of the bedrock aquifers obtained from 548 field measurements is 5.1 × 10–5 m/s with a standard deviation of 0.7 of the logarithms. The modelled area extends from the foothills of the Adirondacks to the St. Lawrence River and covers 2,850 km2. The numerical groundwater flow model was developed using the finite element simulator FEFLOW. The model has 13 layers with layer thicknesses ranging from 5 m for the top layer to 75 m for the bottom layer. The average thickness of the numerical model is 655 m, for a total volume of 1,868 km3. The St. Lawrence River is considered as a specified head boundary; the base and other lateral limits are considered as no-flow boundaries, whereas a head and conductivity-dependent boundary is specified along major streams and wetlands. Spatial recharge rate is applied as a specified flux across the top of the model and was fixed during calibration to reduce model uncertainty. Groundwater withdrawal of 34 Mm3/yr is assigned using sinks for major wells and as a uniform negative flux across the top of the model to account for domestic and other diffuse uses. Calibration was carried out against 153 hydraulic head measurements, with horizontal hydraulic conductivity and vertical anisotropy used as calibration parameters. The regional groundwater flow amounts to 268 Mm3/yr: 12.7% is withdrawn for domestic purposes; aquifer contribution to streams and wetlands is 176 Mm3/yr, and 55 Mm3/yr is discharged to the St. Lawrence River. Groundwater flow appears to be controlled by the sub-horizontal bedding planes contributing to relatively high vertical anisotropy.

Seismic fragility assessment of low-rise stone masonry buildings

Many historic buildings in old urban centers in Eastern Canada are made of stone masonry reputed to be highly vulnerable to seismic loads. Seismic risk assessment of stone masonry buildings is therefore the first step in the risk mitigation process to provide adequate planning for retrofit and preservation of historical urban centers. This paper focuses on development of analytical displacement-based fragility curves reflecting the characteristics of existing stone masonry buildings in Eastern Canada. The old historic center of Quebec City has been selected as a typical study area. The standard fragility analysis combines the inelastic spectral displacement, a structure-dependent earthquake intensity measure, and the building damage state correlated to the induced building displacement. The proposed procedure consists of a three-step development process: (1) mechanics-based capacity model, (2) displacement-based damage model and (3) seismic demand model. The damage estimation for a uniform hazard scenario of 2% in 50 years probability of exceedance indicates that slight to moderate damage is the most probable damage experienced by these stone masonry buildings. Comparison is also made with fragility curves implicit in the seismic risk assessment tools Hazus and ELER. Hazus shows the highest probability of the occurrence of no to slight damage, whereas the highest probability of extensive and complete damage is predicted with ELER. This comparison shows the importance of the development of fragility curves specific to the generic construction characteristics in the study area and emphasizes the need for critical use of regional risk assessment tools and generated results.

Grid-based hydrostratigraphic 3D modelling of the Quaternary sequence in the Chateauguay River watershed, Quebec.

Groundwater recharge, groundwater-surface water interaction, and protection and management of the groundwater resource are strongly constrained by the geological nature of the substratum. A grid-oriented technique has been developed in order to build a 3D stratigraphic model of the Quaternary sediments overlying a regional fractured rock aquifer. The technique is based on the integration of the surficial sediments map and borehole logs with the use of GIS and grid-calculator software Vertical Mapper. The applied methodology focused on estimating the thickness of the stratigraphic units rather than the elevation of the contacts. First, a regular grid was generated over the study area with 30 × 30 m cells. The bulk thickness of the Quaternary sequence for each grid cell was computed as the difference between the terrain digital elevation model and the rock surface krigged over more than 5000 drillers’ logs available. Two computation methods for estimating the discrete thicknesses are discussed and evaluated: the absolute method, in which the thickness of a given unit is computed as an independent value, and the relative method, in which the thickness is computed as a fraction of the bulk thickness. The simplified Quaternary stratigraphy consists, from top to bottom, of: organics (peat), alluvium, lacustrine, aeolian, coarse marine, marine clay and fine silt, fine sandy and silty glacio-fluvial, coarse sandy and gravelly glacio-fluvial, and glacial (silty clayey till) sediments. Their spatial distribution respects fully the surficial sediments map and contacts. At locations with missing stratigraphic data, the borehole logs database was improved with the addition of control points representing the anticipated variation of the successive layers. With the absolute method, derivation of the thicknesses of the layered strata in zones of irregular bedding proved to be difficult. The relative computation method gives more consistent results for various stratigraphic settings and allows rapid, internally consistent estimation of the overburden stratification.

Hazus: A standardized methodology for flood risk assessment in Canada

While Canada is exposed to a variety of natural hazards, most risk and emergency managers presently lack the necessary tools and guidance to adequately undertake rigorous risk assessments. Recently, Natural Resources Canada (NRCan) has adopted Hazus, a standardized methodology for estimating potential losses from natural hazards developed by the US Federal Emergency Management Agency (FEMA, fema.gov/hazus) as one of the best practice methods for risk assessment. Hazus estimates potential losses from earthquakes, floods and hurricanes, and includes a hazard and inventory database needed to conduct baseline risk assessment studies. An agreement has been signed with FEMA to adapt and co-develop a harmonized North American version of the Hazus methodology. At the same time, collaboration has been initiated within the federal government between the departments of Natural Resources, Environment, Defence and Public Safety to promote widespread usage of Hazus among the full range of Canadian decision-makers. This article reports the typical features of the Canadian version of the Hazus flood module and summarizes ongoing activities and potential challenges in implementing this model in Canada.

Regional Sustainability of the Chateauguay River Aquifers

A steady increase in groundwater use in the Chateauguay River watershed has led to potential conflicts between various groundwater users. This study summarizes the quantity and sustainability estimations of the groundwater resources within this basin. Regional sustainability is defined with simulated drawdowns from uniform withdrawal scenarios compared to water levels obtained without any withdrawal. Three sustainable conditions are defined: sustainable withdrawal, withdrawal with increased drawdown, and unsustainable withdrawal. The current withdrawal rate of 34 Mm3/yr results in a median drawdown of 1.5 m, compared to pre-development conditions. This drawdown is well within the range considered sustainable, an indication that regional aquifers are not currently overexploited. A hypothetical pumping rate of 48 Mm3/yr, resulting in an average drawdown of 2.2 m, is the estimated sustainable limit. Increasing exploitation from 48 to 122 Mm3/yr would need tight control and planning. Withdrawal rates beyond 122 Mm3/yr are judged not sustainable as the regional median drawdown would exceed 8 m. The water levels in recharge areas are the most sensitive to groundwater extraction. The combination of aquifer sensitivity to recharge variations, simulated drawdown maps, and aquifer vulnerability to surface contamination reveals the most sensitive areas of the regional aquifers, areas that would need particular attention and protection by groundwater managers.

Groundwater Quality, Geochemical Processes and Groundwater Evolution in the Chateauguay River Watershed, Quebec, Canada

A hydrogeochemical study was carried out in the Quebec portion of the Chateauguay River watershed. The objective was to characterize the chemical composition of groundwater in order to evaluate its quality and assess geochemical variations related to the geological and hydrogeological settings. Bulk groundwater samples were collected from 144 wells distributed evenly over the study area. Nine of the wells were sampled with a multi-level packer system, for a total of 22 multi-level samples. Samples were analysed for a comprehensive set of chemical inorganic parameters: dissolved major, minor and trace constituents, bacterial content, and stable (δ2H, δ13C, δ18O) and radioactive (3H, 14C) isotopes. Major dissolved constituents Ca, Mg, Na, K, Cl, SO4 and HCO3– ions represent more than 92% of total dissolved solids and their concentrations seem controlled by both hydrogeological and geological factors. Most water quality problems are related to aesthetic standards for potable water use (hardness, total dissolved solids (TDS), Fe and Mn concentrations) and TDS and Cl for irrigation use. Analyses of tritium (3H) and 14C confirm the inferred recharge zones and indicate the presence of variable water ages. Groundwater shows a wide range of compositions as indicated by 12 water types defined on the basis of major ions with a weak variation of chemical composition with depth. The predominant water type, Ca-HCO3, occurs in most geological and hydrogeological settings. Principal Component Analysis (PCA) and geochemical graphs were used to identify the major processes that exert a control over the chemical composition of the groundwater. Approximately 80% of the geochemical variation can be explained by mixing between fresh recharge water with more saline water associated with the former Champlain Sea, which invaded the aquifer. Secondary processes are related to ion exchange and the potential dissolution of minerals. A cross-section along a major flow path shows that the geochemical evolution of groundwater leads to relations between water type groups, geochemical processes, and groundwater flow conditions. The hydrogeochemical conceptual model infers that carbonate dissolution during recharge leads to one end-member, a Ca-HCO3 type water, which further evolves along its flow path due through ion exchange and mixing with remnant Champlain Sea water (Na-Cl), the other end-member.

Earth Observation Based Land Cover for Regional Aquifer Characterization

Managing groundwater resources requires quantification of several complex atmosphere/surface properties affecting recharge, including precipitation, temperature, soils, land cover and land cover change. In this review we elaborate on some of the remote sensing techniques commonly applied for generating information about changes in land cover, over large geographical areas, which are often required for hydrogeological studies. Two case studies are presented: the Chateauguay River Basin (Quebec) and Casselman Township (Ontario). Both examples represent agriculture dominated landscapes that require a unique mapping approach to minimize confusion between agriculture and forest classes. The first example demonstrates a procedure for mapping land cover aerial extent and change using remote sensing data acquired off the peak of the growing season where agricultural fields can be most effectively discriminated from forests. In the second example three classification methods are compared for mapping specific crop types. Results did not show substantially greater performance for any one of the three methods. Differences in processing and theoretical advantages are considered the main criteria for selection. The analyzes undertaken highlight the need to minimize phenology effects between image dates, by selecting images before leaf out or after leaf senesce for effective change detection and using several dates of imagery over the growing season to effectively map crop types.