Performance des lieux d'enfouissement sanitaire de Laterrière et de Ste-Sophie, Québec, Canada Performance of two sanitary landfill sites, Laterrière and Ste-Sophie, Québec, Canada

Résumé

Ce travail décrit les caractéristiques hydrogéologiques et chimiques de deux lieux d'enfouissement de déchets municipaux localisés au Québec. Le site de Laterrière est en opération depuis 1971 dans une ancienne sablière. Sous les dépôts meubles, la topographie du sotte rocheux est marquée par une vallée qui forme une limite imperméable à l'aquifère de sable et impose un écoulement convergent de l'eau souterraine vers la rivière Chicoutimi. Le site de Ste-Sophie, en opération depuis 1976, repose sur une couche de 3,5 mètres de sables fins saturés recouvrant un lit d'argile marine. La surélévation de la nappe dans les déchets favorise un écoulement divergent vers le périmètre de la zone d'enfouissement.

Un réseau de piézomètres installé en bordure des deux sites a permis pendant trente mois de prélever des échantillons d'eau et de les analyser. Dans chaque panache de contamination, l'alcalinité, les ions majeurs Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl¯, le fer, le manganèse, l'azote total et ammoniacal, les DCO et DBO₅ montrent des concentrations généralement supérieures à celles de L'eau naturelle ambiante. A proximité des deux Lieux d'enfouissement, les teneurs en cadmium, chrome, nickel et plomb excèdent les normes québécoises de potabiLité de l'eau. Dans chaque cas, ta distribution des contaminants est surtout régie par le réseau d'écoulement. La dispersion hydrodynamique et les réactions géochimiques dans le sous-sol sont des processus importants d'atténuation des contaminants.

Mots-clés:

• Enfouissement sanitaire,
• lixiviat,
• migration des contaminants,
• dispersion,
• modélisation

Abstract

This paper describes the hydrogeological and geochemical features of two sanitary landfill sites located in the Province of Quebec : Laterriere and Ste-Sophie. The main objective is to evaluate the leachate attenuation performance of each site. For this project, the Quebec Ministry of the Environment chose two sites with contrasting hydrogeological settings.

The Laterriere landfill, in operation since 1971, covers an area of about 32 ha. The refuse is deposited in an old sandpit and the landfill boundary is located 400 meters away from the Chicoutimi River, 25 meters above the stream water level. The bedrock topography shows a valley under the landfill which acts an an impervious base for the sand aquifer and induces a convergent groundwater flow towards the river.

The Ste-Sophie landfill covers an area of 48 ha and began operating in 1976. The refuse zone is located on flat ground 350 meters away from the Ruisseau-aux-Castors. The refuse is deposited on a 3.5 meter layer of fine sands nearly saturated overlying a marine clay bed. A water table mound is observed beneath the landfill, resulting in diverging low lines outward from the center of the Landfill.

For each site, the occurence and migration of contaminants was investigated using networks of piezometers. Sampling was performed monthly over a period of thirty months and thirty parameters were analyzed on each sample. The analytical results were entered in a database and treated by statistical methods.

The shape of each contamination plume is controlled mainly by the groundwater flow system. At Laterriere, the plume is restricted to a narrow path between the landfill and the river and inside a bedrock valley. This flow net has been simulated by a two-dimensional finite elements model. At Ste-Sophie, the divergence of flow lines due to groundwater mounding beneath the site causes dispersion of contaminants around the landfill. The use of an analytical solution shows that longitudinal dispersivity is a major process of contaminant migration.

For the two landfills, the highest concentrations of contaminants occur directly beneath or close to the boundaries of the landfills. Total alkalinity, Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl¯, iron, manganèse, total and ammoniacal nitrogen, chemical and biological oxygen demand (COD and BOD) are much above background levels throughout the plume. Sulfate is also a major contaminant at Ste-Sophie with concentration of 1100/mg/l. The concentrations of cadmium, chrome, nickel and lead exceed the drinking water standards near the two landfill sites but they decline rapidly in the direction of flow. At Laterriere, the decline in chloride concentration measured at the discharge point S-23 is about 85 % while the decline of reactive contaminants, such as iron and COD, is respectively 98 and 99.7 %.

Finally, we discuss hydrogeological criteria for landfill site selection in Quebec.

Keywords:

• Sanitary landfill,
• groundwater monitoring,
• leachate,
• contaminant migration,
• dispersion,
• modeling