Hydrogeological Processes and Chemical Reactions at a Landfill

Abstract

Chemical and isotopic analyses were made of water from wells in and downgradient from a landfill to determine chemical and isotopic effects of generation and migration of leachate on ground water. The distribution and wide concentration range of oxygen and methane permit the delineation of an anaerobic zone, a regional oxygenated zone and an intermediate zone. The ratio of reduced nitrogen to nitrate indicates location of reducing fronts as the leachate migrates. The pH of the native ground water is low (≥5.0) primarily because of the low pH of rainfall and the lack of calcareous or other soluble minerals in the aquifer material. The pH is higher (∼6.6) in the leachate because of generation of carbon dioxide, ammonia, and methane. The native ground water has a low TDS (80 mg/l) while the leachate has an average TDS of 2800 mg/l and is primarily a NaHCO type water. Sulfate concentrations are extremely low and H S was not detected. We suggest that a major source of cations may be their exchange from the clays by the ammonium generated in the leachate. High concentrations of Fe and Mn are attributed to a source in the refuse but more important to reduction of oxide cements and coatings resulting from degradation of organic matter. The main source of bicarbonate is from organic degradation with minimal CO from the soil zone. At one landfill site 52% of the total alkalinity is attributed to organic compounds, mainly organic acid anions. The δ C of bicarbonate in the leachate is exceedingly heavy (+18.40 / ) which results from fractionation during the formation of methane. The 10 per mil deuterium enrichment of water may be due to decomposition of deuterium-enriched compounds and bacterial processes that preferentially consume the lighter hydrogen isotope.

Hydrogeology and ground-water-quality conditions at the Emporia- Lyon County Landfill, eastern Kansas, 1988

“…The hydrogen removal prevents buildup that would be toxic to methanogens and would suppress fatty-acid production (Gaudy and Gaudy, 1980). End products of fully completed anaerobic decomposition are methane, water, and carbon dioxide (Baedecker and Back, 1979). These end products probably first appear on the periphery of landfills (Metzler, 1975) where higher pH is more favorable to methanogenic bacteria.…”

“…After depletion of trapped or incoming oxygen by aerobic bacteria, the environment becomes reducing. Degradation processes in the landfill include biologic decomposition, solution, precipitation, sorption, ion exchange, and diffusion of gases (Baedecker and Back, 1979). Sufficient moisture, 40 to 60 percent, is essential, however, for significant degradation rates.…”

“…The depletion of oxygen as the primary oxydizing agent may lead to sulfate, nitrate, manganese oxides, iron oxides, and water acting as oxidizing agents (table 9). In the process of oxidizing organic matter, the oxidizers themselves are reduced to forms that are more soluble and thus are detected in larger concentrations under reducing conditions, except that nitrate nitrogen is reduced to ammonia nitrogen (Baedecker and Back, 1979;Freeze and Cherry, 1979, p. 118). In the oxygen-depleted landfill environment, the oxidation of organic matter also may lead to the production of methane gas (Freeze and Cherry, 1979, p. 118).…”

“…Chemical processes in leachate production are oxidation, reduction, dissolution, precipitation, ion exchange, and sorbtion; these processes probably are affected by the organic environment (Baedecker and Back, 1979). Physical processes are settlement, movement of evolved and ejected water by differential hydraulic heads, entrainment of colloidal and particulate material in flushing water, filtration, change of solute concentration by osmosis and concentration gradients, density separation of immiscible phases, and vertical and horizontal migration of gases.…”

“…Decomposition by hydrolysis allows bacteria to convert complex organic molecules to smaller, soluble ones that the bacteria can use for growth. Net products are primarily carbon dioxide and water, plus sulfate and ammonia (Baedecker and Back, 1979).…”