Calcium and magnesium are two major cations that naturally occur in groundwater and in leachate. Both calcium and magnesium are alkaline earth metals that exhibit similar geochemical behaviors. Each have a single oxidation state and their concentrations in natural waters are controlled by the solubility of calcium or magnesium containing-minerals. In natural waters where geologic materials are primarily sedimentary in origin, like Ohio, calcium concentrations are largely controlled by the solubility of gypsum (CaSO4) and calcite (CaCO3) and magnesium concentrations are largely controlled by dolomite (CaMg(CO3)2) and magnesite (MgCO3) (Hem, 1985). The total concentrations of calcium and magnesium in water is also known as the water hardness. The higher the concentrations of these constituents, the harder the water. When concentrations of calcium and magnesium are low, the water is considered “soft”.
In C&D debris, the dissolution of minerals from drywall and concrete contributes to elevated calcium and magnesium concentrations in leachate. Does leachate recirculation increase the concentrations of calcium or magnesium in C&D leachate? The expected answer would be yes; however, the interaction of recirculated leachate is not that simple, as the following data shows. To answer this question, I compiled and analyzed the relevant analytical data using time series graphs and statistical trend testing from three C&D landfill sites that recirculate leachate in Ohio. The background information regarding the three study sites is presented in the first article in the Effects of Leachate Recirculation Article Series, “Introduction to the Effects of Leachate Recirculation”.
The calcium and magnesium data for Site A consists of 18 data points spanning a period of six years. For Site B there are 30 data points from 12 years of accumulated sampling data. Site C has 16 data points spanning only four years of sampling. Across the three sites, the reported calcium concentrations range from less than 10 mg/L up to 750 mg/L with the majority of results ranging between 300 mg/L and 600 mg/L. Whereas the magnesium concentrations typically range between 100 mg/L and 500 mg/L. In comparison, analytical results from the sand and gravel aquifers in Ohio reported average calcium concentrations of 93.1 mg/L and average magnesium concentrations average 28.3 mg/L (Ohio EPA Division of Drinking and Ground Waters, 2014). These leachate results show an elevated concentration for both calcium and magnesium over drinking water results. However, the results over time tell a different story. The times series graphs for calcium showed decreasing trends at both Site A and Site B. In contrast, Site C shows significant variability in the calcium concentrations with an overall slight increasing trend. The magnesium concentrations in both Site A and Site B exhibit clear decreasing trends. Similar to calcium, Site C showed variability in the magnesium concentrations over time, but no apparent trend. (See figures, below).
With two of the three sites, calcium and magnesium concentrations decrease with recirculation, and with the third, Site C, the concentrations show no statistically significant trends (See table, left). What factors are causing the differences between sites and controlling the changes in concentrations?
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Calcium Trends
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Site A
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Site B
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Site C
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Visual Trend
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Decreasing
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Decreasing
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Increasing
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Sen’s Slope
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None
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Decreasing
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None
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Linear Regression
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Decreasing
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Decreasing
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None
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Magnesium Trends
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Site A
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Site B
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Site C
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Visual Trend
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Decreasing
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Decreasing
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None
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Sen’s Slope
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Decreasing
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Decreasing
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None
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Linear Regression
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Decreasing
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Decreasing
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None
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To answer this question, it is necessary to understand how calcium and magnesium are mobilized. As mentioned previously, calcium and magnesium concentrations are generally controlled by the solubility of the associated minerals such as gypsum, calcite, dolomite, magnesite, etc. In C&D debris, these minerals are common in the form of gypsum drywall and concrete. The solubility of these minerals increases with decreasing pH. With no recirculation, the concentrations of calcium and magnesium in leachate would increase initially due to the effects of acid rain. According to the USGS, rain typically has a pH of 5.6 S.U. However, in Ohio, atmospheric pollution results in acid rain for which the pH can range between 4.1 and 4.5 S.U. (USGS, 2016). Therefore, infiltrating rain dissolves the minerals in drywall and concrete, increasing concentrations of calcium and magnesium in the leachate. However, acid rain is not the only factor controlling the solubility of calcium and magnesium.
Dissolved calcite and other carbonate minerals produce a buffering effect on the solubility pH of the infiltrating leachate. Calcite, CaCO3, dissociates into calcium (Ca2+) and carbonate ions (CO3-2). The carbonate ions react with hydrogen to form bicarbonate (HCO3-). Carbonate and bicarbonate are the primary components of alkalinity, which, by definition, is the capacity of a solution to neutralize an acid. Therefore, dissolution of concrete contributes to the typically high alkalinity in C&D landfill leachate, which in turn has a moderating effect on the pH of the leachate. For the three sites studied, total alkalinity measurements ranged from 260 mg/L to as high as 3600 mg/L. The average total alkalinity across the three sites was around 2100 mg/L. (Of note: there were no observed trends for alkalinity in any of the sites studied, therefore alkalinity trends will not be discussed as a separate article in this series).
One would expect that as the pH increases, the calcium and magnesium would become less soluble and precipitate out. However, pH is not the only factor controlling solubility. If a gas phase is present, the calcium solubility is controlled by the partial pressure of carbon dioxide (CO2) in addition to the pH of the solution. Calcium solubility increases with increased partial pressures of CO2. The landfill environment generates significant amounts CO2.
Sulfate Reducing Bacteria (SRB) generate CO2 as a byproduct of respiration. Gypsum drywall, or calcium sulfate dehydrate (CaSO4 2H2O), is one of the other sources of dissolved calcium in landfill leachate, and produces dissolved sulfate (SO42-). In the reducing landfill environment, SRB use an organic carbon source (i.e. wood, paper, etc.) as a food source to reduce sulfate to sulfide and generate hydrogen sulfide gas. SRB are most efficient at near neutral pH, which is maintained in the leachate by the high alkalinity. As a byproduct of the sulfate reduction, SRB also generate CO2, thereby increasing the partial pressure of CO2 in the landfill (Yang, et al., 2006). Calcium will stay dissolved in this environment.
However, with leachate recirculation, the leachate becomes aerated and encounters atmospheric concentrations of CO2. Under these conditions, the calcium solubility changes and reaches equilibrium with the atmospheric CO2 partial pressure. Atmospheric CO2 partial pressures are less than the elevated landfill CO2 partial pressure and calcite precipitates out of solution as a result.
It should be noted that although magnesium behaves similarly to calcium, some magnesium minerals are more soluble than calcite, and magnesium is less likely to then precipitate out of solution. In comparison to calcium, magnesium has a longer residence time and requires supersaturated conditions to precipitate out of solution. With recirculation, the change from landfill conditions to atmospheric conditions will likely result in super saturation with respect to magnesium and carbonate, therefore, magnesium can precipitate out as a magnesium carbonate mineral. Other mineral species are also possible depending on the composition of the leachate.
In addition to precipitation in atmospheric conditions, both calcium and magnesium exhibit cation-exchange behaviors and are strongly adsorbed onto clay minerals. Therefore, with recirculation, cation exchange removes calcium and magnesium from solution as the leachate migrates through the soil cover.
In summary, calcium and magnesium concentrations are decreasing for Sites A and B. Site C shows no significant trends. Therefore, leachate recirculation does not increase the concentrations of these two constituents. Precipitation and cation exchange are the likely the drivers behind the decreased concentrations observed. The current calcium and magnesium concentrations are still elevated in the leachate compared to “natural” waters. More long-term data is necessary to determine if continued leachate recirculation could improve leachate quality to the point that it is similar to natural waters or if current trends would reverse as the landfill ages.
References
Hem, J. D. (1985). Study and Interpretation of the Chemical Characteristics of Natural Water. U.S. Geological Survey.
Ohio EPA Division of Drinking and Ground Waters. (2014). Major Aquifers in Ohio and Associated Water Quality.
USGS. (2016, December 2). Acid rain. Retrieved from The USGS Water Science School: https://water.usgs.gov/edu/acidrain.html
Yang, K., Xu, Q., Townsend, T. G., Chadik, P., Bitton, G., & and Booth, M. (2006, August). Hydrogen Sulfide Generation in Simulated Construction and Demolition Debris Landfills: Impact of Waste Composition. Journal of the Air and Waste Management Association, 56, 1130-1138.
Previous Articles in Series:
Introduction to the Effects to Leachate Recirculation Series
Changes in Iron and Manganese Concentrations in C&D Leachate with Recirculation
Up next:
The Effects of C&D Leachate Recirculation on Chromium and Nickel Concentrations
Published: October 31, 2017