The Effects of Leachate Recirculation on Barium and Strontium Concentrations in C&D Leachate

The Effects of Leachate Recirculation on Barium and Strontium Concentrations in C&D Leachate

Does leachate recirculation increase constituent concentrations?   This article series, “The Effects of Leachate Recirculation” seeks to answer that question for construction and demolition debris landfills.  In construction and demolition (C&D) leachate specifically, the long-term effects of recirculation remains a topic of study and debate.  Leachate recirculation is a leachate management practice that reintroduces leachate to the landfill instead of the leachate being treated or disposed of off-site.  Its reintroduction to the landfill allows it to percolate through the debris, which can benefit the facility in several ways: improvement of leachate quality, faster stabilization of the landfill, dust control, fire prevention, and reduced leachate generation through absorption and evaporation.  However, does leachate recirculation actually improve leachate quality?  Without data to back-up these claims, the concern remains that recirculating leachate could result in ever-increasing concentrations of hazardous constituents.

To show that this is not the case and that leachate quality actually improves with recirculation, I visually and statistically analyzed quarterly leachate sampling data from three C&D landfills in Ohio for trends in constituent concentrations.  Previous articles in this series have discussed iron, manganese, calcium, magnesium, and arsenic and found either no significant trends or decreasing trends associated with the recirculated leachate.  Does this hold true for barium and strontium?

Both barium and strontium are alkaline earth metals commonly found at elevated concentrations in C&D landfill leachate.  These metals occur naturally in soil and rocks, but C&D leachate concentrations are higher than in “natural” groundwater.  Although not carcinogenic, barium poses several health risks.  Ingestion of high concentrations of barium in water can cause difficulty breathing, increased blood pressure, changes in heart rhythm, stomach irritation, brain swelling, muscle weakness, and damage to the heart, liver, kidney, and spleen (Water Quality Association, 2013).  Based on these risks, the United States Environmental Protection Agency (US EPA) established Maximum Contaminant Levels (MCLs) for barium of 2.0 mg/L. 

In contrast, strontium does not have an established MCL.  Ingesting strontium at high concentrations can result in altered bone mineralization.  Strontium can replace calcium or prevent calcium absorption resulting in bone deformities.  Typically, strontium is not a known carcinogen, although there is a radioactive isotope, strontium-90, that is carcinogenic.  However, given the limitations in the types of debris accepted at C&D landfills, strontium-90 is unlikely in C&D leachate. The USEPA established health advisories for strontium; the Lifetime Health Advisory Level (Lifetime HAL) is 4 mg/L (Ohio Department of Health, 2015). 

Both barium and strontium, as well as, calcium, magnesium, beryllium, and radium are alkaline earth metals, a group of elements, which have a single oxidation state in natural water and in common surface minerals.  In natural water, the solubility of the mineral barite (BaSO4) controls the concentration of barium, which is poorly soluble (Hem, 1985).  Strontium is typically found in sediments as the carbonate mineral strontianite (SrCO3) or the sulfate mineral celestite (SrSO4).  Both strontium minerals are less soluble than the corresponding calcium minerals, calcite (CaCO3) and gypsum (CaSO4 2H2O).  The celestite solubility acts as the primary control on strontium concentrations (Hem, 1985). 

Barium compounds such as barite or witherite (barium carbonate) are used in a variety of industries.  Of note, the following barium products may end up in a C&D landfill: plastics, rubber, textiles, ceramic glaze and enamels, glass, bricks, and steel (World Health Organization, 2004). Strontium containing products that could be part of the C&D waste stream include ceramics, glass, paints, and metal alloys (Watts & Howe, 2010).  Strontium can also be found as a component of gypsum drywall, where strontium has naturally replaced the calcium in gypsum (Townsend, Hwidong, Lott, & Krause, 2013).

In soils, background concentrations of barium and strontium in Ohio soils range from 9.3 mg/kg to 323 mg/kg (Cox & Colvin, 1996) and from 52 mg/kg to 150 mg/kg (Smith, Cannon, Woodruff, Solano, & Ellefsen, 2014), respectively.  As shown on Figures 1 and 2, Ohio barium and strontium concentrations rank between the 20th and 60th percentile range (296 mg/kg – 579 mg/kg) compared to the national

Figure 1: Concentration of Barium in substratum soils (C Horizon) in the conterminous United States. Figure from (Smith, Cannon, Woodruff, Solano, & Ellefsen, 2014)

Figure 2: Concentration of Strontium in substratum soils (C Horizon) in the conterminous United States. Figure from (Smith, Cannon, Woodruff, Solano, & Ellefsen, 2014)


ranges in Horizon C substrate, which is the substrate with the least alteration from surface processes (Smith, Cannon, Woodruff, Solano, & Ellefsen, 2014).   

In Ohio sand and gravel aquifers, barium concentrations average 0.158 mg/L and strontium concentrations average 1.957 mg/L (Ohio EPA Division of Drinking and Ground Waters, 2014).  In contrast, the leachate concentrations from our study sites are elevated above the natural concentrations in groundwater, with barium concentrations averaging 0.47 mg/L and strontium concentrations averaging 4.0 mg/L.  The Overall Barium Concentrations and Overall Strontium Concentrations graphs, next page, present the available leachate data from the three study sites. 

What we want to know is whether the recirculation of leachate increases concentrations for barium and strontium over time.  To determine the possible effects of leachate recirculation on these metals concentrations, I analyzed the available leachate data using time-series graphs to identify visual trends, and two different methods of trend testing in the software Sanitas® to estimate statistical trends. 

The analysis presented here is based on the available data for each of the three-recirculation sites.  Site A has four years of quarterly data or 16 data points for both barium and strontium; Site B has 29 data points for barium, which represents 11 years of sampling results and 16 data points for strontium, which represents four years of sampling results; and, Site C has 16 data points representing four years of results for barium and strontium.   

The time series graphs for barium, created using the data points outlined above, show weak and insignificant decreasing visual trends for sites A and C, and no appreciable visual trend for Site B.  However, the data set from Site B demonstrates extreme temporal variability in barium concentrations from 2010 to present, with concentrations ranging from less than 0.2 mg/L up to 1.14 mg/L.  These temporal variations may represent effects of infiltration associated with seasonal rates of precipitation and recirculation. 

The statistical trend testing results show no statistically significant increasing or decreasing trends for barium in the data from any of the three sites.

Barium Trends

 

Site A

Site B

Site C

Visual

Decreasing

None

Decreasing

Sen’s Slope

None

None

None

Linear Regression

None

None

None

Strontium Trends

 

Site A

Site B

Site C

Visual

None

None

Increasing

Sen’s Slope

None

None

None

Linear Regression

None

None

None

The strontium time-series graphs, similar to barium, show no apparent trends with significant temporal variability for each of the three sites.  The Site C time-series graph appears to show a slight increasing trend, masked by the temporal variation.  The trend testing results reports no statistically significant trends for strontium at any of the three recirculating sites.

Comparison of the barium and strontium time-series graphs show some correlation between constituent concentrations.  These two constituents appear to behave similarly, elevated barium concentrations coincide with elevated strontium concentrations.  The trend analyses suggest that leachate recirculation is not influencing barium or strontium trends in the long-term.  Although potentially not associated with recirculation, the short-term variability may be associated with other elements, such as cell development, precipitation, recirculation schedule, and other factors.  Unfortunately, accurate information on cell development and the leachate recirculation schedule is not available.  Without this information, what other relationships and reactions could we assess to account for the observed barium and strontium trends and concentrations?

Other factors that may influence the concentrations of these constituents in leachate include physical and chemical relationships between the different minerals in the debris and surrounding soils and oxidation-reduction (REDOX) reactions, both biological and chemical.  The dominant physical attribute controlling concentrations of these constituents is the mineral solubility. As mentioned previously, the limited solubility of the mineral barite (BaSO4), a barium sulfate, generally controls the barium concentrations in solution.  In addition, we know that sulfate (SO42-) is commonly elevated in C&D leachate with the primary source being gypsum (CaSO4) drywall.  The composition of the waste stream and the chemical environment directly influences the concentration of sulfate in leachate.  If there is more gypsum drywall in the waste stream, sulfate concentrations should be higher because of the amount of gypsum available for dissolution.  Gypsum demonstrates a limited solubility at normal temperature and pH ranges, more so than barite, which is poorly soluble.  However, the soil may possess barium and be a source of barium.  As shown in Figure 1, Ohio barium concentration in soil rank in the 20-60 percentile range.

The Overall Sulfate Concentrations graph, below, shows the different sulfate concentrations for each of the three sites. 

 

Site A shows the highest concentrations, significant variability, and an overall decreasing visual trend.  Site C demonstrates lower concentrations, less variability, and shows no apparent trend.  Site B shows significantly lower concentrations, significantly less variability, and no apparent trend.  Statistical analysis of the sulfate concentrations at the three site show no statistically significant trends. 

A comparison of the sulfate concentrations to barium concentrations (Barium vs Sulfate Concentrations Graph, below) shows a negative correlation,

 

higher sulfate concentrations correspond with lower barium concentrations.  The solubility of barite accounts for this relationship.  The solubility of barite is very low (0.0002448 g/100 mL @ 20 ºC); therefore, sulfate concentrations must be low to drive dissolution of barite.  If sulfate concentrations are high, such as at Site A, barium will not dissolve readily resulting in lower barium concentrations.   Whereas, sulfate concentrations are low at Site B, which promotes barite dissolution, as demonstrated by the elevated barium concentrations.