The Effects of Leachate Recirculation on Arsenic Concentrations in C&D Leachate

The Effects of Leachate Recirculation on Arsenic Concentrations in C&D Leachate

      Leachate recirculation is one possible method of leachate management at construction & demolition (C&D) debris landfills.  It can be more cost-effective than the alternatives, such as hauling to a treatment plant or discharging to a sanitary sewer.  With leachate recirculation, facilities pump leachate out of the landfill and then reintroduced it to the surface of the landfill with various methods.  However, the long-term effects of leachate recirculation on C&D leachate quality is still a topic of study and debate.  Does recirculation increase constituent concentrations? This article series, “The Effects of Leachate Recirculation” seeks to answer that question using the quarterly leachate sampling data from three Ohio C&D landfills that recirculate leachate as the basis to discuss trends in constituent concentrations and the chemical environments that impact those changes.  Thus far, articles in this series have discussed the effects of leachate recirculation on iron, manganese, calcium, and magnesium concentrations. This is the fourth article in the series, evaluating the effects of recirculation on arsenic concentrations. 

     Arsenic is of particular concern in construction and demolition (C&D) landfill leachate.  It is naturally occurring in soil and rocks, and C&D leachate commonly exhibits arsenic concentrations higher than “natural” groundwater; and, even if not detected in leachate, the leachate can mobilize the naturally occurring constituents.  Any of these occurrences may pose a risk to human health and the environment.

     Long-term exposure to arsenic has been linked to various cancers, including bladder, lung, skin, kidney, nasal passage, liver, and prostate cancers.  Ingestion of arsenic can also cause cardiovascular, pulmonary, immunological, neurological and endocrine issues (USEPA Office of Water, 2001).  Based on these risks, the United States Environmental Protection Agency (US EPA) established a Maximum Contaminant Level (MCL) for arsenic of 0.01 mg/L.

     Arsenic is a nonmetallic element, a group that also includes antimony, selenium, bromine, and iodine.  There are multiple oxidation states, two of which are stable in solution: arsenate (As5+) and arsenite (As3+).  Arsenic is naturally occurring in geologic materials and occurs in C&D waste materials.  The most common source of arsenic in C&D is chromated copper arsenate (CCA) treated wood. CCA treated wood is more resistant to decay than untreated wood, but in the landfill environment can leach chromium, copper, and arsenic into leachate (Townsend, 2001).  Additionally, arsenic is a component of paints, metals, and semi-conductors (USEPA Office of Water, 2001).

     In Ohio soils, arsenic is naturally occurring and can be present in concentrations in exceedance of United States Environmental Protection (USEPA) screening standard of 0.39 parts per million (ppm)


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

 
(Pacific Northwest National Laboratory, 2014).  Figure 1 shows the distribution of arsenic across the conterminous United States.  In much of Ohio, arsenic concentrations in the C horizon soil, which is the substrate with the least alteration from surface processes, rank in the 90th – 100th percentile range compared to the rest of the conterminous United States, with concentrations ranging from 12 to 193 mg/kg (Smith, Cannon, Woodruff, Solano, & Ellefsen, 2014).  Studies of arsenic concentrations in Ohio soils show ranges of 0.5 to 56 parts mg/kg (Cox & Colvin, 1996), which is consistent with the results of a study of the baseline arsenic in Ohio soils which found concentrations ranging from 2 to 45.6 ppm with an average of 9.69 ppm (Pacific Northwest National Laboratory, 2014).

 

     The analysis presented here is based on the available data for each of the three sites recirculating leachate.  For Site A there are 16 data points spanning five years, Site B has 29 data points representing 11 years of sampling results, and Site C has 16 data points representing four years of results.

      In Ohio sand and gravel aquifers, arsenic concentrations average 5.59 µg/L (0.00559 mg/L) (Ohio EPA Division of Drinking and Ground Waters, 2014).  In contrast, the leachate concentrations from our study sites are typically elevated above the natural concentrations in groundwater.  The arsenic concentrations in the study sites vary drastically between the three sites as shown on the Overall Arsenic Concentrations time series graph (left), with an overall range of 0.002 mg/L (Site B) up to 0.54 mg/L (Site A).  Arsenic concentrations in Site A and Site C exceed the drinking water MCL of 0.010 mg/L in all of the historical data.

      The data available for our three study sites show that C&DD leachate contains elevated concentrations of arsenic.  Does leachate recirculation increase arsenic concentrations?  To determine the possible effects of leachate recirculation on arsenic 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 individual arsenic time-series graphs for each of the three sites, below, show visually decreasing trends for Site A and Site B, and no apparent trends for Site C.  The data for Site C shows more temporal variability than observed in the data from the other sites.  Both Sen’s Slope/Mann-Kendall trend testing and linear regression show statistically significant decreasing trends and corroborate the visual trends in the Site A and Site B data.

Arsenic Trends

 

Site A

Site B

Site C

Visual

Decreasing

Decreasing

None

Sen’s Slope

Decreasing

Decreasing

None

Linear Regression

Decreasing

Decreasing

None

     These trend analyses indicate that leachate recirculation does not increase arsenic concentrations.    The arsenic data shows visually and statistically significant decreasing trends in the data for two of the three study sites.  These decreasing trends go against what we have been told will happen if we recirculate leachate and triggers the following questions.  Why is there such a significant variability in arsenic concentrations between the three sites?  What reactions could account for the observed trends?

    Recirculation cannot account for the large disparity in arsenic concentrations between the three sites, as shown in the Overall Arsenic Concentrations graph.  The most plausible explanation for these differences include variabilities in the waste stream, naturally occurring arsenic in the soils used to line the facility and cover the material, and/or different degree of chemical and biological processes occurring at each of the sites.

      The waste stream contains several items that may significantly affect the amount of arsenic in the leachate.  As mentioned previously, one of the primary sources of arsenic in C&D waste stream is CCA-treated wood.  Variations in the quantity of CCA-treated wood in the landfill can account for some of the variation in arsenic concentrations between the three sites. The amount of arsenic leached from waste containing CCA-treated wood has been shown to be positively correlated with the amount of CCA-treated wood contained therein (Zhang, Kim, Dubey, & Townsend, 2017).  However, though the individual C&D landfills keep track of the volume of incoming waste, the sites do not commonly differentiate waste stream by type or source, therefore the effect of this waste stream on the arsenic concentrations cannot be verified. 

      Another factor contributing to the diverse arsenic concentrations are the concentrations of sulfide. The dissolution of gypsum drywall can contribute significant amounts of sulfide to the leachate.  In the presence of high sulfide concentrations, arsenic and sulfide can form soluble thioarsenate species, thus mobilizing arsenic.  However, at low sulfide concentrations, arsenic and sulfide can form an insoluble arsenic-sulfide mineral, orpiment. In laboratory experiments, Zhang et al (2017) found that arsenic concentrations in leachate decreased as sulfide concentrations increased, up to approximately 1,000 µg/L.  Above 1,000 µg/L sulfide, arsenic concentrations increased as sulfide concentrations continued to increase.  

      Sulfide is not included in the leachate parameter monitoring-list required by the Ohio Administrative Code (OAC) 3745-400-21 for C&D landfills, however the monitoring list does include sulfate.  Sulfate and sulfide concentrations are interrelated, as sulfate (SO42-) is generated by the dissolution of gypsum drywall (CaSO4).  Sulfur Reducing Bacteria (SRB) then reduce sulfate to sulfide as hydrogen sulfide (H2S).

The Overall Sulfate Concentrations graph, below, shows that Site A shows an overall decreasing trend.  Site C has a single high point early in the historical data, and the rest shows no apparent trend. 

 

Site B shows significantly lower concentrations.  Statistical analysis of the sulfate concentrations show no significant trends.  Please note that the overall distribution of sulfate concentrations matches that for arsenic, suggesting a relationship between the two.  Site A shows the highest concentrations, Site C second highest, and Site B shows the lowest sulfate and arsenic concentrations.  The most plausible explanation suggests that the overall gypsum drywall content of the landfill is controlling the arsenic concentration, as was suggested in Zhang et al (2017).

      The Sulfate vs Arsenic Concentrations Graph (below), corroborates the strong correlation between arsenic concentrations and sulfate concentrations (i.e. associated with the dissolution of gypsum drywall).

      Another potential source for arsenic in the leachate is naturally occurring arsenic in the soil. The arsenic concentrations in the soils used in the landfill construction can also affect the observed leachate arsenic concentrations.  As discussed previously, arsenic occurs naturally in Ohio soils and can be mobilized in reducing conditions, which are common in landfill environments.  With natural soil arsenic concentrations ranging from 0.5 to 56 parts mg/kg (Cox & Colvin, 1996), the soil used for the landfill construction for liner, cover, or capping can impact the resulting concentrations in leachate.  While incoming soils are tested for permeability, moisture, and compaction, no chemical data is typically collected unless contamination is suspected.  Therefore, the actual arsenic levels in the soils are unknown, but can potentially account for some of the variation in leachate arsenic concentrations observed between the three sites. 

    The discussion thus far has focused on accounting for the large discrepancies in arsenic concentrations between the three study sites.  However, what chemical and biological processes affect the temporal changes (i.e., decreasing trends) in arsenic concentrations observed in Sites A and B?

      Geochemically, arsenic behaves similarly to iron and can be found bound to oxidized iron (Fe(III)) minerals.  Under reducing conditions, which is typical in landfill environment, insoluble Fe(III) is reduced to the more soluble Fe(II).  The reduced Fe(II) dissolves and the bound arsenic is also released into solution and mobilized in the groundwater, which results in elevated concentrations of both (Wang, Sikora, Kim, Dubey, & Townsend, 2012).  The decay of any organic materials (i.e. wood, paper, etc.) provides a food source for biological activity, such as reducing bacteria like SRB, and generates reducing conditions (Wang, Sikora, Kim, Dubey, & Townsend, 2012).  This is why even if C&D Landfill operators exclude arsenic containing wastes from the waste stream, the chemical environment in C&D landfills commonly possess the ability to mobilize naturally occurring arsenic. 

      Conversely, with recirculation, leachate is exposed to air and can become aerated and oxygenated.  Oxygenation can cause the leachate to generate oxidizing conditions, which will cause iron to oxidize to the less soluble Fe(III) and precipitate out of solution.  See the second article in this article series, “Changes in Iron and Manganese Concentrations in C&DD Leachate with Recirculation” for additional information regarding the effects of oxidation and reduction on iron concentrations.  Arsenic is soluble in both common oxidation states.  However, arsenic will co-precipitate with iron oxides and adsorbs onto other minerals (Hem, 1985), thus reducing the arsenic concentrations in recirculated leachate.

      In summary, Arsenic is a naturally occurring metal that can be elevated in C&D landfill leachate.  The arsenic concentrations and variability of these concentrations between landfills are influenced by the sulfate/sulfide concentrations, which are likely dependent on the quantity of gypsum drywall in the landfill, alongside the natural arsenic content in soils and the quantity of CCA-treated wood and other arsenic containing wastes in the waste stream. The process of recirculating leachate does not result in elevated concentrations of arsenic.  Instead, leachate recirculation can potentially result in decreased arsenic concentrations through co-precipitation with iron oxides.

References

Bureau of Environmental Health and Radiation Protection. (2016, June 30). Arsenic: Answers to Frequently Asked Health Questions. Ohio Department of Health.

Cox, C. A., & Colvin, G. H. (1996). Evaluation of Background Metal Concentrations in Ohio Soils. Columbus: Cox-Colvin & Associaties, Inc. Environmental Services.

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.

Pacific Northwest National Laboratory. (2014, April 16). Significant basline levels of arsenc found in soil throughout Ohio are due to natural processes. SciencyDaily. Retrieved from www.sciencedaily.com/releases/2014/04/140416125432.htm

Smith, D. B., Cannon, W. F., Woodruff, L. G., Solano, F., & Ellefsen, K. J. (2014). Geochemical and Mineralogical Maps for Soils of the Conterminous United States: U.S. Geological Survey Open-File Report 2014-1082. Reston: U.S. Geological Survey. Retrieved from http://dx.doi.org/10.3133/ofr20141082

Townsend, T. (2001, January 11). Arsenic and Old Wood. Recycling Today.

USEPA Office of Water. (2001, January). Drinking Water Standard for Arsenic, EPA 815-F-00-015.

Wang, Y., Sikora, S., Kim, H., Dubey, B., & Townsend, T. (2012). Mobilization of iron and arsenic from soil by construction and demolition debris landfill leachate. Waste Management, 32, 925-932.

Zhang, J., Kim, H., Dubey, B., & Townsend, T. (2017). Arsenic leaching and speciation in C&D debris landfills and the relationship with gypsum drywall content. Waste Management, 59, 324-329.

Previous Articles in Series:

Introduction to the Effects to Leachate Recirculation Series

Changes in Iron and Manganese Concentrations in C&D Leachate with Recirculation

Changes in Calcium and Magnesium Concentrations in C&D Leachate with Recirculation

Up next:

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

Published: January 18, 2018















Tags: 

Article Authors

Geologist II View Bio

Download Article