# Risk Evaluation of Leachable Mercury From Concrete Products Made With Fly Ash

Robert McCann 1 

## , Kerri L. M. Hartung 2 

## , Jill Hedgecock 3 

## , Peggy Schuler 4 

## , Steven Putrich 4

1 

### AES Hawaii, Inc., 91-086 Kaomi Loop, Kapolei, HI, 96707; 2 

### URS Corporation, 1600 Perimeter Park Drive, Morrisville, NC 27560; 3 

### URS Corporation, 221 Main Street, San Francisco, CA 94105; 4 

### URS Corporation, 1375 Euclid Avenue, Cleveland, OH, 44115

KEYWORDS : mercury, fly ash, risk, risk ev aluation, concrete, flowable fill

ABSTRACT

AES Hawaii, Inc. operates a 203 Megawatt circul ating fluidized bed coal and alternative fuel-fired cogeneration facility. A portion of the byproduct fly ash may be used as a concrete admixture. Per AES Hawaii’s solid waste management permit (SWMP), composite samples of fly ash are routinely tested for total and leachable metals content. In 2005, several samples exceeded the pe rmit limit for leachable mercury (0.00025 mg/L) determined through the Synthetic Precip itation Leaching Procedure. AES Hawaii prepared a risk evaluation to determine if me rcury from fly ash posed an environmental concern when intermixed in concrete products.

The primary objectives of the risk evaluation were to: 1) ev aluate whether incorporating fly ash into concrete/flowable fill reduces me rcury leachability to acceptable levels; and 2) determine if a threshold fly ash content exists that results in unacceptable mercury concentrations in concrete/flowable fill leachat es. The study demonstrated that, even in cases where mercury in the raw fly ash ex ceeds the SWMP limit, no detectable mercury is leachable from the final concrete products containing such fly ash. In addition, since no mercury was detected in leachates, no thre shold of fly ash content (up to 90 percent cement replacement) appears to exist where leachable mercury exceeds the applicable risk-based limits. As such, mercury in AES Hawaii fly ash used as an admixture in concrete products does not pose an una cceptable risk to human health or the environment. The Hawaii Department of Health therefore proposed an increased mercury limit for such fly ash from 0.00025 mg/L to 0.0055 mg/L.

INTRODUCTION

AES Hawaii, Inc. (hereafter AES Hawaii) op erates a 180 Megawatt net circulating fluidized bed coal and alternative fuel-fired c ogeneration facility, the only such facility on the Island of Oahu and the only operational coal fi red plant in operation in Hawaii. This operation generates approximat ely 15 to 20 percent of O ahu’s electricity. Coal combustion ash is a by-product of AES Ha waii operations from burning coal and alternative fuels for electricity generati on. Fly ash and bed ash are mixed to form

### conditioned ash, some of which was reused in concrete and the surplus was deposited in a private quarry for land reclamation. The quarry had reached capacity and would no longer accept the fly ash, requiring future dis posal at a permitted land fill. In an island environment which by design has land limitati ons, the option of continuing to use a portion of the fly ash as an admixture by Hawa iian Cement in its concrete products was considered highly desirable.

Under AES Hawaii’s solid waste managemen t permit (SWMP) issued by the Hawaii Department of Health (HDOH), the fly ash is routinely tested via the SPLP for suitability for use in concrete by measuring the tota l and leachable metals content of composite samples. The SPLP analysis is designed to measure how likely t he chemicals present in a solid material are to be released into surface water or groundwater when exposed to precipitation. In 2005, several compos ite fly ash samples submitted for routine analysis exceeded the permit limitation for leachable mercury (0.0 0025 mg/L) (refer to Figure 1). AES Hawaii retained URS Corporat ion (URS) to conduct a risk evaluation to determine if mercury from fly ash posed an en vironmental concern when intermixed in concrete products.

The major components of the risk evaluation were to identify potential risks, design a quantitative study to evaluate the potential ri sks, analyze the resu lts of the study, and propose alternate permit conditi ons dependant upon those results.

IDENTIFICATION OF POTENT IAL RISKS FOR EVALUATION

Two potential risk issues were identified fo r mercury in AES fly ash subsequent to its inclusion in concrete products: 1) direct contact with mercury release during mixing or from weathered concrete, and 2) leaching of me rcury from the concre te to surface water or groundwater. Further evaluat ion eliminated the fi rst potential risk as unlikely based on the following rationale:

− During combustion, mercury in coal vola tilizes and is emitted in the flue gas. Some of the mercury contained in the flue gas (predominantly ionic mercuric compounds such as HgCl 2 

### and HgSO 4 

### ) then adsorbs to the carbon phase of the coal ash as it cools. The presence of vo latile elemental mercury in the fly ash is unlikely. As such, volatilization is not considered an important fate and transport mechanism in the fly ash product.

− Because total concentrations of mercury in the fly ash (typically 0.2 to 0.4 milligrams per kilogram [m g/kg]) are well below Hawaii’s risk-based screening criterion for mercury of 13 mg/kg at c ontaminated sites (HDOH, 2005), as well as the United States Envir onmental Protection Agency (USEPA) Region IX residential preliminary remediation goal (PRG) (USEPA, 2004a) of 23 mg/kg (for “mercury and compounds”), direct contact wit h fly ash (dermal, oral, or inhalation of particulates) by residents or workers is also not considered a potential health issue.

### The second potential risk remained for furt her evaluation because the inorganic mercury compounds bound to fly ash particles could hav e the potential to be released into an aqueous phase when exposed to water in the environment. Therefore, the risk evaluation was designed to address potential c oncerns if mercury bound into concrete were to be found leachable into surface water and groundwater.

RISK EVALUATION PURPOSE AND OBJECTIVES

The primary objectives of the risk evaluation were two-fold : 1) to evaluate whether mixing fly ash with cementitious material to form concrete or flowable fill reduces the leachability of mercury to acceptable levels ; and 2) to determine if a threshold of fly ash content exists that results in unacceptable mercury concentrations in the SPLP leachate of the concrete. Secondary objectives inclu ded investigating whether a difference exists in mercury leachability between intact and crushed (recycled/weathered) concrete, and whether an extraction fluid simulating the pH of rainwater on the leeward coast of Oahu influences mercury leachability. These se condary objectives were designed to be studied in the event that me rcury was detected in the SPL P leachate upon mixing with concrete. With HDOH approval, these se condary objectives woul d be dropped from the evaluation if the SPLP leachate, under “worst case” conditions, did not contain mercury.

STUDY DESIGN

The study would be conducted in two stages, each stage designed to assess the primary and secondary objectives, respectively . The first stage would compare “worst case” analytical results from SPLP testing of concrete cylinders containing varying percentages of ash with varying concentra tions of mercury against applicable riskbased criteria. If detectable mercury levels we re to be identified during the first stage, the second stage of the study would then result in collection and comparison of analytical results from SPLP testing procedu res modified to represent “real world” environmental affects on the same concrete cylinders.

Hawaiian cement concrete products are used throughout Hawaii in a variety of land and marine applications, thereby prompting a mu lti-pathway evaluation. Based on these potential exposure pathways, app licable criteria for comparison with SPLP results were identified in advance of analytic al testing. These criteria, in conjunction with the established permit limit, were used to se t the appropriate method detection limits for SPLP testing in an effort to minimize study uncertainties.

TECHNICAL APPROACH

The technical approach involved reviewing anal ytical data from 2005 to select archived samples appropriate to the study’s objec tives, re-analyzing the 2005 results and raw materials from Hawaiian Cement to est ablish a study baseline, and preparing and curing concrete cylinders according to industry standards with varying levels of ash replacement to represent permitted mixtures and flowable fill. The first stage of the

### study then concluded with t he analysis of the cured cylinders per Total Mercury and SPLP method requirements.

Sample Selection and Study Baseline

AES Hawaii’s routine monitoring program incl udes semi-monthly compositing of fly ash samples for analysis. One grab sample is taken of each ash type for each day of shipment. Each composite consists of por tions of these discrete grab samples taken during the designated time period, combi ned by laboratory personnel and analyzed for total and leachable metals. A portion of each grab sample is archived at AES Hawaii.

Using the monitoring data from 2005, eight semi-monthly composites were identified that yielded leachable mercury concentra tions above the method detection limit (MDL) at varying ranges appropriate to the study’s objectives (refer to Figure 1). URS’ laboratory, Geotesting Services, Inc., comb ined the archived grab samples associated with those time periods as appropriate to recr eate the original composite samples. The new composites were then submitted to a subcontracted laboratory for analysis to confirm the original fly ash SPLP mercury resu lts prior to testing concrete made from varying percentages of those composites. Small quantities of Portland cement, #4 basalt, and sand obtained from Hawaiian Cement were submitted for analysis as well, to verify that these consti tuents would not contribute merc ury to the final product.

A summary of the original monitoring data an d the new data is presented in Figure 2. The SPLP mercury results gener ated by the study baseline testing were somewhat different than the original results. Specific ally, where previously six of eight samples exceeded the permit limit of 0.00025 mg/L, in the new data set, only one sample exceeded that level. Althoug h the concentrations differed somewhat, the new analyses confirmed within acceptable precision that there was a trace amount of leachable mercury present in most of the samples test ed, which, in at least one case, exceeded AES Hawaii's permit limit for fly ash. Bas ed on the old and new data, the four samples with the highest leachable mercury content and greatest amount of remaining sample material were selected to conduct the risk evaluation testing (refer to Figure 2)

Cylinder Preparation and Curing

From the three selected com posites, Geotesting Services prepared batches of concrete using the ratio of materials contained in Hawaiian Cement’s concrete, replacing 25 percent and 50 percent of the Portland cement with each fl y ash composite. In addition, batches of flowable fill we re prepared by replacing 90 perc ent of the cement with each of the three fly ash composit es. The proportions of materi als in each of these mixes (based on batch weights provided by Hawaii an Cement) are summarized as follows in Table 1.

### Table 1. Summary of Concrete Mixtures for Analysis By Percent Cement Replacement with Fly Ash

Percent of Material by Weight

Concrete Flowable Fill
Materials 25% Fly ash* 50% Fly ash* 90% Fly ash*

No.4 Basalt 60.9 60.9 61.3

Sand 15.3 15.3 0.0
Water 8.3 8.3 15.0
Cement 11.6 7.7 2.4 Fly Ash 3.9 7.7 21.4 * Percent of cement replaced with fly ash.



### One batch of concrete was prepared containi ng no fly ash to serve as a control. The concrete and flowable fill mixtures were pl aced in 2-inch-diameter cylindrical molds and moist-cured for 28 days (for consistency with industry standards) prior to chemical analysis. At least three cylinders were pr epared for each mixture, plus an additional three cylinders for one 50 percent mixture, to allow duplicate analyses for quality control (refer to Figure 3). To simulate weathered c onditions, a portion of the cylinders were crushed. The crushed cylinders were cons idered a worst case condition because the smaller components would have a larger surf ace area, thereby enhanc ing the potential for leaching of mercury.

ANALYTICAL RESULTS

The analytical data for total mercury in t he crushed concrete samples are summarized in Figure 4. The total mercury concent rations ranged from less than 0.007 mg/kg to 0.037 mg/kg, well below the 13 mg/kg permit limit. No mercury was detected in the SPLP leachates of any of the crushed concrete samples, relative to a reporting limit of 0.0001 mg/L and a method detection limit of 0. 00002 mg/L. As such, it was considered unnecessary to proceed with the previously described Stage 2 of the study. Since the SPLP results were all non-detect, the leachabl e portion of the total mercury content was, for the purposes of this study, effectively zero.

RISK EVALUATION

AES Hawaii’s permit criterion for leachable mercury was promulgated on the State of Hawaii Tier I Environmental Action Level (EAL) for water (HDOH, 2005). In general, Tier I action levels are conservative criteria set to be protective of human health and aquatic life under most exposure scenarios. However, since AES Hawaii fly ash is not reused as a stand-alone product, but rather as a component of a Hawaiian Cement concrete product, and SPLP mercury levels from concrete made with fly ash were all non-detect, AES Hawaii requested the risk eval uation include a proposed alternative permit limit that would be appl icable to fly ash used as a concrete additive.

### Pathway Determination

Since Hawaiian Cement concrete products ar e used in various settings throughout the island of Oahu (and possibly elsewhere in Hawaii), the risk evaluation considered the most sensitive receptors for each pathway de termined to be complete in the conceptual site model. Figure 6 provides a pictorial c onceptual site model (CSM) of how leachable constituents in selected concrete produc ts could be released and reach human and ecological receptors. Thes e receptors were (a) a human drinking water user, (b) a recreational fisherperson, (c) various fres hwater aquatic organisms, and (d) various marine aquatic organisms.

Comparison of SPLP Mercury Resu lts with Applicable Criteria

In addition to the permit limit for mercury of 0.00025 mg/L, which is based on the most stringent water standard (i.e., the protection of marine aquat ic species and habitat under chronic conditions) with an attenuation factor of 10 applied, the SPLP mercury results were compared to other commonly used risk-based criteria for the ident ified receptors of concern as discussed below. These criteria include:

− Environmental Action Levels presented in “Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater,” (HDOH, 2005);

− The State of Hawaii water quality criter ia listed in the Hawaii Administrative Rules, HAR §11-54-4(b)(3);

− The ambient water quality criteria (AWQC) included in “National Recommended Water Quality Criteria : 2004” (USEPA, 2004b); and

− The National Primary Drinking Water Regulations (USEPA, 2002).

Figure 6 presents a summary of the potentially applic able criteria by receptor type and pathway, compared to SPLP mercury results.

Basis for Permit Limit

The existing mercury permit limit (0. 00025 mg/L) is derived from the HDOH marine chronic aquatic habitat goal (based on the Hawaii chronic saltwater surface water quality standard – 0.000025 mg/L) with an att enuation factor of 10 applied. The application of the at tenuation factor is consistent with Buchman (1999), where the Coastal Protection and Restoration Division of the National Oceanic and Atmospheric Administration uses 10 times the applic able AWQC for screening for dilution and attenuation. The Hawaii chronic saltwa ter surface water quality standard of 0.000025 mg/L, however, is based on an outdated USEPA cr iterion. This value appears in “Ambient Water Quality Criteria for Merc ury” (USEPA, 1985), and was derived using the

### Final Residue Value procedure described in that document. Since the publication of the Great Lakes Aquatic Life Criteria Gui delines in 1995 (60FR15393-15399, March 23, 1995), USEPA no longer uses the Final Re sidue Value procedure for deriving AWQCs for new or revised 304(a) aquatic life criteria.

DISCUSSION OF STUDY UNCERTAINTY

A summary of uncertainties associated with t he analytical data and chemical forms of mercury follows. Estimates of exposure an d risk are always subject to a number of uncertainties that could lead to underestimates or overesti mates of risks. Because of the conservative nature of the assumptions us ed in establishing risk- based criteria, it is more likely that risks have been overestimated.

Analytical Data

Some uncertainty associated with the analytical data is expected due to variability in sample matrices and limitations of the analyt ical methods. Uncertainty due to sample heterogeneity is minimized with careful sample preparation and monitored with replicate sample analyses. Some variability, how ever, especially for solid samples, is unavoidable.

Under most circumstances, sufficiently sens itive reporting limits (i.e., reporting limits lower than the applicable standards) were achieved. However, in some cases the applicable criteria (specifically, the fish consumption standard and the chronic marine habitat standard) are below the reporting limit for SPLP mercur y. The laboratory MDL, however, was less than the criteria in each of these cases. Concentrations between the MDL and the reporting limit cannot be accu rately quantified and are considered estimates. Even so, USEPA defines the MDL (determined statistica lly from multiple analyses of a sample in a given matrix c ontaining the analyte of interest) as the minimum concentration of a substance t hat can be measured and reported with 99 percent confidence that the analyte concent ration is greater than zero. Since no mercury was detected above the MDL in any of the concrete sample SPLP leachates, it is reasonably certain that mercury was not present in these samples.

Chemical Form of Mercury

Inorganic mercury entering marine and freshw ater environments can remain dissolved in the aqueous phase, resorb directly onto soil or sediment, or be chemically altered into either of two more toxic organic forms: meth ylmercury and dimethylmercury. Of these two forms, methylmercury is of greater environmental concern because of the biomagnification (increasingly higher concentrations) through food webs to levels that are toxic to fish and wildlife, and subsequently, humans.

### Because no mercury was detected in any of t he concrete sample leachates, it is unlikely mercury mixed with concrete would be c onverted to methylmercury in a marine environment at concentrations that would cause significant envir onmental impact.

STUDY CONCLUSION AND PERMIT LIMIT REVISION

This study demonstrated that, even in cases where SPLP mercury in the raw fly ash exceeds AES Hawaii’s Solid Waste Managem ent Permit (SWMP) limit, no detectable mercury is leachable from the final concrete product containing such fly ash. As such, mercury in fly ash used as an admixture in concrete (up to 90 percent cement replacement) does not appear to pose a health risk to hum ans or the environment. Based on these results, and the fact that t he current SWMP limit is derived from a chronic saltwater criterion calculated usi ng an outdated method, it was concluded that the SWMP limit for SPLP mercury in the ra w fly ash used in concrete products was overly conservative and should be re-evaluated by HDOH.

As noted previously, the basis for the cu rrent permit limit was USEPA the chronic marine AWQC accordingly in 2002 of 0. 000025 mg/L. The current chronic marine AWQC value of 0.00094 mg/L (USEPA, 2004b) and chronic freshwater AWQC value of 0.00077 mg/L are higher than current chroni c freshwater standards promulgated by HDOH. The HDOH 2005 freshwater chronic aquatic habitat goal (based on the Hawaii chronic freshwater surface water quality st andard) is 0.00055 mg/L. Thus, AES Hawaii proposed to HDOH at the conclusi on of the study that the d ilution attenuation factor of 10 be applied to the current HDOH 2005 fres hwater chronic aquatic habitat goal of 0.00055 mg/L, thereby resulting in an incr eased SPLP mercury permit limit from 0.00025 mg/L to 0.0055 mg/L for fly ash when mixed with cement to form concrete and/or flowable fill. This proposed value wo uld be more conservative than that based on USEPA’s standards, and would adopt a curr ent standard as the basis for the mercury limit.

HDOH agreed to consider SPLP concentrations below 0.0055 mg/L (0.00055 mg/L with a dilution attenuation factor of 10 applied) to be acceptabl e for reuse of fly ash in concrete, and that SPLP concentrations below this level would not require confirmation testing or further risk ev aluation. HDOH’s acceptanc e of the study results and recommendations therefore effectively resu lted in an increase to allowable fly ash mercury limits when used as a concrete additive.

REFERENCES

Buchman, M. 1999. NOAA Screening Quick Reference Table . NOAA HAZMAT Report
99-1. Coastal Protection and Restorat ion Division, National Oceanic and Atmospheric Administration, Seattle, WA.

HDOH, 2005. Hawaii Department of Health. Screening for Environmental Concerns at
Sites With Contaminated Soil and Groundwater , Interim Final. Prepared by

### Hawai’i Department of H ealth Environmental M anagement Division, 919 Ala Moana Blvd, Honolulu, Hawai’i. May.

USEPA, 1985. United States Environmental Protection Agen cy. Ambient Water Quality Criteria for Mercury - 1984 . EPA 440/5-85-026. Office of Water, Regulations and Standards. January.

USEPA, 2002. United States Environmental Protection Agency. List of Drinking Water
Contaminants and their MCLs. EPA 816-F-0 13. Office of Water. November.

USEPA, 2004a. United Stat es Environmental Protection Agency. Region IX Preliminary Remediat ion Goals (PRGs.)

, Version 9. Stanford J. Smucker, Technical Support Section (SFD-8-4). 75 Hawthorne Street, San Francisco.
October.

USEPA, 2004b. National Reco mmended Water Quality Criteria

. Office of Water.
Available at http://epa.gov/waterscience/ criteria/wqcriteria.html

.3/27/07 vsa \02 155833 AES H awaii\F ig 1_re sults.cd r Chem ical w as an alyze d for but n ot de tecte d. The n umer ic val ue is the s ampl e rep orting limit

March 2007 AES Hawaii Risk Evaluation of Leachable Mercury from Concrete Product s Made With Fly Ash

FIGURE 1 

### JANUAR

Y -OCT OBER 2005 FL Y ASH SPLP MERCUR Y RESUL TSSPLPMercury,mg/L

U Permit Limit3/28/ 07 vs a \02 1558 33 AES Hawa ii\Fig 2_ba selin e.cdr M ar ch 2 00 7 AE S Ha wa ii Ri sk E va lu at io n of

L ea ch ab le M er cu ry fr om Co nc re te P ro du cts M ad e W ith F ly As h

FI G U R E 2 B A S E LI N E A S H S P LP M E R C U R Y C O M P O S IT E S C R E E N IN G R E S U L TS Permit Limit3/27/07 vsa \02 155833 AES H awaii\F ig 3_cy linder .cdr March 2007 AES Hawaii Risk Evaluation of Leachable Mercury from Concrete Product s Made With Fly Ash

FIGURE 3 

### CYLINDER PREP

ARA TION3/27/0 7 vsa \0215 5833 AES H awaii\ Fig 4_ concr ete.cd r Ma rch 20 07 AE S Ha wa ii Ri sk E va lua tio n o f L ea ch ab le Me rcu ry fro m Co nc re te Pr od uc ts Ma de W ith F ly As h

FI GU RE 4 T O T A L M ER C U R

Y R ES U L TS F O R C O N C R ET E M A D E W IT H F L Y A SH (m g/ kg )P ACIFIC OCEAN

Not to Scale Drinking W ater W ell Sea W all Groundwater Rip Rap

Groundwater T S R E A M Invertebrate R
A Y O A D W

ROA DW A Y

Algae/ Zooplankton

3/27/07 vsa \02 155833 AES H awaii\F ig 5_ris k csm.c dr

Leachable Constituent LEGEND D r i n k i n g W a t e r U s e r * * R e c r e a t i o n a l F i s h e r p e r s o n * *

S e a T u r t l e M a r i n e F i s h

F r e s h w a t e r F i s h C a r n i v o r o u s F i s h

a n d M a m m a l s

I n g e s t i o n o f d r i n k i n g w a t e r D e r m a l c o n t a c t I n g e s t i o n o f s e a f o o d I n c i d e n t a l c o n t a c t w i t h w a t e r

a n d s e d i m e n t *

I n g e s t i o n o f f o o d U p t a k e o f l e a c h a b l e c o n s t i t u e n t f r o m w a t e r a n d / o r s e d i m e n t I n g e s t i o n o f F o o d

U p t a k e o f l e a c h a b l e c o n s t i t u e n t f r o m w a t e r a n d s e d i m e n t I n g e s t i o n o f F o o d I n g e s t i o n o f p r e y

Notes: Leachable constituent potentially present in sea wall, building foundations, cinder blocks, and roadways.

* Not considered a significant exposure p athway

** Inhalation of particulates is not considered a significant
exposure p athway since tot al mercury concentration in fly ash is below the residential preliminary remediation goal. March 2007 AES Hawaii Risk Evaluation of Leachable Mercury from Concrete Product s Made With Fly Ash

FIGURE 5 

### SITE CONCEPTUAL MODEL3/28/07 vsa \02 155833 AES H awaii\F ig 6_co mpariso n_edit.c dr March 2007 AES Hawaii Risk Evaluation of Leachable Mercury from Concrete Product s Made With Fly Ash

FIGURE 6 

### COMP ARISON OF SPLP MERCUR Y RESUL TS FROM CONCRETE MADE WITH FL Y ASH T O APPLICABLE ST ANDARDS Basis of fly ash permit limit when used as concrete admixture

Basis of st andard fly ash permit limit