Risk Evaluation of Leachable Mercury 

From Concrete Products Made With Fly Ash

Robert McCann1, Kerri L. M. Hartung2, Jill Hedgecock3, Peggy 

Schuler4

, Steven Putrich4

1

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

1600 Perimeter Park Drive, Morrisville, NC 27560; 3URS Corporation, 221 Main Street, 

San Francisco, CA 94105; 4URS Corporation, 1375 Euclid Avenue, Cleveland, OH, 

44115 

KEYWORDS: mercury, fly ash, risk, risk evaluation, concrete, flowable fill 

ABSTRACT 

AES Hawaii, Inc. operates a 203 Megawatt circulating 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 permit limit for leachable mercury (0.00025 

mg/L) determined through the Synthetic Precipitation Leaching Procedure. AES Hawaii 

prepared a risk evaluation to determine if mercury from fly ash posed an environmental 

concern when intermixed in concrete products. 

The primary objectives of the risk evaluation were to: 1) evaluate whether incorporating 

fly ash into concrete/flowable fill reduces mercury leachability to acceptable levels; and 

2) determine if a threshold fly ash content exists that results in unacceptable mercury 

concentrations in concrete/flowable fill leachates. The study demonstrated that, even in 

cases where mercury in the raw fly ash exceeds 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 threshold 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 unacceptable 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) operates a 180 Megawatt net circulating 

fluidized bed coal and alternative fuel-fired cogeneration facility, the only such facility on 

the Island of Oahu and the only operational coal fired plant in operation in Hawaii. This 

operation generates approximately 15 to 20 percent of Oahu’s electricity. Coal 

combustion ash is a by-product of AES Hawaii operations from burning coal and 

alternative fuels for electricity generation. Fly ash and bed ash are mixed to form 

2007 World of Coal Ash (WOCA), May 7-10, 2007, Covington, Kentucky, USA Home
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 disposal at a permitted landfill. In an island 

environment which by design has land limitations, the option of continuing to use a 

portion of the fly ash as an admixture by Hawaiian Cement in its concrete products was 

considered highly desirable. 

Under AES Hawaii’s solid waste management 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 total and leachable metals content of composite 

samples. The SPLP analysis is designed to measure how likely the chemicals present 

in a solid material are to be released into surface water or groundwater when exposed 

to precipitation. In 2005, several composite fly ash samples submitted for routine 

analysis exceeded the permit limitation for leachable mercury (0.00025 mg/L) (refer to 

Figure 1). AES Hawaii retained URS Corporation (URS) to conduct a risk evaluation to 

determine if mercury from fly ash posed an environmental 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 risks, analyze the results of the study, and 

propose alternate permit conditions dependant upon those results. 

IDENTIFICATION OF POTENTIAL RISKS FOR EVALUATION 

Two potential risk issues were identified for 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 mercury from the concrete to surface water 

or groundwater. Further evaluation eliminated the first potential risk as unlikely based 

on the following rationale: 

− During combustion, mercury in coal volatilizes and is emitted in the flue gas. 

Some of the mercury contained in the flue gas (predominantly ionic mercuric 

compounds such as HgCl2 and HgSO4) then adsorbs to the carbon phase of the 

coal ash as it cools. The presence of volatile 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 [mg/kg]) are well below Hawaii’s risk-based screening 

criterion for mercury of 13 mg/kg at contaminated sites (HDOH, 2005), as well as 

the United States Environmental Protection Agency (USEPA) Region IX 

residential preliminary remediation goal (PRG) (USEPA, 2004a) of 23 mg/kg (for 

“mercury and compounds”), direct contact with 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 further evaluation because the inorganic mercury 

compounds bound to fly ash particles could have 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 concerns 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 included 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 secondary objectives were designed to be 

studied in the event that mercury was detected in the SPLP leachate upon mixing with 

concrete. With HDOH approval, these secondary objectives would 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 concentrations of mercury against applicable riskbased criteria. If detectable mercury levels were 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 procedures 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 multi-pathway evaluation. Based on these 

potential exposure pathways, applicable criteria for comparison with SPLP results were 

identified in advance of analytical testing. These criteria, in conjunction with the 

established permit limit, were used to set the appropriate method detection limits for 

SPLP testing in an effort to minimize study uncertainties. 

TECHNICAL APPROACH 

The technical approach involved reviewing analytical data from 2005 to select archived 

samples appropriate to the study’s objectives, re-analyzing the 2005 results and raw 

materials from Hawaiian Cement to establish 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 the analysis of the cured cylinders per Total Mercury and 

SPLP method requirements. 

Sample Selection and Study Baseline 

AES Hawaii’s routine monitoring program includes 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 portions of these discrete grab samples taken 

during the designated time period, combined 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 concentrations above the method detection limit (MDL) 

at varying ranges appropriate to the study’s objectives (refer to Figure 1). URS’ 

laboratory, Geotesting Services, Inc., combined the archived grab samples associated 

with those time periods as appropriate to recreate the original composite samples. The 

new composites were then submitted to a subcontracted laboratory for analysis to 

confirm the original fly ash SPLP mercury results 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 constituents would not contribute mercury to the final product. 

A summary of the original monitoring data and the new data is presented in Figure 2. 

The SPLP mercury results generated by the study baseline testing were somewhat 

different than the original results. Specifically, 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. Although 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 tested, which, in at least one case, exceeded 

AES Hawaii's permit limit for fly ash. Based 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 composites, 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 fly ash composite. In addition, 

batches of flowable fill were prepared by replacing 90 percent of the cement with each 

of the three fly ash composites. The proportions of materials in each of these mixes 

(based on batch weights provided by Hawaiian 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 containing no fly ash to serve as a control. The 

concrete and flowable fill mixtures were placed 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 prepared 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 conditions, a portion of the cylinders were 

crushed. The crushed cylinders were considered a worst case condition because the 

smaller components would have a larger surface area, thereby enhancing the potential 

for leaching of mercury. 

ANALYTICAL RESULTS 

The analytical data for total mercury in the crushed concrete samples are summarized 

in Figure 4. The total mercury concentrations 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 leachable 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 evaluation include a proposed alternative 

permit limit that would be applicable to fly ash used as a concrete additive. 
Pathway Determination 

Since Hawaiian Cement concrete products are 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 determined to be complete in the conceptual 

site model. Figure 6 provides a pictorial conceptual site model (CSM) of how leachable 

constituents in selected concrete products could be released and reach human and 

ecological receptors. These receptors were (a) a human drinking water user, (b) a 

recreational fisherperson, (c) various freshwater aquatic organisms, and (d) various 

marine aquatic organisms. 

Comparison of SPLP Mercury Results 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 aquatic 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 identified 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 criteria 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 applicable 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 attenuation factor of 10 applied. The 

application of the attenuation factor is consistent with Buchman (1999), where the 

Coastal Protection and Restoration Division of the National Oceanic and Atmospheric 

Administration uses 10 times the applicable AWQC for screening for dilution and 

attenuation. The Hawaii chronic saltwater surface water quality standard of 0.000025 

mg/L, however, is based on an outdated USEPA criterion. This value appears in 

“Ambient Water Quality Criteria for Mercury” (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 Guidelines in 1995 (60FR15393-15399, March 23, 

1995), USEPA no longer uses the Final Residue Value procedure for deriving AWQCs 

for new or revised 304(a) aquatic life criteria. 

DISCUSSION OF STUDY UNCERTAINTY 

A summary of uncertainties associated with the analytical data and chemical forms of 

mercury follows. Estimates of exposure and risk are always subject to a number of 

uncertainties that could lead to underestimates or overestimates of risks. Because of 

the conservative nature of the assumptions used 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 analytical methods. Uncertainty due to sample 

heterogeneity is minimized with careful sample preparation and monitored with replicate 

sample analyses. Some variability, however, especially for solid samples, is 

unavoidable. 

Under most circumstances, sufficiently sensitive 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 mercury. The laboratory MDL, 

however, was less than the criteria in each of these cases. Concentrations between the 

MDL and the reporting limit cannot be accurately quantified and are considered 

estimates. Even so, USEPA defines the MDL (determined statistically from multiple 

analyses of a sample in a given matrix containing the analyte of interest) as the 

minimum concentration of a substance that can be measured and reported with 99 

percent confidence that the analyte concentration 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 freshwater 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: methylmercury 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 the concrete sample leachates, it is unlikely 

mercury mixed with concrete would be converted to methylmercury in a marine 

environment at concentrations that would cause significant environmental 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 Management 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 humans or the environment. 

Based on these results, and the fact that the current SWMP limit is derived from a 

chronic saltwater criterion calculated using an outdated method, it was concluded that 

the SWMP limit for SPLP mercury in the raw fly ash used in concrete products was 

overly conservative and should be re-evaluated by HDOH. 

As noted previously, the basis for the current 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 chronic freshwater standards promulgated by 

HDOH. The HDOH 2005 freshwater chronic aquatic habitat goal (based on the Hawaii 

chronic freshwater surface water quality standard) is 0.00055 mg/L. Thus, AES Hawaii 

proposed to HDOH at the conclusion of the study that the dilution attenuation factor of 

10 be applied to the current HDOH 2005 freshwater chronic aquatic habitat goal of 

0.00055 mg/L, thereby resulting in an increased 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 would be more conservative than that based 

on USEPA’s standards, and would adopt a current 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 acceptable for reuse of fly ash in 

concrete, and that SPLP concentrations below this level would not require confirmation 

testing or further risk evaluation. HDOH’s acceptance of the study results and 

recommendations therefore effectively resulted 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 Restoration 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 Health Environmental Management Division, 919 Ala 

Moana Blvd, Honolulu, Hawai’i. May. 

USEPA, 1985. United States Environmental Protection Agency. 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-013. Office of Water. November. 

USEPA, 2004a. United States Environmental Protection Agency. Region IX 

Preliminary Remediation Goals (PRGs.), Version 9. Stanford J. Smucker, 

Technical Support Section (SFD-8-4). 75 Hawthorne Street, San Francisco. 

October. 

USEPA, 2004b. National Recommended Water Quality Criteria. Office of Water. 

Available at http://epa.gov/waterscience/criteria/wqcriteria.html. 
3/27/07 vsa \02155833 AES Hawaii\Fig 1_results.cdr

Chemical was analyzed for but not detected.

The numeric value is the sample reporting limit

March 2007

AES Hawaii 

Risk Evaluation of Leachable Mercury from 

Concrete Products Made With Fly Ash

FIGURE 1

JANUARY-OCTOBER 2005 

FLY ASH SPLP MERCURY RESULTS

S

P

L

P

M

erc

ury, m

g/L

U

Permit Limit
3/28/07 vsa \02155833 AES Hawaii\Fig 2_baseline.cdr

March 2007

AES Hawaii 

Risk Evaluation of Leachable Mercury from

Concrete Products Made With Fly Ash

FIGURE 2

BASELINE ASH SPLP MERCURY 

COMPOSITE SCREENING RESULTS

Permit Limit
3/27/07 vsa \02155833 AES Hawaii\Fig 3_cylinder.cdr

March 2007

AES Hawaii 

Risk Evaluation of Leachable Mercury from 

Concrete Products Made With Fly Ash

FIGURE 3

CYLINDER PREPARATION
3/27/07 vsa \02155833 AES Hawaii\Fig 4_concrete.cd

r

March 2007

AES Hawaii 

Risk Evaluation of Leachable Mercury from 

Concrete Products Made With Fly Ash

FIGURE 4

TOTAL MERCURY RESULTS FOR

CONCRETE MADE WITH FLYASH (mg/kg)
PACIFIC OCEAN

Not to Scale

Drinking Water 

Well

Sea

Wall

Groundwater

Rip Rap

Groundwater

STREAM

Invertebrate

R

AY

OADW

ROADWAY

Algae/

Zooplankton

3/27/07 vsa \02155833 AES Hawaii\Fig 5_risk csm.cdr

Leachable 

Constituent

LEGEND

Drinking Water User **

Recreational Fisherperson**

Sea Turtle

Marine Fish

Freshwater Fish

Carnivorous Fish

and Mammals

Ingestion of drinking water

Dermal contact

Ingestion of seafood

Incidental contact with water

and sediment *

Ingestion of food

Uptake of leachable constituent

from water and/or sediment

Ingestion of Food

Uptake of leachable constituent

from water and sediment

Ingestion of Food

Ingestion of prey

Notes:

Leachable constituent potentially present in sea wall, building 

foundations, cinder blocks, and roadways.

* Not considered a significant exposure pathway

** Inhalation of particulates is not considered a significant 

exposure pathway since total mercury concentration in fly 

ash is below the residential preliminary remediation goal.

March 2007

AES Hawaii 

Risk Evaluation of Leachable Mercury from 

Concrete Products Made With Fly Ash

FIGURE 5

SITE CONCEPTUAL MODEL
3/28/07 vsa \02155833 AES Hawaii\Fig 6_comparison_edit.cdr

March 2007

AES Hawaii 

Risk Evaluation of Leachable Mercury from 

Concrete Products Made With Fly Ash

FIGURE 6

COMPARISON OF SPLP MERCURY

RESULTS FROM CONCRETE MADE WITH 

FLY ASH TO APPLICABLE STANDARDS

Basis of fly ash permit limit when used

as concrete admixture

Basis of standard fly ash permit limit