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Environmental Modeling

Coal-fired power plant wet flue gas desulfurization bromide discharges to U.S. watersheds and their contributions to drinking water sources Kelly D Good, and Jeanne M. Vanbriesen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03036 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018

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Environmental Science & Technology

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Coal-fired power plant wet flue gas desulfurization

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bromide discharges to U.S. watersheds and their

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contributions to drinking water sources

9 Kelly D. Gooda*, Jeanne M. VanBriesenb

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a

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Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15213, USA. Email: [email protected].

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ORCID: 0000-0002-2180-955X.

Graduate Research Assistant, Department of Civil and Environmental Engineering, Carnegie

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b

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Systems (Water-QUEST), Department of Civil and Environmental Engineering and Department

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of Engineering and Public Policy, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh,

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PA 15213, USA. Email: [email protected]. ORCID: 0000-0002-2631-0213.

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* corresponding author

Duquesne Light Company Professor, Director of Water Quality in Urban Environmental

ACS Paragon Plus Environment


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ABSTRACT

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Wet flue gas desulfurization (FGD) wastewater discharges from coal-fired power plants may

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increase bromide concentrations at downstream drinking water intakes, leading to increased

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formation of toxic disinfection byproducts (DBPs). Despite this, bromide was not regulated in

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FGD wastewater in the 2015 Steam Electric Effluent Limitations Guidelines and Standards

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(ELGs). Case-by-case management was recommended instead, depending on downstream

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drinking water effects. The present work seeks to identify U.S. regions where power plant

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discharges could affect drinking water. Bromide loads were evaluated for all coal-fired power

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plants operating wet FGD, and flow paths were used to identify downstream surface water sources.

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A population-concentration metric was used to evaluate the effect of wet FGD on downstream

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drinking water and the vulnerability of drinking water to upstream discharges. On a hydrologic

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region level, results indicate the Ohio, South Atlantic Gulf, and Missouri Regions are the most

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likely to see effects of power plant bromide discharges on populations served by surface water.

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Increased refined coal use, which may be treated with bromide, contributes to uncertainty in

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potential bromide effects on drinking water. Measurement of bromide concentrations in wet FGD

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discharges would reduce this uncertainty, and control of bromide discharges may be needed in

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some watersheds.

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TOC ART

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KEYWORDS

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source water, disinfection byproducts, bromide, refined coal, coal-fired power plants, flue gas

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desulfurization

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INTRODUCTION

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Disinfection byproducts (DBPs) are an unintended consequence of drinking water treatment

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caused by reaction of chemical disinfectants with organic matter and other precursors.1 When

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bromide is present in source waters, even at very low concentrations, increases in the rate and

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extent of DBP formation are observed, along with a shift in speciation toward brominated forms.2–4

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Brominated DBPs are more toxic than their chlorinated analogs,5,6 and while a few DBP classes

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(e.g., trihalomethanes) are regulated as surrogates for risk, they are not the toxicity drivers in

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complex DBP mixtures.7–9 Despite this important role for bromide in DBP formation and toxicity,

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understanding of bromide sources, the loads they discharge, and how those loads affect

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concentrations at drinking water intakes, is lacking.

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Bromide is naturally present in many source waters due to its presence in soil and sediment,10–12

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as well as rain13 and surface runoff.14 Several studies have assessed bromide concentrations in U.S.

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drinking water sources.15,16 An analysis of Information Collection Rule (ICR) data (collected in

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1997-1998) reported a range of below detection (< 20 µg/L) to 2,230 µg/L, with a median of 36

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µg/L and a mean of 69 µg/L, for all water sources.17 A separate ICR data analysis reported a

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median of 30 µg/L and mean of 60 µg/L specifically for surface water sources.18

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Anthropogenic sources of bromide include discharges from fossil fuel extraction activities,

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including oil and gas development19,20 and coal mining;21 coal-fired power generation;22,23 and

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flame retardant textile production facilities;23,24 as well as other industrial sources.25 Discharges

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from these sources are associated with elevated bromide concentrations in rivers,26–28 and in some

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cases these sources have increased since the national surveys of bromide concentrations (see


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supporting information (SI)

Figure S2 related to coal-fired power plants and Wilson and

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VanBriesen (2012)29 related to oil and gas discharges). In watersheds with anthropogenic

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discharges, States et al. (2013) report bromide concentrations as high as 299 µg/L while Wilson

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and VanBriesen (2013) report concentrations as high as 599 µg/L. Increased formation and

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bromination of trihalomethanes was reported in both watersheds.27,30 The effect of increasing

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anthropogenic bromide concentrations in source waters on DBP formation and associated drinking

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water risk has also been reported elsewhere.31,32

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Coal-fired power plants are a significant source of bromide discharges to the environment due to

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the presence of bromine in coal,33 bromide addition to coal to reduce air emissions of mercury34,35

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and bromide added to create refined coal to qualify for tax credit.36–38 Bromine that would exit the

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plant in stack gases is captured in wet flue gas desulfurization (FGD) units deployed to reduce

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sulfur dioxide emissions to the air, leading to a wastewater with elevated bromide.39–43 FGD

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wastewater bromide concentrations (typically on the order of 10-100 mg/L), are rarely monitored

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and are currently unregulated.44

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FGD wastewater discharges have been implicated in elevated source water bromide and

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subsequent effects on drinking water.26,27,45 McTigue et al. (2014) used data from 2011 to identify

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118 coal-fired power plants operating wet FGD units; fifty-seven of these power plants had 96

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drinking water treatment plants located in vulnerable downstream locations.23 These results

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suggested that wet FGD installations were likely the cause of increases in brominated DBPs at

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some utilities, including increases that led to DBP compliance violations.23

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Similarly, the United States Environmental Protection Agency (EPA) considered the potential

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effect of bromide discharges from power plants in the supporting technical analysis46 associated

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with the revision to the Effluent Limitations Guidelines and Standards for the Steam Electric

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Power Generating Point Source Category (ELGs).44 This analysis used geographic proximity to

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consider drinking water systems potentially affected only if they were located within 8 kilometers

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(5 miles) of a power plant discharge. Further, the analysis assessed only the number of potentially

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affected systems, without consideration of their size or the size of the population they served.

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Recent work reconsidered these limitations for drinking water utilities in Pennsylvania47 and for a

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utility in the Allegheny River Basin that had been experiencing elevated bromide concentrations.48

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This work demonstrated that bromide concentration contributions are affected by the magnitude

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of the bromide load in the discharge and the dilution capacity at the drinking water intake, and that

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these effects are not controlled by geographic proximity.47 Further, the presence of large drinking

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water facilities in urban areas that are downstream of power plant discharges can lead to significant

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populations being affected by these discharges. Asserting effects would only be relevant inside a

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geographic limit and ignoring population served by drinking water systems led the EPA to

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underestimate the potential downstream effects of wet FGD bromide discharges on drinking water

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sources.47

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Thus, since downstream drinking water effects were presumed to be limited, the ELGs

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recommended that permitting authorities consider bromide regulation only on a case-by-case

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basis.44,49 No details were provided to inform such case-by-case permitting, and key information

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required for establishing discharge standards is not generally available (i.e., identification of


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affected intakes and monitoring data from upstream wet FGD bromide discharges). The present

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work uses information on power plant discharge locations and drinking water system surface water

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facility locations, as well as information on coal consumption and receiving water flow dynamics,

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to identify locations where bromide effects can be expected. This analysis will assist regulators,

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drinking water utilities, and power plant operators in making informed decisions regarding

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bromide use, discharge, and control. Further, the analysis will provide input in advance of the

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planned reconsideration of the ELGs for wet FGD wastewater by the EPA.50

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MATERIALS AND METHODS

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This national-level analysis required collection and analysis of the following data: hydrologic

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boundaries, river conditions and flow paths; public drinking water system surface water facilities;

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coal-fired power plant air pollution controls and receiving waters for wet FGD discharges; and

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coal consumption and characteristics. Figure 1 provides a schematic of the modeling approach for

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(A) GEOSPATIAL MODEL identifying potential downstream surface water effects, (B) LOAD

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MODEL estimating wet FGD Br loads based on coal consumption, (C) CONCENTRATION

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CONTRIBUTION MODEL estimating concentration contributions at downstream drinking water

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facilities, and (D) MODELED EFFECTS of wet FGD discharges on populations consuming

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drinking water downstream.

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INPUT

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OUTPUT

Active surface water facilities by HUC12 watershed (SW-HUC) Stream segments (NHD Flowlines identified by “COMID”) Receiving water NHD Flowline for wet FGD discharges

Spatially joined SW-HUC12 to NHD Flowlines

Streamflow (mean annual) for downstream flow path NHD Flowlines with SWHUC12 joins

Downstream flow paths (NHD Flowlines) for wet FGD discharges

Wet FGD-associated coal consumption, by plant Coal and FGD assumptions

Modeled Br load (high estimate)

Bromide loads discharged from wet FGD, by plant (Low, Mid, High estimates)

Br concentration contribution at each downstream SWHUC12 contributed by wet FGD (modeling for downstream effect stopped when contribution < 1 µg/L)

For each SW-HUC12, vulnerability calculated as product of the facility population served and the sum of all wet FGD Br concentration contributions (≥ 1 µg/L) from upstream discharges

For each wet FGD discharge, sum of all downstream SWHUC12 vulnerability values, calculated as the product of the facility population and wet FGD Br concentration contribution (≥ 1 µg/L)

135 136 137 138 139

Figure 1. Schematic showing model setup for the evaluating wet FGD bromide effects on surface drinking water facilities in the United States (NHD–National Hydrography Dataset; COMID–unique identifier in the NHD; HUC12–12-digit hydrologic unit code watershed; SW-HUC12–a watershed containing a surface drinking water facility).

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Hydrologic data

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In order to identify and summarize potentially vulnerable areas on a national scale, geospatial

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hydrologic data were obtained from the Watershed Boundary Dataset (WBD) as part of National

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Hydrography Dataset Plus Version 2 (NHDPlusV2), including river segments (Flowlines) and

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watershed boundaries of various levels (referred to as Hydrologic Unit Codes [HUC]).51 For

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assessment of dilution capacity in receiving waters and downstream flow paths, attributes for the

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Flowline segments in the NHDPlusV2 dataset were utilized. Data for these segments include flow

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estimates that provide a readily available tool for assessment of dilution capacity (described

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elsewhere in more detail),47 and the analysis presented here used the mean annual flow condition 8 of 36 ACS Paragon Plus Environment

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(“QE_MA”) for the main analysis; a low flow condition (selected as the minimum month mean

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flow for each location, “QE_mm” where “mm” is the two-digit code for each month) was also

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used for a more detailed evaluation in the Ohio Region. Data for the smallest watershed boundary

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level (12-digit HUC, which divides the contiguous U.S. into 83,073 watersheds) were used for

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surface drinking water facility locations. Results are summarized and visualized at the hydrologic

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subregion (4-digit HUC) and region (2-digit HUC) levels.

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Public drinking water system surface water facilities

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Previous work utilized verified drinking water intake locations in Pennsylvania (from Rice and

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Westerhoff52,53) to identify which intakes were located downstream of wet FGD discharges; this

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analysis was limited to large community water systems serving more than 10,000 people.47 Exact

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intake locations for all public water systems in the United States are considered sensitive

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information and are not available to the public.54 However, the U.S. EPA recently developed a

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publicly-available source water protection mapping application called the Drinking Water

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Mapping Application to Protect Source Waters (DWMAPS) that includes drinking water source

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information from the Safe Drinking Water Information System (SDWIS) at the watershed scale

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(12-digit HUC).55,56 Nationwide active surface water facilities were requested and received from

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the EPA for this analysis for all sizes and categories of public drinking water systems.57 This

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drinking water source information was spatially joined to NHD Flowlines in ArcGIS58 to enable

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identification of flow paths downstream of wet FGD receiving waters that intersect watersheds

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containing source waters for drinking water systems. For the contiguous U.S., the dataset included

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9,134 surface water facilities (intakes, reservoirs, springs, infiltration) for 6,802 drinking water

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systems serving 134 million people in 5,177 watersheds, which were all included in this analysis, 9 of 36 ACS Paragon Plus Environment


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regardless of system size or type. Each facility is associated with a water system (identified by

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PWSID); the population associated with the PWSID was used as a surrogate for potential effect,

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following the EPA Risk-Screening Environmental Indicators (RSEI) Methodology.59 Since

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systems may use more than one source water and/or have more than one facility, while the dataset

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includes only a single total population for the system, the RSEI method may result in over-counting

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of potentially affected people by assigning the total population for the system to each water source.

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Systems that sell water to other systems (wholesalers) are included in the spatial analysis, but the

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populations of those sequential systems are not included, which may result in an under-counting

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of potentially affected people.

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Identification of coal-fired power plants with wet flue gas desulfurization bromide loads

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potentially affecting downstream surface water sources

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Year 2016 EIA Form 860 data were used to identify coal-fired power plant electricity generating

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units (EGUs) according to their FGD type.60 The analysis was limited to EGUs in the contiguous

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U.S. with a status of ‘operable,’ a primary fuel source of type ‘coal,’ and North American Industry

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Classification System (NAICS) code of 22 (Utilities–Electric Power Generation, Transmission and

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Distribution), as discussed in more detail in SI Section B.

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In order to assess potential downstream surface water effects, receiving waters for wet FGD

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discharges were identified through review of ELG rulemaking documents; discharges to rivers

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were identified by stream segments while discharges to lakes were identified by their outflow

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stream segment.61–65 There are three notable limitations for these data sources: first, receiving

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waters are not differentiated for individual wastewater streams; second, some discharges were


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excluded from the lists if a change was planned (e.g., retirement, conversion to natural gas, or a

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change in wastewater treatment practices), and these changes may not have occurred prior to 2016;

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and third, the absence of a listed receiving water is not confirmation that a surface water discharge

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does not occur indirectly (e.g., some power plants send wastewater to separate treatment facilities

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that may not remove bromide and that have surface discharge permits).

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Thus, for the current analysis, which was intended to assess potential effects, a wet FGD discharge

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to surface water was assumed to exist unless information explicitly indicated that the ultimate fate

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of FGD wastewater was not surface water, or if the discharge was adjacent to a Great Lake or

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estuary (surface water sources that are not included in the present analysis). If the FGD-specific

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receiving water could not be identified, the discharge was assumed to be into the largest of the

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receiving waters listed for the power plant. This assumption is not anticipated to have a substantial

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effect on the result since bromide contributions at drinking water facilities are assessed along

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downstream flow paths, which are likely downstream of all of the receiving waters listed for the

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power plant. If a receiving water was not included in the list, a spatial join in ArcGIS58 was used

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to identify the nearest NHD Flowline to the facility; this was necessary for 13 plants. This approach

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resulted in the exclusion of 24 out of 140 plants with wet FGD from surface water effect modeling

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for the following reasons: Great Lakes discharge (7 plants); estuary discharge (4 plants); FGD

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wastewater disposal method described in the questionnaire as deep well injection or evaporation

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ponds (13 plants).

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Finally, at the time of ELG rulemaking, EPA estimated that zero liquid discharge (ZLD) for FGD

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wastewater was operational or planned by 2014 at 51 of 139 plants.46 However, since a national



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list of plants operating ZLD in 2016 was not available, and FGD ZLD may still result in surface

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discharges of bromide through other wastestreams,46,66 the national analysis here did not exclude

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plants potentially operating ZLD. A NPDES wastewater discharge permit review for an individual

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power plant could provide this information and the analysis for a specific power plant could then

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be updated. Additional discussion and a sensitivity analysis for this assumption are provided in SI

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Section C.

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Coal consumption and wet FGD bromide load modeling

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The wet FGD bromide load model is based on coal consumption and type; year 2016 data were

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obtained for each plant from EIA Form 923 (see SI Section D).67 Wet FGD bromide loads were

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estimated following Good and VanBriesen (2016),48 with the exception that more recent bromide

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concentration data for the Br naturally present in coal were used (see SI Section E). While bromide

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may also be added at the plant for mercury control, no nationwide source of information on this

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practice is available. For the present analysis, no assumptions were made regarding bromide

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addition for mercury control at power plants.

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The United States Geological Survey (USGS) coal quality (COALQUAL) database was recently

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revised and includes resolution of data integrity and reporting issues,68 and bromine data from

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COALQUAL version 3 were reviewed and analyzed (SI Section F) in order to obtain estimates for

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ranges of Br in U.S. coal by rank. COALQUAL sample data include U.S. County, and since EIA

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Form 923 includes plant-level monthly coal delivery data, also by county, the COALQUAL data

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could be used to estimate bromide loads more accurately at the individual plant level. However,

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the model developed here uses a broad set of assumptions with readily available data for the Br 12 of 36 ACS Paragon Plus Environment

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content expected to be naturally present in bituminous, subbituminous, and lignite coals, as well

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as an assumed value for refined coal, in order to provide a high-level estimate of 2016 bromide

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loading across the U.S. fleet. The Br values are shown in Table S6.

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For refined coal, in the absence of detailed information about the processes for refining, Br addition

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was assumed as explained in SI Section G. The Br addition rate assumed here (100 ppm Br in dry

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coal) was selected based on the distribution of reported Br addition rates reported by the Electric

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Power Research Institute (EPRI) for the Section 45 refined coal tax credit (10-460 ppm).36 While

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the EPRI study may not be representative of national Br application rates, the data indicate that

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bromide addition for refined coal was higher than bromide addition for mercury control alone.

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The selection of 100 ppm for refined coal is at the high end of the distribution assumed for Br

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addition for mercury control in previous work.47,48 Also, while other products could be added

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during refining (including an iodide-based additive),36,69 the intention here is to provide a plausible

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Br load for the case in which all wet FGD-associated refined coal has additional Br in order to

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identify regions of the country with the potential to experience bromide-related problems from

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power plants.

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Evaluating downstream wet flue gas desulfurization bromide effects using flow path modeling

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For each of the 116 wet FGD plants for which receiving waters were identified, flow paths were

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traced using the NHDPlusV2 Flow Navigator Toolbar70 for ArcGIS.58 Flow paths were continued

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downstream until the concentration contribution as calculated from the high Br load estimate under

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mean annual flow conditions reached 1 µg/L. To account for loss of dilution capacity under lower

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than mean annual flows, this threshold was selected as an order of magnitude lower than 10 µg/L, 13 of 36 ACS Paragon Plus Environment


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a concentration contribution found meaningful for risk changes by Regli et al. (2015).31 Setting

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this threshold for each power plant flow path also ensured that conditions leading to higher

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concentration contributions due to cumulative effects from multiple power plants affecting the

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same downstream drinking water facility were not neglected. The flow paths were then used to

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identify downstream watersheds (12-digit HUC) with surface drinking water facilities.

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Drinking water effects of coal power plant discharges are dependent on the bromide loads (which

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may vary as utilities alter use of coal types, particularly increasing use of refined coal), on the river

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dilution capacity at the surface drinking water facility, and on the population using surface water

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as a drinking water source. These drivers were integrated into an effect metric calculated as the

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product of the surface drinking water facility population served and the wet FGD Br concentration

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contribution (µg/L). Thus, once the affected surface drinking water facility watersheds were

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identified for each power plant operating wet FGD, effects were summarized two ways: (1) for

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each power plant wet FGD discharge, cumulative downstream surface water effects, and (2) for

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each surface water-affected watershed, cumulative vulnerability from upstream wet FGD

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discharges (also summarized for the subregion [4-digit HUC] and region [2-digit HUC] levels).

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This allowed for assessment of the effect of multiple FGD discharges on a single drinking water

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source as well as the assessment of the effect of a single FGD discharge on multiple downstream

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drinking water sources.

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RESULTS AND DISCUSSION

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Identification of relevant power plants and affected drinking water utilities

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Of 325 coal-fired power plants, 140 (45%) have wet FGD. Seventy-nine plants have downstream

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drinking water utilities that modeling identified as potentially affected by FGD-associated bromide

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discharges (Table S9; Figure S8). Table 1 provides a summary of the watersheds containing active

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surface water facilities for drinking water systems in the U.S. For the 9,134 surface drinking water

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facilities modeled here, 390 (4.3%) have at least one upstream power plant with wet FGD modeled

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to contribute at least 1 µg/L Br under high estimated load and mean flow conditions. This

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represents 230 of 5,177 (4.4%) of the 12-digit HUC watersheds containing surface drinking water

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facilities. Approximately half of these 12-digit HUC watersheds (112 of 230) are affected by more

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than one discharge, with the cumulative effects most evident in the Ohio Region (watersheds

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beginning with HUC-05 in Table S13) where some watersheds are modeled to be affected by as

297

many as 9 wet FGD discharges. The population associated with the potentially affected drinking

298

water systems is 19.5 million people (or 14.6% of the total 134 million people served by systems

299

with active surface water facilities in the contiguous U.S.).

300



301 302

Table 1. Surface drinking water facilities potentially affected by upstream wet FGD discharges a

Number of watersheds (HUC12) Number of facilities Number of systems Population served 303 304 305 306

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All facilities

Facilities with upstream wet FGD contributing at least 1 µg/L Br (percent of all facilities)

5,177

230 (4.4%)

9,134 5,802 134 million

390 (4.3%) 307 (5.3%) 19.5 million (14.6%)

a

Limited to the contiguous U.S. Does not include individual wet FGD discharges modeled to contribute less than 1 µg/L under mean flow conditions in this analysis.

307 308

Total national modeled wet FGD bromide loads and model sensitivity

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Based on 2016 coal consumption data (Tables S10 and S11) and a range of assumed Br contents

310

(Table S4), total national wet FGD Br loads (Table S12) for all 140 plants operating wet FGD

311

were estimated as 20,200 kg/day (lower), 25,800 kg/day (middle), and 34,300 kg/day (upper). For

312

the 79 power plants with wet FGD modeled to be affecting downstream drinking water sources,

313

wet FGD Br loads were estimated as 13,000 kg/day (lower), 16,500 kg/day (middle) and 21,800

314

kg/day (upper). This model incorporates uncertainty in the Br content naturally present in each

315

coal rank, as well as 100 ppm Br added to refined coal, resulting in wide ranging total load

316

estimates. While not included in this analysis, Br addition for mercury control at specific power

317

plants would increase the load estimates and could result in additional power plants being

318

identified as contributing to downstream drinking water intake bromide concentrations.

319 320

The estimate for Br added to Refined Coal represents a significant source of uncertainty (see Figure

321

S3, which shows that the model is most sensitive to this parameter) since data on the type and


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amount of additive are not available from the EIA or IRS (see SI Section G). Bromide added to

323

coal at power plants for mercury control would also contribute to uncertainty. While no national-

324

level data exist for either Refined Coal bromide levels or bromide addition for Hg control;

325

information may be available from individual refining facilities and power plants that could

326

improve local predictions. The wide range in national modeled wet FGD Br load highlights the

327

need to understand wet FGD contributions and their potential drinking water effects on a more

328

regional scale, so that areas more susceptible to Br loads can be identified and attention focused

329

on FGD discharges in these regions.

330 331

Geospatial distribution of wet FGD coal consumption and bromide loads by coal type

332

Figure 2 shows the 2016 coal consumption at U.S. power plants operating wet FGD by coal type

333

in each hydrologic region (identified by 2 digit HUC) (panel a), the coal consumption associated

334

with wet FGD power plants identified with downstream drinking water effects (panel b), and the

335

estimated bromide load from the power plants identified with downstream drinking water effects

336

(panel c). Values are shown in Tables S10-12.

337 338

Ohio (HUC-05) and South Atlantic-Gulf (HUC-03) were the top two regions for wet FGD-

339

associated coal consumption as well as wet FGD capacity (Table S11). Notably, the Ohio region

340

has much higher wet FGD-associated Bituminous and Refined Coal consumption than the South

341

Atlantic-Gulf region (and all other regions), which is important because these coal types have

342

higher halogen content. Thus, even though power plants in the Ohio region consumed only 32%

343

more coal than power plants in the South Atlantic-Gulf region in 2016 (Figure 2b, Table S11),

344

their modeled wet FGD bromide load is more than double (Figure 2c, Table S12). 17 of 36 ACS Paragon Plus Environment


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The uncertainty in Br content (shown for each coal type as a lower, middle, and upper estimate in

347

Table S4) is larger for Bituminous and Refined Coal compared to Subbituminous and Lignite.

348

Figure 2c demonstrates how the assumption for Br content in coal affects the modeled Br load in

349

each region (values shown in Table S12). For example, the South Atlantic Gulf region (03), plants

350

upstream of drinking water sources burned primarily Bituminous and Subbituminous but also

351

some Refined Coal, with an estimated wet FGD Br load ranging from 1,400 to 3,360 kg/day, or

352

varying by a factor of 2.4. The differences associated with uncertainty in coal bromide content

353

suggest that load contribution predictions would be improved by direct monitoring of bromide in

354

wet FGD discharges, particularly at plants burning Bituminous and Refined Coal. It is also

355

important to note that these estimates are based on coal type burned in 2016. Increased use of

356

Refined Coal in regions that are currently using lower bromide coals could significantly alter this

357

vulnerability assessment and identify different or additional locations as vulnerable to bromide

358

load effects on drinking water systems.


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359

360 361 362 363 364

Figure 2. Regional 2016 wet FGD associated coal consumption (colored by coal type) for all plants (panel a) and plants affecting downstream surface water (panel b). 2016 modeled wet FGD bromide load for the three Br content scenarios (panel c; see Table S4 for range of Br values for each coal type) for surface water (SW) affecting plants. Data are provided in Tables S10 to S12.



365

Bromide concentration contributions in receiving waters

366

For each watershed (12-digit HUC) with surface drinking water facilities downstream of a wet

367

FGD discharge, the cumulative bromide concentration contribution for the high load estimate

368

under mean flow conditions was computed (results provided in Table S13). The concentration

369

contributions would be higher under lower flow conditions, but these values provide a general

370

estimate for potential contributions at each location. Figure 3 shows the histogram of these

371

concentration contributions. The median concentration contribution is 16 µg/L. Sixty-three

372

percent of the affected watersheds (145 of 230) have a bromide concentration contribution from

373

the power plants above 10 µg/L, which is the smallest change in source water bromide

374

concentration identified by Regli et al. (2015) as potentially relevant for DBP formation risk

375

concerns at drinking water intakes.31 All watersheds with bromide concentration contributions

376

above 50 µg/L under mean flow conditions are located on small receiving waters (mean annual

377

flow less than 140 m3/s), except for two facilities (in 0305 and in 0511). Concentration

378

contributions above 100 µg/L under mean flow conditions are rare (14 of 230 watersheds), and

379

exhibit some spatial clustering, with three watersheds in subregion 0207 and four watersheds in

380

subregion 0714.


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Page 21 of 36


381

382 383 384 385

Figure 3. Histogram of modeled bromide concentration contributions from wet FGD to watersheds (12-digit HUC) with active surface drinking water facilities in the contiguous U.S.

386 387

Geospatial distribution of drinking water vulnerability

388

The watershed vulnerability assessment results, based on predicted concentration contributions

389

and drinking water facility populations, are displayed by hydrologic subregion in Figure 4 (values

390

for 4-digit subregions and individual 12-digit watersheds are shown in Table S13). Darker shading

391

in Figure 4 indicates higher vulnerability, which is concentrated in major river basins, including

392

the Ohio, the Missouri, and the Susquehanna, but also includes parts of North and South Carolina

393

and Texas. Five of the top 20 vulnerable subregions are in the Ohio River Basin (HUC beginning

394

with 05 in Figure 4 and Table S13). Figure 4 also shows subregions with wet FGD-associated

395

refined coal consumption in 2016 (thick outline), which as discussed, included an assumption for

396

100 ppm added Br in this analysis and represents a substantial source of uncertainty in Br loads in

397

wet FGD discharges (Figure S3).

398



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399

High vulnerability values can come from high concentration contributions, high drinking water

400

populations, or a combination. At the subregion level, 0305 has the highest vulnerability (67.6

401

million people·µg/L), representing the sum of vulnerability for 14 surface drinking water facility

402

watersheds (containing 18 facilities serving 2.2 million people) that are affected by 5 wet FGD

403

discharges (values shown in Table S13). The high vulnerability is driven predominately by a

404

single upstream FGD discharge (large triangle within subregion 0305 in Figure 4; plant 2727-NC

405

in Figure S9) that is modeled to contribute 30 µg/L to 2 watersheds containing 5 surface drinking

406

water facilities that collectively serve 2 million people. Other watersheds in the region had higher

407

modeled bromide concentration contributions (including from multiple FGD discharges), but they

408

serve much smaller populations and therefore had lower vulnerability values.

409 410

In contrast, subregion 0509, with the third highest vulnerability (55.5 million people·µg/L),

411

represents only 6 surface drinking water facility watersheds (containing 12 facilities serving 2.7

412

million people) that are affected by 10 wet FGD discharges. This effect was dominated by only

413

one affected watershed (containing 3 facilities serving 2.2 million people) receiving a cumulative

414

bromide concentration contribution of 20 µg/L from 7 upstream FGD discharges. This result

415

highlights the importance of multiple upstream discharges, especially for watersheds that contain

416

drinking water utilities that serve large populations.

417 418 419

Identification of power plants with significant downstream effects

420

Figure 4 also shows the results for the power plant-level effect analysis, where larger triangles

421

show power plants with a higher estimated cumulative downstream effect on surface water sources. 22 of 36 ACS Paragon Plus Environment

Page 23 of 36


422

Of particular note in Figure 4 is that power plants may discharge in watersheds where their effects

423

are relatively low due to few drinking water plants using surface waters or low populations.

424

However, since bromide is conservative, the discharged bromide may have a greater effect at

425

significant distance downstream where large surface water treatment plants provide water for

426

many consumers. This is most evident for the two large plants in North Dakota (power plants

427

2823-ND and 2817-ND in Figure S9). These plants have little effect in the subregions into which

428

they discharge (1013) and the first downstream subregion (1014), but further downstream where

429

large surface water treatment systems use this source water (subregions 1023, 1024 and 1030), the

430

bromide contributions, even after dilution, could affect many water consumers.

431 432

The power plant-level results (million people·µg/L) are also shown in Figure S9, which identifies

433

plants by EIA ID and state. The present analysis includes many assumptions and highlights key

434

uncertainties in understanding the effects of wet FGD Br discharges on downstream drinking water

435

sources. To provide context for the uncertainties in the results, Figure S9 also indicates the

436

following: power plants included in the EPA list of potential zero liquid discharge (ZLD) facilities

437

during ELG rulemaking71 (Table S2 and associated discussion in the SI), power plants potentially

438

adding Br for mercury control (Table S8 and associated discussion in the SI), and power plants

439

consuming refined coal in 2016 (Table S7). For instance, plants 2823-ND and 2817-ND discussed

440

above both consumed refined coal in 2016. Thus, while it was assumed that Br was added to

441

refined coal at a rate of 100 ppm in this analysis, these plants could be burning coal that has been

442

refined with a different amount of Br or with another product besides Br. Plant 2823-ND was on

443

the ZLD list (Table S2, as indicated in Figure S9 by an asterisk), and plant 2817-ND was on the

444

list of plants possibly adding Br for mercury control (Table S8, as indicated in Figure S9 by a plus



Page 24 of 36

445

sign). This analysis highlights the need for additional information for specific power plants in order

446

to assess bromide loads that may affect drinking water systems.

447

448 449 450 451 452 453 454

Figure 4. Geospatial distribution wet FGD bromide effects on drinking water, estimated as the product of the surface drinking water facility population served and the estimated wet FGD bromide concentration contributions in µg/L (for contributions at least 1 µg/L under high predicted load and mean annual flow conditions). Surface water vulnerability to upstream wet FGD discharges is summarized by hydrologic subregion (HUC4), and power plant effect on downstream surface water is summarized by power plant.

455


Page 25 of 36


456

Future considerations for the effects of wet FGD discharges on drinking water sources

457

This analysis provides a national assessment of modeled bromide loads and associated in-stream

458

concentration contributions from wet FGD in the United States based on 2016 coal consumption.

459

Using the consumption-based load model presented here, individual power plant wet FGD bromide

460

load estimates can be developed, with opportunities to reduce uncertainty with additional

461

information (e.g., using the Br content from COALQUAL that is specific to the county from which

462

the coal was delivered, and/or an actual Br addition rate provided by a coal refining facility or

463

power plant). Watershed-level analyses incorporating additional bromide sources and measured

464

background levels can be developed to assess the relative contribution of power plant discharges

465

to the bromide at specific drinking water intakes (as previously demonstrated by Good and

466

VanBriesen),47,48 and risk models can be applied to determine the effect of these bromide changes

467

on drinking water.30,31,72

468 469

Changes in the quantity and type of coal consumed, including increased use of refined coal in

470

regions that are currently using lower bromide coals, could significantly alter the results presented

471

here (see SI Section E on sensitivity analysis). Similarly, an increase in bromide addition for

472

mercury control at power plants currently using low bromide coal or discharging to rivers with

473

high dilution capacity could alter the results and lead to effects in areas not identified in this

474

analysis. Thus, the specific results of this retrospective analysis should not be used to predict

475

future levels of bromide discharged from power plants; rather, the method can be applied with

476

updated coal and bromide use information as it becomes available.

477



Page 26 of 36

478

In addition to these model uncertainties, the most significant factor affecting wet FGD Br loads

479

discharged to surface waters is the presence of FGD treatment that affects bromide. Bromide is a

480

small, soluble, generally non-reactive ion, and thus its removal from water and wastewater is

481

challenging.73,74 Reverse osmosis removes 99% of bromide from seawater in conventional

482

desalination;75–77 however, most of the energy investment is removing other dissolved solids.

483

Anion exchange membranes show good bromide removal (91%),78 and electrolysis and

484

volatilization have been shown to have some efficacy (40% removal).73,79,80 Since FGD wastewater

485

contains other constituents, most notably toxic metals (arsenic, selenium, and mercury),46 ZLD

486

technology for FGD wastewater offers additional benefits besides eliminating bromide discharges.

487

ZLD is considered a viable wastewater treatment option for coal-fired power plants81 and was

488

encouraged through a voluntary incentive option in the 2015 ELGs.44 Power plants that elect ZLD

489

for treatment of wet FGD wastewaters will avoid bromide discharges associated with FGD (see SI

490

Section C), provided residuals are not sent to surface-discharging facilities. Identification of ZLD

491

operations is a critical need to determine the extent to which the problem identified in this work

492

has already been ameliorated by FGD treatment at relevant facilities.

493 494

Although not quantified in this analysis, iodide is also expected to be contributed by wet FGD

495

discharges since it is naturally present in coal (Figure S4),82,83 and like bromide, sometimes added

496

to coal to improve mercury removal.69 The effect of iodide on DBP formation is not as well studied

497

as bromide; however, recent work suggests increasing iodide sources in the environment are

498

affecting DBP formation and toxicity.84–87 The bromide load estimates provided here show

499

substantial uncertainty, and iodide load estimates would be even more uncertain due the limited

500

information available on iodine in coal and its behavior in wet FGD systems. Unlike bromine,


Page 27 of 36


501

which is present primarily as Br- in natural waters, iodine is present as iodide (I-), organically-

502

bound iodine (organo-iodine), and iodate (IO3-).88,89 Iodate has not been found to be harmful90–92

503

and is therefore considered a desirable sink for iodine in drinking water treatment.93,94 However,

504

unless it is transformed to iodate prior to discharge, the use of iodide for coal amendment and

505

refining should not be considered a solution to the bromide problem identified in this work; further

506

analysis of the potential role of power plant-associated iodide discharges in DBP formation at

507

downstream drinking water utilities is needed. Monitoring wet FGD effluent, at an adequate

508

frequency and before it is mixed with other waste streams, would significantly improve

509

understanding of the contributions of bromide (and iodide) to surface waters and their potential

510

drinking water effects.

511 512

Managing effects from bromide is particularly challenging for drinking water utilities. Since

513

bromide is unreactive under typical environmental conditions, only dilution and/or reduction of

514

loads will decrease its concentration at drinking water intakes. Within conventional drinking water

515

treatment, bromide, which is small and soluble, is not easily or economically removed.27,73,95

516

Removal of DBPs in drinking water distribution systems (e.g., using mixing and aeration in storage

517

tanks96,97) can change DBP speciation98,99 and may not reduce risk.100 Similarly, alternative

518

disinfectants (e.g., switching from chlorine to chloramine) may reduce regulated DBP

519

concentrations but may increase other DBPs.9,101 Thus, source control of bromide is the most

520

effective option for mitigating downstream effects at drinking water treatment plants. Reducing

521

the concentrations of halogens in source waters will reduce the formation of regulated groups of

522

DBPs (thus reducing the potential for regulatory compliance failure) as well as reducing the

523

formation of unregulated but more toxic brominated and iodinated DBPs. Removal of bromide



Page 28 of 36

524

(and iodide) from FGD wastewater should be prioritized at coal-fired power plants that discharge

525

upstream of watersheds used as drinking water sources.


Page 29 of 36


526

ACKNOWLEDGEMENTS

527

The authors gratefully acknowledge the three anonymous reviewers for their careful attention and

528

suggestions that significantly improved this work; Dr. Jeffrey Quick of the Utah Geological Survey

529

for his input on coal quality data; and Mr. James (Bo) Williams of the Source Water Protection

530

Division of the Office of Ground Water and Drinking Water at U.S. EPA for his input on geospatial

531

drinking water data. Funding from the following sources is acknowledged: National Science

532

Foundation Graduate Research Fellowship Program (DGE1252522); NSF Integrative Graduate

533

Education and Research Traineeship in Nanotechnology Environmental Effects and Policy

534

fellowship program (DGE0966227); Wilton E. Scott Institute for Energy Innovation at Carnegie

535

Mellon University; the Colcom Foundation; Dean’s Fellowship from the Carnegie Institute of

536

Technology. Any opinions, findings, conclusions, or recommendations expressed in this material

537

are those of the authors and do not necessarily reflect the views of the National Science Foundation

538

or any other funding source.

539 540

SUPPORTING INFORMATION

541

Additional details about methods and data sources (coal-fired power plants, wet FGD bromide

542

load model setup, halogens in coal, bromide addition) and additional details for results (including

543

values underlying the visuals).

544



545

REFERENCES

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