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Horizontal cooling towers: riverine ecosystem services and the fate of thermoelectric heat in

the contemporary Northeast US

View the table of contents for this issue, or go to the journal homepage for more

2013 Environ. Res. Lett. 8 025010

(http://iopscience.iop.org/1748-9326/8/2/025010)

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IOP PUBLISHING ENVIRONMENTAL RESEARCH LETTERS

Environ. Res. Lett. 8 (2013) 025010 (10pp) doi:10.1088/1748-9326/8/2/025010

Horizontal cooling towers: riverine

ecosystem services and the fate of

thermoelectric heat in the contemporary

Northeast US

Robert J Stewart1, Wilfred M Wollheim1,2, Ariel Miara3,

Charles J Vor ̈ osmarty  ̈

3,4

, Balazs Fekete3,4, Richard B Lammers1 and

Bernice Rosenzweig3

1

Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham,

NH 03824, USA

2 Department of Natural Resources and Environment, University of New Hampshire, Durham,

NH 03824, USA

3 CUNY Environmental Crossroads Initiative, City University of New York, New York, NY 10031, USA

4 Department of Civil Engineering, The City College of New York, New York, NY 10031, USA

E-mail: rob.stewart@unh.edu

Received 28 January 2013

Accepted for publication 27 March 2013

Published 18 April 2013

Online at stacks.iop.org/ERL/8/025010

Abstract

The electricity sector is dependent on rivers to provide ecosystem services that help regulate excess heat,

either through provision of water for evaporative cooling or by conveying, diluting and attenuating waste

heat inputs. Reliance on these ecosystem services alters flow and temperature regimes, which impact fish

habitat and other aquatic ecosystem services. We demonstrate the contemporary (2000–2010) dependence

of the electricity sector on riverine ecosystem services and associated aquatic impacts in the Northeast US, a

region with a high density of thermoelectric power plants. We quantify these dynamics using a spatially

distributed hydrology and water temperature model (the framework for aquatic modeling in the Earth

system), coupled with the thermoelectric power and thermal pollution model. We find that 28.4% of

thermoelectric heat production is transferred to rivers, whereas 25.9% is directed to vertical cooling towers.

Regionally, only 11.3% of heat transferred to rivers is dissipated to the atmosphere and the rest is delivered

to coasts, in part due to the distribution of power plants within the river system. Impacts to the flow regime

are minimal, while impacts to the thermal regime include increased river lengths of unsuitable habitats for

fish with maximum thermal tolerances of 24.0, 29.0, and 34.0 ◦C in segments downstream of plants by

0.6%, 9.8%, and 53.9%, respectively. Our analysis highlights the interactions among electricity production,

cooling technologies, aquatic impacts, and ecosystem services, and can be used to assess the full costs and

tradeoffs of electricity production at regional scales.

Keywords: thermoelectricity, thermal pollution, water temperature, river ecosystem services, Northeast

USA

S Online supplementary data available from stacks.iop.org/ERL/8/025010/mmedia

Content from this work may be used under the terms of

the Creative Commons Attribution 3.0 licence. Any further

distribution of this work must maintain attribution to the author(s) and the

title of the work, journal citation and DOI.

1. Introduction

Thermoelectric power plants are the largest users of

freshwater in the United States (US) (Averyt et al 2011)

+10$33.00 11748-9326/13/025010 
c 2013 IOP Publishing Ltd Printed in the UK
Environ. Res. Lett. 8 (2013) 025010 R J Stewart et al

and globally (Vassolo and Doll 2005). They provide 90%

of electricity consumed in the US, and an even a greater

proportion in the northeastern part of the country (Averyt et al

2011). Thermoelectricity production relies on biophysical

processes of freshwater ecosystems to provide coolant or for

conveyance, dilution, and attenuation of waste heat loads.

As a result, power generation has the potential to stress

freshwater ecosystems through both reduced river flows

via consumptive use by cooling towers (Sovacool 2009,

MacKnick 2011) and the discharge of thermal pollution. It

is important to quantify anthropogenic water use and its

implications (Postel and Richter 2003, King et al 2005)

as overuse can result in significant degradation to river

flow regime, biogeochemistry (Meybeck and Helmer 1989,

Meybeck 2003) and aquatic habitat (MEA 2005). Here we

apply the ecosystems services concept (Smakhtin et al 2004,

Sweeney et al 2004, Richter et al 2006, Brauman et al

2007) to understand how aquatic ecosystems support a critical

economic activity, thermoelectric power production.

The two most commonly applied methods for thermoelectric plant cooling are ‘re-circulating cooling’ or

RCC, which typically utilize evaporative cooling towers;

and ‘once-through cooling’ or OTC, where waste heat is

transferred directly to rivers (Averyt et al 2011). Both

approaches rely on aquatic ecosystem services, with the

former requiring consumptive water use, and the latter

depending on transport, dilution, and dissipation processes

in natural waterways (Brauman et al 2007). Electricity

production at both OTC and RCC plants are inherently linked

to ambient air and water temperatures, as output efficiencies

decrease with elevated intake temperatures (NETL 2002,

Miara and Vorosmarty 2013). Thermoelectric plants that do

not withdraw or consume water from river systems include

those that implement alternative methods (i.e. dry cooling)

and those that use marine sources, and represent 4.1% and

10.6% of the total number of thermoelectric plants within

the region, respectively. In the face of changing climate and

increasing energy demand (Wilbanks et al 2008), it is essential

to assess the capacity and associated environmental tradeoffs

(Bennett et al 2009) of heat regulating ecosystem services that

support the electricity sector.

If river systems were nothing more than networks of

nonconductive pipes, 100% of heat loads from power plants

would be delivered to receiving oceans and stream temperatures would reflect the conservative mixing (i.e. dilution)

of heat inputs. However, rivers are interactive pathways

that continuously exchange energy with the atmosphere

(Edinger et al 1968, Dingman 1972, Webb and Nobilis

1997, Webb et al 2003, Wilhelm et al 2006, Pedersen and

Sand-Jensen 2007, Austin and Allen 2011). The river’s ability

to mitigate thermal pollution is a function of river length,

channel dimensions, discharge magnitude and velocity, and

atmospheric conditions (i.e. wind speed, difference between

water and air temperature, relative humidity, and solar

radiation) (Edinger et al 1968, Dingman 1972). Entire

river networks thus have the potential to buffer significant

amounts of upstream anthropogenic heat loading and reduce

downstream thermal pollution. Recently developed models

have predicted water temperatures in a changing climate (van

Beek et al 2012, van Vliet et al 2012, Wu et al 2012), but

have not quantified the capacity of river systems to attenuate

anthropogenic heat loads.

Both OTC and RCC cooling methods come at potentially

significant environmental costs, specifically, reduced river

flow and increased water temperatures (Vassolo and Doll

2005, Averyt et al 2011). Re-circulating cooling towers

withdraw relatively small volumes from the river, but these

withdrawals are mostly consumptive, thereby putting fish

habitat and other downstream water uses at risk during

low flow periods. Once-through cooling technologies are

less consumptive of water but require substantial water

withdrawals and return flows are at elevated temperatures.

Increased water temperatures can reduce the abundance and

connectivity of suitable habitats for native fish and can create

refugia for cold-intolerant invasive species (Morgan et al

2003, Rosa et al 2012).

Here we apply a spatially distributed river water

temperature model coupled with a thermoelectric power plant

model (TP2M) to characterize the contemporary (2000–2010)

dependence of electricity production on freshwater ecosystem

services, and the resulting impact on receiving freshwaters.

We apply this model to rivers in the Northeast US, which

contains a high density of thermoelectric power plants serving

the region’s high energy demands (Wilbanks et al 2008).

Specifically, our goals are to (1) calculate how much regional

electricity production depends on engineered cooling towers

versus riverine ‘horizontal cooling towers’ for transferring

heat away from thermoelectric power plants, (2) quantify

the benefit that thermoelectric power plants receive from the

regulation of excess heat by freshwaters, and (3) assess the

consequences of relying on these riverine ecosystem services

in terms of altered freshwater temperatures and flow regimes.

2. Quantifying the heat regulating ecosystem

services of rivers

To assess thermal regulating ecosystem services provided by

river systems, we require spatially distributed models that

integrate the distribution, type and size of power plants, their

water demands and heat loads, and hydrologic and thermal

conditions throughout the river system. Models must account

for natural and anthropogenic heat loading, discharge, and

re-equilibration by aquatic systems. We embedded power

plant and water temperature models in the framework for

aquatic modeling in the Earth system (FrAMES) (Wollheim

et al 2008a, 2008b, Wisser et al 2010, Stewart et al 2011)

to simulate river flows, electricity generation, and water

temperatures in the Northeast US at a spatial resolution of

3 min (latitude/longitude) (figure S.1, available at stacks.

iop.org/ERL/8/025010/mmedia). FrAMES utilizes the water

balance model (WBM) and water transport model (WTM)

(Vorosmarty et al 1998, Wisser et al 2010) for the coupled

simulation of the vertical water exchange between the land

surface and the atmosphere and the Muskingum–Cunge

routing of horizontal water transport (Ponce 1994) through

branching river segments. This spatially distributed, gridded

2
Environ. Res. Lett. 8 (2013) 025010 R J Stewart et al

river network model was updated with modules to account for

transport, mixing and re-equilibration of water temperatures

along river reaches at a daily time step. Model results

match well with United States Geological Survey (USGS)

observations for discharge (n = 694 stations) and water

temperature (n = 243 stations) in basins ranging from 200

to 70 200 km2(figures S.2, S.3, available at stacks.iop.

org/ERL/8/025010/mmedia). Model input data includes total

daily precipitation, average daily air temperature, cloud cover,

and wind speed which were acquired from NASA’s Global

Modeling and Assimilation Office (Modern Era-Retrospective

analysis for Research and Applications or MERRA). Model

output was summarized for quantification of summer (June,

July, August), winter (December, January, February), and

annual impacts. Tidal influences were not incorporated in the

current version of the model; thus, reported values for thermal

impacts are in terms of exported freshwater temperatures only.

A detailed summary of each model, their linkages, input data,

and validation is described in the supplementary material

(available at stacks.iop.org/ERL/8/025010/mmedia).

The non-point thermal loading model (NTLM) was used

to generate runoff heat fluxes as the product of (1) surface

and groundwater runoff inputs to each individual river

grid cell and (2) their associated temperatures. Precipitation

was assigned the mean daily air temperature in each

grid cell, whereas snowmelt was assigned a temperature

of 0 ◦C. Surface runoff temperatures are volume-weighted

mixtures of precipitation and snowmelt temperatures, whereas

groundwater runoff temperatures are the result of volumeweighted mixtures of percolation temperatures that comprise

shallow ground water. Similar models have been developed

for other domains, including global (van Beek et al 2012,

van Vliet et al 2012) and regional northwestern US (Wu et al

2012). Runoff and its water temperature from the local grid

cell are then mixed with routed fluxes from upstream and any

storage existing in the river channel from the previous time

step.

Re-equilibration of water temperatures during flow

routing through the river network is computed using the river

temperature re-equilibration model (RTRM). This method is

based on a combined empirical and deterministic approach

outlined in Dingman (1972). The method is appropriate for

large scale applications (Mohseni and Stefan 1999, Bogan

et al 2003, Pedersen and Sand-Jensen 2007, Austin and

Allen 2011), including lakes and large rivers (Morse 1972)

and is based on the theory of equilibrium temperature—the

temperature at which there is no net exchange of energy with

the atmosphere (Edinger et al 1968, Dingman 1972, Webb

et al 2003).

The thermoelectric power and thermal pollution model,

or TP2M (Miara and Vorosmarty 2013), was applied to

simulate power plant operations including withdrawals,

consumption (evaporation), and resulting return flows and

temperatures to the river network corresponding to electricity

demand. TP2M accounts for reduced power plant efficiency

and electricity generation when cooling water temperatures

in the power plant condenser increase above a threshold

temperature. Historical reported values (2000–2010) for

Figure 1. Locations of freshwater once-through cooling (OTC, red)

and re-circulating cooling (RCC, blue) thermoelectric power plants

within the study domain. Dry cooling and other hybrid technologies

(n = 19 plants) and those that withdraw from marine sources and

the Great Lakes (n = 47 plants) are not shown. Contributing

hydrological drainage areas that fall outside of political boundaries

were simulated but not displayed.

monthly power generation at all 384 thermoelectric power

plants along rivers in the region (figure 1) were used to

simulate the daily heat loads at each location. WBM, WTM,

NTLM, RTRM, and TP2M are fully coupled with one

another in FrAMES (figure S.1) and dynamically simulate

the spatially distributed waste heat loads to the river system

accounting for various cooling technologies (OTC, RCC,

dry cooling, and hybrid combinations), and fuel types.

Thermoelectric power plants that withdraw cooling water

from the Atlantic Ocean were not included in this study. Input

data requirements for TP2M were assembled from a variety of

sources including the US Energy Information Administration

(EIA) and Union of Concerned Scientist (UCS) databases and

application of standard thermodynamic equations.

3. Riverine ecosystem services and the fate of

thermoelectric heat

Of the total heat generated by thermoelectric power plants

in the Northeast US between 2000 and 2010, 34.3% was

converted to electricity, 28.4% was transferred directly to

rivers (OTC plants), 25.9% was dissipated via consumptive

water use in engineered cooling towers (RCC plants), and

the remainder was lost during the cooling process to sinks

(Rutberg et al 2011) other than the condenser (figure 2). The

electricity sector in this region therefore relies as heavily on

rivers as it does on engineered evaporative cooling towers

to convey heat from plants during electricity generation.

Thermoelectric power plants within the region draining

to the Atlantic produced a total of 461.0 TWh yr−1 of

electricity, 54.3% of which was generated by OTC plants

(table 1). The ability of river systems to serve as horizontal

cooling towers involves two supporting ecosystem services:

downstream transport/dilution and heat dissipation in the

3
Environ. Res. Lett.

8 (2013) 025010 R J Stewart et al Table 1. Average annual thermoelectricity produced (2000–2010), the associated waste heat inputs to the river network, the resulting temperature increases at the river mouth due to thermal pollution, and the per cent of heat dissipated by the riverine ecosystem service. Basin Thermoelectricity produced (TWh yr−1) Heat input to river (PJ yr−1) Heat concentration (TJ km−3 yr−1) Freshwater temperature increase at river mouth (◦C) Heat attenuated by riverine ecosystem (%) OTC RCC Total OTC RCC Net Net Sum. Win. Ann. Sum. Win. Ann. Atlantic 250.4 210.6 461.0 1055 −0.6 1054 4.5 1.9 0.9 0.9 11.9 12.9 11.3 Penobscot 1.7 2.9 4.6 7.6 0.0 7.6 0.5 0.2 0.1 0.1 7.6 22.3 10.5 Merrimack 13.3 2.5 15.8 25.5 0.0 25.5 3.1 1.0 0.6 0.6 19.1 33.6 22.2 Connecticut 23.4 2.7 26.1 50.3 0.0 50.3 2.6 0.8 0.4 0.5 20.3 39.6 23.8 Hudson 36.1 15.1 51.2 252.3 0.0 252.3 11.3 5.0 2.4 2.5 6.4 7.5 6.2 Delaware 12.2 58.6 70.8 86.7 −0.1 86.6 5.5 2.7 0.9 1.1 12.9 11.9 12.5 Susquehanna 33.8 36.4 70.2 239.7 −0.3 239.4 7.1 2.9 1.3 1.5 11.1 11.6 9.7 James 24.5 4.8 29.3 177.6 0.0 177.6 19.7 8.2 3.1 3.9 21.5 13.9 18.1 4
Environ. Res. Lett. 8 (2013) 025010 R J Stewart et al

Figure 2. Allocation of total heat (in petajoules) generated in freshwater thermoelectric power plants during electricity production at

selected basins, including heat to evaporative cooling towers (red), heat to electricity generation (green), heat lost to sinks other than the

condenser (gray), heat to river (dark and light blue), heat attenuated by riverine ecosystem services (light blue), and heat conveyed to the

ocean (dark blue).

rivers themselves. Our model suggests that, of the heat

transferred to rivers draining into the Atlantic, 11.3% is

dissipated, and the remainder (935.2 PJ yr−1) is transported

to oceans.

The regulating capacities of river networks vary by

watershed, depending on the magnitude of heat inputs

(table 1), the spatial distribution of power plants within

the basin, and to a lesser extent climate conditions. Long

flow paths from heat source to the river mouth provide

greater opportunity for impacted water temperatures to reach

equilibrium. Temperature regulation also varies slightly with

season (12.9% of total river heat inputs removed during the

winter, 11.9% during the summer). Average annual network

scale heat removal in northeast basins range from 6.2% in the

Hudson (6.4% in summer, 7.5% in winter) to 23.8% in the

Connecticut (20.3% in summer, 39.6% in winter). Predicted

increases in average freshwater temperatures exported at river

mouths due to power plants range from 3.9◦in the James

(8.2◦in summer, 3.1 in winter) to 0.1◦in the Penobscot

(0.2◦in summer, 0.1◦in winter). Actual water temperatures

in the James are likely to be less elevated than simulated due

to tidal dilution, which is not represented in our model. Tidal

dilution impacts approximately the last 233 km and 170 km of

the Hudson River and James River main stems, respectively.

Downstream thermoelectric power plants benefit from the

service of heat dissipation by the upstream river network. To

assess the benefits of this service, we calculated the average

number of days per year in which water temperatures are

below the critical OTC power plant operational threshold

of 22 ◦C (EEA 2008, van Vliet et al 2011, Miara and

Vorosmarty 2013) due to riverine heat dissipation. We made

this calculation using model scenarios where power plant heat

re-equilibration was and was not allowed to occur. Nearly half

(48.6%) of freshwater OTC plants experienced some increase

in the average number of days where intake river temperatures

were optimal because of heat dissipation (figure 4), with the

average benefit small (2.1 days) over the 11 year period.

However, average benefits ranged from 0.1 to 22.7 days across

individual plants depending on the upstream distance and

magnitude of heat loads. Further, benefits increased during dry

years, as three plants in the region gained over 41 optimal days

during 2001, which suggests the thermoelectricity sector may

rely more heavily on riverine ecosystem services under certain

climate conditions, with implications for future climate

changes.

In aggregate, OTC plants produce all of the total net

annual heat loads to rivers (table 1) and many of the largest

of these are located near basin mouths due to the large water

withdrawals they require (Kenny et al 2009). As a result of this

common spatial configuration, a significant amount of thermal

pollution (1) escapes the river network, and (2) impacts other

downstream plants near the basin mouth, with little heat

reduction provided by the river’s ecosystem service. In this

sense, the ecosystem service provided by the regional network

of rivers is generally limited to waste heat conveyance rather

than attenuation. Exceptions include the Connecticut and

Merrimack, where over one-fifth of annual waste heat inputs

to each basin are dissipated (table 1).

Total annual heat inputs to rivers, standardized by

total annual basin discharge, reveals a broad range in the

concentration (defined as total heat/basin runoff) of annual

heat loads (PJ km−3yr−1) and the dilution capacities among

5
Environ. Res. Lett. 8 (2013) 025010 R J Stewart et al

Table 2. Count of thermoelectric power plants in each basin, the average distance for each type of plant to the river mouth (weighted based

on total energy output), and the percentage of average summer flows (2000–2010) that are withdrawn and consumed during electricity

production.

Basin

Thermoelectric

power plant count

Weighted

distance (km)

Avg. summer

withdrawals (%)

Avg. summer

consumption (%)

OTC RCC Ratio OTC RCC OTC RCC Total OTC RCC Total

Atlantic 185 169 1.1 123.4 238.8 22.1 0.5 22.6 0.1 0.4 0.5

Penobscot 6 2 3.0 49.6 65.6 1.1 0.0 1.1 0.0 0.0 0.0

Merrimack 6 4 1.5 104.5 93.1 10.4 0.1 10.5 0.1 0.0 0.1

Connecticut 16 8 2.0 134.9 94.0 30.0 0.0 30.0 0.2 0.0 0.2

Hudson 20 7 2.9 55.8 134.9 49.3 0.1 49.4 0.3 0.1 0.4

Delaware 20 25 0.8 60.4 103.4 25.9 2.4 28.3 0.2 1.9 2.1

Susquehanna 16 10 1.6 122.3 216.7 30.2 0.9 31.1 0.2 0.7 0.9

James 8 8 1.0 120.9 211.8 94.8 0.3 95.1 0.4 0.3 0.6

Figure 3. Increase in average summer water temperatures

(2000–2010) due to thermal pollution from power plants. Callout

boxes show results for average winter conditions in selected regions.

Temperature increases due to plants are more widespread in the

summer because waste heat inputs are dissipated more quickly in

the winter.

Northeast basins (table 1). The relatively high concentration

of heat inputs in the Hudson and James Rivers (table 1)

are likely because plants in these systems rely heavily on

the additional dilution capacity provided by tidal water that

our model does not account for. To better understand the

spatial distribution of heat inputs, for each drainage network

we calculated the average flow path distance from OTC and

RCC power plants to the river mouth, weighted based on

energy production (table 2). The average weighted distance

to the ocean for OTC power plants in the Connecticut

River basin (134.9 km) is the longest of those studied in

the Northeast, whereas the Hudson has the shortest average

weighted distance (55.8 km). These patterns heavily impact

total network heat retention (table 1) and offset the fact that

both the Connecticut and Hudson Rivers are in cooler northern

areas of our regional domain. Thermoelectric plants in both

of these basins predominantly use OTC technologies and

rely extraordinarily on rivers as horizontal cooling towers,

releasing over 48% of the total annual waste heat generated

to the river system (figure 2).

Figure 4. Average increase in the number of optimal operation

days (days with intake river temperatures below 22 ◦C) per year at

OTC plants due to the upstream ecosystem service of heat

dissipation. Data shown were quantified by comparing scenarios

with and without dissipation of upstream power plant heat over the

11 yr model period.

Re-circulating cooling towers are highly effective in

preventing anthropogenic heat from passing into the river

network (table 1). A full 100% per cent of the total net

annual electricity sector heat inputs to the river system are

from power plants that use OTC technologies (table 1). Water

temperatures of blowdown, the effluent from RCC towers,

typically have a negligible affect on river temperatures (Miara

and Vorosmarty 2013) but are shown here to be cool enough

to result in a small net heat loss from the river systems. Cold

effluent from RCC plants is enough to reduce average summer

and even winter water temperatures in a few tributaries, but

not in the larger main stem rivers (figure 3).

4. Impacts on aquatic ecosystems

The environmental costs associated with utilizing river

networks to dissipate heat are reduced flow (from RCC

and OTC) and increased freshwater river temperatures (from

OTC). Total water withdrawals for thermoelectricity in the

Northeast are substantial, corresponding to 11.8% of average

6
Environ. Res. Lett. 8 (2013) 025010 R J Stewart et al

Table 3. Increases in unsuitable thermal habitats for various fish species in river segments downstream of thermoelectric plants. The total

length of all river segments downstream of plants is 7530 km.

Fish species

Maximum

average weekly

tolerancea(

◦C)

Unsuitable habitat

without

thermoelectric

plants

considered (km)

Increase in

unsuitable habitat

due to

thermoelectric

plants (km)

Per cent increase

in unsuitable

habitat (%)

Brook trout Salvelinus fontinalis 22.4 7526.6 3.9 0.1

Rainbow trout Oncorhynchus mykiss 24.0 7451.1 45.5 0.6

Longnose dace Rhinichthys cataractae 26.5 6100.1 191.8 3.1

Creek chub Semotilus atromaculatus 27.1 5712.9 240.3 4.2

Northern pike Esox lucius 28.0 5260.9 227.8 4.3

Walleye Stizostedion vitreum 29.0 4687.1 461.1 9.8

Smallmouth bass Micropterus dolomieui 29.5 4275.3 648.3 15.2

Bluntnose minnow Pimephales notatus 30.1 3852.7 708.7 18.4

Golden shiner Notemigonus crysoleucas 30.9 3177.5 773.4 24.3

River Carpsucker Carpiodes carpio 32.1 2293.8 906.5 39.5

Red shiner Cyprinella lutrensis 34.0 1452.8 782.5 53.9

Largemouth bass Micropterus salmoides 35.5 917.6 738.6 80.5

a From Eaton and Scheller (1996).

annual flows (ranging in basins from 0.7% to 42.9%) and

22.6% of average summer flows (ranging from 1.1% to

95.1%), though most of these are non-consumptive (table 2).

OTC dominates water usage, representing 98.0% of total

annual and summer withdrawals by the sector. The range in

water withdrawal during dry years (i.e. 2001) is considerable,

ranging from 1.1% (Penobscot River) to 223% (James

River) of total freshwater summer flows. Increased water

temperatures and consumption of river discharge can both

degrade natural habitats, especially during summer periods

when flows are naturally low (Schindler et al 2005, Caissie

2006).

RCC withdraws substantially less than OTC technologies, but is nearly three times more consumptive, representing

74.1% of the total water consumed by thermoelectric plants

in the region. Evaporative losses are minor in proportion

(0.5%) to the average summer flow conditions in all

basins in the northeast (table 2). The Delaware River

experiences the greatest reductions in flow due to power plant

consumption, losing 2.1% of average summer discharges.

Water consumption during the driest year (2001) corresponds

to 2.9% of summer flows in the Delaware River, but only 0.7%

of summer flows for all basins draining to the Atlantic and

implies evaporative water use by the thermoelectric sector is

appropriate in this region relative to its hydrologic regime.

Thus, our study aligns with earlier analyses (Averyt et al

2011) on the impact of thermoelectric stress on water supply

in the northeast: the flow regime is minimally affected by

power plants in the region on average, and we found only

slightly more impact during dry years.

In contrast, heat inputs via OTC alter the temperature

regime, sometimes over great distances. To identify the

signature that thermal pollution has on water temperatures,

simulations were conducted with and without TP2M

activated. Heat inputs from thermoelectric power plants

increase average summer and winter water temperatures by at

least one degree in 25.7%, and 16.7% of potentially impacted

river length (segments downstream of OTC and RCC plants

which total 7530 km) in watersheds that drain to the Atlantic.

Localized temperature increases can be extreme, especially in

the winter (up to 27.0 ◦C) but are quickly diluted or dissipated.

Impacts are lower but more widespread in the summer

(figure 3). Peak temperature increases due to power plants are

more significant in the winter due to the large difference in

effluent and ambient temperatures but re-equilibration during

cold months is rapid because heat dissipates quickly to the

atmosphere over short flow path distances. Rivers are more

effective at temperature re-equilibration during the winter than

in the summer despite higher flows that would otherwise

reduce their effectiveness at re-equilibration (Wu et al 2012).

Slower dissipation of anthropogenic heat loads during the

summer means that minor disturbances often propagate

great distances downstream. This is most notable along the

Connecticut River downstream of the Vermont Yankee power

plant (located in the southeast corner of Vermont) where water

temperatures due to thermal pollution are perturbed more than

1

◦C over a short distance in the winter, but extend over the

entire length of Massachusetts in the summer (figure 3).

Increased freshwater temperatures due to anthropogenic

heat loads pose risks to the thermal habitat of native fish

in the region. We conducted a simple analysis of impacts

to fish thermal habitat similar to that of Mohseni et al

(2003). The fish and temperature database matching system

(Eaton et al 1995, Eaton and Scheller 1996) was used to

define maximum average weekly temperatures tolerated by

select cold, cool, and warm water fish species (table 3).

Unsuitable habitat was defined on an annual basis as grid cells

with maximum average weekly temperatures that exceed the

maximum thermal tolerances for each species (Mohseni et al

2003). We quantified the total unsuitable habitat in all river

lengths downstream of OTC and RCC for pristine (without

TP2M) and contemporary (with TP2M) scenarios.

An increase in total unsuitable thermal habitats in rivers

due to power plant discharges is apparent for all species

considered (table 3). But, perhaps counterintuitively, power

plant thermal pollution has a greater impact on the potential

7
Environ. Res. Lett. 8 (2013) 025010 R J Stewart et al

thermal habitat of warm water, rather than cool and cold-water

fish. This is because, in our study region, conditions are

already marginal for cool and cold-water fish in large rivers,

and these species find refuge in headwaters and low-order

streams (Hudy et al 2008) that do not receive power plant

effluent. In segments downstream of plants, thermal pollution

results in increases of 0.6%, 9.8%, and 53.9% in total annual

river lengths where maximum average weekly temperatures

exceed 24 ◦C, 29 ◦C, and 34 ◦C, respectively. In some cases,

thermal pollution raises peak summer water temperatures

to levels above critical ecosystem thresholds (EPA 2011).

These pockets of exceptionally warm temperatures (up to

46 ◦C in freshwater systems during extreme events) could

cause severe disruptions in otherwise healthy ecosystems,

and the magnitude of these impacts is just beginning to be

documented (Hester and Doyle 2011). Warm areas along the

river corridor during winter may also provide refugia for

various warm water invasive species (Dukes and Mooney

1999, Durance and Ormerod 2007, Pandolfo et al 2010, Rosa

et al 2012), further threatening native aquatic wildlife.

Regional (Kaushal et al 2010) and global (van Vliet

et al 2011) water temperature analyses have identified

increasing water temperatures that are correlated with rising

air temperatures. Annual mean water temperatures in some

large streams and rivers in the US are increasing at a rate

between 0.009 and 0.077 ◦C per year (Kaushal et al 2010).

Sensitivities of simulated global mean water temperatures

indicate increases of 1.3 ◦C, 2.6 ◦C, and 3.8 ◦C for air

temperature increases of 2 ◦C, 4 ◦C, and 6 ◦C, respectively

(van Vliet et al 2011). If these reported temperature increases

represent baseline changes due to warming climate, the

increases due to thermal pollution quantified here would

significantly exacerbate the problem in densely populated

regions with increasing energy demands (Hojjati and Battles

2005, Wilbanks et al 2008). Sensitivity analyses using

various policy, climate, and energy demand scenarios indicate

potential changes in impact (Miara et al 2013).

5. Conclusions

Assessment of the interactions among different cooling

technologies, aquatic ecosystem services, and aquatic

ecosystem impacts are critical to identify the full costs and

tradeoffs of electricity production (Bennett et al 2009). OTC

and RCC technologies at thermoelectric power plants are

dependent on natural ecosystem services in river networks

to provide coolant and for conveyance and attenuation of

heat loads. OTC power plants are responsible for 100% of

the regional net waste heat input to the river network, and

given their spatial distribution, most (88.7% or 935.2 PJ yr−1)

is delivered to the ocean. Thus, the general placement of

OTC plants in the Northeast limits the ecosystem service

provided by waterways to conveyance rather than mitigation

of waste heat. Upstream siting of OTC plants results in

greater attenuation of anthropogenic heat loads but longer

impact distances, whereas downstream siting results in less

freshwater impact but greater thermal loads to coastal

zones. The benefit that OTC plants gain from upstream

anthropogenic heat dissipation is small but may increase

with future climate. Thermal habitat loss in river segments

downstream of plants is considerable, and will be exacerbated

with climate change on the horizon (Eaton and Scheller 1996,

van Vliet et al 2011) and increasing energy demands.

OTC cooling technologies withdraw a substantial

proportion of river flows and leave a moderate footprint

on average seasonal river temperatures whereas RCC plants

evaporate considerably more river discharge but appear to

pose minor environmental concern due to the low proportions

of average summer flows they consume. Thus from a

purely aquatic ecosystem standpoint, RCC technologies are

preferred in water rich regions as OTC plants and their

reliance on horizontal cooling towers may put an unnecessary

stress on aquatic ecosystems. However, RCC technologies

have much higher total costs. Total capital costs, operating

costs, the efficiencies of the two cooling technologies, and

the ancillary costs associated with relying on horizontal

cooling towers to buffer heat loads for freshwater and

coastal environments must be considered to fully evaluate the

tradeoffs associated with thermoelectric power plants. These

tradeoffs will vary across regions depending on local climate.

Despite having even higher capital and operating costs,

alternative technologies such as dry cooling may be preferred

from an aquatic ecosystem standpoint. Future management

should also consider the geographic placement of plants in

the river network to minimize environmental impacts given

increasing electricity demands and warming climate.

Acknowledgments

We are grateful to S Glidden and P Yang for data processing

and technical support. This work was supported by the

National Science Foundation through EaSM-1049181, the

Experimental Program to Stimulate Competitive Research

EPS-1101245 and through the Environmental Protection

Agency Grant STAR-RD834187.

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