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4
Clean Water for People and Ecosystems
Most living things--humans and ecosystems--depend on availability of
clean water. The quality of the planet's waters is changing on time scales of
minutes to centuries in ways that are only partially understood. Ensuring
clean water for the future requires an ability to understand, predict, and
manage changes in water quality.
INTRODUCTION
As the "lifeblood" of the planet and the universal solvent, water trans-
ports vast quantities of dissolved chemicals and suspended matter through
the biosphere. The concentration of these constituents defines the quality
of water and is controlled by naturally occurring and anthropogenic phe-
nomena. Natural variations of the water quality of groundwater, streams,
and lakes occur because of geological, climatic, and biological influences.
Rain and snowmelt percolate through organic material near Earth's surface,
then through the mineral soil, and through the rocks making up aquifers.
Surface streams are fed in part by groundwater discharge and the waters
also interact with the streambed. In all of these environments, materials
dissolve into, or are removed from, the water through reactions that often
are mediated by microbes. Water flowing over the land surface also mobi-
lizes sediment, which is then transported in streams and rivers. In this way
the hydrologic cycle is intimately linked with all of Earth's element cycles.
Human activities have changed the natural element cycles in many
ways. Massive alterations in land use and water allocation, as well as the
use of synthetic chemicals, have set in motion a transformation of water
quality in many hydrologic basins that, at best, will lead to enhanced wa-
ter treatment effort or, at worst, to the deterioration of water quality with
potentially adverse impacts on human and ecosystem health. In the United
States, most of these transformations have been under way since the mid-
20th century. In other countries this transformation is just beginning. Yet
for many groundwater systems and surface water bodies, the reversal of
123
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124 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
damages will require a time scale of decades to centuries. For example,
groundwater in many parts of the nation is highly enriched in nitrogen,
leaving a legacy of nitrogen pollution that, even without new inputs, will
take many decades to be diluted by groundwater recharge. Thus, the cur-
rent quality of any water resource reflects the past as well as ongoing con-
tamination, and water, particularly groundwater, may not become clean
even if future sources of contamination are eliminated.
As the global human population grows through this century the de-
mand for clean water will increase. Humans already appropriate more than
half of the annual renewable freshwater supply globally, and there are few
untapped sources of clean freshwater in the places on Earth where most
people live (Postel et al., 1996). Supplying clean water to the growing hu-
man population will require much greater water reuse and water treatment
than in the past. As societies attempt to increase their standard of living,
economic growth will likely exacerbate problems related to water quality
and availability, because cleaning water requires energy and energy genera-
tion consumes and may contaminate water. The agricultural intensification
necessary to feed the growing population and an increasingly urban global
population are trends that are likely to further concentrate human and live-
stock wastes and place additional stress on water treatment systems. The
decisions societies make about how to acquire, clean, and dispose of water
have enormous impacts on the aquatic ecosystems that are the source of
water and the recipient of wastes. The degradation of aquatic ecosystems
and the loss of sensitive aquatic taxa can lead to a reduced capacity for
natural wetlands, streams, and lakes to trap, store, and transform contami-
nants. This provides a positive feedback that can further exacerbate water
quality problems. Finally, numerous opportunities exist for climate change
to impact water quality.
Access to clean water is a political and social problem, but decision
makers who are tasked with resolving the problem should be informed by
the results of hydrologic research. Science will ensure that the knowledge
base necessary to address the challenges of maintaining good water quality
where it exists and restoring it when it has been degraded will be available
in the future. Research opportunities related to water quality stem largely
from a need to know the processes that control the evolution of water
quality in both relatively pristine and in heavily impacted environments.
The requisite research also spans spatial scales from local to global, and
time scales from minutes to decades. Understanding will come through
research on the transport and fate of the constituents that dictate water
quality of surface and groundwater. Similar to the surge in analytical tech-
niques enabling detection of minute amounts of contaminants in water, the
committee anticipates a surge of discoveries from the hydrologic sciences
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CLEAN WATER FOR PEOPLE AND ECOSYSTEMS 125
to advance the scientific understanding needed to promote clean water for
the planet.
RESEARCH OPPORTUNITIES
Many broad research questions fall within the scope of promoting clean
water for the planet and challenge hydrologists and their collaborators in
biogeochemistry, environmental engineering, and chemistry. Thus, hydro-
logic research should pursue more linkages with these disciplines than it has
in the past. Feedbacks among the fluxes of water and a variety of elements
and compounds are at the core of phenomena that should be understood
to advance the Earth sciences. This report presents research opportuni-
ties in three broad topic areas. The first area relates to element fluxes in a
dynamically variable, highly heterogeneous, connected Earth. The second
specifically involves contaminant hydrology and the vexing problems that
should be addressed to understand the evolution of water quality. The third
focuses on the three important, large-scale drivers of water quality: climate
change, energy needs, and agriculture.
4.1. Chemical Fluxes through Complex Environments
Geological materials and surfaces are heterogeneous, which con-
founds the description of fundamentally important hydrologic
processes.
The world is complex and composed of diverse and dissimilar parts,
a concept that is referred to as heterogeneity. Heterogeneity presents chal-
lenges that scientists grapple with in different ways. Geologists identify and
classify rocks by the heterogeneity of their matrix, for example, by grain size
or mineralogy. Chemists work with mixtures of substances and character-
ize compounds by their solubility in one phase of a mixture versus another.
Hydrologists grapple with the flow of water through a heterogeneous land-
scape, both on Earth's surface and in the soils and rocks of the subsurface.
Earth's heterogeneity can be observed at different scales, from pore
size in the subsurface, to the irregularity of a river channel, to patchiness
of ecosystems--all of which impact the flow of water and, in turn, the
movement and concentration of constituents. The flow of water and dis-
solved or suspended mass fluxes can vary many orders of magnitude over
spatial scales ranging from centimeters to kilometers. It is unrealistic to
expect that every detail of heterogeneity can be measured and cataloged
to further understanding; useful scientific theories, after all, are ones that
capture the essence of phenomena simply by setting aside unnecessary
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126 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
complications. Yet, the effects of heterogeneity on hydrological transport
and biogeochemical transformation of substances should be generalized to
some basic principles so effective water management strategies can be de-
vised. Significant scientific effort has directed toward achieving this goal, yet
challenges still remain. With current capabilities, cleanup of contaminated
groundwater sites is difficult and not likely to be achieved in a desirable
timeframe. This difficulty is, in large part, due to the heterogeneity of the
subsurface, which confounds treatment technologies. Finally, heterogeneity
leads to the observation that discrete units of landscape are connected by
water-mediated transport of matter, energy, and organisms. Understanding
the connectivity is one of the goals of hydrologic research related to hetero-
geneity. Opportunities exist to further understand hydrologic connectivity
and its relationship to water quality.
What is the role of subsurface heterogeneity and connectivity in mass
transport, and how can it be characterized?
Much of the subsurface contains well-connected pathways of relatively
high permeability that constitute only a small portion by volume of porous
or fractured media. It is also quite common for most of that portion of the
subsurface, referred to as aquifers, to contain substantial volumes of non-
aquifer materials, such as silts and clays or unfractured rock, which are not
conduits for the flow of water. In the actual three-dimensional world, one
need only have a relatively small volume fraction of high-permeability ma-
terial for that material to fully connect or "percolate" (Harter, 2005). The
well-connected, high-conductivity paths cause earlier arrival of contami-
nants at wells or other receptors than conventional analyses would indicate.
Slow mass transfer between these fast paths and the presence of this lower-
conductivity media lead to broad dispersion of dissolved constituents. This
is consistent with field observations, including the ubiquitous observation
that contaminant cleanup by well extraction methods is difficult and often
requires decades of pumping.
Thus, progress has been made in understanding the impact of connec-
tivity on the dispersion of chemical contaminants, but it reveals information
about connectivity that is of concern. Contaminants are moving faster and
are dispersed more widely in the subsurface than was originally thought.
Given the importance of groundwater as a source of potable water and
in recharging surface water systems, continued improvements in under-
standing fluxes and transformations in such connected networks should be
pursued and will require much new work. Oftentimes, this understanding
is derived from research conducted at experiment sites (Box 4-1). Yet the
overarching question exists: Can connected networks in the subsurface be
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CLEAN WATER FOR PEOPLE AND ECOSYSTEMS 127
characterized to yield a simplified theory, thereby overcoming the hetero-
geneity problem?
Opportunities exist to develop tools to overcome the heterogeneity chal-
lenge. One example involves methods to determine groundwater residence
time, the elapsed time from when a parcel of water enters the groundwater
system to when it reaches a down-gradient location such as a well or spring.
Because groundwater consists of a collection of water and solute molecules
that typically have taken very different pathways to the destination, the
same groundwater sample can potentially contain water molecules with
residence times ranging from years to centuries or millennia. Groundwater
age therefore has an age distribution that represents, on the one extreme,
the fast flow paths through the high-conductivity, well-connected portions
of the system, and on the other extreme, the slow flow paths through the
often nonaquifer portions of the system. The age distribution is rich with
information about the complexity of the surrounding flow regime, the pres-
ence of preferential flow paths, different scales of heterogeneity, and slow
mass exchange between the fast and slow parts of the system.
Methods for determining groundwater ages are based on interpreting
the presence of dissolved materials and/or associated water molecules at the
time they enter the groundwater. The most widely used methods include
those based on measuring concentrations of tritium-helium (3H/3He), chlo-
rofluorocarbon (CFC), and carbon-14 (14C). The key obstacle to measur-
ing the groundwater age distribution is the lack of environmental tracer
methods for dating groundwater in the 50- to 3,000-year range (Figure
4-2). What are the new methods that will allow enhanced tracking of
water quality by age in this "dating gap window?" Can multiple chemical
species, including a variety of isotopes that cover the full range of likely
groundwater ages, be used in tandem with an appropriate reactive transport
model to infer the residence time distribution in three dimensions for an
aquifer system?
Pursuit of these research opportunities will advance understanding of
fluxes and transformations in connected, heterogeneous, subsurface net-
works through field characterization and experimentation as well as new
theories. How can water and solute residence time distributions in sub-
surface hydrologic systems be measured? How can these measurements
promote new theories to understand the impact of heterogeneity and con-
nectivity on contaminant distribution? What are the most appropriate
models to represent solute transport in aquifers containing different degrees
of heterogeneity?
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128 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
BOX 4-1
Revelations in the Complexity of Flow
and Transport Processes
The MADE tracer experiment site at Columbus Air Force Base, Mississippi,
is a contaminated site paired with a highly complex, underlying heterogeneous
aquifer. This heavily studied, and as a result heavily instrumented, site has pro-
vided insights into heterogeneity and connectivity issues at the plume scale over
the past 25 years. Research activities have revealed an extreme complexity of flow
and transport processes in highly heterogeneous porous media. Existing theories
and models have been shown to be incorrect or inadequate, while new ideas and
improved approaches are continuously being proposed and developed (Zheng et
al., 2011; Figure 4-1).
The key questions and hypotheses emerging from these research activities
include the following: Are small-scale preferential flow paths resulting from varia-
tions in hydraulic conductivity (K) the primary cause of the observed "nonideal"
behaviors in tracer plumes at the MADE site and elsewhere? What are the nature,
geometry, and scale of preferential flow channel networks in a highly heteroge-
neous fluvial aquifer such as that at the MADE site? How are such flow channel
networks (and flow barriers) related to the texture, structure, grain size, and facies
distribution of fluvial sediments? What is the most appropriate upscaled model to
represent solute transport in aquifers containing small-scale, connected, high-K
networks without an explicit definition of these networks? How can the parameters
for the upscaled model be obtained from readily available field data?
a
FIGURE 4-1 Observed conservative tracer plumes at the MADE site: (a) bromide plume
(concentration in mg/L) at 503 days after 2-day injection from the MADE-1 test; (b) observed
MADE-2 tritium plume (concentration in picocurie per milliliter [pCi/ml]) at 328 days after 2-day
injection from the MADE-2 test; and (c) observed MADE-3 bromide plume at 152 days after
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CLEAN WATER FOR PEOPLE AND ECOSYSTEMS 129
b
c
trench source release. (The two-dimensional contour map is constructed from the peak concentra-
tions at each multilevel sampler location from the MADE-3 test.) All plumes exhibit a highly complex
("non-Gaussian") pattern that cannot be readily described by classical models. SOURCE: Reprinted,
with permission, from Zheng et al. (2011). © 2011 by John Wiley and Sons.
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130 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
FIGURE 4-2 List of available environmental tracers for estimating water residence
time in geologic systems and the "dating gap" for which no reliable methods yet
exist for estimating age. SOURCE: Courtesy of N. Plummer, U.S. Geological Survey.
How much of the spatiotemporal heterogeneity in linked hydrological-
biogeochemical cycling must be understood to estimate, model, and
predict net watershed solute export?
The increasing ability to monitor chemical and biological conditions in
R02116
real time is revealing previously unknown temporal trends that in turn push
Figure 4-2
the boundaries of process understanding. The more finely watersheds are
subdivided and the morebitmapped, uneditable
frequently hydrologic and biological processes are
measured, the more dynamic ecosystem hydrology and biogeochemistry are
discovered to be. As hydrologists move into the era of high-resolution data
sets, a critical challenge for hydrologists is to determine the data density and
distribution necessary to address critical questions about ecosystem-scale
biogeochemical behavior. How much detail is needed?
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CLEAN WATER FOR PEOPLE AND ECOSYSTEMS 131
For regional fluxes of water, hydrologic models are typically able to
provide reasonable approximations and predictions without highly resolv-
ing properties and processes in space and time. For biogeochemically active
solutes and elements with significant gaseous forms, coupled hydrological-
biogeochemical models often fail to match observations. This is because (1)
hydrologic models can match the flux of water without having to accurately
approximate pathways and mass exchange between regions of fast and slow
flow and (2) representing biogeochemical reaction rates requires reasonable
approximations of reactant supply and environmental conditions (tempera-
ture, pH, and oxidation reduction potential) along flow paths as well as
accurate flow path routing of water. Processes that occur homogeneously
across the ecosystem (e.g., organic matter decomposition) will be easier to
model and upscale than processes occurring under a more limited set of
conditions (e.g., denitrification, which occurs only under conditions of low
oxygen, high carbon, and high nitrate). For heterogeneously distributed
biogeochemical processes, measures of whole ecosystem rates provide little
information about where and when the process occurs.
There can be great spatial and temporal heterogeneity in the availability
of nutrients and the extent of hydrologic connectivity within ecosystems.
Because of this variation, there are places and times within ecosystems
that can play disproportionately large roles in driving whole-ecosystem
biogeochemistry. For example, riparian zones that receive materials from
the surrounding uplands and that are more highly hydrologically connected
to streams than the rest of the catchment tend to have heightened rates
of nutrient transformations. Although scientists conceptually recognize
the likely importance of these rare moments and locations for enhanced
biological activity, in practice it has been challenging to define, locate, and
monitor these ecosystem control points. The increasing ability to map and
monitor hydrology, chemical concentrations, and biological activity in real
time is revealing previously unknown spatiotemporal hydrologic and bio-
geochemical dynamics (Box 4-2) that in turn push the boundaries of process
understanding. What variables control the occurrence and magnitude of hot
spots in terrestrial, riparian, and hyporheic zones? Can knowledge about
the joint distribution of organic carbon and fast flow paths be used to infer
the major control points for ecosystem biogeochemical reactions? How do
human interventions on the landscape change hydrological connectivity and
therefore the spatial distribution of hot spots?
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132 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
BOX 4-2
Understanding of the Correlated Variation
in Hydrologic Science and Biogeochemistry
through Technological Innovations
Technological innovations are providing an unprecedented understanding of
the spatial and temporal covariation in hydrology and biogeochemistry. At land-
scape scales, remote sensing technologies effectively capture hydrologic varia-
tion in space and time. For example, synthetic aperture radar derived vegetative
cover and inundation for a reach of the Solimões River in the central Amazon
Basin, Brazil, is shown in Figure 4-3. On a much smaller scale, microelectrode
profiles illustrate the variability of oxygen, iron, and manganese concentrations
in co-located sediment profiles (Figure 4-4). Measurement techniques have pro-
vided new insights into soil stream interactions, as well. For example, dissolved
carbon dioxide (CO2) concentrations for a peatland stream vary over the course
of a storm hydrograph (Figure 4-5). Finally, high-frequency in situ sensor arrays
in Ichetucknee River, Florida show the close relationship between nutrient cycling
and metabolic activity (Figure 4-6).
FIGURE 4-3Landscape-scale remote sensing technology showing vegetative cover and
inundation for a reach of the Solimões River in the central Amazon Basin, Brazil. SOURCE:
Reprinted, with permission, from Hess et al. (2003). © 2003 by Elsevier.
R02116
Figure 4-3
bitmapped, uneditable
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CLEAN WATER FOR PEOPLE AND ECOSYSTEMS 133
FIGURE 4-4 Voltammetric profiles indicate significant variability in speciation in wetland soils
2 cm apart in sediment cores collected on the same day in Jug Bay, Maryland. SOURCE:
Reprinted, with permission, from Ma et al. (2008). © 2008 John Wiley and Sons.
FIGURE 4-5 Variability of dis-
solved CO2 concentrations (mg
C L1) for a peatland stream
over the course of a storm hy-
drograph (discharge in L s1).
SOURCE: Reprinted, with per-
mission, from Dinsmore and
Billett (2008). © 2008 by the
American Geophysical Union.
FIGURE 4-6 Diel covariation in gross
primary production (GPPR02116
in g O2 m2
Figure
h1) and nitrate uptake 4-5
(autotrophic
bitmapped,
N assimilation or uptake asuneditable
Ua-NO3-pre
in mg N m2 h1) rates in the highly
autotrophic Itchetucknee River, Flor-
ida. The line is best-fit least-mean-
square regression on individual
observations (see Chapter 1, Figure
1-4 for the in situ observations used
to probe this coupling). SOURCE:
Reprinted, with permission, from
Heffernan and Cohen (2010).
© 2010 by the American Society of
Limnology and Oceanography.
R02116
Figure 4-6
bitmapped, constituent components uneditable
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144 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
A more detailed understanding of water quality continues to emerge
from the increasing sophistication of chemical analytical instrumentation
and includes detecting contaminants at increasingly lower concentrations
as well as new contaminants. Yet, when is the risk from the presence of
these constituents so small that it is negligible for all practical purposes?
Merging information about the relationship between hydrologic flux and
contaminant concentration with risk assessment is critical. What are the
contaminant dilution ratios resulting from hydrologic flux, and how should
the findings inform risk analysis?
4.3. The Future of Water Quality in a Hot, Flat,1 and Crowded World
As Earth's human population moves toward 9 billion, as re-
source use intensifies, and as climate changes, maintaining ad-
equate water quality will rely on new knowledge.
The final layer of complexity in the water quality picture involves the
large-scale drivers of water quality. Much conversation revolves around
Earth's growing population and demographic change. Intellectual leaders
debate questions about how the world should deal with the correspond-
ing intensification of resource use and the impact of climate change. The
amount of food required to feed the projected population of 9 billion is
staggering--and currently many people go hungry. Similarly, the energy
required for transportation, to generate electricity, or to provide clean
water for 9 billion people is enormous. And all of these demands should
be considered against the backdrop of a changing climate, providing yet
another challenge to maintaining adequate water quality for humans and
ecosystems.
The layers of complexity in a hot, flat, crowded world are daunting.
The world will need scientific knowledge to even begin to deal with these
issues. The hydrologic research community has an obligation to tackle the
water quality questions embedded within these large-scale drivers.
How do changing flow paths as a result of urbanization correspond to
changes in water quality?
Currently, about half of the global population lives in urban areas. As
global urban migration continues, this is expected to increase to 60 percent
1 The term "flat," coined by the author Thomas Friedman in his books The World is Flat
(2005) and Hot, Flat, and Crowded: Why We Need a Green Revolution--and How It Can
Renew America (2008), is used to describe a new era of globalization that allows people and
entities around the world to compete, connect, and collaborate.
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CLEAN WATER FOR PEOPLE AND ECOSYSTEMS 145
by 2030 (UN, 2006), which will have impacts on the environment. For
example, the increase in impervious surface area associated with urbaniza-
tion slows evaporative cooling, allowing surface temperatures to rise higher
than in rural areas. Impacts on the urban hydrologic cycle also exist; it has
long been recognized that the increase in impervious surface area leads to
a decrease in infiltration and an increase in runoff, thereby changing the
hydrologic response of streams.
The recognition of water quality problems associated with urban devel-
opment has influenced engineering design in the 21st century. Incorporation
of "green" infrastructure into the engineering of cities has become preva-
lent, whereby effects of stormwater and impervious surfaces are mitigated
through designs that attempt to mimic the natural hydrologic cycle and
management of stormwater on a watershed scale (NRC, 2008). Practices in-
clude promoting infiltration of stormwater such as removing hardened sur-
faces and regreening urban lands; incorporating grass swales, rain gardens,
and green roofs into landscape design; using pervious paving materials; and
installing subsurface storage where infiltration practices are not possible.
An opportunity exists to probe the geochemical phenomena associated with
changing urban flow paths, i.e., stormwater.
Knowledge exists in this area. For example, a smaller percentage of
atmospherically introduced contaminants are retained in urban landscapes
compared to forested ones. As a result, chemical concentrations of nitrogen
and metals are higher in urban streams (Figure 4-12). This lack of retention
of contaminants in urban drainages can have effects on downstream aquatic
ecosystems and potable water supplies. Future increased water demand in
urban systems will exacerbate this problem. What are the impacts of urban-
ization on hydrologic connectivity and, therefore, water quality? How do
urban management practices, for example, urban stormwater management,
impact hydrologic connectivity, water quality, and ecosystem health?
What new hydrologic knowledge is needed to enable agriculture and
silviculture to be sustainable with respect to water quality?
The role of agriculture in water quality continues to be a major global
problem. As the global population increases, the need for food and fiber
will increase, leading to the need to convert more marginal land to agricul-
ture and to increase agricultural production per unit land area. In the latter
case, increased fertilizer usage will increase nutrient fluxes, especially fixed
nitrogen compounds, into both groundwater and surface water systems.
These nonpoint sources enhance eutrophication and nuisance algal blooms
in water bodies locally, regionally, and even far afield as demonstrated by
increased hypoxia in the Gulf of Mexico due to input of nutrients from
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146 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
FIGURE 4-12 Increasing annual flow weighted dissolved organic nitrogen concen-
tration (DIN, µM) with increasing residential area (A) and impervious surface (B).
SB and CC represent two catchments of contrasting land use (heavily residential
and heavily forested). SOURCE: Modified, with permission, from Wollheim et al.
(2005). © 2005 by Springer Science +R02116
Business Media.
Figure 4-12
bitmapped, uneditable
Mississippi River waters derived primarily from row-crop agriculture in
the Midwest.
In addition to introducing contaminants into the environment on a
landscape scale, irrigated agriculture also has a large demand for water,
which in turn often leads to groundwater overdraft. Under predevelop-
ment conditions, the groundwater systems flowed from recharge areas to
natural discharge areas, providing an exit for dissolved salt. However, when
overdraft occurs and the pumped groundwater is reapplied to the land for
irrigation, a closed hydrologic basin is potentially created whereby shallow
groundwater of lesser quality (for example, elevated salinity) eventually
makes its way to the pumping wells, only to be withdrawn and reap-
plied to the land, resulting in further concentration of salts and nutrients.
Such a closed system in agricultural basins such as the San Joaquin Valley
of California may lead to long-term salinization of groundwater, which
will eventually render the groundwater unsuitable for irrigation and most
other uses. In certain environments, conversion of natural landscapes to
agriculture also can impact groundwater due to groundwater rise. If the
native vegetation is replaced with less water consumptive crops, then the
subsequent rise in the water table can solubilize previously accumulated
salts, increasing groundwater salinity, mobilizing nitrate, and salinizing soil.
Efforts to ensure global food security will lead to changes in practices
that will need to be assessed, which underscores the importance of ad-
dressing critical research questions. What are the hydrologic and water
quality implications of changes in crop patterns and rotations? What are
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CLEAN WATER FOR PEOPLE AND ECOSYSTEMS 147
the possibly nonlinear feedbacks between irrigation water quality and plant
productivity, and how are these affected by management of the hydrologic
regime? As the approach to "peak phosphorus" leads to resource restric-
tions, it will become necessary to pervasively employ nutrient recycling (for
example, recovery and use of nutrients from municipal and other waste
products). What are the water quality implications related to the use of
different fertilizer forms?
How can the hydrologic sciences inform solutions to the "water-energy
nexus"?
Water and energy are mutually dependent resources. Increased energy
consumption will lead to increased water consumption and water avail-
ability challenges throughout the globe (explored in Chapter 2). Yet, the
"water-energy nexus" has the potential to impact water quality, as well.
Water is needed in the production, refinement, or distribution of a variety
of energy sources, and when water is used in this manner there is an impact
on water quality. For example, 90 percent of all of the electricity used in the
United States is generated by steam, which must then be cooled to condense
and reuse or returned to waterways (Averyt et al., 2011). Power plant cool-
ing uses approximately 30 percent more water than is used in irrigation in
the United States, and although the water use is mostly nonconsumptive,
return flows are warmer than initially extracted, impacting aquatic ecosys-
tems (Averyt et al., 2011). The scope of the water-energy nexus is broad
indeed given the need for water and energy for 9 billion people; changes in
the thermal regime of aquatic ecosystems is just the beginning. How will
the increasing need for energy impact the planet's water quality?
Extraction techniques to obtain a variety of carbon-based fuels have a
long history of causing adverse water quality impacts. For example, drain-
age of acidic waters from coal mines (acid mine drainage) has long plagued
coal-producing regions. Many open questions remain related to the impacts
of energy production on water quality. One current example relates to the
development of natural gas from deep shale beds. Hydraulic fracturing,
a technique that produces natural gas otherwise locked in organic-rich
shales, involves injecting water and chemical additives into shales at high
pressure. The potential environmental impact of hydrofracturing has been
a contentious topic. Accidental releases of contaminants into aquifers have
been documented, likely because of improper well construction. Disposal
of flowback water from wells in surface disposal ponds also has been impli-
cated in water contamination. What research on hydrogeological implica-
tions of hydrofracturing and on the chemical composition of injection fluids
and their interaction with natural minerals is needed to properly inform the
public debate?
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148 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
Marine and hydrokinetic energy--energy from waves, tides, and cur-
rents in oceans and so forth--generates power by harnessing the natural
flow of water without using a dam, a diversionary structure, or an im-
poundment. While in its infancy, this energy source has potential; ma-
rine and hydrokinetic energy could provide almost 10 percent of the U.S.
electricity demands, a step toward the 2020 mandated requirement of 20
percent renewable energy by electric utilities in the United States (American
Clean Energy and Security Act of 2009). Yet, the environmental effects of
marine and hydrokinetic energy are far from understood. It is known that
this technique will reduce water velocities in the vicinity of the project, as
well as increase water surface elevations and decrease flood conveyance
capacity at larger scales (Bryden et al., 2004). As this technology develops
(Figure 4-13), research efforts to provide the scientific understanding of
how harnessing the natural power of water disrupts the water cycle and
alters water quality will become a priority. What are the related changes in
FIGURE 4-13 Federal Energy Regulatory Commission (FERC) marine and hydro-
kinetic alternative energy projects in the United States. The pressure for advancing
marine and hydrokinetic energy is running faster than the knowledge needed to
ensure that these projects are not adversely affecting the environment in the near
or long term. SOURCE: NOAA Fisheries Office of Habitat Conservation, February
11, 2009.
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CLEAN WATER FOR PEOPLE AND ECOSYSTEMS 149
natural flow paths, which could in turn lead to altered water quality and
effects on the aquatic food web? What are the long-term effects in estuary
and lagoon inlets of, for example, changing wave height?
Solutions to offset the buildup of carbon from carbon-based fuels also
are being pursued. Carbon sequestration, processes to remove CO2 from
the atmosphere, is increasingly looked to as a means of mitigating the im-
pact of global warming. Carbon can be stored in various forms: liquid in
the oceans, gas in deep geologic formations, or solid using chemical reac-
tions to produce stable carbonates. Carbon sequestration processes also
can take a variety of forms, from injection of carbon into underground
reservoirs to development of tree plantations. But like with any retooling of
the natural environment, the consequences of these efforts can be surpris-
ing. For example, semipermanent, biological carbon sequestration through
extensive tree plantations will reduce streamflow and impact water quality.
Carbon sequestration in deep geologic formations has the potential to af-
fect groundwater quality. What are the water quality tradeoffs associated
with various carbon sequestration strategies? What data are necessary to
inform risk analysis models assessing the impact of carbon sequestration
techniques on water quality?
What might be the effects of climate change on freshwater quality?
Understanding the impact of climate change on water availability has
received particular attention. Far less attention has been paid to the impact
of climate change on water quality and, as a result, far less is known. Nu-
merous opportunities exist for climate change to produce new regimes that
impact water quality, from increasing sediment, chemical, and pathogen
loading in runoff to saltwater intrusion threatening the quality of coastal
groundwater supplies. One of the most obvious is a change in the thermal
regime of water bodies. How does a changing temperature regime impact
flow dynamics and biogeochemical processes in streams, lakes, and reser-
voirs? How do thermal and hydrologic changes interact to affect freshwater
ecosystems through, for example, increased eutrophication?
Management of urban and agricultural catchments for rapid water
conveyance (i.e., replumbing of the water cycle) has led to increasing
proportions of contaminant loads moving into and through freshwaters
during peak flows. Climate change also is expected to exacerbate this phe-
nomenon. For example, increased precipitation could increase sediment
yield and contaminant runoff in agricultural dominant as well as urban-
and suburban-dominant watersheds. A potential increase in contaminant
loading has important consequences for downstream reservoirs, lakes, and
coastal environments. How does climate change impact river flows and
the flushing of constituents into down-gradient areas? Increased flooding
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150 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES
has the potential not only to flush contaminants downstream, but also to
remobilize contaminants through increased erosion. Will increased fluxes
remobilize contaminants, and, if so, which contaminants are more suscep-
tible to this remobilization?
Similarly, water extraction from surface and groundwaters has led to
prolonged periods of low flows, which can aggravate contamination prob-
lems. Again, climate change is expected to exacerbate this phenomenon. In
some areas there could be a decrease in precipitation leading to decreases
in base flow and evapoconcentration of water, increasing salinity. In oth-
ers, decreased frequency of rainfall events could introduce concentrations
of non-point-source contaminants that have accumulated on the landscape
over longer time periods, leading to enhanced pulses to the aquatic ecosys-
tem. Changes in flow regimes due to changes in precipitation patterns or
changes in percentages of snow versus rain might also impact water quality.
As contaminant loads are added to declining water volumes, chemical
signals are amplified. Low flows in surface waters can also further exacer-
bate problems of low oxygen and alter the toxicity of contaminants. This
concept has a variety of consequences that expand into the realm of Water
and Life, because low flow shrinks habitat for aquatic species and increases
the contaminant concentration and, in turn, exposure. What will be the im-
pact of climate change induced low flows on contaminant concentrations?
In the spirit of the previous chapter how does this translate to ecosystem
impact?
CONCLUDING REMARKS
The committee presented research questions designed to further un-
derstand Earth's water quality in relatively pristine and impacted environ-
ments. Similar to the approach to address the challenges and opportunities
outlined in Chapters 2 and 3, field studies, whenever feasible and appro-
priate, are important to answering the questions proposed in this chapter.
Answering these questions also will require work by hydrologic scientists
and engineers as well as a focused effort from those in related subdisci-
plines such as aquatic geochemistry and biogeochemistry. In the previous
chapters the committee presented research questions probing the dynamic
relationship between water and physical process and life, questions that
also will require work at disciplinary interfaces. Thus, a common theme
extends through the range of research questions noted in this report, that is,
the need for interdisciplinary research that takes advantage of cutting-edge
technological capabilities to grapple with the complex water-related chal-
lenges of today and tomorrow. Examples in the current context include re-
search on groundwater age linking physical hydrologists using Darcy's Law
and chemical hydrologists perfecting age-dating techniques. Or research
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CLEAN WATER FOR PEOPLE AND ECOSYSTEMS 151
using advances in chemical analytical techniques to help disentangle the
coupled hydrological biogeochemcial processes that control the evolution
of Earth's water quality profile.
Many of the research questions discussed here and in the previous
chapters have relevance and applicability to the multitude of stakehold-
ers who are concerned with water resources. For example, environmental
management or stewardship requires an understanding of cause and effect
as well as the ability to predict effects of different management practices
on the environment. Such a predictive analysis requires not only long-
term monitoring and understanding changes in water quality but also
development of sophisticated models capable of representing the effects of
heterogeneity, connectivity, and biogeochemical reactions on water quality
at the regional scale. Thus, while the hydrologic sciences are critical to the
resolution of the issues discussed in this report, it will be interdisciplinary
teams of researchers, including not only physical, chemical, and biological
scientists, but also social scientists, that produce useful scientific results.
Ongoing consultations with those who build and maintain infrastructure
are essential. To ensure that society, in additional to science, benefits from
research results, interactions with policy makers and other decision makers
will have to follow.
The next and final chapter presents findings for the hydrologic com-
munity to consider with respect to both advancing fundamental science and
to contributing to solutions of the complex water issues of today.
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