United States
Environmental Protection
Agency
Environmental Research
Laboratory
Duluth MN 55804
Research and Development
EPA-600/S3-84-011 Mar. 1984
AEPA Project Summary
Watershed Sensitivity
Measurement Strategy for
Identifying Resources at
Risk from Acidic Deposition
Orie L. Loucks
The objectives of this research include
a review of existing literature on the
use of indices for quantifying resource
status and predicting long-term trends
in relation to acidic deposition, a review
of options regarding the appropriate
form of a sensitivity index or a loading
tolerance model for determining resources
at risk, and identifying validation steps
needed to complete testing of the
measure or model and to begin its
application.
One section of the full report describes
the suite of measures which, when
taken together, best identify areas
potentially sensitive to acidic inputs.
Each of the component measures,
when viewed separately, has certain
limitations which prevent it from being
an adequate measure of sensitivity;
when the components are considered
as an integrative measure, however,
the limitations are less significant.
For non-agricultural systems, forest
site index appears to be a well-established
integrative measure capable of respond-
ing to altered soil and water chemistry.
The extent to which site index is related
to changes in cation nutrient storage
(due to cation stripping by acid precipi-
tation) or other pollutant impacts is
incompletely documented, however.
Measurements of aquatic sensitivity
have been developed more fully, and a
number of experimental and field data-
based approaches exist. These include
the Calcite Saturation Index, the
Henrickson nomograph, the Aimer/
Dickson relation and an additional
measure proposed here based on pH
shock effects during acid flushing
events. This integrative response pro-
perty appears to describe a complex
environment leading to species and
population effects associated with
periodic but physiologically important
exposures to H+ and AI3+.
This Project Summary was developed
by EPA's Environmental Research
Laboratory, Duluth. MN, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction and Objectives
In August, 1979, a federally funded
acid rain assessment program was
established by executive order. At about
the same time, the Environmental
Protection Agency awarded a Cooperative
Agreement to North Carolina State
University to conduct a program of
subcontracted studies on biological
effects of acid precipitation. The Institute
of Ecology (TIE) was awarded a subcontract
entitled "Assessment of the Sensitivity
Index Concept for Evaluating Resources at
Risk from Atmospheric Pollutant Deposi-
tion (Acid Rain)," to be carried out in
support of studies at the ERL-Duluth, U.S.
Environmental Protection Agency.
The study focused on developing
sensitivity measures for evaluating
terrestrial and aquatic resources at risk
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from atmospheric pollutant deposition
(oxidants as well as acid rain). The main
objectives were:
(1) To review existing literature on the
use of indices for quantifying resource
status, predicting long-term trends
in ecosystem and resource responses
to acidic deposition, and for assess-
ing overall risks from atmospheric
pollutant deposition in relation to air
emissions management;
(2) To consider options regarding the
appropriate form of a sensitivity
index or pollutant loading tolerance
model for use in determining resourc-
es at risk and to outline how such a
measure would function in a region-
al inventory of risk from pollutant
deposition or in the assessment of
benefit from acid precursor control;
(3) To identify validation steps needed,
data required (existing data or new
measurements), and the steps
required to complete testing and
begin application of the sensitivity
measures or loading tolerance
model m regional and national
energy development decisions.
Approach
Early in the study, possibilities were
explored (see Loucks et al. 1981) for
compressing various acid ram indices
(i.e., the McFee soil sensitivity measure,
the Calcite Saturation Index) into one
sensitivity index. This was eventually
recognized as an unsound approach
because too many dimensions of chemical
mediation in the environment were being
expressed in a single dimensionless
index number. Instead, it was determined
that the first part of the report should
examine and define the scope of the
processes involved in deposition and
acidification in both the terrestrial and
aquatic components of watersheds. To
this end, the relationships involved in
formation, deposition, and subsequent
effects from the input of acidic substances
were summarized in the form of a
descriptive flow model (Figure 1). The
model makes explicit the processes by
which watersheds incorporate and respond
to the multiple pollutants in rural landscapes
and to acidic deposition in particular. It
also relates the transfers between air and
land, and between land and water to
subsequent effects.
Implicit in this approach is a relatively
formal systems structure used as a guide
to the evaluation of the otherwise
fragmented information base. The quali-
tative model of known relationships
between the chemical and biological
subsystems, and controls operating on
the ecosystem as a whole is, in effect, a
suite of hypotheses that ultimately must
be tested sequentially in a comprehensive
research program. As such, it represents
both theory and the general outline of a
report evaluating the prospective signifi-
cance of acidic deposition for terrestrial
and aquatic resources. It also outlines the
principal properties of these systems
most likely to function as measures or
indices of the sensitivity of the resources
involved.
Results
Quantifying pollutant impacts on
resources such as streams, lakes or
forests requires making a distinction
among properties of the system with
varying degrees of sensitivity to pollutant
exposures. Two general groups of mea-
sures can be distinguished: indicators,
defined as individual high-resolution
response measures of components
within a complex system (i.e., a chemical
nutrient stock); and integrators, defined
as measures which reflect the combined
action of several environmental properties
governing a more aggregated response.
Both types of measures are necessary to
present fully the combined effects of
pollutants on the landscape, and both
groups are examined in detail through the
report.
One major section of the report,
entitled "Sub-Components of Lake/Wa-
tershed Sensitivity," contains the back-
ground material on pertinent characteris-
tics of acid precipitation effects on soils,
lakes and watersheds, and lays a founda-
tion for both the recommended sensitivity
measures and the outline of research and
data needs in later sections (see Loucks
1982a). This section also provides an
understanding of the various processes
occurring within the system, information
that is required for developing and
evaluating measures of watershed/lake
sensitivity. Topics of interest include
hydrologic flows, the nitrogen and sulfur
cycles, alkalinity relationships, interactions
of acids with organic material, nutrient
stripping, H+ toxicity, the mobilization and
toxicity of aluminum and heavy metals, and
synergisms between H+, aluminum and
heavy metals
Particularly important are results from the
detailed review of the mobilization of
potentially toxic metals by the elevated H+
concentrations in soil water. Mercury,
copper, lead, cadmium and aluminum are
all considered in some detail, but the
most thorough coverage is devoted to the
known plant and animal toxicities from
ionic free aluminum (Al3*). For terrestrial
systems, aluminum toxicities (in crop
species) are expressed in the formation of
stubby and brittle roots with reduced fine-
root branching, and acute foliar phospho-
rus deficiencies, possibly the result of
strong immobilization by soluble Al3*. In
aquatic environments, effects on animals
are expressed in the altered ionic balance
of materials in the bloodstream in the
presence of AL3+, and in oxygen deficiencies
as AI3+-induced mucus on the gills clog
normal gas exchange with the bloodstream.
The section principally focused on new
results, entitled "Methodologies for
Identifying Sensitive Terrestrial and
Aquatic Areas," describes the various
options for measures that best identify
potentially sensitive areas. Three separate
measures are employed for the terrestrial
component: McFee's soil sensitivity
classification based on cation exchange
capacity, the soil sensitivity classification
based on base saturation, and the forest
site index. The site index (SI) concept has
been accepted as a measure of forest
productivity for many decades and is
examined here as an integrative method-
ology for measuring changes in potential
forest growth due to long-term acidic
inputs (see Loucks 1982b). The magnitude
of site index changes due to a combination
of oxidants, changes in cation nutrient
storage (resulting from cation stripping by
acidic precipitation), and aluminum toxicity
effects still is in completely quantified,
however Further studies using available
data bases on oxidant exposures, apparent
changes in total nutrient stocks and
aluminum mobilization in relation to
acidic inputs'will be required.
Integrative measures for expressing
aquatic sensitivity have been developed
more fully, and a number of experimental
and field-data based methods exist.
These include the Calcite Saturation
Index, the Henrickson nomograph and
the Almer/Dickson relation. An additional
measure, based on pH shock effects
during acid flushing events, attempts to
identify species/population impacts
associated with short-term, physiologically
important exposures of critical life stages
to H+ and AI3+. Research on brook trout
and Atlantic salmon has provided a broad
understanding of the response of several
pH-sensitive fish species to both long-
term and short-term elevated H+exposures
Mortalities of fish eggs, sac fry and adult
fish are viewed as a response to continu-
ing chronic pH depression. Effects on egg
viability, hatching success, and adult
survival are known to occur as a response
to short-interval acute H+ and associated
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Natural Sources
Fossil Fuel Emissions
CO2, SO,, /VO,, M.C.O,
Flux to Watersheds
hT. SO2', NO~3, NH\
Atmospheric Reactions
SO,
/VO,
NOl
H"
03
Flux to Lakes
I Highly Buffered
Soils
Soluble Nutrient
Ions Ca2+, Mg*\ Na\ K*.
P0l~, SOf, HCOl
Elevated H*. Metals,
and NOl Levels in
Groundwater
To Fish and
Aquatic Biota
Soluble Toxic
Ions H\ AI3\ Pb2\
Cu2
Altered
Physiological
Process
V )
I Acid I
i Lakes '
I
I Poorly
i Buffered
Lakes
.1.
Highly Buffered \
(Carbonate) Lakes i
Soluble Toxic
Ions H\ A/3\ Pb2\
Cu2\ Hg2*,
Soluble Nutrient
Ions Ca2*, Mg2\ NOl
P0l~. SO2', HCOl
Disruption of
Organic Matter &
Nitrogen Cycle
NOl, NH\, Org N
Alterea
Physiological
Process
Effects on Crops
and Forests
Effects on
Human Health
__^
Effects on Fish and
Other Aquatic Biota
Figure 1.
Flow diagram showing system linkages for acidprecipitaton formation, deposition and effects as a consequence of nitrogen and sulfur
oxide emissions from fossil fuel combustion
AI3+ exposures. The experimental data
base supports two findings:
(1) The short-term acute exposure, or
shock effect, can be expected when
pH drops in the order of 0.5 to 1.5
units of the pH scale within a background
environment of pH 5.5 to 6.5; and
(2) Episodic shock exposures are sig-
nificant even at pHs above the level at
which chronic effects ordinarily
would be produced.
Taken together, these data suggest that
for waters normally in a range of pH 5.5 to
6.5, a pH depression of 0.5 to 1.0 pro-
duces a physiologically significant,
acid-induced alteration of water chemistry
and biological response. Given this
dose/response relationship, an acid
loading tolerance, or loading threshold,
can be defined as the annual SO?" loading
which, when subjected to a defined
flushing event (e.g., snowmelt or first
major rainfall following drought), leads to
the minimal biologically-significant short-
term H+ and AI3+ exposure.
Present data on pH depression during
flushing events (Table 1) indicate a range
in responses, from 0.8 units during snow-
melt in northern Minnesota to more than
2.0 units in the Adirondacks. Pending
further testing, a significant shock event
response (defined as a A pH of 0.5 to 1.0
unit) may be a useful basis for estimating
the annual SOT loading which produces
only a marginally unacceptable level of
pH depression. This estimate must be
defined for watersheds within a specified
alkalinity range, a specified range of
hydrologic dilution (i.e., stream size), and
a defined return interval for episodes.
Recurrence at an average of once a year
during critical life-cycle stages would be
consistent with the physiological data
base described above. Present results
suggest that a wet-deposition sulfate
loading of 5 to 7 kg/ha-yr produces a
physiologically critical episode response
(A pH) in the range of 0.5 to 1.0 for the
most sensitive streams in poorly buffered
regions. As with the other models, the
applicability of the episode shock model
still is relatively untested at this time, and
research is continuing
The final section, entitled "Concluding
Comments and Research Needs," is a
brief description of the data needed to
achieve a full validation of the sensitivity
measurement options. Insufficient data
presently exist for quantifying fully the
hydrogen ion or sulfate fluxes through a
wide variety of watersheds or the
aluminum mobilization during peak H+
concentrations. Nutrient stripping effects
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Table 1. Episode A pH Data for Flushing Events at Sites Representing a Cross-Section ofSCfc Loading Intensities in the Eastern United States an
Canada, 1977-79
Site
Little Moose Lake. N Y
{Outlet)
Sagamore Lake, N Y.
(Outlet)
Harp Lake Inlet, Ont.
(No 3)
Harp Lake Inlet, Ont
(No. 4)
Filson Creek, MN
Flushing
Event Date
March 9-1 3, 1977
March 25 - April 20, 1978
Mid-March - Mid-Apr//
April 10-20, 1978
April 12-24, 1979
pH Observed
Before (During)
Episode
6.8 (4 8)
6.4 (4.9)
6. 1 (5. 1)
6.4 (5.4)
6.6(5.8)
Aptf
2.0
1.5
1.0
1.0
0.8
Annual SOl
Deposition
(wet only)
(kg/ha)
38
38
30
30
14
on forest productivity, effects from
metal mobilization on plants, from food
chain alterations on mammals and
predatory birds, and the role of organic
matter in mediating acidic deposition
effects are all too poorly known to have a
fully reliable, locally applicable sensitivity
measure at this time. The need for
continuing research on many of these
questions is acknowledged
Publications
Loucks, O.L, R.W. Usher, R.W. Miller, W.
Swanson, and D. Rapport. 1981.
Assessment of Sensitivity Measures
for Evaluating Resources at Risk from
Atmospheric Pollutant Deposition.
Final Report to the U.S. Environmental
Protection Agency, Environmental
Research Laboratory-Duluth. The
Institute of Ecology, Indianapolis. 87
PP-
Loucks, 0 L 1982a. The Concern for
Acidic Deposition in the Great Lakes
Region. In: P.M. D'ltn (ed.) Acid Precipi-
tation - Effects on Ecological Systems, pp.
21-41 Ann Arbor Science Publishers,
Ann Arbor, Michigan.
Loucks, O.L. 1982b. Use of Forest Site
Index for Evaluating Terrestrial Resources
at Risk from Acidic Deposition. In: R.A.
Linthurst (ed ) Direct and Indirect
Effects of Acid Deposition on Vegetation.
Proceedings of ACS Symposium. (In
Press). Ann Arbor Science, Publishers,
Ann Arbor, Michigan.
Orie L. Loucks is with the Institute of Ecology, Indianapolis, IN 46208.
Gary E. Glass is the EPA Project Officer (see below).
The complete report, entitled "Watershed Sensitivity Measurement Strategy for
Identifying Resources at Risk from Acidic Deposition," (Order No. PB 84-141
209; Cost: $11.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
6201 Congdon Blvd.
Duluth, MC 55804
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