United States
                    Environmental Protection
Environmental Research
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
                     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

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.

  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

  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
  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

                            Natural Sources
                                                     Fossil Fuel Emissions
                                                    CO2, SO,, /VO,, M.C.O,
                       Flux to Watersheds
                       hT. SO2', NO~3, NH\
                                                    Atmospheric Reactions
                 Flux to Lakes
I     Highly Buffered
              Soluble Nutrient
          Ions Ca2+, Mg*\ Na\ K*.
             P0l~, SOf, HCOl
    Elevated H*. Metals,
     and NOl Levels in
     To Fish and
    Aquatic Biota
                             Soluble Toxic
                          Ions H\ AI3\ Pb2\
                                                           V   )
I   Acid  I
i  Lakes  '
                                                                        I    Poorly
                                                                        i   Buffered
           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
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

   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
Little Moose Lake. N Y
Sagamore Lake, N Y.
Harp Lake Inlet, Ont.
(No 3)
Harp Lake Inlet, Ont
(No. 4)
Filson Creek, MN
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)
6.8 (4 8)
6.4 (4.9)
6. 1 (5. 1)
6.4 (5.4)
Annual SOl
(wet only)
   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

   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
   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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