189400-24-L 

NASA PROGRES5 REPORT 

SPATIAL CHARACTERIZATION OF ACID RAIN STRESS 

IN CANADIAN SHIELD LAKES 

NASA Contract: NAS5-28779 

Principal Investigator: Fred J. Tanis 

Reporting Period: August 1, 1986 to February 1, 1987 

1 .O OBJECTIVE 

The acidification of lake waters from airborne pollutant5 is of 

continental proportions both in North America and Europe. A major 

concern of the acid rain problem is the cumulative ecosystem damage to 

lakes and forest. The number of lakes affected in northeastern United 

States and on the Canadian Shield is thought to be enormous. Our principal research objective is to examine how seasonal changes in lake 

transparency are related to annual acidic load. Further, the relationship between variations in lake acidification and ecophysical units is 

being examined. Finally the utility of Thematic Mapper (TM) based 

observations to measure seasonal changes in the optical transparency in 

acid lakes is being investigated. 

Previous investigations have suggested that dissolved organic 

carbon (DOC), which originates from the dissolution of humic substances, 

controls transparency in most Canadian Shield Lakes. It has also been 

established that aluminum, which is abundant in the local rocks and 

soi Is, is easily mobilized by acidic components contained in spring 

runoff. The presence of any significant amount of aluminum induces a 

loss of DOC from the water column by coagulation, resulting in increased 

(0 a2 DO 
optical transparency. This process has not been observed in normal lakes 

associated with buffered geologies. In a normal lake, transparency 

would tend to decrease in time with seasonal phytoplankton productivity 

cycles. Thus seasonal changes in “he optical transparency of lakes 

should potentially provide an indication of the stress due to acid 

deposition and loading. 

The potential for this optical response is related to a number of 

local ecophysical factors with bedrock geology being, perhaps, the most 

important. Other important factors include sulfate deposition, veyetative cover, and terrain drainage/relief. The area of southern Ontario 

under study contains a wide variety of geologies from the most acid r*ain 

sensitive granite quartzite types to the least sensitive limestone dolomite sediments. Annual sulfate deposition ranges from 1.0 to 4.0 grams 

per square meter. 

2.0 APPROACH 

Water quality parameters are being measured along with insitu optical data in representative lakes in the Canadian Shield. This is being 

done to calibrate a Bio-Optical Model which defines the linkages between 

the acid rain induced chemical lake processes and the upwelling radiance 

sensed by the Thematic Mapper sensor on Landsat. A spring/surnrner scene 

pair with companion field measurements is being collected in selected 

study sites located in northern Ontario. These data will be used to 

investigate possible forniulations of the mu1 titemporal remote sensing 

causal relationships between pH and observed changes in water 

transparency. 

It is hypothesized that a verifiable relationship exists between 

seasonal changes in water quality associated with the level of lake 

acidification and Thematic Mapper radiometry. The verified Bio-Optical 

Model will be used to establish the limits for which such relationships 

2 
are inherently valid, and together with the field data, the set of ecophysical units and water qual i ty condi tioris where the Laridsat approach 

is valid. Under these restrictions lakes within an ecophysical stratum 

will be assigned a value for the degree of acidification based upon the 

TM multitemporal relationship. These results will permit one to test 

the hypothesis that the severity of lake acidification is not uniform 

over large areas, but rather that variations exist which are strongly 

related to ecophysical units and proximity to probable sources of 

atmospheric inputs. 

3.0 ACTIVITIES BY TASK 

3.1 STRATIFICATION OF ECOPHYSICAL PARAMETERS (TASK 1) 

The Canadian Shield area covered by three Landsat TM scenes has 

been stratified into ecophysical units based upon soil/bedrock' sensitivity, vegetative cover, terrain/drainage, and acid deposition. The 

objective of the stratification is twofold. First, it is intended to 

reveal the location, status and co-occurrence of environmental attributes which influence lake acidification. Second, it provides a basis 

to characterize each lake within the study areas as an aid to the 

sampling design. 

A total of 694 ecophysical units have been mapped in the three 

Landsat scenes which comprise the northern Ontario study area (Figure 

1). Maximum likelihood cluster analysis was used to identify ten 

st at i st i call y si gn i f i cant The mean aci d i f i cat i on 

sensitivity rating of these groups ranged from 3.5 to 7.8. The field 

sampling program involved gathering data from nine of the ten 

ecophysical types. 

ecophys i cal types. 

The stratification procedure itself cannot reveal the physical 

significance of the co-occurring ecophysical attributes deemed iiiiportant 

to lake acidification. The physical significance is established by 
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Kilometers 

FIGURE 1. LOCATION OF LANDSAT TM STUDY SITES 

4 
examining the relationship between the ecophysical strata and the water 

quality (WQ) variables. An analysis was performed to test the hypothesis that the mean WQ parameter was significantly different between pairs 

of strata. Results were compiled1 at the 5% significance level and 

suggest that DOC, pH, sulfate, and aluminum are related to the level of 

ecophysical acidification sensitivity. This analysis is summarized in 

Table 1 and suggests that the most acid sensitive strata (5,7,9) have 

lake waters with lower DOC and pH values and a higher aluminum content 

than those with less sensitivity (1,2,3,4). Thus the clustering analysis appears to have produced strata with lake WQ conditions which are 

characteristic acid sensitive lakes. The most important optical connection is with DOC, which provides a basis for a remote sensing 

identification. 

3.2 SITE SELECTION (TASK 2) 

Site selection was based upon the stratification and clustering 

analysis described above and each of the following considerations: (1) 

availability of historical water quality and remote sensing data, (2) 

existing Canadian initiatives to collect site specific data, (3) site 

accessibi 1 i ty, and (4) coverage of ecophysical lake types. Sites 

selected included (1) Algoma, (2) Sudbury, (3) Wawa, and (4) Dorset. 

During August 1986 field data were gathered from each of these 

sites which included 21 WQ parameters (296 lakes), detailed subsurface 

optical measurements (12 lakes), airborne spectral radiometer measurements (102 lakes), and Landsat data from sites 1, 2, and 3. Most of 

these measurements were made in the Algoma and Sudbury sites which are 

shown as Figures 2 and 3. Water quality data were collected from a 

large number of lakes by the Ministry of Environment. PROBAR spectral 

radiometer measurements were made in most of these lakes which were 

larger than 20 hectares and which were found to be suitable for a he1 icopter measurement. The subsurface optical measurements were made in a 

representative set of lakes at each site. 

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Figure 2. LAKES SAMPLED WITHIN THE SUDBURY STUDY AREA 

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Localioii Map 

Legend: Collection Site for Liniiiological Parameters 

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MER ~ata Collection Site 

Figure3. LAKES SAMPLED WITHIN THE ALGOMA STUDY AREA 

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3.3 LIAISON ACTIVITIES WITH THE CANADIANS (TASK 3) 

A cooperative program with Canadian agencies and universities 

interested in the remote sensing aspects of the acid rain problem have 

resulted in an informal joint program which includes four major Canadian 

participants. These are Professor Roger Pitblado of Laurentian University Sudbury, Ontario, Dr. John Forestcue of the Ontario Geological 

Survey (OGS), Dr. Vernon Singroy of the Ontario Centre for Remote 

Sensing (OCRS), and Professor Michael Dickman from Brock University, 

Saint Catherines, Ontario. The Canadians are funded through The 

Ministry of Environment (MOE) and the Ontario Geological Survey. 

3.4 DEFINITION OF MULTITEMPORAL RADIOMETRIC RELATIONSHIPS TO 

ACIDIFICATION (TASK 4) 

In this task a TM radiative transfer model will be calibrated to 

predict possible multitemporal changes in signal level which result from 

field measured changes in optical and chemical properties. Work has 

proceeded on this model to include specific calibration for the Landsat 

TM sensor. The model treats atmospheric optics, water optics, and the 

wind ruffled air-water interface. Calculations made for typical atmospheric conditions suggest that TM band one (450-520nm) sensitivity to 

changes in the upwelling radiance is 0.06024 mW/cm2.sr.nm/DN (digital 

number). If we assume the downwelling irradiance is 100 mW/cm2.nm count 

at the water surface than the surface reflected signal will change by 

5.5 counts per one percent change in surface reflectance. A maximum of 

2.86 counts change will be observed for a one percent change in subsurface reflectance. The standard deviation of the mean extracted signal 

is typically 1.0 to 1.5 counts in TM band one but was in some instances 

as large as two or three counts. The hypothesized seasonal changes in 

TM DN count levels which result from changes in lake DOC concentrations 

are presently unknown. The projected TM sensitivity suggests that TM 

band one will not be sensitive to subsurface reflectance changes which 

9 
are less than 0.5 percent. More realistically, one or two percent 

changes in reflectance will be necessary to see a seasonal change in the 

TM DN value. 

3.5 DATA PROCESSING (TASK 5) 

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Thus far the extraction software has been applied to the following 

TM scenes; 

Path 22, Row 27 of 27 May 1985, and 18 August 1986, and 

Path 19, Row 27 of 22 May 1985, and 13 August 1986. 

Brightness values were extracted from the approximate center of 

each lake based upon the latitude and longitude of each lake center. A 

3 by 3 pixel area was extracted and the mean brightness in each band was 

recorded. The standard deviation of each sample was also retained which 

had a typical value of one DN count in TM band one. These extracted 

mean values were then correlated to water chemistry data collected 

during August 1986. 

The MER-1000 subsurface upwell ing and downwell ing spectral irradiance data were collected in the field at variable sampling depths below 

the lake water surface. These data were first used to interpolate the 

irradiance data to common depths on a logarithmic scale before computing 

values of subsurface reflectance. The slope of the depth log-irradiance 

regression equation defines the average irradiance attenuation coefficient (K). the mixed layer was easily determined from 

the temperature depth profile. The irradiance attenuation coefficient 

changes very little within the mixed layer, but rapidly within the transition zone (thermocline) . We therefore used only irradiance measurements from the mixed layer to make the K-value determination. Subsurface spectral reflectance was calculated at 2,4,6, and 8 meters below 

the surface. Example reflectance curves are shown in Figure 4, along 

with the DOC and Chlorophyll-a measurements. The impact of DOC and 

The thickness of 

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Chlorophyll-a on reflectance is apparent. As DOC increases the bluegreen portion of the reflectance spectrum is diminished due to the high 

selective absorption. Chlorophyll-a also diminishes the measured 

reflectance below 520 nm, due to absorption. At wavelengths greater 

than 520 nm absorption is reduced and backscattering increases. The 

reflectance calculations at 700 nm are not considered valid since the 

irradiances are very small and contaminated by sensor noise. 

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DETERMINATION RELA 

In this task the 

ity variables, opt 

ometry with lake ac 

Sudbury site showed 

P(Ho)=.0007) as well as 

IONSHIPS WITH FIELD DATA (TASK 6) 

relationships that exist between lake water 

cal measurements, and seasonal changes in TM 

dification are being determined. WQ data from 

good correlation between pH and DOC (rho=.64, 

DOC and Chlorophyll-a (0.92, .0001). Correlation 

of pH with DOC is the greatest for DOC values less than 3.0 mg/l, as 

shown in Figure 5. Also shown in the figure are the data for Algoma, 

which shows virtually no correlation. The extracted TM band one DN 

counts exhibit parallel relationships with DOC. Significant correlations between DN counts were found to be -.623 for DOC; -.371 for pH; 

and -.419 for Chlorophyll-a for the Sudbury data, while no correlations 

were evident with the Algoma data. 

3.7 SPATIAL ASPECTS OF ACIDIFICATION (TASK 7) 

An analysis will be performed to interpret the spatial aspects of 

lake acidification. In this task each lake within an ecophysical 

stratum will be assigned a value based upon a TM multitemporal acidification relationship. These values will be used to assess the significance of atmospheric inputs, and the level of acidification, in relation 

to ecophysical parameters. Currently no work is being performed on this 

task. 

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4.0 TECHNICAL PROBLEMS 

The time needed to complete the scheduled data collection during 

May-June 1987 and analyze the reqlrested TM coincident coverage will 

necessitate a six month no cost time extension to complete this applications research. 

5.0 PLANS FOR THE NEXT REPORTING PERIOD 

A second field trip will begin 1 May and last through 30 June 1987 

to collect the spring portion of the needed spring/summer data pair. 

This trip is envisioned to be structured differently than the first trip 

made last August. Instead of a single visit to many lakes as was the 

case for August 1986, we will make several visits to the same lakes in 

order to observe the expected DOC induced reflectance changes which are 

thought to occur during the spring period. Field methods used during 

the August trip involving use of the Biospherical MER-1000 submersible 

radiometer have been improved upon and it is expected that eight to 

twelve lakes can be covered in the same day. During the previous August 

1986 field work subsurface measurements were gathered usually from only 

a single lake per day. collection of field data will begin 

during the first part of the next period. Field data will be reduced 

after the field trip and data will be exchanged with the Canadian 

investigators. Staff at the MOE will analyze water samples for a 

complete profile of WQ parameters. 

Initial