This addendum is intended to provide additional information on the implications of alternative choices 

of the percent of waterbodies to target when establishing a depositional load (DL%ECO) for a target ANC, either 

for the whole country or for acid-sensitivity categories. For the purposes here, we use the DL%ECO. This topic is 

introduced in Section 5.5 and is expanded on here for the four aggregation scenarios (one population, sensitive 

and less sensitive categories, ecoregions, and a cluster method based on log ANC), using a target ANC of 50 

μeq/L and three example percentiles (90%, 75%, and 50%). These percentiles represent the DL%ECO `values for 

ANC 50 μeq/L calculated to protect 90%, 75% and 50% of waterbodies under each aggregation scenario. 

While those waterbodies with CL less than the selected DL%ECO would not likely receive the same degree of 

protection as the targeted waterbodies, they would likely receive some benefit from the reductions in deposition 

necessary to meet the selected DL. Thus while they would not achieve an ANC of 50, they will see some 

improvement in the ANC. This section focuses on the expected level of protection that would be afforded these 

waterbodies with CL less than the DL%ECO for ANC 50 μeq/L in each aggregation scenario. 

ONE POPULATION 

This aggregation method looks at the entire country as one population of waterbodies and would allow a 

single DL%ECO for the whole population. The following table is identical to the table presented in Chapter 5 as 

Table 5-12. It is included here to provide detail for the figures A-1 through A-4. 

Table 5-12. Comparison of percentage protection from ANC values less than 50 μeq/L and less 

than 20 μeq/L using DL that result when the US is considered one population. 

DL 

(meq/m2

/yr) 

Total 

number 

of Sites 

in 

Analysis 

Total 

Number of 

Sites 

protected 

from ANC 

<50 

Total % 

Sites 

protected 

from ANC 

<50 

Total 

Number of 

Sites 

protected 

from ANC 

<20 

Total % Sites 

protected from 

ANC <20 

DL 

90% 

ANC 50 

27 5280 4778 90 5145 97 

DL 

75% 

ANC 50 

55 5280 3973 75 4394 83 

DL 

50% 

ANC 50 

118 5280 2654 50 2947 56 
Table 5-12 shows a comparison between the percent of waterbodies that would be protected from 

ANC<50 μeq/L using 90%, 75% and 50% DL%ECO `values for a target ANC of 50 μeq/L for the one population 

approach (see Chapter 5 for discussion of one population aggregation method) and those that, while not 

protected from an ANC <50 μeq/L would be protected from at least an ANC<20 μeq/L under the DL%ECO

`values for a target ANC of 50 μeq/L. These waterbodies would have an ANC between 20 and 50 μeq/L. The 

selection of the DL%ECO `values for a target ANC of 50 μeq/L representing 90% of the waterbodies (27 

meq/m2/yr) would likely protect 97% of all waterbodies from having an ANC<20 μeq/L. If the 75% DL%ECO

`values for a target ANC of 50 μeq/L was chosen (55 meq/m2

/yr), 83% of waterbodies would likely be protected 

from an ANC<20 μeq/L and if the 50% DL%ECO `values for a target ANC of 50 μeq/L was chosen (118 

meq/m2/yr) only 56% of waterbodies would likely be protected against an ANC <20 μeq/L. This is an 

important distinction as severe degradation is likely to occur in lakes and streams with ANC<20 μeq/L. 

Figures A-1 through A-3 below show maps of those waterbodies with critical loads for a target ANC of 

50 μeq/L that are less than the DL%ECO `values for a target ANC of 50 μeq/L calculated to protect 90%, 75% and 

50% of the population when the US is considered one population. Thus, the waterbodies shown on the map 

represent the 10%, 25% and 50% of waterbodies that would not be protected from ANC <50 μeq/L. The intent 

is to determine what percentage of those remaining waterbodies, while not protected from ANC <50 μeq/L 

would be protected from ANC <20 μeq/L under each DL%ECO scenario. This is shown on the maps with blue 

and red dots representing those waterbodies with ≥ ANC20 μeq/L and <ANC 20 μeq/L respectively. Under each 

scenario, the waterbodies that would likely fall below ANC 20 μeq/L are spread throughout the US and varied 

in type and function. Figure A-4 is a graphical breakdown of the percent of waterbodies at each ANC level (50, 

20-50, and less than 20 μeq/L) under the one population approach. 
Figure A-1 Map of waterbodies with critical loads for an ANC limit of 50 μeq/L that are less than the DL%ECO `valu

50 μeq/L calculated to protect 90% of the population. The US is considered one population. The dots indicate the 

would not likely achieve an ANC of 50 μeq/L. Given that the country would meet the DL established for a target Adots show waterbodies that would not likely be protected from an ANC of <20 μeq/L and blue dots show waterbodprotected from ANC <20 μeq/L. 
Figure A-2 Map of waterbodies with critical loads for an ANC limit of 50 μeq/L that are less than the DL%ECO `value

μeq/L calculated to protect 75% of the population. The US is considered one population. The dots indicate the 25% would not likely achieve an ANC of 50 μeq/L. Given that the country would meet the DL established for a target ANdots show waterbodies that would not likely be protected from an ANC of <20 μeq/L and blue dots show waterbodieprotected from ANC <20 μeq/L. 
Figure A-3 Map of waterbodies with critical loads for an ANC limit of 50 μeq/L that are less than the DL%ECO `value

μeq/L calculated to protect 50% of the population. The US is considered one population. The dots indicate the 50% not likely achieve an ANC of 50 μeq/L. Given that the country would meet the DL established for a target ANC of 5

waterbodies that would not likely be protected from an ANC of <20 μeq/L and blue dots show waterbodies that woulANC <20 μeq/L. 
One Population

0 20 40 60 80 100

90% 75% 50%

ANC 50

ANC 20-50

ANC <20

Figure A-4. Expected ANC values resulting from a target ANC of 50 μeq/L for specific target percentages of 

waterbodies. Blue represents the percent of lakes expected meet or exceed the ANC 50 μeq/L and by definition 

is equal to the target percentage indicated on the x-axis. Yellow represents the additional percentage of lakes 

expected to exceed the ANC 20 μeq/L. The remaining lakes, indicated in red, would potentially be below the 

ANC 20 μeq/L level. 
SENSITIVE AND LESS SENSITIVE CATEGORIES 

This aggregation method divides the country into two categories based on sensitivity (further discussion 

of this method can be found in Chapter 5) and would allow two DL%ECO values for the whole population. The 

following table is identical to the table presented in Chapter 5 as Table 5-13. It is included here to provide 

detail for the figures A-5 through A-11. 

Table 5-13. Comparison of percentage protection from ANC values less than 50 μeq/L and less than 20 μeq/L 

using DL that result when the US is divided into two categories, sensitive and less sensitive based on ANC data. 

DL 

(meq/m2/yr) 

Total 

number of 

Sites in 

Analysis 

Total 

Number of 

Sites 

protected 

from ANC 

<50 

Total % Sites 

protected 

from ANC 

<50 

Total Number 

of Sites 

protected 

from ANC 

<20 

Total % 

Sites 

protected 

from 

ANC <20 

DL 

90% 

ANC 

50 

26 4553 4104 90 4451 98 

DL 

75% 

ANC 

50 

51 4553 3428 75 3841 84 

Sensitive 

DL 

50% 

ANC 

50 

106 4553 2284 50 2575 57 

DL 

90% 

ANC 

50 

53 727 655 90 672 92 

DL 

75% 

ANC 

50 

117 727 546 75 560 77 

Less 

sensitive 

DL 

50% 

ANC 

50 

277 727 364 50 377 52 

Table 5-13 shows a comparison between the percent of waterbodies that would be protected from 

ANC<50 μeq/L using 90%, 75% and 50% DL%ECO `values for a target ANC of 50 μeq/L for the two category 

approach and those that, while not protected from an ANC <50 μeq/L would be protected from at least an 

ANC<20 μeq/L under the DL%ECO values for a target ANC of 50 μeq/L. These waterbodies would have an 

ANC between 20 and 50 μeq/L. The selection of the DL%ECO `values for a target ANC of 50 μeq/L representing 

90% of the sensitive waterbodies would likely protect 98% of sensitive waterbodies from having an ANC<20 
μeq/L. If the 75% DL%ECO `values for a target ANC of 50 μeq/L was chosen, 84% of sensitive waterbodies 

would likely be protected from an ANC<20 μeq/L and if the 50% DL%ECO `values for a target ANC of 50 μeq/L 

was chosen only 57% of sensitive waterbodies would likely be protected against an ANC <20 μeq/L. The 

selection of the DL%ECO `values for a target ANC of 50 μeq/L representing 90% of the less sensitive waterbodies 

would likely protect 92% of less sensitive waterbodies from having an ANC<20 μeq/L. If the 75% DL%ECO

`values for a target ANC of 50 μeq/L was chosen, 77% of less sensitive waterbodies would likely be protected 

from an ANC<20 μeq/L and if the 50% DL%ECO `values for a target ANC of 50 μeq/L was chosen only 52% of 

less sensitive waterbodies would likely be protected against an ANC <20 μeq/L. 

Figures A-5 through A-10 below show maps of those waterbodies with critical loads for a target ANC of 

50 μeq/L that are less than the DL%ECO `values for a target ANC of 50 μeq/L calculated to protect 90%, 75% and 

50% of the population when the US is considered as two categories based on sensitivity. Thus, the waterbodies 

shown on the map represent the 10%, 25% and 50% of waterbodies that would not be protected from ANC <50 

μeq/L. The intent is to determine what percentage of those remaining waterbodies, while not protected from 

ANC <50 μeq/L would be protected from ANC <20 under each DL%ECO ` scenario. This is shown on the maps 

with blue and red dots representing those waterbodies with ≥ ANC20 μeq/L and <ANC 20 μeq/L respectively. 

Again under each scenario, the waterbodies that would likely fall below ANC 20 μeq/L are geographically 

diverse and likely represent many types of waterbodies. Figure A-11 is a graphical breakdown of the percent of 

waterbodies at each ANC level (50, 20-50, and less than 20 μeq/L) under the two category approach based on 

sensitivity. 
Figure A-5 Map of sensitive waterbodies with critical loads for an ANC limit of 50 μeq/L that are less than 

target ANC of 50 μeq/L calculated to protect 90% of the population. The US is divided into sensitive and lebased on ANC values. The dots indicate the 10% of the population that would not likely achieve an ANC 5country would meet the DL established for a target ANC of 50 μeq/L, the red dots show waterbodies that wfrom an ANC of <20 μeq/L and blue dots show waterbodies that would likely be protected from ANC <20. 
Figure A-6 Map of sensitive waterbodies with critical loads for an ANC limit of 50 μeq/L that are less than the DL

target ANC of 50 μeq/L calculated to protect 75% of the population. The US is divided into sensitive and less sensibased on ANC values. The dots indicate the 25% of the population that would not likely achieve an ANC 50 μeq/Lcountry would meet the DL established for a target ANC of 50 μeq/L, the red dots show waterbodies that would nofrom an ANC of <20 μeq/L and blue dots show waterbodies that would likely be protected from ANC <20. 
Figure A-7 Map of sensitive waterbodies with critical loads for an ANC limit of 50 μeq/L that are less than the D

target ANC of 50 μeq/L calculated to protect 50% of the population. The US is divided into sensitive and less sebased on ANC values. The dots indicate the 50% of the population that would not likely achieve an ANC 50 μecountry would meet the DL established for a target ANC of 50 μeq/L, the red dots show waterbodies that would from an ANC of <20 μeq/L and blue dots show waterbodies that would likely be protected from ANC <20. 
Figure A-8 Map of less sensitive waterbodies with critical loads for an ANC limit of 50 μeq/L that are less than 

the target ANC of 50 μeq/L calculated to protect 90% of the population. The US is divided into sensitive and lebased on ANC values. The dots indicate the 10% of the population that would not likely achieve an ANC 50 μ

country would meet the DL established for a target ANC of 50 μeq/L, the red dots show waterbodies that would from an ANC of <20 μeq/L and blue dots show waterbodies that would likely be protected from ANC <20. 
Figure A-9 Map of less sensitive waterbodies with critical loads for an ANC limit of 50 μeq/L that are less than th

the target ANC of 50 μeq/L calculated to protect 75% of the population. The US is divided into sensitive and less based on ANC values. The dots indicate the 25% of the population that would not likely achieve an ANC 50 μeqcountry would meet the DL established for a target ANC of 50 μeq/L, the red dots show waterbodies that would nfrom an ANC of <20 μeq/L and blue dots show waterbodies that would likely be protected from ANC <20. 
Figure A-10 Map of less sensitive waterbodies with critical loads for an ANC limit of 50 μeq/L that are less tha

for the target ANC of 50 μeq/L calculated to protect 50% of the population. The US is divided into sensitive ancategories based on ANC values. The dots indicate the 50% of the population that would not likely achieve an that the country would meet the DL established for a target ANC of 50 μeq/L, the red dots show waterbodies thprotected from an ANC of <20 μeq/L and blue dots show waterbodies that would likely be protected from ANC 
Sensitive

0 20 40 60 80 100

Less Sensitive

0 20 40 60 80 100

90% 75% 50%

ANC 50

ANC 20-50

ANC <20

Figure A-11. Expected ANC resulting from a target ANC of 50 μeq/L for specific target percentages of 

waterbodies. This classification of lakes categorizes the lakes based on sensitivity. 

ECOREGIONS 

This aggregation method divides the country into Omernick Ecoregion Level 3 categories and would 

allow DL%ECO values for each ecoregion or some combination of ecoregions. This method of aggregation is 

useful in that it provides a biologically relevant grouping of waterbodies but is also data intensive. For this 

example, only ecoregions with greater than 50 observations were included as fewer observations lead to large 
variations in the value of DL%ECO depending on whether this value was calculated by fitted distribution or 

ranking the observations. This is described more fully in Figure A-12. Table A-1 shows a comparison between 

the percent of waterbodies that would be protected from an ANC<50 μeq/L using 90%, 75% and 50% DL%ECO

`values for a target ANC of 50 μeq/L for each ecoregion with greater than 50 observations and those that, while 

not protected from an ANC <50 μeq/L would likely be protected from an ANC<20 μeq/L. 

Figures A-13 through A-15 below show maps of those waterbodies with critical loads for a target ANC 

of 50 μeq/L that are less than the DL%ECO `values for a target ANC of 50 μeq/L calculated to protect 90%, 75% 

and 50% of the population by ecoregion. Thus, the waterbodies shown on the map represent the 10%, 25% and 

50% of waterbodies that would not be protected from ANC <50 μeq/L. The intent is to determine what 

percentage of those remaining waterbodies, while not protected from ANC <50 μeq/L would be protected from 

ANC <20 μeq/L under each DL%ECO ` scenario. This is shown on the maps with blue and red dots representing 

those waterbodies with ≥ ANC20 μeq/L and <ANC 20 μeq/L respectively. Figure A-16 is a graphical 

breakdown of the percent of waterbodies at each ANC level (50, 20-50, and less than 20 μeq/L) under the 

ecoregion approach using mean values for all ecoregions. 
Table A-1 Comparison of percentage protection from ANC values less than 50 μeq/

20μeq/L using DL that result when ecoregions with greater than 50 observations are 

Ecoregion 

Ecoregion Waterbodies N= 

% Protected ANC 50 90% 

Total % Sites protected from ANC<20 Using ANC 50 90% DL 

% Protected ANC 50 75% 

Total % Sites protected from ANC<20 Using ANC 50 75% DL 

% PA5

5.3.1 735 90% 98% 75% 87% 8.4.1 510 90% 97% 75% 86% 5.2.1 469 90% 100% 75% 91% 8.4.4 379 90% 100% 75% 92% 6.2.10 202 91% 98% 76% 84% 8.4.2 200 91% 100% 76% 99% 6.2.14 186 90% 96% 76% 85% 8.1.7 169 91% 95% 76% 83% 8.3.4 168 90% 96% 76% 85% 5.3.3 159 91% 99% 75% 86% 8.1.8 143 91% 97% 76% 90% 6.2.5 133 91% 98% 76% 86% 6.2.9 108 91% 92% 76% 78% 6.2.12 105 91% 100% 76% 96% 6.2.7 88 91% 95% 76% 88% 8.1.3 88 91% 95% 76% 81% 6.2.11 86 91% 92% 76% 76% 6.2.15 86 91% 100% 76% 91% 6.2.13 78 91% 96% 76% 90% 8.3.5 75 92% 100% 76% 87% 6.2.3 73 92% 97% 77% 78% 7.1.8 68 91% 94% 76% 78% 8.5.3 53 92% 100% 77% 87% 13.1.1 52 92% 92% 77% 79% 
0 200 400 600

40 50 60 70 80 90 100

Number of Sites

Percent at ANC 50

90%

75%

50%

Figure A-12. Percent of lakes within an Ecoregion (n = 76) expected to equal or exceed ANC 50 μeq/L 

compared to the number of sites sampled within the Ecoregion. Horizontal gray lines are provided for reference 

to the 50%, 75% and 90% target levels. The vertical gray line shows the cutoff level of n = 50 which was used 

for analyses. 
Figure A-13 Map of waterbodies by ecoregion with critical loads for an ANC limit of 50 μeq/L that are less than th

the target ANC of 50 μeq/L calculated to protect 90% of the population. The US is divided into level 3 ecoregions. 

10% of the population that would not likely achieve an ANC of 50 μeq/L. Given that each ecoregion would meet thtarget ANC of 50 μeq/L, the red dots show waterbodies that would not likely be protected from an ANC of <20 μeqwaterbodies that would likely be protected from ANC <20 μeq/L. 
Figure A-14 Map of waterbodies by ecoregion with critical loads for an ANC limit of 50 μeq/L that are less than th

the target ANC of 50 μeq/L calculated to protect 75% of the population. The US is divided into level 3 ecoregions. 

25% of the population that would not likely achieve an ANC of 50 μeq/L. Given that each ecoregion would meet thtarget ANC of 50 μeq/L, the red dots show waterbodies that would not likely be protected from an ANC of <20 μeqwaterbodies that would likely be protected from ANC <20 μeq/L. 
Figure A-15 Map of waterbodies by ecoregion with critical loads for an ANC limit of 50 μeq/L that are less tha

the target ANC of 50 μeq/L calculated to protect 50% of the population. The US is divided into level 3 ecoregio

50% of the population that would not likely achieve an ANC of 50 μeq/L. Given that each ecoregion would metarget ANC of 50 μeq/L, the red dots show waterbodies that would not likely be protected from an ANC of <20 waterbodies that would likely be protected from ANC <20 μeq/L. 
Eco-Regions

0 20 40 60 80 100

90% 75% 50%

ANC 50

ANC 20-50

ANC <20

Figure A-16. Expected ANC resulting from a target ANC of 50 μeq/L for specific target percentages of 

waterbodies. The yellow bars represent the mean value of all of the ecoregions. The whiskers associated with 

the ANC 20 μeq/L (yellow) bars represent the upper confidence interval of the mean values. 
LOG ANC CLUSTER 

This aggregation method divides the country into statistical clusters based on log ANC values and would 

allow DL%ECO values to be determined for each cluster. This method of aggregation is described more fully in 

Chapter 5 (Section 5.3.2.7). Table A-2 shows a comparison between the percent of waterbodies that would be 

protected from ANC<50 μeq/L using 90%, 75% and 50% DL%ECO `values for a target ANC of 50 μeq/L for 

each cluster and those that, while not protected from an ANC <50 μeq/L would likely be protected from an 

ANC<20 μeq/L. 

Figures A-17 through A-19 below show maps of those waterbodies with critical loads for a target ANC 

of 50 μeq/L that are less than the DL%ECO `values for a target ANC of 50 μeq/L calculated to protect 90%, 75% 

and 50% of the population. Thus, the waterbodies shown on the map represent the 10%, 25% and 50% of 

waterbodies that would not be protected from ANC <50 μeq/L. The intent is to determine what percentage of 

those remaining waterbodies, while not protected from ANC <50 μeq/L would be protected from ANC <20 

μeq/L under each DL%ECO `scenario. This is shown on the maps with blue and red dots representing those 

waterbodies with ≥ ANC20 μeq/L and <ANC 20 μeq/L respectively. Figure A-20 is a graphical breakdown of 

the percent of waterbodies at each ANC level (50, 20-50, and less than 20 μeq/L) under the cluster approach 

using mean values for all clusters.

Table A-2 Comparison of percentage protection from ANC values less that 50 μeq/L and 

less than 20 μeq/L using DL that result when sites are clustered using logANC values. 

Cluster 

Cluster 

N= 

% 

Protected 

ANC 50 

90% 

Total % Sites 

protected from 

ANC <20 

using ANC 50 

90% DL 

% 

Protected 

ANC 50 

75% 

Total % Sites 

protected from 

ANC <20 

using ANC 50 

75% DL 

% 

Protected 

ANC 50 

50% 

Total % Sites 

protected from 

ANC <20 

using ANC 50 

50% DL 

1 2432 90% 98% 75% 85% 50% 58%

2 1113 90% 98% 75% 85% 50% 58%

3 784 90% 95% 75% 81% 50% 54%

4 655 90% 96% 75% 80% 50% 51%

5 216 90% 91% 75% 76% 50% 51%
Figure A-17 Map of waterbodies with critical loads for an ANC limit of 50 μeq/L that are less than the DL%ECO `va

ANC of 50 μeq/L calculated to protect 90% of the population using the cluster method. The US is divided into 5 clThe dots indicate the 10% of the population that would not likely achieve an ANC of 50 μeq/L. Given that each cluestablished for a target ANC of 50 μeq/L, the red dots show waterbodies that would not likely be protected from an dots show waterbodies that would likely be protected from ANC <20. 
Figure A-18 Map of waterbodies with critical loads for an ANC limit of 50 μeq/L that are less than the DL%ECO `v

ANC of 50 μeq/L calculated to protect 75% of the population using the cluster method. The US is divided into 5 The dots indicate the 25% of the population that would not likely achieve an ANC of 50 μeq/L. Given that each cestablished for a target ANC of 50 μeq/L, the red dots show waterbodies that would not likely be protected from dots show waterbodies that would likely be protected from ANC <20. 
Figure A-19 Map of waterbodies with critical loads for an ANC limit of 50 μeq/L that are less than the DL%ECO `va

ANC of 50 μeq/L calculated to protect 50% of the population using the cluster method. The US is divided into 5 clThe dots indicate the 50% of the population that would not likely achieve an ANC of 50 μeq/L. Given that each cluestablished for a target ANC of 50 μeq/L, the red dots show waterbodies that would not likely be protected from an dots show waterbodies that would likely be protected from ANC <20. 
Clusters

0 20 40 60 80 100

90% 75% 50%

ANC 50

ANC 20-50

ANC <20

Figure A-20. Expected ANC resulting from a target ANC of 50 μeq/L for specific target percentages of 

waterbodies. The yellow bars represent the mean value of all of the clusters. The whiskers associated with the 

ANC 20 μeq/L (yellow) bars represent the upper confidence interval of the mean values.