This addendum is intended to pr ovide 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 cate gories. For the purposes here, we use the DL %ECO . This topic is

introduced in Section 5.5 and is expanded on here for th e four aggregation scenario s (one population, sensitive

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

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

ANC 50 μ eq/L calculated to protect 90%, 75% and 50% of waterbodies unde r 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 sel ected DL. Thus while they would not ach ieve an ANC of 50, they will see some

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

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

ONE POPULATION

This aggregation method looks at the entire countr y as one population of wate rbodies and would allow a

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

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



### Table 5-12. Comparison of percentage protection from ANC values less than 50 μ eq/L 

### and less

than 20 μ eq/L 

### using DL that result when the US is considered one population.

DL

(meq/m 2 

### /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 56Table 5-12 shows a comparison between the percent of waterbodies that w ould be protected from

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

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

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

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

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

meq/m 2 /yr) would likely protect 97% of all waterbodies from having an ANC<20 μ eq/L. If the 75% DL %ECO

` values for a target ANC of 50 μ eq/L was chosen (55 meq/m 2 /yr), 83% of waterbodies would likely be protected

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

meq/m 2 /yr) only 56% of waterbodies would likel y be protected against an ANC <20 μ eq/L. This is an

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

Figures A-1 through A-3 below show ma ps of those waterbodies with cri tical loads for a target ANC of

50 μ eq/L that are less than the DL %ECO ` values for a target ANC of 50 μ eq/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 no t be protected from ANC <50 μ eq/L. The intent

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

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

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

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

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

20-50, and less than 20 μ eq/L) under the one population approach.Figure A-1 Map of waterbodies with critical loads for an ANC limit of 50μeq/L that are less than the DL%ECO`values for the target ANC of50μeq/L calculated to protect 90% of the population. The US is considered one population.The dots indicate the 10% of the population thatwould not likely achieve an ANC of 50μeq/L. Given that the country would meetthe DL established for a target ANC of 50μeq/L, the reddots show waterbodies that would not likely be protected from an ANC of <20μeq/L and blue dots show waterbodies that would likely beprotected from ANC <20μeq/L.Figure A-2 Map of waterbodies with critical loads for an ANC limit of 50μeq/L that are less than the DL%ECO`values for the target ANC of 50μeq/L calculated to protect 75% of the population. The US is considered one population. The dotsindicate the 25% of the population thatwould not likely achieve an ANC of 50μeq/L. Given that the country would meetthe DL established for a target ANC of 50μeq/L, the reddots show waterbodies that would not likely be protected from an ANC of <20μeq/L and blue dots show waterbodies that would likely beprotected from ANC <20μeq/L.Figure A-3 Map of waterbodies with critical loads for an ANC limit of 50μeq/L that are less than the DL%ECO`values for the target ANC of 50μeq/L calculated to protect 50% of the population. The US is considered one population. The dotsindicate the 50% of the population that wouldnot likely achieve an ANC of 50μeq/L. Given that the country would meetthe DL established for a target ANC of 50μeq/L, the red dots showwaterbodies that would not likelybe protected froman ANC of <20μeq/L and blue dots show waterbodiesthat would likely be protected fromANC <20μeq/L.

### One Population0 20 40 60 80 100



### 90% 75% 50% ANC 50

ANC 20-50

ANC <20

Figure A-4. Expected ANC values resul ting from a target ANC of 50 μ eq/L for specific target percentages of

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

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

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

ANC 20 μ eq/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 Chap ter 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 pr otection from ANC values less than 50 μ eq/L and less than 20 μ eq/L

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

DL

(meq/m 2 /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 w ould be protected from

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

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

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

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

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

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

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

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

would likely protect 92% of less sensi tive waterbodies from having an ANC<20 μ eq/L. If the 75% DL %ECO

` values for a target ANC of 50 μ eq/L was chosen, 77% of less sensitiv e waterbodies would likely be protected

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

less sensitive waterbodies would likel y be protected against an ANC <20 μ eq/L.

Figures A-5 through A-10 below show maps of those wa terbodies with critical lo ads for a target ANC of

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

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

shown on the map represent the 10%, 25% and 50% of waterbodies that w ould not be protec ted from ANC <50

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

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

with blue and red dots repres enting those waterbodies with ≥ ANC20 μ eq/L and <ANC 20 μ eq/L respectively.

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

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

waterbodies at each ANC leve l (50, 20-50, and less than 20 μ eq/L) under the two category approach based on

sensitivity.Figure A-5 Map of sensitive waterbodies withcritical loads for an ANC limit of 50μeq/L that are less than the DL%ECO`values for thetarget ANC of 50μeq/L calculated to protect 90% of the population. The US is divided into sensitive and less sensitive categoriesbased on ANC values. The dots indicate the 10% of the population that would not likely achieve an ANC 50μeq/L. Given that thecountry would meet the DL established for a target ANC of 50μeq/L, the red dots show waterbodiesthat would not likely be protectedfrom an ANC of <20μeq/L and blue dots show waterbodies thatwould likely be protected from ANC <20.Figure A-6 Map of sensitive waterbodies withcritical loads for an ANC limit of 50μeq/L that are less than the DL%ECO`values for thetarget ANC of 50μeq/L calculated to protect 75% of the population. The US is divided into sensitive and less sensitive categoriesbased on ANC values. The dots indicate the 25% of the population that would notlikely achieve an ANC 50μeq/L. Given that thecountry would meet the DL established for a target ANC of 50μeq/L, the red dots show waterbodiesthat would not likely be protectedfrom an ANC of <20μeq/L and blue dots show waterbodies thatwould likely be protected from ANC <20.Figure A-7 Map of sensitive waterbodies withcritical loads for an ANC limit of 50μeq/L that are less than the DL%ECO`values for thetarget ANC of 50μeq/L calculated to protect 50% of the population. The US is divided into sensitive and less sensitive categoriesbased on ANC values. The dots indicate the 50% of the population that would notlikely achieve an ANC 50μeq/L. Given that thecountry would meet the DL established for a target ANC of 50μeq/L, the red dots show waterbodiesthat would not likely be protectedfrom an ANC of <20μeq/L and blue dots show waterbodies thatwould likely be protected from ANC <20.Figure A-8 Map of less sensitive waterbodies with critical loads for an ANC limit of 50μeq/L that are less than the DL%ECO`values forthe target ANC of 50μeq/L calculated to protect 90% of the population. The US is divided into sensitive and less sensitive categoriesbased on ANC values. The dots indicate the 10% of the population that would not likely achieve an ANC 50μeq/L. Given that thecountry would meet the DL established for a target ANC of 50μeq/L, the red dots show waterbodiesthat would not likely be protectedfrom an ANC of <20μeq/L and blue dots show waterbodies thatwould likely be protected from ANC <20.Figure A-9 Map of less sensitive waterbodies with critical loads for an ANC limit of 50μeq/L that are less than the DL%ECO`values forthe target ANC of 50μeq/L calculated to protect 75% ofthe population. The US is divided into sensitive and less sensitive categoriesbased on ANC values. The dots indicate the 25% of the population that would notlikely achieve an ANC 50μeq/L. Given that thecountry would meet the DL established for a target ANC of 50μeq/L, the red dots show waterbodiesthat would not likely be protectedfrom an ANC of <20μeq/L and blue dots show waterbodies thatwould likely be protected from ANC <20.Figure A-10 Map of less sensitive waterbodies with critical loads for an ANC limit of 50μeq/L that are less than the DL%ECO`valuesfor the target ANC of 50μeq/L calculated to protect 50% ofthe population. The US is divided into sensitive and less sensitivecategories based on ANC values. The dots indicate the 50%of the population that would not likely achieve an ANC 50μeq/L. Giventhat the country would meet the DLestablished for a target ANC of 50μeq/L, the red dots show waterbodies that would not likely beprotected from an ANC of <20μeq/L and blue dots show waterbodies thatwould likely be protected from ANC <20.Sensitive0 20 40 60 80 100

Less Sensitive0 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 μ eq/L for specific target percentages of

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

ECOREGIONS

This aggregation method divides th e country into Omernick Ecoregi on 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 biologica lly 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 largevariations 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 w ould be protected from an ANC<50 μ eq/L using 90%, 75% and 50% DL %ECO

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

not protected from an ANC <50 μ eq/L would likely be pr otected from an ANC<20 μ eq/L.

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

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

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

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

percentage of those remaining waterbodi es, while not protected from ANC <50 μ eq/L would be protected from

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

those waterbodies with ≥ ANC20 μ eq/L and <ANC 20 μ eq/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 μ eq/L) under the

ecoregion approach using mean values for all ecoregions.Table A-1 Comparison ofpercentage protection from ANC values less than 50μeq/Land less than20μeq/Lusing DL that result when ecoregions with greater than 50 observations are used.EcoregionEcoregionWaterbodiesN=% ProtectedANC 50 90%Total % Sitesprotected fromANC<20Using ANC 5090% DL% ProtectedANC 50 75%Total % Sitesprotectedfrom ANC<20Using ANC50 75% DL%ProtectedANC 5050%Total % Sitesprotected fromANC<20Using ANC 5050% DL5.3.173590%98%75%87%50%60%8.4.151090%97%75%86%50%56%5.2.146990%100%75%91%50%56%8.4.437990%100%75%92%50%69%6.2.1020291%98%76%84%50%53%8.4.220091%100%76%99%51%63%6.2.1418690%96%76%85%51%57%8.1.716991%95%76%83%51%62%8.3.416890%96%76%85%51%57%5.3.315991%99%75%86%50%62%8.1.814391%97%76%90%50%63%6.2.513391%98%76%86%51%56%6.2.910891%92%76%78%51%52%6.2.1210591%100%76%96%51%57%6.2.78891%95%76%88%51%63%8.1.38891%95%76%81%51%55%6.2.118691%92%76%76%51%53%6.2.158691%100%76%91%51%65%6.2.137891%96%76%90%51%64%8.3.57592%100%76%87%51%59%6.2.37392%97%77%78%52%55%7.1.86891%94%76%78%51%56%8.5.35392%100%77%87%53%57%13.1.15292%92%77%79%52%52%0 200 400 60040 50 60 70 80 90 100

Number of SitesPercent at ANC 50

90%

75%

50%

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

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

to the 50%, 75% and 90% targ et levels. The vertical gr ay line shows the cutoff leve l of n = 50 which was used

for analyses.Figure A-13 Map of waterbodiesby ecoregion with critical loads for an ANC limit of 50μeq/L that are less than the DL%ECO`values forthe target ANC of 50μeq/L calculated to protect 90% of the population. The US is divided into level 3 ecoregions. Thedots indicate the10% of the population that wouldnot likely achieve an ANC of 50μeq/L. Given that each ecoregion would meet the DL established for atarget ANC of 50μeq/L, the red dots show waterbodies that would not likely beprotected from an ANC of <20μeq/L and blue dots showwaterbodies that would likelybe protected from ANC <20μeq/L.Figure A-14 Map of waterbodiesby ecoregion with critical loads for an ANC limit of 50μeq/L that are less than the DL%ECO`values forthe target ANC of 50μeq/L calculated to protect 75% of thepopulation. The US is divided into level 3 ecoregions. The dots indicate the25% of the population that wouldnot likely achieve an ANC of 50μeq/L. Given that each ecoregion would meet the DL established for atarget ANC of 50μeq/L, the red dots show waterbodies that would not likely beprotected from an ANC of <20μeq/L and blue dots showwaterbodies that would likelybe protected from ANC <20μeq/L.Figure A-15 Map of waterbodiesby ecoregion with critical loads for an ANC limit of 50μeq/L that are less than the DL%ECO`values forthe target ANC of 50μeq/L calculated to protect 50% of thepopulation. The US is divided into level 3 ecoregions. The dots indicate the50% of the population that wouldnot likely achieve an ANC of 50μeq/L. Given that each ecoregion would meet the DL established for atarget ANC of 50μeq/L, the red dots show waterbodies that would not likely beprotected from an ANC of <20μeq/L and blue dots showwaterbodies that would likelybe protected from ANC <20μeq/L.

### Eco-Regions0 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 μ eq/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 μ eq/L (yellow) bars represent the upper c onfidence 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 wa terbodies that would be

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

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

ANC<20 μ eq/L.

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

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

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

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

those remaining waterbodies, whil e not protected from ANC <50 μ eq/L would be protected from ANC <20

μ eq/L under each DL %ECO ` scenario. This is shown on the maps w ith blue and red dots representing those

waterbodies with ≥ ANC20 μ eq/L and <ANC 20 μ eq/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 μ eq/L) under the cl uster approach

using mean values for all clusters.



### Table A-2 Comparison of pe rcentage protection from ANC values less that 50 μ eq/L 

### and

less than 20 μ eq/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 50μeq/L that are less than the DL%ECO`values for the taregetANC of 50μeq/L calculated to protect 90% of the population using the cluster method. The US isdivided into 5 clusters (see Chapter 5).The dots indicate the 10% of the population thatwould not likely achieve an ANC of 50μeq/L. Given that each cluster would meet the DLestablished for a target ANC of 50μeq/L, the red dots show waterbodies that would not likely be protected from anANC of <20 and bluedots show waterbodies that wouldlikely be protected from ANC <20.Figure A-18 Map of waterbodies with critical loads for an ANC limit of 50μeq/L that are less than the DL%ECO`values for the taregetANC of 50μeq/L calculated to protect 75% of the population using the cluster method. The US is divided into 5 clusters (see Chapter 5).The dots indicate the 25% of the population thatwould not likely achieve an ANC of 50μeq/L. Given that each cluster would meet the DLestablished for a target ANC of 50μeq/L, the red dots show waterbodies that would not likely be protected from anANC of <20 and bluedots show waterbodies that wouldlikely be protected from ANC <20.Figure A-19 Map of waterbodies with critical loads for an ANC limit of 50μeq/L that are less than the DL%ECO`values for the taregetANC of 50μeq/L calculated to protect 50% of the population using the cluster method. The US is divided into 5 clusters (see Chapter 5).The dots indicate the 50% of the population thatwould not likely achieve an ANC of 50μeq/L. Given that each cluster would meet the DLestablished for a target ANC of 50μeq/L, the red dots show waterbodies that would not likely be protected from anANC of <20 and bluedots show waterbodies that wouldlikely be protected from ANC <20.

### Clusters0 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 μ eq/L for specific target percentages of

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

ANC 20 μ eq/L (yellow) bars represent the upper conf idence interval of the mean values.