TECHNICAL MEMORANDUM 

EPA-REGION 10 CAPTAINS BAVWASP MODEL: 

RESULTS FOR INCREASED BOD LOADING 

Prepared For: 

Prepared By: 

Date: 

Distribution: 

Summary 

Westward Seafoods, Inc. 

Steve Costa, gde 

David Wilson, CH2M HILL 

11 May 2000 

Dave Boisseau, Westward Seafoods 

Burney Hill, EPA-Region 10 

CH2MHILL 

gde 

This Technical Memorandum presents the approach and results of the Captains Bay 

WASP model simulation using the 1992 data for minimum dissolved oxygen and the 

effluent BOD loading as defined in EPA's April 28, 2000 letter to Westward Seafoods 

(Attachment I). The WASP model parameters specified by EPA as acceptable to the 

agency are very conservative values, and they represent a rather unexpected 

decision by EPA reversing prior agreements. The objective of the modeling was to 

determine the loading capacity and effluent limitations based on EPA's model 

parameters and consistent with the dissolved oxygen (DO) water quality standard of 

5.0 mg/1. 

The Captains Bay WASP model was recalibrated and executed using the same input 

file and procedures used by EPA in the original 1993 formulation. The only change 

in the modeling was that a different loading, as directed by EPA, was used to 

calibrate the model. Therefore, the results obtained are completely consistent with 

the original EPA modeling. 

The results indicate that a wasteload biological oxygen demand (BOD) discharge 

loading increase of 35% is justified, based on the BOD loading and ambient 

dissolved oxygen levels specified by EPA to calibrate the model. This result is 
EPA-REGION 10 ';APTAINS BAVWASP M ODEL: 

RESULTS FOR INCREASED BOD L OADING 

predicated on a DO water quality standard of 5 mg/l as defined for estuarine waters 

by the State of Alaska. 

Background 

In 1993, EPA Region 10 developed a WASP model to investigate and develop the 

allowable BOD loading capacity and effluent limitations for Westward Seafoods 

discharge to Captains Bay. This model has recently been re-examined both in terms 

of formulation and available data for calibration. The final result of this model reexamination effort included the following points: 

• Although the WASP model is intended to deal with average values of DO 

for defined model segments, EPA has directed that the model be 

calibrated using the lowest observed DO in the water column in the 

discharge segment, over a specified seasonal period, as was done for the 

1993 modeling report This essentially results in the model being used as 

a correlation tool (or model) rather than as an actual simulation (or model) 

of physical processes. When the model is calibrated in this fashion it 

cannot be used to represent the physical behavior of the Captains Bay as a 

physical hydrodynamic system. 

• EPA Region 10 has directed that the data to be used for calibration be 

restricted to data collected during the summer of 1992 and that the model 

be executed in a steady-state mode. This precludes any ability to verify or 

validate the model or data set used for calibration using the extensive 

additional water column data available for the 1993-1999 time period. 

• EPA Region 10 has concluded that the appropriate loading to be used for 

calibration is not the loading used by EPA in 1993 but rather the average 

loading over the time period that DO was depressed below the water 

quality standard level of 5.0 mg/] during the summer of 1992. This 

loading was higher than that used in the previous 1993 modeling and 

results in a higher loading capacity and higher effluent limitations than 

were calculated and applied by EPA in 1993. 

The model was run and correlated using the original EPA model formulation as 

described in the 1993 Fact Sheet (Attachment II) for the increased loading sceanario. 

The 1993 Fact Sheet provides the basic description of management objectives, 

technical basis of the model, and uncertainties in the modeling. These factors will 

not be repeated here, and the reader is referred to the fact sheet for those specific 

technical details. 

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EPA-REGION 10 ";APTAINS BAVWASP MODEL: 

RESULTS FOR INCREASED BOD LOADING 

ModeJ Formulation 

The model was run using the original EPA model formulation as described in the 

1993 Fact Sheet (Attachment II). EPA (Attachment I) indicated that "minor 

corrections" should be made to errors in geometry, boundary exchanges, and typos 

found in the original models. However, exactly what changes to make and what 

changes would be unacceptable was not specified. Since the model, as formulated 

by EPA, is basically used as an elaborate correlation scheme, the changes indicated 

do not cause any meaningful changes to the results. Therefore, the model was run 

in its original 1993 EPA formulation. Using that approach there is full consistency 

with the previous modeling done by EPA and there can be no argument about 

whether something was changed that should not have been changed. 

With two minor exceptions, the only changes to the EPA input file other than title 

entries to identify specific model cases, were the loading and the calibration 

coefficients. The exceptions were the elimination of the advective flow in Data 

Group D of the WASP input and the time step for one of the calibration cases 

considered. The justifications for these exceptions are as follows: 

• It was previously demonstrated that the exclusion of the advective flow data 

makes no difference whatsoever in the model DO predictions, within the 

accuracy of the model output (±O.Ol mg/I). Eliminating this data block 

eliminates approximately 13 error messages at runtime, making the model 

more convenient to use, and the results are identical to those obtained if the 

Data Group D are included. 

• For vertical diffusion coefficients, Kv, of greater than about 0.004 m2/sec, the 

EPA coded time step of 0.05 days is too large and the model will not run. 

Therefore a time step of 0.01 was used for some model runs. This will have 

no significant effect except to make the model runtime about five times 

longer. The original time step of 0.05 was used in all simulations except 1 set 

of runs with a Kv = 0.01. 

The model source code, the model executable file, and the input file, originally used 

by EPA Region 10 could not be located by EPA. A listing of the input file was 

included as Appendix D in the 1993 Fact Sheet, and this input file was reproduced 

(Attachment III). The Fact Sheet indicates that a WASP4 model Guly 1991 version) 

was used. However, as mentioned above the model was not available from EPA 

and the input file was incompatible with the WASP4 code in our possession. 

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EPA-REGION 10 ";APTAINS BAY WASP MODEL: 

RESULTS FOR INCREASED BOD LOADING 

However, the input file was compatible with the WASPS (September 1993 version). 

It is likely that EPA Region 10 used a version of the analysis engine that was 

intermediate between the two versions available to us. The WASPS formulation was 

used throughout the study reported in this memorandum. 

ModeJ Calibration Approach 

The calibration process for an analysis engine such as WASP requires the internal 

adjustment of various parameters until the input forcing function(s) result in a target 

output response(s). The model, as formulated by EPA Region 10, was recalibrated 

to match the DO response in model Segment 76 to BOD loading as requested by the 

28 April 2000 EPA letter (Attachment I). Model Segment 76 is the defined discharge 

segment where the BOD load is placed. The segment is 200 by 220 meters in 

horizontal extent and extends vertically from 5 meters below the surface to 15 

meters below the surface. 

The effluent BOD loading used for calibration was the average of all Westward 

Seafood~s BOD load values for the time period 21 July through 10 September 1992, 

as directed by EPA. The BOD loading included the stickwater component, which 

was not included in the original 1993 model analysis but EPA has indicated it 

should have been included. The letter from EPA indicates that the ultimate BOD 

should be used. The WASP model requires ultimate BOD as input, but EPA 

previously approximated this parameter with the BODzo (20-day BOD) value. 

Therefore, we used BOD20 as the loading, calculated based on the conversion factor 

EPA provided in the 1993 Fact Sheet. 

The effect of not using BODzo should not be substantial, given that the model, in its 

original EPA formulation, is basically being used as a correlation tool and it is not 

simulating realistic physical processes. It appears that the results from using the 

BODzo value, as derived by EPA, would provide a conservative estimate of effects on 

DO (that is, the model overstates the DO depression caused by a given loading). 

The difference between EPA BODzo conversion and the generally accepted ultimate 

BOD conversion is 1.31xBODs compared to 1.46xBODs, respectively. The EPA 

procedure appears to understate the actual ultimate BOD loading by approXimately 

10 percent. Therefore, this approach provides an additional "safety factor" of about 

10 percent to an already very conservative model. The model loading was 

calculated as shown in Table 1. 

As directed by EPA Region 10, the model was calibrated assuming that the average 

BOD loading from Table 1 results in an arbitrary average DO of 3.1 mg/I in model 

Segment 76 (D076 = 3.1). Based on a review of the DO monitoring data requested 

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EPA-REGION 10 <;APTAINS BAY WASP MODEL: 

RESULTS FOR INCREASED BOD LOADING 

earlier by EPA-Region 10, the actual average DO in this model segment for the time 

period in question was found to be 7.63 mg/1. This value was based on the 

monitoring stations representative of the discharge segment for the time period 25 

July to 7 September 1992. EPA's assumption of the low DO is based on the lowest 

two or three point values of over three hundred measurements observed in the 

vertical profiles. The process of forcing the model to simulate the low DO point 

measurements rather than the average value, as the WASP model is designed to do, 

is what makes the modeling a correlation exercise rather than a simulation of 

physical processes. This is considered an extremely conservative approach. 

Table 1 

1992 Critical Period BOD Loading 

Date BOD. (mg/I) Flow (mgd) BOD. (Ibs/day) 

21 July 5775 3.62 174,351 

3D July 5082 4.24 179,707 

3 August 5070 3.67 155,181 

10 August 3542 4.16 122,887 

20 August 4854 3.57 144,522 

2 September 3788 3.59 113,414 

10 September 4296 4.03 144,389 

Average BOD5 147,779 

Average BOD20 (BOD20 : BOD5 x 1.31) 193,590 

The calibration of WASP, as for any model of this type, is performed by adjusting a 

variety of selected parameters until the required output for given input is achieved. 

Typically the parameters used include horizontal and vertical diffusion, reaeration, 

and other model input parameters. As described in the 1993 Fact Sheet, EPA set all 

parameters to constants and varied two parameters to calibrate the modeL The 

calibration for this study was done in the same fashion as indicated in the 1993 Fact 

Sheet, by adjusting the vertical diffusion coefficient, Kv, and the deoxygenating 

coefficient, KDc. 

The use of two calibration parameters to match a single response value does not 

have a unique solution. Because of the way the model is formulated and calibrated, 

there is no strictly correct way to select the appropriate calibration pair or resultant 

loading values to use. Such a situation is often considered a fatal flaw in modeling 

exercises, but is acceptable to EPA in this case. Typically, one would match the DO 

over a number of model segments throughout the Bay to provide a model that is 

simulating physical processes and to at least partially account for uncertainties in 

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EPA-REGION 10 ';APTAINS BAY WASP MODEL: 

RESULTS FOR INCREASED BOD LOADING 

the measured target DO values. However, such a process is not meaningful for 

WASP unless the average segment DO values are considered. 

When using the model as a correlation tool for minimum DO values, other 

approaches to supplement calibration must be used. As described in the 1993 Fact 

Sheet EPA selected a calibration pair that resulted in a DD76 = 3.1 mg/l and a DO in 

model Segment 26, immediately above the Segment 76, of D026 = 5.5 mg/1. This 

value of predicted D026 is 1.3 mg/llower than the "actual" D026 listed in EPA's 

1993 Fact Sheet. No explanation is provided in the Fact Sheet as to why EPA 

selected the calibration pair that was eventually applied and used. However, for 

consistency we used the same procedure (D026 value of 5.5 mg/l) to select the 

appropriate calibration pair. 

Model Calibration 

As mentioned above, there are an infinite number of solutions to this calibration 

problem in terms of pairs of {Kv, Koc} values that will result in a DD76 = 3.1 mg/I for 

a given BOD loading. We considered a range of Kv from 0.01 to 0.00001 and 

adjusted Koc for each value of Kv considered to obtain DD76 = 3.1 mg/1. EPA had 

previously used a value of Kv = 0.00013. Typically, vertical diffusion is one to two 

orders of magnitude less than horizontal diffusion. EPA used a horizontal diffusion 

coefficient of 0.57 for spatial scales 20 times the vertical scale. This would 

correspond to expected vertical diffusion coefficients on the order of 0.001. 

Therefore, the range considered in the current modeling includes the appropriate 

range based on EPA's previous application of the model, and extends an order of 

magnitude on either end of that range. 

A summary of calibration runs and results is shown in Table 2, with DO levels 

indicated for both model Segments 76 and 26. At the loading indicated in Table l, 

for each Kva series of Koc values was considered until the target DD76 of 3.1 mg/I 

was bracketed. A second order polynomial was fit through the curve of Koc versus 

D076 to determine the value resulting in 3.1 mg/1. These graphical results are 

provided in Attachment IV and summarized in Table 3. The calibration pair that 

matches the EPA value of both DD76 and D026 is highlighted in Table 3. 

Model Results - Predicted Effluent BOD Loading Capacity 

For each pair of calibration parameters that resulted in a D076 of 3.1 mg/I, the model 

was run for a series of loadings to bracket the BOD20 loading that resulted in a DO of 

5.0 mg/I in Segment 76 (DD76 = 5 mg/l). The loading that results in a DD76 of 5.0 

mg/l was then determined by using a second order polynomial curve fit. The 

loading capacity so determined is obviously a function of the calibration parameter 

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EPA-REGION 10 IjAPTAINS BAVWASP MODEL: 

RESULTS FOR INCREASED BOD LOADING 

pair used. A summary of the model runs and results is shown in Table 4. Table 5 

shows the loading capacity that would result in Do,6 of 5.0 mg/I and 4.0 mg/1. 

The loading that will result in a predicted Do,6 = 5.0 mg/I, over the range of 

calibration pairs considered, ranges from 90,500 Ibs/ day to 122,200 Ibs/ day of 

BOD20 (Table 5). The lower value is about the same as that predicted by the 1993 

EPA modeling (90,000 Ibs/ day), which used a much smaller BOD20 loading. The 

higher value is about 35% greater than the 1993 EPA modeling results. These results 

are summarized in Figure 1. 

As discussed above, there is no unique calibration for the model as formulated. It is 

suggested, as mentioned above, that a reasonable procedure is to use the same 

approach as that provided in the EPA 1993 Fact Sheet. This would maintain full and 

corn plete consistency with EPA's previous modeling. The calibration pair for the 

current modeling that results in a DO,6 = 3.1 mg/I and a D026 = 5.5 mg/I is Kv = 

0.0001 and Koc = 0.022. The simulations using this pair predict a value of BOD20 

loading of 122,200 Ibs/ day that results in a DO,6 = 5.0 mg/1. Therefore, an increase 

of 35% in effluent BOD limitations is justified based on using the more 

"appropriate" loading values for calibration in the original EPA model. The 

reasoning for using a smaller BOD loading for calibration value in the 1993 model is 

unknown. 

Assuming the same procedure for setting effluent limitations is followed as was 

previously done, existing permit limitations for maximum daily load and average 

monthly load should be multiplied by 1.35 to be consistent with the revised loading 

capacity. The values are calculated are an increase of monthly average BODs load 

from 58,000 Ibs/ day to 78,300 Ibs/ day and an increase in daily maximum loading 

from 90,000 Ibs/ day to 121,500 Ibs/ day. 

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EPA-REGION 10 (;APTAINS BAY WASP MODEL: 

RESULTS FOR INCREASED BOD LOADING 

Table 2 

Summary of Calibration Runs: 800zo Loading = 193,600 Ibs/day 

Case Kv Koc 0076 (mgtl) 0026 (mgtl) 

_01 0.0400 1.01 3.95 

_02 0.0350 1.38 4.29 

03 0.0200 3.47 5.80 - 0.00010 

_04 0.0250 2.59 5.21 

05 0.0275 2.22 4.95 -

06 0.0225 3.01 5.49 

11 0.0400 7.20 7.46 -

12 0.0600 6.69 7.02 -

13 0.1200 5.36 5.88 - 0.00100 

14 0.2400 3.34 4.20 -

15 0.3000 2.60 3.57 -

16 0.2700 2.95 3.87 -

21 0.0020 4.64 8.82 -

22 0.0030 3.64 8.93 - 0.00001 

23 0.0035 3.18 9.02 -

24 0.0040 2.76 9.07 

31 0.4000 4.17 4.71 -

- 32 0.5000 3.49 4.12 

0.00200 

33 0.6000 2.91 3.62 -

34 0.7000 2.43 3.20 

42 0.0100 3.89 7 .03 -

43 0.0200 1.25 5.57 - 0.00005 

44 0.0120 3.16 6.67 -

45 0.0140 2.54 6.35 

51 2.0000 4.89 4.83 -

_52 2.4000 4.45 4.39 

53 0.01000 3.4000 3.55 3.50 -

_54 4.4000 2.87 2.82 

_55 4.0000 3.12 3.07 

Table 3 

Summary of Calibration Results 

Kv Koc 0076 (mgtl) 0026 (mgtl) 

0.00001 0.0036 3.1 8.76 

0.00005 0.0122 3.1 6.64 

0.00010 0.0220 3.1 5.55 

0.00100 0.2585 3.1 4.00 

0.00200 0.5672 3.1 3.78 

0.01000 4.0320 3.1 3.05 

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EPA-REGION 10 CAPTAINS BAY WASP MODEL: 

RESULTS FOR INCREASED BOD LOADING 

Table 4 

Summary of Model Runs with Calibrated Model 

Case Kv Koc Loading (Ibstday) 0076 (mgtl) 

07 170,000 3.68 -

- 08 130,000 4.77 

0.00010 0.0220 

09 100,000 5.65 -

10 80,000 6.25 

17 - 170,000 4.47 

- 18 130,000 5.33 

0.00100 0.2585 

19 - 100,000 6.01 

20 - 80,000 6.47 

27 - 170,000 3.49 

- 28 130,000 4.22 

0.00001 0.0036 

- 29 100,000 4.81 

30 80,000 5.21 

37 - 170,000 3.61 

- 38 130,000 4.57 

0.00200 0.5672 

39 - 100,000 5.35 

_40 80,000 5.89 

_47 170,000 3.67 

_48 130,000 4.72 

0.00005 0.0122 

49 100,000 5.58 

50 80,000 6.17 

57 170,000 3.56 -

- 58 130,000 4.51 

0.01000 40320 

59 100,000 5.28 -

_60 80,000 5.81 

Table 5 

Summary of Model Results 

Calibration Parameter Pair Loadin'g (lbstday BOD .. ) Resulting in : 

Kv Koc 0076 = 5.0 mgtl 

0.00001 0.0036 90,500 

0.00005 0.0122 120,233 

0.00010 0.0220 122,159 

0.00100 0.2585 115,949 

0.00200 0.5672 113,461 

0.01000 4.0320 111 ,200 

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EPA-REGION 10 CAPTAINS BAyWASP MODEL: 

RESULTS FOR INCREASED BOD LOADING 

·---r-·------...,...----· 

12S000 

Loading (Ibs/day) 

1S0000 

. Kv = 0.01000 

e Kv = 0.00200 

• Kv = 0.00100 

• Kv = 0.00010 

. Kv = O.OOOOS 

)f. Kv = 0.00001 

17S000 200000 

Figure 1 

Segment 76 DO as a Function of 80020 Loading 

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