UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

REGION IX 

75 Hawthorne Street 

San Francisco, CA 94105 

Memorandum 

May 9, 2016 

From: Diane E. Fleck, P.E., Esq. 

Wat er Qu ality Assessment Section 

Water Divi sion, US EPA Region 9 

To: Admi nistrat ive Record for Proposed Seleniu m Criteri a for t he Sa n Francisco Ba y and De lt a 

Re: Ca lculations to Det erm ine Corbufa Value in the Technica l Support Docum ent for t he Proposed 

Aquatic Life and Aq uatic-Dependent Wildlife Se lenium Criteri a for t he Sa n Franci sco Bay and Delta 

{2016) 

Ca lculati ons t o determine the maximum Corbufa se lenium concent ration from t he Predicted 

Invertebrate Selenium Concent rat ions in Table 4-9, to protect ea ch category of d arn -eat ing species: 

1. For Clam-eating fish: 

The Predict ed Invertebrat e (prey) Di et ary Concentration = 7.7 1J.g/ g dw (from Ta ble 4-9); 

7.7 1J.g/ g dw =sum of: (% prey diet) x (prey TIF) x (particulate concent rat ion ). 

Using a diet of 40% Corbufa (TIF=17), 60% Annelids/ Cru stacean s (TIF=1.3) and a predicted particulate 

conce nt rati on of 1.017 IJ.g/ g dw (f rom Table 4-9): 

The predicted dietary contribution from Corbufa = 0.40 x 17 x 1.017 = 6.92 IJ.g/ g dw; and 

the predicted diet ary contribut ion from Annelid/ Crustaceans = 0.6 x 1.3 x 1.017 = 0.79 1J.g/ g dw. 

(To Verify: 6.92 + 0.79 IJ.g/ g dw = 7. 71 or 7. 7 ~J. g dw) 

Since t he Clam-eat ing Fish Diet is compri sed of 40% Corbula, t he Target Concentrat ion in Corbula t o 

protect d arn-eating fi sh is: 

(Predi cted dietary contribut ion from Corbu/a) I (% of diet ) = 6.92 1J.g/ g dw I 0.40 = 17.30 IJ.g/ g dw 

-+ Target Concentration in Corbufa ~ 17 1J.g/ g dw (or rn g/kg dw). 

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2. For Clam-eating birds: Rails : 

The Predict ed Inverte brate (prey) Diet ary Concentration = 15.6 g dw (from Table 4-9); 

15.6 ~g g dw = sum of: (% diet) x (TIF) x (p articulate concentration). 

Using a diet of 20% Corbu!a (TIF=17), 40% Other Mollusk (TIF=2.8), 30% Insect (TTF=2.8), 10% Pl ant 

(negligible cont ribut ion ) and a predicted particulate concentrat ion of 2.894 ugl g dw (from Table 4-9): 

The predicted dietary cont ribut ion from Corbula = 0.20 x 17 x 2.894 = 9.84 g dw; 

t he predicted dietary contribut ion from other mollusks = 0.40 x 2.8 x 2.894 = 3.24 ~gig dw; 

t he predicted dieta ry cont ribut ion from insects= 0.30 x 2.8 x 2.894 = 2.43 g dw; and 

the predi cted diet ary contribut ion from plant=> negligible (0). 

(To Verify: 9.84 + 3.24 + 2.43 = 15.51 or 15.6 g dw) 

Since th e Clam-eat ing Ra il Diet is compri sed of 20% Corbula, t he Target Concent ration in Corbula t o 

prot ect cl am-eat ing rai ls is: 

(Predict ed dietary contribut ion from Corbula) I(% of diet ) = 9.84 ~g g dw I 0.20 = 49.20 g dw 

-+ Target Concent ration in Corb ula ~ 49 g dw (or mglkg dw). 

3. For Clam-eating birds: M igratory Ducks (50% of time spent in Estuary): 

The Predi cted Invertebrate (prey) Concentration= 6.9 g dw (from Table 4-9); 

6.9 ~g/g dw = sum of: (% diet ) x (TIF) x (particulat e concentration ); 

Using a diet of 45% Corbu/a (TIF=17), 5% Ot her An imal (TIF=2. 8), 50% Outside of Estuary (negligible 

cont ribut ion) and a predicted particu late concentration of 0.890 ~g g dw (from Table 4-9): 

The predicted dietary contribution from Corb u/a = 0.45 x 17 x 0.890 == 6. 81 /k g dw; 

t he predicted dietary cont ribut ion from Other An imal = 0.05 x 2.8 x 0.890 = 0.12 g dw; and 

t he predicted diet ary contribution from outside of estuary => negligible (0). 

(To Verify: 6.81 + 0.12 = 6.93 or 6.9 g dw) 

Since t he Clam-eating Duck Diet is comprised of 45% Corbu/a, t he Target Concentrat ion in Corbu/a to 

protect cl am-eat ing du cks is: 

(Predicted diet ary contribut ion from Corb ula) / (%of diet ) = 6.81 g dw I 0.45 = 15.13 g dw 

-+Target Concentration in Corb ula :S 15 g dw (or mg/ kg dw ). 

In order to protect all three categories of species using the proposed dietary contributions, the 

concentration in Corbu/a should be::::; 15 ~g g dw (or mg/ kg dw). 

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