Nothing creative.  Use any reptile effects data you can find.  If no reptiles use birds (reptiles with fluffy scales). I assume that you have access to Sparling et al. 2010 Ecotoxicology of Amphibians and Reptiles, 2nd edition.  Also, Gardner and Oberdorster 2006. Toxicology of Reptiles.

You can read the follow publications from Kristine L Richardson:

1)Biotransformation of 2,2 ',5,5 '-Tetrachlorobiphenyl (PCB 52) and 3,3 ',4,4 '-Tetrachlorobiphenyl (PCB 77) by Liver Microsomes from Four Species of Sea Turtles.   Chemical Research in Toxicology . 05/2011; 24(5):718-25. DOI: 10.1021/tx1004562

2)Accumulation patterns, biotransformation enzymes, and in vitro biotransformation of polychlorinated biphenyls in several species of sea turtle. Comparative Biochemistry and Physiology - Part A Molecular & Integrative Physiology (Impact Factor: 2.37). 09/2010; 157:S9. DOI: 10.1016/j.cbpa.2010.06.024

3)The characterization of cytosolic glutathione transferase from four species of sea turtles: Loggerhead (Caretta caretta), green (Chelonia mydas), olive ridley (Lepidochelys olivacea), and hawksbill (Eretmochelys imbricata).  Comparative Biochemistry and Physiology Part C Toxicology & Pharmacology (Impact Factor: 2.83). 06/2009; 150(2):279-84. DOI: 10.1016/j.cbpc.2009.05.005

https://www.researchgate.net/researcher/58839960_Kristine_L_Richardson

If you want a creative way and testing is out of the question them probably environmental systems analysis type of investigation needs to be conducted, specifically either through environmental modelling (systems dynamics approach but time-step based data is hard to come by) or conducting more at the systems level e.g. material flows analysis (which is like a snapshot of the system).

Pat, I have used sea turtles as receptors when conducting bioaccumulation risk assessment analysis with the models Trophic Trace and BEST.  There are only a limited amount of literature on the toxic effects of contaminants on sea turtles as you know.  However, birds are reasonable proxies for reptiles in terms of biochemistry of the liver and eggs, their metabolic and excretory processes are similar, and because these assumptions are standard in sea turtle risk assessments according to Lam et al. (2006) and van de Merwe et al. (2009).  Further, the toxic effects of metals and other contaminants are believed to be similar between birds and reptiles (Perrault in review).   Also, don't forget that there is data available on freshwater turtles, especially redear sliders, and these data can be extrapolated when comparing body mass between the study animal and your sea turtle species for modeling.  Also, other large aquatic reptiles (alligators, crocodiles) can be used as proxies given their phylogenetic, life history, and body mass similarities in relation to marine turtles.  

There are currently no NOAEL or LOAEL values available for sea turtles. However, recent studies in refereed journals include PNECs for several toxic metals in the green turtle and the leatherback turtle. These PNECs can be used as surrogates for NOAELs in sea turtles.  Because sea turtles are similar to birds in terms of metabolic and excretory processes and in terms of liver and egg biochemistry, it is possible that bird toxicity inputs may be used with sea turtles in the absence of more appropriate data in certain circumstances.  

I use the USACE Environmental Residue Effects Database a lot to find toxicity inputs for risk assessments:  http://el.erdc.usace.army.mil/ered/index.html

Here are some studies on sea turtle bioaccumation possible effects:

 Lam, J.C.W., S. Tanabe, S.K.F. Chan, M.H.W. Lam, M. Martin, and P.K.S. Lam. 2006. Levels of trace elements in green turtle eggs collected from Hong Kong:  Evidence of risks due to selenium and nickel. Environmental Pollution 144:790–801.

 Perrault, J.R., D.L. Miller, J. Garner, and J. Wyneken.  2013.  Mercury and selenium concentrations in leatherback sea turtles (Dermochelys coriacea): Population comparisons, implications for reproductive success, hazard quotients and directions for future research.  Science of the Total Environment 2013:61–71.

 Perrault, J.R. In review. Mercury and selenium ingestion rates for leatherback sea turtles (Dermochelys coriacea):  A cause for concern in this species?  Manuscript submitted to Marine Environmental Research.

van de Merwe, J.P., M. Hodge, H.A. Olazowy, K. Ibrahim, and S.Y. Lee. 2009. Chemical contamination of green turtle (Chelonia mydas) eggs in peninsular Malaysia:  Implications for conservation and public health. Environmental Health Perspectives 117:  1397–1401.

Here's a paper on sliders.  The data can be extrapolated for use with sea turtles based on relative body mass differences:

Burger, J., C. Carruth-Hinchey, J. Ondroff, M. McMahon, J.W. Gibbons, M. Gochfeld.  1998.  Effects of lead on behavior, growth, and survival of hatchling slider turtles.  Journal of Toxicology and Environmental Health Part A 55:495–502.