To determine aqueous pyrethroid partitioning across a salinity gradient 20 mL Pyrex beakers were filled with 8 mL of deionized water at varying salinities (0, 0.5, 2, 6 ppt). Salinity was adjusted using a 10 ppt stock solution (made by diluting Instant Ocean into deionized water). The water samples were spiked with pyrethroids (bifenthrin, cyhalothrin, cyfluthrin, cypermethrin, deltamethrin, esfenvalerate, fenpropathrin) at 500 ng/L (pyrethroid stock solution was 0.25 ng/µL in acetone, 16 µL added to each beaker). Samples were equilibrated in the dark (beaker covered with aluminum foil and placed in a cardboard box; the first set was equilibrated for 24 hr, the second set for 120 hr). Each set of samples at a particular salinity and time were run in triplicate. An additional set of samples were run for 24 hr with the addition of 30 mg/L of suspended sediment. At the end of the specified time, the water was gently removed from the beaker with a pipette and run through a solid phase extraction (SPE) cartridge (Water Oasis HLB, 3 cc 60 mg) and eluted with ethyl acetate (3 mL) to determine the fraction of pyrethroids in the water. After the removal of the water, the beaker was then rinsed twice with 0.5 mL of dichloromethane (rinses were combined) and exchanged into ethyl acetate to give fraction associated with the beaker surface. Both the SPE eluent and beaker rinses were reduced to 50 µL in ethyl acetate and analyzed via gas chromatography-mass spectrometry (GC-MS). The fraction of pyrethroids in the aqueous phase increased with increasing salinity.