Rain deposition of pesticides in coastal waters of the South Atlantic Bight

Concentrations of currently used pesticides were measured in surface waters of the South Atlantic Eight immediately after a major storm event that deposited significant rainfall (200-300 mm) in the coastal region. A plume of freshened water (salinity at least 2 parts per thousand lower than surrounding 36.2 parts per thousand salinity waters) was detected offshore. A concurrent study determined that the freshened plume was the result of rainfall. Concentrations of atrazine, metolachlor, and trifluralin were enriched inside the freshened plume (by a factor of approximately 2-6) as compared to surface waters outside the plume, providing direct evidence of rain deposition of these pesticides. Calculated rain end-member concentrations were higher than local river end-members. Estimated quantities of pesticides deposited in the freshened plume during the 2-3-day event ranged from 130 to 490 kg of atrazine, from 95 to 120 kg of metolachlor, and from 10 to 27 kg of trifluralin. These Values represent approximately 4-10%, 3-5%, and 1-3% of the estimated yearly riverine discharge of atrazine, metolachlor, and trifluralin, respectively. Our results indicate that short-duration rain events may produce major pulses in the pesticide input to surface waters of the coastal ocean. Our estimates suggest that this mechanism may represent an important source of pesticides to the coastal ocean. No other pesticide was detected in offshore samples, and only alachlor and simazine were detected in local estuarine waters of Winyah Bay and Charleston Harbor.