Pesticides in surface water downstream of and near agricultural and developed land in Hawai‘i, 2015–19

Pesticides and pesticide degradates (herein referred to as pesticides) in surface water were assessed at 78 sites on 4 Hawaiian Islands (Kauaʻi, Oʻahu, Maui, and Island of Hawaiʻi) during 2015–19. Each site was downstream of or near agricultural land, developed land, or both. Most (58) sites were streams; the remaining sites were canals, ditches, anchialine pools, coastal ponds, and the nearshore ocean. Pesticides in water at each site were assessed by collection of one to four water samples, by a weeks-long deployment of a passive sampler, or both. Passive-sampler extracts and water samples, which consisted of fair-weather samples and storm samples, were analyzed for as many as 253 pesticides that consisted of 129 herbicides, 101 insecticides, and 23 fungicides.

A total of 117 pesticides were detected in water. Of these, 30 pesticides were detected at more than 20 percent of their assessment sites and thus were considered “common” pesticides. The common pesticides included 17 herbicides (ametryn, atrazine, bentazon, bromacil, diuron, hexazinone, metolachlor, propazine, simazine, triclopyr, prometryn, and degradates of atrazine [4], diuron, hexazinone, and prometryn); 8 insecticides (carbaryl, dinotefuran, fipronil, flubendiamide, imidacloprid, methoxyfenozide and 2 degradates of fipronil); and 5 fungicides (azoxystrobin, metalaxyl, propiconazole, a degradate of chlorothalonil, and a degradate of thiophanate-methyl and benomyl). Common pesticides typically were present more frequently in storm samples than in fair-weather samples. A mixture of two or more pesticides was detected in 86 percent of the water samples and in water during every passive-sampler deployment.

About 92 percent of all pesticide detections had concentrations less than 100 nanograms per liter. Pesticide concentrations were less than Federal aquatic-life benchmarks (ALBs) for vertebrates and typically were less than ALBs for invertebrates, nonvascular plants, and vascular plants. Acute ALBs were exceeded by acetochlor, atrazine, carbaryl, chlorpyrifos, cis-permethrin, diazinon, diuron, halosulfuron methyl, and imidacloprid in storm samples at one to four sites. ALBs for invertebrates were exceeded by clothianidin, diazinon, fipronil, and imidacloprid in fair-weather samples, during passive-sampler deployments, or both at 1 to 17 sites. Federal drinking water standards were available for only five pesticides detected in water, and the standard for atrazine was exceeded in one storm sample. Pesticide concentrations did not exceed any Federal human-health benchmarks, but many of the detected pesticides did not have a benchmark.