Pesticide levels in streams and sediments on the islands of Oahu and Kauai, Hawaii

A survey water quality study was conducted between 2014 and 2017 to investigate the presence and concentration of pesticides in the nearshore marine environments on the islands of Oahu and Kauai. On Kauai, a total of 32 surface water and 16 sediment samples were collected from four streams and one irrigation ditch over multiple sampling events. On Oahu, a total of 27 surface water and 16 sediment samples were collected from six streams on the leeward side of the island over multiple sampling events, typically under baseflow conditions. The samples were analysed for 197 compounds spanning eight chemical classes of pesticides. Five herbicide (glyphosate, metolachlor, atrazine, imazapyr and MCPA) and one insecticide (imidacloprid) compounds were detected in the surface water samples collected. Seven insecticide (imidacloprid, carbaryl, chlordane, p,pʹ-DDD, p,pʹ-DDE, p,pʹ-DDT and dieldrin), five herbicide [glyphosate, aminomethylphosphonic acid (AMPA), diuron, DCPMU and pendimethalin] and one fungicide (azoxystrobin) compound were detected in the stream bed sediments collected. Detected pesticides spanned the pesticide class list, with seven of the eight classes of pesticides tested discovered. This study found widespread, low level contamination by both legacy and currently used pesticides in nearshore waters and river bed sediments on the islands of Oahu and Kauai. Based on the pesticide data obtained during the study, five streams on the island of Oahu (two leeward and three windward streams) were selected for additional high-frequency sampling for the broad-spectrum herbicide glyphosate (Roundup), due to this compound’s prevalence and the elevated concentration levels (compared to other pesticides) measured. These five streams flow through watersheds dominated by agricultural, residential and mixed-use land use and were sampled under both baseflow and storm conditions. The pervasiveness and overall concentration levels of glyphosate detected are greater than any other pesticide currently or historically present in Hawaiian streams. Glyphosate was detected in 95% of stream samples collected during storm events (59 samples, 798 ng/L median, 1,308 ng/L mean detects) and 60% of stream samples collected under baseflow conditions (103 samples, 152 ng/L median, 462 ng/L mean detects), respectively (detection limit = 50 ng/L). In addition, either glyphosate or its degradation product AMPA was detected in 100% of the stream bed sediment samples collected on Oahu and Kauai during the study. The higher glyphosate concentrations in measured stream samples collected under storm versus baseflow conditions is believed to result from the release of adsorbed glyphosate present in stream-bed sediments as they become re-suspended during the rapid rises in stream volumes that characterize Hawaiian stream during runoff events. The mean glyphosate concentration measured in streams that drain urban and mixed-use areas (1,020 and 1,050 ng/L, respectively) was slightly higher than concentration levels measured in streams that drain agricultural areas (760 ng/L). Glyphosate was detected more frequently in stream samples collected from agricultural areas than from urban and mixed-use areas (82%, 69% and 63% detection rates, respectively). The mean glyphosate concentrations measured during this study in stream waters and their associated bed sediments are more than 7 and 10 times higher than the maximum mean detect concentration of the most prevalent persistent organic pollutants (α-chlordane in sediment and pentachlorophenol in stream and bay waters) measured in waters and sediments in urban and mixed-use areas on Oahu in the mid-1970s.