Dissolved pesticides were measured in weekly water samples from 85 wadeable streams in Central Coastal California over a variable six-week period during March–May, 2017, as part of the California Stream Quality Assessment (CSQA) study conducted by the U.S. Geological Survey's (USGS) Regional Stream-Quality Assessment (RSQA) Project. The 85 streams consisted of 40 urban sites (5–100% urban land in the lower basin), 9 agricultural sites, 24 mixed land-use sites, and 12 undeveloped sites. Water samples were filtered (0.7 micrometers) and analyzed for 253 pesticide compounds by direct-injection liquid chromatography with tandem mass-spectrometry (LC-MS/MS). Two similar LC-MS/MS methods were used: a broad-spectrum (223 compounds) method in use since 2012 and a newly developed method for 30 new-generation fungicides and diamide and neonicotinoid insecticides. This Data Release provides sampling-site locations, analyte information, concentration data for pesticide compounds in environmental weekly water samples, quality-control data for the new method (to supplement previously published quality control data for the standard method), aquatic-life benchmark and Pesticide Toxicity Index toxicity concentration values that were used to assess potential toxicity, estimates of agricultural and nonagricultural pesticide-use data, and streamflow data for gaged sites, in support of the journal article, “New-generation pesticides are prevalent in California’s Central Coast streams,” by Sandstrom, M.W., Nowell, L.H., Mahler, B.J., and Van Metre, P.C.

These datasets are one component of the multistressor studies conducted in Midwest streams in 2013 (MSQA) and in Southeast streams in 2014 (SESQA) by the U.S. Geological Survey National Water Quality Assessment Project. High-frequency small-volume autosamplers were deployed at 7 sites each in MSQA and SESQA that collected daily and weekly composite water samples, which were analyzed for 225 pesticides and pesticide degradates. Five of the MSQA autosampler sites were in agricultural watersheds and two in urban watersheds, whereas all seven SESQA autosampler sites were in urban watersheds. The daily and weekly composite samples were compared with results from traditional discrete water samples collected weekly at the sites. Mixtures of pesticides were present in most samples and the Pesticide Toxicity Index (PTI) and acute invertebrate benchmarks were used to evaluate the potential for acute invertebrate toxicity of mixtures. This Data Release provides concentration data for pesticide compounds in environmental weekly and daily composite water samples, pesticide analyte information, summaries of quality control data, and PTI scores, in support of the journal article “Daily-composite stream samples reveal highly complex pesticide occurrence and potential toxicity to aquatic life,” by Norman, J.E., Mahler, B.J., Nowell, L.H., Van Metre, P.C., Sandstrom, M.W., Corbin, M.A., Qian, Y., Pankow, J.F., Luo, W., Fitzgerald, N.B., Asher, W.E., and McWhirter, K.J.

These datasets are one component of the multistressor studies conducted in Midwest streams in 2013 (MSQA) and in Southeast streams in 2014 (SESQA) by the U.S. Geological Survey National Water Quality Assessment Project. High-frequency small-volume autosamplers were deployed at 7 sites each in MSQA and SESQA that collected daily and weekly composite water samples, which were analyzed for 225 pesticides and pesticide degradates. Five of the MSQA autosampler sites were in agricultural watersheds and two in urban watersheds, whereas all seven SESQA autosampler sites were in urban watersheds. The daily and weekly composite samples were compared with results from traditional discrete water samples collected weekly at the sites. Mixtures of pesticides were present in most samples and the Pesticide Toxicity Index (PTI) and acute invertebrate benchmarks were used to evaluate the potential for acute invertebrate toxicity of mixtures. This Data Release provides concentration data for pesticide compounds in environmental weekly and daily composite water samples, pesticide analyte information, summaries of quality control data, and PTI scores, in support of the journal article “Daily-composite stream samples reveal highly complex pesticide occurrence and potential toxicity to aquatic life,” by Norman, J.E., Mahler, B.J., Nowell, L.H., Van Metre, P.C., Sandstrom, M.W., Corbin, M.A., Qian, Y., Pankow, J.F., Luo, W., Fitzgerald, N.B., Asher, W.E., and McWhirter, K.J.

Dissolved pesticides were measured in weekly water samples from 100 wadeable streams across eleven states in the Midwestern U.S. during May-August, 2013, as part of the Midwest Stream Quality Assessment study conducted by the U.S. Geological Survey's (USGS) National Water Quality Assessment (NAWQA) Project. Of the 100 stream sites, 12 were urban indicator sites and the remaining 88 sites were located along an agricultural gradient of watershed land use. Twelve depth- and width-integrated samples were collected at each site within the 14-week study period. Water samples were filtered (0.7 micrometers) and analyzed for 227 pesticide compounds by direct-injection liquid chromatography with tandem mass-spectrometry, and for glyphosate by Enzyme-Linked Immunoassay (ELISA) in a separate analysis. Potential aquatic toxicity was evaluated using the Pesticide Toxicity Index and by comparison to U.S. Environmental Protection Agency aquatic-life benchmarks. A mesocosm experiment was conducted to investigate the effects of the insecticide imidacloprid on natural stream communities transported to experimental streams in the laboratory, where they were exposed to imidacloprid under controlled conditions. This Data Release provides sampling site locations and watershed characteristics, pesticide analyte information, agricultural pesticide use data, summaries of quality control data, concentration data for pesticide compounds in environmental weekly water samples, and a summary of invertebrate metrics and imidacloprid concentration data from the mesocosm study, in support of the journal article, "Complex mixtures of dissolved pesticides show potential aquatic toxicity in a synoptic study of Midwestern U.S. streams," by Nowell, L.H., Moran, P.W., Schmidt, T., Norman, J.E., Nakagaki, N., Shoda, M.E., Mahler, B.J., Van Metre, P.C., Stone, W.W., Sandstrom, M.W., and Hladik, M.L.

Dissolved pesticides were measured in weekly water samples from 100 wadeable streams across eleven states in the Midwestern U.S. during May-August, 2013, as part of the Midwest Stream Quality Assessment study conducted by the U.S. Geological Survey's (USGS) National Water Quality Assessment (NAWQA) Project. Of the 100 stream sites, 12 were urban indicator sites and the remaining 88 sites were located along an agricultural gradient of watershed land use. Twelve depth- and width-integrated samples were collected at each site within the 14-week study period. Water samples were filtered (0.7 micrometers) and analyzed for 227 pesticide compounds by direct-injection liquid chromatography with tandem mass-spectrometry, and for glyphosate by Enzyme-Linked Immunoassay (ELISA) in a separate analysis. Potential aquatic toxicity was evaluated using the Pesticide Toxicity Index and by comparison to U.S. Environmental Protection Agency aquatic-life benchmarks. A mesocosm experiment was conducted to investigate the effects of the insecticide imidacloprid on natural stream communities transported to experimental streams in the laboratory, where they were exposed to imidacloprid under controlled conditions. This Data Release provides sampling site locations and watershed characteristics, pesticide analyte information, agricultural pesticide use data, summaries of quality control data, concentration data for pesticide compounds in environmental weekly water samples, and a summary of invertebrate metrics and imidacloprid concentration data from the mesocosm study, in support of the journal article, “Complex mixtures of dissolved pesticides show potential aquatic toxicity in a synoptic study of Midwestern U.S. streams,” by Nowell, L.H., Moran, P.W., Schmidt, T., Norman, J.E., Nakagaki, N., Shoda, M.E., Mahler, B.J., Van Metre, P.C., Stone, W.W., Sandstrom, M.W., and Hladik, M.L.

Dissolved pesticides were measured in weekly water samples from 100 wadeable freshwater streams across eleven states in the Midwestern U.S. during May-August, 2013, as part of the Midwest Stream Quality Assessment study conducted by the U.S. Geological Survey's (USGS) National Water Quality Assessment (NAWQA) Project. Of the 100 stream sites, 12 were urban indicator sites and the remaining 88 sites were located along an agricultural gradient of watershed land use. Twelve depth- and width-integrated water samples were collected at each site within the 14-week study period. Water samples were filtered (0.7 micrometers) and analyzed for 227 pesticide compounds by direct-injection liquid chromatography with tandem mass-spectrometry, and for glyphosate by Enzyme-Linked Immunoassay in a separate analysis. Potential aquatic toxicity was evaluated using the Pesticide Toxicity Index and by comparison to U.S. Environmental Protection Agency aquatic-life benchmarks. This data release provides sampling site locations, method information, summaries of quality-control data, and concentration data for pesticide compounds in environmental weekly water samples, in support of the journal article, "Complex mixtures of dissolved pesticides show potential aquatic toxicity in a synoptic study of Midwestern U.S. streams," by Nowell, L.H., Moran, P.W., Schmidt, T., Norman, J.E., Nakagaki, N., Shoda, M.E., Mahler, B.J., Van Metre, P.C., Stone, W.W., Sandstrom, M.W., and Hladik, M.L.

Dissolved pesticides were measured in weekly water samples from 100 wadeable freshwater streams across eleven states in the Midwestern U.S. during May-August, 2013, as part of the Midwest Stream Quality Assessment study conducted by the U.S. Geological Survey's (USGS) National Water Quality Assessment (NAWQA) Project. Of the 100 stream sites, 12 were urban indicator sites and the remaining 88 sites were located along an agricultural gradient of watershed land use. Twelve depth- and width-integrated water samples were collected at each site within the 14-week study period. Water samples were filtered (0.7 micrometers) and analyzed for 227 pesticide compounds by direct-injection liquid chromatography with tandem mass-spectrometry, and for glyphosate by Enzyme-Linked Immunoassay in a separate analysis. Potential aquatic toxicity was evaluated using the Pesticide Toxicity Index and by comparison to U.S. Environmental Protection Agency aquatic-life benchmarks. This data release provides sampling site locations, method information, summaries of quality-control data, and concentration data for pesticide compounds in environmental weekly water samples, in support of the journal article, “Complex mixtures of dissolved pesticides show potential aquatic toxicity in a synoptic study of Midwestern U.S. streams,” by Nowell, L.H., Moran, P.W., Schmidt, T., Norman, J.E., Nakagaki, N., Shoda, M.E., Mahler, B.J., Van Metre, P.C., Stone, W.W., Sandstrom, M.W., and Hladik, M.L.

Surface water samples were collected from four stream/agricultural drain sites in the Sacramento Valley of California to measure pesticides commonly applied to rice. Samples were collected weekly from May through August 2010 to capture the rice pesticide application season. Water samples were filtered (0.7 µm) and extracted via solid-phase extraction. Additionally, the filter paper was solvent extracted to measure suspended sediment-associated pesticides. Both fractions were analyzed via gas chromatography-mass spectrometry. A total of 89 pesticides and pesticide degradates were measured in the dissolved water phase, while 14 pyrethroid insecticides were measured in the suspended sediment-associated fraction (pyrethroids are hydrophobic and known to associate with particulates). For the dissolved phase, 18 compounds were detected. The most frequently detected compounds included those pesticides applied to rice: azoxystrobin (detection frequency 100%, maximum concentration 128,000 ng/L), clomazone (96%, 19,400 ng/L), 3,4-dichloroaniline (propanil degradate; 92%, 13,600 ng/L), thiobencarb (83%, 12,400 ng/L), and propanil (57%; 6,500 ng/L). Other frequently detected dissolved pesticides not applied to rice included boscalid (45%, 75.7 ng/L) and metolachlor (43%, 600 ng/L). Only two pyrethroids were detected in the suspended sediment fraction; this included lambda-cyhalothrin (17%, 14.8 ng/L), which was applied to rice, and bifenthrin, which is not specific to rice (6%, 26.3 ng/L).

Surface water samples were collected from four stream/agricultural drain sites in the Sacramento Valley of California to measure pesticides commonly applied to rice. Samples were collected weekly from May through August 2010 to capture the rice pesticide application season. Water samples were filtered (0.7 µm) and extracted via solid-phase extraction. Additionally, the filter paper was solvent extracted to measure suspended sediment-associated pesticides. Both fractions were analyzed via gas chromatography-mass spectrometry. A total of 89 pesticides and pesticide degradates were measured in the dissolved water phase, while 14 pyrethroid insecticides were measured in the suspended sediment-associated fraction (pyrethroids are hydrophobic and known to associate with particulates). For the dissolved phase, 18 compounds were detected. The most frequently detected compounds included those pesticides applied to rice: azoxystrobin (detection frequency 100%, maximum concentration 128,000 ng/L), clomazone (96%, 19,400 ng/L), 3,4-dichloroaniline (propanil degradate; 92%, 13,600 ng/L), thiobencarb (83%, 12,400 ng/L), and propanil (57%; 6,500 ng/L). Other frequently detected dissolved pesticides not applied to rice included boscalid (45%, 75.7 ng/L) and metolachlor (43%, 600 ng/L). Only two pyrethroids were detected in the suspended sediment fraction; this included lambda-cyhalothrin (17%, 14.8 ng/L), which was applied to rice, and bifenthrin, which is not specific to rice (6%, 26.3 ng/L).

This Data release presents results for pesticide constituents in 460 samples collected from domestic and public-supply wells in August 2013-May 2018 for the California Groundwater Ambient Monitoring and Assessment Program Priority Basin Project (GAMA-PBP). The samples were analyzed for 225 pesticide constituents on USGS National Water Quality Laboratory analytical schedule 2437. Results from quality-control samples collected with the environmental samples and results from laboratory quality-control samples were used to re-process the data to meet project data-quality objectives. The data release includes the final approved results tabulated by result and by sample, the original and final values for results affected by the quality-control re-processing, ancillary and summary information about the pesticide constituents, and results from field replicate and field blank samples.