Dissolved organic carbon concentration and dissolved organic matter characteristics in surface water samples from the U.S. Geological Survey National Water Quality Network rivers, 2008 to 2018

The sampling of 41 hydrologically diverse rivers that are monitored through the National Water Quality Network (NWQN) by the U.S. Geological Survey (USGS) took place during the water years of 2008 through 2018. Water samples were collected and filtered in the field (unless otherwise noted) using 0.45 micrometer pre-rinsed capsule filters (Versapor membrane), silicon tubing, and a peristaltic pump. Water samples were then shipped on ice to the USGS in Boulder, Colorado and chilled to approximately 4 to 6 degrees Celsius until analysis. Dissolved organic carbon (DOC) was measured on an OI700 Analytical total organic carbon analyzer by wet-oxidation; each sample was measured in replicate and the average was reported. Ultraviolet (UV) absorbance at the wavelength of 254 nanometers was measured with an Agilent HP8453 UV-visible spectrophotometer. Specific UV Absorbance (SUVA) at 254 nanometers is a calculated parameter defined as the UV absorbance at a wavelength of 254 nanometers in absorbance units per centimeter divided by the DOC concentration in milligram per liter and then multiplied by 100. SUVA at 254 nanometers is reported in units of liter per milligram carbon per meter and correlates with the percent aromatic carbon of the dissolved organic matter (DOM). Iron(III) absorbs light in the UV spectrum and, if present at appreciable concentration, can influence UV absorbance values at 254 nanometers and calculated SUVA values. For sites believed to exhibit iron(III) concentrations sufficient for the UV absorbance at 254 nanometers of the DOM to be influenced, the filter-passing total iron concentrations of whole water samples were determined using a Hach spectrophotometer and FerroVer method. SUVA values were calculated using iron(III)-corrected UV absorbance at 254 nanometers for samples that were determined to have filter-passing total iron concentrations greater than 0.02 milligram per liter. Utilizing the UV-visible absorbance spectral scans, spectral slopes at wavelengths 275 to 295 nanometers and at wavelengths 350 to 400 nanometers were determined. The spectral slope ratio is a calculated parameter defined as the spectral slope at wavelengths 275 to 295 nanometers divided by the spectral slope at wavelengths 350 to 400 nanometers. Fluorescence measurements were made on a Jobin-Yvon Horiba Fluoromax-3 fluorometer. Fluorescence measurements included the wavelength of peak emission intensity at 370 nanometers excitation and the fluorescence index (FI). FI is defined as the ratio of fluorescence emission at 470 nanometers divided by 520 nanometers at excitation 370 nanometers. Additional fluorescence indices determined included the humification index (HIX) and the freshness index (β:α). Humification index values increase with the proportion of humic substances. The humification index is defined as the area under the emission spectrum from 435 to 480 nanometers divided by the summation of areas under the emission spectrum at 300 to 345 nanometers and 435 to 480 nanometers, at an excitation of 254 nanometers. The freshness index increases with the proportion of recently produced DOM and is defined as the intensity of fluorescence emission at 380 nanometers divided by the maximum fluorescence emission intensity between 420 and 435 nanometers at excitation 310 nanometers. DOM was separated into fractions by an isolation method that passes aqueous sample at pH less than 2 sequentially through Amberlite XAD8 and XAD4 resins using low pressure liquid chromatography. The hydrophobic organic acid (HPOA) fraction is the eluate from the XAD8 fractionation column, and the transphilic organic acid (TPIA) fraction is the eluate from the XAD4 fractionation column. The hydrophilic organic (HPI) fraction is the effluent of the sample passed sequentially through the XAD8 and XAD4 fractionation columns. The hydrophobic organic neutral (HPON) fraction is that retained on the XAD8 column after passing the sample through at pH less than 2 and back eluting with sodium hydroxide; this fraction is determined by difference. All other operational DOM fraction percentages (HPOA, TPIA, HPI) are determined by measurement of DOC concentration. Associated laboratory quality assurance and quality control measures have been taken to produce final data.