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<br />tions were the vertical-profile samples collected from <br />the Road 8 site in October 1992 (samples were col- <br />lected from below the middle of the channel) and <br />November 1992 (samples were collected from below <br />the channel near the right bank). The locations of the <br />vertical-profile sampling sites at Road 8 were varied to <br />examine vertical profiles of water quality at different <br />locations in the channel. <br />Selected samples of filtered surface water and <br />ground water and riverbed sediment were sent to the <br />USGS National Water-Quality Laboratory in AIVada, <br /> <br />Colorado, for analysis of the lsN/14N ratios of nitrate <br />and ammonium in the samples. Unfiltered samples of <br />surface water and ground water were collected for the <br />analysis oftotal organic carbon once during the study. <br />Samples wcre analyzed by thc USGS National Water- <br />Quality Laboratory in AIVada, Colorado, using estab- <br />lished analytical procedures (Wershaw and others, <br />1987). <br />Samples of surface watcr and ground water were <br />collected for quality-assurance assessment. Quality- <br />assurance samples consisted of replicates, equipment <br />blanks, and splits, Replicates consisted of sequential <br />samples for each type of analysis and were used to <br />monitor the precision of the laboratory analytical meth- <br />ods. Equipment blanks consisted of distilled-water <br />samples that were processed through the sampling <br />equipment, after the equipment had been washed, in <br />the same order as environmental samples, and were <br />used to monitor the potential for cross contamination of <br />samples between sampling sites. Equipment-blank and <br />replicate samples usually were collected each week <br />that the cross-section sites were sampled, and they <br />were collected each month that the monitoring wells <br />were sampled. Selected samples also were split and <br />sent to the MWRD laboratory and the USGS National <br />Water-Quality Laboratory in AIVada, Colorado, for <br />analysis of the same constituents so that the analytical <br />perfonnance of the laboratories could be compared. <br /> <br />Sediment Sampling <br /> <br />Riverbed and aquifer sediments were collected <br />one time for analyses of percent sediment (by weight) <br />that was less than 0.079 in, in size, percent organic mat- <br />ter (by weight) in the less than 0.079-in size fraction, <br />and rates of microbial denitrification. Riverbed sedi- <br />mcnt was collected from thrce locations in the river <br />channel at each of two sites (Henderson and Road 8). <br />These locations included the right edge of water, center <br />of thc channel, and left edge of water. Riverbed sedi- <br />ment was collected at the right edge of water and center <br />of the channel at the McKay Road site. Riverbed sed- <br />iment also was collected at the left edge of water at <br /> <br />88th Avenue for microbiological analyses, but not for <br />grain-size or organic-carbon analysis. Aquifer sedi- <br />ment was collected during installation of monitoring <br />wells 2B at McKay Road, 2B at Henderson, and I B at <br />Road 8 (pI. I), <br />Samples ofrjverbed sediment used for grain-size <br />and organic-matter measurements were collected using <br />PYC tubes hammered into the sediment to a depth of 0 <br />to 2 ft and withdrawn by hand to retrieve the sediment. <br />Grain size was detennined by drying the sediment at <br />I 050C for 48 hours and passing the sediment through a <br />0.079-in sieve. The sieved material was weighed and <br />divided by the weight of the starting material to deter- <br />mine the percent of sediment (by weight) that was <br />sand, silt, or clay sized. Organic-matter content of the <br />less than 0.079-in size fraction was detennined by <br />weighing the sediment before and after combustion at <br />6500C for 12 hours and attributing the change in mass <br />to organic material. Riverbed sediment used for micro- <br />biological analysis was collected using the same proce- <br />dure, except that PYC tubes were methanol-rinsed <br />prior to use, The samples were placed in sterilized <br />glass jars and shipped to the USGS South Carolina <br />District for measurement of denitrification rates using <br />the acetylene-block technique (Bradley and others, <br />1992). Preliminary experiments were done with no <br />additions of nitrate and organic carbon to estimate rates <br />of denitrification under in-situ conditions. Additional <br />experiments were perfonned to detennine if denitrifi- <br />cation rates were limited by the availability of nitrate <br />or organic carbon by adding nitrate and glucose to <br />the sediment~ and repeating the experiments, Incuba- <br />tions were done in triplicate with killed (autoclaved) <br />controls to account for abiotic production of nitrous <br />oxide (N20) by the sediment. <br />Samples of aquifer sediment used for grain-size <br />and organic-matter content were collected from the <br />alluvial aquifer at the time that the PYC monitoring <br />wells werc installed. Sediment samples were collected <br />at the locations of monitoring wells McKay Road 2B, <br />Henderson 2B, and Road 8 IA. Sediment at these sites <br />was collected every 5 ft starting at land surface and <br />ending at the base of the alluvium. Because ofthe lack <br />of cohesion of the sediments, samples were not col- <br />lected using standard coring devices such as the split <br />spoon or Shelby tube. Instead, sediment was collected <br />directly from the auger flights as they were brought to <br />land surface. These samples were analyzed for grain- <br />size and organic-matter content using procedures <br />described in the previous paragraph. Samples of aqui- <br />fer sediment for microbiological analysis were col- <br />lected from the water table in a hand-dug hole near <br />we1l2A at the Henderson site. Sediment was collected <br />using an alcohol-cleaned PYC tube that was pushed <br />into the sediment. <br /> <br />10 QuanUty and Quality of Ground-Watar Dlacharga to tha South Plana Rlvar, Danvar to Fort Lupton, Colorado, <br />Auguat 1992 Through July 1993 <br />