Although numerous studies of hyporheic exchange and denitrification have been conducted in pristine, high‐gradient streams, few studies of this type have been conducted in nutrient‐rich, low‐gradient streams. This is a particularly important subject given the interest in nitrogen (N) inputs to the Gulf of Mexico and other eutrophic aquatic systems. A combination of hydrologic, mineralogical, chemical, dissolved gas, and isotopic data were used to determine the processes controlling transport and fate of NO3− in streambeds at five sites across the USA. Water samples were collected from streambeds at depths ranging from 0.3 to 3 m at three to five points across the stream and in two to five separate transects. Residence times of water ranging from 0.28 to 34.7 d m−1 in the streambeds of N‐rich watersheds played an important role in allowing denitrification to decrease NO3− concentrations. Where potential electron donors were limited and residence times were short, denitrification was limited. Consequently, in spite of reducing conditions at some sites, NO3− was transported into the stream. At two of the five study sites, NO3− in surface water infiltrated the streambeds and concentrations decreased, supporting current models that NO3− would be retained in N‐rich streams. At the other three study sites, hydrogeologic controls limited or prevented infiltration of surface water into the streambed, and ground‐water discharge contributed to NO3− loads. Our results also show that in these low hydrologic‐gradient systems, storm and other high‐flow events can be important factors for increasing surface‐water movement into streambeds.

No abstract

What kind of water-quality information does the NAWQA Program provide? Water-quality assessments by a single program cannot possibly address all of the Nation's water-resources needs and issues. Therefore, it is necessary to define the context within which NAWQA information is most useful. • Total resource assessment-NAWQA assessments are long-term and interdisciplinary, and include information on water chemistry, hydrology, land use, stream habitat, and aquatic life. Assessments are not limited to a specific geographic area or water-resource problem at a specific time. Therefore, the findings describe the general health of the total water resource, as well as emerging water issues, thereby helping managers and decision makers to set priorities. • Source-water characterization-Assessments focus on the quality of the available, untreated resource and thereby complement (rather than duplicate) Federal, State, and local programs that monitor drinking water. Findings are compared to drinking-water standards and health advisories as a way to characterize the resource. • Compounds studied-Assessments focus on chemical compounds that have wellestablished methods of investigation. It is not financially or technically feasible to assess all the contaminants in our Nation's waters. In general, the NAWQA Program investigates those pesticides, nutrients, volatile organic compounds, and metals that have been or are currently used commonly in agricultural and urban areas across the Nation. A complete list of compounds studied is on the NAWQA Web site at water.usgs.gov/nawqa. • Detection relative to risk-Compounds are measured at very low concentrations, often 10 to 100 times lower than Federal or State standards and health advisories. Detection of compounds, therefore, does not necessarily translate to risks to human health or aquatic life. However, these analyses are useful for identifying and evaluating emerging issues, as well as for tracking contaminant levels over time. • Multiple scales-Assessments are guided by a nationally consistent study design and uniform methods of sampling and analysis. Findings thereby pertain not only to water quality of a particular stream or aquifer, but also contribute to the larger picture of how and why water quality varies regionally and nationally. This consistent, multiscale approach helps to determine if a water-quality issue is isolated or pervasive. It also allows direct comparisons of how human activities and natural processes affect water quality in the Nation's diverse environmental settings.

No abstract

For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprodTo order this and other USGS information products, visit http://store.usgs.gov Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.Suggested citation: Degnan, J.R., and Brayton, M.J., 2010, Preliminary investigation of paleochannels and groundwater specific conductance using direct-current resistivity and surface-wave seismic geophysical surveys at the Standard Chlorine of Delaware, Inc., Superfund Site, Delaware City, Delaware, 2008: U.S. Geological Survey Open-File Report 2010-1058, 27 p., at http://pubs.usgs.gov/of/2010/1058/. iii AcknowledgmentsThe authors of this report would like to recognize the work and dedication of Bernice Pasquini Altitude, as used in this report, refers to distance above the vertical datum.Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25°C).Resistivity is given in Ohm meters. ACRONYMS USED IN AbstractThe U.S. Geological Survey (USGS), in cooperation with Region III of the U.S. Environmental Protection Agency (USEPA) and the State of Delaware, is conducting an ongoing study of the water-quality and hydrogeologic properties of the Columbia and Potomac aquifers and the extent of cross-aquifer contamination with benzene; chlorobenzene; 1,2-dichlorobenzene; 1,4-dichlorobenzene; and hydrogen chloride (hydrochloric acid when dissolved in water) in the vicinity of the Standard Chlorine of Delaware, Inc. (SCD), Superfund Site, Delaware City, Delaware. Surface geophysical surveys and well data were used to identify and correlate low-permeability units (clays) across the site and to search for sand and gravel filled paleochannels that are potential conduits and receptors of contaminated groundwater and (or) Dense Non-Aqueous Phase Liquid (DNAPL) contaminants. The combined surveys and well data were also used to characterize areas of the site that have groundwater with elevated (greater than 1,000 microsiemens per centimeter) specific conductance (SC) as a result of contamination.The most electrically conductive features measured with direct-current (DC) resistivity at the SCD site are relatively impermeable clays and permeable sediment that are associated with elevated SC in groundwater. Many of the resistive features include paleochannel deposits consisting of coarse-grained sediments that are unsaturated, have low (less than 200 microsiemens per centimeter) SC pore water, or are cemented. Groundwater in uncontaminated parts of the Columbia aquifer and of the Potomac aquifer ...

Methods for groundwater sampling have evolved over time. This evolution has been driven by changing theories on how to obtain representative aquifer water samples. Passive sampling is a fairly recent method that relies on the natural flushing capacity of a well to obtain representative samples. The use of diffusion samplers is one method of passive sampling and works well under certain conditions. As part of a 2-year study to determine the temporal variability and trends in concentrations of volatile organic compounds (VOC's) in a large plume (0.5 mi2 area) of contaminated ground water in a glacial-drift aquifer, results of VOC analyses of samples collected with diffusion bag samplers were compared with those of samples collected with other types of samplers. The area of study is the primary source area of the large VOC plume and is located adjacent to a a river that losses flow and recharges the aquifer. The concentrations of VOC's, primarily tetrachloroethylene (PCE), trichloroethylene (TCE), and cw-l,2-dichloroethene (cw-l,2DCE), in samples collected with diffusion samplers show a strong positive linear correlation (root-mean square error of 0.94 and above) with concentrations from purged samples following low-flow sampling procedures. A total of 20 coupled diffusion and peristaltic-pump samples were collected from 7 wells completed in high-permeability glacial-drift. The mean concentration of PCE in the diffusion samples was 1,152 parts per billion (ppb) and the mean from the peristalticpump samples was 1,119 ppb. The standard deviations also were similar. The mean concentrations of TCE were slightly higher in diffusion samples (89.2 ppb) than peristaltic-pump samples (75.4 ppb). The mean concentration of cw-l,2DCE in diffusion samples (95.0 ppb) was virtually identical to the mean in peristaltic-pump samples. Although VOC concentrations changed dramatically at several wells over the sampled period, trends in VOC's detected using diffusion samplers corresponded with trends in VOC's from other lowflow sampling methods. For example, at two wells where coupled diffusion and peristaltic-pump samples were collected, VOC concentrations varied by a half order of magnitude over a two-month period. Although the diffusion sampler was installed and left in the well for the entire period, VOC concentrations in the diffusion sampler at the time of retrieval generally matched those in the instantaneous samples collected with the peristaltic pump on the same day, suggesting relatively rapid equilibration of the diffusion sampler to VOC concentrations in the well. The use of diffusion samplers allowed for the understanding of contaminant transport conditions at the study site because it allowed for an increase in the frequency of sampling without an associated increase in labor cost. For example, spatially variable declines in PCE concentrations were identified over the two-year study that are related to spatial variations in sediment lithology and the location of the plume within the groundwater flow system. Wells screened ...

Vertical Datum: In this report "NGVD-29" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929) a geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929. Contents

Passive diffusion samplers have been tested at a number of sites where volatile organic compounds (VOC's) are the principal contaminants in ground water. Test results generally show good agreement between concentrations of VOC's in samples collected with diffusion samplers and concentrations in samples collected by purging the water from a well. Diffusion samplers offer several advantages over conventional and low-flow groundwater sampling procedures: • Elimination of the need to purge a well before collecting a sample and to dispose of contaminated water. • Elimination of cross-contamination of samples associated with sampling with non-dedicated pumps or sample delivery tubes. • Reduction in sampling time by as much as 80 percent of that required for "purge type" sampling methods. • An increase in the frequency and spatial coverage of monitoring at a site because of the associated savings in time and money. The successful use of diffusion samplers depends on the following three primary factors: (1) understanding site conditions and contaminants of interest (defining sample objectives), (2) validating of results of diffusion samplers against more widely acknowledged sampling methods, and (3) applying diffusion samplers in the field.