Abstract. Three-component (throughfall, soil water, groundwater) hydrograph separations at peak flow were performed on 10 storms over a 2-year period in a small forested catchment in north-central Maryland using an iterative and an exact solution.Seven pairs of tracers (deuterium and oxygen 18, deuterium and chloride, deuterium and sodium, deuterium and silica, chloride and silica, chloride and sodium, and sodium and silica) were used for three-component hydrograph separation for each storm at peak flow to determine whether or not the assumptions of hydrograph separation routinely can be met, to assess the adequacy of some commonly used tracers, to identify patterns in hydrograph-separation results, and to develop conceptual models for the patterns observed. Results of the three-component separations were not always physically meaningful, suggesting that assumptions of hydrograph separation had been violated. Uncertainties in solutions to equations for hydrograph separations were large, partly as a result of violations of assumptions used in deriving the separation equations and partly as a result of improper identification of chemical compositions of end-members. Results of three-component separations using commonly used tracers were widely variable. Consistent patterns in the amount of subsurface water contributing to peak flow (45-100%) were observed, no matter which separation method or combination of tracers was used. A general conceptual model for the sequence of contributions from the three end-members could be developed for 9 of the 10 storms. Overall results indicated that hydrochemical and hydrometric measurements need to be coupled in order to perform meaningful hydrograph separations.

Urban settings are a focal point for environmental contamination due to emissions from industrial and municipal activities and the widespread use of motor vehicles. As part of the National Water-Quality Assessment Program of the U.S. Geological Survey, streambedsediment and dated reservoir-sediment samples were collected from the Chattahoochee River Basin and analyzed for total lead (Pb) and zinc (Zn) concentrations. The sampling transect extends from northern Georgia, through Atlanta, to the Gulf of Mexico and reflects a steep gradient in population density from nearly 1000 people/ km 2 in the Atlanta Metropolitan Area to fewer than 50 people/ km 2 in rural areas of southern Georgia and northern Florida. Correlations among population density, traffic density, and total and anthropogenic Pb and Zn concentrations indicate that population density is strongly related to traffic density and is a predictor of Pb and Zn concentrations in the environment derived from anthropogenic activities. Differences in the distributions of total Pb and Zn concentrations along the urban-suburban-rural gradient from Atlanta to the Florida Panhandle are related to temporal and spatial processes. That is, with the removal of leaded gasoline starting in the late 1970s, peak Pb concentrations have decreased to the present. Conversely, increased vehicular usage has kept Zn concentrations elevated in runoff from population centers, which is reflected in the continued enrichment of Zn in aquatic sediments. Sediments from rural areas also contain elevated concentrations of Zn, possibly in response to substantial power plant emissions for the region, as well as vehicular traffic.

[1] Concentration-discharge (c-Q) plots have been used to infer how flow components such as event water, soil water, and groundwater mix to produce the observed episodic hydrochemical response of small catchments. Because c-Q plots are based only on observed streamflow and solute concentration, their interpretation requires assumptions about the relative volume, hydrograph timing, and solute concentration of the streamflow end-members. Evans and Davies [1998] present a taxonomy of c-Q loops resulting from three-component conservative mixing. Their analysis, based on a fixed template of endmember hydrograph volume, timing, and concentration, suggests a unique relationship between c-Q loop form and the rank order of end-member concentrations. Many catchments exhibit variability in component contributions to storm flow in response to antecedent conditions or rainfall characteristics, but the effects of such variation on c-Q relationships have not been studied systematically. Starting with a ''baseline'' condition similar to that assumed by Evans and Davies [1998], we use a simple computer model to characterize the variability in c-Q plot patterns resulting from variation in end-member volume, timing, and solute concentration. Variability in these three factors can result in more than one c-Q loop shape for a given rank order of end-member solute concentrations. The number of resulting hysteresis patterns and their relative frequency depends on the rank order of solute concentrations and on their separation in absolute value. In ambiguous cases the c-Q loop shape is determined by the relative ''prominence'' of the event water versus soil water components. This ''prominence'' is broadly defined as a capacity to influence the total streamflow concentration and may result from a combination of endmember volume, timing, or concentration. The modeling results indicate that plausible hydrological variability in field situations can confound the interpretation of c-Q plots, even when fundamental end-member mixing assumptions are satisfied.

Trace-element concentrations in 541 streambed-sediment samples collected from 20 study areas across the conterminous United States were examined as part of the National Water-Quality Assessment Program of the U.S. Geological Survey. Sediment samples were sieved and the <63-µm fraction was retained for determination of total concentrations of trace elements. Aluminum, iron, titanium, and organic carbon were weakly or not at all correlated with the nine trace elements examined: arsenic, cadmium, chromium, copper, lead, mercury, nickel, selenium, and zinc. Four different methods of accounting for background/ baseline concentrations were examined; however, normalization was not required because field sieving removed most of the background differences between samples. The sum of concentrations of trace elements characteristic of urban settingsscopper, mercury, lead, and zincswas well correlated with population density, nationwide. Median concentrations of seven trace elements (all nine examined except arsenic and selenium) were enriched in samples collected from urban settings relative to agricultural or forested settings. Forty-nine percent of the sites sampled in urban settings had concentrations of one or more trace elements that exceeded levels at which adverse biological effects could occur in aquatic biota.

Monthly mean air temperature (AT) at 85 sites and instantaneous stream-water temperature (WT) at 129 sites for 1960-2010 are examined for the mid-Atlantic region, USA. Temperature anomalies for two periods, 1961-1985 and 1985-2010, relative to the climate normal period of 1971-2000, indicate that the latter period was statistically significantly warmer than the former for both mean AT and WT. Statistically significant temporal trends across the region of 0.023°C per year for AT and 0.028°C per year for WT are detected using simple linear regression. Sensitivity analyses show that the irregularly sampled WT data are appropriate for trend analyses, resulting in conservative estimates of trend magnitude. Relations between 190 landscape factors and significant trends in AT-WT relations are examined using principal components analysis. Measures of major dams and deciduous forest are correlated with WT increasing slower than AT, whereas agriculture in the absence of major dams is correlated with WT increasing faster than AT. Increasing WT trends are detected despite increasing trends in streamflow in the northern part of the study area. Continued warming of contributing streams to Chesapeake Bay likely will result in shifts in distributions of aquatic biota and contribute to worsened eutrophic conditions in the bay and its estuaries.