Kenneth Hyer

Quantifying watershed‐scale groundwater loading and in‐stream fate of nitrate using high‐frequency water quality data

We describe a new approach that couples hydrograph separation with high-frequency nitrate data to quantify time-variable groundwater and runoff loading of nitrate to streams, and the net in-stream fate of nitrate at the watershed scale. The approach was applied at three sites spanning gradients in watershed size and land use in the Chesapeake Bay watershed. Results indicate that 58–73% of the annual nitrate load to the streams was groundwater-discharged nitrate. Average annual first-order nitrate loss rate constants (k) were similar to those reported in both modeling and in-stream process-based studies, and were greater at the small streams (0.06 and 0.22 day−1) than at the large river (0.05 day−1), but 11% of the annual loads were retained/lost in the small streams, compared with 23% in the large river. Larger streambed area to water volume ratios in small streams results in greater loss rates, but shorter residence times in small streams result in a smaller fraction of nitrate loads being removed than in larger streams. A seasonal evaluation of k values suggests that nitrate was retained/lost at varying rates during the growing season. Consistent with previous studies, streamflow and nitrate concentrations were inversely related to k. This new approach for interpreting high-frequency nitrate data and the associated findings furthers our ability to understand, predict, and mitigate nitrate impacts on streams and receiving waters by providing insights into temporal nitrate dynamics that would be difficult to obtain using traditional field-based studies.

Comparison of two regression-based approaches for determining nutrient and sediment fluxes and trends in the Chesapeake Bay watershed

Nutrient and sediment fluxes and changes in fluxes over time are key indicators that water resource managers can use to assess the progress being made in improving the structure and function of the Chesapeake Bay ecosystem. The U.S. Geological Survey collects annual nutrient (nitrogen and phosphorus) and sediment flux data and computes trends that describe the extent to which water-quality conditions are changing within the major Chesapeake Bay tributaries. Two regression-based approaches were compared for estimating annual nutrient and sediment fluxes and for characterizing how these annual fluxes are changing over time. The two regression models compared are the traditionally used ESTIMATOR and the newly developed Weighted Regression on Time, Discharge, and Season (WRTDS). The model comparison focused on answering three questions: (1) What are the differences between the functional form and construction of each model? (2) Which model produces estimates of flux with the greatest accuracy and least amount of bias? (3) How different would the historical estimates of annual flux be if WRTDS had been used instead of ESTIMATOR? One additional point of comparison between the two models is how each model determines trends in annual flux once the year-to-year variations in discharge have been determined. All comparisons were made using total nitrogen, nitrate, total phosphorus, orthophosphorus, and suspended-sediment concentration data collected at the nine U.S. Geological Survey River Input Monitoring stations located on the Susquehanna, Potomac, James, Rappahannock, Appomattox, Pamunkey, Mattaponi, Patuxent, and Choptank Rivers in the Chesapeake Bay watershed. Two model characteristics that uniquely distinguish ESTIMATOR and WRTDS are the fundamental model form and the determination of model coefficients. ESTIMATOR and WRTDS both predict water-quality constituent concentration by developing a linear relation between the natural logarithm of observed constituent concentration and three explanatory variables—the natural log of discharge, time, and season. ESTIMATOR uses two additional explanatory variables— the square of the log of discharge and time-squared. Both models determine coefficients for variables for a series of estimation windows. ESTIMATOR establishes variable coefficients for a series of 9-year moving windows; all observed constituent concentration data within the 9-year window are used to establish each coefficient. Conversely, WRTDS establishes variable coefficients for each combination of discharge and time using only observed concentration data that are similar in time, season, and discharge to the day being estimated. As a result of these distinguishing characteristics, ESTIMATOR reproduces concentration-discharge relations that are closely approximated by a quadratic or linear function with respect to both the log of discharge and time. Conversely, the linear model form of WRTDS coupled with extensive model windowing for each combination of discharge and time allows WRTDS to reproduce observed concentration-discharge relations that are more sinuous in form. Another distinction between ESTIMATOR and WRTDS is the reporting of uncertainty associated with the model estimates of flux and trend. ESTIMATOR quantifies the standard error of prediction associated with the determination of flux and trends. The standard error of prediction enables the determination of the 95-percent confidence intervals for flux and trend as well as the ability to test whether the reported trend is significantly different from zero (where zero equals no trend). Conversely, WRTDS is unable to propagate error through the many (over 5,000) models for unique combinations of flow and time to determine a total standard error. As a result, WRTDS flux estimates are not reported with confidence intervals and a level of significance is not determined for flow-normalized fluxes. 2 Comparison of Two Regression-Based Approaches for Determining Nutrient and Sediment Fluxes and Trends The differences between ESTIMATOR and WRTDS, with regard to model form and determination of model coefficients, have an influence on the determination of nutrient and sediment fluxes and associated changes in flux over time as a result of management activities. The comparison between the model estimates of flux and trend was made for combinations of five water-quality constituents at nine River Input Monitoring stations. The major findings with regard to nutrient and sediment fluxes are as follows: (1) WRTDS produced estimates of flux for all combinations that were more accurate, based on reduction in root mean squared error, than flux estimates from ESTIMATOR; (2) for 67 percent of the combinations, WRTDS and ESTIMATOR both produced estimates of flux that were minimally biased compared to observed fluxes (flux bias = tendency to over or underpredict flux observations); however, for 33 percent of the combinations, WRTDS produced estimates of flux that were considerably less biased (by at least 10 percent) than flux estimates from ESTIMATOR; (3) the average percent difference in annual fluxes generated by ESTIMATOR and WRTDS was less than 10 percent at 80 percent of the combinations; and (4) the greatest differences related to flux bias and annual fluxes all occurred for combinations where the pattern in observed concentrationdischarge relation was sinuous (two points of inflection) rather than linear or quadratic (zero or one point of inflection). The major findings with regard to trends are as follows: (1) both models produce water-quality trends that have factored in the year-to-year variations in flow; (2) trends in water-quality condition are represented by ESTIMATOR as a trend in flow-adjusted concentration and by WRTDS as a flownormalized flux; (3) for 67 percent of the combinations with trend estimates, the WRTDS trends in flow-normalized flux are in the same direction and magnitude to the ESTIMATOR trends in flow-adjusted concentration, and at the remaining 33 percent the differences in trend magnitude and direction are related to fundamental differences between concentration and flux; and (4) the majority (85 percent) of the total nitrogen, nitrate, and orthophosphorus combinations exhibited longterm (1985 to 2010) trends in WRTDS flow-normalized flux that indicate improvement or reduction in associated flux and the majority (83 percent) of the total phosphorus (from 1985 to 2010) and suspended sediment (from 2001 to 2010) combinations exhibited trends in WRTDS flow-normalized flux that indicate degradation or increases in the flux delivered. Introduction Excessive nutrient (nitrogen and phosphorus) and sediment transport to the Chesapeake Bay from the watershed is detrimental to the overall structure and function of the bay ecosystem and is a major concern for local, State, and Federal entities that benefit from and work to protect the living resources of the bay. The flux (also called load) of nutrients to the Chesapeake Bay is in part natural but has been accelerated as a result of anthropogenic inputs of these nutrients through sewage disposal, agricultural runoff, urban runoff, and acid rain (Officer and others, 1984; Nixon, 1987; Schlesinger, 1997). Accelerated eutrophication through excessive nutrient flux has been linked to the loss of critical habitat for living resources within the Chesapeake Bay estuary (U.S. Environmental Protection Agency, 1983). Cooper and Brush (1991) found that accelerated algal production resulting from elevated nutrient fluxes has led to an increased occurrence of anoxic conditions in bottom waters and associated sediment throughout the Chesapeake Bay estuary. Similarly, the flux of sediment to the Chesapeake Bay results from both natural processes associated with upland erosion, lateral movement of channels into streambanks, and downcutting of streambeds (Waters, 1995) as well as anthropogenic processes such as agriculture, logging, mining, and urbanization. Anthropogenically derived sediment can overwhelm the natural assimilative capacity of the aquatic system (Cairns, 1977) and may bury filter-feeding organisms, reduce habitat available for macroinvertebrates, contribute to decreased fish populations, and impair growth of aquatic vegetation by reducing available light (Lenat and others, 1981; Dennison and others, 1993; Box and Mossa, 1999; Madsen and others, 2001). The Chesapeake Bay Program (CBP) was initiated in 1983 to direct the restoration and protection of the Chesapeake Bay. The CBP is composed of various Federal, State, academic, and local watershed organizations. In 1987, the CBP established its first nutrient reduction goal, which was to reduce nitrogen and phosphorus fluxes to the Chesapeake Bay. In 2000, the CBP recommitted to achieve the nutrient and sediment reduction goals established in 1987and established criteria for dissolved oxygen, chlorophyll, and water clarity (Chesapeake Bay Program, 2000). Despite extensive restoration efforts made by the CBP, however, established waterquality goals were not being obtained for the Chesapeake Bay and associated tributaries (Chesapeake Bay Foundation, 2010). As a result, in 2010, the U.S. Environmental Protection Agency (USEPA) established the Chesapeake Bay total maximum daily load (TMDL) for nitrogen, phosphorus, and sediment (U.S. Environmental Protection Agency, 2010). This TMDL assigns accountability for nutrient and sediment fluxes to New York, Pennsylvania, Maryland, Delaware, Virginia, West Virginia, and the District of Columbia and serves as a catalyst for rigorous implementation of management actions to mitigate the transport of excessive nutrients and sediment to the Chesapeake Bay and tidal estuaries. Since the early 1990s, the U.S. Geological Survey (USGS), in cooperation with the Virginia Department of Environmental Quality (VADEQ) and the Maryland Department of Natural Resources (MDDNR), has been responsible for monitoring nutrient and sedime

Contaminants in urban waters—Science capabilities of the U.S. Geological Survey

Streams and estuaries with urban watersheds commonly exhibit increased streamflow and decreased base flow; diminished streamchannel stability; excessive amounts of contaminants such as pesticides, metals, industrial and municipal waste, and combustion products; and alterations to biotic community structure. Collectively, these detrimental effects have been termed the “urban-stream syndrome.” Water-resource managers seek to lessen the effects on receiving water bodies of new urban development and remediate the effects in areas of existing urbanization. Similarly, the scientific community has produced extensive research on these topics, with researchers from the U.S. Geological Survey (USGS) leading many studies of urban streams and the processes responsible for the urban-stream syndrome. Increasingly, USGS studies are evaluating the effects of management and restoration activities to better understand how urban waters respond to the implementation of management practices. The USGS has expertise in collecting and interpreting data for many physical, chemical, and ecological processes in urban waters and, thus, provides holistic assessments to inform managers of urban water resources.