Hydrologic Benchmark Network Stations in the U.S. 1963-95 (USGS Circular 1173)

Analytical Methods

        Historical water-quality records for stations in the HBN are available from the USGS National Water Information System (NWIS). Water measurements made at HBN stations have included a comprehensive suite of about 85 characteristics, including physical properties, major dissolved constituents, trace elements, radiochemical constituents, nutrients, and biological constituents (Alexander and others, 1996). Data for 63 of these characteristics were recently compiled on a CD-ROM for 1962 through 1994 (Alexander and others, 1996). The parameter codes used in NWIS and the periods of record of physical properties and water-quality constituents retrieved for this report are listed in table 1. In addition to those items listed in table 1, fluoride data (code 00950) were retrieved for Falling Creek, Ga., and total aluminum (code 01106), total iron (code 01046), and dissolved organic carbon (code 00681) data were retrieved for McDonalds Branch, N.J. The following modifications were made to the data before statistical and graphical analyses were performed: (1) Alkalinity concentrations stored under parameter code 00410 before 1981 were combined with alkalinity concentrations stored under parameter code 90410 to obtain a continuous record of laboratory alkalinities; (2) dissolved and total nitrite plus nitrate and dissolved and total ammonium were combined to obtain more continuous records of these two nutrient species; (3) concentrations reported as less than were set equal to the reporting limit for the time-series plots and charge-balance calculations; (4) concentrations reported as zero were retained in the data files; (5) concentrations were converted from units of milligrams per liter to microequivalents per liter, except silica, aluminum, and iron, which were converted to units of micromoles per liter by using the conversion factors listed at the front of this report; and (6) outliers greater than five standard deviations from the mean were removed from the data sets. All periods of record discussed in this report refer to the water year defined as October 1 through September 30 unless otherwise indicated.

Table 1. Parameter codes and periods of record for physical properties and water-quality constituents retrieved from the U.S. Geological Survey National Water Information System

^a Prior to 1981, laboratory alkalinity data were stored under parameter code 00410.

^b Not reported at all stations for entire period indicated.

        The quality of chemical analyses was checked on the basis of ion balance, which was calculated as the total cation charge minus the total anion charge divided by the total charge in solution. The cation charge was calculated as the sum of the hydrogen ion (calculated from field pH measurements), calcium, magnesium, sodium, and potassium concentrations and the anion charge as the sum of laboratory alkalinity, chloride, and sulfate concentrations. Ion balance was calculated only for samples that had complete chemical analyses. The inorganic nitrogen species, nitrite plus nitrate and ammonium, were excluded from the calculation because neither of these species was measured during the entire period of record. Omission of these constituents from the ion-balance calculation was not expected to produce a substantial bias because concentrations were usually at or near the reporting limit of the analytical methods. The water-quality records also were inspected for bias that may have been introduced by sampling or analytical methods and were compared to the chronology of activities and analytical methods that were used by USGS laboratories (Durum, 1978; Fishman and others, 1994) and to operational guidelines for the HBN that were documented in a series of USGS technical memoranda available at URL http://water.usgs.gov/admin/memo/. Summary statistics of physical properties and water-quality constituents were calculated from the historical water-quality records at each station. Spearman rank correlation coefficients (rho values) were calculated to measure the strength of monotonic associations among discharge and the major solutes. Values of rho fall between -1.0 to 1.0 with a value of zero indicating no correlation between constituents.

        Temporal trends in stream discharge and water-quality constituents were calculated by using the computer program, Estimate Trend (ESTREND), which was written by Schertz and others (1991). ESTREND uses two trend-detection techniques, the nonparametric seasonal Kendall test and the parametric Tobit test (Schertz and others, 1991). The seasonal Kendall test for uncensored data was used when less than 5 percent of the observations were censored. The seasonal Kendall test for censored data was used when more than 5 percent of the observations were censored. In cases where a large number of detected concentrations fell between multiple reporting limits, as often occurred for the nitrite plus nitrate concentrations, the Tobit test was used to calculate the trend. Trends were calculated by using unadjusted concentrations and flow-adjusted concentrations. Removal of flow-related variability in the water-quality data not only improved the power of the statistical test but decreased the possibility that the observed trends were an artifact of the sampling discharge record (Hirsch and others, 1982; Schertz and others, 1991). Automated procedures are provided in ESTREND to adjust concentrations for flow-related variability. Flow adjustment was not made if the concentration-discharge model was not significant at the 0.10 probability level (Schertz and others, 1991) or if the data were highly censored. Because the minimum sampling frequency at most HBN stations was quarterly, trends were calculated by using four 3-month seasons beginning in mid-December, except for the Tallulah River in Georgia, which was tested by using two 6-month seasons per year. Trends were considered statistically significant at the 0.01 probability level. The ESTREND procedure also computes a trend slope, which represents the median rate of change in discharge or concentrations for the selected period of record. Interpretation of the trends was aided by the use of a locally weighted scatterplot smoothing technique (LOWESS), which graphically smooths the pattern of the data over time (Schertz and others, 1991). The LOWESS curves are presented in the time-series plots only in cases where the trend in discharge or the unadjusted concentration was statistically significant at the 0.01 probability level.

        Stream chemistry at each HBN station was compared to precipitation chemistry at the closest National Atmospheric Deposition Program (NADP) station. The NADP is a monitoring network of almost 200 stations nationwide that provides long-term records of weekly precipitation chemistry. Sampling, analytical, and quality-assurance protocols for the NADP network are summarized in Peden (1983). Annual volume-weighted mean (VWM) concentrations for the NADP stations presented in this report were obtained at URL http://nadp.sws.uiuc.edu.

        In addition to the historical water-quality records and NADP data, stream samples were collected at the gaging stations and from several major tributary streams in each basin between December 1990 and October 1991 to evaluate the spatial variability of surface-water chemistry as a function of subbasin characteristics, particularly geology and land use. The selection criteria for each sampling site are listed in tables throughout the report. Visits to each HBN drainage basin were scheduled during low-flow periods, and samples generally were collected for 1 to 3 days. Grab samples were collected in 2-L polyethylene bottles and filtered within 12 hours of collection. When possible, field measurements of dissolved oxygen, water temperature, and stream discharge were made at the time of sample collection. Measurements of pH were made within 12 hours of collection on unfiltered sample aliquots by using an electrode designed for low ionic-strength waters. The pH electrode was calibrated with pH 4 and pH 7 buffer solutions, then checked against a dilute sulfuric acid standard (pH 4.75) and deionized water. Samples for chemical analysis were filtered through a 0.45-mm filter and preserved within 12 hours of collection. Samples were analyzed for pH, specific conductance, major cations and anions, silica, and alkalinity at the USGS National Water Quality Laboratory (NWQL) in Arvada, Colo., by using methods developed for low-ionic-strength waters (Friedman and Fishman, 1989). Chemical data for the synoptic samples are stored in NWIS.

References Cited

Alexander, R.B., Ludtke, A.S., Fitzgerald, K.K., Briel, L.I., and Schertz, T.L., 1996, Data from the U.S. Geological Survey National Stream Water-Quality Monitoring Networks (WQN) on CD-ROM: U.S. Geological Survey Open-File Report 96-337 and Digital Data Series DDS-37.

Durum, W.H., 1978, Historical profile of quality of water laboratories and activities, 1879-1973: U.S. Geological Survey Open-File Report 78-432, 235 p.

Fishman, M.J., Raese, J.W., Gerlitz, C.N., and Husband, R.A., 1994, U.S. Geological Survey approved inorganic and organic methods for the analysis of water and fluvial sediment, 1954-94: U.S. Geological Survey Open-File Report 94-351, 55 p.

Friedman, L.C., and Fishman, M.J., 1989, Evaluation of methods used from 1965 through 1982 to determine inorganic constituents in water samples: U.S. Geological Survey Water-Supply Paper 2293, 126 p.

Hirsch, R.M., Slack, J.R., and Smith, R.A., 1982, Techniques of trend analysis for monthly water-quality data: Water Resources Research, v. 18, no. 1, p. 107-121.

Peden, M.E., 1983, Sampling, analytical, and quality assurance protocols for the National Atmospheric Deposition Program, in Campbell, S.A., ed., Sampling and analysis of rain: American Society for Testing and Materials, Special Technical Publication 823, p. 72-83.

Schertz, T.L., Alexander, R.B., and Ohe, D.J., 1991, The computer program Estimate Trend (ESTREND)-A system for the detection of trends in water-quality data: U.S. Geological Survey Water-Resources Investigations Report 91-4040, 60 p.

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Last updated July 17, 2000.