Hydrologic Stations – 02 Historical Water-Quality Data and Time-Series Trends

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

 Historical Water-Quality Data and Time-Series Trends

The data set for the Crater Lake HBN station analyzed for this report includes 91 water quality samples that were collected from June 1967 through September 1995. Sampling frequency averaged only three samples per year because of closure of the access road in winter. Samples from the early part of the period of record probably were analyzed at a USGS district laboratory in Portland, Oreg. (Durum, 1978). After establishment of the central laboratory system, samples were analyzed at the Salt Lake City, Utah, laboratory from 1973 to 1975 and the NWQL in Arvada, Colo., from 1976 to 1995. Lake stage for Crater Lake (station 11492200) is available beginning in October 1961, and daily water temperature at the station has been measured since October 1963.

Calculated ion balances for 86 samples that have complete major-ion analyses are shown in figures 19a and 19b. Graphs showing temporal variation of discharge, field pH, majorion concentrations, and ion balance in Crater Lake, Oregon. Ion balances ranged from – 10 to +9.6 percent, and 85 percent of the samples had values within the ±5-percent range, indicating that the analytical results are of high quality. The average ion balance for all samples was -1.0 percent, and 65 percent of samples had a slight excess of measured cations compared to measured anions, indicating that unmeasured constituents, such as organic anions, may have contributed a small amount to the ionic content of lake water at this HBN station. Time-series plots of the major dissolved constituents were inspected for evidence of method-related effects (fig. 19). The most notable pattern is in field pH, which increased rather abruptly in 1988. Several uncharacteristically high calcium and sodium concentrations were measured during the 1970’s. In addition, the scatter in calcium, alkalinity, and chloride concentrations decreased noticeably during the period of record. Because the surface-water chemistry at this station should be relatively stable due to the long residence time of water in the lake, these patterns probably are caused by sampling or analytical artifacts rather than by natural variability in lake chemistry.

Table 34. Minimum, first quartile, median, third quartile, and maximum values of physical properties and major dissolved constituents measured in water-quality samples from Crater Lake, Oregon, June 1967 through September 1995, and volume-weighted mean concentrations in bulk precipitation collected at Crater Lake National Park, Oregon[Concentrations in units of microequivalents per liter, lake stage in feet, specific conductance in microsiemens per centimeter at 25 degrees Celsius, pH in standard units, and silica in micromoles per liter; n, number of lake samples; VMW, volume-weighted mean; spec. cond., specific conductance; <, less than; –, not reported]

a Values are volume-weighted mean concentrations for 1987-88 (Nelson and others, 1993).