More than 100 US Geological Survey (USGS) StandardReference Water Samples (SRWSs) were analyzed for numerous trace constituents, including Al,

Two water-quality synoptic studies were made on the Colorado River in the Grand Canyon, Arizona. Field measurements and the collection of water samples for laboratory analysis were made at 10 mainstem and 6 tributary sites every 6 hours for a 48-hour period on November 5-6, 1990, and again on June 18-20, 1991. Field measurements included discharge, alkalinity, water temperature, light penetration, pH, specific conductance, and dissolved oxygen. Water samples were collected for the laboratory analysis of major and minor ions (calcium, magnesium, sodium, potassium, strontium, chloride, sulfate, silica as SiO 2), trace elements (aluminum, arsenic, boron, barium, beryllium, cadmium, cobalt, chromium, copper, iron, lead, lithium, manganese, molybdenum, nickel, selenium, thallium, uranium, vanadium and zinc), and nutrients (phosphate, nitrate, ammonium, nitrite, total dissolved nitrogen, total dissolved phosphorus and dissolved organic carbon). Biological measurements included drift (benthic invertebrates and detrital material), and benthic invertebrates from the river bottom. Laboratory Several laboratory instrumental methods were used for the analysis of samples collected during the synoptic studies. Techniques were selected based on their optimal use for specific measurement criteria, primarily accuracy, precision, sensitivity, and selectivity. These techniques have been developed over a period of several years for the specific application to environmental water analysis. Alkalinity was determined using an automated Gran titration method (Kramer, 1982). Titrations were performed on a Radiometer auto-titrator using the supernatant portions of the sample after letting the sample settle for at least 24 hours and titrated with a 0.1 N standard sulfuric acid solution. Major cations were determined using inductively coupled plasma-atomic emission spectrometric (ICP-AES) analysis. Specific details of the method and techniques are described in Garbarino and Taylor (1979). The elements calcium, iron, magnesium, silica (as SiO 2), sodium, and strontium were determined simultaneously on a sample aliquot that had been filtered (0.4-µm) and acidified (HNO 3) in the field, using ultraclean procedures. Potassium was determined on the same aliquot by an air/acetylene flame atomic absorption spectrometric technique (Fishman and Friedman, 1985).

Two sampling trips were undertaken in 1994 to determine the distribution of trace elements in the Upper Rio Grande and several of its tributaries. Water discharges decreased in the main stem of the Rio Grande from June to September, whereas dissolved concentrations of trace elements generally increased. This is attributed to dilution of base flow from snowmelt runoff in the June samples. Of the three major mining districts (Creede, Summitville, and Red River) in the Upper Rio Grande drainage basin, only the Creede District appears to impact the Rio Grande in a significant manner, with both waters and sediments having elevated concentrations of some trace elements considerably downriver. For example, dissolved zinc concentrations upriver of Willow Creek, which primarily drains the Creede District, were about 2-3 microg/L; immediately downstream of the Willow Creek confluence, concentrations were above 20 microg/L; and elevated concentrations occurred in the Rio Grande for the next 100 km. The Red River District does not significantly impact the Upper Rio Grande for most trace elements. Because of current water management practices, it is difficult to assess the impact of the Summitville District on the Upper Rio Grande. There are, however, large increases in many dissolved trace element concentrations as the Rio Grande passes through the San Luis Valley, coincident with elevated concentrations of those same trace elements in tributaries. Among these elements are As, B, Cr, Li, Mn, Mo, Ni, Sr, U, and V. None of the trace elements exceeded U.S. EPA primary drinking water standards in either survey, with the exception of cadmium in Willow Creek. Secondary drinking water standards were frequently violated, especially in tributaries draining areas where mining has occurred. Dissolved zinc (in Willow Creek in both June and September) was the only element that exceeded the EPA Water Quality Criteria for aquatic life of 120 microg/L.

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