Nitrate‐nitrogen (NO3‐N) concentrations at public water‐supply intakes on the Des Moines and Raccoon Rivers in Iowa exceeded the maximum contaminant level (MCL) of 10 mg L−1 for public water supplies established by the USEPA for extended periods of time from March through early August 1990. The excessive NO3‐N levels followed 2 yr of less than normal precipitation in 1988 and 1989. The largest daily NO3‐N load (771 t) transported during the last 17 yr in the Raccoon River occurred in June 1990. The streamflow hydrograph for the Raccoon River for March 1990 prior to seasonal fertilizer application indicates that high NO3‐N concentrations characterize the recession side of the hydrograph. High NO3‐N concentrations in streamflow persisted as streamflow decreased to baseflow conditions. This implies that substantial quantities of NO3‐N were being leached from the soil and transported by subsurface flow during early 1990. A multiple linear‐regression model was developed to predict NO3‐N concentrations in the Raccoon River from readily‐obtainable streamflow and climatic data. The four‐variable model explained about 70% of the variability in the concentration of NO3‐N. The mean streamflow for the previous 7‐d period accounted for about 50% of the total variability.

The U.S. Geological Survey conducts an external blind sample qualityassurance project for its water-analysis laboratories in Denver, Colorado, and Ocala, Florida, based on the analysis of reference water samples. Reference samples containing selected inorganic and nutrient constituents are disguised as environmental samples and are sent periodically through Survey offices to each laboratory. The results of this blind sample project indicate the quality of analytical data produced by the laboratories. This report provides instructions for the use of QADATA, an interactive, menu-driven program that allows data users to retrieve the results of the blind sample quality-assurance project. The QADATA program, which is available through the U.S. Geological Survey's national computer network, accesses a blind sample data base that contains more than 50,000 determinations from the last five water years for approximately 40 constituents at various concentrations. The data can be retrieved from the database for any user-defined time period and for any or all available constituents. After the user defines the retrieval parameters, the program prepares statistical tables, control charts, and precision plots and generates a report which can be transferred to the user's office through the computer network. A discussion of the interpretation of the program output is also included in this report. This quality-assurance information will permit users to document the quality of the analytical results received from the laboratories. The blind sample data is entered into the database within weeks after being produced by the laboratories and can be retrieved to meet the needs of specific projects or programs.

The U.S. Geological Survey maintains a quality-assurance program based on the analysis of reference sam¬ ples for its National Water-Quality Laboratory located in Denver, Colorado. Reference samples containing selected inorganic constituents are prepared at the Survey's Water Quality Services Unit in Ocala, Florida, disguised as rou¬ tine samples, and sent daily or weekly, as appropriate, to the laboratory through other Survey offices. Nutrient sam¬ ples and precipitation samples also were submitted as samples of unknown concentration. The results are stored permanently in the National Water Data Storage and Retrieval System (WATSTORE), the Survey's data base for all water data. These data are analyzed statistically for precision and bias. The results of these statistical analyses are discussed for data collected during water-year 1987. An overall evaluation of the major and trace constituent data for water-year 1987 indicated a lack of precision in the National Water-Quality Laboratory for the determination of 6 out of 58 constituents: chloride; chromium; iron, total recoverable; zinc, dissolved (atomic absorption spectroscopy); zinc, (inductively coupled plasma emission spectroscopy); and zinc, total recoverable. There were fewer constituents having positive or negative bias during water-year 1987 than during water-year 1986. A lack of precision was indicated in the determination of three of the six nutrient constituents: nitrate + nitrite nitrogen as N, orthophosphate as R and phosphorus as R A biased condition was indicated in the determination of ammonia + organic nitrogen as N. There was acceptable precision in the determination of all 10 precipitation-level constituents. One precipita¬ tion-level constituent, sodium, indicated a biased condition.

No abstract

An analysis of available hydrologic data was conducted to evaluate the effects on groundwater levels in the Missouri River alluvial aquifer caused by changes in Missouri River stage at selected sites in Fremont and Monona Counties in western Iowa. Daily mean groundwater levels and river stage measured during November 1995-September 1996, simulated daily mean river stage for November 1995-December 1996 derived from simulated daily mean discharge for eight alternative water-management plans for the Missouri River, and simulated daily mean groundwater levels for November 1995-December 1996 for selected water-management plans were used in the study. The measured data represent hydrologic conditions for the Current (1998) Water-Control Plan of the U.S. Army Corps of Engineers. Groundwater levels in the alluvial aquifer vary in response to river stage, precipitation, proximity to drainage ditches, evapotranspiration, and pumpage. In Fremont County, measured groundwater levels generally were lower than river stage during the spring, summer, and fall months. In Monona County, measured groundwater levels generally were higher than river stage. Water levels in wells at distances greater than about 8,000 feet from the river in Fremont County and about 6,500 feet in Monona County likely were more affected by precipitation Abstract 1 itation, discharge from evapotranspiration, aquifer properties, and land-surface altitude.