Distribution and Mass of Nitrate in the Unconfined Aquifer Beneath the Intensively Cultivated Area North of the Rio Grande, San Luis Valley, Colorado, 1997 through 2001

By Robert W. Stogner, Sr.

USGS Scientific Investigations Report 2004-5290, 62 p., 14 figs.--ONLINE ONLY

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The citation for this report, in USGS format, is as follows:
Stogner, R.W., Sr., 2005, Distribution and mass of nitrate in the unconfined aquifer beneath the intensively cultivated area north of the Rio Grande, San Luis Valley, Colorado, 1997 through 2001: U.S. Geological Survey Scientific Investigations Report 2004-5290, 62 p.

Abstract

Current (1997–2001) and historical (1948–49, 1968–69) nitrate-concentration and water-level data collected from wells completed in the unconfined aquifer in the intensively cultivated area north of the Rio Grande in south-central Colorado were used to determine the distribution and mass of nitrate and to determine short- and long-term trends in the concentration and mass of nitrate in the unconfined aquifer.

The distribution of nitrate concentrations in the unconfined aquifer indicates that nitrate concentrations increased considerably between the 1940s and 1990s and were relatively stable between 1997 and 2001. However, evaluation of estimates of mass of nitrate indicated increases in the estimated mass of nitrate in the unconfined aquifer from the 1940s (6,900 tons) to the late 1960s (34,000 tons) to the late 1990s (75,000 tons). Trend analyses indicated no significant trend in annual estimates of the mass of nitrate during 1997–2001, averaging 75,000 tons.

Ground-water samples collected from selected wells with large nitrate concentrations skewed the results of nitrate distribution and mass when nearby wells were not sampled. Estimates of nitrate mass varied by 3 to 8 percent when outliers were removed from the analysis.

Contents

Abstract

Introduction

Purpose and Scope

Description of the San Luis Valley and Study Area

Acknowledgments

Methods

Historical Data

Design of the Ground-Water Monitoring Network, 1997–2001

Collection and Analyses of Ground-Water Samples

Geographic Information Systems

Calculating Variations in Concentration of Nitrate (as Nitrogen)

Estimation of the Mass of Nitrate (as Nitrogen) in the Unconfined Aquifer

Estimated Ground-Water Withdrawals

Hydrogeology

Confining Unit

Unconfined Aquifer

Depth-to-Water Table and Ground-Water Table Altitude

Saturated Thickness

Hydraulic Gradients, Ground-Water Velocity, and Estimated Traveltime

Estimated Volume of Water in Unconfined Aquifer

Distribution of Nitrate (as Nitrogen) in the Unconfined Aquifer

Distribution of Nitrate (as Nitrogen) During 1997–2001

Spatial Variability

Temporal Variability in Nitrate (as Nitrogen) Concentrations

Seasonal Variability

Variability during mass-sampling period

Annual Variability

Distribution of Nitrate (as Nitrogen) During 1968–69

Distribution of Nitrate (as Nitrogen) During 1948–49

Variations in the Distribution of Nitrate (as Nitrogen)

Variations in the Distribution of Nitrate (as Nitrogen) during 1997 through 2001

Variations in the Distribution of Nitrate (as Nitrogen) during 1940s through 2001

Mass of Nitrate (as Nitrogen) in the Unconfined Aquifer

Estimation of Error

Mass of Nitrate (as Nitrogen) during 1997–2001

Influence of High Outliers

Estimated Mass of Nitrate (as Nitrogen) Pumped from the Unconfined Aquifer

Hypothetical Effects of Nutrient Management

Mass of Nitrate (as Nitrogen) during 1968–69 and 1948–49

Variations in the Mass of Nitrate (as Nitrogen)

Variations in the Mass of Nitrate (as Nitrogen) during 1997 through 2001

Variations in the Mass of Nitrate (as Nitrogen) during 1940s through 2001

Summary

Selected References

Appendixes

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