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

Current and historical 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 through 2001, averaging 75,000 tons. Ground-water samples collected from sel