Long-Term Trend Analysis of Non-Point Source Contamination to Shallow Alluvial Groundwater in Colorado.

Land management practices involving the use of fertilizer and pesticide products have led to nonpoint source contamination of some underlying shallow, unconfined aquifer systems.  Few projects exist that are capable of monitoring the intensity, duration, and long-term trend of these contaminated aquifer systems with respect to agricultural chemicals.  The Agricultural Chemicals and Groundwater Protection Program, in the Colorado Department of Agriculture, have one such monitoring effort in place within the South Platte River Basin of northeastern Colorado.  The primary land use for the study area is agricultural production; however, concentrated animal feeding operations, rural/urban development, septic leach fields, industrial/commercial development, railroad, right-of-way, and other land use types provide ample non-point sources of fertilizer and pesticide products.  In 1995, monitoring well and irrigation well networks, with a fairly uniform distribution, were established to monitor the alluvial groundwater for potential contamination with agricultural chemicals.  Annual sampling of these well networks and laboratory analysis for a suite of pesticides and nitrate has provided the sample numbers needed to facilitate statistical trend analysis using non-parametric techniques.  Viewed as a whole, the irrigation well network shows a significant downward trend in nitrate concentration, while the monitoring well network reports no significant trend.  Trend analysis on individual wells shows significant upward trends in 8 of 36 irrigation wells and 3 of 19 monitoring wells.  Significant downward trends were observed in 13 of 36 irrigation wells and 6 of 19 monitoring wells.  Within a Geospatial Information System, trend responses will be compared to other available data such as crop rotations, soil characteristics, and well proximity to concentrated animal feeding operations, in an effort to explain the existence of trends.