Livestock agriculture (e.g., dairy, beef, pork, poultry) in the United States is tending rapidly toward operations where a large number of animals are concentrated in a relatively small area. The economies of scale are counterbalanced by the dangers of pollution from inadequate treatment of animal waste. Traditional methods of treatment involve lagoon retention and subsequent spreading on fields, but the sheer volume of production seems to be outstripping these and other technologies. Surface-water runoff finds its way into streams and rivers, ultimately polluting all downstream segments of the watershed. The topic of this paper is spatio-temporal statistical modeling of (log) nitrate concentration in the upper North Bosque watershed, which is a region of concentrated dairy operations. A model is fitted from daily data collected over a period of 15 months at 17 stream monitoring sites throughout the watershed. Optimal predictions of unknown nitrate concentration at all stream locations at any given time are obtained, along with a measure of their variability.

Key Words
Database construction; Geographic Information System; Geostatistics; GIS; Heteroscedasticity; Kriging; Nonpoint source; Upper North Bosque watershed; Variogram.

Noel Cressie is Distinguished Professor, Department of Statistics, Snedecor Hall, Iowa State University, Ames, IA 50011. James J. Majure is Senior Analyst, Miner and Miner, Consulting Engineers, Fort Collins, CO 80632.