Determination of Agricultural Chemical Impacts on Shallow Groundwater Quality in the Rio Grande Valley: Las Nutrias Groundwater Project

A comprehensive assessment was made of water and chemical relationships at a commercial farm in the central Rio Grande valley during 1994, 1995 and 1996. A highly instrumented 15-acre tile-drained field (Las Nutrias Groundwater Project) was used to collect areally averaged data on recharge rates and nitrate and pesticide leaching to shallow groundwater.

Because its outlet to a surface drain was submerged, the tile-drainage system did not perform to design specifications. The resultant high water table below the field served as s source of water for crops during the summer, but also resulted in salinization of the field. Due to its submerged condition the tile drain collected primarily ambient groundwater rather than recharge water from the overlying field. The amount of recharge water captured by the drain was less than 5%.

In the spring of 1994, nitrate-nitrogen concentrations in recharge water periodically exceeded the drinking water standard of 10 mg/l nitrate-nitrogen; in subsequent years, nitrate never exceeded the standard. The greater leaching of nitrate in 1994 resulted from anomalously high residual nitrogen levels in the topsoil at the beginning of that year’s growing season, which allowed significant levels of nitrate (up to 38% of the fertilizer nitrogen applied that season) t be transported to groundwater by preferential flow processes. Due to rapid dilution of the recharge by ambient groundwater; however, groundwater flowing offsite had nitrate-nitrogen concentrations of only a few mg/l, well below the drinking water standard.

No pesticides were detected in the tile drain water beneath the project field. Pesticides applied by the landowner included chlorpyrifos (Lorsban), dimethoate (Dimate 4E), and cyfluthrin (Baythroid 2). Groundwater samples were analyzed for these chemicals as well as other pesticides applied in the area. A few monitoring well samples showed chlorpyrifos at levels of less than 1 æg/l. Similarly, no pesticides applied offsite were detected in groundwater beneath the field.

The flow rate and chemical signature in the tile changed rapidly in response to an irrigation event. These responses suggest rapid transmission of water and surface-applied chemical via preferential flow. The preferential flow hypothesis was supported by measurements of soil hydrologic properties and by visual observations of soil cracks and animal burrows.

This study provides the first detailed information on agricultural leaching below a commercially managed New Mexico farm. The results suggest that current management practices do not present a threat to shallow groundwater quality in the Rio Grande basin.