Bowman: Effects of Farming on Groundwater

LAS NUTRIAS, N. M. -- A groundwater research project being conducted in northern Socorro County by a New Mexico Tech hydrologist is determining how current agricultural practices impact groundwater quality and is shedding more light on related water-quality issues around the state and the Southwest.

Data collected from Las Nutrias Groundwater Project by Robert Bowman and his research colleagues at Tech is being used to discover how nutrients, pesticides, and other agricultural chemicals move through soil into shallow groundwater.

In addition to providing new information for water resources management to several government agencies and organizations, Las Nutrias Groundwater Project also will provide farmers along the entire Rio Grande valley with recommendations as to how they can modify farming practices to minimize chemical leaching into groundwater, while at the same time saving them money as they irrigate and apply fertilizers and pesticides to crops.

Since the summer of 1992, Bowman and his group have been conducting their study on a 60-acre commercial farm at the northern end of Socorro County. This particular site was chosen because the private landowner uses a tile drainage system--a unique system of underground drainage pipes--to return irrigation water to the Rio Grande. After obtaining the farmer's consent to allow his farmland to be used in the field study, Tech hydrologists adapted the existing tile drainage system so that samples of irrigation return flows could be easily collected year-round under typical farming conditions.

Bowman says Las Nutrias Groundwater Project was started after a review of available technical materials indicated that very little hydrologic and water-quality data had ever been obtained from actively farmed areas. Moreover, before the study of the farmland in Las Nutrias was mounted, there was almost no available information regarding agricultural impacts on groundwater quality in New Mexico and other Southwestern states.

Some preliminary findings that have come out of Las Nutrias Groundwater Project show that water table contours and groundwater flow patterns change rapidly in response to irrigations; soil salinities and related poor crop yields are influenced by the particular makeup of soils; and nitrate levels in shallow groundwater rise rapidly following an irrigation, even during periods between applications of nitrogen fertilizer.

"We saw a marked response in the nitrate levels in our groundwater samplings right after the first irrigations following fertilizations," Bowman observes. "This is something we expected. However, we were not expecting to see subsequent irrigations resulting in even higher levels of nitrate concentrations in the tile drain lines."

Further observations of "spiking," as the rapid increase in concentrations of nitrates is called, also were made later in the growing season immediately after irrigations, even though months had passed since the last fertilization.

Many factors may have contributed to the unexpected higher concentrations of nitrate, including variable crop uptake rates, residual nitrate in the soil profile from previous growing seasons, and variations in rainfall, Bowman says. However, he believes the most significant factor was that a large portion of the nitrogen fertilizers which were applied to the field tended to remain near the soil surface until subsequent irrigations flushed them further down.

"Our preliminary analyses show that only five to 10 percent of the nitrogen in the ammonium-based fertilizers which are commonly applied to farmland is lost to groundwater," Bowman points out, "and, at those levels, federal drinking water standards for allowable levels of nitrates are not being exceeded.

"From the farmer's standpoint, the fertilizer being lost is not a significant economic loss; and from the environmentalist's viewpoint, there's not a significant degradation of the groundwater," he adds.

Based on some of the initial findings of Las Nutrias Groundwater Project, Bowman proposes that farmers themselves can adjust their fertilizing practices to minimize groundwater contamination by agricultural chemicals, while at the same time maximizing crop yields.

"I think farmers along the Rio Grande valley could achieve the same benefits they now get from fertilizing their crops with more frequent, less heavy fertilizations," Bowman notes, "so that nitrogen levels at or near the soil surface remain constant and moderate throughout the growing season."

Bowman also encourages commercial farmers to more directly incorporate their fertilizers into the soil when they do apply fertilizers, perhaps by discing it into the first six inches of topsoil. "Just these two simple modifications to the typical way nitrogen fertilizers are applied would cut down significantly on the loss of nitrates to groundwater," he says.

Hydrology research in Las Nutrias Groundwater Project will continue, Bowman says, and will expand to include water chemistry analyses which will look for pesticides commonly used on crops.

"We've already been collecting samples for pesticide analyses," he notes, "and we will soon have some data available on Lorsban chlorpyrifos, a pesticide commonly used on alfalfa crops to control weevils."

The field-study portion of the research project also will be augmented with tracer tests to determine the manner in which chemicals travel in soil and groundwater and with further monitoring of other agricultural pesticides used on the field and surrounding farms.

Further laboratory experiments conducted at New Mexico Tech by Bowman and his associates will determine how these types of pesticides are absorbed into soils and groundwater and what the characteristics are of the chemicals as they degrade.

As more and more data are collected and assessed through the Las Nutrias Groundwater Project, best management practices (BMPs) will be evaluated by the Socorro Soil and Water Conservation District (Socorro SWCD). Shortly thereafter, an educational outreach campaign will be mounted by the Socorro SWCD, beginning with the development and distribution of an informational brochure on the project, as well as list of BMPs, for Rio Grande valley farmers.

Additional documents, videotapes, and tours also will be provided as the educational outreach campaign progresses.

Many individuals and agencies, including the Socorro SWCD, have contributed to Las Nutrias Groundwater Project. Primary financial support has come from New Mexico Tech, the U. S. Bureau of Reclamation, and from a special water quality grant from the Cooperative State Research Service of the U. S. Department of Agriculture.

-NMT-

(George Zamora)