|Chemicals and Water Quality - 72
Project Leader and Principal UC Investigators
D.W. Henderson, J.W. Biggar, K.K. Tanji,Water Management for Rice Production
A major focus is on continued use of chemicals while meeting increasingly
stringent standards of water quality. The research goals are maximum
efficacy of chemicals and minimum release of their residues into drains and
river systems. Information is being gathered from district-wide operations,
commercial fields, and small field plots.
Full knowledge is lacking on the fate of fertilizers, herbicides, and other pesticides in flooded rice fields and drains. Strip plots at Davis are used to determine how fast chemicals are degraded and consumed, where they go, and how fast they get there. A new sampling technique was developed for close following of chemical distribution, persistence, and movement in flood and seepage waters.
For the first time, different experimental water management practices are being examined for their effects on the quality of irrigation return flows. This is the most comprehensive experiment on persistence and transport of agricultural chemicals in flooded rice culture, according to state and federal water quality enforcement agencies, chemical industries, and rice researchers.
The water management practices investigated are static, flow-through, and recycled systems. The static (stagnant) system has no spill of water, for only enough is applied to replenish losses to crop use and seepage; in flow-through, a fraction of applied water is spilled continuously; and in recycling, the spill water is recirculated to the inflow end for reuse.
With these management systems and with various rates and methods of chemical application, plus the new sampling equipment developed, it is now possible to determine rather comprehensively the fate of a chemical in any combination of circumstances. Thus, for example, soil-incorporated granular Ordram is present at 0.1-1.0 ppm in flood waters up to 3-4 days regardless of water management, whereas it persists at 0.5-3.3 ppm for about 6 weeks in the seepage water in the submerged soil, and thereafter in trace amounts through the growing season. Where granular Ordram was applied postflood, recycling the spill water gave more uniform distribution and more effective concentrations than did other methods. Under static and flow-through systems, chemicals applied postflood quickly lose their original uniformity of distribution as they move downstream with the water.
When ammonium sulfate is top-dressed, ammonium nitrogen is released into flood waters at exceedingly high concentrations (255 ppm as nitrogen or more) as the fertilizer dissolves, but this ammonium ion is quickly tied up by bottom mud, so that within a few days any ammonium remaining is nitrified to nitrate. With soil incorporation, in contrast, release of ammonium and nitrate ions into flood waters is minimal except in the first day or two, and much of the fertilizer remains in the ammonium form, which is fixed by the soil and not readily leached by seepage waters. It may thus persist for about 5 weeks, gradually being taken up by plants or nitrified to nitrate.
The implications are clear: Escape of chemicals into surface drain waters is minimized by restricting runoff from a rice field for 3-4 days after top-dressing with nitrogen fertilizer or applying a pesticide.
Factors (besides nitrogen fertilizer and Ordram residues) that have been measured periodically in this research include dissolved mineral salts, dissolved oxygen, phosphorus, organic carbon, biochemical oxygen demand, and turbidity all important measures of water quality and pollution.
Water flow patterns and the mixing and transport of chemicals are followed also with dye tracers, injected or sprayed. Supplementing these field studies are computer simulation models under development to predict chemical persistence in the flooded field and the concentrations of residues expectable in drain waters.
With these refinement of techniques now completed, the work will be extended to other herbicides, to basin systems more complex than rectangular strip plots, and also to selected commercial rice fields and drain systems. (RM 4)