![photo of a center pivot](images/irrigation1.jpg)
USFWS Photo
by Pedro Ramirez, Jr.Most of the agricultural products produced in
the western United States would not be possible without the use of irrigation. In
semi-arid areas of the West excess water is applied to fields to flush out salts leached
onto the surface soils. This excess water (drainwater or return flow) either infiltrates
into the soil or runs off into nearby basins, ponds, or streams. Pesticides applied to the
irrigated crops can be mobilized and transported by drainwater and return flows to surface
waters. Irrigation water applied to soils containing soluble trace elements can leach
trace elements, such as selenium, and arsenic from the soil. Generally, soils derived from
marine Cretaceous shales can contain elevated concentrations of selenium and other trace
elements. Irrigation projects with a high risk of selenium toxicity to fish and wildlife
have the following characteristics:
- wetlands located on upper Cretaceous marine shales;
- soils derived from upper Cretaceous marine shales;
- a high evaporation index (>2.5);
- terminal lakes & ponds; or
- they are located downstream from a major selenium source(s).
High evaporation rates can concentrate these trace elements to levels that are toxic to
fish and sensitive bird species. Selenium is typically the trace element of particular
concern. Although small concentrations of selenium are necessary in the diet of most
animals, high concentrations are toxic to sensitive species of fish and birds. Waterfowl
or fish can ingest large concentrations of selenium when they consume plants and insects
from ponds that receive selenium-contaminated irrigation runoff or drainwater. The effects
of selenium toxicity to fish and birds include impaired reproduction and deformed embryos.
Selenium and other trace elements as well as salts in irrigation drainwater and return
flows can cause farmland to become non-productive and can pollute groundwater. Irrigation
systems, such as surge and sprinkler systems, create less runoff and reduce deep
percolation consequently reducing the problems associated with irrigation drainwater and
return flows. |