|
|
| Central Columbia Plateau - Yakima River Basin NAWQA Study - Publications |
Water-quality conditions in the Central Columbia Plateau were evaluated as part of the National Water Quality Assessment Program on the basis of analysis of nutrient, suspended-sediment, and pesticide data collected from 1959 to 1991 at 105 surface-water sampling locations. Three subunits were delineated based on differences in surface-water hydrology and water-quality conditions in the study unit. The movement of surface water through the Quincy-Pasco subunit in the southwestern part of the study unit is artificially controlled by the Columbia Basin Irrigation Project. The highly variable streamflows in the Palouse subunit in the eastern part of the study unit are more naturally influenced by storm runoff and snowmelt. The North-Central subunit in the central and northern parts of the study unit has few perennial streams and an extensive network of ephemeral streams.
Non-point sources account for at least 97 percent of the estimated inputs of nitrogen and phosphorus to the study unit. Locally, however, point sources may dominate: most of the nutrient loading to the South Fork Palouse River during low streamflows comes from the Moscow and Pullman sewage treatment plants (STPs). The Quincy-Pasco subunit receives the highest rates of fertilizer and pesticide applications (approximately 132 pounds of nitrogen, 17 pounds of phosphorus, and 2.3 pounds of pesticides per acre per year) in the study unit. The combined annual loads of total nitrogen and phosphorus from four large drainages from the Quincy- Pasco subunit (which together represent approximately 0.8 percent of the drainage area of the Columbia River below Priest Rapids Dam) are about 3.4 percent and 0.7 percent, respectively, of the corresponding annual loads in the Columbia River below Priest Rapids Dam.
Nutrient concentrations in the Quincy-Pasco subunit vary seasonally and spatially as a result of the operations of the irrigation project and of changing proportions of irrigation return flows, unused irrigation water, and ground-water seepage in the surface drains and wasteways. Higher phosphorus concentrations occur during the irrigation season, when suspended-sediment concentrations also are highest. Detections of pesticide compounds throughout the year in the Quincy-Pasco subunit indicate that pesticides may be transported to surface waters by agricultural and storm runoff and influx from the ground- water system. Variations in nitrogen-species concentrations in the Quincy-Pasco subunit during and after the irrigation season indicate interactions between ground and surface waters: constituents are carried to the ground- water table by artificial recharge, shallow ground water then discharges to surface drains and wasteways, and concentrations are diluted by unused excess water during the irrigation season. Concentrations generally increase downstream as a result of contributions from agricultural runoff and ground-water seepage, except when mitigated by dilution and (or) biological assimilation of nutrients.
Variations in water quality in the Palouse subunit relate to runoff from agricultural lands during storms and to dilution of STP discharges. There is more precipitation in this part of the study unit, and the loess soils are particularly subject to erosion during storms. Runoff transports nutrients, sediment, and possibly pesticides to the streams. Nutrient and suspended-sediment concentrations are highest from November through April in most of this subunit. Several STPs operate in the Palouse subunit, and nutrient concentrations in tributaries affected by these STPs are highest during low streamflows.
The chronic water-quality criterion for the protection of aquatic life was exceeded at 9 of the 10 sites in the study unit where water samples collected during the 1970's were analyzed for DDT and its metabolites; the overall frequency of detection was low, however. DDT was detected in 97 percent of the whole-fish samples collected in the irrigation project. U.S. Environmental Protection Agency and Washington State surface-water- quality chronic standards for dieldrin were exceeded at four of nine sites; dieldrin was detected in 43 percent of the whole-fish samples collected in the irrigation project, and the human health criterion for edible fish fillets was exceeded in four fish.
Better information is needed about the timing, amounts, and new types of pesticides applied in the study unit. Many of the pesticides investigated in previous studies currently are not used, or are not used as much as other compounds. Some of the compounds currently in use are less stable and more soluble than pesticides used in the past and may result in a larger flux of pesticides to surface waters from the shallow ground-water system.
More data are needed to further improve understanding of water quality in the Central Columbia Plateau. Little is known about surface-water quality outside of the irrigation project and the Palouse River Basin or about water-quality conditions in barren and range lands and ground-water-irrigated farm lands. Because streamflows in the irrigation project are relatively uniform from year to year, it is possible to estimate constituent loads in this part of the study unit with fewer samples than are required for the rest of the study unit.
CITATION: Greene, K.E., Ebbert, J.C., and Munn, M.D., 1996, Nutrients, suspended sediment, and pesticides in streams and irrigation systems in the Central Columbia Plateau in Washington and Idaho, 1959-1991: U.S. Geological Survey Water-Resources Investigations Report 94-4215, 125 p.
For a paper copy of the full report, send email to: rwblack@usgs.gov