Vaccaro, J. J., U.S. Geological Survey, Suite 600, 1201 Pacific Ave, Tacoma, WA 98402; jvaccaro@usgs.gov
The Columbia Plateau aquifer system underlies 50,600 square miles in eastern Washington, northcentral Oregon, and northwestern Idaho. The hydrologic data were analyzed for a sub-area of 32,700 square miles. The aquifer system is composed of the Columbia River Basalt Group and unconsolidated overlying sedimentary materials (named the overburden). The basalt composes more than 99 percent of the volume of the system and averages 3,300 feet in thickness. For study purposes the system was divided into seven regional hydrogeologic units, including the pre-basalt 'basement' rocks that are considered the base of the system.
Lateral hydraulic conductivities average about 40 feet per day for the overburden and 2 feet per day for the basalt. Calculated ground-water velocities average about 2.0 foot per day in the overburden and 0.2 foot per day in the basalt units. Ground-water recharge varies widely due to spatial variations in precipitation and irrigation, and was estimated to be 6,570 cubic feet per second for predevelopment conditions and 10,210 cubic feet per second for current conditions. Discharge from the system, excluding pumpage, is mainly to surface-water features. Ground-water pumpage was about 56,000 acre-feet in 1945, increased to a peak of 940,000 acre-feet in 1979, and was about 823,000 acre-feet in 1984.
Water levels have risen over about 1.2 million acres due to surface-water irrigation, and have declined over about 1 million acres due to pumping; declines began in the 1960's and are continuing today. Locally, both declines and rises have exceeded 200 feet since predevelopment conditions.
Increased recharge from surface-water irrigation is mainly balanced by an increase in leakage to rivers (1,055 cubic feet per second) and to drains (1,645 cubic feet per second), and by ground-water pumpage (1,135 cubic feet per second). Using 1980's pumpage, the aquifer system was simulated to have future long-term water-level declines ranging from 1 to more than 250 feet; the largest effects would be in the Wanapum unit, where declines would extend over 400 square miles.
Covert, John J., Washington State Department of Ecology, 4601 N. Monroe, Spokane, WA 99205-1295; jcov461@ecy.wa.gov
Static ground-water levels, and spring stage have been measured at the Baring Spring site adjacent to Sinking Creek in Lincoln County, Washington, over a number of years. This long-term monitoring program has provided sufficient data for an analysis of the pumping effects of irrigation wells on the surface and subsurface hydrology in the area.
Water-level measurements indicate that the water levels in both the Priest Rapids and Roza Members of the Wanapum Basalt fluctuate in accordance with the pumping of irrigation wells (irrigation cycles). Discharge at the Baring Spring also fluctuates with the irrigation cycles. This clearly demonstrates hydraulic continuity between the ground water and surface water at the site. Continuous records indicate that stage fluctuations in the spring lag three to four days behind the response observed in the basaltic aquifers.
Williamson, Alex K., Central Columbia Plateau NAWQA, 1201 Pacific Avenue, Suite 600, Tacoma, WA 98402; (206) 593-6530, x235; akwill@usgs.gov; http://wwwdwatcm.usgs.gov/ ccpt.nawqa.html
Sampling was done during 1993-95 to describe and explain the distribution of 84 pesticides in ground and surface water and 60 volatile organic compounds (VOCs) in ground water. Many pesticides were detected, mostly at very low concentrations. More pesticides were detected in surface water, and they were detected more frequently than in ground water.
Ground Water Land Use Studies
Dry-Farmed Grains in the Palouse Subunit (Roberts and Wagner, 1996)
Pesticides or VOCs were detected in 25% of 53 ground-water samples. Pesticides were detected in 97% of 72 surface-water samples; they were detected at all surface-water sites except for the site in the headwaters of the Palouse River. Six pesticides, one pesticide breakdown product (metabolite), and five VOCs were detected in samples from ground water, and 29 pesticides or their breakdown products were detected in surface-water samples. No pesticide concentrations in ground water (the only source of drinking water in the area) exceeded drinking-water standards or guidelines, but four pesticides (diazinon, triallate, lindane, and carbaryl) were detected in surface water at concentrations that exceeded the freshwater-chronic criteria for protection of aquatic life.
None of the pesticides commonly used in the area were detected in ground water, but 10 of these were detected in the surface water.
Irrigated Row Crops in Quincy and Pasco Basins (Roberts and Jones, 1996a)
Low concentrations of pesticides and(or) VOCs were detected in 69% of ground-water samples collected during 1993-94 from 49 wells (30 shallow domestic wells and 19 monitoring wells). Twenty-four compounds (pesticides, pesticide degradation products, or impurities in active ingredients) were detected in ground water underlying irrigated row crops. Only EDB and dieldrin concentrations exceeded their human health standard and guideline, respectively, for drinking water. Neither pesticide is still used.
Orchards in the Quincy and Pasco Basins (Roberts & Jones, 1996b)
Water from 68% of 40 wells had detectable concentrations of at least one of 20 different pesticides and VOCs. Almost 50% of the wells sampled had detectable concentrations of more than one pesticide or VOC. Of the 16 detected compounds with existing health standards or guidelines for drinking water, only 1,2-dichloropropane exceeded the standard and in only one well. More pesticides were detected in water from shallow monitoring wells than in water from shallow domestic wells. Most of the pesticides detected (except the insecticides) are not used extensively on orchards in this study area. However, they all have been used in the past on either the orchards or the surrounding cropland.
Public Supply Wells (Ryker and Williamson, 1996)
Very low concentrations of pesticides were detected in 45% of 138 public supply wells sampled in the Central Columbia Plateau. 66% of wells with detections had more than one pesticide detected. Triazine herbicides were most commonly detected. No pesticides were detected at concentrations exceeding U.S. Environmental Protection Agency (EPA) drinking water standards. Dieldrin exceeded one EPA health guideline in one well. Shallow wells with elevated concentrations of nitrate had the highest rate of pesticide detection.
Surface Water (Wagner, Ebbert, and Roberts, 1996)
Surface-water sites representing four drainage basins with different farming practices were sampled. Samples were collected one to five times a month from March 1993 through May 1994. Nearly all samples had pesticide detections. Of the 84 compounds targeted for analysis, 45 compounds were detected. Atrazine, Simazine, DCPA, and EPTC were detected in more than half of the samples. Five pesticides (chlorpyrifos, azinphos-methyl, carbaryl, diazinon, and triallate) were detected during this time period at concentrations that could adversely affect aquatic life. No pesticides were detected at concentrations that exceeded EPA drinking water standards, but concentrations of dieldrin and alpha-HCH exceeded their drinking water guidelines. Detections of pesticides usually, but not always, were related to pesticides used on crops in the drainage basins.
Ebbert, James C., and Williamson, Alex K., U.S. Geological Survey, Central Columbia Plateau NAWQA, 1201 Pacific Avenue, Suite 600, Tacoma, WA 98402; (206) 593-6530 x234; jebbert@usgs.gov
Analyses of suspended-sediment and suspended-solids data indicate that the ongoing transition from furrow irrigation to sprinkler irrigation in the Quincy-Pasco Basins has reduced the transport of soil from cropland to surface waters. About 67 percent of the differences in amounts of suspended sediment exported from drainage basins during the irrigation season can be attributed to irrigation method. Other factors that affect soil erosion, such as soil type and slope, were not evaluated in this study. Average daily yields of suspended sediment in surface-water discharges from nine drainage basins sampled during the 1994 irrigation season ranged from 0.4 pound per acre of irrigated cropland in Mattawa Drain, where the contributing drainage area has almost no furrow irrigation, to about 17 pounds per acre of irrigated cropland in Lind Coulee Wasteway, where the furrow method is used on about 60 percent of cropland in the contributing drainage basin (not including areas irrigated by ground water). The data show a downward trend in yields of suspended solids at Crab Creek near Beverly associated with a decrease over time in the use of furrow irrigation in the Lower Crab Creek Basin. Although there are fewer data to support the conclusion, the use of sprinkler irrigation also reduces losses of pesticides and nitrogen from cropland in surface runoff. Because it is associated with sediment, phosphorus losses are probably reduced also, but reliable application data are not available to confirm this.