Research about Pasco Basin

USGS2016

Simulation of groundwater storage changes in the eastern Pasco Basin, Washington

The Miocene Columbia River Basalt Group and younger sedimentary deposits of lacustrine, fluvial, eolian, and cataclysmic-flood origins compose the aquifer system of the Pasco Basin in eastern Washington. Irrigation return flow and canal leakage from the Columbia Basin Project have caused groundwater levels to rise substantially in some areas, contributing to landslides along the Columbia River. Water resource managers are considering extraction of additional stored groundwater to supply increasing demand and possibly mitigate problems caused by the increased water levels. To help address these concerns, the transient groundwater model of the Pasco Basin documented in this report was developed to quantify the changes in groundwater flow and storage. The MODFLOW model uses a 1-kilometer finite-difference grid and is constrained by logs and water levels from 846 wells in the study area. Eight model layers represent five sedimentary hydrogeologic units and underlying basalt formations. Head‑dependent flux boundaries represent the Columbia and Snake Rivers to the west and south, respectively, underflow to and (or) from adjacent areas to the northeast, and discharge to agricultural drains, springs, and groundwater withdrawal wells. Specified flux boundaries represent recharge from infiltrated precipitation and anthropogenic sources, including irrigation return flow and leakage from water-distribution canals. The model was calibrated with the parameter ‑ estimation code PEST++ to groundwater levels measured from 1907 through 2013 and measured discharge to springs and estimated discharge to agricultural drains. Increased recharge since pre-development resulted in a 6.8 million acre-feet increase in storage in the 508-14 administrative area of the Pasco Basin. Four groundwater-management scenarios simulate the 7-year drawdown resulting from withdrawals in different locations. Withdrawals of 2 million gallons per day (Mgal/d) from a hypothetical well field in the upper Ringold Formation along the Columbia River could generate 30–70 feet of drawdown, which may reduce landslide susceptibility along the White Bluffs. Drawdowns resulting from a 1 Mgal/d withdrawal from wells screened in either Pasco gravels, upper Ringold Formation, or both Ringold Formation and underlying basalt are simulated in the other three scenarios, and differ because of the contrasting hydraulic conductivities within the screened intervals.

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USGS1995

Distribution and sources of nitrate, and presence of fluoride and pesticides, in parts of the Pasco Basin, Washington, 1986-88

Ground water was sampled in a 900-square-mile agricultural area in the Pasco Basin, which includes parts of eastern Benton County and western Franklin County, Washington, to determine distributions of nitrate and fluoride. Additional data were obtained to determine if fertilizers, irrigation water, septic systems, and naturally occurring nitrate are sources of nitrate in ground water. Limited sampling also was done to determine if pesticides were present in the ground water. Nitrate concentrations in ground water ranged from less than 0.1 to 100 milligrams per liter as nitrogen, and median concentrations of nitrate nitrogen in ground water were 3.2 and 6.7 milligrams per liter for Benton and Franklin Counties, respectively. In Franklin County, where a large percentage of the land is used for irrigated agriculture, nitrate nitrogen concentrations in water from 31 percent of sampled wells were equal to or greater than the maximum contaminant level for drinking water of 10 milligrams per liter. In Benton County, nitrate concentrations in water from about 10 percent of the sampled wells exceeded the maximum contaminant level. Nitrate concentrations in ground water at some locations in Franklin County have increased by as much as two orders of magnitude since the early 1950's. Historical data generally were not available to evaluate changes of nitrate concentrations in ground water in Benton County, except for the area around the town of Finley. A comparison of data collected during this study with data collected during 1976-77 indicate that nitrate concentrations in ground water of the Finley area probably have not changed over the intervening period. Applied nitrogen fertilizers are a major source of nitrate in ground water at many locations in the study area. Surface water used for irrigation does not contain sufficient nitrate to cause elevated concentrations in ground water. Instead, canal seepage, which makes up about 50 percent of the ground-water recharge in the study area, tends to dilute the nitrate present in ground water. Septic systems in the Finley area of Benton County are a source of nitrate in ground water, but analyses of data and results of a numerical model analysis of nitrate concentrations in the unconfmed ground-water system indicate that they are not the primary source of nitrate in ground water in this area. Naturally occurring nitrate may be a source of nitrate in ground water underlying Badger Coulee in Benton County. Average masses of natural nitrate per unit volume of sediment in two boreholes in Badger Coulee were equivalent to 2,590 and 964 pounds of nitrogen, respectively, in a block of sediments 50 feet thick underlying an acre of land. At most other locations in the study area, the amount of natural nitrate in ground water is probably small compared with nitrate from anthropogenic sources. Fluoride concentrations in ground water in the study area ranged from less than 0.1 to 4.7 milligrams per liter; the median concentration was 0.5 milligram per liter. The concentration of fluoride in water from only two of 143 wells equalled or exceeded 2.0 milligrams per liter, which is the secondary maximum contaminant level for drinking water. Both are deep wells open to the Saddle Mountains Basalt in Franklin County. Large concentrations of fluoride in deep ground waters of the Pasco Basin are apparently the result of natural conditions in the deeper basalt aquifers. One or more pesticide compounds were detected in 10 of 29 ground-water samples, which were analyzed for selected chlorophenoxy acid herbicides, triazine herbicides, carbamate insecticides, organophosphorus insecticides, and a few other types of pesticides. The sampling locations did not represent a random distribution, but instead, most were wells open to unconfined, shallow ground water in irrigated areas. The pesticides found include the herbicides atrazine, dicamba, metribuzin, picloram, and 2,4,5-T. Also present were aldicarb sulfone and aldicarb sulfoxide, which are degradation products of the insecticide aldicarb. Except for metribuzin, pesticide concentrations were at or near the analytical reporting limits. In all instances, the concentrations of pesticides detected were below the health advisory levels that are issued by the U.S. Environmental Protection Agency

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