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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2009–5030

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Study Methods

Hydrologic and water-quality data were collected from 38 sites in the study area: the irrigation delivery canal (W15), 18 wells (W20p–W32q), 17 streambed sites in 2 transects located near the mouth of DR2 (W51a-W64c), a field drain (W42), and the surface-water outflow of DR2 (W13) (fig. 1). Data collected included ground-water levels, stream stage, stream discharge, water-quality parameters, estimated ground-water recharge dates, stable isotope abundances in water and nitrate, and soil mineralogy.

Collection and Analysis of Hydrologic Data

Wells, piezometers, and drive points used for collection of ground-water and streambed data were installed as part of this study, following procedures described in Lapham and others (1997). Wells were constructed of 5-cm-diameter PVC with 1.5-m slotted PVC screens and were installed using either a hollow-stem auger or a mud rotary drill. The annulus around the screen was filled with silica sand, the remaining annulus was grouted with bentonite, and a cement cap was placed at land surface. Shallow wells were generally screened approximately 1 m or less below the water table; deeper wells were screened 7–12 m below the water table except at site W21, where wells also were screened at 16 and 26 m below the water table.

Piezometers and drive points at the streambed transects were installed in 10-cm-diameter hand-augered holes. Piezometers for measuring water levels had screened open intervals of 15 cm, and drive points for water-sample collection had screened open intervals of 0.95 cm. The annular space was filled with sand to a height of approximately 6–15 cm above the top of the screen and then with bentonite pellets to the level of the streambed. During well installation at most sites, samples of aquifer material were collected with a split-spoon sampler from the depth of the screened interval and submitted for mineralogical analysis by X-ray diffraction. After installation, wells were developed by pumping and surging and were allowed to equilibrate for at least 45 days before the first water samples were collected. Additional installation details are described by Capel and others (2008) and Puckett and others (2008).

Water-level data were collected manually from 16 wells, and water-level and temperature data were collected by electronic recorder from 6 shallow wells, the W23 well pair, and 1 streambed site. Recorder data were collected using Solinst Leveloggers (Model 3001 F15; range: 4 m; resolution: 0.1 cm; accuracy: 4 mm; Solinst Canada Ltd, Georgetown, Ontario, Canada) and Solinst Barologgers (Model M5), for atmospheric-pressure-change compensation. The temperature of water in the stream and at multiple depths below the streambed was recorded by suspending StowAway TidbiT Temperature Loggers (Onset Computer Corp. Pocasset, Massachusetts; range: -4 to 30°C; accuracy: ±0.2°C at 20°C) within the piezometer clusters.

Discharge from the field drain was measured volumetrically, using a calibrated 19 L bucket and stopwatch. Stream discharge was measured near the catchment outlet using an AquaRod water-level sensor (Sequoia Scientific, Inc., Bellevue, Washington). The AquaRod was mounted in an existing, calibrated flume that is maintained by the Sunnyside Valley Irrigation District (Sunnyside, Washington). Irrigation and canal-leakage data were obtained from the Sunnyside Valley Irrigation District.

Precipitation data and daily crop-specific evapotranspiration data were obtained from the Harrah, Washington, station of the Pacific Northwest Cooperative Agricultural Weather Network (U.S. Department of the Interior, Bureau of Reclamation, Pacific Northwest Region; http://www.usbr.gov/pn/agrimet/, accessed January 6, 2009), located approximately 35 km from the study catchment. These evapotranspiration data were coupled with field-scale crop data obtained from land-use surveys (Payne and others, 2007) to estimate total catchment evapotranspiration. (Precipitation data from an onsite weather station were available, but the period of record was inadequate for use in the work reported here. The onsite data that were available agreed well with data from Harrah station, with differences in total precipitation less than 10 percent for the common periods of record.)

The quantities of ground water flowing into and out of the catchment were estimated from numerical simulations (Leon J. Kauffman, U.S. Geological Survey, written commun., 2006) using the MODFLOW code (Harbaugh and others, 2000). Ground-water flow paths were simulated (Leon J. Kauffman, U.S. Geological Survey, written commun., 2006) using the particle-tracking code MODPATH (Pollock, 1994).

Collection and Analysis of Water-Chemistry Data

During 2004, water samples were collected 6 times from the Sunnyside Canal (W15), 4 times from wells (W20p–W32q) and streambed sites (W51a–W64c), 17 times from the field drain (W42), and 27 times from DR2 at the catchment outflow (W13) (fig. 2). Measurements of temperature, pH, specific conductance, alkalinity, and dissolved oxygen were obtained onsite. Water samples were subsequently analyzed for nitrogen (N) and phosphorus (P) species, and for major solutes (calcium, magnesium, sodium, potassium, chloride, and silica). Selected samples also were analyzed for dissolved organic carbon (DOC), stable isotopes of water (²H and ¹⁸O), and stable isotopes of nitrate (¹⁵N and ¹⁸O). Selected ground-water and streambed samples were analyzed for age-dating constituents, including CFCl₃ (CFC-11), CF₂Cl₂ (CFC-12), and C₂F₃Cl₃ (CFC-113), sulfur hexafluoride (SF₆), and tritium (³H), and for concentrations of dissolved N gas (N₂) and argon in order to estimate the amount of N present as a result of denitrification (Heaton and Vogel, 1981; Vogel and others, 1981; Böhlke and others, 2002; Green and others, 2008).

Further details of the instrumentation and the methods used for data collection, analysis, and quality control are described by Capel and others (2008, supplemental material). Additional information on sample-collection, analyses, and interpretation of dissolved-gas, stable-isotope, and ground-water-age data are available in Green and others (2008).

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