U.S. Geological Survey

By Rodney R. Caldwell

USGS Water-Resources Investigations Report 97-4233, 49 pages, 1 plate, 16 figures, 1 table

Available from U.S. Geological Survey, 10615 S.E. Cherry Blossom Drive, Portland, OR 97216 (ph: 503-251-3201, e-mail: or-info@usgs.gov) and from U.S. Geological Survey, Branch of Information Services, Box 25286, Denver, CO 80225 (303-202-4210).

Abstract

Water-chemistry characteristics were used to assist in the understanding of regional ground-water flow and the interaction of ground water and surface water in the upper Deschutes Basin in central Oregon. Water samples were collected from sites along approximate ground-water flow paths inferred from water table maps to investigate chemical evolution and mixing, and at wells close to canals to investigate canal/ground-water interaction. Samples from 26 wells, 7 springs, and 5 surface-water sites were analyzed for common ions, nutrients, and stable isotopes. Selected samples were analyzed for tritium and carbon-14 to estimate residence times and relative ages. Historical water-chemistry data from more than 550 sites were retrieved from the Environmental Protection Agency's Storage and Retrieval data base (STORET) and the U.S. Geological Survey's National Water Inventory System data base to aid in the investigation.

Samples analyzed for this study were predominantly calcium-magnesium bicarbonate to mixed calcium-magnesium bicarbonate/sodium-potassium carbonate type waters with total dissolved solids concentrations ranging from 44 to 562 mg/L (milligrams per liter). Ground-water with the lowest total dissolved solids concentrations (44 to 60 mg/L) originated from Quaternary lavas in the regional ground-water recharge area on the eastern flank of the Cascade Range. Water from sites in the northern and eastern parts of the study area, which was withdrawn from the Deschutes Formation and volcanic units of non-Cascade Range origin, had the highest total dis solved solids concentrations.

Specific conductance data from more than 550 ground-water sites (wells and springs) were examined. Most of the sites that had water with relatively low specific conductance values (less than 100 microsiemens per centimeter) are located in the western part of the study area, where most of the ground-water recharge occurs. Values increased to the east and northeast in the direction of ground-water flow. Specific conductance values were commonly greater than 250 microsiemens per centimeter in the eastern part of the study area, which is an area of low recharge, low permeability, and longer ground-water residence times.

Tritium values of samples from 14 ground-water sites ranged from less than 1 to 10.3 tritium units (TU). Samples with the highest tritium values (8.5 to 10.3 TU) were from sites in the regional recharge area on the eastern flank of the Cascade Range. Samples with tritium concentrations of less than 1 TU, indicative of water recharged prior to 1953, were from sites in or near a regional dis charge area and in the eastern part of the study area. Locally, wells and springs in close proximity had differing tritium values and ionic concentrations. This may have been due to local recharge from surface water, most notably from the numerous canals that traverse the area. Local recharge from irrigated lands and the mixing of waters of differing origins, residence times, or material encountered along their flow paths may also contribute to the local variability of ground-water chemistry.

Water-level fluctuations resulting from canal leakage have been observed in several area wells. Samples from four canals and five wells adjacent to those canals were collected near the end of the summer irrigation season (September 1994) to examine possible ground-water chemistry effects resulting from canal leakage. The wells were sampled again (March 1995) prior to the operation of the canals for the season. Isotopic evidence indicates that canals (and possibly the Deschutes River from which these canals originate) are likely the predominant source of water in these wells. The deuterium and oxygen-18 composition of water from the canals and adjacent wells was tightly constrained within a range of 5 per mil delta D (-95 to -100 per mil) and less than 1 per mil delta oxygen-18 (-12.56 to -13.32 per mil), respectively. Data from the study area as a whole show a much larger range of values. The canal water samples were nearly identi cal chemically, with total dissolved solids concentrations ranging from 51 to 52 mg/L. Total dissolved solids concentrations of the ground-water samples ranged from 93 to 307 mg/L. Water/rock interaction is a likely mechanism for the increased dissolved ion concentrations in the ground water relative to the canals. No significant chemical variation was observed in comparison of the ground-water samples collected in September 1994 and March 1995. However, multiple specific conductance measurements over a 1-year period (September 1994 to August 1995) indicated a temporal water-chemistry variation in two of the five wells.

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