River-aquifer exchanges in the Yakima River basin, Washington

Abstract

Five categories of data are analyzed to enhance understanding of river-aquifer exchanges-the processes by which water moves between stream channels and the adjacent groundwater system-in the Yakima River basin. The five datasets include (1) results of chemical analyses of water for tritium (3H, a radioactive isotope of hydrogen) and the ratios of the stable isotopes of hydrogen (2H/1H) and oxygen (18O/16O), (2) series of stream discharge measurements within specified reaches (seepage investigations or 'runs'), (3) vertical hydraulic gradients (between stream stage and hydraulic heads the underlying aquifer) measured using mini-piezometers, (4) groundwater levels and water temperature in shallow wells near stream channels, and (5) thermal profiles (continuous records of water temperature along river reaches). Exchanges are described in terms of streamflow, vertical hydraulic gradients, groundwater temperature and levels, and streamflow temperature, and where appropriate, the exchanges are discussed in terms of their relevance to and influence on salmonid habitat. The isotope data shows that the ultimate source of surface and groundwater is meteoric water derived from atmospheric precipitation. Water from deep wells has a different isotopic composition than either shallow groundwater or surface water, indicating that the deep groundwater system contributes, at most, only a small component of the surface-water discharge. The isotope data confirms that river-aquifer exchanges involve primarily modern streamflow and modern, shallow groundwater. Net exchanges of water for 46 stream sections investigated with seepage runs ranged from nearly zero to 1,071 ft3/s for 28 gaining sections, and -3 to -242 ft3/s for 18 losing sections. The magnitude of the upper 50 percent of the net gains is an order of magnitude larger than those for net losses. The sections have a normalized net exchange (as absolute value) that fully ranged from near 0 to 65.6 (ft3/s)/mi. Gaining-section values ranged from about 0.1 to 65.6 (ft3/s)/mi, and losing section values ranged from about -0.1 to -35.4 (ft3/s)/mi. Gains are much more vigorous than the losses with 55 percent being larger than 3.0 (ft3/s)/ mi, whereas, only 6 percent of the negative net exchanges were larger than 3.0 (ft3/s)/mi. Gains and losses for 167 measured reaches within the 46 sections ranged from about 70 to -75 (ft3/s)/mi, and ranged more than 5 orders of magnitude. The median values for the gains and losses were 5.1 and -4.4 (ft3/s)/mi, respectively. The magnitude of the gains was larger than the losses; more than 40 percent of the gains were greater than 10 (ft3/s)/mi, and only about 25 percent of the losses were greater than 10 (ft3/s)/mi. Reaches with large gains are identified and these reaches represent potentially important areas for various life stages of salmonids and possibly for preservation or restoration of that habitat. Ninety-nine measurements of vertical hydraulic gradients (VHGs) were made using mini-piezometers. The median for the measurements was -0.35 ft/ft (negative values indicate downward flow), and in terms of absolute values, the median was 0.05 ft/ft. The VHGs tended to be small. Seventy VHG values were negative (indicating streamflow losses), and 29 were positive (indicating streamflow gains). VHGs vary more than 4 orders of magnitude, and in terms of magnitudes, 65 percent were less than 0.1 ft/ft. The negative VHG values are not only more prevalent but are larger than the positive values. The magnitudes of almost 50 percent of the negative VHGs are greater than 0.05 ft/ft and only 33 percent of the positive VHGs are greater than 0.05 ft/ft. The percentile distribution of the VHG data, which is similar to the shape of the seepage data distribution, shows that beyond the 80th percentile, the positive values become much larger, indicating that the largest VHGs have a different controlling mechanism. The VHGs were formulated in terms of fluxes per unit are