Scientific Investigations Report 2006-5136
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5136
Several methods are used for quantifying ground-water discharge in riverine systems. The most common method is to make a series of discharge measurements. Such measurements provide useful information on net gain or loss in a reach defined by the bounding measuring sites for some particular time. When used in conjunction with dye-dilution gaging measurements (Kilpatrick and Cobb, 1985), discharge measurements also can provide information on ground-water discharge and recharge (sum equals net gain or loss) over a reach (Harvey and Wagner, 2000). In large systems, many discharge measurements, which can be costly, are needed to identify ground-water discharge locations, especially in extensively modified systems or when investigating the temporal variability in the discharge.
Seepage meters were used in lake studies to measure ground-water discharge (Lee, 1977; Lee and Cherry, 1978). Seepage meters are best suited for sandy lakeshores, and installation in rivers is problematic (Lee and Hynes, 1978; Harvey and Wagner, 2000). Using mini-piezometers was described early in the literature and has been oriented to studies of salmonid habitat (Terhune, 1958; Gangmark and Bakkala, 1958; Coble, 1961; Vaux, 1962). This method was modified for the reach survey using manometer-style measurements (Fokkens and Weijenberg, 1968; Lee and Cherry, 1978; Winters and others, 1988), which use mini-piezometers to measure the pressure head in shallow ground water with a concurrent measurement of the river head; these are done conjunctively using a portable manometer. Detailed information is provided by such surveys. For example, Simonds and others (2004) determined that, for some gaining reaches in the Dungeness River, Washington, identified from discharge measurements at bounding sites, the river was losing water over most of the reach and the ground-water discharge was local. Jackman and others (1997) measured large variations in discharge across the width of a small stream. Data presented in White and others (1987) also suggested variations in discharge across a 7-m wide river.
Monitoring of streamflow and vertical distribution of temperature below the streambed (Lapham, 1989; Silliman and Booth, 1993; Constanz, 1998; Constanz and others, 2001) yielded data for estimating ground-water discharge. Such monitoring with concurrent modeling provided detailed ground- and surface-water interactions at the diurnal to seasonal scale. The method is most easily applied to the ephemeral and lower order parts of the stream network and is difficult to use in larger stream reaches with high streamflow, especially with limited boat access. As with fixed station data, the usefulness of results are highly dependent on locations selected for monitoring.
Introduced stream tracers can be used to estimate discharge areas (Bencala and Walters, 1983; Jackman and others, 1984; Kilpatrick and Cobb, 1985; Triska and others, 1989). Tracers can include environmental tracers such as temperature (White and others, 1987), and tracer studies may or may not include modeling. Tracers are best applied to short reaches (less than the length scale of the longitudinal dispersion coefficient) and mainly for hyporheic flow analysis (Harvey and Wagner, 2000).
Continuous measurements of water levels and temperature in ground water in shallow piezometers near and in the river also are used to estimate ground-water discharge (or river losses) (Harvey and Wagner, 2000). The latter two methods become intractable over long reaches or for an entire riverine system.
Electrical conductivity profiling also can be used to locate ground-water discharge areas. Lee and others (1997) used two electrodes encased in a tubular shell with a brass nose cone towed from the back of a motorized watercraft. Electrodes were connected to a laptop computer (LTC) by a cable. The electrodes’ output, which was converted to a value of electrical conductivity, was used as a measure of the difference between river water and ground water. Location of the shell was determined by a continously logging GPS. For many riverine systems, using a towed measurement device connected to an LTC would not be applicable due to channel configuration, water depths, rapids, various bottom materials, log-jams, diversion dams, riprap, and partially submerged or buried tree limbs. However, the basic concept of profiling provides a technique that can be modified for most riverine systems.
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