GAGING STREAMFLOW GAIN AND LOSS
ABERJONA RIVER, WOBURN, MASSACHUSETTS
Introduction
As described in the problem setup, the connection of the groundwater flow system with the Aberjona River and the possible pathway of well contamination being derived from the river were critical issues in the "A Civil Action" trial. This exercise examines how the Aberjona River interacts with the underlying glacial outwash aquifer, which is tapped by wells G and H (see Chapter 1 for more details concerning the geology and hydrogeology of this site).
Streamflow measurements were made by the U.S. Geological Survey during the 30-day aquifer test in 1985-86 at several temporary gaging stations including the two locations seen in the above map. Upstream of the wells G and H on the north side of Olympia Avenue, USGS employees used flow meters to measure the river discharges under Olympia Avenue. They also measured river discharges downstream of wells G and H, immediately south of the Salem Street bridge. By comparing the stream discharges between these two gaging locations, one can determine if the stream is gaining or losing water across the streambed as it flows past wells G and H, and can estimate the quantity of surface water gained or lost between Olympia Avenue and Salem Street.
Instructions - Part I Streamflow Gain or Loss
The objective of this exercise is to better understand the interaction between the Aberjona River and the shallow groundwater system. To accomplish this objective, you will calculate the stream discharge at two gaging locations (Olympia Avenue and Salem Street) at two times (Dec. 4, 1985 and Jan. 3, 1986).
1. On the "Olympia Avenue" and "Salem Street" worksheets, calculate the stream velocity in each compartment. Use the pygmy flow meter conversion formula given below.
Velocity (ft/s) = 0.977R + 0.028
where R is the number of revolutions of the flow meter measured per second.
2. Calculate the cross-sectional area of each compartment (width times depth).
3. The discharge of each compartment is determined by multiplying the average velocity times the cross-sectional area. Most compartments were measured at one depth (0.6 of the water depth from the stream bottom) whereas others were measured at two depths (0.2 and 0.8 of the water depth from the stream bottom). In the latter case, first the discharge is determined by finding the average of the two velocities in the compartment then multiplying it by the cross-sectional area of the compartment.
4. The last column allows for calculation of an ongoing summation of discharge (cumulative discharge) from all compartments in stream profile.
5. Calculate the total cross-sectional area and discharge by summing the cross-sectional areas and discharges at the bottom of each column.
6. Answer the questions provided in the "Questions" worksheet.
Instructions - Part II Temporal Variation of Streamflow
Additional measurements of streamflow gain/loss were made at the temporary Olympia Avenue and Salem Street gaging stations periodically during the 30-day aquifer test as well as at three small contributing tributaries ("Streamflow Change" worksheet). Figure 1 and the accompanying table illustrate the measured temporal variations in streamflow from December 1985 to January 1986. Study the gaining or losing characteristics of the stream using the table and the accompanying graph; then answer the questions on the "Questions" worksheet. Note that the flow contributions from the three tributaries between the gaging stations are small but not insignificant. See the Reference Book for more background concerning stream gain and loss.