SALSA Poster:

Quantification of Groundwater-Surface Water Interaction, Mac Nish et al.

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	QUANTIFICATION OF GROUNDWATER-SURFACE WATER INTERACTION IN A SOUTHWESTERN RIPARIAN SYSTEM
R. D. Mac Nish (1),C. J. Peters (1), M. A. Schulte (1), D. C. Goodrich (2), D. R. Pool (3), T. Maddock III (1), C. L. Unkrich (2), M. P. L. Whitaker (1), B. F. Goff (2)
1 Department of Hydrology & Water Resources, University of Arizona Tucson, Arizona, USA 2 Southwest Watershed Research Center, USDA-ARS, Tucson, Arizona, USA 3 Arizona District, USGS, Water Resources Division, Tucson, Arizona, USA email: macnish@hwr.arizona.edu

INTRODUCTION This poster presents preliminary findings on stream-aquifer interaction on a 425 meter long reach of the San Pedro River in Cochise County, Arizona. We present stream aquifer flux estimates made from streamflow measurements, and ground water head distributions acquired over 48 hour periods of intensive data collection (synoptic studies) in April and June of 1997. These two synoptic studies represent different degrees of stress created by the transpiration of riparian vegetation. This work is an important step towards attaining our ultimate goal of developing a better understanding of the interaction between streams and aquifers and improving our capability to simulate stream-aquifer systems. The study reach at Lewis Springs (about 13 km east of Sierra Vista, AZ) is underlain by a floodplain aquifer deposited by the San Pedro river in a valley carved in the regional aquifer. While both aquifers are unconsolidated sediments, the floodplain aquifer is more permeable than the underlying regional aquifer.

DATA ACQUISITION Data were collected during synoptic collection events which lasted from 32 to 48 hours in length in March, April, June, and August. During these synoptic periods water level information was obtained hourly from 25 piezometers and continuously recorded at six wells and another piezometer. The piezometers and wells were arranged in clusters whose distribution is shown on the site map above. In each cluster, two or three piezometers and/or wells were finished in the floodplain aquifer at various depths, ranging from about 4 to 7 meters in depth. There was only one piezometer at the West South Far (WSF) cluster, as the floodplain aquifer had less than one meter of saturation at that site. The diagram to the right shows the details of piezometer construction for the 25 piezometers. Four of the wells were located along the West Middle clusters(WMC to 150 meters west of WMF) and penetrated the regional aquifer to depths of 50 to 60 meters. The ground water network provided means to obtain three dimensional data on groundwater levels in the floodplain aquifer in the study reach.

Water level measurements were made and recorded at approximately one hour intervals over the course of each synoptic period by two person teams, with most of the personnel involved being volunteers or students at the University of Arizona.

Surface water data collection included hourly measurements of stream stage in stilling tubes or staff gages at sites X1 to X5 shown on the site map above. During the later synoptic periods, bubble gages were installed at most cross-sections. To relate the stage data to streamflow, measurements of flow were taken by pygmy meter at irregular intervals at the same sites, with most of the measurements made at sites X1 and X3. During the June synoptic period, a flume was installed immediately upstream of X1, and continuous stage and flow data were recorded.

In addition to the flow measurements made by pygmy meter and flume, tracer dilution gaging was also employed to obtain hourly flow data at the five cross-sections. The principles of the constant injection rate technique we employed are shown in the schematic to the left. To assure complete mixing of the injected rhodamine WT dye, the dye injection equipment was located about 200 meters upstream of site X1. Utilizing devices used in hospitals for controlling intravenous fluid injection (shown in the photo to the right), the dye injection was started several hours before the beginning of the synoptic period and continued throughout the duration of the event. Unfortunately the dye mixture viscosity was dramatically affected by variations in the ambient temperature, and so the dye injection rate varied in a sinusiodal fashion which severely complicates the determination of flow rates.

To calculate the flow rates, samples of stream water were taken hourly, concurrently with the stage measurements at the stilling tubes/staff gages at each cross-section. The samples were taken from the approximate center of the channel at each cross-section as shown in the photo to the left. By analyzing the dye concentration in the sample, the amount of water required to dilute the injected dye solution to the sample concentration can be determined. The amount of water determined includes the amount of stream flow passing the injection site, plus any baseflow contributions the stream has received from the groundwater in the reach between the injection and sample sites. The degree of mixing was evaluated at several sites in the reach during the April and June synoptic periods by taking a number of samples at various positions in the cross-section and comparing the concentrations. The variations in concentrations were within acceptable limits in all cases.

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