Characteristics of shallow deposits beneath Rillito Creek, Pima County, Arizona

Characteristics of the stream-channel and basin-fill deposits beneath a 12-mile reach of Rillito Creek, Pima County, Arizona, were obtained to describe the geohydrologic system. The findings presented here are part of a larger project to improve the understanding of recharge processes beneath ephemeral streams.

The stream-channel deposits, which range in thickness from 15 to 40 feet, generally are sandy gravels or gravelly sands. On average, the stream-channel deposits are 44 percent gravel, 51 percent sand, 2 percent silt, and 3 percent clay. The underlying basin-fill deposits also are sandy gravels or gravelly sands but have, on average, a larger component of silt and clay than the stream-channel deposits?about 9 percent silt and 6 percent clay.

Porosity values for the stream-channel and basin-fill deposits are similar: about 31 and 34 percent on average, respectively. Volumetric moisture content and percent saturation, however, generally were lower in the stream-channel deposits than in the basin-fill deposits. Moisture content in the stream-channel deposits ranged from 2 to 40 percent and averaged about 18 percent, whereas moisture content in the basin-fill deposits ranged from 7 to 47 percent and averaged about 24 percent. Saturation in the stream-channel deposits ranged from 9 to 100 percent and averaged about 58 percent; saturation in the basin-fill deposits ranged from 30 to 100 percent and averaged about 69 percent. Porosity and moisture content correlate with silt and clay content. Cumulative thickness of water in the 100- to 125-foot thick unsaturated zone obtained by integrating the moisture content over depth, ranged from 17.2 to 40.4 feet.

Matric potential for saturation levels at the time of sample collection generally was less than -1 bar for deposits that were less than 35 percent saturated. Matric potential generally was greater than -0.1 bar for deposits that were more than 65 percent saturated. Moisture-retention curves are a function of the physical properties, such as porosity and grain size, of the sediments. The shapes and van Genuchten fitting parameters of moisture-retention curves for the stream-channel deposits are different from those of the basin-fill deposits. For instance, the fitting parameter, a, for stream-channel deposits ranged from 4.56 to 1,220 bar-1 and averaged 220 bar-1, whereas a for basin-fill deposits ranged from 4.22 to 67.9 bar-1 and averaged 22.8 bar-1. The residual water content for the basin-fill deposits is greater than that for the stream-channel deposits. Relative hydraulic conductivity of the stream-channel deposits is less than relative hydraulic conductivity of the basin-fill deposits at the same matric potential. Unsaturated hydraulic conductivity for moisture conditions that existed at the time of sample collection typically was more than two orders of magnitude less than saturated hydraulic conductivity.

Saturated vertical hydraulic conductivity of the stream-channel deposits is about an order of magnitude greater than that of the basin-fill deposits. The equivalent hydraulic conductivity of stream-channel deposits ranges from 2 to 7.3 feet per day, and averages about 4 feet per day, whereas the equivalent hydraulic conductivity of the basin-fill deposits ranges from 0.06 to 1.5 feet per day and averages 0.61 foot per day. The equivalent vertical hydraulic conductivity of the entire unsaturated zone cored is 0.75 foot per day. Assuming no vertical to horizontal anisotropy, the equivalent horizontal hydraulic conductivity generally is about two to three times that of the equivalent vertical hydraulic conductivity. The difference between average equivalent vertical and horizontal hydraulic conductivity values results from the differences in methods used to calculate the respective values.

Electrical methods were useful in
discriminating between stream-channel deposits
and basin-fill deposits. In general, electrical
conductivity of the stream-channel deposits was
less than 30 millimhos per meter and averaged
27 millimhos per meter. The conductivity of the
basin-fill deposits was greater than that of the
stream-channel deposits and averaged
44 millimhos per meter. The greater conductivity
probably is related to factors such as greater
moisture content and fraction of fine sediments in
the basin-fill deposits. Apparent resistivity
measured with two-dimensional resistivity
soundings generally decreased with depth.
The resistivity values from the near-surface
measurements represent dry stream-channel
deposits and averaged 303 ohm meters. The
resistivity values for basin-fill deposits generally
were less than 140 ohm meters and less than
100 ohm meters when saturated.

Seismic-velocity values for the recent alluvium
(stream-channel and terrace deposits) ranged from
1,150 to 2,200 feet per second, whereas values for
basin-fill deposits ranged from 2,000 to
11,650 feet per second. The average seismic
velocity for the stream-channel deposits
(1,300 feet per second) was less than that for the
terrace deposits (1,600 feet per second). Saturated
basin-fill deposits had an average velocity of
7,800 feet per second, whereas unsaturated basin-fill
deposits had an average velocity of
2,750 feet per second.