WSP04273

• File Number: 8277.400.10
• Description: Las Vegas Wash Unit- Nevada
• State: NV
• Basin: Colorado Mainstem
• Date: 1/1/1993
• Author: USGS
• Title: Shallow Ground Water in Whitney Area - Southeastern Las Vegas Valley - Clark County NV -- Part II. Assessment of a Proposed Strategy to Reduce the Contribution of Salts to Las Vegas Wash
• Water Supply Pro - Doc Type: Report/Study

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Estimated conductivities (table 2)
were used to produce a vertical profile of
horiwnral hydraulic conductivity through
the entire aquifer and along a line con-
necting four cluster-well sites (fig, 5),
This was accomplished by first averaging
horizonral and vertical hydraulic conduc-
tivities over 5-ft depth intervals for the
entire 30-ft saturated thiclcness of the
aquifer at each well cluster (table 3), and
then interpolating the averaged hydraulic
conductivities between cluster-well sites.
The average horizonral hydraulic conduc-
tivities were used to estimate the quantity
of flow at each cluster-well site. Because
the cluster-well sites are aligned nearly
parallel to the flow path, the quantity of
flow at each site could be readily estimat-
ed for a unit area (thickness times unit width of aquifer) using Darcy's law, The hydraulic gradient was
estimated from measured water levels at wells upgradient from. downgradient from, and at the cluster well
being assessed.

T.ABLE 3.--Estimated horizontal and vertical hydraulic
conductivities, hydraulic gradi~nt, and ground-water flow
rate for entire aquifer thickness of unit width at each cluster.
well site

Hydraulic conductivity
(feet per day)

Hydraulic Flow
gradient (cubic feet
(feet per foot) per day)

Site

Horizontal

Vertical

WG035N

30

3

0,0045
,.

4.1

WG044N

17

2

,0077

3,9

WG053N

36

3

.0083

9,0

WG062

8

.7

.0167

4.0

Specific Yield

As the water table declines in response 10 evapotranspiration. the quantity of water removed from the
aquifer is related to the specific yield of the sediments. Specific yield of a sediment is defined as the ratio
of the volume of water that the sediment, after being saturated, will yield by gravity to the volume of
sediment (Lohman, 1972, p. 6).

The quantity of water removed by evapotranspiration can affect simulation results. Thus, estimates
of specific yield were detennined from single core samples collected at cluster-well sites WG044N,
WG053N, WG062, and WG071. Cores of 1.5-in. diameter were collected from the zone of water-table
fluctuation, usually 1-2 ft below land surface at these cluster-well sites. The cores were sealed and taken
to the laboratory. where they were cut into 2-in. lengths. The 2-in. samples were saturaled in deionized
water for 48 hours and then weighed. Once saturated, the samples were subjected to incremenral pressures
over a period of I week. Each pressure increment was held constant for a period of 24 hours, after which
the sample was reweighed, An estimate of drainable water was detennined by subtracting the weight of
the sample prior to the increase in pressure, from the weight after a 24-hour period.

Figure 6 shows the results of the laboratory tests. Data for cluster-well sites WG044N and WG053N
and for sites WG062 and WG071 are grouped together, because the sample pairs had similar drainage
curves, The curves were used to estimate an approximate specific yield by defining the point on the curve
where the slope becomes noticeably flatter. The specific yield ranged from 8 to 12 percent for all cores.

This technique works well for sandy soils, where the cumulative quantity of drainable water ceases
to increase with increasing pressure. It is less satisfactory for silt and clay. which are common in the upper
part of the aquifer in the Whitney area. because the cumulative quantity of drainable water from silt and
clay generally does not cease with increasing pressure. This phenomenon is known in soil mechanics as
creep,

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