PROJ00326

• Contract/PO #: C153498
• Contractor Name: U. S. Geological Survey
• Water District: 0
• County: Boulder
• Bill Number: SB 87-15
• Loan Projects - Doc Type: Contract Documents

the Colorado Department of Natural Resources, Divi-
sion of Water Resources, Office of the State Engineer,
and the Castle Pines Metropolitan District then began a
study to better define the hydrologic characteristics of
the bedrock aquifers in the area. The purpose of this
work was to obtain detailed geologic and hydrologic
data pertaining to the bedrock aquifers and to develop
and evaluate new techniques for estimating aquifer
characteristics by use of core analyses, aquifer testing,
and geophysical logging.

Purpose and Scope

This report describes techniques for estimating
the specific yield and specific retention of an aquifer
from grain-size analyses of aquifer samples and from
borehole geophysical logs. The techniques described
are based on empirical correlations between indepen-
dent and dependent variables that are analyzed by
least-squares linear regression. Regression analyses of
specific retention on grain-size characteristics, and
regression of specific yield or specific yield divided by
porosity on geophysical-log response each produced
statistically significant relations. The validity of some
of the techniques is demonstrated, and a specific-yield
log calculated from other geophysical logs is described.
Data used in this report are contained in Robson
and Banta (1993); sample handling and analyses proce-
dures are described in McWhorter and Garcia (1990),
Raforth and Jehn (1990), and Robson and Banta
(1993). A previous report pertaining to this test site
(Robson and Banta, 1990) describes techniques for
estimating aquifer specific storage on the basis of
barometric efficiency, aquifer testing, or aquifer-
compression measurements.

Background

Data for work described in this report were
obtained at a test site located about 20 mi south of Den-
ver (fig. I) at an altitude of6,61O ft on the hilly western
margin of the Denver basin. In July 1985, an irrigation
well (well A3) was completed in the Arapahoe aquifer
at a total depth of 2,398 ft. In February 1987, Jehn
Water Consultants undertook drilling of a core hole
(hole CI) at a site 58 ft southeast of well A3 (fig. I).
Core was recovered to a depth of 1,957 ft, at which
point the core barrel was lost, forcing abandonment
of the hole after the upper 1,955 ft of hole was logged
(Raforth and Jehn, 1990). A second core hole
(hole CIA) was drilled to 1,895 ft at a site 68 ft south-
southeast of well A3 (fig. I). Coring continued from

1,895 to 3,1l0 ft and was followed by geophysical
logging. Both core holes were subsequently filled by
injection of cement grout and abandoned. In October
1987, well USGS was drilled for the U.S. Geological
Survey to a depth of 2,400 ft at a site 50 ft east of
well A3 (fig. 1). After extensive geophysical logging
of well USGS, 2,400 ft of steel casing was grouted into
the hole and gun perforated through the Arapahoe aqui-
fer to form an observation well. Well USGS was
capped at the completion of the study.
The test site is underlain by the four principal
bedrock aquifers of the Denver basin aquifer system
(Robson, 1987). The Dawson aquifer is the shallowest
of these aquifers and is underlain by the Denver aqui-
fer, the Arapahoe aquifer, and the Laramie-Fox Hills
aquifer. The three uppermost aquifers were considered
in this work. The three aquifers consist of poorly to
moderately consolidated, interlayered conglomerate,
sandstone, siltstone, and mudstone. Sandstone is the
most prevalent water-yielding material and ranges
from very fine- to coarse-grained, poorly to well-sorted
arkosic and quartzose sandstone. Mudstone units form
confining layers within and between the principal aqui-
fers. Mudstone generally consists of poorly to moder-
ately sorted, very fine sand, silt, and clay. No carbonate
rocks are present in these Denver basin aquifers.
Specific yield is defined (Lohman and others,
1972) as the ratio of (1) the volume of water that a sat-
urated rock will yield by gravity drainage to (2) the vol.
ume of the rock. This definition implies that gravity
drainage has reached equilibrium. In drainage experi-
ments, equilibrium is difficult to achieve because of the
incomplete drainage of heterogeneous aquifer materi-
als and the slow drainage of the unsaturated zone, par-
ticu
ar
y in fine-grained materials. As a result, specific
yield calculated from field-drainage experiments of
limited duration (such as aquifer tests) usually is
smaller than the true specific yield of the aquifer mate-
rial. Specific yields determined through use of centri-
fuge or porous-plate laboratory techniques have been
shown (Neuman, 1987) to be less affected by slow
drainage and, thus, are better indicators of the equilib-
rium specific yield of aquifer materials. Porosity, spe-
cific yield, and specific retention are related and
commonly are expressed in dimensionless units:

$ = SY + SR,

(1)

where
$ = the porosity,
SY = the specific yield, and
SR = the specific retention.

2 Techniques for Estlmstlng Specific Yield and Specific Retanllon from Graln-S1ze DBla snd Geophysical Logs Irom
Clastic Bedrock Aquifers