1

 

Prepared in cooperation with Coconino County, Arizona, and the U.S. Forest Service

Depth of Cinder Deposits and Water-Storage Capacity at 

Cinder Lake, Coconino County, Arizona 

By Jamie P. Macy, Lee Amoroso, Jeff Kennedy, and Joel Unema 

Open-File Report 2012–1018 

U.S. Department of the Interior 

U.S. Geological Survey 

 
COVER: 

Photograph of San Francisco Mountain, nearby neighborhoods, and Cinder Lake area, Coconino County, Arizona. 

(USGS photograph taken by Jon Mason.) 
 

Prepared in cooperation with Coconino County, Arizona, and the U.S. Forest Service

Depth of Cinder Deposits and Water-Storage Capacity at 

Cinder Lake, Coconino County, Arizona 

By Jamie P. Macy, Lee Amoroso, Jeff Kennedy, and Joel Unema 

Open-File Report 2012–1018 

U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

KEN SALAZAR, Secretary 

U.S. Geological Survey 

Marcia K. McNutt, Director 

U.S. Geological Survey, Reston, Virginia: 2012 

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its natural and living resources, natural hazards, and the environment—visit 

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visit http://www.usgs.gov/pubprod

To order this and other USGS information products, visit http://store.usgs.gov

Suggested citation: 

Macy, J.P., Amoroso, L., Kennedy, J., and Unema, J., 2012, Depth of cinder deposits and water-storage capacity at 

Cinder Lake, Coconino County, Arizona: U.S. Geological Survey Open-File Report 2012-1018, 20 p., available at 

http://pubs.usgs.gov/of/2012/1018/. 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report. 
 iii

Contents 

Abstract ......................................................................................................................................................................... 1

Introduction .................................................................................................................................................................... 1

Purpose and Scope .................................................................................................................................................. 4

Methods ....................................................................................................................................................................... 4

Local Hydrogeology ................................................................................................................................................. 4

Transient Electromagnetics (TEM) Surveys ............................................................................................................. 6

Borehole Drilling ....................................................................................................................................................... 7

Porosity Calculations ................................................................................................................................................ 9

Gravity ...................................................................................................................................................................... 14

Results ......................................................................................................................................................................... 14

Transient Electromagnetics (TEM) Surveys ............................................................................................................. 14

Borehole Drilling ....................................................................................................................................................... 15

Estimates of Total Porosity and Storage .................................................................................................................. 15

Gravity ...................................................................................................................................................................... 18

Conclusions ................................................................................................................................................................. 18

Acknowledgments .................................................................................................................................................... 18

References Cited ......................................................................................................................................................... 19

Figures 

Figure 1. Map of Flagstaff, Arizona, and area north of Flagstaff, including Cinder Lake. ...................................... 2

Figure 2. Map of the study area, Cinder Lake, Coconino County, Arizona, including borehole locations, USGS 

stream-flow gages, transient electromagnetic transect, ditches, and the extent of the area used to calculate 

total storage................... ........................................................................................................................... 3

Figure 3. Geologic map of the study area, Cinder Lake, Coconino County, Arizona. ......................................... 5

Figure 4. A’ to A cross-section (fig. 2) of transient electromagnetic smooth-model inversion results using Zonge 

STEMINV modeling software. Numbers across the x-axis indicate the TEM station number. .................... 8

Figure 5. Lithologic log of borehole CL-1. ........................................................................................................... 10

Figure 6. Lithologic log of borehole CL-2. ........................................................................................................... 11

Figure 7. Lithologic log of borehole CL-3. ........................................................................................................... 12

Figure 8. Lithologic log of borehole CL-4. ........................................................................................................... 13

Tables 

Table 1. Estimated total porosity values for units from borehole CL-1. .............................................................. 16

Table 2. Estimated total porosity values for units from borehole CL-2. .............................................................. 17

Table 3. Estimated total porosity values for units from borehole CL-3. .............................................................. 17

Table 4. Estimated total porosity values for units from borehole CL-4. .............................................................. 18
 iv

Conversion Factors and Datums 

Inch/Pound to SI 

Multiply By To obtain 

Length

inch (in.) 2.54 centimeter (cm) 

inch (in.) 25.4 millimeter (mm) 

foot (ft) 0.3048 meter (m) 

mile (mi) 1.609 kilometer (km) 

Area

acre 4,047 square meter (m2) 

acre 0.4047 hectare (ha) 

acre 0.4047 square hectometer (hm2

) 

acre 0.004047 square kilometer (km2

) 

Volume

acre-foot (acre-ft) 1,233 cubic meter (m3) 

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3) 

liter (L) 61.02 cubic inch (in3

) 

liter (L) 33.82 ounce, fluid (fl. oz) 

liter (L) 2.113 pint (pt) 

liter (L) 1.057 quart (qt) 

liter (L) 0.2642 gallon (gal) 

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s) 

Time

Second (s) 0.001 millisecond (msec) 

Second (s) 0.000001 microsecond (μsec) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: 

°F=(1.8×°C)+32. 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: 

°C=(°F-32)/1.8. 

Electrical resistivity ρ in ohm-meters [ohm-m] can be converted to electrical conductivity σ in siemens per meter [S/m] as 

follows: σ = 1/ ρ. 

Electrical resistivity ρ in ohm-meters [ohm-m] can be converted to electrical conductivity σ in millisiemens per meter 

[mS/m] as follows: σ = 1,000/ρ. 

Electrical resistivity ρ in ohm-meters [ohm-m] can be converted to electrical conductivity σ in microsiemens per centimeter 

[μS/cm] as follows: σ = 10,000/ ρ. 

Datums

Vertical coordinate information is referenced to theNorth American Vertical Datum of 1988 (NAVD 88). 

Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83). 

Altitude, as used in this report, refers to distance above the vertical datum. 
Depth of Cinder Deposits and Water-Storage Capacity at 

Cinder Lake, Coconino County, Arizona 

By Jamie P. Macy, Lee Amoroso, Jeff Kennedy, and Joel Unema 

Abstract 

The 2010 Schultz fire northeast of Flagstaff, Arizona, burned more than 15,000 acres on the east 

side of San Francisco Mountain from June 20 to July 3. As a result, several drainages in the burn 

area are now more susceptible to increased frequency and volume of runoff, and downstream areas are 

more susceptible to flooding. Resultant flooding in areas downgradient of the burn has resulted in 

extensive damage to private lands and residences, municipal water lines, and roads. Coconino County, 

which encompasses Flagstaff, has responded by deepening and expanding a system of roadside ditches 

to move flood water away from communities and into an area of open U.S. Forest Service lands, known 

as Cinder Lake, where rapid infiltration can occur. Water that has been recently channeled into the 

Cinder Lake area has infiltrated into the volcanic cinders and could eventually migrate to the deep 

regional groundwater-flow system that underlies the area. How much water can potentially be diverted 

into Cinder Lake is unknown, and Coconino County is interested in determining how much storage is 

available. The U.S. Geological Survey conducted geophysical surveys and drilled four boreholes to 

determine the depth of the cinder beds and their potential for water storage capacity. Results from the geophysical surveys and boreholes indicate that interbedded cinders and alluvial deposits are underlain by basalt

at about 30 feet below land surface. An average total porosity for the upper 30 feet of deposits was 

calculated at 43 percent for an area of 300 acres surrounding the boreholes, which yields a total potential 

subsurface storage for Cinder Lake of about 4,000 acre-feet. Ongoing monitoring of storage change in the 

Cinder Lake area was initiated using a network of gravity stations. 

Introduction 

The 2010 Schultz fire northeast of Flagstaff, Arizona, burned more than 15,000 acres on the east 

side of San Francisco Mountain from June 20 to July 3 (fig. 1). As a result, several drainages in the 

burn area are now more susceptible to runoff and flooding events of greater frequency and volume. 

Despite a rapid response from the U.S. Forest Service (USFS) to mitigate some of the expected flooding 

and debris effects of the fire, one of the largest flood events associated with the Schultz Fire burn area 

occurred on July 20, 2010. Massive damage to private lands and residences, municipal waterlines, and 

roads in and downgradient of the burn areas have occurred, and greater than expected flooding persists. 

Although the fire occurred on USFS lands, there are a number of subdivisions located on unincorporated 

lands in Coconino County within drainage basins downgradient of the burn area, including the 

communities of Timberline Estates, Fernwood Estates, Pine Mountain Estates, and Wupatki Trails (fig. 

1). In response to the flooding effects, the County deepened and expanded a system of roadside ditches 

to move flood water away from these communities and into open USFS lands where rapid infiltration 

can occur. The Copeland, Campbell, and Girl’s Ranch Ditches were designed to safely reroute the 

runoff from two of the largest drainages originating in the burn area through developed lands and 

distribute the water into the Cinder Lake area (fig. 2). The Copeland, Campbell, and Girl’s Ranch 

ditches are designed to carry about 1,600, 1,200, and 500 ft3

/s of water, respectively. 
 2

Figure 1. Map of Flagstaff, Arizona, and area north of Flagstaff, including Cinder Lake. 
 3

Figure 2. Map of the study area, Cinder Lake, Coconino County, Arizona, including borehole locations, 

USGS stream-flow gages, transient electromagnetic transect, ditches, and the extent of the area used to 

calculate total storage. 

Cinder Lake is a dry cinder bed located on USFS land surrounded by cinder cones to the north, 

east, and south and alluvial fans at the base of San Francisco Mountain to the west. Water that has been 

channeled into the Cinder Lake area infiltrates into the volcanic cinders and alluvium and could follow 

three flowpaths out of the Cinder Lake area, including: (1) recharging the deep regional groundwater-flow 

system that underlies the area, (2) development of a perched aquifer that discharges to the Rio de Flag 

through the subsurface by moving laterally along confining layers such as basalt or clay, or (3) 

accumulating in the subsurface to the point where the cinders and alluvium are filled to capacity, forming 

a surface lake discharging to the Rio de Flag outside of the area to the southeast (fig. 1). The third 
 4

flowpath could potentially impact the City of Flagstaff’s unlined landfill, which is downgradient and to 

the southeast of Cinder Lake. 

The ultimate flow path for the runoff may be largely determined by the lithology and storage 

capacity of near-surface materials in the Cinder lake area. Although a cinder bed occurs at the surface, 

little is known about the thickness or potential water storage capacity of those highly porous materials 

and underlying materials. Therefore in 2010, Coconino County entered into a cooperative program with 

the U.S.Geological Survey (USGS) to determine the depth and storage capacity of cinder deposits in the 

Cinder Lake area for distributing and infiltrating water from flow events associated with the Schultz 

Fire burn area. 

Purpose and Scope 

The purpose of this report is to describe and define the depth of interbedded cinders and alluvial 

deposits to the uppermost basalt layer in the study area, and to provide estimates of potential water storage 

in the interbedded deposits above the basalt in the Cinder Lake area. The scope of the report includes a 

description of methods used to determine the depth of the interbedded deposits to the top of the basalt 

layer and calculate water storage for the Cinder Lake study area. 

Methods 

Surface geophysical surveys were conducted and four boreholes were drilled to assess the nearsurface materials and collect lithologic information, to provide control for the geophysical surveys, and 

to monitor for possible water storage following infiltration events. Surface geophysics are nonintrusive 

methods that can indicate changes in electrical properties and characteristics of the subsurface. Transient 

Electromagnetics (TEM) surveys were made to determine the total depth of cinders to basalt. Drill cores 

from the boreholes were used to estimate storage capacity of the cinder deposits. In addition to the 

subsurface investigations, a network of gravity monuments were established to monitor changes in 

subsurface water storage. 

Local Hydrogeology 

Flagstaff is located on the southern edge of the Colorado Plateau in north-central Arizona. The 

eastern part of the San Francisco volcanic field covers most of the area and provides much of the 

topographic relief. Cinder cones and hills, basalt flows, and San Francisco Mountain are the 

principal features superimposed onto the consolidated sedimentary rocks of the Colorado Plateau (Bills 

and others, 2000). In northern Arizona, the Colorado Plateau is composed of Cambrian through 

Mesozoic sedimentary rocks overlain by Tertiary and Quaternary volcanic and sedimentary rocks. 

Volcanic rocks of Miocene to Quaternary age overlie sedimentary strata of the Colorado Plateau 

and are prevalent near Flagstaff and Cinder Lake. These volcanic rocks are aphanitic basalt and cinder 

cones; dacite flows and domes; dacite pyroclastic-flow breccias; andesite flows, flow breccias, and tuff 

breccia; and benmoreitic flows, cinder cones, and domes. These rocks range in thickness from zero to 

more than 5,000 ft beneath San Francisco Mountain. The average thickness of the volcanic rocks is 

about 150 ft (Bills and others, 2000). The hills surrounding Cinder Lake primarily are basalt cinder 

cones (Moore and Wolfe, 1976; figs. 1 and 3). Remnant basalt flows also are exposed to the north, east, 

and south of Cinder Lake (Moore and Wolfe, 1976; fig. 3). There are a few exposures of basalt within 

Cinder Lake that resemble push-up features where basalt is exposed at the surface (figs. 2 and 3). These 

basalt features could be remnants of basalt flows or eroded basalt flows. On the southern edge of Cinder 

Lake, basalt exposures form a ridge-like feature. 
 5

Figure 3. Geologic map of the study area, Cinder Lake, Coconino County, Arizona. 

Information from three deep groundwater wells developed in the Cinder Lake area provide a 

context for the regional aquifer near Flagstaff and for what is to be expected in the subsurface near 

Cinder Lake. Those wells include the Sunset Crater Well, the Marijka Well, and the Doney Park Well 1 

(DPW1) (fig. 1). All three wells were developed for groundwater withdrawal, but driller’s logs from the 

wells provide information about the subsurface and the depth of the regional aquifer. The regional 

aquifer near Flagstaff typically is 1,200 to 1,400 ft below land surface. The regional aquifer is composed 
 6

of the Kaibab Formation, Toroweap Formation, Coconino Sandstone, Schnebly Hill Formation, and 

Upper and Middle Supai Formation. The regional aquifer is heterogeneous and anisotropic and has a 

complex groundwater-flow system. The most productive water-bearing material tends to be fine- to 

medium-grained sandstone, and groundwater flow and potential well yields are related to geologic 

structure. Static water in wells around Flagstaff typically is in the Schnebly Hill Formation or the Upper 

Supai Formation. 

The driller’s log for the Sunset Crater Well (fig. 1) shows an interval of 700 ft of volcanic 

material at the surface. The Kaibab Limestone underlies volcanic material and extends to a depth of 

1,000 ft below land surface. An 830-ft interval of Coconino Sandstone is beneath the Kaibab Limestone. 

Then from 1,830 to 2,200 ft below land surface, the well is completed in the Supai Formation. 

Another well with similar characteristics in the area is the Marijka Well (fig. 1). Alluvium and 

cinder material extend from the surface to a depth of about 210 ft below land surface. Basalt continues 

another 90 ft to the top of the Kaibab Limestone. Kaibab Limestone extends from 300 to 740 ft below 

land surface. The Coconino Sandstone is found in this well from 740 to 1,400 ft, where it contacts the 

Schnebly Hill Formation and upper Supai Formation. 

A third well in the area is well DPW-1 (fig. 1). The driller’s log from well DPW-1 showed 

basaltic sand, basalt fragments, and basaltic scoria to a depth of 55 ft below land surface and an interval 

of basalt from 55 to 205 ft. An interval of Moenkopi Formation from 205 to 260 ft is beneath the 

basalt. Driller’s logs from the Sunset Crater and Marijka Wells in the area do not show an interval of 

Moenkopi Formation. In DPW-1, the Moenkopi Formation overlies Kaiaba Formation from 260 to 480 

ft and the Coconino Sandstone extends from 560 to 1,740 ft beneath the Kaibab Formation. The 

Schnebly Hill Formation is at a depth of 1,740 ft, and the well is completed in this unit. The stratigraphy 

described in three wells near Cinder Lake provides background information that was used to determine 

possible depth of study for geophysical techniques and what would be expected from a borehole drilled 

from the surface to the water table near Cinder Lake. 

Transient Electromagnetics (TEM) Surveys 

A TEM sounding is made by transmitting an intermittent electrical signal through an 

ungrounded wire that is laid in a square loop on the surface of the earth. The transmitter loop (Tx) is 

energized periodically at regular intervals at frequencies of 4 to 32 hertz and creates a time-varying 

magnetic field in the earth below it through Ampere’s law (Fitterman and Stewart, 1986). According to 

Faraday’s Law of Induction (Fitterman and Stewart, 1986), rapidly switching the electric signal on and 

off produces a primary electromagnetic field that diffuses into the ground and causes (induces) eddy 

currents in the ground and in nearby buried conductors (Sharma, 1997). The eddy currents create 

secondary magnetic fields, some portion of which travel back to the surface (Nabighian and Macnae, 

1987) where they can be measured as a decaying magnetic field. This usually is done by using a 

receiver composed of a smaller secondary loop or a vertical coil of highly conductive material. 

Although the secondary magnetic field can be measured anytime, many instruments focus on detecting 

the fields after the instrument stops transmitting. This gives the instrument greater sensitivity than other 

electrical methods because the decay of these currents occurs when there is no primary field to interfere 

with detection (Nabighian and Macnae, 1987). 

Measurements of the decaying secondary field are made at the land surface by sampling the field 

during multiple short time windows. Measurements for this survey are made with the receiver coil 

inside of the transmitter loop to provide a 1-dimensional (1-D) sounding of the subsurface response to 

the primary field. The depth of investigation of a TEM sounding is dependent on the size of the primary 

loop, the amount of current excited into the loop, and the ability of the subsurface to conduct an 
 7

electrical field. A general guideline is that the depth of investigation is about two to three times the 

size of the transmitter loop of wire. In conductive areas, the depth of investigation may only be 

equivalent to the size of the loop; and in more resistive areas, the depth of investigation may be more 

than three times the size of the primary loop (Zonge, 1992). Each TEM sounding is comparable to a 

single drilled borehole, although the sample volume is significantly larger because of the size of the 

transmitter loop and the outward spreading of the primary field with depth (smoke-ring effect). The 

results of an individual TEM sounding can be modeled and plotted as resistivity values with depth. 

Multiple TEM soundings adjacent to or near each other can be plotted together as a TEM profile or 

cross section. 

Raw TEM data were collected and then processed, and 1-D layered earth resistivity models were 

developed using Zonge Engineering’s DATPRO (Zonge Engineering, Tucson, Arizona) suite of 

software. Raw TEM data were averaged using Zonge’s TEMAVG program. Averaged data were 

imported into Zonge’s STEMINV inversion software to produce a 1-dimensional model of subsurface 

resistivity for each sounding. 

A Zonge Engineering NanoTEM system (Zonge Engineering, Tucson, Arizona) was used for 

this investigation. The system was set up in a configuration that consisted of a Zonge ZT-20 transmitter 

connected to a transmitter loop (Tx) that was excited with between 3.0 and 3.5 amps for each sounding. 

The receiver consisted of a smaller loop of wire located in the center of the transmitter loop used in 

conjunction with a Zonge GDP-32II multifunction receiver (Rx). Thirty-two hertz data were collected 

for each sounding. Thirty-one time windows of data were collected at each sounding from 0.0 μsec to 

2.859 msec. Measured data were observed in the field as averaged normalized returns for each time 

window that were then graphed for visual inspection of irregularities. Three repetitions of each sounding 

were measured and averaged together. 

TEM soundings were conducted at Cinder Lake during May and June 2011. Although different 

TEM loop sizes were tested to determine which size would most appropriately profile the depth to 

basalt, a 60 ×60 ft transmitter loop with a 15 ×15 ft receiver loop showed the best results and was 

therefore used for the survey. A north-south profile, A–A , of 49 adjacent TEM soundings were made 

from borehole CL-3 in the north to about 600 ft south of borehole CL-2 (figs. 2 and 4). 

TEM data can be influenced negatively by nearby metal conductors, such as fences, pipes, 

underground wires, overhead or buried power lines, and train tracks. There were no cultural 

interferences such as these in the vicinity of Cinder Lake. 

Borehole Drilling 

Boreholes were drilled at Cinder Lake to provide control for geophysical techniques and to 

characterize the subsurface materials found above the first basalt layer (fig. 1). Four boreholes were 

drilled by Boart LongyearTM (2010), using sonic drilling methods. The sonic drilling method produces a 

continuous, relatively undisturbed core sample and is beneficial for investigative studies where 

information is needed about subsurface lithology. Sonic drilling uses an outer steel casing and an inner, 

smaller diameter steel drill pipe connected to a hollow pipe that fits inside the outer steel casing. Highfrequency vibrations are sent down the drill stem as it rotates into the subsurface. As the outer case 

advances into the subsurface, the inner casing forms a sleeve around the subsurface material that can be 

extracted as a core. The sleeve is removed from the core and core materials are loaded into clear plastic 

sleeves and described. Each bag typically holds about 2.5 ft of core sample and the clear bags are laid 

on a tarp in sequential order where they can be described and documented. Borehole cores were initially 

described on-site and further described in the laboratory (Schoeneberger and others, 2002; Birkeland 

and others, 1991). 
Figure 4. Subsurface lithologic and electrical profile A’ to A (fig. 2). Results of 51 one-dimensional transient electromagnetic smooth-model 

inversions are contoured at irregular resistivity intervals. Numbers across the x-axis indicate the TEM station number.



6287+

$


6630 

6560 

6490 

6420 

6350 

Borehole CL-2 Borehole CL-1 

6630 

6560 

6490 

6420 

6350 

h h h  h  h 

0RGHO5HVLVWLYLW\

RKPP

1585 

rn 

CD 

 4



0 

O

-=;:; 

c.. 

13 

)LJXUH

8
 9

A total of four boreholes were drilled to depths of 40–45 ft between June 3 and June 6. Three of 

the boreholes were drilled in a north to south line across Cinder Lake and named from north to south 

CL-3, CL-1, and CL-2 (fig. 2). The fourth borehole was drilled offset to the east from borehole CL-1 

and CL-4 (fig. 2). The boreholes were completed into the top 5 ft of the uppermost basalt layer using 4-

in. polyvinyl chloride (pvc) casing with perforations in the bottom 10 ft. The perforated casings are 

intended to provide a means to measure future water accumulation that may occur following runoff and 

infiltration events. 

Porosity Calculations 

A component of the study was to determine the total porosity of subsurface materials and to 

calculate the total potential subsurface storage capacity beneath Cinder Lake. The total porosity of the 

major lithologic units from the cores of wells CL-1, CL-2, CL-3, and CL-4 were estimated using a direct 

water saturation technique (Yu and others, 1993; Nimmo, 2004). Plastic bags containing the cores were 

cut open and allowed to partially dry. Samples of 2–3 Lwere collected from six or seven individual units 

within each core (labeled porosity sample number) and air-dried for several hours. Each sample was 

divided into two test samples of 1,000 mL each, which were placed in an 80°C oven on an aluminum 

baking sheet until completely dry. Each dry sample was added to a 1,000 mL glass beaker and 

compacted by shaking the beaker by hand. Compaction often reduced the volume from the original 

1,000 mL test sample, so the post-compaction sediment volume of each sample was recorded. Volumes 

ranged from 870 to 1,000 mL. Water was added to each test sample from a 1,000 mL graduated cylinder 

until the sediment was saturated to the bottom of the container and the water level rose to the level of 

the top of the sediment. Saturation occurred in a few minutes for coarse-grained cinder samples but 

required several hours in finer grained units including clay- and silt-sized particles. The total amount of 

water added to each test sample was recorded and divided by the test sample sediment volume to 

estimate the porosity of each test sample. The porosities estimated from the two test samples were 

averaged for each unit. 

The water-saturation technique for estimating porosities provides an estimate for total porosity 

minus the micro-pore space for the sampled sediments. The time allowed for sample saturation was 

limited, and therefore small-scale pores may have remained filled with air and could contribute to an 

underestimate of total porosity. An average total porosity for each core section was determined by an 

average of the porosity of each layer weighted by layer thickness. Layers in each core were either 

measured or assigned an estimated minimum total porosity based on the measured values of similar 

layers. The products of the total porosity and thickness of each layer within a core were summed and 

divided by the total thickness of the cinder deposits above the basalt to calculate the weighted average 

total porosity. This calculation was performed for all cores (figs. 5–8). 
10

Figure 5. Lithologic Log of Borehole CL-1 

42.2%

40

42

44

46

20

22

24

26

28

30

32

34

36

Depth 

(ft) Description Munsell color

Porosity 

sample 

number

2

38

4

6

8

10

Total porosity 

(blue shaded cells 

estimated from similar 

units of other Cinder 

Lake boreholes) 0

12

14

16

18

Basalt bedrock, increasingly vesicular N3 (dry)

Loose fine to medium sand with ~2% coarse-grained 

quartz sand

Clayey, slightly consistent fine to medium sand

Loose, very fine to fine well-sorted sand

Moderately well-sorted fine to medium sand with some 

soil development and clay films

Basalt clasts 1 to 4 inches in diameter (some 

weathered) with a matrix of finer basalt fragments and 

cinders

Clasts: 2.5Y 7/1 (dry) 

Matrix: 5YR 4/1 (dry)

Competent vesicular (0.15-0.3 inch) basalt bedrock with 

sub-vertical fractures with weathering rinds and subvertical voidsup to 1 inch wide and 4 inches long

Basalt: N6 (dry) 

Weathering rind: 

5Y 6/1 (dry)

Basalt rubble and fine material with discontinuity; 

possible flow base N3 (dry)

Loose, moderately sorted medium to coarse quartz, 

feldspar, and lithic sand with some clay films on grains 7.5YR 3/2 (moist)

Dense, soft, plastic, clayey, moderately sorted medium 

to coarse sand 7.5YR 4/3 (moist)

Basalt cinders with some sand-size material 5R 2/2 (moist)

Clayey fine to medium sand 10YR 4/4 (moist)

Basalt clasts up to 6 inches diameter, gravels, and 

cinders with very little sand-size material N4 to N2 (moist)

Loose, fine to medium sand

Moderately well-sorted, loose fine to medium sand with 

some coarse mafic volcanic fragments 

5

7.5YR 4/3 (moist)

7.5YR 4/3, (moist)

7.5YR 6/3 (moist)

10YR 4/4 (moist)

7.5YR 4/6 (moist)

Basalt bedrock becoming less vesicular N4 (dry)

7.5YR 4/4 (moist)

Basalt cinders and clasts to 1.5-4 inches with some 

clayey fine to medium sand

Loose fine sand and silt 7.5YR 6/4 (moist)

Average Column Porosity (weighted average of upper 31 ft)

40%

44%

39%

minimal

minimal

minimal

55%

40%

40%

30%

38%

49%

6

7

1

2

3

4
11

Figure 6. Lithologic Log of Borehole CL-2 

Average Column Porosity (weighted average of upper 24.5 ft) 44.1%

minimal

minimal

minimal

6 28% 5R 4/6 to 5R 3/4 

(moist)

5YR 4/1 (moist)

N7 (dry)

N5 (dry)

Basalt bedrock, slightly vesicular, no fractures

Basalt bedrock with 0.15 to 0.25 inch diameter vesicles, 

no fracturing

Basalt bedrock, upper part of core appears abraded by 

drilling, no fractures

Basalt cinders 0.15 to 0.4 inch in diameter

5R 3/4 to 10R 4/3 

(moist)

10R 3/4 (moist)

N2 (dry)

10YR 5/6 (dry)

7.5YR 4/3 (moist)

7.5YR 4/4 (moist)

N3 to N1 (moist)

7.5YR 4/4 (moist)

5

3

4

2

Basalt cinders up to 1.5 inches in diameter with 

overlying basaltic rubble

Basalt cinders 0.10 to 0.25 inches in diameter, a few 

clasts to 1 inch

Basal cinders and clasts up to 5 inches in diameter

Clayey medium to coarse sand with 5% basalt cinders

Fine to medium clayey sand

Basalt cinders and fine basalt fragments 0.1 to 0.25 inch 

in diameter.

Loose medium to coarse sand with silt and cinders 

Medium to coarse clayey sand

Medium to coarse sand

Upper 3 inches loose basalt cinders. 

Clayey loose coarse sand with volcanic fine gravels

22

20

18

16

14

12

10

8

6

40

38

36

34

32

30

28

26

24

50%

Basalt cinders 0.2 to 0.6 inches in diameter with clasts 

to 1.8 inches

N2 to N1 (moist)

10YR 4/5 (moist)

10YR5/3 (dry)

10R 5/4 (moist)

46%

46%

51%

Total porosity 

(blue shaded cells 

estimated from similar 

units of other Cinder 

Lake boreholes) 0

Depth 

(ft) Description Munsell color

Porosity 

sample 

number

1 51%

39% 4

2
12

Figure 7. Lithologic Log of Borehole CL-3 

43.3%

4

2

40

38

36

34

32

30

28

26

24

minimal

38%

53%

40%

50%

38%

7 43%

6

5

3

4

1 56%

2 58%

50%

34%

N2 (moist)

7.5YR 3/3 (moist)

7.5 YR 3/4 (moist)

10YR 3/4 (moist)

N3 (moist)

5YR 4/4 (dry) to N6 (dry)

N3 (moist)

N2 (moist)

10YR 4/4

10YR 5/6 (moist)

10YR 4/2 (moist)

7.5YR 3/4 (moist)

N3 (dry)

Loose, well-sorted fine sand

10YR 3/4 to 10YR 4/4 

(moist)

7.5YR 5/4 to 7.5YR 3/3 

(moist)

10YR 6/6 (dry)

10YR 4/4 (dry)

N2 (moist)

10YR 4/3 (moist)

Basalt clasts, 0.2-0.5 diameter, up to 1 inch with a 

matrix of fine sand and cinders

Loose, well sorted very fine to fine sand with fine gravelsized lithic clasts at base

Basalt cinders 0.1 to 0.5 inches in diameter

Loose, moderately well-sorted medium to coarse sand 

with basalt cinders throughout

Basalt cinders 0.1 to 0.25 inches in diameter

Vesicular basalt bedrock with some sub-vertical 

fracturing

Basalt cinders

Basalt bedrock, few vesicles 0.1 to 0.2 inches in 

diameter, no fractures other than those induced by 

drilling

Basalt bedrock, fine vesicles 0.1 to 0.15 inches in 

diameter, no fractures

Coarse sand with basaltic cinders.

Soft, clayey medium to coarse sand

Soft, slightly clayey medium to very coarse sand with 

some basalt cinders throughout.

Clayey fine to medium sand

Loose, well sorted fine to medium sand

Moderately sorted medium to coarse clayey sand

Basalt cinders 0.15 to 0.4 inches in diameter

Loose basalt cinders, average 0.1 inches in diameter

Average Column Porosity (weighted average of upper 29 ft)

22

20

Loose medium to coarse, slightly clayey sand with fine 

gravels (0.2-0.4 inches in diameter)

Weathered basalt cinders with poorly-sorted sand

Loose, moderately well sorted medium to coarse sand 

with some gravel-sized cinders

18

16

14

12

10

8

6

Total porosity 

(blue shaded cells 

estimated from similar 

units of other Cinder 

Lake boreholes) 0

Depth 

(ft) Description Munsell color

Porosity 

sample 

number

N3 (moist)

10YR 7/6 to 10YR 4/4 

(moist)
13

Figure 8. Lithologic Log of Borehole CL-4 

47.5%

Depth 

(ft)

Average Column Porosity (weighted average of upper 32 ft)

4

5

6

7

60%

44%

57%

35%

minimal

53%

49%

50%

50%

10YR 7/1 (dry)

1

2

3

7.5YR 4/4 (moist)

N2 (moist)

7.5YR 4/3 (moist)

7.5YR 4/4 (moist)

7.5YR 6/8 (moist)

N3 (moist)

7.5YR 4/3 (moist)

N2 (dry)

7.5YR 5/4-3/3 (moist)

N2 (moist)

N2 (moist).

7.5YR 4/3 (moist)

Hard and dense basalt

Loose basalt cinders up to 1⁄2 inches in diameter

Basalt cinders to 6 1⁄2 inches in diameter, some very 

well weathered, with matrix of finer cinders

Basalt cinders up to 2 3⁄4 inches in diameter, some are 

highly weathered

Basalt cinders up to 3 inches in diameter in a loose 

matrix of well-sorted fine sand

Loose basalt cinders, average 0.1 inches in diameter

Loose, well sorted fine to very fine sand with rare basalt 

cinders up to 3⁄4 inches in diameter.

Loose basalt cinders, average 1⁄4 inches in diameter

Well sorted fine to very fine sand with weak soil 

development. Grades into sand and basalt cinders at 

base of interval. 

Moderately well-sorted very fine to fine sand

Loose basalt cinders, average 0.2 inches in diameter 

with clasts to 1 inch

Angular basalt clasts, some clasts show strong 

weathering

Basalt cinders to 1 1⁄4 inches in diameter with a trace of 

very fine-grained sand.

Cinders mixed with waxy plastic clay

Porosity 

sample 

number

Total porosity 

(blue shaded cells 

estimated from similar 

units of other Cinder 

Lake boreholes)

N2 (dry)

Description Munsell color

Basalt cinders, average 0.1 inches in diameter

14

16

28

30

32

34

36

38

40

42

44

18

20

22

24

26

8

10

12

0

2

4

6
 14

Gravity 

Gravity data are a measure of mass below the measurement location. Changes in gravity over 

time, determined from repeat measurements at the same location, indicate changes in subsurface mass 

when elevation change and other sources of subsurface mass change are negligible. At Cinder Lake, the 

dominant source of any measurable gravity change would be due to a change in water storage in the 

subsurface. As stormwater enters the basin, water storage and gravity increase. Over time, if 

stormwater moves deeper and spreads laterally, gravity would decrease because the distance between 

infiltrated stormwater and the gravimeter has increased (following Newton’s law of gravitation, gravity 

decreases as 1/r2, where r is the radial distance to the mass under consideration). The Cinder Lake area 

is well suited to gravity monitoring because of the unsaturated-zone storage change by introducing large 

amounts of infiltration and unconfined aquifer conditions, both of which result in a large-magnitude 

signal. To quantify the spatial extent of water storage change, a network of 19 gravity stations has been 

established. An initial round of gravity measurements was made in June 2011, prior to the summer 

monsoon season. Subsequent measurements were made over the course of the summer of 2011. 

Measurements are made using both an absolute gravimeter, which determines gravity by measuring the 

acceleration of a falling mass in a vacuum, and a spring-based relative gravimeter, which measures the 

difference in gravity between stations by measuring the difference in length of a mechanical spring 

among the stations. The absolute gravimeter provides reference values at a small number of stations, 

much like benchmarks in a leveling network; the relative gravimeter, with which measurements can be 

collected more quickly, is used to measure the relative gravity difference between the absolute gravity 

stations and the remaining stations throughout the study area. As with leveling surveys, the two types of 

measurements are combined in a least-squares network adjustment to take full advantage of redundant 

observations. 

Results 

Transient Electromagnetics (TEM) Surveys 

TEM soundings at Cinder Lake were useful in determining the lateral continuity of materials 

found at depths of 40–45 ft in boreholes and in estimating the thickness of the basalt layer. Adjacent 

TEM soundings were made every 60 ft from north to south along a transect between wells CL-3, CL-1, 

and CL-2 (fig. 2). TEM modeling consisted of calculating a 1-D, smooth-model inversion of resistivity 

versus depth for each sounding. The 1-D inversions are then interpolated to form a cross section of 

estimated resistivity with depth. Inversion results from the 60-ft loops produced models with three 

distinct layers (fig. 4). The uppermost layer is a moderately resistive unit with resistivity values between 

200 and 300 ohm-m (blue) that extends from the surface to a depth of about 30 ft (fig. 4). The 

uppermost layer corresponds to the overlying cinder, sand, silt, and clay material observed in the boreholes 

at Cinder Lake. The second layer is a highly resistive unit between 300 and 1,000 ohm-m (purple) that 

extends from about 30 to about 90 ft in depth (fig. 4). This layer corresponds with the basalt layer found 

in the boreholes and is consistently observed along the geophysical cross section. The basalt layer crops 

out in two areas near the southern end of the TEM profile, which is consistent with observed basalt 

exposures at Cinder Lake. Along the basalt layer there are areas near TEM soundings 17, 20, and 21 

where the resistivity is about 300 ohm-m compared to the more typical 400 ohm-m throughout the rest 

of the layer. The slightly lower resistivity could indicate that the basalt near these soundings is more 

fractured and has some component of saturation. Water infiltrates into the ground from the Copeland 

Ditch near this area and could provide the additional water found in the fractured basalt. The third layer 
 15

of this inverse model begins at a depth greater than 90 ft and is characterized by resistivity values 

ranging from 50 to 300 ohm-m (green; fig. 4). There is no supporting borehole data for the layers 

beneath the basalt and therefore no interpretation of this layer is made, although it represents a lithology 

with much lower resistivity values than cinders and sediments or basalt. 

Borehole Drilling 

Drilling in all four boreholes encountered interbedded cinders, sand, silt, and clay overlying 

hard, fractured basalt at depths between 29 and 32 ft. Many high-angle fractures were observed in basalt 

samples recovered during the drilling process, which implies that the basalt layer is not a confining 

layer. Interbedded cinders, sand, silts, and clays were above the basalt. Diagrams of the boreholes 

describe the cinder deposits and illustrate the interbedded relationship of the cinders and sediments 

(figs. 5–8). Intervals of clay in each of the boreholes could indicate a layer that could potentially cause 

perching conditions. Layers of clay are found in borehole CL-1 at depths of 2.5, 15, and 25 ft; in 

borehole CL-2 at depths of 5 and 9 ft; in borehole CL-3 at depths of 12 and 25 ft; in borehole CL-4 at a 

depth of 5 ft. 

Moore and Wolfe (1976) reported the age of basalt and pyroclastic deposits in the Cinder Lakes 

area as Middle Pleistocene. The fact that basalt, scoria, and cinders were found in the boreholes below a 

depth of about 15 ft shows some evidence of weathering, as these deposits were subaerially exposed for 

perhaps 500 k.y. 

The alluvium (medium- to coarse-grained sand and clayey sand) found in the cores above the 

Middle Pleistocene volcanic rocks shows that waterborne sediments were transported to the Cinder 

Lake area by water, hyper-concentrated flow, or debris flow during and between episodes of volcanic 

activity. Lithic clasts and mafic volcanic rocks with an andesite affinity are a minor component

of the alluvial deposits. Upgradient from Cinder Lake, there are multiple andesite flows on the 

eastern side of San Francisco Mountain (Wolfe and others, 1987) and a small andesite cinder cone 

(Moore and Wolfe, 1987) about 3.5 km upslope that might have been the source of the andesite clastic 

material. The clays may have been transported by fluvial action, although some of the clay might be the 

result of soil development based on clay films and bridging between sand grains seen in some samples 

that resulted from pedogenesis. The degree of pedogenesis corroborated by the minor clay and 

carbonate accumulation seen in the samples suggests that alluvial deposition occurred during the middle 

Holocene to perhaps the latest Pleistocene. 

 The fine-grained to very fine grained sand and silt suggests that there was some eolian activity 

that deposited thin sand sheets on the volcanic deposits. Small calcareous nodules found within these

fine-grained sands appear to be Stage I pedogenic carbonate (Machette, 1985). In the southern 

Colorado Plateau region, Stage I soil carbonates have been found to form within 2–10 k.y. (L. Amoroso, 

unpub. data, 2012), so there may have been a hiatus in deposition after the mid- to early Holocene. The 

eruptions from Sunset Crater began about A.D. 1040 and continued to about A.D. 1180 (Smiley, 1958; 

Ort and others, 2002); pyroclastic materials from these eruptions are found at the surface and to a 

depth of about 15 ft in the cores. 

No radiocarbon-datable organic material or charcoal was found in the samples that could 

constrain the age of the alluvial deposits. 

Estimates of Total Porosity and Storage 

The amount of water that can be accommodated in the cinders and sediments during a given time 

interval is a function of not only the volume of available storage (capacity) of the deposits, but also the 

volume of groundwater inflow and outflow. Water-storage capacity for this study refers to the total 
 16

available water storage in the deposits and does not take into consideration inflows and outflows from 

the system. 

Porosity estimates were calculated for each borehole. The weighted average porosity for cores 

from boreholes CL-1, CL-2, CL-3, and CL-4 are 42.2, 44.1, 43.3, and 47.5 percent, respectively (tables 

1– 4; figs. 5–8). An overall average porosity for the 30 ft of material above the uppermost basalt flow is 

44 percent. The estimated porosity values for these cores are within the reported range for these types of 

unconsolidated deposits, which are between 25 and 50 percent (Todd, 1964; Freeze and Cherry, 1979; 

Driscoll, 1986). 

Porosity 

sample 

number Test 

Sediment Volume 

(mL)

Water Volume 

(mL)

Estimated 

porosity 

(percent)

Porosity 

sample 

number 

average 

(percent)

Thickness 

(ft)

CL-1-1 1 1000 550 55.0 54.5 2.5

2 1000 540 54.0

CL-1-2 1 1000 400 40.0 40.0 7.0

2 1000 400 40.0

CL-1-3 395 39.5 40.0 2.5

2 1000 405 40.5

CL-1-4 1 1000 400 40.0 38.0 7.5

2 1000 360 36.0

CL-1-5 1 1000 500 50.0 48.5 4.0

CL-1-5 2 1000 470 47.0

CL-1-6

CL-1-6

CL-1-7

CL-1-7

 

Table 1. Estimated total porosity values for samples from borehole CL-1
 17

Porosity 

sample 

number Test 

Sediment volume 

(mL)

Water volume 

(mL)

Estimated 

porosity 

(percent)

Porosity 

sample 

number 

average 

(percent)

Thickness 

(ft)

CL-2-1 1 1050 525 50.0 50.8 2.0

2 1000 515 51.5

1 1000 430 43.0 38.5 3.0

2 1000 340 34.0

CL-2-3 1 1000 460 46.0 46.0 1.5

2 1000 460 46.0

CL-2-4

CL-2-4

CL-2-5

CL-2-5

CL-2-6

CL-2-6

Table 2. Estimated total porosity values for samples from borehole CL-2 

Porosity 

sample 

number Test 

Sediment volume 

(mL)

Water volume 

(mL)

Estimated 

porosity 

(percent)

Porosity 

sample 

number 

average 

(percent)

Thickness 

(ft)

CL-3-1 1 1000 579 57.9 56.4 3.5

2 1000 548 54.8

CL-3-2 1 950 360 37.9 37.8 1.5

CL-3-2 2 940 355 37.8

CL-3-3 1 1000 340 34.0 34.2 6.0

CL-3-3 2 1000 343 34.3

CL-3-4

CL-3-4

CL-3-5

CL-3-5

CL-3-6

CL-3-6

CL-3-7

CL-3-7

Table 3. Estimated total porosity values for samples from borehole CL-3
 18

Porosity 

sample 

number Test 

Sediment volume 

(mL)

Water volume 

(mL)

Estimated 

porosity 

(percent)

Porosity 

sample 

number 

average 

(percent)

Thickness 

(ft)

CL-4-1 1 1000 520 52.0 52.5 5.5

2 1000 530 53.0

CL-4-2 1 1000 490 49.0 48.7 0.5

2 950 460 48.4

CL-4-3 1 1000 480 48.0 50.0 1.0

CL-4-3 2 1000 520 52.0

CL-4-4 1 1000 600 60.0 59.5 4.5

CL-4-4 2 1000 590 59.0

CL-4-5

CL-4-5

CL-4-6

CL-4-6

CL-4-7

CL-4-7

Table 4. Estimated total porosity values for units from borehole CL-4

The lateral extent of the area used to calculate storage at Cinder Lake is about 300 acres (fig. 2), 

and the cinder deposits are about 30 ft deep with an average total porosity of 44 percent. These 

dimensions and average porosity yield a total storage for Cinder Lake of 3,960 acre-ft or about 4,000 

acre-ft. 

Gravity 

Gravity measurements are ongoing at Cinder Lake, and results from gravity measurements will 

not be available until data have been collected through multiple infiltration events and multiple 

quiescence periods during fiscal year 2012. 

Conclusions 

The U.S. Geological Survey drilled four boreholes and conducted geophysical surveys to 

determine the depth of the cinder beds and their potential for water storage capacity. Results indicate 

that the uppermost basalt layer at Cinder Lake is approximately 30 ft below land surface in an area that 

has little topography. Total porosity for the upper 30 ft of sediments was estimated as 43 percent, 

which yields a total storage for the 300-acre extent of Cinder Lake of about 4,000 acre-ft. 

Acknowledgments 

 The authors extend their appreciation to James Guidatti and Randy Ryan from Coconino 

County who provided field support throughout this study. A special thanks to Steve Skotnicki from 
 19

HydroSystems Inc. for his geologic expertise during the drilling process and overall discussion of the 

subsurface geology at Cinder Lake. The authors thank Ken Robinson and Matt Morales from the City of 

Flagstaff for working hand-in-hand with the USGS and for allowing the USGS access to all of their data 

from Cinder Lake. An invaluable person to this study is Don Bills and his many areas of expertise 

during the entire project are truly appreciated. The authors also thank the field crews from the 

USGS Arizona Water Science Center who helped in collecting the borehole and geophysical data for 

this study. 

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