WATER RESOURCES OF THE TOWN OF ROCKVILLE.
EASTERN WASHINGTON
COUNTY, UTAH:
A REPORT FOR THE
WASHINGTON COUNTY WATER CONSERVANCY DISTRICT
Peter D. Rowley,
Consulting Geologist
Geologic Mapping,
Inc. P.O. Box 651, New
Harmony, UT 84757
435-865-5928,
pdrowley@accesswest.com
www.geologicmappinginc.com
September 10, 2004
Introduction
On August 25, 2004, Barbara Hjelle, Counsel for the Washington
County Water Conservancy District (WCWCD), St. George. Utah,
requested that I do a quick assessment of the water resources of the town of Rockville, Utah. She provided the names of two persons to
contact, who had recently met with her and Ron Thompson, Manager of the
WCWCD. These persons are: Dan McGuire,
Mayor of Rockville, and Pete Mortensen, President of the Rockville Pipeline
Company. On August 27, I set up a meeting
with them.
At 9:00 a.m., August 31, Corey Cram, Hydrogeologist with WCWCD,
and I met with Dan McGuire and Pete Mortensen at Pete's home in Rockville. This was followed by a tour of Rockville's well field and
water tanks. At our meeting Dan and Pete
summarized Rockville's
situation. The town currently gets all
of its drinking water from 3 shallow wells, but the flow from these wells has
decreased by half in the last 3 years.
They interpreted that this decrease is due to the continuing drought
that has affected this and other parts of Utah.
The drought has substantially reduced the recharge to the wells. The town is now rationing water, and Dan and
Pete conclude that they must find additional or alternative sources of water
before the wells dry up completely.
Accordingly, they initiated the meeting a couple weeks ago with WCWCD.
Rockville is doubtless named for its many car- and garage sized
boulders of the Shinarump Conglomerate Member of the Chinle Formation (Triassic
age) scattered throughout the town and littering the slopes of mesas (called
Rockville Bench on the north side of town) that rise about 4OO feet above the
town. These boulders are derived from
the mesas themselves, which are held up by a resistant cliff of the Shinarump
Member. Curiously, Pete's home is next
door to Jack Burn's home, which was largely destroyed
by rock fall (Rowley and others, 2001)
at about 6:00 a.m. on October 18, 2001.
This rock fall resulted when the edge of the Shinarump cliff
gave way just north of Jack's home, broke apart as it hit the slope below the
Bench, and sent a spray of boulders toward Jack's home. One of those boulders, estimated at about
200-300 tons and of greater diameter than the height of the house, smashed into
the home and missed Jack by several yards.
Jack suffered no injuries and, at the time of our meeting, his home had
been repaired. The Shinarump is
noteworthy for purposes of this report because it is the aquifer for Rockville’s water
supply.
Background
The Shinarump Member forms a thin (60-135 feet thick)
resistant bed suitable as a ground-water aquifer because of (1) primary
permeability as a sandstone and conglomerate, and (2) secondary permeability
due to faulting and fracturing because of its resistant and brittle
nature. The bed dips eastward at several
degrees and goes underground just east of town.
Rockville
has an elevation of 3,730 feet. Rockville's small well
field is about 1 mile southeast of town at an elevation of 4,000 feet on its
north side and 4,150 feet on its South.
All wells penetrate the Shinarump.
The recent history of the town illustrates a Conservative
approach to growth and development because of its limited water supply. The Utah Division of Water Rights web page
lists Rockville’s
water rights, which date to 1925, at a total flow of 0.15 cfs (67 gpm) or 108.6
acre- feet/year. Pete could not confirm
the accuracy of these values, but he agreed that they are in the right
ballpark. The town occupies about 9
Square miles, extending as far west as west as Grafton. The town extends about a mile eastward past
the cemetery on the south side of the canyon, but the homes on the north side
that include Anasazi development (less than a mile east of the cemetery) are in
Springdale. House lots in Rockville are restricted
in size to no less than 1 acre, although most people would prefer that they be
able to develop half-acre lots. The town
has allowed only 152 water shares (connections), 9 of which have been permitted
but not yet hooked up. In contrast,
Virgin to the west is annexing land and growing rapidly; it is at the east end
of the WCWCD pipeline. Springdale
to the east gets its culinary water by treating water from the Virgin River
and, after passing the sewage through settling ponds, the town returns the
treated waste to the Virgin River down stream
of Rockville. Springdale's
water is some of the highest costing water in the State, whereas Rockville's water is among
the cheapest in the State. Some ranchers
in Rockville have water rights to water from the
Virgin River and from shallow wells into Virgin River
alluvium, but this water is not of culinary quality. Such agricultural water uses are relatively
small; no pivot irrigating, for example, is done in the Rockville area.
Recently Rockville's
water system was upgraded through a million-dollar loan. The improvements involved new pipelines and a
new 300,000-gallon concrete water tank, placed adjacent to the old tank
(100,000 gallons), which continues in service.
These tanks are about a quarter mile north of the well field. The piped water is automatically chlorinated
as it emerges from the well field, on the way to the tanks.
Rockville's Well Field
The well field for Rockville is shown on the Utah Division
Of Water Rights web site as consisting of 5 wells and one spring, Rimrock (or
Buttermilk) Spring. Pete, however, said
that one of these “wells” (well no. 2) is the spring inasmuch as the spring is
piped. Rimrock Spring is shown on the
7.5-minute quadrangle of the area, the Springdale West quadrangle. The spring is in an unnamed dry gulch that
drains northward to the Virgin River. Although as recently as three years ago, the
spring flowed year round, currently it dries up in May, and returns in the late
winter. By the spring of the year, when
good flows return, the water contains bacteria.
One well (well no. 1), apparently the oldest, is just
downstream of the spring. The well is no
longer used because its water quality is poor and has a sulfur smell. The three producing wells are in the gulch
upstream of the spring. The first of
these to be drilled, well no. 3, is several hundred yards south of the
spring. It is about 80 feet deep and has
a yield of about 35 gpm. The pump turns
on when the water level is 31 feet below the surface, and it turns off when the
water level drops to 65 feet.
The next well to be drilled, another couple hundred yards to
the south, is well no. 4. It also is
about 80 feet deep and produces about 22 gpm.
The pump starts when the water level is 57 feet, and turns off at 67
feet. The most recent well, no. 5, is
another couple hundred yards father south.
It is about 82 feet deep and produces at 30 gpm; it was producing at 60
gpm three years ago. Its pump starts
when the water level is 63 feet and shuts off when the level is at 80
feet. Pete noted that when each new well
was drilled, the flow in the wells to the south decreased once pumping in the
new well began.
Hydrogeology of the Area
On the south side
of the Virgin River, the Shinarump Conglomerate Member forma a bench at the same height and comparable in size to the Rockville Bench north
of Rockville. Below this rock unit, here as well as beneath
Rockville Bench, is an impermeable, gypsiferous mudstone, siltstone, .and
fine-grained sandstone belonging to the upper red member of the Moenkopi Formation, which also is Triassic. The upper red member
is 200 to 280 feet thick, but additional members of the Moenkopi, similarly all
aquitards, under lie the red member. The
Moenkopi is about 1700 feet thick
(Willis and others, 2002).
Above the Shinarump is a similar impermeable rock unit, the
Petrified Forest Member of the Chinle Formation. It is 400 to 500 feet thick. This, the upper unit
of the Chinle Formation, is an aquitard.
It also contains bentonite, an unstable and swelling clay that caused the Member to be the most susceptible rock unit in this part of Utah to land
sliding. The Member is overlain by
Jurassic
rocks, the lower parts of which (Moenave Formation
and Kayenta Formation above it) are largely aquitards. But upper parts (upper Kayenta and the
overlying Navajo Sandstone) of the Jurassic sequence are permeable rocks that arc
important aquifers where buried. Unfortunately, these Jurassic aquifers are all above the elevation of Rockville, where
they form the spectacular and colorful topography of Zion National Park
(Biek and others, 2000). These aquifers
are thus not available as sources of water for Rockville
or Springdale.
At the Rockville
well field, lower parts of the soft Petrified Forest Member are exposed above the
Shinarump cliff. The impermeable shale of the Petrified Forest Member is in turn overlain by
an unconsolidated Pleistocene landslide that mantles and covers the rest of the
Petrified Forest Member. The landslide
is as much as 200 feet thick, and is derived from collapse of the Petrified
Forest Member and overlying Jurassic rocks
that
tower above the well field on up to the pinnacles of Eagle Crags, at an elevation of more than 6,300 feet, more than 2,000 feet above, and a
little more than a mile southeast of
the well field (Solomon, 1996; Willis and others, 2002). Rimrock Spring probably results from rain
water derived from these uplands that
seeps through the unconsolidated landslide until it hits the impermeable shale of the Petrified Forest Member, where this ground
water then moves horizontally to the ground surface and issues as a
spring. The wells are spudded (sunk) in
the basal parts of the Petrified Forest Member, and tap ground water from the
underlying Shinarump. The main source of
the recharge to the part of the Shinarump aquifer beneath the sell field is
precipitation on the uplands to the south, including
Smithsonian Butte (Moore and Sable, 2001) and Eagle Crags. Precipitation and runoff sink into the
unconsolidated landslides and other surficial deposits and move through
fractures into and through the Shinarump.
Locally, the slope of the gulch and of the hillsides near the well field
is northward, and thus the local ground-water gradient is northward. Therefore, each new well drilled southward of
existing wells will tap ground water destined for the northern wells, lowering
their yield.
The Shinarump Member is the lowest aquifer developed for
water in the area, but it is not commonly tapped for culinary water. This is because it is overlain and underlain
by shale aquitards that contain significant amounts of gypsum. Gypsum is water soluble, releasing salts into
the ground water. This gives high total
dissolved solids (TDS) to the water, so that it is at best marginal as culinary
water. In the Apple Valley development a
half dozen miles south of Rockville, in fact, such water is non culinary. A new center- pivot irrigation system on the
east side of the Apple Valley community appears to tap the Shinarump; the
yields are good but the water can only be used for growing hay, for it is unfit
for human consumption. Perhaps the
reason why Rockville's
water is potable is because rainfall onto the hummocky landslides in the
uplands south of the well field infiltrates rapidly into the aquifer, so has
less residence time to get salty.
Additional or Alternative Sources of Ground Water
There are several options for drilling additional or
alterative sources of culinary water for Rockville. These options are in addition to two other
options that WCWCD and the town of Rockville
will consider, namely tapping into Springdale's
water supply to the east or into the WCWCD pipeline to the west. Unfortunately, most drilling options are
risky. These are listed numerically
below, from I to 6. The wells suggested
in the first five options are annotated in red on the attached (mailed to
Barbara) geologic map of Willis and others (2002).
The first option (Option #1) that came to mind is to
drill the unconsolidated Quaternary sand and gravel alluvial deposits that
underlie the Virgin River. Because the river is a perennial stream,
these deposits are saturated throughout their thickness and should yield
sizeable amounts of ground water. Some
shallow wells on the flood plain adjacent to the river at Rockville tap the alluvial deposits, but this
water is used only for agriculture.
Four of these wells are shown on the Utah Division of Water Rights
website (for T. 42 S., R. 11 W., Section 1) penetrating to depths of 150 feet
or less and with yields of 25 to 60 gpm.
This option appears to be unsatisfactory because, as Pete and Dan noted,
the water is of non-culinary grade and would have to be substantially treated
to be potable. .l1'Iis is how Springdale
gets its culinary water, of course, but this use also lowers the quality of
this water for downstream users by returning the water through its
sewage treatment plant downstream of Rockville.
Because the few agricultural wells that penetrate the alluvium near Rockville are shallow,
apparently the alluvium is not thick enough at these wells to naturally cleanse
the water to culinary grade. Corey
suggested that perhaps a driller should test the thickness of the alluvium by
tapping ground water from the base of the river alluvium and screening out the
upper part of the well casing. In the Apple Valley area, most culinary water is derived from the
unconsolidated alluvium of Gould
Wash that is as much as 300 feet
thick. In the Rockville area, it does not appear to be this
thick. Saturated
river alluvium also underlies the East
Fork of the Virgin River, which enters the main (North Fork) stem of the Virgin
River about 1.5 miles east of the center of
Rockville (just east of the entrance to Anasazi housing development). In a later phone
conservation, Pete told me that water from the East Fork and from wells in its
alluvium is non-culinary. Furthermore,
most of the water rights for this water is owned by a
large orchard there; the main well for the orchard is at Shunesburg, a small
community about 2 miles east of the mouth of the East Fork. The owners of the orchard have offered to
sell some of the water rights to Rockville,
but Pete says that the asking price is steep.
Thus Option #2 would be to drill into this alluvium, from the
floodplain or adjacent river terraces anywhere east of the mouth of the East
Fork, depending on available land and permits.
The cost of a treatment plant would have to be added to this option.
The Shinarump Member extends east from Rockville's well field. It dips gently (2 degrees) east, so it
decreases in elevation the farther east it goes, and it goes underground about
a half mile east of where the East Fork joins the North
Fork. The Shinarump could
be drilled at several places between the well field and just cast of where it
passes beneath the East Fork. One
possible spot (Option #3) would be between 2,500 feet and 3,000 feet east and northeast of Rimrock Spring, where a gulch similar in
size to the one at Rimrock Spring, as well as the landslide above the gulch,
supplies recharge to the Shinarump from Eagle Crags. Here a modest flow, similar to that of wells
#3-5, could be expected. In fact, two
wells are shown in this gulch on the Utah Division of Water Rights website (for
T .42 S., R. l 0 W., Section 7), with depths of III and 119 feet and yields of
65 and 32 gpm. Presumably these wells
are on private land, so Rockville's
wells should not be sited upgradient (south) of them. Some of the other land in this area must
belong to BLM, which probably would approve the use. Sites farther to the east also should be
considered.
An alternative site (Option #4) to tap the Shinarump
would be to spud a well just east of where the sandstone and conglomerate ledge
disappears beneath the East Fork. In fact,
here the Shinarump would likely be partly recharged by the river itself so the
water would likely have fewer salts than at the present well field, where recharge has had to pass through the Petrified Forest Member.
And because it is partly recharged by the
river, it is likely that the yield of the well would be higher
than that of option #3. On the Utah
Division of Water Rights website (for T .42 S., R. 10 W., Section 5), drill
records for a well owned by the town of Springdale, located on the west floodplain
of the North Fork just northwest of where it is joined by the East Fork,
indicated that it penetrated through river alluvium, entered the upper member of the Chinle Formation at 110 feet and the Shinarump at 130
feet. It has a total depth of 192 feet
and a yield of 20 gpm. Under Option #4,
a well sited 3,000 to 4,000 feet east of this Springdale well would likely have a higher
yield.
Drilling faults is a method I generally prefer in the search
for water resources. The area of Rockville, however, is
only sparsely faulted. Nonetheless, in
looking through my collection of aerial photos, I spotted one north-south fault
that crosses the Virgin River just east of Grafton. And, happily, Willis and others (2002) mapped
it as well. So
this gives us Option #5.
Positioning a drill rig on the downthrown (western) side of the fault
about 100 feet west of the fault would hopefully intersect the west dipping
fault zone (Willis and others show it dipping west at 70 degrees) below the
water table. There are a few flies in
this ointment, of course. The fault
passes beneath Quaternary surficial deposits in the vicinity of the river, so
its location on flood plains and river terraces along the river is nowhere near
as certain as where it cuts bedrock north and south of the river, where we can be sure of its location. Another fly is that at several hundred feet
below the surface, we would be in a thick sequence of gypsiferous Moenkopi
shale. Thus water in this
"aquifer" of fault breccia and fractured and broken Moenkopi shale
might contain salts. Finally, there can
be no drilling north of the flood plain, for this area is in Zion National Park.
Option #6
is mentioned only for completeness, for it is far too risky to attempt. Drilling anywhere in the canyon of the Virgin
River, even beneath Rockville,
would encounter the carbonate (Paleozoic) aquifer beneath the Moenkopi. Even if we drilled at the lowest point we
could find, we would have 1,200-1,400 feet of Moenkopi to go through before we
hit the Kaibab Limestone. The Kaibab
likely contains ground water but we would want to go at least several hundred
feet into it. Unfortunately, in the
nearby area, the Kaibab and underlying Toroweap Formation (also a limestone)
arc gypsiferous. This is seen at Pah
Tempe Hot Springs, which issue from the Toroweap. But Pah Tempe has high TDS, some if not most
of which derives from the gypsum.
Clearly it would be too risky and too expensive to test the carbonate
aquifer in the Rockville
area!
Of the various options presented, the ones I favor arc
options #3 and #4. Option #3 would
likely be only a stopgap measure, for yields would not be much more than those
from existing wells. The choice of these
options would depend largely on the availability (ownership) or the land for
drilling sites and the difficulty of getting a drill rig to the site. Drill locations for each of these options
could be moved to accommodate the availability of the land for drilling and
roads. I would be glad to help site the
drill rig.
References
Bick, R.F., Willis,
G.C., Hylland, M.D., and Doelling, H.H., 2000, Geology of Zion National Park,
Utah, in Sprinkel, D.A., Chidsey, T .C., Jr., and Anderson, P .B., editors,
Geology of Utah's Parks and Monuments: Utah Geological Association Publication
28, p. 107-138.
Moore, D. W., and Sable, E.G., 2001, Geologic map of the
Smithsonian Butte quadrangle, Washington County,
Utah and Mohave
County, Arizona: Utah
Geological Survey Miscellaneous Publication 01-1, 30 p., scale
1:24,000.
Rowley, P.O., Hamilton, W.L., Lund, W.R.,
and Sharrow, David, 2002, Rockfall and landslide hazards of the canyons of the
upper Virgin River
basin near Rockville and Springdale, Utah:
Geolological Society of America 2002 Abstracts with Programs, v. 34, no. 4, p.
A-50.
Solomon, B.J.,
1996, Engineering geologic map folio, Springdale,
Washington County, Utah: Utah Geological Survey Open-File
Report 340, scale 1:14,400.
Willis, G.C.,
Doelling, H.H., Solomon, B.J., and Sable, E.G., 2002, Interim geologic map of the Springdale West
quadrangle, Washington County, Utah: Utah Geological Survey Open-File Report
394, 18 p., scale 1:24,000.
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