Water Resources of the Inner Basin of SanFrancisco Volcano, Coconino County, Arizona

Item Type text; Proceedings

Authors Montgomery, E. L.; DeWitt, R. H.

Publisher Arizona-Nevada Academy of Science

Journal Hydrology and Water Resources in Arizona and the Southwest

Rights Copyright ©, where appropriate, is held by the author.

Download date 13/06/2018 21:05:32

Link to Item http://hdl.handle.net/10150/300367

WATER RESOURCES OF THE INNER BASIN OFSAN FRANCISCO VOLCANO, COCONINO COUNTY, ARIZONA

E. L. MontgomeryNorthern Arizona University

R. H. DeWittCity of Flagstaff Water Department

GENERAL

The highest peak of San Francisco Mountain rises to an altitudeof 12,633 feet, approximately 6,000 feet above the surrounding plateau.The several high peaks are arranged in a horseshoe pattern which isopen to the northeast and forms the periphery of the Inner Basin. Thelocation of the Inner Basin is shown on Figure 1.

One of the earliest sources of municipal water for Flagstaff wasfrom springs located in the Inner Basin which were first used by theCity in 1899. From that date to the present the springs have beendeveloped to supply significant amounts of water in the spring andsummer of most years. Beginning in 1967, the groundwater supplyfrom the Inner Basin was further developed with a drilling programwhich, by 1970, resulted in five producing wells. Figure 2 showsquantities consumed and sources of all municipal water for Flagstafffrom 1949 through 1973.

HYDROGEOLOGIC FEATURES

The Inner Basin is a collapse and erosional feature in San FranciscoMountain,an extinct volcano of Late Cenozoic age, which lies approximatelyeight miles north of Flagstaff. The outcrop patterns of geologic unitsin the Inner Basin are shown in Figure 3.

The oldest rocks exposed in the Inner Basin are a sequence ofandesitic pyroclastics, breccias, and lava flow rocks which are assignedto stage 1 of Robinson's (1913) second general period of eruption in theSan Francisco Volcanic Field. The stage 1 rocks are believed to bemoderately permeable, particularly where they have been fractured bystructural movements. The younger volcanic rocks of stages 2, 3, 4,

and 5 contain less pyroclastics and breccias and are believed to havea lower permeability. Intrusive igneous rocks are well exposed in theCore Ridge, which is believed to be the principal vent area for thevolcano. Other intrusive rocks occur as dikes which crop out in thewalls of the Inner Basin. Intrusive igneous rocks and stage 2, 3, 4,and 5 lava flow rocks are believed to be aquitards and provide perchingconditions for local perched groundwater reservoirs which occur in thewalls of the Inner Basin.

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Unconsolidated rocks in the Inner Basin comprise colluvial depositswhich mantle the inner and outer slopes of San Francisco Mountain, andglacial morainal and alluvial deposits which crop out on the floor andare interbedded in the subsurface of the basin. The unconsolidatedrocks range in size from clay or silt to boulders, and are poorly sorted.The colluvial rocks have a high porosity and permeability and areefficient infiltration media. Morainal deposits are poorly permeableand where they occur above the water table they provide perchingconditions for local perched groundwater reservoirs. Where morainaldeposits occur below the water table they retard water movement andcause artesian conditions where they overlie permeable alluvial rocks.Alluvial deposits are moderately permeable and comprise the principalaquifer which extends to depths greater than 500 feet in the Inner Basin.Figures 4 and 5 are hydrogeologic sections of the Inner Basin and showsubsurface hydrogeologic relations.

Holm (1974) suggests that the Inner Basin and Interior Valley maybe the modern expression of a graben which formed during late stages ofvolcanic activity in the San Francisco Volcano. The locations of faultswhich are inferred to bound the graben are uncertain as those faults arecovered by colluvial, glacial, and alluvial rocks. Stage 1 rocks whichunderlie the alluvial aquifer in the Inner Basin and Interior Valley arebelieved to be intensely fractured in the graben and may be a potentialaquifer for future Inner Basin wells.

INNER BASIN SPRINGS

Between 1899 and 1961 seven large springs and several small springswere developed to produce from perched groundwater reservoirs for theFlagstaff systems. The springs were developed as infiltration galleriesor combinations of infiltration galleries and inlet boxes.

Discharge from Inner Basin springs is large during the March -Julyperiod which is the period during and shortly after the snowmelt season.Discharge is small during the November - February period when the perchedreservoirs are largely depleted. Flow from Inner Basin springs during1973 is given in Figure 6.

INNER BASIN WELL FIELD

Between 1966 and 1971 thirteen cased wells numbered IB -1, IB -2,IB -3, IB -4, IB -5, IB -6, IB -7, IB -8, IB -9, IB -11, IB -13, IB -14, and IB -15were constructed in the Inner Basin. Eight were exploration holes.Five wells (19 -2, IB -6, IB -9, IB -li, IB -14) range in depth from 248 to502 feet and were constructed to produce water for the Flagstaff system.Of these five, one (IB -11) is a flowing artesian well, two (TB-9 and

IB -14) are equipped with pumps and engines, and two (IB -2 and IB -6)

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are not presently (1974) equipped. The locations of Inner Basin wellsare shown on Figure 3.

Inner Basin wells have been used in all summers of the past eightyears when municipal water demand was high and when flow from InnerBasin springs was low. The season in which water can be produced fromInner Basin wells is limited due to difficult access. Roads to theInner Basin are usually closed by snow in October or November and re-main closed until mid May or early June. Discharge from Inner Basinwells during 1973 is shown in Figure 6.

LONG -TERM PUMP TEST

A long -term test of the Inner Basin well field was begun onSeptember 15, 1973 after pre -test water levels and flow from springshad been monitored and recorded. The test began with pumpage of IB -9at a rate of 859 gpm (gallons per minute). After water level responsewas monitored in the other wells, IB -14 was placed into productionat a rate of 551 gpm on September 22. Except for artesian flow fromIB -11 which ranged from 230 gpm early in the test to 20 gpm at the endof the recovery phase, the remaining wells were used solely as obser-vation wells to monitor water level response to the pumpage of IB -9and IB -14. During the pumping phase of the test, measurements weremade of discharge from wells, discharge from springs, water leveldecline, water temperature, and conductance.

The recovery phase for IB -9 began on November 6, 1973 when thediesel engine, power supply for that well, failed. Water level recoverymeasurements for IB -14 began on November 10. Recovery measurementscontinued through November 18, 1973, when roads to the Inner Basin wereclosed by snowfall. A summary of water level drawdown and recoverydata is listed in Table 1.

ANALYSIS

After the pumping and recovery phases of the test were completed,water level drawdown and water production data were analyzed to determinecoefficients of transmissibility and storage of the aquifer, andinterference effects. During both the drawdown and recovery periodswater level data showed the influence of boundaries and of leakage fromthe perched reservoir into the principal aquifer and from the principalaquifer into the underlying volcanic rocks.

Inspection of water level data from IB -4 did not indicate adiscernible response to pumpage from wells. Data from IB -4 were usedsolely to determine rates of leakage from the principal aquifer.

Water level data from IB -11 were inspected for use in computingaquifer parameters and boundary effects. Because the portion of waterlevel decline due to artesian discharge could not be separated from thatdecline due to pumpage from wells, water level data from IB -11 were notused.

191

Tab

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COEFFICIENT OF TRANSMISSIBILITY

Drawdown and recovery data were used to compute transmissibilityusing the methods of Theis (1935)and Jacob (1950). The graphic plotsof drawdown and recovery data indicated a series of line segments,each of which was used to compute transmissibility. The computedvalues ranged from 2,900 to 328,000 gpd /ft (gallons per day per foot).The early drawdown and recovery data indicated transmissibility valueswhich are larger than those indicated by later data. The largestvalues are attributed to the influence of high permeability pathsprovided by interbeds of alluvial deposits between wells. The smallestvalues are attributed to the influence of boundaries.

The most reliable transmissibility values are believed to be thosedetermined using the latest drawdown and recovery data collected priorto the time when the influence of boundaries became significant.

COEFFICIENT OF STORAGE

Coefficient of storage of the principal aquifer in the Inner Basinwas computed from drawdown and recovery data using both the Theis andJacob methods of analysis. Only data from observation wells were usedin computations for coefficient of storage. The computed values rangedfrom 0.2 to 22 percent using data from IB -1, IB -2, IB -6, IB -8, andIB -14.

Early drawdown and recovery data, which indicated large trans-missibility values, usually indicated the smallest coefficient ofstorage values. Late data, which were influenced by boundaries, alsoindicated small coefficient of storage values. Drawdown data which werejudged most reliable for computing transmissibility were also believedto be most reliable for use in computing coefficient of storage.

DISCUSSION AND SUMMARY

The areal distribution of aquifer parameters indicates that averagevalues of coefficient of transmissibility and storage are greatest inthe southwestern part of the aquifer in the vicinity of wells IB -1,IB -2, IB -8, and IB -9 and are less in the central part in the vicinityof IB -6 and IB -14. Average transmissibility and coefficient of storagein the northwestern part of the aquifer are not known; however, no waterlevel decline in IB -4 in response to pumpage of IB -9 and IB -14 indicatesthat the transmissibility of the aquifer near IB -4 is small. Theseareal variations of aquifer parameters are interpreted to indicate thatthe abundance of interbeds of glacial morainal deposits is less in thesoutheastern part and is greater in the central and northwestern partof the aquifer.

193

Average coefficients of transmissibility and storage of theprincipal aquifer are estimated to be 14,000 gpd /ft and 0.08 respectively.

INFLUENCE OF BOUNDARIES

The response of water levels to pumpage from wells in the InnerBasin aquifer is influenced by aquifer boundaries where the alluvialrocks abut less permeable volcanic rocks. Projected locations of the

Inner Basin aquifer boundaries are shown on Figure 7.

In the analysis of drawdown for the principal aquifer, image wells

for IB -9 and IB -14 were located across the southeast, southwest, andnorthwest boundaries. Analysis using the theory of images indicated:

1) The influence of the southeast boundary was an increasedrate of drawdown in the southeastern part of the aquifer and increasedrates of drawdown in wells IB -1, IB -2, IB -8, IB -9, and IB -14.

2) The duration of the Inner Basin aquifer test was not sufficientfor the southwest aquifer boundary to influence rates of drawdown. With

longer periods of pumping this boundary would cause rates of drawdownin the southwestern part of the aquifer to increase.

3) The northwest boundary caused an increased rate of drawdown inthe northwestern margin of the aquifer but did not influence water levels

in wells. With longer periods of pumping the influence of this boundarywould cause increased rates of drawdown in IB -4, IB -6, and IB -14.

OCCURRENCE AND FLOW PATTERN

The average quantity of water which enters the Inner Basin aquifereach year as recharge is equal to the average amount of water whichleaves the Inner Basin as discharge from springs and wells, fromevapotranspiration and sublimation, and from leakage.

INFLOW

The amount of water available for recharge to the Inner Basin aquifersystem is equal to the average annual precipitation in the watershed area.The area of the Inner Basin watershed is 2,420 acres and is shown onFigure 7. Average annual precipitation for the Inner Basin watershedis estimated to be 36.4 inches, based on the 1957 -1973 average; halfof this precipitation occurs as winter snowfall. Potential recharge isapproximately 7,350 acre -feet per year. Recharge is believed to occur

chiefly from infiltration from snowmelt.

194

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CIRCULATION AND STORAGE

Groundwater contours of the static water level conditions in theprincipal aquifer are shown on Figure 7. The general pattern ofgroundwater flow is from recharge areas at the margins of the aquifertoward natural discharge areas below and northeast of the well field.

Groundwater storage in the principal aquifer is restricted becausethe areal extent and the depth of the aquifer are limited,because theaquifer media are poorly sorted, and because the interbedded morainaldeposits yield only meager amounts of water. The average quantity ofrecoverable water in the principal aquifer was estimated using computedspecific yield and estimated aquifer volume to be 8,000 acre -feet.Additional quantities of recoverable water may occur in fracturedstage 1 volcanic rocks below the Inner Basin and Interior Valley.

DISCHARGE

Outflow from the Inner Basin comprises evaporation, transpiration,sublimation from the winter snowpack,discharge from springs and wells,and leakage from the principal aquifer to the underlying volcanics.Discharge from evaporation has not been estimated but is believed tobe large. Outflow via transpiration and sublimation is estimatedfrom analysis of data in an unpublished report by the U. S. Bureau

of Reclamation to be 3,200 acre -feet per year. Average discharge from

springs from 1951 through 1973 was 445 acre -feet per year. Averagedischarge from wells from 1969 through 1973 was 440 acre -feet per year.Leakage from the principal aquifer into the underlying fracturedvolcanic rocks was estimated from average rate of water level declinein wells which are not influenced by pumpage of other Inner Basin wells,and from estimates of aquifer volume and specific yield and is approxi-mately 1,150 acre -feet. Estimated inflow and outflow are summarized

below:

Average Annual Inflow from Precipitation = 7,350 acre -feet

Average Annual Outflow

From Wells = 440 acre -feet

From Springs = 445 acre -feet

From Transpiration and Sublimation = 3,200 acre -feet

Leakage from Principal Aquifer = 1,150 acre -feet

Evaporation = Unknown

Average Annual Outflow (incomplete) = 5,235 acre -feet

196

The difference between the inflow and outflow values given aboveis 2,115 acre -feet and is approximately equal to evaporation.

WATER LEVEL CONTOURS

Figure 8 shows the composite effect of the pumping wells and ofboundaries on water levels in the principal aquifer at the conclusionof the pumping phase of the test. The positions of the drawdown lineswere computed using estimated average aquifer parameters of 14,000gpd /ft and 0.08 for coefficients of transmissibility and storage.

INFLUENCE OF POMPAGE FROM WELLS ON DISCHARGE FROM SPRINGS

Analysis of the aquifer systems in the Inner Basin indicates thatsprings issue from perched reservoirs which occur in the walls of theInner Basin, beyond the margins of the principal aquifer, or issuefrom perched reservoirs which lie above the water table in the principalaquifer; hence, rates of leakage are not increased by a lowering ofwater levels in the principal aquifer. This conclusion is supportedby records of discharge from springs in years prior to development ofwells and in years when Inner Basin wells were pumped. Inspection ofFigure 6 indicates that the decline in rate of discharge from springswas not accelerated through the period in which the Inner Basin wellswere pumped.

CONCLUSIONS

The salient conclusions from the Inner Basin water resourcesstudy are:

1) The average coefficients of transmissibility and of storageof the principal aquifer are approximately 14,000 gpd /ft and 0.08respectively.

2) The influence of aquifer boundaries increased rates of draw -down of water levels in the Inner Basin well field.

3) Inner Basin springs issue from perched reservoirs. Pumpageof Inner Basin wells does not influence discharge from Inner Basinsprings.

4) The quantity of recharge water to the Inner Basin aquifersystem is greater than the average yield from springs and wells. Theaverage quantity of water in storage in the principal aquifer is8,000 acre -feet.

5) A large amount of water is lost from the Inner Basin aquifersystem via leakage into underlying fractured volcanic rocks. A part ofthe water lost via leakage could be intercepted by pumpage from a wellconstructed in the Interior Valley.

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REFERENCES CITED

Holm, R. F., 1974, Volcanic Geology of the Inner Basin, San FranciscoPeaks Volcano, Arizona (abs.), in Abstracts with Programs, RockyMountain Section 27th Annual Meeting, Geol. Soc. Amer., p. 448.

Jacob, C. E., 1950, Flow of Groundwater, in Rouse, Hunter (ed.),Engineering Hydraulics, Chap. 5: New York, John Wiley & Sons,p. 321 -386.

Robinson, H. H., 1913, The San Franciscan Volcanic Field, Arizona:U. S. Geol. Survey Prof, Paper 76, 213 p.

Theis, C. V., 1935, The Relation Between the Lowering of the PiezometricSurface and the Rate and Duration of Discharge of a Well UsingGroundwater Storage: Am. Geophys. Union Trans., p. 519 -524.

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