Geochemistry of Ground Water in Avra Valley, Pima County, Arizona

Geochemistry of ground water in Avra Valley, Pima County, Arizona

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Authors Conner, Leslee Lynn,1957-

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/191892 GEOCHEMISTRY OF GROUND WATER IN AVRA VALLEY, PIMA COUNTY, ARIZONA

Leslee Lynn Conner

A Thesis Submitted to the Faculty of the DEPARTMENT OF HYDROLOGY AND WATER RESOURCES In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE WITH A MAJOR IN HYDROLOGY In the Graduate College THE UNIVERSITY OF ARIZONA

1986 STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained by the author.

SIGNED:

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below:

gCnrn, Wtt E. S. SIMPSO Date Professor of Hydrology and Water Resources ACKNOWLEDGMENTS

I am grateful to my advisor, Dr. Eugene S. Simpson for his guidance and laboratory support through the course of this research. I am also indebted to Dr. Austin Long for isotope analyses, and to Drs.

Long and Stanley N. Davis for their constructive comments as members of my graduate committee.

I gratefully acknowledge the information provided by the City of

Tucson Water Department, Arizona Department of Water Resources, U.S.

Geological Survey and Jim Posedly. In addition, I extend my gratitude to the City of Tucson and the residents of Avra Valley, who readily and generously allowed me to sample their wells.

Heartfelt thanks are due Darcy Campbell, June Fabryka-Martin and

Daniel Matthews, who each so generously offered support and guidance, as only dear friends can.

I am also grateful to Sally Adams and Harding Lawson Associates for their technical support. TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS vi

LIST OF TABLES vii

ABSTRACT viii

1. INTRODUCTION 1

2. AVRA VALLEY 4

Geology 6 Mountains 6 Structure 8 Alluvium 10 Hydrology 12 Climate 12 Surface Water 12 Ground Water 13 Ground-water development 18 Ground-water chemistry 18 Past studies 21

3. METHODS 22

Sampling Program 22 Laboratory Analyses 25

4. RESULTS AND DISCUSSION 27

General Trends in Ground-water Chemistry 27 Ground Water in Southern and Mid Avra Valley 32 Ground Water in Mountain-front Samples 40 Ground Water in Northern Avra Valley 42 Minor Ion Chemistry 43 Potassium 43 Nitrate 44 Fluoride 47 Iodide 50

iv V TABLE OF CONTENTS--Continued

Page

Stable Isotopes 51 Deuterium and oxygen-18 51 Fractionation 52 Units 53 Isotopes in southern Arizona 53 Isotopes in Avra Valley 54 Equilibrium Approach 56 Saturation indices 57 Mineral stability diagrams 60

5. SUMMARY AND CONCLUSIONS 65

Recommendations 66

REFERENCES 68 LIST OF ILLUSTRATIONS

Figure Page

1. Location of Avra Valley, Arizona 2

2. Avra Valley 5

3. Geology of the Avra Valley area 7

4. Depths to bedrock, Avra Valley 9

5. Depths to water, Avra Valley 15

6. Elevation of the water table and approximate flow lines, Avra Valley 16

7. Transmissivities of the ground water aquifer, Avra Valley 19

8. Location of sampled wells, Avra Valley 23

9. Piper trilinear diagram 31

10. Stiff diagrams 33

11. Bicarbonate - TDS scattergram 36

12. Sodium distribution in Avra Valley 39

13. Nitrate - chloride scattergram 46

14. Fluoride and iodide distribution in Avra Valley 48

15. Isotope ratios of samples from Avra Valley 55

16. Mineral stability diagram for the H20-Al203-K2 0-Si02 system 62

17. Mineral stability diagram for the H20-A103-Na20-Si02 system 63

v i LIST OF TABLES

Table Page

1. Estimated volumes of flow into Avra Valley 17

2. Sampled wells in Avra Valley 24

3. Results of chemical analyses 28

4. Recommended limits for drinking water 29

5. Ca:Na ratios in Avra Valley 38

6. Saturation indices 59

v ii ABSTRACT

A geochemical evaluation of the ground water in Avra Valley,

Arizona using major ions and the stable isotopes deuterium and oxygen-18 verifies that ground-water recharge from the surrounding mountains is negligible and that most ground water enters the valley as underflow from Altar Valley. Primarily granitic sediments result in soft waters with elevated pH in southern Avra Valley. Increasing sedimentary mineralization in northern reaches produces slightly harder waters.

Elevated nitrate in the ground water results from intense local agriculture; uneven distribution of nitrate and corresponding ground-water mounding suggest leakage around irrigation well casings as the primary pathway for nitrates to the ground water.

viii CHAPTER 1

INTRODUCTION

This report evaluates ground-water geochemistry in Avra Valley, west of Tucson, Arizona (Figure 1). The objectives are to characterize the chemistry of the ground water and to understand the processes which determine the chemistry. In particular, the influence of mountain-front recharge, sediment lithology, and agricultural activity on the chemistry of Avra Valley ground water are examined. In addition, the extent to which ground-water chemistry supports or changes the present understanding of ground-water movement in Avra Valley is reviewed.

For the study, ground-water samples were collected from Avra

Valley and analyzed for major ions, selected minor ions, and stable isotope ratios of hydrogen and oxygen. The chemical results were evaluated with reference to principles of water chemistry and geochemical processes.

The value of geochemical investigations of ground water lies in the contributions of such studies to our knowledge of the occurrence and movement of the water in the subsurface environment (see Garrels, 1967;

GarrelS and MacKenzie, 1967; Helgeson, 1968). Similar research in southern Arizona has included the Cortaro (Martin, 1980) and Oracle

(Winstanley, 1984) areas, San Pedro Valley (Usunoff, 1984; DeWald,

1984), the Santa Rita Mountains (Merz, 1984) and the Tucson Basin

(Bostick, 1978; Gallaher, 1979; Thorne, 1982; Olson, 1982; Koglin, 1984;

1 2

0 50 100 MILES

Figure 1. Location of Avra Valley, Arizona. 3 and Mohrbacher, 1984). The approach in these studies was similar to that in the present evaluation: characterization of the major ion chemistry of ground water and identification of geochemical trends, often supplemented by use of stable isotopes. Knowledge of the local geology in such studies is helpful, because the composition of the aquifer materials through which the ground water passes plays a major role in the chemistry of the ground water. CHAPTER 2

AVRA VALLEY

Avra Valley is a north-trending alluvial basin in south-central

Arizona, 12 miles west of the city of Tucson. Covering roughly 520

square miles, the valley extends 40 miles from south of Three Points,

Arizona to the Pima-Pinal County line north of Marana (Figure 2). East to west, Avra Valley ranges from 7 to 20 miles (White, Matlock and

Schwalen, 1966).

The Tucson Mountains form the eastern boundary of Avra Valley, separating it from the Tucson Basin. The Silverbell, Waterman, and

Roskruge Mountains bound the valley along the west, and the Sierrita

Mountains lie to the south. These mountains are of moderate height,

rising up to 2000 feet above the valley floor.

The sediments which fill Avra Valley derive from mountains bounding the valley on the east, west and south. The floor of Avra

Valley slopes gently to the north with an average gradient of 26 feet per mile, from an elevation of 2550 feet near Three Points to 1950 feet outside Marana.

The study area is defined as that portion of Avra Valley lying between the southern line of Township 11 South and State Highway 86

(Figure 2). This area is 23 miles in length and covers approximately

345 square miles.

4 5 R 9 E / \fl io E R 11 E R 12 E I r z -"*/-

f-J-r .-"s•• Marana ,,,4 . I-10

1,12. 091.,;6•>._

o te-)7'S ,r 1r

TWIN PEAKS o.

TERM\ °MOUNTAINS ..-/ / \\ 1... • n n"'"-'".:-.:-. . -7-- 7 '‘v\. ( • '• / ' ,, ...-K... / • z. – ;:- f //.„1 k t 13 . ,../. • f 1'..—...( /(•• S. 404, r A-- 4t ( / ( r ( \ 4.1e IT%.4 ) ... +4, \\

., 4-, ff 1..---. I 1•.- qt • Jf(7,9 .). : : •••Y

• ./ /

"N stei>. co .."...... 1.7 ,6tae-e..

Three Points

4Z. V- 4 I (c, - \.n • ,/ t• • 4 ERR I TA 0 MILES 5 MC/017AM S SCALE: 1:250,000

Figure 2. Avra Valley. 6

Geology

Avra Valley lies in the Basin and Range Lowlands physiographic province which extends across southern Arizona. This province is characterized by predominantly northwest-trending alluvial basins flanked and separated from each other by steeply rising mountains of volcanic origin. The topography of the area is believed to be the result of an extensive period of block-faulting which occurred in

Tertiary time, during which down-thrust blocks created grabens roughly parallel to adjacent uplifted mountains. Subsequently, these grabens filled with sediments eroded from the mountain slopes, creating the deep alluvial basins present today (Eberly and Stanley, 1978).

Mountains

The bordering mountain ranges influence the ground water system in Avra Valley in several ways. As noted by White and others (1966), the relatively impermeable rocks of the mountains serve as barriers to ground-water flow and thereby determine the lateral limits of the aquifer. In addition, the zone of contact between the mountains and valley sediments acts as an area for recharge of storm runoff. Also, and of particular significance for this study, the mountains are the source of the sediments through which ground water in the valley flows.

Consequently, the chemistry of the ground water can be expected to be influenced by variations in the composition of these source rocks.

On the west side of the Tucson Mountains, Cretaceous sedimentary rocks predominate (Figure 3). The bulk of these rocks are arkosic sandstones and conglomerates (Whallon, 1983), although Cretaceous- LEGEND

COt Carboniferous and Devonian limestone, shale, sandstone, and quartzite

Et Cambrian quartzite and limestone

Ka Cretaceous andesite

Ks Cretaceous shale, sandstone, conglomerate, and limestone

Kvs Cretaceous volcanic and sedimentary, undifferentiated

Lgr Laramide granite

Li Laramide dikes and plugs, granitic to dioritic in composition

Qb Quaternary basalt

Qs Quaternary sediments

Qts Quaternary sand, gravel, and conglomerate

Ta Tertiary andesite

Tb Tertiary basalt

Tr Tertiary rhyolite 7

R12E,/ .//

LEGEND - SEE FACING/ PRECEEDING PAGE

MILES 5 SCALE: 1:250,000 Figure 3. Geology of the Avra Valley area after Wilson and others, 1969). 8

Tertiary volcanics and granitic intrusives are exposed on the northern

slopes and high on the southern and central slopes. At the northwestern

edge of the Tucson Mountains is an outlier, Twin Peaks, composed of

Paleozoic limestones, shales, sandstone and quartz (Wilson, Moore and

Cooper, 1969).

To the west, the portion of the Silverbell Mountains within the

study area consists primarily of Tertiary volcanic flows and granitic

intrusives similar to those in the Tucson Mountains. In the Waterman

Mountains, just south of the Silverbell Mountains, Paleozoic limestones,

Cretaceous sedimentary rocks and Tertiary volcanics are found. In the

Roskruge Mountains, Cretaceous volcanic and sedementary rocks

predominate at the northern end and along the lower slopes of the

eastern flank. Higher up are Cretaceous-Tertiary volcanics and granitic

intrusives, and at the southern end, Tertiary volcanics. The northern

end of the Sierrita Mountains, which lie south of Avra Valley, consists

of Precambrian, Paleozoic, and Cretaceous granites (Wilson and others,

1969).

Structure

In their study of alluvial basins in southern Arizona, Eberly

and Stanley (1978) noted that at depth, the grabens produced by the

late-Miocene block faulting are quite steep and narrow. The deep central parts of the troughs are not often interconnected, but rather are separated by buried horst blocks. Depositional continuity was found to occur only at shallow depths, above the horst blocks. All of these features are apparent in Avra Valley. First, as Figure 4 shows, 9

10E R 11 E

1 \ 60\--. r sto.-8, soot) -10 cp s .../

W PEAK 0 •

hmePoints

Depths Below Land Surface (ft)

0 MILES 5 SCALE: 1 :250,000

Figure 4. Depths to bedrock, Avra Valley (after Whallon, 1983). 10

northern Avra Valley is indeed a deep, narrow graben. A saddle (West,

1970) extending across the center of the valley from east to west can

also be seen in Figure 4. The saddle likely is a buried horst block.

And as described in the section on Alluvium, the continuity of the

shallow-most sediments closely follows that noted by Eberly and Stanley.

Alluvium

The depth of the sediments in Avra Valley varies from just a few

feet along the pediments of the mountains to more than 9000 feet in the

center of the basin. Lenses of highly permeable gravels, silts and

sands alternate with discontinuous lenses of fine-grained, nearly

impermeable sediments.

The sediments in Avra Valley are fairly similar from north to

south to a depth of 300 feet. Below this depth, the northern sediments

are distinctly different from those in the south (Whallon, 1983). For example, sediments below 300 feet in the north have scattered gypsum

nodules and a higher percentage of fines than sediments in the south.

As explained below, these differences are believed to be the result of the differing depositional histories of the northern and southern parts of the valley.

Lacustrine fossils and thick sequences of evaporites found in deep sediments throughout southern Arizona support the theory that for a significant period following the late-Miocene block faulting, external drainage from the newly formed basins did not exist (Eberly and Stanley,

1978). The presence of gypsum nodules at depth in the northern part of

Avra Valley is consistent with this theory and suggests an environment 11 dominated by evaporation (Whallon, 1983). The difference in the percentage of fines at depth in the northern and southern parts of Avra

Valley suggest also that discontinuous drainages existed within the valley. In contrast to the playa-like environment of the northern part of the valley, the southern drainage appears to have more closely resembled a fluvial system, in which coarser sediments were deposited near the mountain-valley contact and finer particles were carried into the central valley (Whallon, 1983). The upper 300 to 400 feet of alluvium, although locally heterogeneous, is fairly uniform from north to south and is believed to have been deposited after flow-through drainage was established in the valley. The upper alluvium in Avra

Valley appears to agree with the depositional continuity described by

Eberly and Stanley (1978) as typical of block-fault alluvial basins.

Local heterogeneities noted in the upper alluvium in Avra Valley are predominantly due to interfingering of coarse sediments of varying thicknesses with finer sediments (White and others, 1966). The fines include clays, siltstone and mudstone, whereas the coarser materials are composed of igneous (granite, tuffs, porphyrys, andesites, and basalts), metamorphic (gneiss, mica and schist), and sedimentary (limestone, sandstone, siltstone) gravels. Caliche, which is common in arid environments, is also found in the sediments. As expected, the sediment mineralogy reflects the diverse geologic makeup of the surrounding mountains. 12

Hydrology

Climate

The arid climate of Avra Valley is like that of much of the southwestern United States: hot, dry summers and moderate winters.

Precipitation averages 9 1/2 inches per year in the valley proper, although as much as 17 inches can fall annually in the surrounding mountains (White and others, 1966). Summer storms, which develop locally, account for forty percent of the annual precipitation. They are quite intense but short-lived. In contrast winter storms, which are the result of cold fronts from the Pacific, are significantly less intense and of longer duration. Consequently, winter precipitation in southern Arizona and hence Avra Valley is more likely to contribute to ground-water recharge than summer rainfall (Keith and Rasmussen, 1980).

Evaporation rates in southern Arizona greatly exceed precipitation. Values for annual evaporation in Avra Valley are not available; however, they are likely to be close to the 100 inches per year observed at the University of Arizona, 12 miles east of the study area (Clifton,

1981).

Surface Water

Streamflow in Avra Valley is intermittent, limited to runoff following summer and winter storms. Runoff from the mountains usually recharges along the pediments or evaporates before reaching the valley proper. As a result, the numerous stream channels which originate in the mountains surrounding Avra Valley become less distinct as the channels approach the valley floor and many disappear completely. The 13 major drainage in the valley is Brawley Wash. Brawley Wash originates in Altar Valley and enters Avra Valley just west of Three Points. The wash extends the length of the valley, joining Los Robles Wash just before it joins the Santa Cruz River north of Marana. The Santa Cruz

River flows northwesterly across the northern reaches of Avra Valley outside the study area. Flow has been perennial in the Santa Cruz River since the introduction of sewage effluent from Tucson in 1969 (Matlock and Morin, 1976).

Ground Water

As noted earlier, the alluvium that fills Avra Valley extends to an average depth of 3000 feet, with depths greater than 9000 feet in the northern part of the valley. The result is a basin-wide, nearly continuous unconfined aquifer with substantial storage capacity (White and others, 1966). Isolated areas of artesian pressure have been reported (Andrews, 1937; White and others, 1966) in wells drilled in the valley. Localized occurrences of confining pressures could result from the irregular distribution of clay lenses in the aquifer. Recent evidence of localized high water levels in northern Avra Valley has also been reported (Babcock, Cameron, and Brumbaugh, 1985). This could be the result of zones of perched water developing due to a lowering of the overall water table around clay lenses (Robertson, 1984). Alterna- tively, perhaps ground-water mounds are forming in response to high volumes of recharge from irrigated acreage (Babcock and others, 1985).

Generally, depths to ground water in Avra Valley are most shallow in the center of the valley (Figure 5). Within the study area, 14

Figure 5. Depths to water, Avra Valley (after Babcock and others, 1985). 15 the depth to water ranges from less than 200 feet near Three Points to over 400 feet along the Tucson Mountains.

Water-table elevations in Avra Valley in 1984 are shown on

Figure 6. Flow lines, normal to the lines of equal ground water elevation, were constructed. These flow lines indicate that ground water enters Avra Valley from the south, flowing to the northeast past

Three Points. The groundwater then begins a mainly north-northwesterly course through the remainder of the valley, bounded by and roughly parallel to the surrounding mountains.

The primary source of ground water in Avra Valley is underflow from Altar Valley, and to a lesser extent recharge along the mountain fronts and Brawley Wash (White and others, 1966; Whallon, 1983).

Several investigators have estimated volumes of the various flow components; these are summarized in Table 1. Recharge along the length of Brawley Wash is believed to contribute very little to the aquifer

(less than 10 percent), a result which is not surprising given the sporadic nature of flows in the wash and the local imbalance between precipitation and evaporation. Recharge along the mountain fronts is estimated to be roughly a third of the volume which enters the valley as underflow from the south.

Hydraulic properties of the aquifer in Avra Valley were calculated by several investigators. Most recently, Whallon (1983) estimated transmissivity in the top 1000 feet of sediments: values range from 1000 ft 2/day to 44,000 ft 2/day, with the zones of highest and lowest transmissivities corresponding to the areas of predominantly 16

Figure 6. Elevation of the water table and approximate flow lines, Avra Valley (after Babcock and others, 1985). 17

Table 1. Estimated volumes of flow into Avra Valley.

Volume Source (acre-feet/year) Reference

Ground-water 9,000 Moosburner, 1972 underflow from 14,700 Turner, 1959

Altar Valley { 16,600 Whallon, 1983

Brawley Wash 1,100 Osterkamp, 1973

Mountain-front recharge from:

Tucson Mountains 1,000 Osterkamp, 1973

Sierrita Mountains 1,200 Osterkamp, 1973

Roskruge, Waterman and Silverbell Mountains 3,300 Osterkamp, 1973

TOTAL 15,600 - 23,200 18 coarse and fine sediments, respectively (Figure 7). Whallon also estimated specific yield of the sediments to be 0.14.

Ground-water Development. Use of ground water in Avra Valley has been dominated by agriculture for almost 50 years. The first irrigation wells in the valley were drilled in 1937 near Marana, and by

1954, more than 100 wells were in service, providing 30,000 acres of farmland with 90,000 acre-feet of water annually (White and others,

1966). Concern over rapidly declining water levels from this pumping led to a public hearing on the issue in 1954. As a result, most of Avra

Valley was declared a critical ground-water area and therefore closed to new wells. However, restrictions on the installation of new wells were apparently not accompanied by restrictions on the volume of water pumped from existing wells. By the early 1960's, annual pumping had increased to 150,000 acre-feet (Clifton, 1981). Data from wells throughout Avra

Valley verify the dramatic decline in water levels since 1940. By 1979, declines of up to 140 feet had occurred in the valley (Whallon, 1983).

Recent stabilization of the water table is attributed to the gradual retirement of more than 14,000 acres of farmland since the early 1970's by the City of Tucson. Tucson bought the land for the associated water rights in order to supplement the city's water supplies. Annual withdrawal by the city varies roughly between 6000 and 9000 acre-feet

(City of Tucson, 1985), substantially less than the volume previously used to irrigate the retired land.

Ground-water Chemistry. The quality of the ground water in Avra

Valley is generally good and, with few exceptions, the water is suit- able for domestic and agricultural use. High levels of fluoride 19

2 <2600 ft /day F1- 8000- 13,000

2600 -8600 El >13,000 ft?/da y

N 6 MILES 5 SCALE: 1:250,000 I Figure 7. Transmissivities of the ground-water aquifer, Avra Valley (after Whallon, 1983). 20

(concentrations greater than 1.4 mg/1), which are common throughout

southern Arizona, were noted by Osterkamp and Laney (1974) northwest of

Three Points and in a small area in northeastern Avra Valley. Fluoride

in excess of approximately 0.7 mg/1 in local drinking water poses a

potential health problem (Osterkamp and Laney, 1974).

Irrigated agriculture often results in an accumulation of salts

in the soil. Building up from repeated applications of irrigation water

and chemical fertilizers, the salts are left behind after water is used

by plants or evaporated from the soil. During extended periods of

irrigation such as those resulting from multiple planting seasons in

Avra Valley, water infiltrating soils mobilizes the accumulated salts

and can in time carry them into the ground water. In Avra Valley,

ground-water recharge as infiltration from rainfall penetrating the

valley floor is unlikely, primarily because evapotransporation so

outweighs precipitation. But in northern Avra Valley, year-round

agriculture has resulted in a fairly constant application of irrigation

water to the soil, greatly increasing infiltration and the likelihood of

ground-water recharge from irrigation return flow.

Improperly operating septic systems can contribute nitrates to

ground water, and many residences in Avra Valley operate septic systems.

But compared to the volume of water applied to agricultural land in Avra

Valley, the contribution of nitrates from septic systems is believed to

be negligible. In contrast, the volume of treated sewage effluent

released into the Santa Cruz River by Pima County is enough to have caused elevated levels of nitrate in ground water in far northeastern

Avra Valley. 21

Past Studies. Several hydrologic studies are available on Avra

Valley's ground water. In none of the studies, however, was the geochemistry of the ground water in the valley evaluated.

One of the first ground-water studies in Avra Valley was published in 1937 (Andrews). In that report, Andrews catalogued the results of a field reconnaisance, presenting available well logs, depths to water and limited water chemistry. Several subsequent publications included Avra Valley as part of regional ground-water surveys (Cushman,

1952; Heindle and White, 1965), but not until 1966 was a systematic assessment of water resources in Avra Valley completed (White, and others, 1966). In their report, White and others focused on the history of ground-water use in the valley and its resultant effect on the water table. Subsequent hydrologic studies of Avra Valley included water budgets (Brown, 1976), calculation of recharge along surface drainages and bajadas (Osterkamp, 1973), and the feasibility of artificial recharge of the aquifer (Wicke, 1978). Matlock and Morin (1976) presented an update on the effect of continued pumping on the water table, assembled an extensive compilation of historic water quality analyses and presented results of well-cuttings analyses from a number of wells throughout Avra Valley. Ground-water modeling studies include an electric analog (Moosburner, 1972) and a numerical model (Clifton,

1981). Most recently, Whallon (1983) evaluated the hydraulic properties of the aquifer in Avra Valley. However, as noted previously, a geochemical evaluation of the ground water is lacking. CHAPTER 3

METHODS

Sampling Program

Thirty ground-water samples were collected by the author from

Avra Valley wells in March and April, 1985. Six samples were from wells owned and operated by the City of Tucson; the balance were from private wells. Of the latter, seven were from actively pumping irrigation wells, four were from well taps of actively pumping domestic wells, and thirteen were from domestic pressure tanks. Figure 8 shows the loca- tions of the sampled wells. Information on elevations, depths and perforated intervals are tabulated in Table 2. Wells to be sampled were selected so as to provide as complete coverage of Avra Valley as possible, with emphasis on wells which might indicate the presence or absence of recharge along the mountain fronts. However, within this framework, accessibility of individual wells became the primary factor in the final selection of sampling sites.

Field procedures included collection and preservation of each sample, plus measurement of temperature, pH and alkalinity. Alkalinity was measured by acid titration, to a pH less than 3.00.

In the laboratory, each sample was analyzed for major cations and anions, as well as potassium, nitrate, iodide and fluoride. Seven samples from actively pumping wells were analyzed for the stable isotope ratios 2H/ 1 H and 180/160.

22 23

TWIN PEAKS

13 A

Am1-5 -...... --.....;74 / AV-1 A AV-2 $1'.44 A AV-3

Three Points

P. 1 1

/ 10. (t.) MILES SCALE. 1:250,000

Figure 8. Location of sampled wells, Avra Valley.

24 Table 2. Sampled wells in Avra Valley

Total Top of Bottom Well Elevation Depth Screen of Screen Sample Number Location (feet) (feet) (feet) (feet) Type

01 14-12-27-a 3084 210 D,p 02 13-11-12-adb 2500 450 D,pt 03 14-12-27-ac 3084 250 D,pt

04 13-11-29-ddd 2240 509 400 503 D,p 06 13-11-29-dda 2240 530 ------D,pt 07 13-11-29-dcc 2210 550 530 550 D,pt

08 13-11-33-daa 2340 520 D,pt 10 15-10-29-acd 2526 305 190 300 D,pt 11 14-11-26-ddd 2342 480 D,pt

12 13-09-08-ccc 2450 25 D,p 13 12-11-34-bda 2178 540 420 530 D,pt 14 13-10-09-ddd 2098 800 I,p

15 13-10-25-bcd 2174 1090 D,pt 16 12-11-19-aa 2015 500 I,p 17 12-11-07-cdd 199 606 260 600 I,p

18 12-10-04-a 1957 400 D,pt 19 12-10-04-bcc ------D , pt 20 15-10-33-bbc 2523 617 I,p

21 15-10-33-bcc 2525 710 I,p 22 15-10-29-dad 2519 300 D,pt 24 13-11-24-ccc 2327 560 400 560 D,pt 28 12-10-30-abb 2050 670 400 670 D,p 29 12-10-23-ac 2020 550 I,p 30 12-10-23-dc 2025 1050 I,p

AV-01 15-11-15-ccc 2440 1005 400 1005 C,p AV-02 15-11-15-ddd 2455 900 370 900 C,p AV-03 15-11-22-ddd 2515 997 C,p

AV-05 15-11-15-bba 2390 861 348 609 C,p AV-14 14-11-28-dcc 2310 495 285 490 C,p AV-19 14-11-27-bcc 2300 1123 411 890 C,p

D,p = Domestic well, pumping I,p = Irrigation well, pumping D,pt = Domestic well, pressure tank C,p = City well, pumping 25 Laboratory Analyses

All of the analyses listed below except isotope measurements were performed by the author. Cations were analyzed on a Varian Model

475 atomic absorption spectrophotometer with detection limits of 0.05 mg/l. Chloride, sulfate and nitrate were analyzed with a Dionex 10 ion chromatograph. Detection limits of the ion chromatograph were 0.5 mg/1 chloride, 0.1 mg/1 sulfate and 1.0 mg/1 nitrate. Fluoride and silica concentrations were measured using colorimetric methods provided by the

Hach Chemical Company; detection limits were 0.5 and 1.0 mg/1, respectively. Concentrations of iodide were determined colorimetrically, by the ceric-arsenious oxidation procedure outlined by Skougstad and others

(1979) and modified by Fabryka-Martin (1985). Total dissolved solids

(TDS) were calculated either through the use of the computer code

WATEQF, which is explained in Chapter 4, or with the epm:TDS relationship described by Davis and DeWiest (1966). Stable isotope ratios were analyzed in the Laboratory of Isotope Geochemistry in the

Dept. of Geosciences, University of Arizona.

Charge balances were calculated for each sample to check the accuracy of the analytic results. Charge imbalances most frequently arise in two ways: poor laboratory techniques may yield erroneous results for individual species or a chemical analysis may be incomplete, overlooking a significant cation or anion. Charge balance is evaluated by summing the equivalents per million (epm) of cations and anions. If the two sums are not equal, error is calculated according to

1 cations - 1 anions 1 % Error = x 200. r 1 cations + 1 anions 26

By this formula, errors greater than 10% indicate inaccurate or incomplete analyses. If the source of the imbalance is not found the results should be discarded.

The error in the charge balances for samples 1, 3, 10, 17, 24, and 30 exceeded 10 percent. Four of the samples (10, 17, 24, and 30) were dropped from further consideration, because the source(s) of the error could not be identified. The error in the analyses for samples 1 and 3 is believed to be from incomplete analyses, as suggested by the high TDS concentrations. These samples were included in the geochemical evaluation which follows. CHAPTER 4

RESULTS AND DISCUSSION

Measured concentrations of major and minor ions in the ground- water samples from Avra Valley are presented in Table 3 along with field pH and temperature. Several anomalous values are readily apparent. In the samples from wells near the mountains, concentrations of calcium

(samples 2, and 12), bicarbonate (samples 1, 3, and 12), sulfate (sample

2), and silica, as Si02, (sample 12) are higher than the average. Also, samples 20 and 21 from southwestern Avra Valley have high pH values and a near absence of calcium and magnesium.

General Trends in Ground-water Chemistry

Several of the samples have measured concentrations of TDS, sulfate, nitrate and fluoride above recommended water quality standards for human consumption (Table 4). Samples 2, 3, 12, 19, 28, and 29 exceed the recommended limit for TDS; sample 2 also exceed the sulfate limit. Nitrate is high in samples 12 and 29, and elevated fluoride was measured in samples 2 and 21.

The TDS concentrations in Avra Valley increase toward the edges of the valley, with an extreme of 2010 mg/1 in sample 2. An increase in

TDS concentration from south to north corresponding to the general direction of ground-water flow is also evident in Avra Valley.

Increases in TDS along flow paths are often observed in ground water

(Freeze and Cherry, 1979); observed elevated TDS values in northern Avra

27 28

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Table 4. Recommended limits for drinking water.

Recommended Chemical Limit Constituent (PPm)

TDS 500

Cl - 250

F - 1.4

SO42 250

NO3 45

Source: Freeze and Cherry, 1979 and U.S. Public Health Service, 1962. 30 Valley may also result from agricultural return flow reaching the water

table or from the increased percentage of fines in the north. The

increase in TDS in Avra Valley generally corresponds to increases, from

south to north, of calcium, chloride, sulfate and nitrate.

Figure 9 shows the distribution of the sample chemistries on a

trilinear diagram. Trilinear diagrams are useful because they display

data from a large number of water samples and group the waters on the

basis of major ions. Two distinct types of water are seen in the

diagram: Na-HCO3 waters designated Group A, and the Ca-HCO3(-SO4)

waters of Group B. Although removed from Group A, samples 20 and 21 are

also classified according to the trilinear diagram as Na-HCO3 waters.

The exceptionally high sulfate noted earlier in sample 2 is apparent on

the trilinear diagram; sample 2 is a Ca-SO4 water.

The division of the samples into groups on the trilinear diagram

roughly corresponds to a spatial differentiation between waters in

southern and mid Avra Valley (Group A) and northern Avra Valley (Group

B). Exceptions to this are samples 3 and 8, which are southern and mid

valley samples in Group B, and sample 14 from northern Avra Valley in

Group A. The lower triangles of the trilinear diagram suggest that the

differentiation between Group A and Group B waters is more a function of

the anions than the cations. This may be helpful later in distinguish-

ing dominating geochemical processes.

One drawback of the trilinear diagram is the loss of information

involved in reducing a full chemical analysis to a single point. Also, the trilinear plots percentages of cations and anions, not true

concentrations, and ignores the presence of many ions.

, +i9 113, X4.29 GROUP B - ,/ 1 \1571,4184D 8 \ A 1 El sV j \ 12_i . , AV-14 16 iig 13 /

\e., GROUPA 1.--- /\ 7..... 461Z/Aly-16.\\ k 147 A 4\ * \''< ril \ \ ' \ \...-BOk \ AV-4 A ) 20A \A / /AV-6 co A- \ \ V AV-3,AV-1,11 0 6 0 /

\ . 21A A\ '/\ 110 / •t* 1,4 \

Figure 9. Piper trilinear diagram. 32 To overcome these limitations, modified Stiff diagrams (Figure 10) were

drawn. Stiff diagrams illustrate true ion concentrations and display

characteristic shapes for chemically different waters. The area of each

diagram is proportional to TDS and, as modified, the relative lengths of

bars below each pattern correspond to silica and nitrate concentrations.

Comparison of the trilinear and Stiff diagrams allows classifi-

cation of the Avra Valley ground-water samples into three basic cate-

gories: (1) southern and mid valley water, (2) mountain-front and (3)

northern water. Further geochemical analyses are conducted within each

of these groups.

Ground Water in Southern and Mid Avra Valley

Weathering of alkali feldspars appears to exert a strong

influence on ground-water chemistry in southern Avra Valley. This

conclusion is supported by observed high silica:TDS ratios, relatively

high pH values, bicarbonate:TDS relationships and low Ca:Na ratios.

Ground-water samples from southern Avra Valley contain relatively high bicarbonate concentrations. Elevated bicarbonate in ground water is often associated with recently recharged water. Water

infiltrating the sub-surface readily dissolves carbon dioxide gas (CO2)

in soil zones, where the partial pressure of CO2 is much greater than that in the atmosphere. This process is represented by:

CO2(g) + H20 = H2CO3 where CO2 combines with water to form carbonic acid. The acid then dissociates to hydrogen and bicarbonate ions: 33

CATIONS (epm) ANIONS (eprri) 12 10 8 6 4 2 0 2 4 6 8 0 12 I 1 1 1 1 1 I 1 1 1 1 1 1

SOUTHERN AVRA VALLEY

Na+ + K+ Cr Ca+2 20 HCO; Mg+2 SCV

Figure 10. Stiff diagrams. 34 H2CO3 = HCOs + H +

The resultant higher acidity of recharging water increases dissolution and weathering of minerals in the sub-surface.

When CO2-charged water encounters silicic minerals such as feldspars, incongruent dissolution takes place, during which cations and silica are leached from the igneous mineral and an alumino-silicate residue (clay) is left behind. Concurrently, both the pH and the bicarbonate concentration of the ground water increase. An example of this is the incongruent dissolution of the alkali feldspar albite to kaolinite (Freeze and Cherry, 1979):

2NaAlSi308 + 11H20 = Al2Si205(OH)4 + 2Na+ + 2(OH - ) + 4H4SiO4 (albite) (kaolinite) and:

2(OH - ) + 2CO2 = 2HCOs.

If, instead of a silicate mineral, the CO2-charged water encounters a carbonate such as calcite, a similar increase in bicarbonate concentration and pH occurs, without the release of silica:

CaCO3 + H+ = Ca+ 2 + HCOs.

Because both processes raise pH values and increase bicarbonate concentrations, the distinction between carbonate or feldspar weathering as the dominant influence on the ground-water chemistry is not apparent from only the pH and bicarbonate values. Evaluating the relative influences of the carbonates and feldspars requires examination of relationships such as silica:TDS, HCO3:TDS and Ca:Na. 35

The Stiff diagrams for southern Avra Valley ground-water samples

reflect high silica concentrations relative to total dissolved solids.

Because of their high silica content, dissolution of igneous rocks and

in particular, of feldspars is suggested. With such high silica:TDS

ratios, alkali feldspars (rich in sodium or potassium) are reasonably

the dominant weathering igneous minerals, since alkali feldspars release

a greater proportion of silica per cation than do the calcic (calcium)

or ferromagnesian (iron or magnesium) silicates (Hem, 1959). And

because, within alkali feldspars, the sodium feldspars such as albite

are considerably more soluble than potassium feldspars, incongruent

dissolution of albite shown earlier is believed to be a fair

representation of weathering feldspars in southern Avra Valley.

The relative contribution of calcite dissolution to the

feldspathic-dominated ground-water chemistry in southern Avra Valley can

be evaluated by noting that actively dissolving calcite results in a

positive correlation between bicarbonate and TDS concentrations. In

addition, Ca:Na ratios in such an environment would generally be greater than 1.

A scattergram of bicarbonate and TDS (Figure 11) reveals no

strong linear correlation for the samples in southern Avra Valley. The southern Avra Valley samples do, however, congregate together and mountain-front samples 3 and 12 show a moderate correlation (r 2 = 1.00) with sample 29. Figure 11 provides several insights into the geochemistry of the ground water in southern Avra Valley. First, with the southern ground water samples so similar in bicabonate and IDS, it is probable that the ground water in southern Avra Valley is not subjected 36

4.0 - +2 9 AV-3 + AV-5 11\ / 14 16 AV-1 \ 6 0/-.7 3.0 - \ •6 ; 14 + AV-2 AV 28 21; .AV-19 +13 22 + 15 18 •8 iv 2.0 - •20

• Southern and Mid Valley Wells

• Northern Wells + Mountain Front Wells

Figure 11. Bicarbonate - TDS scattergram. 37 to substantial variations in lithology, nor is it affected by significant additional sources of water, one of which would be recharge.

Another clue to geochemical processes found in Figure 11 is in the lack of linear correlation between all but three mountain-front and one of the northern samples. This confirms that calcite dissolution is not as dominant a geochemical process in the ground water in Avra Valley as dissolution of igneous mineralization. Interestingly, the aberrant correlation for the mountain-front samples is reasonable if those wells are intercepting recently recharged water. The correlation with sample

19 is, however, puzzling, until the minor ions are reviewed in a later section.

Low Ca:Na ratios in ground water can result from base exchange

(in dilute waters such as the ground water in Avra Valley, calcium is the primary exchangeable cation, replacing sodium on mineral surfaces).

According to Dreyer (1982), base exchange is likely to occur in sediments. The change in the Ca:Na ratio noted in southern Avra Valley

(Table 5) is not, however, the result of base exchange, since relatively uniform sodium concentrations are observed in the ground water (Figure

12).

In summary, evidence provided by ground-water chemistry in southern Avra Valley suggests that weathering of feldspathic minerals is the predominant geochemical process in southern Avra Valley.

To this point, assumptions, based on water-table gradients, have been made regarding the flow of ground-water in the valley. These are that ground water flows northerly through the valley, after entering the southern portion as underflow from Altar Valley and that the

Table 5. Ca:Na ratios in Avra Valley.

Ca/Na Sample (e pm)

21Southern: 0.1 20 0.1 22 0.6 AV-19 0.6 AV-1 0.7 AV-3 0.7 AV-5 0.7 AV-2 0.8 11 0.8 AV-14 1.1

Mid-valley: 8 0.7 4 0.7 6 0.8 7 0.8

Mountain-front: 1 0.2 3 0.5 12 1.0 2 3.5

Northern: 13 0.4 28 0.8 14 0.9 16 0.9 18 1.0 15 1.2 19 1.4 29 2.0

./ # r.-- •

-P. . -9:• :.P-. -..7

(2 , 17, n 1 ? (2.2) r .7)

• T IW N PEAK S .) ( 0 f . .f A a TERM \ - T. 11.1 ) (3.9) o mOuNTAirsis : \ ki ''.-1 ---' .' \\ it... (1.8) 10 ••. ,;...n•.77.":" n, .. . : l -7.- 7 •••:\ ( _ 10 \;\. 1 j .( ...1 k. l 7 \\ f •-•-• I /...' 1.1\ (2.0) n , .. ' (2.0) g= (3.3) ./ f . 7 (1.811.10o_ /.. I 1.1.-) ( •.... ( 1.' : i r ,A7 r t \ ,-' '

14 ;71 .• •

) ! (1.8)

Figure 12. Sodium distribution in Avra Valley. 40

contribution from mountain-front recharge is relatively minor. The

observed ground-water chemistry near Three Points and throughout

southern Avra Valley is consistent with these assumptions. Well logs

from southern Avra Valley confirm that the local sediments are

predominantly granitic and volcanic in origin, with increased basalts

towards the Roskruge Mountains and increased occurrences of sedimentary

sediments towards the Tucson Mountains. Significant recharge from these mountains may be reflected in the chemistries of samples 1, 3 and 12,

but is not seen in the valley proper. The slight increase of the Ca:Na

ratios in the ground water along the presumed flow path corresponds to the observed increase in sedimentary mineralization from southern to

northern Avra Valley.

Ground Water in Mountain-front Samples

The mountain-front samples from Avra Valley include samples from wells 1, 2, 3, and 12. These samples were obtained to examine the

possibility that ground water recharges along the mountain pediments.

The high TDS concentrations of the mountain-front samples

indicate that recharge is occurring. Samples 1, 3 and 12 have the highest bicarbonate concentrations in the valley, although, of these, only sample 12 has a high bicarbonate concentration relative to total ions in solution (Figure 9).

Sulfate concentrations in samples 1, 3 and 12 are significantly higher than those in southern Avra Valley, although in samples 1 and 3, chloride concentrations are much lower in relation to sulfate than those

in sample 12. The location of samples 1 and 3 adjacent to Cretaceous 41

shales and sandstones of the Tucson Mountains could account for the

higher sulfate, since sedimentary rocks generally contribute more

sulfate to ground water than do igneous rocks. It is possible that some

of the sulfate is from oxidizing pyrites, although this would require

buffering or other pH control, since the pH of samples 1 and 3 are not

as low as would be expected with pyrites. However, iron released in the

oxidation of pyrite in the area could account for some of the error in

the chemical analyses of samples 1 and 3.

The chemistry of sample 12 suggests both recharge and, possibly,

evaporation. A moderately high Si02:TDS ratio, and lowest pH (7.06) and

calcium and bicarbonate percentages indicate moderate carbonate influ-

ence. The high concentration of chloride with high sulfate in sample 12 may result from evaporation, which is possible if the ground water is as

close to the surface as the depth of the well (only 25 feet) suggests.

The unusually high silica concentration in sample 12 is probably a

result of the well's proximity to the Recortado ash flow in the Roskruge

Mountains.

Sample 2 exhibits extremely high calcium, sulfate, and TDS concentrations, with an unusually low concentration of bicarbonate.

These trends are indicative of gypsum (CaSO4) dissolution dominating the

local water chemistry. The presence of gypsum in Avra Valley is possible, as gypsiferous mudstones in the Pantano Formation were cited by Laney (1972) and Mohrbacher (1984) as sources of high sulfate in ground water in the Tucson Basin. Usunoff (1984) also encountered

ground water which was influenced by considerable amounts of gypsum in the San Pedro Valley southeast of Avra Valley. 42

Ground Water in Northern Avra Valley

The chemistry of ground-water samples from northern Avra Valley suggest a different mineralogical environment than was encountered in the south. Relative proportions of calcium, sulfate and chloride increase in northern ground-water samples, whereas the proportions of bicarbonate and sodium decrease. TDS also increases causing silica:TDS ratios to decrease. These trends reflect an increasing occurrence and influence of non-silicic rocks, such as carbonates. Also, the lack of substantial recharge along the mountain fronts appears to continue in northern Avra Valley.

Increased calcium in ground water from northern Avra Valley may result from two sources. First, the silicic mineralization in the north may be changing to calcic feldspars. As noted by Hem (1959), igneous rocks are diverse enough that one group (alkali feldspars) can produce waters which are soft (low calcium and magnesium) with high pH and silica:TDS ratios, and another group (calcic or ferromagnesi an silicates) can produce hard waters with lower pH and S102:TDS ratios.

The suspected shift in igneous mineralization is supported by slightly lower pH values in the northern part of Avra Valley, as well as by lower silica:TDS ratios.

Observed increases of calcium, sulfate and chloride along the flow path suggest an increase in carbonate mineralization in northern

Avra Valley. Carbonates are a much more significant source of calcium in ground waters than igneous rocks and generally, sulfates and chlo- rides are not a major constituent of igneous rocks. Well cuttings confirm that the sediments in northern Avra Valley contain gravel lenses 43 of up to 40% metamorphics and up to 80% mudstones and siltstones

(Matlock and Morin, 1976). This is reasonable, as the geology of the

Tucson, the northern Roskruge and the Waterman Mountains show significantly more sedimentary and metamorphic mineralization than the southern

Roskruge and Sierrita Mountains and these differences should be seen in the sediments of the valley.

Diminished as they may be, the role of igneous mineralization on the chemistry of ground water in northern Avra Valley is still a dominant one. Relatively lower silica:TDS ratios in the north are still of such magnitude as to preclude the dominance of any other mineralization in the valley.

Recharge from the mountain fronts in northern Avra Valley appears to be low. Bicarbonate:calcium ratios decrease in a northerly direction, which is reasonable as ground water progresses farther along a particular flow path, as long as no significant recharge occurs along the path.

Minor-ion Chemistry

Potassium (K+ ), nitrate (N0s), fluoride (F - ) and iodide (I - ) are the minor ions which were measured in the ground-water samples from Avra

Valley. Laboratory values are given in Table 3.

Potassium

Although the abundance of potassium in igneous rocks lies between that of sodium and magnesium (Hem, 1959), high concentrations of potassium in ground water are unusual. This is because potassium silicates tend to be much more resistant to weathering than sodium and 44 magnesium silicates. Also, once in solution, potassium recombines

easily with other weathering products, keeping concentrations in ground water low.

The potassium concentrations measured in Avra Valley are, as

expected, all low. Samples 20 and 21 have the lowest concentrations

(0.8 and 0.3 mg/1, respectively). These concentrations are probably due

to the relatively slow weathering of potassium feldspars. The highest

concentration, 5.2 mg/1 in sample 28, is still reasonably low in

proportion to that sample's TDS concentration.

Nitrate

The relatively high nitrate concentrations in (samples 3, 12,

15, 16, 18, 19, 28, 29 and AV-14) deserve attention, because sources of nitrate concentrations greater than 10 ppm in ground water are seldom from natural sources.

As noted earlier, nitrate often enters the environment from leaking septic systems or from irrigation water return flow. The scale of irrigation compared to the number of septic systems in Avra Valley, however, precludes septic systems as a major source of nitrate in the ground water. The mountain-front samples, however, may be an exception to this, as the density of residences in these areas is much higher than in the rest of the valley. Confirming this, elevated chloride and sodium concentrations in sample 3 correspond to elevated nitrate, as would be expected with contamination from sewage. The same correspond- ence occurs in sample 12, but the proximity of stables to well 12 and the extremely shallow water level in that well suggest the nitrates in 45 sample 12 are more likely from livestock than a residential septic

system.

Elevated nitrate from agricultural sources appears to be present

in ground water from wells 15, 16, 18, 28, 29 and AV-14. Samples 29 and

AV-14 have very high nitrate concentrations, whereas the increased

levels of nitrate in 15, 16 and 28 are less dramatic. None of these

wells has a chloride concentration high enough to suspect sewage

contamination (Figure 13) and all are from wells located in or downgra-

dient from areas of irrigated agriculture.

The moderate increase in nitrates in groundwater from wells 15,

16 and 28, with the lack of significant nitrate in wells 14, 21 and 22

is, at first, puzzling. These wells are all located in the middle of

intensive agriculture and would be expected to show increases in nitrate

proportional to those seen in wells 29 and AV-14. However, it appears

that nitrate contamination in wells 29 and AV-14 is occurring not so much as a result of infiltration of irrigation water through the

unsaturated zone above the water table, but from short-circuiting of

irrigation water down the irrigation wells. Most irrigation water is

applied to fields immediately adjacent to the irrigation well. The

possibility of vertical leakage of irrigation water is therefore quite

likely. The location of the observed ground-water mound in northern

Avra Valley (see Figures 5 and 6) corresponds to the location of sample

29, which has the highest nitrate concentration, further substantiating the theory that these or other local irrigation wells are transmitting nitrate-laden irrigation water directly to the ground water. 46

4.5

4.0 -

+ 3.5 - 19 +1 2 + 3.0 - 2 + 3

15 .+ 4'1 8 • 8 16 +2 + 8 29 "AV-14 +1 1.5 - +1 3 '20

1.0 - AV.-19 21 AV-3 AV-2 22 \ 4.6/ 0.5 - • • • + 7 /11 14 AV-5 AV-1 0.0 1111111111111

0.0 0.2 0.4 02 02 tO 1.2 14

(epm) NO3

• Southern and Mid Valley Wells * Northern Wells + Mountain Front Wells

Figure 13. Nitrate - chloride scattergram. 47

Fluoride

Fluoride concentrations in the ground water samples from Avra

Valley are variable, ranging from 0.40 to 2.3 mg/l. Higher concentrations are generally found in the mountain-front wells and near

Three Points (Figure 14). Fluoride concentrations exceeding the maximum recommended 1.4 mg/1 (Table 4) were measured in samples 2 and 21, which are located in the areas noted by Osterkamp and Laney (1974) as having high fluoride in the ground water.

Fluoride is associated with igneous and metamorphic rocks such as granite, gneiss and schist, all of which are found in the mountains which surround Avra Valley (Kister and Hardt, 1966). Although fluoride is slightly more abundant in igneous rocks than is chloride, low solubilities of fluoride-bearing rocks and minerals are a major factor in the difference in the concentrations of chloride and fluoride in most ground water. The minerals most responsible for fluoride concentrations in ground water are fluorite, CaF2, and apatite, Ca5(PO4)3(F,C1). Anion

exchange, in which OH - replaces F - from amphiboles and micas, is another source of fluoride in ground water (Hem, 1959).

The concentration of fluoride in ground waters seems to corre- late poorly with other dissolved minerals (Usunoff, 1984). An exception is a correlation between fluoride concentrations and alkalinity, with an accompanied inverse relationship between fluoride and hardness. The fluoride-alkalinity relationship is based on the dissociations of fluorite and calcite:

CaF2 = Ce 2 + 2F -

CaCO3 + H+ = Ca+2 + HCO1 48

r z \

rt . 1 (.53) re,49, 513(

1.48) (.50) ( .02

./ .02 7 •• --K.:7/ ./' ., 1 i./...1 t. la ...... i t 7 (.49) .03 Z. Z . . ! .1.. (.67) . (.50) ..--'....-1 / .02 ( .48)) k.,,c,„‘ ( 7 .02 % ____S)4 / -- • C.)4, / • . r : 3 ( t 419(,,s, r ( 1 I/ (.651' 4,,s, ..08' , 1. ) \

/ r. .-- -n.-• (.. /.. ... Y 1 .r ;.55) (.40) .01 .02

/ • (.50)

LEGEND

(1.5) Fluoride Concentration (PPm)

.02 Iodide Concentration (ppm)

IERRITA 0 MILES 5 OUNTA IN S SCALE: 1:250,000 Figure 14. Fluoride and iodide distribution in Avra Valley. 49

In this system, once the saturation of calcium in the ground water is reached, continued dissolution of one of the calcium minerals will result in the precipitation of the other.

High alkalinities and fluoride concentrations correspond with low calcium and hardness concentrations in the ground-water samples from near Three Points. This indicates that locally, fluorite is dissolving in equilibrium with calcite. Elevated fluoride concentrations in this area are also consistent with the strong local influence of granitic sediments.

The high fluoride in samples 2 and 13 in northeastern Avra

Valley also correspond to the occurrence of granites, which predominate on the northwest flank of the Tucson Mountains. But, although the chemistry of sample 13 reflects the relatively low calcium and magnesium expected with granites, low bicarbonate concentrations and pH do not correspond to the conditions seen in areas where fluorite is dissolving.

As noted earlier, high calcium concentrations in sample 2 result from the dissolution of gypsum:

CaSO4•2H20 = Ca+ 2 + SO4 2 + 2H20.

With gypsum dissolving, dissolution of fluorite can only occur if calcite is precipitating:

Ca+2 + CO 2 = CaCO3.

In this environment, the low pH and bicarbonate concentration in sample

2 can be explained by the conversion of bicarbonate to carbonate needed to sustain the precipitation of calcite:

HCOs = H+ + COs2. 50

Elevated concentrations of fluoride in samples 1, 3, 13 and 28

are less easily explained. Calcium and bicarbonate concentrations of

these samples do not strongly indicate the same calcite-fluorite trend

seen near Three Points. Yet the geology of the mountains near these

wells, and therefore presumably the local sediments, are viable sources

of fluorite: samples 1 and 3 are located near rhyolite and metamor-

phosed sedimentary rocks, and sample 28 is near outcrops of andesite.

As with the ground water from well 2, complex reactions may be allowing

dissolution of fluorite without the accompanied trends in hardness and

alkalinity.

Iodide

Iodide concentrations in Avra Valley range from 10 parts per billion (ppb) in sample AV-2 to 230 ppb in sample 1. The higher concentrations occur around the edges of the valley, whereas concentrations in the center of the valley remain low and fairly constant from south to north.

Iodide is not found in significant quantities in igneous rocks

(Lloyd and others, 1982) and what iodide is in these rocks is very soluble, leading to fairly rapid iodide depletion in weathering by-products (Fuge, Johnson and Phillips, 1979). High iodide concentrations in ground water are usually associated with limestones and shales, especially those of marine origin (Fuge and others, 1979).

Low and constant iodide values through southern Avra Valley where granites strongly influence the ground-water chemistry may indicate that depletion of what iodide was available occurred soon after 51 deposition of the granitic alluvium. High iodide concentrations in samples 1 and 3 likely are due to sediments from adjacent non-marine

Cretaceous shale, sandstone, limestone and Tertiary rhyolite in the

Tucson Mountains. The relatively high concentration of iodide in sample

12 is probably derived from sediments of similar rocks in the adjacent

Roskruge Mountains. Elevated iodide in sample 2 may be from granites or sedimentary rocks, since gypsum is likely to be associated with mudstones or shales.

Stable Isotopes

Isotopes arise from different numbers of neutrons in an element's nucleus. In some cases, the added neutrons induce instability, and the nucleus becomes radioactive. This report considers only the major non-radioactive isotopes of hydrogen and oxygen. A difference in the number of neutrons changes atomic weight but not atomic number and as it is the atomic number which determines chemical properties; isotopes of a given element will behave nearly identically in chemical reactions. Differences do arise, however, in processes in which atomic weight plays a role. For elements of low atomic weight such as hydrogen and oxygen, an extra neutron significantly changes the relative atomic weights of the isotopes and their relative abundance can be detected with a mass spectrometer.

Deuterium and Oxygen-18

The value of the isotopes of hydrogen and oxygen as water tracers lies not only in their low atomic weights but also because hydrogen and oxygen are components of the water molecule. Thus, more 52 than any other tracer, these isotopes reflect processes which directly

affect the water molecule. In this study, the stable isotopes deuterium ( 2 H) and oxygen-18 ( 180) were measured in ground-water samples

from Avra Valley.

Fractionation

Differences in atomic weight between isotopes affect thermodynamic properties such as vapor pressures and bonding energies.

Changes in the relative amounts of isotopes in different phases arising from such differences are called fractionations. Fractionation is the basis for using isotopes as "tracers," as waters with different histories of evaporation and precipitation can be distinguished from one another by differences in their ratios of the heavier isotope to the lighter isotope (isotopic signatures). Comparing isotopic signatures of different waters can lead to an understanding of precipitation, recharge and subsurface flow characteristics in the area.

As water evaporates, differences in vapor pressures cause the vapor to be isotopically lighter than the remaining liquid, and following condensation, the condensate is heavier than the remaining vapor.

It follows that the more evaporations and condensations water undergoes, the more isotopically light the vapor will become. Isotopic ratios of hydrogen and oxygen are thus controlled primarily by the number of evaporation and condensation stages which occur (Fontes, 1964). Ambient temperatures during evaporation and condensation affect the fractionation pattern, with fractionation increasing as temperature decreases

(Dreyer, 1982). 53 Rainwater that recharges ground-water aquifers near the ocean is generally isotopically heavier than the water that recharges farther inland, and water that recharges at higher elevations is generally lighter than recharge occurring in the valleys. However, once precipitation infiltrates the saturated zone of the subsurface, fractionation essentially stops and the water retains its isotopic signature.

Units

Stable isotopes are not measured in absolute concentration units, but rather in relation to a standard. The reported units of the stable isotopes deuterium and oxygen-18 are 6 , the relative difference in parts per thousand (per mil, or %.). This is derived from the relationship

[(180/160) sample - (180/160) x 1000 = 6180 (180/160)standard standard

The standard reference, SMOW (Standard Mean Ocean Water), is representa- tive of water in the mid-latitude oceans and by definition, D = 6 180 .

O. Condensation and evaporation occurring once the water leaves the mid-latitude oceans is either enriched in heavier isotopes ( 6 becomes less negative) or depleted ( 6 becomes more negative).

Isotopes in Southern Arizona

In the Tucson Basin, isotopes of hydrogen and oxygen in precipitation and recharge have been shown to vary with season (Simpson,

Thorud and Friedman, 1970) and with elevation (Gallaher, 1979). Simpson 54

and others (1970) found that winter precipitation, for which the moisture generally originates in the northern Pacific Ocean, was

isotopically lighter than summer precipitation derived from the Gulf of

Mexico. Colder temperatures of the northern Pacific and increased distances from the mid-latitudes are believed to be the major factors

affecting the winter isotopic concentrations. Gallaher (1979) attributes an observed elevation distinction to lower temperatures of

condensate as moisture ascends the Santa Catalina Mountains. Gallaher

(1979) and Mohrbacher (1984) used these differences to demonstrate the

usefulness of isotopic signatures in understanding subsurface flow.

They found that water recharging the ground-water system in the Tucson

Basin along the mountain-alluvial contact of the Santa Catalina

Mountains can be distinguished from recharge from valley washes on the basis of the isotopic concentrations in the ground water.

Isotopes in Avra Valley

Seven ground-water samples from Avra Valley were selected for measurement of their deuterium and oxygen-18 ratios. The samples represent ground water from throughout the valley: samples 1 and 12 are from the mountain-front, samples 20 and AV-5 are from southern Avra

Valley, sample 4 is from mid-valley, and samples 16 and 28 are from the north. The samples provide a general idea of isotopic variations of the ground water in Avra Valley.

The Craig Meteoric line, which represents worldwide precipitation, is shown in Figure 15, along with results of the seven isotope analyses. All samples plot within the confidence interval of this 55

*SMOW

CRAIG METEORIC LINE (6 D =8 6 18 0 + 10)

.12 16 * AV-5 1 .4 •28

6 18o70,3 (SMow)

Figure 15. Isotope ratios of samples from Avra Valley. 56 global line, indicating a relatively minor influence of evaporation or condensation on the land surface. However, the observed consistant offset from the global rainfall line may reflect the "amount effect"

seen in precipitation which occurs in the deserts of the southwestern

United States. This effect is attributed to evaporation of precipitation as it is falling, which is often a significant process in arid environments (Simpson, 1986). The relatively light isotopic content of

sample 20 is reasonable in light of the chemical nature of the ground water near Three Points. With most of the ground water at Three Points

resulting from underflow from Altar Valley, sample 20 may represent water recharged from the significantly higher elevations of the Sierrita or Baboquivari Mountains. This assumes that Altar Valley receives only minor recharge from the valley floor, which is reasonable.

Samples 4 and AV-5 plot essentially on the Craig Meteoric line.

Substantially heavier than sample 20, their occurrence on the meteoric

line suggests less of an "amount effect" during precipitation, suggest-

ing recharge from winter precipitation.

Equilibrium Approach

An equilibrium model of the ground water in Avra Valley was used to gain a final perspective on the geochemical processes occurring in the subsurface environment. Equilibrium models are based on the chemical thermodynamics of a system; equilibrium is the point of maximum

stability (in terms of energy) towards which every system proceeds

(Stumm and Morgan, 1970). Few ground water-rock systems reach equili-

brium (Dreyer, 1982) but the usefulness of the approach lies as much in 57 exploring the factors which cause disequilibrium as in providing an idea

of the direction in which the system may be heading.

Saturation Indices

Chemical reactions that occur in the subsurface environment can

be quantified, in terms of energy expended or produced, by an

equilibrium constant, K eg . The equilibrium constant for the

dissociation of calcite:

CaCO3 = Ca+ 2 + COs 2

is defined as:

[aca+2 ] [acci 2 ] Keg = = [aCa+2 ]EaC03- 2] [aCaCO3]

where "a" is the activity of a given chemical species. In dilute waters, activities of aqueous species are assumed equal to concentration

in moles/liter and the activities of solid phases are defined as 1.

Equilibrium constants are related to free standard energy of formation:

-L G° ln Keg = RT •

Because changes in the Gibb's free energy of the system, G, equal zero

at equilibrium:

AGR = AG° + RT ln Keg = 0.

Thus, by comparing the activities of aqueous species found in a ground-

water sample to the activities that would occur at equilibrium, the

degree of saturation with respect to a given mineral can be calculated. 58

A convenient quantity by which to express this condition is the

Saturation Index, which compares the Ion Activity Product (calculated from the true concentrations of ions in the sample) to the equilibrium constant:

IAP = [aca4-2][aC0-32]

SI = log e[ IAPK j.

When the saturation index is equal to 0, equilibrium exists in the system with respect to calcite, calcium and carbonate. When the saturation index is negative, the reaction, as written, proceeds to the right and calcite continues to dissolve. If the saturation index is positive, the reaction as written goes to the left, and calcite precipi- tates. These conclusions all assume that calcite is present in the system.

Saturation indices and log activities for the mineral stability diagrams were calculated for the ground-water samples from Avra Valley using the geochemical equilibrium model WATEQF (Truesdell and Jones,

1974). WATEQF solves the complex series of simultaneous thermodynamic equations which describe a system, using the method of successive approximations (Nordstrum and others, 1979). Selected saturation indices for the ground-water samples from Avra Valley are given in Table

6, along with calculated partial pressures of CO2.

The gypsum saturation index for sample 2 confirms the strong influence that gypsum exerts in that ground water. Approaching 1 and hence equilibrium, the index is several orders of magnitude above that 59

Table 6. Saturation Indices

Well Log Saturation Indices Number (PCO2 ) Quartz Calcite Dolomite Gypsum Fluorite

1 2 -2.5 0.8 0.2 -0.2 -0.1 -0.1 3 -2.0 0.3 0.7 -1.5 -1.3 4 -1.9 0.7 -0.6 -4.8 -2.5 -1.5 6 -2.1 0.9 -0.5 -1.5 -2.3 -2.0 7 -2.0 0.9 -0.6 -1.6 -2.8 -2.0 8 -2.6 0.7 -0.1 -0.6 -1.6 -1.9 11 -2.5 0.7 0.2 -0.4 -2.7 -2.2 12 -1.5 1.2 0.03 -0.1 -1.5 -1.5 13 -2.8 0.2 14 -2.7 4.7 0.4 0.01 -2.6 -1.8 15 -2.5 0.8 -0.1 -0.8 -2.3 -1.6 16 -2.5 0.8 0.2 0.2 -2.2 -1.9 18 -2.5 0.7 0.1 -0.2 -2.0 -1.8 19 -2.4 0.7 0.1 -0.5 -1.8 -1.7 20 -3.4 0.8 0.2 -0.1 -2.8 -1.7 21 -3.9 0.6 0.5 -3.3 -1.6 22 -3.0 0.6 0.2 0.01 -2.6 -2.2 28 -2.8 0.8 0.5 0.6 -1.3 -1.2 29 -2.1 0.7 0.2 -0.2 -1.7 -1.7 AV-1 -2.8 0.6 0.3 1.0 -2.8 -2.0

AV-2 -2.8 0.6 0.3 0.01 -2.8 -1.7 AV-3 -2.6 0.6 0.2 -0.1 -2.8 -1.8 AV-5 -2.6 0.6 0.1 -0.6 -2.7 -2.3

AV-14 -2.4 0.7 0.1 -0.2 -1.9 -1.8 AV-19 -2.7 0.7 0.2 -0.3 -2.8 -2.0 60

for the other samples. Sample 2 also has fluorite and calcite satura-

tion indices that support the theory that fluorite is dissolving,

accompanied by a loss of calcium to calcite (calcite is shown to be

precipitating by a positive saturation index). Saturation indices for the mountain-front sample 12 indicate supersaturation with respect to

quartz, undersaturation with respect to calcite and dolomite, and a

fairly high Pco . These values support the assumption that sample 12 is 2 primarily influenced by local volcanic intrusives, with a high Pco 2 indicative of recent recharge.

These samples also have rather high Pc0 2 values. These high values may be due to the fact that samples 4, 6, and 7 were tank samples. The possibility also exists that since these samples are

located near or just downgradient of the bedrock saddle (see Figure 3), mixing of deeper waters may be occurring in this area. The empirical

relationship between P,CO2 and ac a+2 is not a linear one (Dreyer, 1982). It is possible for two waters that were separately saturated with respect to calcite to be undersaturated with respect to calcite

after mixing. Information available in this area, however, is not sufficient to test this hypothesis.

Mineral Stability Diagrams

As noted, the equilibrium approach is useful in understanding the complexities of the alumino silicate-water system. Alumino-silicate minerals are abundant in igneous rocks, and as shown, are quite

influential in determining chemistry of Avra Valley ground water. The weathering of albite presented earlier, for instance, shows that the 61 dissolution of alumino silicates yield clays that participate in base exchange. Knowing which silicates are weathering and to which products helps in understanding the dynamics of a hydrogeochemical system.

Alumino silicate-water systems are represented by stability diagrams such as those shown on Figures 16 and 17. Stability diagrams are constructed using free energies of various reactions in a system.

Plotting the log of ion activities from ground water samples indicates the status of the water chemistry with respect to the weathering silicates.

Plots of ground water samples from Avra Valley on Figures 16 and

17 (which represent weathering of alkali feldspars) show that alumina silicates are likely to be weathering in Avra Valley. In an idealized system freshly recharging water is farthest from equilibrium with respect to the weathering mineral (feldspar) and therefore plots to the lower left. As the water progresses through the system it tends towards equilibrium with feldspar, and plots along an increasing slope towards the feldspar boundary, or that of an intermediate mineral.

Nearly all samples from Avra Valley plot in the kaolinite field, which is reasonable in that kaolinite is often the primary weathering product of feldspars. Samples 20, 21, and 28 are closest to saturation with respect to alkali feldspars.

Of note on Figures 16 and 17 is the distribution of the Avra

Valley samples in relation to each other. The positions of samples 13,

20, 21, and 28 on the diagrams suggest longer residence times in a feldspathic system. This is realistic, since these samples are likely to represent underflow from Altar Valley (samples 20 and 21) or are 62

MUSCOVITE PYROPHYLLITE

6.0-

K - 5.0- FELDSPAR (ADULARIA)

GIBBSITE KAOLINITE

28+

21 • 13 AV-1 • 20 22•• AV-2 AV-3 (.%M-19 AV-5 - 5. 14,16,18 +19 •AV -14

3 + le 29 • SOUTHERN AND t MID-VALLEY WELLS 6 3.0- + NORTHERN WELLS • 7 • 4 + MOUNTAIN-FRONT WELLS + 2

+12

1 f 1 -7 -6 4 LOG 1[H4 &04 ]

Figure 16. Mineral stability diagram for the H 20-Al 20 3 -K 20-Si0 2 system. 63

• 21 ALBITE

• 20 6.0

29 13++

022 KAO LIN ITE AV-1 AV-19 AV-23 --8-.3++1164 0 AV-5* + 2 5.0 - 11.4:4 a 14t AV-14 +15 +12 +29

• g Na - MONTMOR I LLON ITE (BE IDE LLITE)

4.0 - • SOUTHERN AND MID-VALLEY WELLS • NORTHERN WELLS + MOUNTAIN-FRONT WELLS

- ; -2 LOG a[11 4 S104 J

Figure 17. Mineral stability diagram for the H20-Al 203-Na20-Si0 2 system. 64 located farther along the flow path in Avra Valley (samples 13 and 28).

However, the isolation of samples 13 and 28 from other northern samples is significant, in that most of the remaining northern samples reflect a general movement away from saturation with respect to feldspars. The reasons for this are most likely complex, however it is reasonable to theorize that the stability diagrams reflect the increasing influence of non-igneous sediments and the introduction of fresher water along the flow path; both of these are consistent with the observed chemical trends presented earlier. An increasing influence of non-feldspathic mineralization is evidenced in the progression of southern and mid valley samples from wells 20 and 21 to the congregation corresponding to the AV-wells, with a nearly constant activity of silica. Between these samples, and those that apppear to be farthest from saturation, lie the wells that were noted to be influenced to some degree by nitrate contamination. Introduction of "fresher" water would push the ground water away from equilibrium with feldspars. The extreme plots of samples 4, 6, and 7 from saturation can not be explained by either of these two processes alone. A strong influence of recently recharged water is not probable, since other chemical trends show no significant increase from average Avra Valley samples. It is possible that equilibrium with respect to feldspar in these samples is strongly influenced by a local change in mineralization not detected in the analyses performed in this study. CHAPTER 5

SUMMARY AND CONCLUSIONS

Groundwater geochemistry in Avra Valley was evaluated to

determine the extent to which factors that influence the subsurface flow

regime can be observed in ground-water chemistry. Ground water in Avra

Valley occurs in a nearby continuous water-table aquifer ranging in

depth from approximately 200 to 400 feet below ground surface. Based on

water level data, most of the ground water is assumed to enter the

valley as underflow from the south; mountain-front recharge is presumed

to be negligible. The sediments through which the ground water flows

are primarily igneous, with increasing occurrances of sedimentary and metamorphic mineralization in the northern portion of the valley.

In this study, ground-water samples were obtained from thirty

wells in Avra Valley. Major and minor ions were measured in each

sample; ratios of the stable isotopes deuterium and oxygen-18 were measured in seven samples. Resulting constituent concentrations were evaluated using geochemical principles of solubility, ion ratios and equilibrium.

Results of the geochemical evaluation show that, as expected, the chemistry of the ground-water in Avra Valley is a function of mineralogy of the sediments through which the water flows. Ground water

in southern Avra Valley reflects the primarily granitic sediments of

northern Altar and southern Avra Valleys; high silica:TDS ratios,

65 66 elevated pH and low calcium:sodium ratios suggest the weathering of alkali feldspars. In northern Avra Valley, silica:TDS ratios and pH decrease and calcium, sulfate and chloride increase. Such trends are reasonable in areas where sedimentary and metamorphic mineralization increase, as is the case in northern Avra Valley. Saturation indices support the conclusion that isolated occurrences of high fluoride and sulfate found in ground-water samples are due to weathering of localized fluorite and gypsum, respectively.

The geochemical evaluation also supports the theory that recharge to Avra Valley ground water from surrounding mountain pediments is low. Although samples from mountain-front wells did reflect recent recharge, gradients of chemical composition were not observed from the mountains into the central valley, and no mixing zones were identified.

Elevated nitrate was observed in several well samples, verifying that local agriculture is affecting ground water quality. The observed uneven distribution of nitrate and a corresponding mounding of ground water suggests borehole leakage around irrigation wells rather than deep infiltration as the primary pathway for nitrates to the ground water.

Recommendations

Limitations on access to many wells in Avra Valley and a lack of deep wells to sample prevented analysis of vertical gradients in Avra

Valley ground-water quality. Further studies in the valley should emphasize higher spatial densities and better vertical control, especially in the vicinity of the bedrock saddle. Better definition of sediment stratigraphy, with emphasis on clay mineralogy, would provide 67 much-needed data for future geochemical interpretations. Finally, seasonal changes in recharge, pumping by the City of Tucson, and agricultural practices may influence the chemistry of the ground water; all of these factors should be addressed in future studies of Avra

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