HWR 431 / 531 HYDROGEOLOGY LAB SECTION

LABORATORY 6

TUCSON BASIN PROJECT – HYDROGEOCHEMISTRY

Introduction

In this laboratory we will be investigating the hydrogeochemistry of the Tucson Basin. It will be your job to produce a report based upon your analysis of available data. The following section contains information compiled from numerous studies which have been performed on the Tucson Basin aquifer. Use this information to support your assessment of Tucson Basin quality.

Background

The Tucson Basin is a broad northwest trending alluvial valley in southeastern Arizona. It is bounded by mountains ranging in elevation from 900 to 2800 meters above mean seal level. The basin floor ranges from 610 meters in the northwest to 1070 meters in the southeast. The area of the basin comprises approximately 3600 km2.

Drainage in the Tucson basin is to the northwest. There are four primary drainages in the basin: the Santa Cruz River which flows northward along the western edge of the basin, the Pantano River which drains the Rincon Mountains in the east and the Rillito River and Tanque Verde creek which drain the Catalina Mountains along the northern edge of the basin.

Figure 1 - Tucson Basin Site map

2 Metropolitan Tucson occupies the north central one-third of the basin. Its expanding population is expected to exceed 1.5 million by the year 2025. This population will require 290 to 360 millions of m3 of water per year. At present, Tucson depends primarily upon groundwater for its potable water supply. Although CAP (Central Arizona Project) water could potentially provide an alternative water resource, it will not be sufficient to meet all of Tucson's water supply needs.

Geology:

The Tucson Basin is part of the Basin and Range province of the southwestern United States. Basin sediments consist of Oligocene to Pleistocene age alluvium deposited in the deep grabens formed by high-angle normal faulting during the Basin and Range crustal extension. The central grabens extend to depths of more than 3000 meters. Figure 2 is a representative cross section of the Tucson Basin.

There are three main lithologic units in the basin. The lowest unit, the Pantano Formation is consolidated and contains clasts of Catalina gneiss and volcanic flows yielding ages of 31.4 to 24.9 million years. The central lithologic unit, the Tinaja beds, Miocene to Pliocene in age and ranges in thickness from100 meters near the periphery of the basin to 600 meters near the center. This formation consists of volcanics from the Tucson Mountain complex, metamorphics from the Catalina and Rincon mountains and interbedded clay-rich anhydrite/gypsum beds. The uppermost sedimentary unit, the Ft. Lowell formation, unconformably overlies the Tinaja beds. These sediments were deposited during the early to mid Pleistocene and are from 60 to 125 meters thick. The formation consists of gravel to clayey silt. A thin layer of Pleistocene and Holocene sediments overlie the Ft. Lowell formation. This layer consists of coarse sand and gravel and is generally above the water table.

Figure 2 - Idealized geological cross-section for the Tucson basin

3 Hydrogeology:

Recharge to the basin occurs mainly through riverbeds and along the mountain fronts. Water is removed from the basin by evapotranspiration, groundwater flow to the northeast and by pumping of groundwater. The average precipitation is approximately 300 mm/yr over the basin and as high as 650 mm/yr on the mountains. The potential evaporation in the basin, estimated at 2000 mm/year, far exceeds the precipitation. Mining of groundwater exceeds all sources of input and has resulted in decrease in groundwater levels up to 50 meters in some areas.

Most wells in the Tucson basin draw water from the Ft. Lowell formation. This formation has an average saturated thickness of 30 meters. The hydraulic conductivity of this formation varies between 0.3 and 60 m/day and the transmissivities vary between 270 and 15,500 m2/day. Regions of high transmissivity within the formation are believed to be paleoalluvial fans and deposits of streams that drained the mountains.

Inorganic chemicals in groundwater:

Because groundwater is in contact with a variety of materials, it naturally contains dissolved species. Total dissolved solids TDS in groundwater can range from less than100 mg/L to more than 500,000 mg/L. The major that occur naturally in groundwater include: , , , , chloride, , and bicarbonate/carbonate. The major gases include nitrogen, carbon dioxide, methane, oxygen, and hydrogen sulfide. The presence of these constituents in groundwater may be the result of many processes including reactions with aquifer material, mixing with of different sources (rivers, deep groundwater...) and anthropogenic contamination. In this project you will evaluate the data in order to discern the sources of these constituents in Tucson Basin groundwater.

Exchange with aquifer materials:

In order to constrain the hydrogeochemical reactions controlling groundwater chemistry, a number of studies have examined the mineralogy of basin sediments. Analysis of basin sediments has employed X-ray diffraction, X-ray fluorescence, microscopic and petroscopic analysis. Quartz, plagioclase, calcite, orthoclase, muscovite, biotite, chlorite and clay are the dominant minerals whereas garnet, epidote, parisians and amphiboles occur in smaller quantities.

There are a number of chemical reactions which may occur between recharged groundwater and basin fill sediments along the flow path. The relative importance of each reaction is depends on many factors and is for the most part beyond the scope of this project. However, in examining the differences in water chemistry between samples taken at different locations, you should keep in mind the composition of recharging water, how the groundwater changes along its flow path, what you would expect the composition to be at the sampling location and what other factors might cause changes in

4 composition. Several important reactions involving minerals known to occur in Tucson Basin sediments are cited below.

Plausible chemical reactions for Tucson basin sediments:

(1) Congruent dissolution of high Mg calcite: CaMg(CO3)2 + H2O = Ca + Mg + HCO3 +OH

(2) Formation, dissociation of carbonic acid: CO2 + H2O = H2CO3 = H + HCO3 = H + CO3

(3) Incongruent dissolution of Oligoclase and formation of Smectite clay: (Ca,Na)AlSi3O8 + H2O = (Ca,Mg,K,Fe)Al2Si3O10(OH)2H2O +Ca + Na

(4) Incongruent dissolution of biotite K(MgFe)3(AlSi3O10)(OH)2 + H2O +H = K + Mg + FeOOH + Al(OH)3 +H4SiO4

(5) Incongruent dissolution of Orthoclase and formation of Smectite clay: KAlSi3O8 + H2O + H = K(Ca,Mg,K,Fe)Al2Si3O10(OH)2H2O +K

(6) Hydrolysis of silica: SiO2 + H2O = H4SiO4

(7) Congruent dissolution of gypsum/anhydrite: CaSO4-2H2O = Ca + SO4

(8) Formation of goethite or ferrihydroxides: Fe + O2 + H2O = FeO(OH)2

(9) exchange: (Na,K)2 – clay + (Ca, Mg) = (Ca, Mg) – clay + (Na,K)

Potential anthropogenic sources of groundwater constituents:

Anthropogenic sources of groundwater contamination in the Tucson Basin include: agriculture, wastewater treatment plants and industrial development. Some of these sources of contamination are shown on the Tucson Basin basemap. Others may not be immediately obvious and may need to be identified through changes in groundwater chemistry in the basin. Sources of groundwater contamination may have distinct chemical compositions. Contaminated groundwater should be either similar in composition to the contaminant source or should have a chemistry which is a mixture of the natural water chemistry and the contaminant source. It should be noted however, that changes in groundwater chemistry may also result from anthropogenically induced changes in the aquifer characteristics such as decreased water table elevations caused by groundwater mining.

5 HYDROGEOCHEMICAL ANALYSIS

Graphical analysis:

In this laboratory, it will be necessary to employ graphical methods to analyze and present chemical data. Such methods provide a convenient means for visually illustrating differences and similarities between water samples. A description of each of these methods is provided below.

Bar graph:

In a bar graph, it is often preferable to plot all the samples next to one another for visual analysis. Each sample is separated into two columns. The first column shows relative amounts of the cations: Na + K, Mg, Ca. The second column illustrates concentrations of Cl + NO3, SO4, CO3/HCO3 and B. Bar graphs may be created using a typical graphing program such as Quattro Pro or Excel.

Figure 3- Bar Graph

Stiff diagram:

A second common method of graphical analysis is the Stiff diagram illustrated below. This analysis provides a way of rapidly comparing the degree of similarity between samples using shapes.

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Figure 4- Stiff Diagram

For convenience, these shapes are often included directly on the site map for quick comparison. An alternative method for comparison of generalized samples (i.e. typical effluent water, typical river water…) is to plot all of the samples on a single page. As illustrated in the figure, the shapes are constructed by plotting cation concentrations on the left and anion concentrations on the right. The cations Na + K are plotted vs Cl, Ca is plotted against HCO3 and Mg is plotted against SO4.

Piper Diagrams:

The Piper or trilinear diagram shows the relative abundance of the major ions of + + +2 +2 - -2 -2 - groundwater analyses (Na , K , Ca , Mg , Cl , SO4 , CO3 , HCO3 ) and produces a single point for each sample.

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Figure 5 - Sample Trilinear Diagram for groundwater samples A and B

This plotting technique allows numerous groundwater analyses to be plotted on a single diagram, and hence, provides the user with a quick and concise means of visually comparing chemical similarities and differences among groups of groundwater analyses, as well as mixing trends between different groundwaters. (mixed waters should plot along a straight line between end member groundwater compositions).

Piper diagram consists of two triangles and a rhombus. On the left-hand triangle, cations are plotted as a percentage Ca (along base), percentage Na + K (along right side) and a percentage Mg (along left side). On the right-hand triangle, anions are plotted as a percentage Cl (along base), percentage SO4 (along right side), and percentage CO3 + HCO3 (along left side). Once these points are plotted, lines are drawn along diagonal paths (as shown in the sample diagram) and connected in the rhombus. The intersection point represents the chemical composition of the individual sample.

In this laboratory project, you will be asked to produce trilinear diagrams for numerous groundwater samples. By plotting these on a single trilinear diagram you should be able to illustrate similarities and differences between the samples. It may be a good idea to produce several diagrams (a summary diagram with all samples + several other diagrams that illustrate the points you are trying to make).

8 PROJECT GOALS

Introduction: As stated in the introductory memo, the goal of the Tucson Basin Project is to make a preliminary assessment of the groundwater quality in the Tucson Basin based upon existing hydrogeological data. The project should be constructed in the standard format outlined at the beginning of the semester. Unless otherwise noted, the final project should include all standard figures, maps and written sections. All writing should be in your own words and all maps and figures should be your own. Although discussion of the project is allowed, reports should be done separately.

Written Report: The written report should focus on examining the maps, figures and data and drawing conclusions about the quality of water, the sources of chemical constituents present and how and why groundwater quality has changed over time. Try to understand why the project data was selected, what it represents and what it indicates about groundwater quality. The most important thing in this section is to explain why you made a certain assessment and how it relates to the quality of water in the basin. Separate the written report into the following sections: Background, Hydrogeology, Water Quality, Sources of Pollution, Conclusions and Recommendations, Data, Maps and Figures.

Background: Summarize the background information about the Tucson Basin in your own words. The important thing in this section is to display an understanding of the regional and local setting including basin location, climate and geology.

Hydrogeology of the Tucson Basin: You have been given several maps displaying information about the hydrogeology of the Tucson Basin, including groundwater contour and transmissivity maps. In addition, there is a background section on the hydrogeology of the basin. Talk about the general hydrogeology of the basin. Answer the following questions:

How does the shape and spacing of the contours reflect the heterogeneity of the aquifer?- What would cause heterogeneity in aquifer transmissivity and conductivity? How should this alter groundwater chemistry? Where are the principle sources of groundwater recharge? (Natural and man-made)

Water Quality Analysis Section: In this section, you need to discuss the results of your data analysis. This includes looking for trends in the data. Look at trends that occur both temporally and spacially. Use the different types of diagrams (trilinear, bar graph, stiff) and the maps to help you identify these trends. Examine the causes of water chemistry differences. Look at what each water sample represents (recharge water, influenced by man, undisturbed, reacted with basin mineralogy). Do you see similarities between the data for different samples? Why might that be? Make sure to discuss the following:

9 What do the various water samples represent? What is the significance of differences in their chemistry? Which samples plot in the same area of the trilinear diagram? Do you see groupings? Where do they occur geographically? What do they represent? Are there changes in water chemistry over time? Can you use chemistry to identify recharge zones? Why or why not? Calculate and discuss the anion-cation balance for Roger Road Plant and estimate the electrical conductivity of the effluent

Sources of Pollution: The water sample data should indicate some sources of pollution in the basin. Define what we mean by pollutant and what we mean by contaminant. What are some of the sources of pollution in the basin? How do these sources manifest themselves as changes in chemistry? Why? Answer the following questions:

Prior to the late 1800’s the Santa Cruz was perennial along some reaches and ground water beneath the flood plain occurred at shallow depths. Evapotranspiration was the main process causing groundwater discharge. Since then, pumping has slowly lowered the water table and the Santa Cruz now only flows after heavy rains or where it receives sewage effluent. The situation is deemed to be responsible for increasing TDS concentrations in groundwater near the Santa Cruz. Why?

Compare the 1944 and 1977 groundwater analyses in the area north of the Canada del Oro and Santa Cruz confluence. What might have caused the changes? List at least 2 possible reasons.

The contour maps show high concentrations of Cl and NO3 parallel to the Santa Cruz River. The two significant sources for these contaminants are sewage effluent and agriculture. What are the two specific sources of high Cl in ground water? The farms downstream of the Cortaro Basin are irrigated with ground water. What are the three specific sources of high nitrogen in this area? Boron is being increasingly used as a substitute for phosphates in detergent. What does his reveal about the nitrogen sources above? The total nitrogenin sewage effluent is about 30 mg/L 20 mg/L occurs as ammonia and 10 mg/L occurs as organic compounds. What is the source of high nitrate near the points of effluent discharge?

How do the concentration contour maps reflect the lateral and vertical movement of groundwater?

Conclusions and Recommendations: Summarize your results including the major sources of contamination in the basin. Suggest some solutions to these problems. Also, if further study could improve our understanding of basin water quality, what types of work would you propose and why? Name at least two analytical methods or pieces of data that would improve water quality analysis.

10 Data Tables/Figures:

Tables/Data: All the data needed for this lab is provided on electronic format. Paper copies of the data are also included at the end of this section. A copy of each of these data tables should be included in the project report. Each table should be labeled and referenced within the body of the report. Tables should be included as an appendix at the end of the report rather than within the report itself. However, it will be necessary to include data within the report to support your statements.

Maps/Figures: Maps and figures are of central importance to assessing the groundwater quality issues in the Tucson Basin. These should be included in appendices at the end of the lab and should be referenced within the body of the report. The project report should include the following maps:

- Site map which shows the basin, major rivers and well locations. - Transmissivity map for the basin. - Contour maps of the following: - 1977 nitrate concentrations; - A contour map of the 1977 chloride concentrations; - A contour map of 1977 TDS levels; - A contour map of 1977 boron concentrations;

Note: These should be drawn by hand on separate base maps. Use the data included in the data tables to produce the maps. The contour interval is your choice but make sure enough contours are drawn so that an assessment of the data may be made.

The project should also include the following figures:

- Bar graphs for all data in Table 1, Table 2 and Table 3. - Stiff diagrams for all data in Table 1, Table 2 and Table 3 including (these may all be put on one page for comparison). - Trilinear diagrams for all data in Table1, Table 2 and Table 3: (Note: Do at least one trilinear diagram by hand to demonstrate you know how to do them).

Make sure to do all three diagrams for each of these data sets:

_ Roger Road effluent _ Ft. Lowell water (typical) _ Tinaja water (typical) _ Mountain front water _ Santa Cruz River water _ Rillito River water _ Pantano River water _ Tanque Verde River _ CAP water _ Selected wells (Table2)

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