GROUND-WATER GEOCHEMISTRY OF THE OGALLALA AQUIFER IN THE SOUTHERN HIGH PLAINS OF TEXAS AND NEW MEXICO by VICTORIA YEKO POTRATZ, B.S. A THESIS IN GEOSCIENCES
Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE
J / AnnrnvpH
December, 1980 ^r
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ACKNOWLEDGMENTS
I wish to express my sincere gratitude to Dr. W. W. Wood for suggesting this thesis problem and for his assistance. I would also like to thank Dr. C. C. Reeves, Jr., and Dr. Necip Gliven for serving on my thesis committee. A special thanks is extended to Randy Bassett for supplying a portion of the data used in this thesis. I would also like to thank John Yeko for his help in producing the computer drawn maps.
n TABLE OF CONTENTS
Page ACKNOWLEDGMENTS i i ABSTRACT vi i
LIST OF TABLES ix
LIST OF ILLUSTRATIONS x I. INTRODUCTION 1 Ob j ecti ve 1 Location and General Features of Study Area... 2 Previous Studies 5 Geology 7 Permi an 8 Triassic 8 Cretaceous 11 Terti ary 11 Quarternary 20 Hydrology 20 Permi an 21 Triassic 21 Cretaceous 23 Tertiary 23 Quarternary 25 II. CHEMISTRY OF THE GROUND WATER IN THE OGALLALA AQUIFER 26 Cal ci um 27
m Magnesi um 28 Sodi um 28 Potassium 29 Bi carbonate 30 Sul fate 30 Chi ori de 31 Silica 31 Flouride 32 Nitrate 32 Uranium, Arsenic, Barium, Boron, Lithium, Vanadi um, and Zi nc 33 III. PROCESSES CONTROLLING THE COMPOSITION OF GROUND WATERS 35 Mineral Equilibrium by Mass Action 35 Equi 1 i bri um Constant 40 Ion Activity Product 41 Temperature Correcti ons 42 Mineral Equilibrium by Oxidation and Reduction 44 Ion Exchange 46 Adsorption 47 Ion Filtration 47 Mixing by Diffusion 48 Mixing by Convection 49 IV. GEOCHEMISTRY OF GROUND WATER IN THE OGALLALA AQUIFER 51
TV Introduction 51 Mineral Equilibrium 57 Cal ci um 57 Magnesium 61 Sodium 61 Potassium 62 Bi carbonate 63 Sulfate 65 Chi ori de 65 Silica 65 Flourite 66 Ni trate 67 Uranium, Arsenic, Barium, Boron, Lithium, Vanadium, and Zinc 67 Mixing of Saline Water with the Ogallala Ground Water 68 Ion Exchange and Adsorption 72 Mass Balance 73 Northern Section of Study Area 75 Southern Section of Study Area 83 V. MANMADE POLLUTANTS INFLUENCING THE CHEMICAL QUALITY OF THE GROUND WATER IN THE OGALLALA AQUIFER 86 Oil and Gas Field Brines 86 Fertil izers 88 Liquid Wastes from Animal Feedlots 88 VI. CONCLUSIONS 92 LIST OF REFERENCES 96 APPENDIX .-... 107
VI ABSTRACT
Concentrations of calcium, magnesium, potassium, sulfate, and chloride ions in the ground water generally increase from the north to the south in the Ogallala Aquifer of the Southern High Plains of West Texas and eastern New Mexico. In the south, higher concentrations of solutes generally occur in areas which are underlain by Cretaceous rocks and are located beneath large saline lakes. The distribution of bicarbonate and flouride ions and also silica molecules are variable throughout the aquifer. High concentrations of nitrate ions usually occur south of the northern edge of the Cretaceous boundry and along the northeastern edge of the aquifer.
The primary sources of solutes in the ground water north of the Cretaceous subcrop are the weathering and dissolution of potassium feldspar, biotite, plagioclase feldspar, calcite, and dolomite. Minor amounts of sodium, sulfate, and chloride may have been added to the ground water by seepage of oil and gas field brines into the aquifer. It is proposed that ground water south of the Cretaceous subcrop obtains additional sodium, sulfate, and chloride ions from the mixing of saline Cretaceous water with the Ogallala water. The mixing between the two aquifers may result from a difference in head between the two aquifers or by diffusion. In these areas of mixing, the sodium ions from the Cretaceous water may be exchanging for calcium and magnesium ions on clay minerals in the Ogallala
vn Formation. Some of the calcium ions produced from the exchange are probably combining with bicarbonate ions and precipitating calcium carbonate. Amorphous silica and flourite may be precipita ting in several areas of the aquifer along with calcite and is probably limiting the concentrations of silica, flourine, calcium, and to a minor degree bicarbonate.
vn T LIST OF TABLES Table Page 1. Chemical analyses of ground water from Permian, Triassic, Cretaceous, and Tertiary rocks 22 2. Ion-pair and complex ion dissociation constants for the major ions in the Ogallala ground water at 25°C and 1 atm 38 3. Saturation index for several minerals 58 4. Chemical analyses of water from saline lakes 71 5. Ratios of sodium, calcium, and magnesium in water from saline lakes and nearby Ogallala ground water 74 6. Source minerals for ground water in the Ogallala Aquifer 78 7. Chemical analyses of the average southern ground water with the average northern ground water from the study area 84
IX LIST OF ILLUSTRATIONS Figure Page 1. Index map of study area in West Texas and eastern New Mexi co 3 2. County map of the Southern High Plains 4 3. Rainfall map, in centimeters, of the Southern Hi gh PI ai ns 6 4. Composite strati graphic column of the Wasson Field, Yoakum and Gaines counties, Texas 9 5. Thickness of Triassic sediments, in meters, within the Southern High Plains 10 6. Subsurface extent of the Cretaceous deposits in the Southern High Plains 12 7. Location of the Ogallala Formation 14 8. Distribution of clays in the Ogallala Formation 16 9. Distribution of sands in the Ogallala Formation 17 10. Distribution of gravels in the Ogallala Formation 18 11. Saturated thickness map, in meters, of the Ogallala Aquifer in the Southern High Plains of Texas 24 12. Average calcium concentration in rainwater in the conterminous United States 52 13. Average sodium concentration in rainwater in the conterminous United States 53 14. Average potassium concentration in rainwater in the conterminous United States 53 15. Average sulfate concentration in rainwater in the contermi nous United States 54 16. Average chloride concentration in rainwater in the conterminous United States 54 17. Percentages of total dissolved carbon dioxide species in solution at 25°C and 1 atm. as a function of pH 64 18. Schematic representation of a saline lake in the southern section of the study area 70 19. Stability relation of muscovite, microcline, kaolinite, gibbsite at 250C and 1 atm. as a function of K"*", pH and H4Si04 76 20. Stability relation of albite, Na-montmorillonite, kaolinite, gibbsite at 25°C and 1 atm. as a function of Na"*", pH and H4Si04 76 21. Stability relation of anorthite, Ca-montmorillonite, kaolinite, gibbsite at 25^0 and 1 atm. as a function of Ca'''^, pH and H4Si04 76 22. Location of major oil and gas fields in the Southern High Plains in Texas 87 23. Location of general soil types in the Southern High Plains in Texas 89 24. Location of cattle feedlots in the northern half of the Southern High Plains in Texas 90
PLATE* I. Location of wells II. Concentration of total dissolved solids III. Concentration of calcium IV. Concentration of magnesium V. Concentration of sodium VI. Concentration of potassium VII. Concentration of bicarbonate VIII. Concentration of sulfate IX. Concentration of chloride X. Concentration of silica XI. Concentration of flouride XII. Concentration of nitrate
XI XIII. Concentration of uranium XIV. Index map for Plate XV
XV. Concentration of Arsenic, Barium, Boron, Lithium, Vanadium, Zinc
XVI. Concentration of sodium and potassium *A11 Plates in pocket.
MICROFICHE*
0001. Southern High Plains Groundwater 0002. Southern High Plains Groundwater *A11 Microfiche in pocket.
xn CHAPTER I
INTRODUCTION
Objective
Generally the chemical composition of ground water is a function of the aquifer's lithologic framework (Hem, 1978). The mechanisms controlling the chemical quality of the water, however, are not adequately understood for clastic aquifers consisting primarily of unweathered rock fragments, although studies by Plummer and Mackenzie (1974), Plummer and others (1976), and Plummer (1977) have identified the mechanism operating in a monomineralic aquifer composed of limestone. A recent study by Thorstenson and others (1979) on the Hell Creek Aquifer dealt with the general water chemistry of a clastic aquifer, however, additional studies on better hydrologically controlled clastic aquifer systems is necessary. The Ogallala Aquifer, located in the Southern High Plains of Texas and New Mexico, was chosen for this type of study for several reasons: 1) the aquifer is essentially geologically and hydrologically isolated; 2) no previous work has been done on the complete regional geochemistry of the ground water; 3) the aquifer is composed of clastic rocks of diverse mineralogy; 4) vast numbers of chemical analyses of the ground water from the aquifer are available from a variety of different sources. The objectives of this study are: 1. Describe the distribution of major and several minor elements found in the ground water of the Ogallala Aquifer. 2. Determine the source of the solutes in the ground water. 3. Determine the major processes controlling the concentra tion of ions in the aquifer.
Location and General Features of Study Area
The Ogallala Aquifer is situated in the Southern High Plains of West Texas and eastern New Mexico (Fig. 1). The Southern High Plains, also known as the Llano Estacado, extends over an area of 77,700 square kilometers. It is a topographically isolated plateau bounded on the east by the "Caprock" escarpment, on the west by the Mescalero escarpment, on the north by the southern side of the Canadian River Valley, and on the south by a gentle transition into the Edwards Plateau. Figure 2 shows the major political boundaries of the area. Regionally the surface of the Southern High Plains exhibits a gentle slope of 1.5 to 2.0 meters per kilometer to the south east. Scattered across the surface are numerous shallow depres sions locally referred to as playa lakes which contain water only after rainfall events. Large saline lake basins which nearly always contain some water are located in Bailey, Lamb, Lubbock, Cochran, Yoakum, Terry, Lynn, Dawson, and Gaines counties in Texas and the southern half of Roosevelt County and the northern rj OKLAHOMA 50 [- TEXAS MILES 0 n 0 KILOMETERS
Figure 1.--Index map of the study area in West Texas and eastern New Mexico. N
NEW I^IExfco ,'- TEXAS
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Figure 2.--County map of the Southern High Plains. half of Lea County in New Mexico (Reeves, 1968). This area is in the central portion of the Southern High Plains (Fig. 6).
The climate of the Southern High Plains is semi-arid, with an average annual temperature of approximately 15 degrees Centigrade and strong winds. Evaporation rate on the Llano Estacado ranges between 2.0 meters and 2.5 meters per year. Figure 3 depicts the average amount of precipitation falling on the Southern High Plains; seventy percent of the rain for the year falls between months of April and September (see Orton, 1966).
Previous Studies
Most regional geochemical investigations have dealt with ground water in carbonate aquifers (Back, 1966; Wallace and Cooper, 1970; Back and Hanshaw, 1971a, 1971b; Langmuir, 1971; Plummer and Mackenzie, 1974; Plummer, 1975, 1977; Plummer and others, 1976; Plummer and Wigley, 1976; Plummer and others, 1978; Plummer and Back, 1980). Several studies have dealt with aquifers composed of igneous material (Feth and others, 1974; Garrels and Mackenzie, 1967; Garrels, 1967; Cleaves and others, 1970; Wallace and Cooper, 1970; Drever, 1971; Miller and Drever, 1977). Most investigations concerning aquifers composed of clastic rocks deal with waters exhibiting brine concentrations (Billings and others, 1969; Hitchon and others, 1971).
Several studies of water quality in the Ogallala Aquifer KILO METERS
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NB/V iV1EX3C0 i'^,^ N- —: - - -\7^ - -i - "EXAS ^ ^ -^ ' ^ ^ 41 'aU£5CCCX
Figure 3.--Rainfall map, in centimeters, of the Southern High Plains. Modified after Reynolds, 1956; Orton, 1966. have been made. George and Hastings (1951) related the concentrations of nitrate detected in the ground water to the geology of the Southern High Plains. Goolsby (1975) and Reeves and Miller (1978) associated the concentration of nitrate, chloride, and dissolved solids to the type of soil overlying the area and to the location of Cretaceous rocks underlying the aquifer. Geochemical studies by Barker and Scott (1958) noted that the concentration of chloride, sulfate, and bicarbonate observed in the water was dependent upon the type of material directly overlying the aquifer and that uranium increased in the direction of flow. Bassett (1973) and Bassett and Wood (1978) determined that the concentrations of silica and flouride detected in the water are probably controlled by the solution of amorphous silica and calcium flouride.
Several maps depicting the concentrations of various ions within the ground water have been prepared for various areas in the Ogallala Aquifer (Goolsby, 1975; Nichols and others, 1976, 1977; McReynolds, 1978a, 1978b; Reeves and Miller, 1978; Uranium Resources Evaluation Project, 1978, 1979a, 1979b; Warren and Nunes, 1978).
Geology
Rocks ranging in age from the Paleozoic to the Cenozoic crop out in the Southern High Plains. Subsurface sediments in the study area range in age from the Precambrian to the Paleozoic 8
(Fig. 4). The lithologies and environments of deposition relevant to this report begin with rocks from the Permian System which are the oldest rocks that subcrop beneath the Miocene-Pliocene age Ogallala Aquifer system. Sediments older than this time period are beyond the scope of this study and will not be discussed further.
Permi an
Permian sediments were deposited in a restricted marine environment and are approximately 2440 meters thick (Nicholson, 1960; Cronin, 1961). The rocks underlie all of the Southern High Plains and are divided into four series. At the base of the Permian is the Wolfcamp Series and the rocks consist of limestone and dolomite. Overlying the Wolfcamp is the Leonard Series and the sediments are composed predominantly of dolomite. The Guadalupe Series overlies the Leonard and the rocks in the Guadalupe are composed of dolomite, anhydrite, sandstone, and shale. At the top of the Permian is the Ocha Series and the sediments are composed of anhydrite, sandstone, and shale.
Triassic
In the Southern High Plains, Triassic sediments rest unconformably on Permian rocks (Fig. 5). The rocks in the Dockum Group were formed during late Triassic time in a continental environment as a thick flood plain alluvial fan deposit (Adams,
A (/» O) •r— +-> c 3 O u (/) iim'i "O o c o ff' 100 Miles 150 Kilorn e t ers Figure 5.—Thickness of Triassic sediments in meters, within the Southern High Plains. Modified after McKee and others. 1959; Reeves, 1970c. 11 1929; Cronin, 1961). Sandstone, shale, and conglomerate with minor amounts of interbedded limestone and dolomite compose the Dockum Group. Cretaceous Cretaceous sediments were deposited in a marine environment and rest unconformably on Triassic material. The sediments were laid down over the entire Southern High Plains but due to subsequent erosion the rocks remain only in the central portion of the study area (Fig. 6) (Brand, 1953). Cretaceous rocks range up to 60 meters thick and are composed of sandstone, limestone, shale, and clay. Tertiary Tertiary sediments were laid down unconformably on the Permian, Triassic, and Cretaceous rocks during late Miocene through middle Pliocene time (Darton, 1898; Johnson, 1901; Frye and Leonard, 1957, 1959). The sediments were deposited in a continental environment by stream and eolian activity (Johnson, 1901; Frye and others, 1956; Frye and Leonard, 1957, 1959; Cronin, 1961; Frye, 1970). During late Cretaceous and early Cenozoic, uplifting, orogenic movement, and volcanic activity occurred in Colorado and New Mexico forming the Rocky Mountains. The area underwent a period of erosion with streams having their headwaters in the Rocky Mountains. These streams carried 12 E ^ P L A N A Tl 0 H C r e t a c 8 0 u $ NEW MEX Rocks TEXAS• Saline Lakes Lubbock Figure 6.- 13 sediment eastward and deposited coarse gravel in pre-Ogallala valleys trending east-west. Following this period, meandering streams deposited finer material in the divides between the valleys until a large flat plain of alluviation was formed (see Johnson, 1901; Frye, 1970). The area remained at equilibrium during late Pliocene allowing a thick soil zone to form (Frye, 1970). Rejuvenation of streams, probably due to renewed upwarping (Frye, 1970), ended the deposition of the Ogallala Formation. Subsequently, the streams re-entered the area and removed part of the coalescent plain leaving an area of uneroded sections of the plain (Johnson, 1901). The entire Ogallala Formation, first named by Darton (1898) from type localities in Nebraska, extends approximately 1290 kilometers north-south and 485 kilometers east-west (Fig. 7). Evans and Meade (1945) raised the Ogallala sediments to Group status in the Southern High Plains by dividing it into the basal Couch Formation and the overlying Bridwell Formation. The Strati graphic Commission did not acknowledge this division (U.S. Geological Survey in Keroher and others, 1966), therefore, the Ogallala material is considered to be a Formation in this report. The composition of the Ogallala Formation in the Southern High Plains is predominantly sand and clay with minor amounts of gravel. The sediments are frequently cemented with calcium carbonate locally called caliche (Frye and Leonard, 1959). 14 Figure 7.--Location of the Ogallala Formation. Modified after Frye, 1970. 15 Lithology of the formation varies both laterally and vertically within short distances. Individual beds or lenses of the sediments are not continuous over vast areas but tend to pinch out or grade into finer or coarser material. Buika (1979) reported that the cumulative thickness of clay within the Ogallala Aquifer ranges from 0 to 100 meters. He also noted that the cumulative thickness of sand, 0 to 140 meters, is generally greater than the total thickness of either clays or gravels. In the Ogallala Aquifer, clays generally thin to the east-northeast while both sands and gravels thin to the southeast (Buika, 1979). (See figures 8, 9, and 10). Highly altered volcanic ash beds of Pliocene age are scattered throughout the Ogallala Formation. The Pliocene ash beds, however, are difficult to distinguish from the Pleistocene ash beds. Caliche, defined by Howell (1957) as "gravel, sand, or desert debris cemented by porous calcium carbonate; also the calcium carbonate itself," occurs as stringers and nodules throughout the Ogallala Formation and as single to multiple layers at the top of the formation (Frye and Leonard, 1957). The caliche occurring in the uppermost part of the formation is resistant to weathering and forms the "caprock" of the Southern High Plains. In the Ogallala Formation the caliche formed as a pedogenic horizon under aggrading conditions (Brown, 1956; Reeves, 1970d). Brown (1956) reported that caliche was probably produced from carbonate sands and quartz which were brought into 16 C. I. ~ Pred* tar m i n«d M«t«rs O MILES 50 O KILOMETERS 75 Figure 8.—Distribution of clays (thickness) in the Ogallala Formation. Modified after Buika, 1979. 17 C. I. - Pr • d • t « r m i nad Meti MILES 50 O KILOMETERS 75 Figure 9.--Distribution of sands (thickness) in the Ogallala Formation. Modified after Buika, 1979. 18 C. I. = 6M«t art MILES 50 O KILOMETERS 75 Figure 10.--Distribution of gravels (thickness) in the Ogallala Formation. Modified after Buika, 1979. 19 the area from the west by wind and rain. The caliche source material was deposited on the soil surface. Calcium carbonate and silica gel from the sands were leached downward by percolating rainwater and deposited as a subsurface evaporite, primarily around roots and organisms. Reeves (1970d) indicated that lithification of the evaporite is probably interrupted by periods of heavy rainfall which causes extensive solution and redeposition of the evaporite. The sand and gravel deposits in the Ogallala Formation are composed of sedimentary, igneous, and metamorphic material (Foster, 1952; Reeves, 1980, personal communication). Foster (1952) reported that the lower Pliocene gravels are composed of chert, sandstone, limestone, quartz, orthoclase, and feldspar. Sands from this time period generally consist of quartz, chert, and orthoclase feldspar. Gravels of middle Pliocene age are composed of chert, limestone, saadstone, and caliche with minor amounts of quartzite and orthoclase feldspar while the sands consist of quartz, orthoclase feldspar, and numerous volcanic glass shards (Foster, 1952). Heavy minerals present in the Ogallala Formation are predominantly magnetite, garnet, hematite, epidote, and ilmenite (Foster, 1952). Minor amounts of sphene, hornblende, biotite, zircon, tourmaline, pyroxene, and leucoxene are present in the soils and sediments directly overlying the Ogallala Aquifer (Rogers, 1969; Davis, 1970; Wood, 1980, personal communication). 20 Quarternary Sediments deposited during the Quarternary consist of eolian sands, lacustrine deposits, and fluvial elastics of Pleistocene age (Evans and Meade, 1945; Reeves, 1963, 1972; Cronin, 1964) and windblown sediments, sand dunes, and valley fills, Holocene in age (Evans and Meade, 1945). Pleistocene sediments, zero to 40 meters thick (Frye and Leonard, 1957), are composed of sand, clay, diatomaceous earth, volcanic ash, and limestone. Volcanic ash beds, Pearlette ash, are usually present in the Tule Formation of Pleistocene age (Swineford, 1949). Material from the Holocene ranges in thickness from zero to 5 meters and consists predominantly of windblown sand and silt (See Evans and Meade, 1945). Hydrology In the Southern High Plains of Texas and New Mexico, saline ground water occurs in the sediments from the Wolfcamp Series (Handford, 1979), Clear Fork Group (Schneider, 1955), and Glorieta, San Andres, Yates, and Rustler Formations (Burnitt and Crouch, 1964) of Permian age. Potable ground water is usually present in the rocks from Santa Rosa Formation (Schneider, 1955) of Triassic age and in the sediments from the Paluxy Formation (Schneider, 1955; Burnitt and Crouch, 1964) of Cretaceous age. Ground water occurs primarily in sediments from the Ogallala Formation (White and others, 1940; Garrett, 1953; Schneider, 1955) of Tertiary age. 21 A comparison of the chemical quality of the ground water from these aquifers is shown in Table 1. Permian Ground water from the Permian is highly mineralized (Table 1) (Leggat, 1957; Cronin, 1961). Handford (1979) determined the potentiometric surface of ground water from the lower Permian and concluded that the general direction of flow in the area of the Southern High Plains was from west to east. He also noted that there was some evidence of vertical flow occurring, however, he was not able to determine if the water was migrating upward into overlying formations or downward. Triassic In the Triassic sediments, ground water occurs in the lower sandy phase which contains coarse sand with some gypsum and calcite crystals (Adams, 1929). The water is slightly saline to saline, as shown in Table 1 (Leggat, 1957; Cronin, 1961). Generally, ground water in the Triassic beds flows westward into the Midland Basin (Leggat, 1957). Garrett (1953) stated that the head is lower in the Triassic Aquifer than in the Ogallala Aquifer. Therefore, under natural, unpumped conditions no upward flow of water from the Triassic to the Ogallala should occur. 22 C\J ^1- ^ to o 00 o 00 to lO 00 00 00 CM o o o cn o o o o o VO o Q tn 00 cn CO C\J to to CM 00 CM cn 00 cn O CM o CM CM O CO t/1 LO o> LU < •o s- «o o O o o CO o o o o o CM 00 00 LO o CM o CM 00 CM CO CQ ro CM 00 CM CM CM to CJ CO 00 CM 00 t/O LO CO <1) E Q. O o CO CO O to O o o to I— to CO o CM to LO 00 I— 00 > o cy> ^ 00 00 rv 00 to CO CM to CM CM CM CM I—I Q) s- s; E cu I •«-> Q- OO fO o O CT> 00 VO 00 LO s: c_> o LO CM CM o to LO o o to to (/J CO CO LO LO CM LU a: cn CM CM CO CO CO CO fO _l LL. X CQ >- < cn on LU <: 00 cn I— t-H CO < I— VO LU CM 00 LO 00 00 cn Q t— to CO «!3- o 42 1 to m 67 2 CM CO ,00 0 ,80 0 ,90 0 ,99 0 CM ,42 0 =) Q CO ^ r^ o 2: CO LO CT> CM CM CTi CO to LO oc: 00 LO 06 9 58 0 10 0 CM 12 2 23 7 LU f— un CO 00 (O >- -J < CM o o o Cretaceous Water from Cretaceous rocks is slightly saline to saline (Table 1) (Leggat, 1957) and is thought to be recharged through the Ogallala Aquifer (Cronin, 1961). Cooper (1960) noted that ground water in Cretaceous sediments is in hydraulic continuity with ground water in the Ogallala Formation near Causey and Lingo in Roosevelt County, New Mexico. This phenomenon is also observed in southeastern Hale County, north-central Lubbock County (Cronin, 1961), and eastern Gaines County (Rettman and Leggat, 1966) in Texas. Brand (1953) suggested that a cavernous aquifer may exist in Cretaceous sediments below the city of O'Donnell in Lynn County, Texas. Tertiary Ogallala Aquifer is hydrogeologically isolated by the four boundaries segregating the Southern High Plains from the surrounding area and is continuous throughout the Southern High Plains. The water in the aquifer is under water table or unconfined conditions, that is, the upper surface of the saturated material is unconfined. Saturated thickness of the aquifer in Texas is shown in Figure 11. In the Ogallala Aquifer of the Southern High Plains, ground water moves in from the northwest to the southeast (White and others, 1940; Cronin, 1961) toward the eastern plains escarpment where natural discharge occurs through springs and seeps (Theis, 1937; Garrett, 1953; Hale, 1962; Cronin, 1961). 24 Figure 11.--Saturated thickness map, in meters, of the Ogallala Aquifer in the Southern High Plains of Texas. Modified after Wyatt, Bell, and Morrison in Wyatt, 1978. 25 Prior to the ground water development of the Southern High Plains, the average amount of natural discharge from the Ogallala was equal to the average amount of natural recharge, and the aquifer was in a state of equilibrium (Theis, 1937; Havens, 1966; Handford, 1979). The Ogallala Aquifer is recharged from the precipitation that falls on the Southern High Plains. Most of the rain does not reach the aquifer due to the high rate of evapotranspiration for the area and because the rain is unable to penetrate the ground's soil layer. Theis (1937) determined the average annual amount of natural recharge for the Ogallala Aquifer in the Southern High Plains to be 1.3 centimeters. Havens (1966) reported a 2.0 centimeters annual recharge for New Mexico. A minor amount of evaporation occurs from lakes whose bottoms are below the water table, although the majority of water is being discharged artificially by wells (Theis, 1937; Garrett, 1953). The increase in water being discharged from the aquifer while the amount of water recharging the aquifer remains constant has caused a decline in the quantity of ground water stored in the aquifer (Theis, 1937; Leggat, 1954a, 1954b). Quarternary Pleistocene sediments are not known to yield any water to wells in the Southern High Plains. Nor is ground water evident in material of Holocene age (see Leggat, 1957; Cronin, 1961). CHAPTER II CHEMISTRY OF THE GROUND WATER IN THE OGALLALA AQUIFER The ions discussed in this study are calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, and silica, and several of the minor ions, also present in ground waters for which chemical data was available, flouride, nitrate, uranium, arsenic, barium, boron, lithium, vanadium, and zinc. Contour maps of these ions were constructed by a standard contour package using trend surface analyses. Chemical analyses of ground water used in this study were determined by Los Alamos Scientific Laboratory (Los Alamos), New Mexico Bureau of Mines and Mineral Resources, New Mexico State Engineer Office, and New Mexico State Geological Survey (United States Geological Survey, New Mexico), Texas Tech University, Texas Department of Water Resources and Texas State Geological Survey (Texas Water Development Board), and Union Carbide Corporation, Nuclear Division (Bendix-Union Carbide Corp.) (microfiche 0001 and 0002). Water analyses from these sources were not used when a value of greater than 4,000 mg/1 dissolved solids were reported as it was assumed that they were obtained from another aquifer or represented pollution in the aquifer. The location of the wells used is depicted in Plate I. Generally, ground water from the area north of the Cretaceous subcrop in the Ogallala Aquifer (Fig. 6) has a lower concentration 26 27 of dissolved solids than water south of this zone (Plate II). In the northern section, dissolved solids in the water are generally less than 500 milligrams per liter (mg/1). Concentration of dissolved solids in the southern section are generally greater than 500 mg/1 and in several areas exceed 3000 mg/1. Calcium The concentration of calcium ions (Ca"''^) in ground water from the Ogallala Aquifer in the study area increases from the northwest to the southeast (Plate III). Calcium values in the northern section rarely exceed 50 mg/1, however, a few areas with high concentrations do exist. Higher concentrations of calcium occur in the southern section of the study area, where values may exceed 100 mg/1. In the southern section, areas with high calcium concentra tions overlie Cretaceous sediments and occur beneath sandy and loamy soils. Generally, the Ogallala Formation in these areas consists of moderately thick clay and sand deposits. Generally areas with high calcium values coincide with areas of high magnesium, sodium and potassium concentrations (Plates IV, V, and VI). Several areas with high concentrations also correspond to areas of high bicarbonate values (Plate VII) and, in the southern half, areas of high sulfate concentrations (Plate VIII). Areas with low calcium values tend to coincide with areas of high flouride values (Plate XI). 28 Magnesium Plate IV shows the distribution of magnesium ions CMg"*"^) in the study area. Areas having high magnesium concentrations, greater than 50 miligrams per liter, generally occur in the southern section of the study area. The majority of high magnesium values occur in ground water underlain by Cretaceous sediments and overlain by sandy and loamy soils. The Ogallala Formation in these areas consists primarily of moderately thick clay and sand deposits. Many areas with high magnesium concentrations coincide with areas having large concentrations of calcium, sodium, and potassium (Plates III, V, and VI). A few areas with high concentrations coincide with areas of high bicarbonate values (Plate VII) and, in the southern half, areas with high sulfate concentrations (Plate VIII). Sodium The concentration of sodium ions (Na"*") in the ground water is usually higher in the southern section of the study area than in the northern section (Plate V and XVI). In the northern section values seldom exceed 40 mg/1, although a few isolated areas above 100 mg/1 do occur. Sodium values for the southern section are varied; concentrations range from 40 mg/1 to greater than 400 mg/1. Most areas with high sodium concentrations occur beneath sandy and loamy soils and generally overlie Cretaceous rocks. The Ogallala Formation in these areas is primarily composed of moderately 29 thick clay and sand deposits. Generally all areas with high sodium concentrations correspond with areas of high calcium, magnesium, and potassium concentrations (Plates III, IV, and VI). The majority of high sodium values also coincides with areas of high sulfate, chloride, and flouride concentrations (Plates VIII, IX, and XI). Some areas with high silica and bicarbonate values (Plates VII and X) also correspond to high sodium concentrations. Potassium The distribution of potassium ions (K"'") in the ground water is depicted in Plate VI. In the northern section of the study area the average concentration of potassium in the water is approximately 5 mg/1, while the concentration of potassium exceeds 10 mg/1 in several areas in the southern section of the study area. High potassium values generally occur in water which is overlying Cretaceous sediments and underlying sandy and loamy soils. Normally these areas are composed of moderately thick clay and sand deposits. Most areas with high potassium concentrations correspond with areas of high calcium, magnesium, and sodium concentrations (Plates III, IV, and V). Areas with high chloride and sulfate values (Plates VIII and IX) also coincide with high potassium concentrations, Several areas containing high concentrations of potassium correlate with areas of high bicarbonate and silica values (Plates VII and X). 30 Bicarbonate Plate VII shows the distribution of bicarbonate ions (HCO^ )• The average concentration of bicarbonate is approximately 300 mg/1. In the northern section of the study area, high bicarbonate concentrations generally occur in areas composed of moderately thick clay deposits and thin sand and gravel deposits. Most high concentrations in the southern section, however, occur in areas consisting of medium thick clay and sand deposits. Several areas with high bicarbonate concentrations correspond with areas of high calcium, magnesium, sodium, and potassium values (Plates III, IV, V, and VI). A few areas with high bicarbonate concentrations also coincide with areas of high silica concentrations (Plate X). Sulfate In the Ogallala Aquifer sulfate ions (S04"^) generally increase in concentration from the northwest to the southeast (Plate VIII). Sulfate concentrations seldom exceed 100 mg/1 in the northern section of the study area, however, in the southern section sulfate concentrations may surpass 600 mg/1. Most high sulfate concentra tions occur in the aquifer in areas which are overlying Cretaceous rocks. In these areas the Ogallala Formation consists primarily of medium thick clay and sand deposits. The majority of high sulfate concentrations corresponds with areas of high sodium and potassium concentrations (Plates V and VI). 31 Several areas with high sulfate concentrations, in the southern section, coincide with high calcium, magnesium, and chloride concentrations (Plates III, IV, and IX). Chloride The dispersion of chloride ions (CI") in the study area is shown in Plate IX. Chloride concentrations in the northern section of the area seldom surpass 100 mg/1, although a few areas with high concentrations do exist. Most high chloride concentrations occur in the southern section where chloride values may exceed 300 mg/1 in several areas. Generally high concentrations of chloride ions occur in water which is overlying Cretaceous sediments. The Ogallala Formation in these areas is composed of relatively thin to moderately thick deposits of clay, sand, and gravel. Areas with high sodium and potassium concentrations (Plates V and VI) tend to correspond with the majority of high chloride concentrations. In the southern section, areas with high chloride values correspond to areas with high sulfate concentrations (Plate VIII). High concentrations of chloride also correspond with calcium and magnesium concentrations (Plates III and IV) in the southeast section of the study area. Silica The dissolved form of silicon is commonly termed "silica," referring to the oxide SiO^, although the actual dissolved species 32 is usually represented as H^SiO^. The concentration of silica in the ground water varies throughout the study area (Plate X). In several areas silica concentrations exceed 60 mg/1, however, the average concentration of silica is approximately 40 mg/1. High silica values generally occur in areas composed of medium thick clay and sand deposits. A few areas with high sodium and potassium values (Plates V and VI) coincide with areas of high silica concentrations. Several areas with high bicarbonate concentrations (Plate VII) in the southern half also correspond with areas of high silica concentration Flouride The distribution of flouride ions (F") in the ground water varies throughout the study area (Plate XI). High concentrations occur primarily in water located in the southern section of the study area. The Ogallala Formation in these areas is generally composed of moderately thick deposits of sand and clay. High concentrations of flouride tend to coincide with areas of high sodium concentrations (Plate V). Areas with high flouride values also correspond with areas of low calcium concentrations (Plate III). Nitrate Plate XII shows the distribution of nitrate (NO^") in the study area. High concentrations of nitrate predominantly occur 33 in the southern section and along the northeast edge of the study area. In areas of high concentrations, the Ogallala Formation is composed of medium thick clay and thick sand deposits in the northern section and moderately thick clay and sand deposits in the southern section. Most of the high nitrate concentrations occur in ground water which is underlying sandy and loamy soils. Uranium, Arsenic, Barium, Boron, Lithium, Vanadium, and Zinc The distribution of uranium is shown in Plate XIII. Plates XIV and XV depict the dispersion of arsenic, barium, boron, lithium, vanadium, and zinc in a small area in the northeast section of the study area. No conclusive correlations between the ions could be determined due to the lack of sufficient data. Concentrations of calcium, magnesium, sodium, potassium, sulfate, and chloride ions generally increase from the northwest to the southeast in the study area. The areas with high concentrations of these ions usually occur in the section of the study area which is underlain by Cretaceous sediments and overlain by sandy and loamy soils. The Ogallala Formation in these areas is generally composed of medium thick sands and clays. Contamination of the Ogallala water by oil and gas field brines may contribute significant quantities of calcium, sodium, sulfate, and chloride ions in the northeast section of the study area and calcium and chloride ions in several localized areas in the southern section. In a few localized areas. 34 large quantities of chloride ions in the ground water may be due to liquid wastes from animal feedlots. Concentrations of bicarbonate, silica and flouride ions are variable throughout the study area. High concentrations of these ions tend to occur in areas which are composed of moderately thick clay and sand deposits. The concentration of nitrate ions is generally high in the southern section and along the northeastern edge of the study area. These areas usually underlie sandy and loamy soils and the leaching of fertilizers from these soils may contribute large quantities of nitrate ions. In several localized areas, liquid wastes recharged into the aquifer from animal feedlots may supply large amounts of nitrate ions. CHAPTER III PROCESSES CONTROLLING THE COMPOSITION OF GROUND WATERS The chemical quality of ground water in an aquifer system is the result of a variety of processes which control the concentrations of the ions in the water. The processes which control the water's ionic composition are mineral solution and precipitation by both mass action and oxidation-reduction, ion exchange, adsorption or desorption, and ion filtration. The mixing of ground water from an aquifer with water from other formations by diffusion or convection may supply additional ions into the aquifer's ground water. Mineral Equilibrium by Mass Action Dissolution of rock forming minerals may account for the majority of dissolved solutes present in ground waters. A mineral will dissolve when it comes in contact with a solution which is thermodynamically undersaturated with respect to the chemical constituents composing the mineral. If the solution is super saturated with respect to the chemical components forming the mineral, the mineral may precipitate. The saturation index, a ratio between the ion activity product, Kiap, and the equilibrium constant, Keq, of a mineral is used to determine if the mineral will dissolve or precipitate. When the saturation index is equal to 1 the reaction is in a state of equilibrium; 35 36 a• = Y-m- where a. = activity of component i Y^ = activity coefficient of component i m^. = free molal concentration of component i. Activity coefficients of ions in dilute solutions, less than 7000 miligrams per liter of dissolved solids, can be determined by using an extended form of the Debye-HUckel equation: 2 Ti _ -AT Z^ \^z logv. = J; + B..0T I '• 1 + a°i B-p tz ' where Aj and Bj = temperature dependent constants Bj = temperature dependent function z. = charge of the ith ion a? = measure of the radius of the hydrated ion I = ionic strength. Ionic strength is defined as: 2 I = ^ s m^ z. where m. refers to the molality of each and every ion in solution. If the solution is highly concentrated the activity coeffic ients are determined by using the Mclnnes assumption. For an indepth discussion of this method see Truesdell and Jones (1974). The free molal concentration of an ion is determined by the 37 following equation. m^ = m-r - 2 m^ , free T complex where m_ is equal to the total molal concentration of the ion and "^complex ^^ equal to the molal concentration of the ion bounded in complex ions and ion pairs. The total molal concentration of an ion is determined experimentally. The molal concentration of complex ions and ion pairs is calculated from a series of equations defined by Berner (1971). The following is an example from Truesdell and Jones (1974) depicting the ion complexing for Ca +2 . The major ion - complexing and ion - pairing reactions are: Ca"^^ + OH" = CaOH"*" Ca"*"^ + HCO3" = CaHC03'^ Ca"^^ + CO^"^ = CaCO^^ +? -7 0 Ca^ + SO, = CaSO/ 4 4 The equilibrium constants for these equations are computed from Table 2: ^ ^ ^CaOH"^ "I a +0 X a Ca^^ OH" 38 TABLE 2 ION-PAIR AND COMPLEX ION DISSOCIATION CONSTANTS FOR THE MAJOR IONS IN THE OGALLALA GROUND WATER AT 25^0 AND 1 ATM. PRESSURE (-log Keq)* anions: HCO3' CO3-2 S04"^ OH" CI cations: Ca*2 1.26 3.20 2.30 1.23 Mg^2 .90 3.40 2.25 2.60 Na* -.25 1.27 1.06 - -2.00 K+ . 0.00 .83 - -.85 *Data modified from Kharaka and Barnes, 1973. 39 ^CaHCO^"^ K = ^ 2 a ,« X a Ca+2 Hco • ^CaCO 0 K. = I ^ a .o X a _7 Ca^ CO. ^ ^CaSO ° K. = 4 a « X a o Ca^'^ SO^"^ Setting activity equal to molality times activity coefficient the equations can be written as: ^1 ^OH- \a+2 ^Ca+2 m = CaOH \aOH+ ^2 'HCO3- \a+2 ^Ca+2 m CaHC03'- ^caHCO + •^ 3 ^3 ^003-2 \a+2 Yca^2 ^ 0 " Y CaCO ° \aC0 0 •^4 ^SO -2 \a+2 Yca+2 m = CaSO 0 \aSO ° ^ 4 The above equations may be combined to determine the free molality 40 Kiap _ 1 Keq • Dissolution of the mineral occurs when the ion activity product is less than the equilibrium constant; Kia£ ^ ^ Keq '• Precipitation of the mineral occurs when the ion activity product is greater than the equilibrium constant. Kia£ ^ Keq '• Equilibrium Constant The mineral disassociation reaction can be represented by the general statement: aA + bB = cC + dD in which the lower case letters are equal to the number of moles of the chemical species represented by the upper case letters. At equilibrium the reaction may be rewritten as: [C] ^ [D] ^ 41 in which Keq is the equilibrium constant and the brackets symbolize the thermodynamic activities or the effective concentrations of the chemical species. The equilibrium constant for a mineral disassociation reaction can be determined from the standard free energy change, G, of the reaction: AG = -RT In Keq where AG = (cG^. + dGQ) - (aG^ + bGg) R = gas constant (1.99 x 10'^ Kcal mole"^ deg'b T = absolute temperature in degrees Kelvin In = natural logarithm. The equilibrium constants for many of the common minerals are available from standard geochemical texts such as Krauskopf (1979) and are contained within the SOLMNEQ computer program (Kahraka and Barnes, 1973) and the WATEQ computer program (Truesdell and Jones, 1974). Ion Activity Product The ion activity product is determined by the activities of the ionic species present in the water: ^ [Ala [Bib The activity of an ion is determined by the following equation: 42 of Ca+2 Ca+2 free Ca'^2 ^^^^^ QaOH"^ CaHC03'^ CaC03O CaSO^o which can be rewritten as m .o Ca-*-2 total "'ca-'^ ^:^ee ^ '' "• \^+2 ( S^OH- + ^2^HC03- + ^3^C03-2 + VSO4-2 ^CaOH+ ^CaHC03+ '^CaC03O "'CaSO^o To compute the free molality of a specific ion, a series of steps or iterations must be performed. In the first step, the free molality of the ion is determined from the available data. This value is then used to determine the molality of the complex ions and ion pairs. New values for ionic strength and activity coef ficients are determined from the molal concentrations of the ion pairs and complex ions. The free molality of the ion is then recalculated using the new data. The new value for free molality is then compared to the previous value. If the new molality value agrees within a suitable value, usually 0.5 percent of the previous value, the iteration is stopped. Temperature Corrections The chemical reaction between a mineral and a solution is 43 dependent upon the temperature and pressure of the system. Equilibrium constants are determined for systems at 25 degrees Centigrade or 298.15 degrees Kelvin and one atmosphere pressure. In most potable ground water systems the pressure effects are small and are not needed to be corrected. The temperature of a system may not be at 25 degrees Centigrade, therefore, both the equilibrium constant and the ion activity product will have to be corrected. The van't Hoff equation is used to correct for the temperature difference: In K = I^+ C (eg or aip) RT ^ 1-1 where R = the gas constant (1.99 x 10""^ Kcal mole"' deg ) T = absolute temperature in degrees Kelvin C = a variable constant H = the change in enthalopy or the amount of heat given off or absorbed during the reaction. The change in enthalopy is defined as: AH = i: H products - z H reactants. To compute the equilibrium constant or ion activity product at a temperature other than 25 degrees Centigrade, the variable constant is solved for at T equal to 298.15 degrees Kelvin and K equal to the 44 value at 298.15 degrees Kelvin. Then T is changed to the temperature of the system and a new equilibrium constant or ion activity product is solved for. Whether a mineral will dissolve in ground water or be precipitated from it may be determined from the saturation index of the mineral. Gibbs free energy of the mineral must be known in order to determine the equilibrium constant for the dissociation reaction of the mineral. The activity product of the reaction may be determined from the activity coefficients and free molality of the individual ions composing the mineral. Activity coefficients of the ions can be described by equations dependent only upon the temperature and ionic strength of the ground water while equations representing the free molality of the ions must also take into account the formation of ion pairs and complex ions. Both the equilibrium constant and the ion activity product must be corrected for the temperature of the water when the temperature is not equal to 25 degrees Centigrade. Mineral Equilibrium by Oxidation and Reduction Oxidation or reduction, redox, occurs when an element loses or gains an orbital electron during a chemical reaction. An oxidation reaction can be represented by the general equation: aA + bB ^ cC + dD + ne" 45 where A and B = oxidized species C and D = reduced species a, b, c, d = number of moles ne" = number of electrons lost during the reaction. At equilibrium, the reaction is defined by the Nernst equation: Eh = EO + BI In Keq nF where Eh = redox potential E° = standard potential of the reaction at standard temperature and pressure R = universal gas constant (1.99 X 10-3 Kcal mole"^ deg"^) T = absolute temperature in degrees Kelvin n = number of electrons transferred in the reaction F = Faraday constant (23.06 Kcal volt" equiv." ). The standard potential of the reaction is defined by: n -AG ° E° =—n. nF where AG ° is equal to the free energy of reaction at standard temperature and pressure. 46 Ion Exchange Ion exchange is the replacement of ions contained on a mineral with ions from solution (Kunin, 1960; Helferich, 1962). When a mineral is immersed in ground water, the free ions in the water diffuse through a film surrounding the mineral and onto the mineral The ions replace ions with the same sign (+ or -) on the mineral. The replaced ions diffuse through the film surrounding the mineral and out into the ground water so that equilibrium is attained. The amount of exchange that occurs is dependent on the temperature, the physical and chemical properties of the ion exchanger, and the size and valence charge of the exchangeable ions in solution. Based on these factors, Kunin (1960) was able to derive a set of eight rules governing the exchange process (p. 12). 1. At low concentrations (aqueous) and ordinary temperatures, the extent of exchange increases with increasing valency of the exchanging ion: (Na+ Ca+2 AI+3 Th+4). 2. At low concentrations (aqueous), ordinary temperatures, and constant valence, the extent of exchange increases with increasing atomic number of the exchanging ion: (Li Na K Rb Cs; Mg Ca Sr Ba). 3. At high concentrations, the differences in the exchange "potentials" of ions of different valence (Na"*" versus Ca+2) diminish and, in some cases, the ion of lower valence has the higher exchange "potential." 4. At high temperatures, in non-aqueous media, or at high concentrations, the exchange "potentials" of the ions of similar valence do not increase with increasing atomic number but are wery similar, or even decrease. 5. The relative exchange "potentials" of various ions may be approximated from their activity coefficients--the higher the activity coefficient, the greater the exchange "potential." 6. The exchange "potential" of the hydrogen (hydronium, H3O+, ion) and hydroxyl ions varies considerably with the 47 nature of the functional group and depends on the strength of the acid or base formed between the functional group and either the hydroxyl or hydrogen ion. The stronger the acid or base, the lower the exchange "potential." 7. Organic ions of high molecular weight and complex metallic anionic complexes exhibit unusually high exchange "potentials." 8. As the degree of cross-linking or the fixed ion concen tration of an ion-exchange material is lowered, the exchange equilibrium constant approaches unity. Samuelson (1953) and Helfferich (1962) also derived a similar set of rules governing ion exchange. Adsorption Adsorption is a nonstoichiometric process in which electrolytes or nonelectrolytes are taken up by clay minerals or adsorbers. The removal of a solute from a clay mineral without being replaced by another solute is termed desorption (see Helfferich, 1962). Helfferich (1962) stated that in natural systems common adsorbers can also act as ion exchangers and that ion exchange and atomic adsorption occurs concurrently with one another. Kharaka and Berry (1973) reported that the amount of adsorption that can occur is mainly dependent on the ionic charge and size of the electrolyte. Generally, maximum adsorption should occur for an electrolyte which has a high ionic charge and a small ionic radii (Wayman, 1965). Ion Filtration Clay minerals may behave as semi-permiable membranes. As a 48 solution passes through a compacted layer of clay particles the charged species are retarded while the noncharged solutes pass through (Chow, 1965). This process is termed ion filtration and permits high concentrations of ions to exist on one side of the clay particles and dilute solutions on the other side. Kharaka and Berry (1973, p. 2591) derived several sequences of retardation for monovalent and divalent cations. The sequences tend to vary depending upon the temperature and the type of clay minerals present. For illite the monovalent and divalent retardation sequences are: Li < Na < K < NH < Rb < Cs, Mg < Ca < Sr < Ba. These sequences indicate that Cs and Ba are the most likely ions to be retarded. Kharaka and Berry (1973) also noted that monovalent cations are more likely to be retarded than divalent cations. At room temperatures, a consistent sequence for anions could not be determined, although several sequences were calculated for higher temperatures. Mixing by Diffusion Diffusion is the migration of ions from zones of high concentrations to zones of low concentrations (Fried and Combarnous, 49 1971). In an aqueous solution, the diffusion of dissolved solutes as a function of concentration can be described by Fick's first and second laws of diffusion (Crank, 1956), J = J£ -D8x — = D— at 3x2 where J = diffusion flux vector C = concentration D = diffusion coefficient t = time X = distance. The gradients driving the diffusion may also be a function of temperature or pressure differentials. Mixing by Convection Mixing by convection is the transport of solutes by and with flowing water from one aquifer to another. The mixing of two or more solutions with different chemical compositions results in the formation of a new solution. The ratio of mixing between the solutions and the chemical constituents composing the solutions controls the chemical composition of the new solution. The primary processes which control the ionic concentrations 50 of ground waters are mineral solution and precipitation by mass action, ion exchange, and adsorption or desorption. The other processes are generally not important in controlling the chemical quality of ground waters, however, they may act as the primary control or strongly influence the water chemistry in some aquifer systems. CHAPTER IV GEOCHEMISTRY OF GROUND WATER IN THE OGALLALA AQUIFER Introduction The chemical characteristics of natural waters are derived from the interaction of solids, liquids, and gases with the water at various stages of the earth's hydrologic cycle. Most ground waters obtain their chemical compositions from the weathering and dissolution of igneous, metamorphic, and sedimentary rocks. The dissolved solutes in rainwater, derived from constituents contained in sea spray, dust, and the atmosphere, also contribute to the chemical composition of the ground water (Figs. 12, 13, 14, 15, 16). The two major natural sources of solutes in the ground water of the Ogallala Aquifer are the dissolution and weathering of rock forming minerals from the Ogallala Formation and overlying Pleistocene deposits and the addition of solutes from strati- graphically lower formations. This latter source is dramatically illustrated in the southern section of the Southern High Plains where high concentrations of dissolved solids are located near large saline lake basins. In the Ogallala Aquifer, mineral equilibrium is probably the major process controlling the concentration of solutes in the ground water. When the saturation index of a mineral is greater than one the mineral may be precipitating. If this occurs the ions necessary for the formation of the mineral will be depleted from the 51 52 V Ca"^"^mg/liter o Averoq* July 1955 — June 1956 Figure 12.—Average calcium concentration in rainwater in tKe conterminous United States. After Junqe and Werbv 1958. 53 Uter 1955 956 Figure 13.--Average sodium concentration in rainwater in the conterminous United States. After Junge and Werby, 1958. V\K"mg/liter average July 1955 — June 1956 Figure 14.—Average potassium concentration in rainwater in the conterminous United States. After Junge and Werby, 1958. 54 mg / Liter ^vera9« July 1955 - Jun* 1956 Figure 15.--Average sulfate concentration in rainwater in the conterminous United States. After Junge and Werby, 1958. -A CI. mg/Liter " " Avwaqe Juty 1955 - Jun« 1956 Figure 16.--Average chloride concentration in rainwater in the conterminous United States. After Junge and Werby, 1958. 55 ground water. Even if the mineral is not precipitating, further dissolution of the mineral cannot occur. When the saturation index of a mineral is less than one, dissolution of the mineral may occur. If the mineral dissolves, the ground water will become increased in the ions composing the mineral. Oxidation is a significant factor controlling the water chemistry of ions possessing more than one oxidation state in aquifers. The water in the Ogallala Aquifer contains free oxygen and therefore is an oxidizing environment. Consequently, if carbon, nitrogen, sulfur, iron, manganese, uranium, and other multivalent ions are present in the ground water they will occur in their most oxidized states. Ion exchange and adsorption may act as a minor influence on the solutes present in the Ogallala water. Ion exchange is probably not important because of the history of the clay minerals. The clay minerals were probably formed in a continental environment, therefore, all their exchange sites were initially saturated with calcium and magnesium ions. Because the dominant cations in the ground water are calcium and magnesium only a minor amount of cations on the clay minerals will be replaced by ions from the ground water. Ion filtration is assumed to be insignificant in controlling the water chemistry in the Ogallala Aquifer. The theory of ion filtration explains abnormally high brine concentrations in isolated geologic basins (De Sitter, 1947; White, 1965) and high 56 concentrations of dissolved solids in some potable aquifer systems (Wood, 1976). There is no evidence of either phenomenon occurring in the Ogallala Aquifer. The mixing of water by convection and/or diffusion may be strongly influencing the water chemistry in the Ogallala Aquifer. Mixing by convection may occur in areas where water from strati graphically lower formations flows into the Ogallala Formation due to the difference in head. The solutes in the Ogallala water, originating from the dissolution of minerals, will be altered depending upon the amount and chemical composition of the introduced solutes. A quantitative geochemical evaluation of mixing can be made, if the amount and components can be identified, using the computer program MIX2 written by Plummer and others (1976). Mixing by diffusion may occur in areas in which mixing by convection could not occur due to the difference in head. However, diffusion would only be detectable in areas in the Ogallala Aquifer which have low hydraulic conductivity. The movement of water and solutes is more rapid than diffusion in areas with high hydraulic conductivity, therefore, even though diffusion may be occurring the solutes are moving away and it cannot be detected. Diffusion of solutes into the Ogallala Aquifer may be an important factor influencing the water quality of the ground water in a few localized areas where a chemical gradient exists between solutes in lower formations and those in the Ogallala Formation. 57 Mineral Equilibrium Saturation indexes for 85 minerals were calculated by the mineral equilibrium program SOLMNEQ (Kharaka and Barnes, 1973) which takes into account the ionic strength and temperature of the water and the concentration of ion pairs and complex ions present in the water. Three hundred water analyses were evaluated using this program in an attempt to define the minerals which may be controlling the solute concentration in the ground water. Only complete chemical analyses in which the potassium concentration was reported were used. A dummy value of .02 mg/1 was used for the aluminum concentration to enable the determination of saturation indexes for clay minerals. The dummy value is the average of 100 analyses in which the aluminum concentration was determined by Union Carbide Corporation, although complete chemical information was not available for these analyses. Recorded in Table 3 are the saturation indexes for 16 common minerals determined from typical water samples from Hale County, in the northern section of the study area, and from Dawson County in the southern section of the study area. A third sample, representative of water chemistry near a saline lake, was taken from Terry County in the southern portion of the study area and is included in Table 3. Calcium Calcium occurs in the Ogallala water as the ion Ca'^2^ j^e calcium ions present in the ground water were probably derived from 58 >> fO +-» —I I CM LO cr 1 1 LO CO o o O <_o3 f- iii O O o >) c o •»-> 4-> c u CM CM 3 O) o 1 1 LO CO O CyO o o o o C/1 O —J c s- a X X X X O CD fO o LO 00 CO o LO r«v LO CT» CO x: .1- CO LO 00 O fO 3 cn o Q O CM CO 00 CM 1— LO oo CO o CC CO O E o) cr CO 00 X 3 oo OJ I I LO CO UJ o i.:: o o o a 00 00 CM 00 O o <0 (O o ^—*• CVJ CM CO U I— o CO •r— 31 LO 00 CM •I- 3 •1— 00 o o o E E CM CO o • CM CO oo *'~-^ '^ (U s. f^ ^— CM CM -c o o CM CT CM O r^ CO CO ca: <: o U. 00 CJ c_> u. (O lO (O •ro— O CM •f— s (O cC CO ro ro ro z o o CO OO CJ CJ CJ "Z. 5 ^ cu 4cJu 4-> 2 >, •r— 'r— *—" o cu c CU 4-> CU ^— o rO ro rO cu 0) *'— (U XJ -Q -C J3 ro •M ^.J .c 4-) +J r- -M a> cu u S- c/1 ro CO .Q Miner s bit e to Ic i al e o o lo m LO 1 o 10 " o X X X CM o CO CO 00 o CO CT> 00 00 CO I o O X I— LO 00 LO LO X3 CU O CJ LO I CO O o X CQ CO o LO CT> o o LO CU CO 3 •r- 3 O i~ -r- O O CJ N ro CX ro Er- 4J CJ S_ tj ro +J ro t. -f— O -r— , -M C CJ «a r— E f— CU ro O 3 •!— <0 •!— CT) Q €2*00 oo * 60 the dissolution and weathering of calcite, plagioclase feldspar, albite and anorthite, and minor amounts of dolomite: CaC03 + CO2 + H2O = Ca"*"^ + 2HCO3 2NaAlSi30Q + 2CO2 "^ 2H2O = Al2Si^OTo(OH)2 + 2Na+ + 2HCO3 + 2H4Si04, 2CaAl2Si20g + 400^ + 6H2O = Al2Si40"|Q(0H)2 + 2Ca'^^ + 4HCO3 + 2A1(0H)3: CaMg(C03)2 "^ ^^2 "^ ^^2^ " ^^^^ '*' ^^^^ "^ ^^^°3' The Ogallala ground water is supersaturated with respect to these three minerals (Table 3). Even though the water is supersaturated with the ions composing plagioclase feldspar and dolomite, these minerals are probably not precipitating because of the lack of other conditions in the system which are necessary before precipitation can occur (Mason and Berry, 1968; Deffeyes and others, 1967; IIling and others, 1967). Calcite may be precipitating throughout the study area or at least not dissolving and may be the primary factor controlling the calcium concentration in the Ogallala water. 61 Magnesium Magnesium is present in the Ogallala water as the charged Mg''"2 ion. Dissolution of dolomite and the weathering of ferromagnesium minerals has contributed magnesium ions into the ground water. The weathering of olivine is an example of such a reaction. MgFeSiO^ + 4CO2 "^ ^^2° " ^^^^ ^ ^^"^^ ^ H4SIO4 + 4HCO3'. The ground water in the Ogallala Aquifer is supersaturated with respect to dolomite (Table 3). Dolomite may not be precipitating due to the lack of other conditions in the aquifer necessary before precipitation can occur (Deffeyes and others, 1967; IIling and others, 1967). However, because dolomite is supersaturated, the water is not dissolving any dolomite which may be present in the aquifer. Sodi um Sodium occurs in the Ogallala ground water as the ion Na"*". The weathering of plagioclase feldspar, previously described, has probably contributed a large portion of the sodium ions present in the ground water. The ground water is supersaturated with respect to plagioclase feldspar, therefore, dissolution of plagioclase is no longer occurring (Table 3). Even though the ground water is supersaturated with the ions necessary for the formation of plagioclase feldspar, plagioclase is probably not precipitating due 62 to the lack of other conditions in the aquifer necessary for the precipitation of plagioclase (Masson and Berry, 1978). The Ogallala water is undersaturated with respect to the mineral halite (Table 3), therefore, the mineral equilibrium of halite is not limiting the concentration of sodium in the ground water. Potassium Potassium occurs in the Ogallala ground water as the ion K . Weathering of potassium feldspar and biotite are probably the major sources of potassium ions in the ground water: 2KAlSi30g + 2CO2 + IIH2O = 2K'^ + 4H4Si04 + 2HCO3" + Al2Si205(0H)4; 2KMg3AlSi30io(OH)2 + 14C02 + I5H2O = 2K'^ + 6Mg'^^ + 4H4Si04 + I4HCO3- + Al2Si205(0H)4. Precipitation of potassium feldspar and biotite is not controlling the potassium ion in the ground water. The water is supersaturated with respect to microcline, therefore, further dissolution of this mineral cannot occur. No thermodynamic data is available to evaluate biotite. Incorporation of potassium ions in clay minerals may have a minor influence on the potassium concentration in the 63 ground water (Garrels and Mackenzie, 1967). Bicarbonate In water from the Ogallala Aquifer, the element carbon occurs predominantly as the bicarbonate ion, HCO3". Dissolution of carbonate minerals, previously discussed, and carbon dioxide, which is brought into the aquifer from the overlying soils by rainwater, are probably the primary sources of bicarbonate ions in the ground water. The dissociation of carbon dioxide can be represented by the following three equations: CO2 + H2O = H2CO3, H2CO3 = H"^ + HCO3-, HCO3" = H"^ + ^03'^. The percentage of undissociated carbonic acid, bicarbonate ions, and carbonate ions in ground waters is dependent upon the pH of the water (Fig. 17). As depicted in Figure 17, at certain pH values, undissociated carbonic acid and bicarbonate ions or bicarbonate ions and carbonate ions will exist together in ground waters in varying percentages. In waters with a pH value of 8.3 it is possible for all three carbon species to occur. 64 (O -o ro O O LO CM 4-> 3 O CO LO (U a. to cu -o •r— X o •r- T3 C; CO o r^ JO CT> s- ^— ro CJ fs E -o cu cu ^ > ^— s- o cu to +J to «4- — <: 03 OI +-> Q. o -M Cf_ O O C O t/» •!- CU 4-> C7> U ro C +-> 3 C M- (U CJ ro S- CU CO Q- ro I NOiinaiblSlQ 30>rj.N33a3d CU s- 3 65 Sulfate In the water from the Ogallala Aquifer, the element sulfur forms the anion sulfate, SO."2. Reduced forms of sulfur, H2S and HS" are probably not present in the aquifer due to the presence of dissolved oxygen in the water. Consequently, any reduced forms of sulfur ions will be oxidized into sulfate. As shown in Table 3 the ground water in the Ogallala Aquifer is undersaturated with respect to the common mineral gypsum, therefore, precipitation of the gypsum mineral is not controlling the concentration of sulfate ions in the ground water. Chloride In the Ogallala water, the element chlorine occurs as the chloride ion, CI". As shown in Table 3 the ground water in the Ogallala Aquifer is undersaturated with respect to the common mineral halite, therefore, precipitation of halite is not controlling the chloride ions in the ground water. Although chloride can occur in some igneous and metamorphic rocks the weathering of this material could not act as a limit on the concentration of chloride ions in the ground water due to the rarity of chloride in these rocks. Silica In the Ogallala Aquifer, the dissolved form of the element 66 silicon is usually represented by the uncharged H4Si04 molecule. Weathering of silicate minerals, plagioclase and potassium feldspars, previously discussed, is probably the predominant source of silica in the ground water. As shown in Table 3 the ground water in the aquifer is supersaturated with respect to many common silicate minerals, therefore, these minerals are no longer supplying silica into the ground water. Mineral equilibrium of amorphous (am.) silica or silica gel appears to be the dominant factor controlling the silica concentration in the ground water (Table 3). H4Si04 = Si02(am. or gel) + 2H2O. Both amorphous silica and silica gel are slightly undersaturated with respect to the ground water and may be dissolving in some areas of the aquifer and precipitating in others. Flouride In the Ogallala water the element flourine forms the anion flouride, F". Dissolution of calcium flouride salts is probably the major source of flouride ions in the ground water, CaF^ = Ca"^^ + 2F". The mineral flourite is slightly supersaturated in some areas of the aquifer and undersaturated in others (Table 3). Precipitation 67 and dissolution of flourite is probably the major factor limiting the concentration of flouride ions in the ground water. Nitrate In the Ogallala water, the element nitrogen generally occurs in its most oxidized state as the anion nitrate, NO3". There are no known nitrate minerals in the Ogallala Aquifer and it is unlikely that any exist due to their extreme solubility. The equilibrium of nitrogen bearing minerals is assumed to have no control over the nitrate ions in the ground water. Uranium, Arsenic, Barium, Boron, Lithium, Vanadium, and Zinc In the Ogallala water uranium generally occurs in its highly +2 oxidized state as the uranyl ion, UO2 , arsenic primarily occurs -2 as hydrated arsenate, H2ASO4 below a pH of 7.2 and HASO4 above a pH of 7.2, boron generally occurs as orthoboric acid, H3BO3, and vanadium generally exists in its anionic form of V *". The alteration of Cenozoic volcanic material is assumed to be the primary source of these seven elements in the Ogallala water. Izett and others (1972) and Block and Johnson (1980) noted appreciable amounts of uranium, barium, and zinc contained in some volcanic ash deposits in the Southern High Plains. The minerals controlling these elements in the ground water could not be determined due to the lack of complete chemical data for the analyses. Mineral equilibrium studies, dealing with areas outside the Southern High 68 Plains, have been done on uranium (Langmuir, 1978), arsenic (Sill^n and Marten, 1964, Wedepohl, 1978), barium (Chow and Goldberg, 1960; Sill^n and Martell, 1964), boron (Goldberg and Arrhenius, 1958; Oborin and Zalkind, 1964), vanadium (Krauskopf, 1956; Evans and Garrels, 1958; Garrels and Larsen, 1959), and zinc (Jurinak and Thorne, 1955, Krauskopf, 1956; Takahashi, 1960; Sill^n and Martell, 1964). Mixing of Saline Water with the Ogallala Ground Water The mixing of saline water with the Ogallala ground water is probably one of the major processes controlling the chemical composition of the ground water in the southern section of the study area. Beneath large saline lakes (Fig. 6), the ground water contains high concentrations of calcium, magnesium, sodium, potassium, sulfate, chloride, and silica (Plates III, IV, V, VI, VIII, IX, and X). There are at least two possible explanations for this occurrence. The first hypothesis is that solutes from the underlying Cretaceous sediments (Table 1) are mixing with the Ogallala water adjacent to the saline lakes. Under most of the saline lakes the Ogallala Formation is absent and the lakes rest on Cretaceous rocks (Reeves, 1970a). The top of the water table in these areas coincides with the top of the Cretaceous rocks. If water in the Cretaceous sediments is at the same hydraulic head as the Ogallala water, the hydraulogic continuity between the formations would 69 allow only very little flow into the Ogallala Formation (Fig. 18a). However, evaporation of water from the lakes causes a decrease in the head, thereby allowing the water and solutes from the Cretaceous aquifer to flow into the Ogallala (Fig. 18b). Another hypothesis is that the solutes in the lake water are derived from the evaporation of Ogallala water (Fig. 18c). Ground water containing solutes from the Ogallala Formation may move laterally into the lake basins. Then, due to evaporation, the water in the lakes may become concentrated in the ionic species present in the original water (Table 4). Seepage of this highly concentrated water back into the Ogallala ground water, because of increased density, could result in the observed increase of these ions in the ground water located adjacent to the saline lakes. An argument against this latter hypothesis is that in some of the saline lakes large amounts of sodium sulfate are being precipitated (Reeves, 1963), and as the Ogallala water contains very small amounts of sulfate ions, even after evaporation of the water, it is not probable that the sulfate concentration would be high enough to allow for precipitation of sodium sulfate. Therefore, it appears that the solutes in the saline lakes came from the Cretaceous rocks. It is possible that diffusion is the mechanism by which solutes from the Cretaceous flow into the Ogallala. Even if the difference in head will not permit flow of the Cretaceous water, solutes may move into the Ogallala Aquifer across a chemical gradient between the two aquifers. 70 PetantionMtrie turfac* of K aquifar Ogallala Ogollala 18a. No evaporation of lake water. •nttomatne turfac* of K oquifar Ogallala Ogallala 18b. Evaporation of lake water, inflow of Cretaceous water. Ogallala 18c. Evaporation of lake water, inflow of Ogallala water on up stream side of the lake. Figure 18.--Schematic representation of a saline lake in the southern section of the study area. 71 oo ,20 0 ,36 0 ,40 0 Q ,00 0 1— o CM LO LO CO n: ^ LO CM LO • • • • r>. r*-. r>v r>. o O>0O l r- CM CTt 00 CM cn rt oo ^ CO CM CO 1 o O o o '^^ o CM CO o <;!- O CO 00 ^T A rt o oo «« Cd CQ CC LU I— o •=^ o o < r— CO LO oCO CO CO «!:^ LL. o CTt CM CM o CO LU 00 O 00 O O >- O CM O CO .J (O CO CTv CM < CO o <: CJ CM O LO o CM LO LO to CO 00 cu > cu CJ cu CC E CM O CM CO o O O s- ro O CM ^- CJ X3 cu >^ >-i >» CU -o •M (U r— o C r— .^ s- 3 •r- tJ O (O (U ro CQ o o oc ro O 72 To positively confirm that the water in the saline lakes is Cretaceous in origin a study of the ionic concentration of the ground water surrounding the lakes must be done. If the water in the lakes is due to the difference in head between the two aquifers then the ground water surrounding the lake should be highly concen trated. However, if the water in the lakes is due to evaporation and subsequent concentration of ions in Ogallala water only ground water from the down stream of the lake should be highly concentrated. Isotopic studies would probably answer this question definitely as the ^H and ^^0 values would be expected to increase in a system undergoing evaporation. Ion Exchange and Adsorption In the Ogallala Aquifer, ion exchange and adsorption are probably influencing the ionic composition of the ground water. However, a comparison between the location of clay deposits (Fig. 8) and the distribution of calcium, magnesium, sodium, and flouride ions (Plates III, IV, V, and XI) did not reveal any major areas in which ion exchange and adsorption are occurring. The reason for this may be that the exchange sites are saturated with calcium and magnesium due to the continental origin and transport of the clays. The effect of ion exchange appears to be most noticeable in areas of the Ogallala Aquifer which are directly underlying large saline lake basins which may be receiving large amounts of sodium ions (Fig. 6). In areas of the Ogallala Aquifer adjacent to saline 73 lakes the ratio of calcium or magnesium to sodium ions is generally greater than in the saline lake waters. The increase of calcium and magnesium ions in the ground water is probably due to the exchange of sodium ions being introduced into the ground water for calcium and magnesium ions on the clay minerals. This ratio cannot be explained by evaporation of the lake water followed by precipita tion of calcium carbonate and magnesium salts could explain the observed ratios. The ratio of magnesium to calcium ions appears to be slightly greater in the saline lake water than in the nearby ground water (Table 5). The ratio of magnesium to calcium increased in the saline lake water probably because of a reduction of calcium ions due to the precipitation of calcium carbonate and the lack of precipitation of any known magnesium minerals (Zherebtsova and Vokaova, 1966), therefore, the increase of calcium and magnesium ions in the ground water near the saline lakes cannot be explained by evaporation of the lake water and subsequent precipitation of calcium carbonate and magnesium salts in the saline lake basins. Mass Balance Mass balance in a closed aquifer system may be defined by the general equation (Miller and Drever, 1977), Original material + Atmospheric input = Solid weathering products + Ions in solution. 74 ro ro r— CO 1— . r— ^— CM ro Ol 0 CQ ro OC CJ &- 0 cu M- •r" CM CO LO LO A a r". • CM ro LO CM LO 0 <: 00 0 to -K 0 LLJ CC ^ LU •a c ro ro ro , LU CO LO 0 2: OC 0 I—I LU ro 0 _l Q. O CU 00 00 sz • ro 0 &- •1^ CU O LU ^- -M OC _i CU 0 ro Ll. <: &- •s. CTt CO Qi t-H tj •^ CU LU => ro E ^ f— cr 00 ro The chemical evolution of an aquifer system may be determined by this equation. In an aquifer, the original rock-forming minerals react with the atmospheric input to produce solid weathering products and dissolved ionic species. Other factors, such as ion exchange, adsorption, mixing, etc., may also influence the dissolved species in the water. Plummer (1977) stated that in order to obtain precise results using this method, the exact mineralogy and hydrology of the aquifer along with the chemical analyses of the water must be known. He also noted that in diversified mineralogical systems, it is frequently not possible to determine the true weathering reactions which are occurring because of the lack of detailed mineralogical data. An attempt was made to determine the probable dissolution and weathering reactions which occurred in the Ogallala Aquifer. Unfortunately detailed mineralogical and isotopic data for the aquifer is not available and consequently only general reactions could be determined. Northern Section of Study Area The original rock-forming minerals assumed to be present in the Ogallala Formation were reacted with the rainwater chemistry reported by Junge and Werby (1958) and carbon dioxide, derived from the overlying soils, to produce the dissolved solutes and weathering products presumed to be forming in the aquifer (Figs. 19, 20, 21, Table 1). The above method is based on a modification of the 76 3 3 <' 1 - Hal* 2 - Oowton I County I ogjH^SiO^T Figure 19.--Stability relation of muscovite, microcline, kaolinite, gibbsite at 25^0 and 1 atm. as a function of K"*", pH and H4Si O4. After Tardy, 1971. u- 12- ^S^^ AI b'j f e ' 10- kr"*><*^ ' 8' X. 1 ^^^i^^^ 1 X^la-Mentmorf llonil* 6- |\ '. 1 1-Hole , N. '••' , County •Cool 1 n te >v Gibbsit* 2- ' >JS 2-C>aw»on 1 1= County 0« i/> r g3-T.r,y -2. '-' 'x County -4 ^ -5 -4 -3 loB[H4SiO^'] Figure 20.--Stability relation of albite, Na-montmorillonite, kaolinite, gibbsite at 25^0 and 1 atm. as a function of Na+, pH and H4Si04. After Tardy, 1971. '0^23 I-Hal* County 2- Dowion County log CH4S'04"] Figure 21.--Stability relation of anorthite, Ca-montmorillonite, kaolinite, gibbsite at 25°C and 1 atm. as a function of Ca'^^^ pH and H4Si04. After Tardy, 1971. 77 process used by Garrels and Mackenzie (1967). The resulting ionic composition of the water was then compared to an average of 500 chemical analyses of the ground water from the northern section of the study area (Table 6) to determine the validity of the reactions. The reactions and products are depicted in Table 6. Due to the lack of detailed mineralogical data the ratio of potassium feldspar to biotite weathered was varied, 75% to 25%(1), 66% to 33%(2), 50% to 50%(3). In step one, potassium feldspar was weathered to produce potassium ions, bicarbonate ions, dissolved silica and kaolinite (Fig. 19). Next, biotite was weathered to produce potassium, magnesium, and bicarbonate ions, dissolved silica and kaolinit'e (Fig. 19). In step three, plagioclase feldspar weathered to produce sodium, calcium, and bicarbonate ions, dissolved silica and calcium montmorillonite (Figs. 20 and 21) (Garrels and Mackenzie, 1967). Next, due to the increase in the silica concentration above the concentration present in the water, silica was precipitated from the water. Calcite was dissolved, in step five, to produce calcium and bicarbonate ions. In step six, dolomite was dissolved to produce calcium, magnesium, and bicarbonate ions. The exact amount of dolomite which is dissolving is not known due to the lack of complete mineralogical data. Only a small amount of the magnesium ions, 10%, are believed to be produced from the dissolution of dolomite. In step nine, calcium carbonate was precipitated from the water to account for the additional calcium ions produced from step eight. The resulting chemical composition was then compared to 78 TABLE 6 SOURCE MINERALS FOR GROUND WATER IN THE OGALLALA AQUIFER (moles X 10"3/liter) Concentration of Rainwater (RW) Step 1 Weathering of potassium feldspar a) .10KAlSi30g + .IOCO2 + .5OH2O = .lOK"^ + .20H4Si04 + .10HC03" ^ .05Al2Si205(0H)4 b) .09KalSi30Q + .O9CO2 + .45H2O = .09K+ + .18H4Si04 + .O9HCO3" + .045Al2Si205(0H)4 c) .07KAlSi30Q + .O7CO2 + .35H2O = .07K"^ + .14H4Si04 + .O7HCO3" + .035Al2Si205(0H)4 Step 2 Weathering of biotite « a) .03KMg3AlSi30^Q(0H)2 + .2ICO2 + .225H2O = .03K"^ + .09Mg"''^ + .06H4Si04 + .2IHCO3" + .015Al2Si205(0H)4 b) .04KMg3AlSi30^Q(0H)2 + .28CO2 + .3OH2O = .04K'^ + .12Mg'^2 + .08H SiO. + .28HCO3" + .02Al2Si205(0H)4 c) .06KMg3AlSi30^Q(0H)2 + .42C02 + .45H2O = .06K+ + .18Mg+2 + .12H Si04 + .42HC03" + .03Al2Si205(0H)4 Step 3 Weathering of plagioclase feldspar 1.50Na ggCa,33AlT 33Si2.6608 ' ^'^^^2^ ^ ^•^^^°2 = '^^^^ + .35Ca'*'2 + .83H4Si04 + I.69HCO3" + -SeCa .,g^Al2 33Si3 g^ 0^o(OH)2 Step 4 Precipitate silica .69H4Si04 = .69Si02 + I.38H2O Step 5 Dissolution of calcite .0 .82CaC03 + .82H2O + .82CO2 = .82Ca + I.64HCO3" Step 6 Dissolution of dolomite .0 .lCaMg(C03)2 + .2H2O + .2CO2 = .lCa^2 + j^g"''^ + .4HC03' 79 TABLE 6 (Continued) Step 7 Precipitation of calcite .lCa+^ + .2HCO3- = .lCaC03 + .ICO2 + .IH2O Step 8 Average water analysis from northern section of the study area Step 9 Difference between calculated water composition and average water analysis 80 CO r- 00 O o O o :»^ CM I— O o o o o CO ,— I— O o o o o o o O o o o o + ro CM f— ,— o o o o o ZOO o o o o o 4- o o o o o o o o o o •o CU CO o o 00 o o 00 00 00 00 00 LO CM CM 00 o CM CJ -•- cncM o o o CM CM CM CM CM CM LO o CVJ oo LO CTt CQ o o cn C3^ o o CT» cr» cn CT> LO LO -=c o o o o (Tt cn LO LO LO cn CO CO CO o o o o CM CO CM CM o CM -»- ro CM LO LO CO CO CO CJ O CO O O CM CO CM CM Oo LO LO LO CO CO CO CO o o o o CM CO CM CM o r— CM CO LO LO CO CT> a. a. QL a. CL a. Q. cu cu O) cu cu •M 4-> cu +-> 4-> +-> cu +-> cu ai OO oo oo oo oo oo oo oo oo 81 CO Or-CMCD'**-*:j-^^^0 o ••- CM ocotocnoooooo OO S'^'^c^oooooo r— CO Or;:.CT>°0°OOsJOgCMCMO I CO CJO^r-r— C0CMOr--r— o CM CM CO .?r «:j- uo ,_ CJ CM -(- ocjcooor^i— cr>i— CM CM CM CO «;3- CO LO (—^ + T3 CU gc^r-oo^^^cMoo 3 Oi— COOOLOOOOI— CM CO . CM CM (>0 ^ CO LO I CM -f- O I COOOCOCOCOOOCOCOOOLO CJ «^ OOOOOOOOCOCO O CM CO CO CO CO CO CO CO CO 00 LO OO CO CO LO o o o o o o o o CQ cn CO CO oo CO o o CO CO CO 00 LO o o o o o o CO CO CO o CM CO LO LO 00 C7^ a. Q. Q_ Q. GL OL CL o. Q. cu CU a> CU OJ CU CU 4-> •M 4-> •M •M •M •M ^ OC oo CO CO OO 00 00 CO to oo 82 an average chemical analysis of the ground water from the northern section of the study area. The analysis, number 3, in which the percentage of weathered potassium feldspar to biotite is 50% to 50% appears to be the closest to the average chemical analysis of the ground water. The excess magnesium ions in the ground water indicated by Table 6, may be attributed to the formation of caliche on the Southern High Plains. Based on data from Horn and Adams (1966) the ratio of calcium to magnesium in the carbonate dust which was brought into the area is approximately .78 moles to .22 moles. The calcium ions were then precipitated as caliche, therefore, the magnesium to calcium ratio increased thus permitting more magnesium to attach to the ion exchange sites on clay minerals. When resolution of the caliche occurred during the wetter Pleistocene period, the newly released calcium ions exchanged for the magnesium ions on the clay minerals resulting in the increased magnesium to calcium ratio observed in the water. The calculated excess sodium, sulfate, and chloride ions in the ground water may be due to the addition of oil field brines. However, a minor amount of the excess ions could have resulted from evaporation and subsequent concentration of these ions in rainwater. That is, some evaporation of rainwater almost certainly occurs prior to recharge but the exact amount is presently unknown and the amount that occurred when the water was recharged 10,000 years ago is impossible to estimate accurately. 83 Excess bicarbonate ions are probably due to the solution of carbon dioxide as the water migrates through the soil zone. Southern Section of the Study Area As stated above, the chemical composition of the Ogallala water ^ WWWWWW in the northern section of the aquifer is primarily derived from the dissolution and weathering of rock-forming minerals composing the Ogallala Formation. If the mineralogy of the formation is assumed to be moderately consistent throughout the Southern High Plains, then water from the southern section of the study area should be composed of approximately equal quantities of the dissolved solutes contained in the water from the northern section. Table 6 shows the comparison between water from the northern and southern sections of the aquifer. A substantial increase in the concentration of sodium, potassium, magnesium, chloride, and sulfate is observed in an average analysis of water located in the southern section based on 500 samples (Table 7). The increase in these ions is probably the result of mixing of solutes derived from lower formations with those from the Ogallala water. The solutes from the lower formations are probably high in sodium, chloride, and sulfate ions. As this water enters the Ogallala Aquifer an increase in the sodium, chloride, and sulfate concentrations occurs. The sodium ions may exchange for calcium and magnesium ions on clay minerals. The calcium ions then precipitate, thereby increasing the magnesium to calcium ratio in the ground water. The bicarbonate concentration 84 OC col to O LO OC LO o CM O C^ -K O <'— O OC s- CO LU Q) LO -o o CO c > +J CJ) ro <: -r- LO O zz: i- o h- cu o CO CO LO a. CO CO I—CO o - O cn I— 00 CQ 3: Q ro CM r>v LO • • o oo LO I— oo CU E LO LO CL. O I— CM o • OC s (U CM LU O > > QC cu < LL. Q LO CO CM s_ LU CC ro LO CM ro to ro 00 Z X LU =5 CO O CU >- OC o o —J cs o o LO LO o J- .c 4-> 4-> 3 u CJ o o CU to c CJ c cu cu cu 3= an CT> S- O CJ ro (O cu L. M- cu cu c+- ro > > •r— ro -a (O ro Q 85 in the southern section supports this hypothesis as it is slightly less than the average concentration observed in the water from the northern section. Thus, the concentration of bicarbonate ions along with calcium ions will decrease in the Ogallala ground water in these areas. The dissolution and weathering of minerals from the Ogallala Formation cannot account entirely for the chemical composition of the ground water in the southern section of the study area. The difference in the anion and cation concentration in the ground water from the southern section is probably due to the mixing of saline water from Cretaceous sediments in areas associated with saline lakes. In summary, the chemical concentration of the solutes in the northern section of the study area appears to be influenced by chemical equilibria of weathering reactions. The addition of small amounts of dissolved solids from oil field brines is probably affecting the chemical composition of the Ogallala water to a minor degree. In the southern section of the aquifer the major controls influencing the chemical constituents in the ground water appear to be the chemical equilibrium of calcite, silica, and flourite and the mixing of solutes from underlying aquifers with the ground water in the Ogallala Aquifer. Ion exchange and adsorption are also major factors which are influencing the solute composition in areas of large saline lakes. CHAPTER V MANMADE POLLUTANTS INFLUENCING THE CHEMICAL QUALITY OF THE GROUND WATER IN THE OGALLALA AQUIFER Several types of solutes produced by man may affect the chemical composition of ground water in the Ogallala Aquifer. A detailed discussion on pollution is described in "Proceeding 1961, A symposium on ground water contamination" by the United States Public Health Service (1961). In the Ogallala Aquifer, oil and gas field brines, fertilizers, and liquid wastes from animal feedlots appear to be dominant sources of man induced solutes. Oil and Gas Field Brines Contamination of the ground water from the disposal of brines produced with oil and gas may locally affect the quality of the Ogallala water. Several major oil and gas fields are located in the Southern High Plains (Fig. 22). In the past many of the brines produced with the oil and gas were recharged through shallow "evaporation" pits. This is no longer permitted and the brines are now injected into the subsurface in lower formations. Percolation of the brines through the bottom of the pits or through the sides of poorly cased disposal wells almost certainly has resulted in a local increase in the concentrations of most ionic species. Ground water beneath some of the disposal pits in Borden, Cochran, Garza, Hockley, Howard, Lamb, and Lynn counties in Texas and also in Lea County, New 86 87 Figure 22.--Location of major oil and gas fields in the Southern High Plains in Texas. Modified after Vlissides, 1964. 88 Mexico, contain high ionic concentrations of calcium, magnesium, sodium, chloride (Whittemore and Pollack, 1979), and sulfate (Broadhurst, 1957; Ash, 1963; Burnitt and Crouch, 1964; Crouch and Burnitt, 1965; McMillion, 1965). Fertilizers Locally the chemical quality of the ground water in the Ogallala Aquifer may be affected by the addition of fertilizers onto the overlying soils. Fertilizers may be leached from the soils by rainwater and carried downward into the ground water. Reeves and Miller (1978) suggested that the expanded use of fertilizers during the past 10 to 20 years has resulted in an increase of nitrate ions in the Ogallala water which underlies areas of sandy soils (Fig. 23). Liquid Wastes from Animal Feedlots The downward percolation of liquid wastes from animal feedlots may be locally influencing the chemical quality of the Ogallala water. Over the past 20 years, the number of cattle feedlots operational in the Southern High Plains of Texas has increased dramatically (Fig. 24). Miller (1971) reported that runoff water from feedlots tends to have higher concentrations of calcium, magnesium, sodium, chloride, and nitrogen than ground water from the Ogallala Aquifer. If the runoff water was recharged into the aquifer an increase in the ground water's total dissolved solids 89 0 MR 50 0 KILOMETERS Too Figure 23.—Location of general soil types in the Southern High Plains in Texas. Modified after Godfrey and others, 1973. 90 FEEDLOT LOCATION 0 ^^^'^ 20 S KITOTAETWS—?0 Figure 24.--Location of cattle feedlots in the northern half of the Southern High Plains in Texas. Modified after Miller, 1971. 91 could result, however. Miller (1971) found no evidence of this phenomenon occurring. In localized areas in the aquifer, the chemical composition of the ground water is probably affected by man induced solutes. Contamination by oil and gas field brines in the northeast section of the study area may produce high sodium, calcium, sulfate, and chloride concentrations in the ground water. Several areas in the southern section with high sodium and chloride concentrations may also be attributed to the addition of oil and gas field brines. The leaching of nitrogen fertilizers from sandy and loamy soils by rainwater is probably a minor source of solutes in the aquifer. However, high concentrations of nitrate ions in the southern section and along the northeastern edge of the study area may be attributed to nitrogen fertilizers. Another source of solutes introduced by man in the Ogallala Aquifer is the liquid wastes from cattle feedlots. In several localized areas of the Southern High Plains the migration of liquid wastes into the Ogallala Aquifer may add large amounts of nitrate and chloride ions to the ground water. The addition of solutes by man's activities are probably affecting the chemical quality of the ground water locally in the Ogallala Aquifer but not as yet on a large scale. CHAPTER VI CONCLUSIONS Assuming that the results of the mass balance analyses reasonably represents the chemistry of the Ogallala water in equilibrium with the skeletal framework then dissolution of the framework cannot account for the entire concentration of solutes present in the ground water. The concentration of solutes in the ground water is probably affected by chemical equilibria, mixing of solutes from saline water probably from Cretaceous rocks, and ion exchange and adsorption of calcium and magnesium ions on clay minerals for sodium ions in the ground water. Mineral solution is the major factor controlling the water chemistry in the Ogallala Aquifer. Calcium ions are probably derived from the dissolution and weathering of calcite, plagioclase feldspar, and dolomite. The saturation index of calcite is greater than one, therefore, the precipitation of calcite is probably limiting the calcium concentration in the ground water. Minor amounts of magnesium ions are derived from the dissolution of dolomite. Even though the saturation index for dolomite is greater than one, dolomite is probably not precipitating due to the lack of other factors necessary before precipitation can occur. Weathering of plagioclase feldspar supplies some of the sodium ions present in the water. The saturation index for plagioclase is greater than one, however, plagioclase is not precipitating because it only forms in 92 93 high temperatures. Weathering of potassium feldspar and biotite are the primary sources of potassium ions in the ground water. The most probable control on the potassium ions is mineral equilibria, however, potassium ions in the ground water may be incorporated in clay minerals. The dominant sources of bicarbonate ions are the dissolution of carbonate minerals and the solution of soil generated carbon dioxide. The primary controls on the bicarbonate ions is the mineral equilibrium of calcite and the chemical equilibrium of carbon dioxide. There are no known sulfate minerals in the Ogallala Formation which supply sulfate ions into the ground water. The saturation index of gypsum is less than one, therefore, the mineral equilibrium of gypsum is not limiting the sulfate ions in the ground water. There are no known chloride minerals in the Ogallala Aquifer. The mineral halite is undersaturated with respect to the Ogallala water, therefore, the mineral equilibrium of halite is not limiting the chloride concentration in the ground water. The primary source of dissolved silica is the weathering of silicate minerals. The saturation indexes for silica amorphous and silica gel are both slightly less than one, therefore, it appears that the mineral equilibrium of silica amorphous or silica gel is limiting the dissolved silica in the ground water. The major source of flouride ions is probably the dissolution of calcium flouride salts. The saturation index for the mineral flourite may be greater than one in some areas of the aquifer and less than one in other areas, therefore, it is assumed that the mineral equilibrium of flourite is 94 limiting the flouride concentration in the ground water. There are no known nitrate minerals in the Ogallala Formation, and it is assumed that the mineral equilibrium of nitrate minerals have no limit on the nitrate concentration in the aquifer. The dissolution of Cenozoic volcanic ash deposits is believed to be the primary source of uranium, arsenic, barium, boron, lithium, vanadium, and zinc. Ion exchange and adsorption appear to be a significant factor controlling the calcium, magnesium, and sodium concentrations in the southern section of the study area. Due to the high sodium concentrations in the ground water, sodium ions may replace calcium and magnesium ions originally present on the exchange sites of the clay minerals. In the Ogallala water located near saline lakes the ratio of calcium or magnesium ions to sodium ions is greater than the ratio in the saline lake water. This suggests that ion exchange is occurring in the ground water. A major factor controlling the solute concentration in the southern section of the study area is the mixing of saline water with the Ogallala water in areas located near saline lakes. Due to the high sulfate concentrations in the saline lake waters the water in the lakes is presumed to be from the underlying Cretaceous rocks. Evaporation of the lake waters may have resulted in a difference of head between the Ogallala and Cretaceous Aquifers, thus allowing water and solutes from the Cretaceous to flow into the Ogallala. Although the Cretaceous water may have flowed into the Ogallala by 95 diffusion. If this is the case, a difference in head is not needed before flow can occur. Several areas exhibiting high concentrations of calcium, sodium, sulfate, and chloride occur near major oil and gas fields in the southern section and northeastern section of the study area. These high concentrations indicate the possible addition of solutes to the ground water by seepage of oil and gas field brines through the bottoms of surface disposal pits and/or through the sides of poorly cased disposal wells. Downward percolation of liquid wastes from animal feedlots may have locally contributed large quantities of nitrate and chloride ions into the ground water. 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APPENDIX In New Mexico, water analyses were obtained from the National Water Data Storage and Retrieval System (1929-1979) which stores analyses determined by New Mexico Bureau of Mines and Mineral Resources, New Mexico State Engineer Office, and New Mexico State Geological Survey. Water analyses were also determined by Los Alamos Scientific Laboratory and were obtained from a report written by Warren and Nunes (1978). In Texas, water analyses were obtained from Texas Tech University (1970-1972), Texas Department of Water Resources (1946- 1979), and Union Carbide Corporation. Data determined by Union Carbide Corporation were obtained from reports written by Nichols and others (1976), Nichols and others (1977), and Uranium Resource Evaluation Project (1978). 107