Geology topics
W. Back
Publications and source records attributed to W. Back.
American hydrogeology at the millennium: An annotated chronology of 100 most influential papers
Hydrogeology developed as scientists undertook activities to describe how a groundwater system functions to explain why it is that way, in order to solve practical problems of water supply. This paper demonstrates the evolutionary nature and growth of hydrogeology in the United States on the basis of a selection of one hundred papers that had a significant impact on subsequent activities. We have identified three revolutionary concepts that resulted directly from this evolutionary understanding and have selected papers that demonstrate important consequences. These three concepts are 1) that the mathematical expression for heat flow can be paraphrased for groundwater and used in transient flow conditions to determine aquifer characteristics; 2) that the distribution of fluid potential can be formulated in mathematical equations suitable for solution by various analytical techniques; and 3) that chemical thermodynamics can be applied to hydrogeologic systems in order to understand the processes controlling the chemical character of groundwater. One purpose of this paper is to encourage scientists to gain an additional dimension of satisfaction from their work by being aware of the contributions of those who went before them and to see how their own work fits into the current understanding of hydrogeology.
Relation of carbon 14 concentrations to saline water contamination of coastal aquifers
Naturally occurring stable or radioactive isotopes may be used in some places to identify the origin of saline water that contaminates some coastal aquifers. In a recent study to determine the origin of saline water in the Ocala Limestone aquifer near Brunswick, Georgia, the following sources were analyzed for C 14 and deuterium concentrations: potable water from the Ocala Limestone, contaminated water from the Ocala Limestone, saune water from the underlying Claiborne Group, and nearby ocean water. The chloride concentration of the groundwater ranges from about 25 ppm in the potable water to more than 2000 ppm in the deeper part of the Claiborne Group. From an interpretation of piezometric maps and other hydrologic data, previous investigators had concluded that the source of the contaminating water was the Claiborne Group and not the nearby ocean. The essentially uniform range of low values of −965 to −987‰ of the modern standard (National Bureau Standard C 14 oxalic acid) for the C 14 activity of the groundwater samples (regardless of the degree of contamination) is in agreement with this conclusion. If recent ocean water, which had a C 14 value of +285‰, were the source of contamination, the contaminated water would have had a C 14 activity higher than the activity of the fresh water. Deuterium analyses are not inconsistent with the interpretation that water from the Claiborne Group, rather than recent ocean water, is the source of the contaminating water. The concurrence of the hydrologic and the isotopic data in this area where the hydrology is well known suggests that isotopic analysis may be used to identify the origin of water in different portions of a hydrologic environment.
Deciphering hydrological systems by means of geochemical processes
Interpretation of geochemical reactions and isotopic composition of groundwater provides a method to determine hydrological parameters such as porosity, hydraulic conductivity, and groundwater flow rates. This geochemical method is largely independent of the more conventional approach of determining these parameters by an evaluation of physical properties of aquifer systems. -from Authors
Water management by early people in the Yucatan, Mexico
The Yucatan Peninsula is a coastal plain underlain by permeable limestone and receives abundant rainfall. Such hydrogeologic conditions should provide major supplies of water; however, factors of climate and hydrogeology have combined to form a hydrologic system with chemical boundaries that limits the amount of fresh water available. Management of water resources has long had a major influence on the cultural and economic development of the Yucatan. The Mayan culture of the northern Yucatan developed on extensive use of groundwater. The religion was water oriented and the Mayan priests prayed to Chac, the water god, for assistance in water management, primarily to decrease the severity of droughts. The Spaniards arrived in 1517 and augmented the supply by digging wells, which remained the common practice for more than 300 years. Many wells now have been abandoned because of serious problems of pollution. A historical perspective of a paper such as this provides insight into the attitudes concerning water of early people and perhaps provides insight into current attitudes concerning water. Hydrogeologists possess the expertise to generate relevant information required by water managers to arrive at management programs to achieve sustainable development. ?? 1995 Springer-Verlag.
Groundwater use: Equilibrium between social benefits and potential environmental costs
In many countries groundwater resources are under-appreciated and, therefore, underutilizied; whereas, in some areas they are inappropriately exploited and, therefore, over-utilized. “Over utilization” can lead to depletion in quantity or a degradation in quality or both. Obstacles to effective management include: (1) lack of knowledge of basic principles of groundwater science among water planners, (2) in many, if not in most countries, ownership of groundwater is in the private domain with the result that codependence is unrecognized, and (3) a misunderstanding by water planners of the concepts of “overexploitation%rdquo; and conjunctive use. The economic, social, and hydrologic constraints and procedures for management for sustainable development of groundwater are significantly different from those for surface water because these differences result from such things as (1) groundwater development is not dependent on large scale collective projects (unlike the utilization of surface water that requires engineering structures for diverting, regulating and transporting water), (2) the activities of many different groups can affect the quality of water, and (3) users of groundwater often are not aware of their co-dependence on the groundwater heritage in which each participates. Hydrogeologists should try to identify those governmental policies that have a detrimental environmental effect, promote those policies that are beneficial, and demonstrate the need for a policy in matters where a policy is lacking.
Chemical hydrogeology in natural and contaminated environments
Chemical hydrogeology, including organic and inorganic aspects, has contributed to an increased understanding of groundwater flow systems, geologic processes, and stressed environments. Most of the basic principles of inorganic-chemical hydrogeology were first established by investigations of organic-free, regional-scale systems for which simplifying assumptions could be made. The problems of groundwater contamination are causing a shift of emphasis to microscale systems that are dominated by organic-chemical reactions and that are providing an impetus for the study of naturally occurring and manmade organic material. Along with the decrease in scale, physical and chemical heterogeneity become major controls. Current investigations and those selected from the literature demonstrate that heterogeneity increases in importance as the study site decreases from regional-scale to macroscale to microscale. Increased understanding of regional-scale flow systems is demonstrated by selection of investigations of carbonate and volcanic aquifers to show how applications of present-day concepts and techniques can identify controlling chemical reactions and determine their rates; identify groundwater flow paths and determine flow velocity; and determine aquifer characteristics. The role of chemical hydrogeology in understanding geologic processes of macroscale systems is exemplified by selection of investigations in coastal aquifers. Phenomena associated with the mixing zone generated by encroaching sea water include an increase in heterogeneity of permeability, diagenesis of minerals, and formation of geomorphic features, such as caves, lagoons, and bays. Ore deposits of manganese and uranium, along with a simulation model of ore-forming fluids, demonstrate the influence of heterogeneity and of organic compounds on geochemical reactions associated with genesis of mineral deposits. In microscale environments, importance of heterogeneity and consequences of organic reactions in determining the distribution and concentrations cf. constituents are provided by several studies, including infiltration of sewage effluent and migration of creosote in coastal plain aquifers. These studies show that heterogeneity and the dominance of organically controlled reactions greatly increase the complexity of investigations.Current investigations and those selected from the literature demonstrate that heterogeneity increases in importance as the study site decreases from regional-scale to macroscale to microscale. Increased understanding of regional-scale flow systems is demonstrated by selection of investigations of carbonate and volcanic aquifers to show how application of present-day concepts and techniques can identify controlling chemical reactions and determine their rates; identify groundwater flow paths and determine flow velocity; and determine aquifer characteristics. The role of chemical hydrogeology in understanding geologic processes of macroscale systems is exemplified by selection of investigations in coastal aquifers. Ore deposits of manganese and uranium, along with a simulation model of ore-forming fluids, demonstrate the influence of heterogeneity and of organic compounds on geochemical reactions associated with genesis of mineral deposits. In microscale environments, importance of heterogeneity is illustrated by studies of infiltration of sewage effluent and migration of creosote in coastal plain aquifers.
A ground-water mixing model for the origin of the Imini manganese deposit (Cretaceous) of Morocco
Three beds of manganese oxide ore in a 10 m-thick dolomite unit are associated with diagenetic features, and, are accordingly also diagenetic in their present aspect. Whether primary or introduced, the Mn mineralogy is attributed to reactions between fresh and saline ground-waters as the zone of mixing passed through the dolomite unit during a period of falling sea-level. The succession of diagenetic changes is: 1) precursor carbonates, 2) dolomite-janggunite, 3) hollandite-pyrolusite-chert-calcite.-G.J.N.
Role of aquitards in hydrogeochemical systems: A synopsis
Aquitards exert significant influence on the hydrogeochemistry of aquifer systems. This influence is manifested somewhat differently depending on the relative position of aquitards within a system. In the deeper regimes, they are influential in the origin and distribution of brines and the development of geopressured zones. In intermediate regimes, they form multi-layered aquifer systems and provide a source of reactive minerals and exchangeable ions. In shallow regimes, aquitards can influence the topography and drainage patterns; this influences the relationship between the water table and the potentiometric surface of confined aquifers, controls the rates of infiltration and discharge, and controls whether the geochemical system is open or closed to exchange of carbon dioxide gas. In coastal areas, aquitards can determine the depth of the saltwater-freshwater interface, its distance from the shoreline, and the position of the mixing zone that causes geochemical alteration of minerals and development of porosity. ?? 1986.
Differential dissolution of a Pleistocene reef in the groundwater mixing zone of coastal Yucatan, Mexico
Mixing of fresh groundwater with subterranean Caribbean seawater generates a highly reactive geochemical zone that enhances aragonite and calcite dissolution and permits neomorphism of aragonite.
Regional ground-water flow concepts in the United States: Historical perspective
A number of important ideas, developed during the past 100 years, form the framework of the present understanding of regional ground-water flow. The most important of these ideas are: Differences in topographic elevation provide the principal driving force for regional flow. Flow through confining layers forms an essential element of regional flow systems. Chemical evolution within the flow systems can be used to understand the flow. Moving ground water is an efficient transport mechanism for heat within the Earth. We trace the evolution of these ideas in the United States and demonstrate their influence on the present-day understanding of flow systems with examples taken primarily from the American literature.
Chemical constraints of groundwater management in the Yucatan peninsula, Mexico
Two critical objectives of water management in the Yucatan are: (1) to develop regional groundwater supplies for an expanding population and tourism based on the Mayan archeological sites and excellent beaches; and (2) to control groundwater pollution in a chemically sensitive system made vulnerable by geologic conditions. The Yucatan peninsula is a coastal plain underlain by permeable limestone and has an annual rainfall of more than 1000 mm. Such a setting should provide abundant supplies of water; however, factors of climate and hydrogeology have combined to form a hydrologic system with chemical boundaries that decrease the amount of available fresh water. Management of water resources has long had a major influence on the cultural and economic development of the Yucatan. The Mayan culture of the northern Yucatan developed by extensive use of groundwater. The religion was water-oriented and the Mayan priests prayed to Chac, the water god, for assistance in water management primarily to decrease the severity of droughts. The Spaniards arrived in 1517 and augmented the supplies by digging wells, which remained the common practice for more than 300 years. Many wells now have been abandoned because of serious problems of pollution resulting from the use of a sewage disposal well adjacent to each supply well. The modern phase of water management began in 1959 when the Secretaría de Recursos Hidráulicos (S.R.H.) was charged with the responsibility for both scientific investigations and development programmes for water-supply and sewage-disposal systems for cities, villages and islands.
Chemical mass-wasting of the northern Yucatan Peninsula by groundwater dissolution
The northern part of the Yucatan Peninsula is a relatively flat, low-lying carbonate terrane with no geomorphic expressions of stream channels. It is estimated that mean annual recharge to the groundwater system is 150 mm. For the 65,500 km 2 study area, mean annual discharge (equivalent to recharge) is 9.8 × 10 9 m 3 , or 8.6 × 10 6 m 3 for each 1 km of the 1,100-km-long coastline. In the interior of the peninsula, the recharging water annually dissolves about 37.5 t (metric tons) of calcite per 1 km 2 . When the groundwater has become saturated with calcite, little additional water-rock interaction occurs until the active mixing (dispersion) zone is reached near the coastline. Theoretical calculations and laboratory experiments have shown that when two waters, each calcite saturated and with different salinities, are mixed, the resulting solution generally becomes undersaturated with calcite and, therefore, is capable of dissolving additional calcite. On the basis of our study of the Xel Ha lagoon on the east coast of Yucatan, we calculate that as much as 1.2 mmol/L additional calcite can be dissolved in the brackish groundwater zone of dispersion. This indicates that if the total solution potential of the amount of water discharging at Xel Ha has focused within the lagoon area, the lagoon could be chemically incised in less than 3,000 yr. We postulate that chemical mass wasting by dissolution in the zone of groundwater mixing is an important geomorphic process in coastal areas of limestone terranes.
The mass balance approach: application to interpreting the chemical evolution of hydrologic systems
Mass balance calculations are applied to observed chemical and isotopic data of three natural water systems involving carbonate reactions in order to define mineral stoichiometry of reactants and products, relative rates of reactions, and mass transfer. One study evaluates reactions in a lagoon on the east coast of the Yucatan Peninsula, Mexico.
Modern marine sediments as a natural analog to the chemically stressed environment of a landfill
Chemical reactions that occur in landfills are analogous to those reactions that occur in marine sediments. Lateral zonation of C, N, S, O, H, Fe and Mn species in landfills is similar to the vertical zonation of these species in marine sediments and results from the following reaction sequence: (1) oxidation of C, N and S species in the presence of dissolved free oxygen to HCO 3 - , NO 3 - and SO 4 2 ; (2) after consumption of molecular oxygen, then NO 3 - is reduced, and Fe and Mn are solubilized; (3) SO 4 2- is reduced to sulfide; and (4) organic compounds become the source of oxygen, and CH 4 and NH 4 + are formed as fermentation products. In a landfill in Delaware the oxidation potential increases down-gradient and the redox zones in the reducing plume are characterized by: CH 4 , NH 4 + ,Fe 2+ . Mn 2+ , HCO 3 - and NO 3 - . Lack of SO 4 2- at that landfill eliminates the sulfide zone. Although it has not been observed at landfills, mineral alteration should result in precipitation of pyrite and/or siderite downgradient. Controls on the pH of leachate are the relative rates of production of HCO 3 - , NH 4 + and CH 4 . Production of methane by fermentation at landfills results in 13 C isotope fractionation and the accumulation of isotopically heavy σ CO 2 (+10 to +18 0 / 00 PDB). Isotope measurements may be useful to determine the extent of CO 2 reduction in landfills and extent of dilution downgradient. The boundaries of reaction zones in stressed aquifers are determined by head distribution and flow velocity. Thus, if the groundwater flow is rapid relative to reaction rates, redox zones will develop downgradient. Where groundwater flow velocities are low the zones will overlap to the extent that they may be indeterminate.
A geochemical hypothesis for dolomitization by ground water
Most modern disordered dolomite has been found in dynamic environments. However, solutions associated with modern dolomite formation do not have a common Mg/Ca ratio; the ratio ranges from about 3 to 100. Ground-water circulation may have a significant role in formation of regional dolomites; one of the primary requirements for regional dolomite formation is a large supply of magnesium ions. An X-ray study of well cuttings from the Tertiary limestone aquifer of central Florida indicates that it is composed primarily of calcite and dolomite with minor amounts of quartz and apatite. The magnesium content of the calcite is slightly lower (0-2 percent MgC03) in the recharge areas than in the deeper confined parts of the aquifer system (2-4 percent MgC03). Our data support other recent work and indicate an equilibrium constant for dolomite of 2 X 10-17. Inasmuch as this value is exactly the square of the calcite equilibrium constant (10~8-35), the Mg/Ca ratio must be unity for the three-phase equilibrium, calcite-dolomite-water. The Mg/Ca ratio in water from the aquifer is as low as 0.05 in the recharge area where the water is also undersaturated with respect to both calcite and dolomite. With time and length of travel path in the system, the water increases systematically in Mg/Ca ratio, which approaches unity; saturation with respect to the two carbonates also increases downgradient until the solution apparently becomes over-saturated with respect to both carbonates. In Tertiary limestones of the Yucatan Peninsula, the Mg/Ca range in water is similar to that for Florida. The small amount of magnesium available from the solution of magnesium calcites and dolomite in the potable zone of active circulation is insufficient to provide the amount required for extensive dolomitization unless enormous quantities of rock are available for dissolution. However, dolomite may be forming in the zones of brackish water that underlie the Florida and Yucatan Peninsulas. The required magnesium may be derived from the readily available ocean water or reflux brines as the hydrologic regimen is changed because of relative fluctuations of sea level.