William Back

Comparison of chemical hydrogeology of the carbonate peninsulas of Florida and Yucatan

Abstract Aquifers of the peninsulas of Florida and northern Yucatan are Tertiary marine carbonate formations showing many lithologic and faunal similarities. In addition, the tropical to subtropical climates of the two areas are similar, each having annual rainfall of about 1000 to 1500 mm. Despite similarities in these fundamental controls, contrasts in the hydrologic and geochemical systems are numerous and striking. For example, Florida has many rivers; Yucatan has none. Maximum thickness of fresh ground water in Florida is about 700 meters; in the Yucatan it is less than 70 meters. In Florida the gradient of the potentiometric surface averages about 1 meter per kilometer; in the Yucatan it is exceedingly low, averaging about 0.02 meter per kilometer. In Florida the chemical character of water changes systematically downgradient, owing to solution of minerals of the aquifer and corresponding increases in total dissolved solids, sulfate, calcium, and Mg-Ca ratio; in the Yucatan no downgradient change exists, and dominant processes controlling the chemical character of the water are solution of minerals and simple mixing of the fresh water and the body of salt water that underlies the peninsula at shallow depth. Hydrologic and chemical differences are caused in part by the lower altitude of the Yucatan plain. More important, however, these differences are due to the lack of an upper confining bed in Yucatan that is hydrologically equivalent to the Hawthorn Formation of Florida. The Hawthorn cover prevents recharge and confines the artesian water except where it is punctured by sinkholes, but sands and other unconsolidated sediments fill sinkholes and cavities and impede circulation. In the Yucatan the permeability of the entire section is so enormous that rainfall immediately infiltrates to the water table and then moves laterally to discharge areas along the coasts.

Hydrogeological Processes and Chemical Reactions at a Landfill

Chemical and isotopic analyses were made of water from wells in and downgradient from a landfill to determine chemical and isotopic effects of generation and migration of leachate on ground water. The distribution and wide concentration range of oxygen and methane permit the delineation of an anaerobic zone, a regional oxygenated zone and an intermediate zone. The ratio of reduced nitrogen to nitrate indicates location of reducing fronts as the leachate migrates. The pH of the native ground water is low (≥5.0) primarily because of the low pH of rainfall and the lack of calcareous or other soluble minerals in the aquifer material. The pH is higher (∼6.6) in the leachate because of generation of carbon dioxide, ammonia, and methane. The native ground water has a low TDS (80 mg/l) while the leachate has an average TDS of 2800 mg/l and is primarily a NaHCO3 type water. Sulfate concentrations are extremely low and H2 S was not detected. We suggest that a major source of cations may be their exchange from the clays by the ammonium generated in the leachate. High concentrations of Fe and Mn are attributed to a source in the refuse but more important to reduction of oxide cements and coatings resulting from degradation of organic matter. The main source of bicarbonate is from organic degradation with minimal CO2 from the soil zone. At one landfill site 52% of the total alkalinity is attributed to organic compounds, mainly organic acid anions. The δ13 C of bicarbonate in the leachate is exceedingly heavy (+18.400 /00 ) which results from fractionation during the formation of methane. The 10 per mil deuterium enrichment of water may be due to decomposition of deuterium-enriched compounds and bacterial processes that preferentially consume the lighter hydrogen isotope.

Process and rate of dedolomitization: Mass transfer and 14C dating in a regional carbonate aquifer

Regional dedolomitization is the major process that controls the chemical character of water in the Mississippian Pahasapa Limestone (Madison equivalent) surrounding the Black Hills, South Dakota and Wyoming. The process of dedolomitization consists of dolomite dissolution and concurrent precipitation of calcite; it is driven by dissolution of gypsum. Deuterium and oxygen isotopic data from the ground water, coupled with regional potentiometric maps, show that recharge occurs on the western slope of the Black Hills and that the water flows northward and westward toward the Powder River Basin. A significant part flows around the southern end of the Black Hills to replenish the aquifer to the east of the Hills. Depth of flow was inferred from interpretation of the silica geothermometer based on the temperature-dependent solubilities of quartz and chalcedony in water. Chemical effects of warm water in the Pahasapa Limestone include changes in the solubility products of minerals, conversion of gypsum to anhydrite, solution and precipitation of minerals, and increases in the tendency for outgassing of carbon dioxide. Where sulfate reduction is not important, sulfur isotope data show that (1) in the Mississippian aquifer, most of the sulfate is from dissolution of gypsum and (2) some wells and springs have a hydrologic connection with overlying Permian and Pennsylvanian evaporites. Sulfate ion concentration, a progress variable, shows a strong correlation with pH as a result of the combined effects of the dedolomitization reactions. Mass-balance and mass-transfer calculations were used to adjust 14C values to determine a range of ground-water flow velocities between 2 and 20 m/yr. These velocities are characteristic of carbonate aquifers. The average rates of dolomite and gypsum dissolution are 1.7 × 10−4 and 3.4 × 10−4 mmol/kg of H2O/yr, respectively. The precipitation of calcite is occurring at the rate of 3.4 × 10−4 mmol/kg of H2O/yr. The close agreement among the model results demonstrates that dedolomitization is controlling water-rock interactions in this regional carbonate aquifer system.

Differential dissolution of a Pleistocene reef in the ground-water mixing zone of coastal Yucatan, Mexico

A geochemical explanation is provided for the extensive dissolution observed along the carbonate coast of the Yucatan Peninsula, Mexico. Mixing of fresh ground water with subterranean Caribbean seawater generates a highly reactive geochemical zone that enhances aragonite and calcite dissolution and permits neomorphism of aragonite. Mixing-zone dissolution caused by ground-water discharge is a major geomorphic process in developing caves, coves, and crescent-shaped beaches along the Yucatan coast. Such dissolution has probably been a significant control on permeability and porosity distribution in carbonate rocks in the geologic record.

Major geochemical processes in the evolution of carbonate-Aquifer systems

Abstract As a result of recent advances by carbonate petrologists and geochemists, hydrologists are provided with new insights into the origin and explanation of many aquifer characteristics and hydrologic phenomena. Some major advances include the recognition that: (1) most carbonate sediments are of biological origin; (2) they have a strong bimodal size-distribution; and (3) they originate in warm shallow seas. Although near-surface ocean water is oversaturated with respect to calcite, aragonite, dolomite and magnesite, the magnesium-hydration barrier effectively prevents either the organic or inorganic formation of dolomite and magnesite. Therefore, calcareous plants and animals produce only calcite and aragonite in hard parts of their bodies. Most carbonate aquifers that are composed of sand-size material have a high initial porosity; the sand grains that formed these aquifers originated primarily as small shells, broken shell fragments of larger invertebrates, or as chemically precipitated oolites. Carbonate rocks that originated as fine-grained muds were initially composed primarily of aragonite needles precipitated by algae and have extremely low permeability that requires fracturing and dissolution to develop into aquifers. Upon first emergence, most sand beds and reefs are good aquifers; on the other hand, the clay-sized carbonate material initially has high porosity but low permeability, a poor aquifer property. Without early fracture development in response to influences of tectonic activity these calcilutites would not begin to develop into aquifers. As a result of selective dissolution, inversion of the metastable aragonite to calcite, and recrystallization, the porosity is collected into larger void spaces, which may not change the overall porosity, but greatly increases permeability. Another major process which redistributes porosity and permeability in carbonates is dolomitization, which occurs in a variety of environments. These environments include back-reefs, where reflux dolomites may form, highly alkaline, on-shore and continental lakes, and sabkha flats; these dolomites are typically associated with evaporite minerals. However, these processes cannot account for most of the regionally extensive dolomites in the geologic record. A major environment of regional dolomitization is in the mixing zone (zone of dispersion) where profound changes in mineralogy and redistribution of porosity and permeability occur from the time of early emergence and continuing through the time when the rocks are well-developed aquifers. The reactions and processes, in response to mixing waters of differing chemical composition, include dissolution and precipitation of carbonate minerals in addition to dolomitization. An important control on permeability distribution in a mature aquifer system is the solution of dolomite with concomitant precipitation of calcite in response to gypsum dissolution (dedolomitization). Predictive models developed by mass-transfer calculations demonstrate the controlling reactions in aquifer systems through the constraints of mass balance and chemical equilibrium. An understanding of the origin, chemistry, mineralogy and environments of deposition and accumulation of carbonate minerals together with a comprehension of diagenetic processes that convert the sediments to rocks and geochemical, tectonic and hydrologic phenomena that create voids are important to hydrologists. With this knowledge, hydrologists are better able to predict porosity and permeability distribution in order to manage efficiently a carbonate—aquifer system.

Opportunities in the Hydrologic Sciences

Hydrologists can take heart that our profession has matured to the point of having its respectable reputation recognized by the National Academy of Sciences. Opportunities in Hydrology follows the publication of Opportunities in Biology and Opportunities in Chemistry, and was prepared by a committee composed of prestigious water-oriented scientists. I am writing this review because the book is extremely important, and its basic premise—that there is such a thing as a single “discipline” of hydrologic sciences—is contrary to the thinking of many hydrogeologists. The committee proposes that students can obtain adequate training and be prepared to develop a career in “hydrologic sciences.” Such an approach may be suitable for many aspects of hydrology, but it does not represent the interests, needs, goals, history, or future of “hydrogeology,” a clearly recognized subdiscipline of hydrology. The various aspects of hydrology are so wide ranging that, from my personal viewpoint and the viewpoints of many of my colleagues, it takes a person of extremely narrow focus to see hydrology as a single discipline.

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.

Modern marine sediments as a natural analog to the chemically stressed environment of a landfill

Abstract Baedecker, M.J. and Back, W., 1979. Modern marine sediments as a natural analog to the chemically stressed environment of a landfill. In: W. Back and D.A. Stephenson (Guest-Editors), Contemporary Hydrogeology — The George Burke Maxey Memorial Volume. J. Hydrol., 43: 393-414. 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.

The origin and isotopic composition of dissolved sulfide in groundwater from carbonate aquifers in Florida and Texas

The δ34S values of dissolved sulfide and the sulfur isotope fractionations between dissolved sulfide and sulfate species in Floridan ground water generally correlate with dissolved sulfate concentrations which are related to flow patterns and residence time within the aquifer. The dissolved sulfide derives from the slow in situ biogenic reduction of sulfate dissolved from sedimentary gypsum in the aquifer. In areas where the water is oldest, the dissolved sulfide has apparently attained isotopic equilibrium with the dissolved sulfate (Δ34S = 65 per mil) at the temperature (28°C) of the system. This approach to equilibrium reflects an extremely slow reduction rate of the dissolved sulfate by bacteria; this slow rate probably results from very low concentrations of organic matter in the aquifer. In the reducing part of the Edwards aquifer, Texas, there is a general down-gradient increase in both dissolved sulfide and sulfate concentrations, but neither the δ34S values of sulfide nor the sulfide-sulfate isotope fractionation correlates with the ground-water flow pattern. The dissolved sulfide species appear to be derived primarily from biogenic reduction of sulfate ions whose source is gypsum dissolution although upgradient diffusion of H2S gas from deeper oil field brines may be important in places. The sulfur isotope fractionation for sulfide-sulfate (about 38 per mil) is similar to that observed for modern oceanic sediments and probably reflects moderate sulfate reduction in the reducing part of the aquifer owing to the higher temperature and significant amount of organic matter present; contributions of isotopically heavy H2S from oil field brines are also possible.

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.