Carbonate Terrains Research Articles

Regional hydrochemistry of North American carbonate terrains

The chemical variation of water samples drawn from carbonate terrains between southern Canada and northern Mexico is shown to be divisible into, first, variations at the local level due to (1) hydrogeologic conditions (time independent), (2) short‐term fluctuations of hydrologic factors (stochastic in time), and (3) seasonal fluctuations of such factors as temperature, precipitation, and plant growth (systematic in time) and, second, regional climatic effects. These regional climatic effects can be satisfactorily represented by mean annual groundwater temperature variation. The regional variation only emerges clearly if care is taken to compare water samples from the same hydrologic class, in this case, springwater. The partial pressure of carbon dioxide with which a springwater sample is in equilibrium is well represented by log Pco2 = a + bT, where a is of the order of −3.5 and b is approximately 0.07 with Pco2 in atmospheres and T in degrees Celsius.

The effect of climate on the chemistry of carbonate groundwater

The temperature coefficient of the log Pco2‐temperature relationship for springwater from carbonate terrains in North America is approximately 0.07/°C. Consideration of a simple model of soil air behavior and various published data suggests that the temperature coefficient of the same relationship for soil air is of the order of 0.04/°C. A model where the soil zone is the source of carbon dioxide for water entering a groundwater system and where the subsequent chemical evolution of the water occurs under closed system conditions is consistent with these relationships.

Groundwater tracing with post sampling activation analysis

A tracing technique is described which uses low concentrations of a nonradioactive tracer, the bromide ion. Concentrations may be so low that they do not constitute a health or pollution problem when added to segments of the hydrologic cycle. The tracer is detected by post sampling activation analysis techniques. Water samples containing the tracer are made radioactive when irradiated with neutrons. Radiation detectors are used to analyze the radiation emitted by each sample and the unique radiation characteristics of the tracer ( 80Br with a half life of 17.6 min) are sought. No chemical separation techniques are required. Given a “rabbit system” to transport irradiated samples to a Ge(Li) detector 3–5 samples per hour can be analyzed. The technique is safe, and tracers can be detected as low as 20 parts per billion (p.p.b.) over that of background concentrations. The bromide ion does not appear to be lost by precipitation, absorption or adsorption and is biologically stable. A spike concentration of 200 p.p.m. was used in field studies because it is lower than the limit set for drinking water. The technique was tried on sinkhole, conduit and spring systems in a carbonate terrain, in central Pennsylvania. Surface water entering a sinkhole had a background bromine concentration of 0.020 μg/ml and 0.027 μg/ml at the conduit outlet 0.6 miles distant. Although the flow rate into the sinkhole was 50 ft. 3/min, no tracer was detected in the spring 16 h after the tracer was added. In a second study ammonium bromide was added in a conduit 500 ft. from its discharge point. Tracer first appeared at the discharge point 1 h after injection (0.11 p.p.m. vs. an average background of 0.03 p.p.m.). The maximum passed at 1 h 30 min and the spread of the spike was estimated to be over 5 h. For a peak maximum of 0.1 p.p.m., only 3 g of ammonium bromide would have been required in the second study. The inlet concentration would have been only 2.8 p.p.m. indicating the sensitivity of the method. Nine of many possible potential field applications are listed for this superior tracing technique.

Hydrochemical environments of carbonate terrains

Waters within a carbonate aquifer have often been classified on a hydrologie basis. The significance of such grouping to the understanding of the geochemical processes acting in the aquifer is dependent on the classes having a distinctive chemistry. One hundred sixty‐six reliable chemical analyses of carbonate waters from Pennsylvania were grouped on a hydrologic basis, and the groupings were tested by a stepwise linear discriminant function analysis. Two parameters (degree of calcite saturation and equilibrium carbon dioxide partial pressure) are sufficient to distinguish the groups at the 0.005 confidence level and provide a reliable way to examine the geochemical evolution of the waters in the carbonate drainage basin.

Preliminary study on the Ruiz geothermal project (Colombia)

The Ruiz region, located in the median part of the Colombian Central Cordillera, is featured by several Tertiary and Quaternary volcanic edifices and by a number of hot manifestations. This situation suggested that a feasibility study could be made for a research project of endogenous fluids. The project region stretches over an area of about 15,000 km 2. Research carried out was intended to reconstruct: lithological-stratigraphical features of the region, in terms of reservoir and cap-rock; volcanologic and structural events; hydro-geological situation; type and nature of manifestations. It was thus possible to single out the existence of two major circulation systems, separated from each other by a thick complex of impervious rocks. The former system chiefly develops at rather shallow depths within volcanic rocks; the latter develops at considerable depths within fractured quartzitic and carbonate terrains of the metamorphic regional basement. The deep circulation system is supplied by a wide absorption area located at altitudes exceeding 3500 m, close to the main volcanic edifices. All waters of the analyzed manifestations surely have a meteoric origin and appear to be merely involved in the shallow circulation.

Conceptual Models for Carbonate Aquifers

AbstractThe very diverse types of ground‐water behavior in carbonate terrains can be classified by relating the flow type to a particular hydrogeologie environment each exhibiting a characteristic cave morphology. The ground water may move by diffuse flow, by retarded flow, or by free flow. Diffuse flow occurs in less soluble rocks such as extremely shaley limestones or crystalline dolomites. Integrated conduits are rare. Caves tend to be small, irregular, and often little more chan solutionally widened joints. Retarded flows occur in artesian environments and in situations where unfavorable stratigraphy forces ground water to be confined to relatively thin beds. Network cave patterns are characteristic since hydrodynamic forces are damped by the external controls. Solution occurs along many available joints. Free flowing aquifers are those in which solution has developed a subsurface drainage system logically regarded as an underground extension of surface streams. These streams may have fully developed surface tributaries as well as recharge from sinkholes and general infiltration. Characteristic cave patterns are those of integrated conduit systems which are often truncated into linear, angulate, and branchwork caves. Free Flow aquifers may be further sub‐divided into Open aquifers lying beneath karst plains and Capped aquifers in which significant parts of the drainage net lie beneath an insoluble cap rock. Other geologic factors such as structure, detailed lithology, relief, and locations of major streams, control the details of cave morphology and orientation of the drainage network.

Clay Mineralogy of some Lower Tertiary (Paleogene) Sediments from Denmark

108 samples from the Paleogene rocks of Denrnark were examined by standard X-ray techniques and their clay suites form the basis of the present study. The X-ray data have permitted a three-fold mineralogical zonation of the Danish Paleogene.The oldest zone (Zone I) is characterized by a high montmorillonite content, minor or trace amounts of illite and segregated mixed-layer clay minerals and the absence of kaolinite. The boundaries of Zone I coincide with the Paleocene Series. The clay minerals are detrital and are derived from a carbonate terrain bordering the Fennoscandian massif. Diagenetic changes in the marine environment and segregation by sorting and floculation are important factors accounting for the high montmorillonite content.A middle zone (Zone Il) is characterized by the predominance of amorphous material. Minor amounts of montmorillonite, illite and kaolinite are present and represent alteration produets of volcanic ash. Zone II is restricted to the Mo Clay Formation.The youngest zone (Zone III) contains variable amounts of montmorillonite, illite, kaolinite and random mixed-layer montmorillonite-illite. The boundaries of Zone III appear to extend from the lower Eocene to the Miocene. The clay minerals of Zone III are products of the alteration of the volcanic material of the Fennoscandian massif. Changes in the conditions of the source area and the physical environment of deposition account for the mineralogical variations.