Chemical and mechanical clogging of groundwater abstraction wells at well field Heel,the Netherlands

Well field Heel, in the south east of the Netherlands, consists of a row of wells drilled in an anoxic pyrite-containing aquifer alongside a former gravel pit, which now serves as a recharge basin, where water is actively aerated. All wells are seriously affected by chemical (screen slot) and/or mechanical (well bore) clogging. The objective of this study is to explain this combined occurrence. A combination of chemical, hydraulic and well-maintenance data indicate three groundwater quality types: (1) oxic basin water, (2) anoxic iron-containing basin water after oxidation of the traversed aquifer, and (3) deeply anoxic native groundwater. Wells abstracting a mixture of oxic basin water and anoxic basin water and/or native groundwater experience chemical well clogging, whereas wells abstracting (only or partly) native groundwater are vulnerable to mechanical well clogging. In the end, after oxic basin water has completely oxidized the traversed the aquifer, only two groundwater quality types will be present. Wells abstracting only oxic basin water will show no clogging, and wells abstracting a mixture of native groundwater and oxic basin water will experience chemical and possibly also mechanical well clogging. In this reasoning, the sequence in abstracted groundwater quality types coincides with a sequence in well clogging: from mechanical to chemical to no clogging. As well field Heel is situated in sloping terrain, the interplay between regional hydraulic gradient and different water qualities results in one-sided chemical clogging in the upper part of the well screen during abstraction, and in the lower part during the resting phase.     

Knowledge Recovery for Continental-Scale Mineral Exploration by Neural Networks

This study is concerned with understanding of the formation of ore deposits (precious and base metals) and contributes to the exploration and discovery of new occurrences using artificial neural networks. From the different digital data sets available in BRGM's GIS Andes (a comprehensive metallogenic continental-scale Geographic Information System) 25 attributes are identified as known factors or potential factors controlling the formation of gold deposits in the Andes Cordillera. Various multilayer perceptrons were applied to discriminate possible ore deposits from barren sites. Subsequently, because artificial neural networks can be used to construct a revised model for knowledge extraction, the optimal brain damage algorithm by LeCun was applied to order the 25 attributes by their relevance to the classification. The approach demonstrates how neural networks can be used efficiently in a practical problem of mineral exploration, where general domain knowledge alone is insufficient to satisfactorily model the potential controls on deposit formation using the available information in continent-scale information systems.     

Modeling of cadmium speciation and dynamics in the Seine estuary (France)

A complexation model (MOCO) was used to describe cadmium (Cd) speciation during estuarine transit in the Seine estuary. This model was developed from field data. Laboratory experiments based on the use of¹⁰⁹Cd enabled checking of certain model simplifications and hypotheses and evaluation of parameters which could not be measured directly. MOCO was coupled with a 3D multivariable hydrosedimentary model (SAM3D) to simulate Cd dynamics in the estuary. These results were compared with measurements (dissolved and particulate Cd) obtained during cruises representative of various hydrodynamic conditions. The purpose of this article is to present the modeling approach used, and its expected applications and limits.     

Mercury transformations and exchanges in a high turbidity estuary:: The role of organic matter and amorphous oxyhydroxides

The speciation and partition of mercury in a macrotidal estuary (Seine estuary, France) was studied in order to explore the role of the high turbidity zone (HTZ) in mercury transfer to the adjacent coastal waters. Water and particles were analyzed to determine the concentrations of various mercury species, including monomethylmercury and the inorganic fraction. The exchangeable particulate mercury, which varies with salinity, and the mercury fraction associated with the amorphous oxyhydroxides were evaluated. The distribution of dissolved mercury species during early mixing suggests non-conservative behavior of organically bound mercury at the head of the estuary. Mercury in the particles covaried positively with suspended particulate matter concentrations up to a threshold, which constitutes the typical mercury load of particles and deposited sediments of the HTZ. This distribution pattern is well explained by a dilution model: a slowly settling, low metal population of particle, characterized by relatively invariant turbidity, becomes admixed with a variable amount of higher metal content particles derived from the resuspension in the HTZ. In addition, in the HTZ, which acts as a degradation reactor for particulate organic matter, particulate mercury concentrations increase with increasing C:N ratios and amorphous oxyhydroxides particles. Mercury reaches the estuarine HTZ mainly associated with the riverine and marine particles, including organic matter and oxyhydroxides, which are temporarily trapped in the HTZ and mixed with autochthonous HTZ particles. The largest particles periodically settle and undergo diagenetic reactions and resuspensions, which lead to their mercury enrichment. Depending upon hydrodynamic conditions, mercury escapes seaward as fine particulate within the plume, partially associated with the oxyhydroxides. A surface complexation model reproduces most of the partitioning observed. In the dissolved phase the model simulation suggests that a very strong ligand must be present to explain the field observations.     

The effect of thermal treatment on the flotation of chrysocolla

Flotation studies using a Hallimond tube have been carried out on purified samples of chrysocolla. The results confirm that by heating the sample to 550°C, flotation of the sulphidized sample with amyl xanthate is considerably improved. Flotation with sodium dodecyl sulphate is also considerably higher. The recovery with cationic collectors is not modified by the thermal treatment.Electrophoretic mobility measurements and flotation studies using benzene instead of air for the collection of the particles suggest that the increase in flotation after thermal treatment is a consequence of (1) an increase in the solubility of copper ions and their adsorption on to the surface of the particles, and (2) an increase of the intrinsic hydrophobicity of the particles due to condensation of some of the free silanol groups on their surface.     

An integrated paleomagnetic study of Rio Grande de Santiago volcanic succession (trans-Mexican volcanic belt): revisited

We carried out an integrated paleomagnetic, rock-magnetic and paleointensity study of Miocene volcanic succession from the trans-Mexican volcanic belt (TMVB) north of Guadalajara. A total of 37 consecutive basaltic lava flows (326 oriented standard paleomagnetic cores) were collected at Lazo locality. Continuous susceptibility measurements with temperature and hysteresis experiments yield in most cases reasonably reversible curves with Curie points close to that of pseudo-single-domain magnetite. Two geomagnetic reversals were observed in the 300 m thick composite section. Paleosecular variation was lower than the one observed in general during Miocene. It appears that the volcanic units have been emplaced during a relatively short time span of about 1 Ma. The mean paleomagnetic directions obtained from this study do not differ significantly from that expected for the middle Miocene. The mean paleomagnetic direction calculated from all data is I=31.1°, D=354.6°, k=124 and ₉₅=2.1°, N=37. Seventy-two samples with apparently preserved primary magnetic mineralogy and without secondary magnetization, mostly belonging to reverse polarity chron were pre-selected for Thellier paleointensity determination. The flow-mean paleointensity values are ranging from 22.4±3.4 to 53.8±6.0 μT and the corresponding virtual dipole moments (VDMs) are ranging from (5.4±0.8) to (12.0±1.4)×10²² A m². This corresponds to mean value of (7.7±2.2)×10²² A m², which is close to present day geomagnetic field strength. Altogether, our data suggest the existence of relatively high geomagnetic field strength undergoing low fluctuations.     

Palaeosecular variation in Central Mexico over the last 30 000 years: the record from lavas

13 lava flows of known age (ages from ¹⁴C dating), which have been erupted in the last 30 000 years, have been studied to determine the palaeosecular variation of the geomagnetic field in Central Mexico. Samples were taken from two different monogenetic volcanic fields: the Michoacan-Guanajuato volcanic field (six sites) and the Chichinautzin Formation (seven sites), both part of the Transmexican Volcanic Belt. The lavas were studied in detail using rock magnetic methods (magnetic susceptibility at room temperature, low-temperature susceptibility behaviour, hysteresis loops, Curie temperatures), combined with reflected light microscopy, in order to deduce their magnetic mineralogy and the domain states of the magnetic minerals. The magnetic carriers are titanomagnetites, which show differing degrees of high-temperature deuteric oxidation, and seem to be predominantly pseudo-single domain (PSD), though in many cases are probably a mixture of domain states. Mean palaeomagnetic directions and palaeointensity values using Shaw and Thellier techniques were obtained using several specimens from each flow. Our data seem to indicate a sharp easterly swing in declination about 5000 years ago, which is also observed in lake sediments from Central Mexico. The calculated values of the virtual dipole moment (VDM) range from 3.1 to 14.9 × 10²² A m². Our data indicate that the virtual dipole moment seems to have increased gradually in magnitude over the last 30 kyr, with a peak at about 9000 years BP. These are features that have been observed in other parts of the globe and are probably caused by variations in the dipole part of the geomagnetic field.     

Study of the Santa Catarina aquifer system (Mexico Basin) using magnetotelluric soundings

A tensor magnetotelluric test survey was carried out in the region of Santa Catarina, located in the Chalco sub-basin of the Mexico Basin. The objective was to define the stratification at depth with an emphasis on the geometry of the main aquifer of that region which is partially known from DC resistivity soundings and drilling. High-quality magnetotelluric soundings could be recorded in the immediate vicinity of large urban zones because the sub-surface is very conductive. Interpretation shows that the solid bedrock is located at a depth of at least 800 m to the south and 1300 m to the north; it could, however, be much deeper. Using complementary DC resistivity sounding and well-logging data, three main layers have been defined overlying the bedrock. These layers are, from surface to bottom, an unsaturated zone of sand, volcanic ash and clay about 10 m thick, followed by a very conductive (1.5 ohm·m) 200 m thick layer of sand and ash with intercalated clay, saturated with highly mineralized water, and finally a zone with resistivity increasing gradually to 60 ohm·m. The investigated deep aquifer constitutes most of this third layer. It consists of a sequence of sand, gravel, pyroclastites and mainly fractured basalts. MT resistivity soundings and magnetic transfer functions also indicate that a shallow resistive structure is dipping, from the northwest, into the lacustrine deposits of the basin. This geologic feature is likely to be highly permeable fractured basaltic flows, which provide a channel by which water contaminated by the Santa Catarina landfill may leak into the basin.     

Growth mechanism and surface defects in the {210} form in pyrite crystals

This paper is a study of the etch pits on the {210} surfaces of pyrite. There is great similarity between them and the etch pits which appear on the {100} and {111} forms. The presence of six-fold growth spirals, frequent on the {210} form surfaces, result from a cooperation of two three-fold growth spirals generated from screw dislocations with the same sign of their Burgers vectors and different modulus.