Salinity contamination response to changes in irrigation management. Application of geochemical codes

Salinity contamination caused by irrigation has been widely studied but the analysis of geochemical processes regarding agronomic variables has not adequately been considered yet. The research presented here analyzes the influence of changes in irrigation management on salinity contamination, through the use of geochemical modeling techniques, in an agricultural basin during the hydrological year of 2001 and within the period 2005-2008. The results indicate that the changes implemented in irrigation management reduced the masses of salts exported in 72%, although water salinity increased by 25% (this salinity level does not restrict its use for irrigation). The different ionic ratios in drainage water, the results of the salinity balances, and the results of geochemical calculations (mass balances and speciation-solubility) indicate, mainly, precipitation of calcite, dissolution of gypsum and halite and cation exchange. The salt contamination index decreased from approximately 70% to levels close to those presented in modern irrigation areas, indicating that the changes in irrigation management were effective. Petrocalcic genesis and punctual sodification of soils can constitute an agroenvironmental problem that requires adequate management of irrigation and drainage considering future modernization of irrigation areas

Hydrogeochemical characterisation and modelling of groundwaters in a potential geological repository for spent nuclear fuel in crystalline rocks (Laxemar, Sweden)

Two sites in the eastern coast of Sweden have been investigated by the Swedish Nuclear Fuel and Waste Management Company (SKB), within the framework of the site characterisation programme, as possible candidates for hosting the proposed repository for the long-term storage of spent nuclear fuel: Forsmark and Laxemar. This study presents the main results concerning the hydrogeochemical characterisation of the groundwaters in the second site, Laxemar. The distribution of the main chemical variables in groundwaters are shown and interpreted in combination with the results from speciation-solubility and reaction-path simulations, together with the available mineralogical information. The results indicate that the main processes determining the overall geochemical evolution of the Laxemar groundwaters are advective/diffusive mixing and water-rock interactions driven by past and present climatic changes inducing the input of different recharge waters over time (glacial meltwater, old marine water and modern meteoric water) and affecting the preexisting very old saline groundwaters in the bedrock. The superimposed effects of these mixing events, deduced from the behaviour of the conservative elements (Cl and δ18O), have generated a rather steep salinity gradient in the groundwater system, with diluted waters in the upper part, brackish waters in the middle, and saline waters in the lower part of the bedrock. The resultant successive disequilibrium states imposed by mixing have conditioned the water-rock interaction processes that have affected the non-conservative elements to different degrees. The main chemical reactions found to be important in controlling some of the variability of these elements and some important parameters like pH and alkalinity, are: aluminosilicate and carbonate dissolution/precipitation, quartz and fluorite equilibrium, cation exchange, and gypsum dissolution. These reactions and their importance in the system are presented in this paper. Once the main hydrogeochemical features of the Laxemar groundwaters and the potentially controlling water-rock interactions in the system have been identified and justified with the help of thermodynamic simulations, a general geochemical conceptual model has been proposed. This model will be used as the basis for predicting the future evolution of the groundwater chemistry as an essential part of the safety assessment of the future repository

The Pliocene Ixtacamaxtitlan low sulfidation epithermal deposit (Puebla, Mexico A case of fossil fungi consortia in a steam-heated environment

The Ntacamaxtithin area in northern Puebla (central Mexico) contains middle Miocene Cu-Mo-Au porphyry/skarn and Pliocene low-sulfidation Au-Ag epithermal deposits that are geologically associated with the evolution of the Timis-Mexican Volcanic Belt (TATVB). In this paper, a new Ar-40/Ar-39 age (2.87 +/- 0.41 Ma) is provided for rhombohedral alunite from a kaolinite + alunite +/- opal +/- cristobalite +/- sin ectite advanced argillic alteration assemblage. This age contributes to the definition of a metallogenic province that is confined to the TIVIVB, a relevant feature for regional exploration. A similar to 12 My gap is established between the formation of the Cu-Mo-Au porphyry/ skarn and low-sulfidation Au-Ag epithermal deposits, which rules out the possibility that their overlapping was the result of telescoping Advanced argillic alteration is conspicuous throughout the mineralized area. This alteration assemblage consists of a widespread kaolinite-rich blanket that underlies silica sinters, polymictic hydrothermal breccias, and an alunite-rich spongy layer that consists of vertical tubular structures that are interpreted as the result of gas venting in a subaerial environment. The above indicate a shallow hypogene origin for the advanced argillic alteration assemblage that is, formation by the partial condensation within a phreatic paleoaquifer of acidic vapors that were boiled-off along fractures that host epithermal veins at depth. The formation of the spongy alunite layer and silica sinters is interpreted to have been synchronous. Within the alunite-rich spongy layer, tubular structures hosted microbial consortia dominated by fungi and possible prokaryote (Bacteria or Archaea) biofilms. Such consortia were developed on previously formed alunite and kaolinite and were preserved due to their replacement by opal, kaolinite, or alunite. This means that the proliferation of fungi and prokaryotes occurred during a lull in acidic gas venting during which other organisms (i.e., algae) might have also prospered. Periodic acidic gas venting is compatible with a multi-stage hydrothermal system with several boiling episodes, a feature typical of active geothermal systems and of low-sulfidation epithermal deposits. The microstructures, typical for fungi, are mycelia, hyphae with septa, anastornoses between branches, and cord-like groupings of hyphae. Possible evidence for skeletal remains of prokaryote biofilms is constituted by cobweb-like microstructures composed of <1 mu m thick interwoven filaments in close association with hyphae (about 2.5 pm thick). Bioweathering of previously precipitated minerals is shown by penetrative biobrecciation due to extensive dissolution of kaolinite by mycelia and by dissolution grooves from hyphae on alunite surfaces. Such bioweathering was possibly predated by inorganically driven partial dissolution of alunite, which suggests a lull in acidic gas venting that allowed living organisms to thrive. This interpretation is sustained by the occurrence of geometrical dissolution pits in alunite covered by hyphae. Fungal bioweathering is particularly aggressive on kaolinite due to its relatively poor nutrient potential. Such delicate microstructures are riot commonly preserved in the geological record. In addition, numerous chalcopyrite microcrystals or microaggregates are found within the alunite layer, which could be related to sulfate reduction due to bacterial activity from the sulfate previously released by fungal bioweathering of alunite. Iiydrogeochernical modeling constrains pH to between similar to 3.2 and similar to 3.6 and temperature to between 53 and 75 degrees C during the stage in which fungi and other organisms thrived. These waters were cooler and more alkaline than in earlier and later stages, which were characterized dominantly by steam-heated waters. The most likely process to account for this interlude would be mixing with meteoric water or with upwelling mature water that did not undergo boiling. El área de Ixtacamaxtitlán en el norte de Puebla (México central) contiene depósitos de tipo pórfido/ skarn de Cu-Mo-Au del Mioceno medio y depósitos epitermales de baja sulfuración de Au-Ag del Plioceno, que están geológicamente asociados a la evolución de la Faja Volcánica Trans-Mexicana (FVTM). En este trabajo se presenta una nueva edad 40Ar/39Ar (2.87 ± 0.41 Ma) en alunita romboédrica procedente de una asociación de alteración argílica avanzada constituida por kaolinita + alunita ± ópalo ± cristobalita ± esmectita. Esta edad contribuye a la definición de una provincia metalogenética circunscrita a la FVTM, lo cual constituye un rasgo relevante para la exploración regional. Se ha determinado un lapso de ~12 millones de años entre la formación de los depósitos de tipo pórfido/skarn de Cu-Mo-Au y los depósitos epitermales de baja sulfuración de Au-Ag, lo cual invalida la posibilidad de que la superposición existente entre dichos depósitos constituya un auténtico telescopaje, contrariamente a interpretaciones previas. Además, dentro de dicho lapso se produjo la formación de un estratovolcán en el área de estudio, que habría interferido en cualquier actividad hidrotermal existente. La asociación de alteración argílica avanzada es reconocible en un área extensa de la zona mineralizada. Dicha asociación consiste en un amplio cuerpo subhorizontal rico en kaolinita que subyace a sínteres silícicos, brechas hidrotermales polimícticas, y un horizonte de aspecto esponjoso rico en alunita que consiste en estructuras verticales tubulares que se interpretan en este trabajo como debidas al escape de gases en un ambiente subaéreo. Tales características son compatibles con un ambiente de formación hipogénico somero para la asociación de alteración argílica avanzada—esto es, formación en terrenos calentados por vapor derivados de la condensación parcial en un paleoacuífero freático de vapores ácidos generados por ebullición a lo largo de fracturas que eventualmente alojaron vetas epitermales en profundidad. Se interpreta que la formación del horizonte esponjoso de alunita y de los sínteres silícicos fue sincrónica. En el interior de las estructuras tubulares de alunita se desarrollaron consorcios dominados por hongos que también incluyen posibles biofilmes de procariontes (bacterias o arqueas). Dichos consorcios se desarrollaron sobre alunita y kaolinita previamente precipitadas, y fueron preservados debido a su reemplazamiento por ópalo, kaolinita o alunita. Ello conlleva que la proliferación de hongos y procariontes se produjo en periodos de pausa en la emanación de gases ácidos, durante los cuales otros organismos (i.e., algas) pudieron haber igualmente prosperado. Este rasgo es compatible con un sistema hidrotermal multiepisódico con diversas etapas con ebullición, lo cual concuerda con el ambiente de formación de los depósitos epitermales de baja sulfuración. Las microestructuras observadas típicas de hongos son micelios, hifas septadas, anastomosis entre ramificaciones, y agrupaciones de hifas en forma de cable o cordón. La posible evidencia de restos esqueléticos de biofilmes de procariontes la constituyen microestructuras semejantes a telarañas formadas por el entramado de filamentos con grosores <1 µm, que se encuentran íntimamente asociadas a hifas (éstas, con grosores del orden de ~2.5 µm). La biometeorización de minerales previamente precipitados se muestra en forma de biobrechificación penetrativa debida a la extensa disolución de kaolinita generada por micelios y por el desarrollo de surcos de disolución generados por hifas en la superficie de los cristales de alunita. Dicha biometeorización vino antecedida por la disolución parcial de la alunita, posiblemente de origen inorgánico, lo cual denota la instalación de un ambiente más benéfico (menos ácido) para el desarrollo de organismos vivos y, por tanto, de un periodo de pausa en la exhalación de gases. Dicha interpretación se argumenta con la presencia de mellas geométricas de disolución en alunita, cubiertas por hifas. La biometeorización fúngica es particularmente agresiva en kaolinita debido a su relativamente pobre potencial nutritivo. Estos tipos de microestructuras delicadas no se preservan habitualmente en el registro geológico. Asimismo, se encuentran numerosos microcristales y microagregados de calcopirita en el horizonte de alunita, que pueden ser hipotéticamente asociados a reducción de sulfatos debida a actividad bacteriana, a partir del sulfato previamente liberado por medio de la biometeorización de alunita. El modelado hidrogeoquímico permitió constreñir el pH entre ~3.2 y ~3.6 y la temperatura entre 53° and 75 °C durante el estadio en que los hongos y otros organismos prosperaron en asociación con aguas más frías y alcalinas que en los estadios precedente y posterior, que se caracterizaron por la presencia dominante de aguas calentadas por vapor. Tales variaciones en temperatura y pH con respecto a los fluidos precedentes pudieron haber sido consecuencia de la mezcla entre éstos y otros fluidos de nueva incorporación. Los candidatos más verosímiles para permitir dicho interludio serían el agua meteórica o agua ascendente madura que no experimentó ebullición