NDLI: Function of a deltaic silt deposit as a repository and long-term source of sulfate and related weathering products in a glaciofluvial aquifer derived from organic-rich shale (North Dakota, USA)

Content Provider	Springer Nature Link
Author	Schuh, W. M. Bottrell, S. H.
Copyright Year	2013
Abstract	A shallow unconfined glaciofluvial aquifer in North Dakota (USA) has largest groundwater sulfate concentrations near the bottom boundary. A deltaic silt layer underlying the aquifer, at >16 m, is the modern proximate sulfate source for the aquifer. The original sulfate source was pyrite in the organic-rich shale component of the aquifer and silt grain matrix. An oxidizing event occurred during which grain-matrix pyrite sulfur was oxidized to sulfate. Thereafter the silt served as a “conserving” layer, slowly feeding sulfate into the lower part of the aquifer and the underlying till. A method was developed for estimating the approximate initial sulfate concentration in the source layer and the redistribution time since the oxidizing event, using a semi-generic convection–dispersion model. The convection–dispersion model and a model for the evolution of modern sulfate δ $^{34}$S in silt-layer pore water from the initial grain-matrix pyrite δ $^{34}$S, both estimated that the oxidizing event occurred several thousand years ago, and was likely related to the dry conditions of the Hypsithermal Interval. The silt layer also serves as an arsenic source. Results indicate that deltaic silts derived from organic-rich shale parent materials in a glacial environment can provide long-term sources for sulfate and arsenic and possibly other related oxidative weathering products.Un aquifère fluvioglaciaire libre, peu profond dans le Dakota du Nord (Etats Unis) a les plus fortes concentrations en sulfates dans ses eaux souterraines à proximité du substratum. Une couche de silts deltaïques située en dessous de l’aquifère, à plus de 16 m, constitue la source moderne de sulfate la plus proche. La source d’origine du sulfate est la pyrite présente dans les schistes riches en matière organique qui composent la matrice des grains silteux de l’aquifère. Il s’est produit un épisode d’oxydation durant lequel le sulfure des grains pyriteux de la matrice s’est oxydé en sulfate. Ensuite, le silt a servi de couche de « conservation », relâchant lentement les sulfates dans la partie inférieure de l’aquifère et des tillites sous-jacentes. Pour estimer la quantité approximative de sulfate initial dans la couche source et le temps de redistribution depuis l’épisode d’oxydation, une méthode a été développée, utilisant un modèle semi générique de convection–dispersion. Le modèle de convection–dispersion et un modèle pour l’évolution des sulfates modernes δ $^{34}$S dans l’eau des pores de la couche silteuse provenant du δ $^{34}$S initial de la pyrite des grains de la matrice estiment tous deux que l’épisode d’oxydation est survenu il y a plusieurs milliers d’années, et qu’il est probablement en lien avec les conditions sèches de l’intervalle Hypsithermal. La couche silteuse sert également de source d’arsenic. Les résultats indiquent que les silts deltaïques dérivés des matériaux parents constitués de schistes riches en matière organique dans un environnement glaciaire peuvent produire sur le long terme des sources de sulfate et d’arsenic et probablement d’autres produits liés aux processus oxydants d’altération.Un acuífero somero glacifluvial no confinado en Dakota del Norte (EEUU) tiene las mayores concentraciones de sulfato en el agua subterránea cerca del límite del fondo. Una capa de limos deltaica que subyace al acuífero, a >16 m, es una fuente próxima de sulfato moderno del acuífero. La fuente original de sulfato era pirita en la componente de esquistos ricos en materia orgánica del acuífero y en la matriz del grano de limo. Un evento oxidante ocurrió durante el cual la matriz de grano del sulfuro de la pirita fue oxidada a sulfato. De allí en adelante el limo sirvió como una capa conservante, alimentando lentamente el sulfato en la parte más baja del acuífero y el till subyacente. Se desarrolló un método para estimar la concentración inicial aproximada de sulfato en la capa fuente y en el tiempo de redistribución desde el evento oxidante usando un modelo semigenérico de convección – dispersión. Tanto el modelo de convección – dispersión como el modelo para la evolución del sulfato moderno δ $^{34}$S en el agua poral de la capa limosa a partir de la matriz inicial del grano de pirita, δ $^{34}$S, estimaron que el evento oxidante ocurrió varios miles de años atrás, y estuvo probablemente relacionado a las condiciones de sequías del intervalo hipsitermal. La capa de limo también sirve como una fuente de arsénico. Los resultados indican que los limos deltaicos derivados del los materiales parentales de esquistos ricos en materia orgánica en un ambiente glacial pueden proporcionar fuentes a largo plazo de sulfato y arsénico y posiblemente otros productos de la meteorización oxidativa relacionados.美国北大卡达州浅层非承压冰水含水层底部边界附近地下水中的硫酸盐含量最高。下伏于含水层的三角洲粉砂层,大约在》16米的地方,是含水层当代最接近硫酸盐源的地方。原来的硫酸盐源为含水层富含页岩成分中的黄铁矿和粉砂颗粒基质。氧化事件发生时,颗粒基质的黄铁矿硫磺氧化成硫酸盐。因此,粉砂成为“保护”层,慢慢输送硫酸盐进入含水层下部及下伏的冰碛层。开发了使用半-类属的对流-分散模型评价源层内近似初始硫酸盐含量及氧化事件以来的再分布事件的方法。对流-分散模型及从初始颗粒基质黄铁矿δ $^{34}$S演化为粉砂层孔隙水中的当代硫酸盐δ $^{34}$S的模型都预计几千年前发生过氧化事件,并且氧化事件可能与大热时期的干燥条件有关。粉砂层也充当着砷源。结果显示,冰川环境下源自富含有机物的页岩母质的三角洲粉砂可以为硫酸盐和砷以及可能的其他氧化风化物提供长期的来源。Um aquífero livre glacio-fluvial pouco profundo no Dakota do Norte (EUA) apresenta as maiores concentrações de sulfato em água subterrânea próximo do limite inferior. Uma camada de silte deltaico situada sob o aquífero, a >16 m, é a fonte próxima atual de sulfato para o aquífero. A origem primitiva do sulfato é a pirite da componente rica em matéria orgânica do argilito e na matriz granular siltosa do aquífero. Em dada altura ocorreu um evento oxidante durante o qual o enxofre da pirite na matriz granular foi oxidado a sulfato. Posteriormente, o silte funcionou como uma camada “conservativa”, alimentando lentamente de sulfato a parte inferior do aquífero e o till subjacente. Foi desenvolvido um método para estimar a concentração inicial aproximada de sulfato na camada origem e o tempo de redistribuição desde o evento oxidante, utilizando um modelo semi-genérico de convecção-dispersão. Quer o modelo semi-genérico de convecção-dispersão, quer um modelo para a evolução da concentração de δ $^{34}$S do sulfato moderno na fase aquosa da camada de silte a partir da concentração de δ $^{34}$S inicial na pirite da matriz granular, ambos estimaram que o evento oxidante ocorreu há vários milhares de anos atrás e que provavelmente esteve relacionado com as condições secas do Intervalo Hipsitérmico. A camada de silte também funciona como uma origem de arsénio. Os resultados indicam que os siltes deltaicos provenientes de argilitos ricos em matéria orgânica em ambiente glacial podem fornecer origens a longo prazo de sulfato e de arsénio e possivelmente outros produtos de alteração oxidante.
Starting Page	565
Ending Page	585
Page Count	21
File Format	PDF
ISSN	14312174
Journal	Hydrogeology Journal
Volume Number	22
Issue Number	3
e-ISSN	14350157
Language	Portuguese
Publisher	Springer Berlin Heidelberg
Publisher Date	2013-12-19
Publisher Institution	International Association of Hydrogeologists
Publisher Place	Berlin, Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Paleohydrology Pyrite oxidation Groundwater sulfate Arsenic USA Hydrogeology Hydrology/Water Resources Geology Waste Water Technology Water Pollution Control Water Management Aquatic Pollution
Content Type	Text
Resource Type	Article
Subject	Earth and Planetary Sciences Water Science and Technology

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