Patrick Longmire

Identification of hydrochemical facies in the Roswell Artesian Basin, New Mexico (USA), using graphical and statistical methods

Analysis of groundwater chemistry can yield important insights about subsurface conditions, and provide an alternative and complementary method for characterizing basin hydrogeology, especially in areas where hydraulic data are limited. More specifically, hydrochemical facies have been used for decades to help understand basin flow and transport, and a set of facies were developed for the Roswell Artesian Basin (RAB) in a semi-arid part of New Mexico, USA. The RAB is an important agricultural water source, and is an excellent example of a rechargeable artesian system. However, substantial uncertainties about the RAB hydrogeology and groundwater chemistry exist. The RAB was a great opportunity to explore hydrochemcial facies definition. A set of facies, derived from fingerprint diagrams (graphical approach), existed as a basis for testing and for comparison to principal components, factor analysis, and cluster analyses (statistical approaches). Geochemical data from over 300 RAB wells in the central basin were examined. The statistical testing of fingerprint-diagram-based facies was useful in terms of quantitatively evaluating differences between facies, and for understanding potential controls on basin groundwater chemistry. This study suggests the presence of three hydrochemical facies in the shallower part of the RAB (mostly unconfined conditions) and three in the deeper artesian system of the RAB. These facies reflect significant spatial differences in chemistry in the basin that are associated with specific stratigraphic intervals as well as structural features. Substantial chemical variability across faults and within fault blocks was also observed.RésuméL’analyse de la chimie des eaux souterraines peut amener d’importantes informations concernant les conditions du sous-sol, et fournir une méthode alternative et complémentaire pour caractériser l’hydrogéologie d’un bassin, spécialement dans des zones où les données hydrauliques sont limitées. De manière plus spécifique, les faciès hydrochimiques ont été utilisés depuis des décennies pour aider à la compréhension des écoulements et des processus de transport dans les bassins, et un ensemble de faciès a été développé pour le bassin artésien de Roswell (BAR) dans la partie semi-aride du Nouveau Mexique, Etats-Unis d’Amérique. Le BAR constitue une ressource en eau importante pour l’agriculture, et est un excellent exemple d’un système artésien rechargé. Cependant, il existe d’importantes incertitudes concernant l’hydrogéologie du BAR et la chimie des eaux souterraines. Le BAR représente une grande opportunité pour explorer la définition des faciès hydrochimiques. Un ensemble de faciès, dérivé des diagrammes d’empreintes digitales (approche graphique), constitue une base pour tester et comparer des analyses en composantes principales, analyses factorielles et analyses par grappe (approches statistiques). Des données géochimiques de plus de 300 puits de la partie centrale du bassin ont été examinées. Le test statistique de faciès basé sur les diagrammes d’empreintes digitales a été utile en termes d’évaluation quantitative des différences entre les faciès, et pour la compréhension des contrôles potentiels sur la chimie des eaux souterraines du bassin. Cette étude suggère que la présence de trois faciès hydrochimiques dans la partie la moins profonde du BAR (conditions essentiellement d’aquifère libre) et de trois dans le système artésien profond du BAR. Ces faciès reflètent des différences spatiales importantes dans la chimie au sein du bassin, qui sont associées aussi bien à des intervalles stratigraphiques qu’à des caractéristiques structurales. Une importante variabilité chimique au travers des failles et à l’intérieur des blocs délimités par des failles a également été observée.ResumenEl análisis de la química del agua subterránea puede dar importantes conocimientos sobre las condiciones del subsuelo, y proporcionar un método alternativo y complementario para la caracterización hidrogeológica de una cuenca, especialmente en las zonas donde los datos hidráulicos son limitados. Más específicamente, las facies hidroquímicas se han utilizado durante décadas para ayudar a entender el flujo y el transporte en la cuenca, y un conjunto de facies fueron desarrolladas para la cuenca artesiana de Roswell (RAB) en una zona semi-árida de Nuevo México, EEUU. La RAB es una importante fuente de agua para la agricultura, y es un excelente ejemplo de un sistema recargable de pozos artesianos. Sin embargo, existen grandes incertidumbres sobre la hidrogeología y la química del agua subterránea en la RAB. La RAB fue una gran posibilidad para explorar la definición de facies hidroquímicas. Un conjunto de facies, derivados de diagramas de huellas digitales (métodos gráficos), existen como una base para la prueba y para la comparación de componentes principales, análisis factorial y análisis de cluster (métodos estadísticos). Se examinaron los datos geoquímicos de más de 300 pozos en la cuenca central de la RAB. La prueba estadística de facies basadas en diagramas de huellas digitales fue útil en términos de evaluar cuantitativamente las diferencias entre las facies, y para la comprensión de posibles controles sobre la química en la cuenca de agua subterránea. Este estudio sugiere la presencia de tres facies hidroquímicas en la parte menos profunda de la RAB (condiciones mayormente no confinadas) y tres en el sistema artesiano más profundo de la RAB. Estas facies reflejan diferencias espaciales significativas en la química de la cuenca que están asociados con intervalos estratigráficos específicos, así como con características estructurales. También se observó una variabilidad química sustancial entre las fallas y dentro de bloques de fallas.摘要地下水化学分析可以帮助人们深入了解地表以下的情况,并为描述盆地水文地质特征、尤其是水力资料有限的地区的水文地质特征提供选择性的和补充性的方法。更具体地说,水化学相使用了几十年帮助了解盆地的水流及水的传输,并且针对美国新墨西哥州半干旱地区罗斯维尔自流盆地提出了一套水化学相。罗斯维尔自流盆地是重要的农业水源地,也是可补给自流系统的极好榜样。然而,罗斯维尔自流盆地水文地质状况和地下水化学存在着大量的不确定性。罗斯维尔自流盆地是探索水化学相的重要机遇。由指纹图表(图解法)导出的一套水化学相作为主要成份的测试和比较、因子分析和聚类分析(统计法)的基础。对罗斯维尔自流盆地中心地区300多口井的地球化学资料进行了调查。基于指纹图表的水化学相统计测试在定量评估相之间的差别以及在了解对盆地地下水化学潜在控制时非常有用。本研究显示,在罗斯维尔自流盆地(大部分为非承压条件)浅部存在着三个水化学相,在盆地较深的自流系统中也存在着三个水化学相。这些水化学相反映了盆地内与特殊地层间隔层以及构造特征相关的化学状况的重要空间差别。还观测了穿过断层及断层地块内的实际化学变化。ResumoAnálises químicas das águas subterrâneas podem gerar informações importantes sobre as condições do subsolo, e fornecer um método alternativo e complementar para a caracterização da bacia hidrogeologia, especialmente em áreas onde dados hidráulicos são limitados. Mais especificamente, fácies hidroquímicas têm sido utilizadas por décadas para ajudar a entender o fluxo e o transporte em bacia, e um conjunto de fácies foram desenvolvidos para a Bacia Artesiana de Roswell (BAR), na porção semiárida no Novo México, EUA. A BAR é uma importante fonte de água para a agricultura, e um excelente exemplo de um sistema artesiano recarregável. Entretanto, existem incertezas substanciais sobre a hidrogeologia e química das águas subterrâneas na BAR. A BAR foi uma grande oportunidade para explorar as definições de fácies hidroquímicas. Testes estatísticos baseados em diagramas padrões de fácies hidroquímicas (abordagem gráfica), existem como base para testes e para comparação de componentes principais, fatores de análises, e análises de clusters (abordagem estatística). Foram examinados dados geoquímicos de mais de 300 poços na BAR, em sua porção central. O teste estatístico, com base em diagramas padrões, foi útil em termos de avaliar quantitativamente as diferenças entre fácies, e para compreender os possíveis controles na composição química das águas subterrâneas. Este estudo sugere a presença de três fácies hidroquímicas na parte rasa da BAR (em sua maioria sob condições de não confinamento), e outras três no sistema artesiano mais profundo da BAR. Estas fácies refletem diferenças espaciais significativas na química da bacia, que estão associados à intervalos estratigráficos específicos, bem como características estruturais. Também foram observadas variações químicas substanciais, ao longo de falhas bem como no interior dos blocos de falha.

Water quality and hydrogeochemistry of a basin and range watershed in a semi-arid region of northern New Mexico

Hundreds of domestic wells in northern New Mexico, have concentrations of U, As, and NO3− that exceed the Environmental Protection Agency’s (EPA) maximum contaminant level (MCL) for drinking water consumption. As part of a case study in groundwater quality, we collected groundwater samples from 749 domestic wells throughout the eastern half of the Española Basin. All water samples were analyzed for major ions, trace metals, and alkalinity. Selected samples were also analyzed for stable isotopes of O, H, and N. Of the wells we measured, 15, 173, and 99 had respective NO3−, U, and As concentrations that exceeded the EPA’s MCL. Total dissolved solids (TDS), U, and HCO3− were elevated in the Sangre de Cristo mountain block and around the town of Nambé. Our findings suggest that roll-front U deposits and devitrification of volcanic ash result in elevated U near Nambé, while weathering of granitic rocks accounts for high U in the mountain block. Arsenic concentrations were high in much of the study area with the exception of the Santa Fe metro region and the mountain block. Elevated As concentrations can be explained by devitrification of volcanic ash, anion exchange with clays, and mixing with hydrothermal fluids. In wells with high NO3− concentrations, analysis of N isotopes are consistent with contamination from domestic wastewater effluent. Our findings suggest that the geochemistry of the region is largely influenced by local geology while groundwater contamination from domestic water treatment and wastewater effluent is an emerging issue.

Design and Construction of Multi-Layered Permeable Reactive Barrier for Removing Radionuclides, Nitrate, and Perchlorate at Los Alamos National Laboratory

Los Alamos National Laboratory (LANL) and Shaw Environmental, Inc. (Shaw) designed and constructed a multilayered permeable reactive barrier (PRB) to remove contaminants from shallow alluvial groundwater within Mortandad Canyon at LANL. This project was developed as a pilot project for LANL to conduct research and development and proof of concept and as such does not meet all identified target contaminant concentrations, but provides LANL data for future applications of the technology. Shaw worked jointly with LANL scientists in selecting the site, conducting a geotechnical and hydrogeologic investigation with contaminant characterization for waste disposal, preparing a design basis report, conducting geochemical and groundwater flow modeling, and preparing both conceptual and final detailed engineered designs. Geochemical modeling of the PRB multibarrier processes was conducted to predict influent and effluent contaminant concentrations and evaluate the potential for mineral precipitation and reduction of effective porosity in the barrier. A numerical model of groundwater flow was constructed to simulate hydrogeologic conditions in Mortandad Canyon and then used to simulate flow with the PRB in place. The Mortandad Canyon PRB is designed to remove radionuclides (americium-241, plutonium-238 and 239/240, and strontium-90), nitrate, and perchlorate from alluvial groundwater. The PRB consists of a funnel and gate constructed of sealable sheet piling driven through the alluvium and into the underlying volcanic tuff. The gate is designed as a braced cofferdam. The gate contains four sequential media cells consisting of lava rock gravel, mineral apatite (a calcium phosphate), biobarrier, and limestone gravel. The lava rock gravel will sorb colloids (sorbed with americium, plutonium, and strontium) from the alluvial groundwater. The apatite will remove soluble metals and radionuclides through sorption processes. The biobarrier serves as a host microorganisms that biodegrade nitrates and perchlorate. The limestone gravel functions to buffer the biobarrier effluent. In addition, there will also be sorption of soluble plutonium, americium, and metals within the biobarrier and limestone layer. A series of sampling ports and monitoring wells were installed within the reactive media cells. The purpose of the funnel is to direct shallow alluvial groundwater through the gate. This project was a joint effort between LANL and Shaw. The initial feasibility studies and bench scale treatability were conducted at LANL. The LANL laboratory data was used as the basis for design criteria. The hydrogeologic and geochemical modeling, engineering design, and construction were performed by Shaw with LANL guidance and input.Copyright