Susan K. Swanson

Two-way cluster analysis of geochemical data to constrain spring source waters

Abstract The purpose of the study was to use geochemical characteristics and apparent ages of sampled groundwaters to determine which of the two regionally extensive bedrock aquifers, the lower bedrock aquifer or the upper bedrock aquifer, is a more likely source of water discharging to springs in the Nine Springs watershed. The use of summary statistics and our knowledge of the regional hydrostratigraphy resulted in the identification of three groups of monitoring points that are representative of groundwaters with distinct geochemical characteristics. Two-way cluster analysis of the geochemical data supports these groupings and further identifies subtle geochemical characteristics of the groups. One spring, which is representative of smaller springs and seeps found in the watershed, belongs to a group that is characterized by variable nitrate and chloride concentrations. Water discharging from this spring has a groundwater residence time of approximately 8 years based on the tritium/helium 3 dating method. The water discharging to this small spring is thought to have traveled primarily through the unlithified aquifer, as opposed to either of the major bedrock aquifers. Most of the springs in the watershed belong to a group that is characterized by elevated, but consistent, nitrate, sodium, and chloride concentrations. In addition, cluster analysis revealed that potassium and alkalinity concentrations are somewhat low. Apparent groundwater ages for this group range from 10 to 15 years. The water discharging from the majority of the springs in the watershed is thought to have traveled primarily through the unlithified aquifer and the upper bedrock aquifer before discharging into the former glacial lakebed wetland complex. Due to the relatively short groundwater residence times, spring water quality and flow in the Nine Springs watershed are likely to be vulnerable to the rapid urban expansion occurring within the watershed.

Spatial and temporal constraints on regional-scale groundwater flow in the Pampa del Tamarugal Basin, Atacama Desert, Chile

Aquifers within the Pampa del Tamarugal Basin (Atacama Desert, northern Chile) are the sole source of water for the coastal city of Iquique and the economically important mining industry. Despite this, the regional groundwater system remains poorly understood. Although it is widely accepted that aquifer recharge originates as precipitation in the Altiplano and Andean Cordillera to the east, there remains debate on whether recharge is driven primarily by near-surface groundwater flow in response to periodic flood events or by basal groundwater flux through deep-seated basin fractures. In addressing this debate, the present study quantifies spatial and temporal variability in regional-scale groundwater flow paths at 20.5°S latitude by combining a two-dimensional model of groundwater and heat flow with field observations and δ18O isotope values in surface water and groundwater. Results suggest that both previously proposed aquifer recharge mechanisms are likely influencing aquifers within the Pampa del Tamarugal Basin; however, each mechanism is operating on different spatial and temporal scales. Storm-driven flood events in the Altiplano readily transmit groundwater to the eastern Pampa del Tamarugal Basin through near-surface groundwater flow on short time scales, e.g., 100–101 years, but these effects are likely isolated to aquifers in the eastern third of the basin. In addition, this study illustrates a physical mechanism for groundwater originating in the eastern highlands to recharge aquifers and salars in the western Pampa del Tamarugal Basin over timescales of 104–105 years.RésuméLes aquifères de la pampa du bassin du Tamarugal (désert d’Atacama, Nord du Chili) constituent la seule source d’approvisionnement en eau pour la ville côtière d’Iquique et l’industrie minière, importante sur le plan économique. Malgré cela, le système régional des eaux souterraines demeure mal compris. Bien qu’il soit largement accepté que la recharge de l’aquifère trouve son origine dans les précipitations de l’Altiplano et de la Cordillère des Andes à l’est, le débat demeurre pour savoir si la recharge est conduite principalement par des écoulements proches de la surface en réponse aux événements périodiques d’inondation ou bien par un écoulement souterrain empruntant des fractures ancrées en profondeur dans le bassin. En s’attaquant à cette discussion, la présente étude mesure la variabilité spatiale et temporelle des cheminements des eaux souterraines à l’échelle régionale, à la latitude de 20.5°S en combinant un modèle bidimensionnel des transferts d’eaux souterraines et de chaleur avec des observations de terrain et des valeurs de l’isotope δ18O dans les eaux souterraines et les eaux de surface. Les résultats suggèrent que les deux mécanismes de recharge précédemment proposés influencent probablement les aquifères dans la pampa du bassin du Tamarugal; cependant, chaque mécansime fonctionne sur différentes échelles spatiales et temporelles. Des inondations provoquées par des événements orageux dans l’Altiplano se propagent aisément aux eaux souterraines vers l’est de la pampa du bassin du Tamarugal Basin par un écoulement des eaux souterraines proche de la surface sur des échelles de temps courtes, par exemple, 100–101 ans, mais ces effets sont probablement localisés dans les aquifères du tiers oriental du bassin. En complément, cette étude illustre le mécanisme physique pour des eaux souterraines provenant des montagnes orientales pour recharger des aquifères et des salars dans l’ouest de la Pampa du bassin de Tamarugal dans des échelles de temps de 104–105 années.ResumenLos acuíferos de la cuenca de la Pampa del Tamarugal (Desierto de Atacama, norte de Chile) son la única fuente de agua para la ciudad costera de Iquique y para la industria minera de importancia económica. A pesar de esto, el sistema regional de agua subterránea sigue siendo poco conocida. Aunque está ampliamente aceptado que la recarga de los acuíferos se origina en la precipitaciones en el Altiplano y la Cordillera de los Andes al este, sigue habiendo un debate si la recarga es determinada principalmente por el flujo subterráneo cercano a la superficie en respuesta a eventos de inundación periódicos o por el flujo subterráneo basal a través de las fracturas situadas en profundidad en la cuenca. Para el tratamiento de este debate el presente estudio, cuantifica la variabilidad espacial y temporal de las trayectorias de flujo subterráneo a escala regional a una latitud de 20.5 ° S mediante la combinación de un modelo bidimensional del flujo subterráneo y de calor con las observaciones de campo y los valores isotópicos de δ18O en las aguas superficiales y subterráneas. Los resultados sugieren que tanto los mecanismos de recarga de los acuíferos previamente propuestos son probablemente influyentes en los acuíferos de la cuenca de la Pampa del Tamarugal; sin embargo, cada mecanismo está operando en diferentes escalas espaciales y temporales. Las inundaciones por las tormentas en el Altiplano transmiten fácilmente al agua subterránea en la cuenca oriental de la Pampa del Tamarugal través del flujo subterráneo cercano a la superficie en escalas de tiempo corto, por ejemplo, 100–101 años, pero estos efectos probablemente son aislados en los acuíferos del tercio oriental de la cuenca. Además, este estudio ilustra un mecanismo físico para las aguas subterráneas que se originan en las tierras altas del este para recargar los acuíferos y salares en la Cuenca de la Pampa del Tamarugal occidental en escalas de tiempo de 104–105 años.摘要(智利北部阿塔卡马沙漠)Pampa del Tamarugal盆地内的含水层是沿海城市伊基克及经济上重要的开采行业的唯一水源。尽管如此,人们对区域地下水系统仍然知之甚少。虽然普遍认为含水层补给来源于东面的阿尔蒂普拉诺高原和安第斯山脉,但是否主要由周期性洪水事件造成的近地表地下水流或者由穿过深部盆地断裂的基岩地下水通量补给仍存在着争议。为探寻这个争议,目前的研究把地下水与热流二维模型与野外观测数据及地表水和地下水中的同位素δ18O值相结合,量化了纬度20.5°S区域尺度地下水流通道的时空变化。结果显示,上述两种含水层补给机理很可能影响着Pampa del Tamarugal盆地内的含水层;然而,每一种机理处在不同的时空尺度上。阿尔蒂普拉诺高原暴雨引起的洪水事件很容易把地下水通过短时尺度如100–101年近地表地下水流输送到Pampa del Tamarugal盆地,但是这些效果很可能被分散到了盆地东部三分之一区域的含水层。此外,本项研究论述了起源于东部高地的地下水在104–105年时间尺度上补给Pampa del Tamarugal盆地西部含水层和干盐湖的物理机理。ResumoAquíferos dentro da bacia do Pampa do Tamarugal (Deserto do Atacama, norte do Chile) são a única fonte de água para a cidade costeira de Iquique e economicamente importante para a indústria de mineração. Apesar disso, o sistema de águas subterrâneas regional permanece pobremente entendido. Embora seja amplamente aceito que a recarga do aquífero origina-se como precipitação no Altiplano e Cordilheira Andina para o leste, há debates remanescentes se a recarga é oriunda primeiramente pelo fluxo de águas subterrâneas próximo da superfície em resposta a eventos periódicos de inundação ou pelo fluxo basal de águas subterrâneas através de bacias fraturadas profundas. Em resposta a este debate, o presente estudo quantifica a variabilidade espacial e temporal em escala regional dos padrões de fluxo das águas subterrâneas no trajeto em 20.5°S de latitude combinando um modelo bidimensional de águas subterrâneas e fluxo de calor com observações a campo e valores isotópicos de δ18O em águas superficiais e subterrâneas. Os resultados sugerem que ambos mecanismos de recarga do aquífero anteriormente propostos estão provavelmente influenciando dentro da bacia do Pampa do Tamarugal; porém, cada mecanismo está operando em diferentes escalas espaciais e temporais. Eventos de inundação gerados por tempestades no Altiplano prontamente transmitem águas subterrâneas para o leste da bacia do Pampa do Tamarugal através do fluxo próximo à superfície das águas subterrâneas numa curta escala de tempo, p.ex., 100–101 anos, mas estes efeitos são provavelmente isolados para aquíferos do terço leste da bacia. Além disso, este estudo ilustra um mecanismo físico para a origem das águas subterrâneas nas terras altas orientais para recarregar aquíferos e salares no oeste da bacia do Pampa do Tamarugal sob escalas de tempo de 104–105 anos.

A Solid Waste Landfill Siting Exercise for an Introductory Environmental Geology Course

A solid waste landfill siting exercise has been developed for an introductory environmental geology course. Students are asked to site a county landfill using the actual state guidelines developed and enforced by the Wisconsin Department of Natural Resources. The students work in small groups and synthesize information from a variety of resources, including maps of geology, hydrology, land use, and topography. After choosing a site, students explain the benefits and drawbacks of the chosen location in oral and written formats. The exercise asks students to draw on a variety of topics and skills explored earlier in the term and to apply them to a real-world environmental problem. It has been successfully implemented as a capstone project at a liberal arts college and a large public institution.