John R. Nimmo

Modeling of soil water retention from saturation to oven dryness

Most analytical formulas used to model moisture retention in unsaturated porous media have been developed for the wet range and are unsuitable for applications in which low water contents are important. We have developed two models that fit the entire range from saturation to oven dryness in a practical and physically realistic way with smooth, continuous functions that have few parameters. Both models incorporate a power law and a logarithmic dependence of water content on suction, differing in how these two components are combined. In one model, functions are added together (model “sum”); in the other they are joined smoothly together at a discrete point (model “junction”). Both models also incorporate recent developments that assure a continuous derivative and force the function to reach zero water content at a finite value of suction that corresponds to oven dryness. The models have been tested with seven sets of water retention data that each cover nearly the entire range. The three-parameter sum model fits all data well and is useful for extrapolation into the dry range when data for it are unavailable. The two-parameter junction model fits most data sets almost as well as the sum model and has the advantage of being analytically integrable for convenient use with capillary-bundle models to obtain the unsaturated hydraulic conductivity.

Soil water retention and maximum capillary drive from saturation to oven dryness

This paper provides an alternative method to describe the water retention curve over a range of water contents from saturation to oven dryness. It makes two modifications to the standard Brooks and Corey [1964] (B-C) description, one at each end of the suction range. One expression proposed by Rossi and Nimmo [1994] is used in the high-suction range to a zero residual water content. (This Rossi-Nimmo modification to the Brooks-Corey model provides a more realistic description of the retention curve at low water contents.) Near zero suction the second modification eliminates the region where there is a change in suction with no change in water content. Tests on seven soil data sets, using three distinct analytical expressions for the high-, medium-, and low-suction ranges, show that the experimental water retention curves are well fitted by this composite procedure. The high-suction range of saturation contributes little to the maximum capillary drive, defined with a good approximation for a soil water and air system as HcM = ∫0∞krw dhc, where krw is relative permeability (or conductivity) to water and hc is capillary suction, a positive quantity in unsaturated soils. As a result, the modification suggested to describe the high-suction range does not significantly affect the equivalence between Brooks-Corey (B-C) and van Genuchten [1980] parameters presented earlier. However, the shape of the retention curve near “natural saturation” has a significant impact on the value of the capillary drive. The estimate using the Brooks-Corey power law, extended to zero suction, will exceed that obtained with the new procedure by 25 to 30%. It is not possible to tell which procedure is appropriate. Tests on another data set, for which relative conductivity data are available, support the view of the authors that measurements of a retention curve coupled with a speculative curve of relative permeability as from a capillary model are not sufficient to accurately determine the (maximum) capillary drive. The capillary drive is a dynamic scalar, whereas the retention curve is of a static character. Only measurements of infiltration rates with time can determine the capillary drive with precision for a given soil.

Centrifugal techniques for measuring saturated hydraulic conductivity

Centrifugal force is an alternative to large pressure gradients for the measurement of low values of saturated hydraulic conductivity (Ksat). With a head of water above a porous medium in a centrifuge bucket, both constant-head and falling-head measurements are practical at forces up to at least 1800 times normal gravity. Darcys law applied to the known centrifugal potential leads to simple formulas for Ksat that are analogous to those used in the standard gravity-driven constant- and falling-head methods. Both centrifugal methods were tested on several fine-textured samples of soil and ceramic with Ksat between about 10−10 and 10−9 m/s. The results were compared to falling-head gravity measurements. The comparison shows most measurements agreeing to within 20% for a given sample, much of the variation probably resulting from run-to-run changes in sample structure. The falling-head centrifuge method proved to be especially simple in design and operation and was more accurate than the constant-head method. With modified apparatus, Ksat measurements less than 10−10 m/s should be attainable.

Kilometer-Scale Rapid Transport of Naphthalene Sulfonate Tracer in the Unsaturated Zone at the Idaho National Engineering and Environmental Laboratory

To investigate possible long-range flow paths through the interbedded basalts and sediments of a 200-m-thick unsaturated zone, we applied a chemical tracer to seasonally filled infiltration ponds on the Snake River Plain in Idaho. This site is near the Subsurface Disposal Area for radioactive and other hazardous waste at the Idaho National Engineering and Environmental Laboratory. Within 4 mo, we detected tracer in one of 13 sampled aquifer wells, and in eight of 11 sampled perched-water wells as far as 1.3 km away. These detections show that (i) low-permeability layers in the unsaturated zone divert some flow horizontally, but do not prevent rapid transport to the aquifer; (ii) horizontal convective transport rates within the unsaturated zone may exceed 14 m d−1, perhaps through essentially saturated basalt fractures, tension cracks, lava tubes, or rubble zones; and (iii) some perched water beneath the Subsurface Disposal Area derives from episodic surface water more than 1 km away. Such rapid and far-reaching flow may be common throughout the Snake River Plain, and possibly occurs in other locations that have a geologically complex unsaturated zone and comparable sources of infiltrating water.

Discrete-Storm Water-Table Fluctuation Method to Estimate Episodic Recharge

We have developed a method to identify and quantify recharge episodes, along with their associated infiltration-related inputs, by a consistent, systematic procedure. Our algorithm partitions a time series of water levels into discrete recharge episodes and intervals of no episodic recharge. It correlates each recharge episode with a specific interval of rainfall, so storm characteristics such as intensity and duration can be associated with the amount of recharge that results. To be useful in humid climates, the algorithm evaluates the separability of events, so that those whose recharge cannot be associated with a single storm can be appropriately lumped together. Elements of this method that are subject to subjectivity in the application of hydrologic judgment are values of lag time, fluctuation tolerance, and master recession parameters. Because these are determined once for a given site, they do not contribute subjective influences affecting episode-to-episode comparisons. By centralizing the elements requiring scientific judgment, our method facilitates such comparisons by keeping the most subjective elements openly apparent, making it easy to maintain consistency. If applied to a period of data long enough to include recharge episodes with broadly diverse characteristics, the method has value for predicting how climatic alterations in the distribution of storm intensities and seasonal duration may affect recharge.

Hydraulic Property and Soil Textural Classification Measurements for Rainier Mesa, Nevada Test Site, Nevada

This report presents particle size analysis, field-saturated hydraulic conductivity measurements, and qualitative descriptions of surficial materials at selected locations at Rainier Mesa, Nevada. Measurements and sample collection were conducted in the Rainier Mesa area, including unconsolidated sediments on top of the mesa, an ephemeral wash channel near the mesa edge, and dry U12n tunnel pond sediments below the mesa. Particle size analysis used a combination of sieving and optical diffraction techniques. Field-saturated hydraulic conductivity measurements employed a single-ring infiltrometer with analytical formulas that correct for falling head and spreading outside the ring domain. These measurements may prove useful to current and future efforts at Rainier Mesa aimed at understanding infiltration and its effect on water fluxes and radionuclide transport in the unsaturated zone.

Vegetation influences on infiltration in Hawaiian soils: Infiltration in Hawaiian Soils

Ecohydrology. 2018;11:e1973. https://doi.org/10.1002/eco.1973 Abstract Changes in vegetation communities caused by removing trees, introducing grazing ungulates, and replacing native plants with invasive species have substantially altered soil infiltration processes and rates in Hawaii. These changes directly impact run‐off, erosion, plant‐available water, and aquifer recharge. We hypothesize that broad vegetation communities can be characterized by distributions of field‐saturated hydraulic conductivity (Kfs). We used 290 measurements of Kfs calculated from infiltration tests from 5 of the Hawaiian Islands to show this effect. We classified the data using 3 broad ecosystem categories: grasses, trees and shrubs, and bare soil. The soils of each site have coevolved with past and present ecological communities without significant mechanical disturbance by agriculture or urban development. Geometric mean values Kfs are 203 mm/hr for soils hosting trees and shrubs, 50 mm/hr for grasses, and 13 mm/hr for bare soil. Differences are statistically significant at the 95% confidence level. These examples show that it is feasible to make maps of relative Kfs based on field and ecosystem data. These ecosystem trends can be used to estimate ongoing changes to run‐off and recharge from climate and land use change. Greater Kfs for ecosystems with primarily trees and shrubs suggests that management decisions concerning reforestation or other changes of vegetation can have substantial hydrologic impacts.

Preferential flow, diffuse flow, and perching in an interbedded fractured-rock unsaturated zone

Layers of strong geologic contrast within the unsaturated zone can control recharge and contaminant transport to underlying aquifers. Slow diffuse flow in certain geologic layers, and rapid preferential flow in others, complicates the prediction of vertical and lateral fluxes. A simple model is presented, designed to use limited geological site information to predict these critical subsurface processes in response to a sustained infiltration source. The model is developed and tested using site-specific information from the Idaho National Laboratory in the Eastern Snake River Plain (ESRP), USA, where there are natural and anthropogenic sources of high-volume infiltration from floods, spills, leaks, wastewater disposal, retention ponds, and hydrologic field experiments. The thick unsaturated zone overlying the ESRP aquifer is a good example of a sharply stratified unsaturated zone. Sedimentary interbeds are interspersed between massive and fractured basalt units. The combination of surficial sediments, basalts, and interbeds determines the water fluxes through the variably saturated subsurface. Interbeds are generally less conductive, sometimes causing perched water to collect above them. The model successfully predicts the volume and extent of perching and approximates vertical travel times during events that generate high fluxes from the land surface. These developments are applicable to sites having a thick, geologically complex unsaturated zone of substantial thickness in which preferential and diffuse flow, and perching of percolated water, are important to contaminant transport or aquifer recharge.RésuméDes couches avec de forts contrastes géologiques dans la zone non saturée peuvent contrôler la recharge et le transport de contaminants vers les aquifères sous-jacents. Les écoulements diffus lents dans certaines couches géologiques et les écoulements préférentiels rapides dans d’autres compliquent la prévision des flux verticaux et latéraux. Un modèle simple est présenté, conçu pour ne requérir que des données géologiques limitées sur le site étudié, pour prédire ces processus de subsurface critiques vis-à-vis d’une source d’infiltration continue. Le modèle est développé et testé en utilisant les données spécifiques du site du laboratoire national de l’Idaho dans la plaine de la Eastern Snake River (ESRP), USA, où se trouvent des sources anthropiques et naturelles de forts volumes d’infiltration à partir des crues, déversements, fuites, rejets d’eaux usées, bassins de rétention et expériences hydrologiques de terrain. L’épaisse zone non saturée surmontant l’aquifère ESRP est un bon exemple d’une zone non saturée nettement stratifiée. Les interlits sédimentaires sont interstratifiés entre des unités de basalte massif et fracturé. La combinaison de sédiments superficiels, basaltes et interlits détermine les flux d’eau à travers le sous-sol variablement saturé. Les interlits sont généralement moins perméables, occasionnant parfois des nappes perchées au-dessus d’eux. Le modèle prédit avec succès le volume et l’extension des nappes perchées et estime les temps de transit vertical pendant les événements qui génèrent de forts flux depuis la surface du sol. Ces développements sont applicables aux sites présentant une épaisse zone non saturée, complexe géologiquement, d’une épaisseur substantielle au sein de laquelle des écoulements préférentiels et diffus et des nappes perchées sont importants en ce qui concerne le transport de contaminants ou la recharge de l’aquifère.ResumenLas capas de fuerte contraste geológico dentro de la zona no saturada pueden controlar la recarga y transporte de contaminantes a los acuíferos subyacentes. El lento flujo difuso en ciertas capas geológicas, y el rápido flujo preferencial en otras, complica la predicción de los flujos verticales y laterales. Se presenta un modelo simple, diseñado para utilizar la limitada información del sitio geológico para predecir estos procesos críticos del subsuelo en respuesta a una fuente de infiltración sostenida. Se desarrolló y probó el modelo usando información específica del sitio del Laboratorio Nacional de Idaho en el este de Snake River Plain (PEIS), EEUU, donde hay fuentes naturales y antropogénicas de altos volúmenes de infiltración a partir de inundaciones, vertidos, filtraciones, disposición de aguas residuales, estanques de retención, y experimentos hidrológicos de campo. El espesor de la zona no saturada sobre el acuífero PEIS es un buen ejemplo de una zona no saturada fuertemente estratificada. Las intercalaciones sedimentarias se intercalan entre las unidades masivas y fracturadas de basalto. La combinación de los sedimentos superficiales y las intercalaciones de basaltos determina los flujos de agua a través del subsuelo variablemente saturado. Las intercalaciones son generalmente menos conductoras, a veces causando que el agua se cuelgue por encima de ellos. El modelo predice con éxito el volumen y el alcance del agua colgada y aproxima los tiempos de desplazamiento verticales durante los eventos que generan altos flujos desde la superficie. Estos desarrollos son aplicables a los sitios que tienen una espesa zona no saturada, geológicamente compleja de espesor considerable en la cual el flujo preferencial y difuso, y el colgado del agua percolada, son importantes para el transporte de contaminantes o la recarga de los acuíferos.摘要非饱和带内强大的地质对比层可以控制对下伏含水层的补给和污染物运移。某一特定的地质层缓慢的弥散流及其他地质层的快速优先流使预测垂直和侧向通量变得更加复杂。这里展示了一个简单的模型,就是利用有限的地质场地信息预测针对持续渗入源产生响应的关键的地表以下的过程。利用美国斯内克河平原东部爱达荷国家实验室提供的现场特定信息建立了模型并对其进行了测试,斯内克河平原东部地区具有天然和人为的大流量的渗入源,包括洪水、泄漏、渗漏、废水处理、澄清池以及水文野外实验等。覆盖斯内克河平原东部含水层的厚层非饱和带是急剧分层的非饱和带一个很好的例子。巨大的和断裂的玄武岩单元之间散布着沉积互层。地表沉积层、玄武岩及互层的组合确定着通过饱和度变化着的地表下岩层的水通量。互层通常传导性较小,有时引起上层滞水在互层上方积聚。模型成功地预测了上层滞水积聚的量和范围,粗略估算了产生地表很高通量事件期间的垂直行程时间。这些进展可应用于具有很厚的、地质复杂的非饱和带的场地,在此非饱和带中,优先流、弥散流和渗透水的积聚对污染物的运移和含水层补给非常重要。ResumoO mecanismo de recarga e transporte de contaminantes de aquíferos sotopostos é controlado por intercalações rochosas de propriedades altamente contrastantes ao longo da zona não saturada. A previsibilidade dos fluxos verticais e laterais nessas formações é baixa por causa do escoamento difuso e lento em determinadas formações, e o escoamento preferencial e rápido em outras. Este trabalho apresenta um modelo simples para avaliação de processos subterrâneos críticos para a recarga a partir de dados geológicos de campo limitados, o qual considera uma fonte de infiltração constante. O modelo foi desenvolvido e testado utilizando-se dados de campo do Laboratório Nacional de Idaho, na Planície do Rio Eastern Snake (PRES), EUA, onde existe fontes naturais e antropogênicas de infiltração por inundações, derramamentos, vazamentos, lançamentos de água residuária, bacias de retenção e experimentos hidrológicos de campo. A espessa camada não saturada sobreposta ao aquífero PRES é um bom exemplo de formação não saturada constituída de estratificações delgadas. São observadas intercalações sedimentares entre unidades basálticas maciças e fissurais. Este padrão de distribuição de camadas, portanto, determina o padrão de escoamento através destas camadas com diferentes graus de saturação. Intercalações de rocha são normalmente hidraulicamente menos condutivas, ocasionando aquíferos suspensos sobre si. O volume e a extensão destes aquíferos suspensos são reproduzidos com sucesso pelo modelo, assim como o modelo também é capaz de reproduzir o tempo de percurso de escoamentos verticais ocasionados por eventos abruptos originados na superfície. Este modelo é válido para sítios que têm geologia espessa, complexa e insaturada em que o escoamento aconteça tanto de forma difusa quanto de forma preferencial, em cujos aquíferos suspensos e drenagem profunda sejam relevantes para o transporte de contaminantes ou para a recarga de aquíferos sotopostos.

Recharge in karst aquifers: from regional to local and annual to episodic scale

The assessment of groundwater recharge for karst aquifers of southern Italy is a major scientific task due to the relevant socio-economic and environmental role of the related groundwater resources. In this paper, the results of two methods, applied at different spatial and time scales, are reported.At regional and mean annual scales, through a multidisciplinary approach, the mean Annual Groudwater Recharge Coefficient (AGRC) was estimated for four sample karst aquifers, with available long-lasting spring discharge time series. Such estimations were extended to other karst aquifers of southern Italy by means of an empirical law that was found linking the AGRC to percentages of outcropping lithologies and endorheic/summit plateau areas.At local and episodic scales, the groundwater recharge of a test perched karst aquifer, belonging to the Mount Terminio hydrogeological structure (Campania region, southern Italy) was estimated. For such a purpose, an improvement of the Water Table Fluctuation (WTF) method, known as Episodic Master Recession (EMR), was applied to estimate the Recharge to Precipitation Ratio (RPR) coefficient, which represents the amount of precipitation recharging groundwater.Results obtained through the first approach furnished AGRC values varying between 50% and 79% and well matching with estimations of infiltration coefficients known in literature for other karst aquifers of Europe. Moreover, the mean value of RPR determined for the local karst aquifer (73%) resulted well matching with the AGRC estimated for the whole Mount Terminio karst aquifer (79%).By the comparison of these outcomes, at the regional and mean annual scales, the groundwater recharge of karst aquifers was found as mainly controlled by both the extension of outcropping lithologies and endorheic/summit plateau zones. While at the local and episodic scales, the groundwater recharge was recognized as chiefly influenced by the rainfall intensity and soil hydrological condition.

Summary of a Panel Discussion at the International Groundwater Symposium held on March 25-28, 2002 in Berkeley, California, USA

ANGELOS N. FINDIKAKIS, B echt el National, Inc., San Francisco, USA RAINER HELMIG, University of Stuttgart, Stuttgart, Germany PETER K1TANIDIS. Stanford University, Stanford, USA JOHN NIMMO, United States Geological Survey, Menlo Park, USA KARSTEN PRUESS, Lawrence Berkeley National Laboratory, Berkeley, USA YORAM RUBIN, University of California, Berkeley, USA FRITZ STAUFFER, ETH, Zurich, Switzerland CHIN-FU TSANG, Lawrence Berkeley National Laboratory, Berkeley, USA