Reactive transport with wellbore storages in a single-well push–pull test

Using the single-well push–pull (SWPP) test to determine the in situ biogeochemical reaction kinetics, a chase phase and a rest phase were recommended to increase the duration of reaction, besides the injection and extraction phases. In this study, we presented multi-species reactive models of the four-phase SWPP test considering the wellbore storages for both groundwater flow and solute transport and a finite aquifer hydraulic diffusivity, which were ignored in previous studies. The models of the wellbore storage for solute transport were proposed based on the mass balance, and the sensitivity analysis and uniqueness analysis were employed to investigate the assumptions used in previous studies on the parameter estimation. The results showed that ignoring it might produce great errors in the SWPP test. In the injection and chase phases, the influence of the wellbore storage increased with the decreasing aquifer hydraulic diffusivity. The peak values of the breakthrough curves (BTCs) increased with the increasing aquifer hydraulic diffusivity in the extraction phase, and the arrival time of the peak value became shorter with a greater aquifer hydraulic diffusivity. Meanwhile, the Robin condition performed well at the rest phase only when the chase concentration was zero and the solute in the injection phase was completely flushed out of the borehole into the aquifer. The Danckwerts condition was better than the Robin condition even when the chase concentration was not zero. The reaction parameters could be determined by directly best fitting the observed data when the nonlinear reactions were described by piece-wise linear functions, while such an approach might not work if one attempted to use nonlinear functions to describe such nonlinear reactions. The field application demonstrated that the new model of this study performed well in interpreting BTCs of a SWPP test.</p

Memory Response: Exposure history dependence of microbial mediated transformations of substrates in groundwater

The flow, transport, and reactivity of dissolved-phase constituents in an unconfined and shallow aquifer were characterized, in situ, by utilizing the single-well push-pull test method. In the first study, the re-oxidation/mobility of uranium, in the presence of nitrate oxidant, was shown to be mitigated by preferential oxidation/mobilization of solid-phase, reduced, sulfur-bearing species. These results indicated that establishing conditions conducive to uranium reduction and the formation of reduced sulfur-bearing species can increase the efficacy of sustained uranium reduction/immobility in the presence of re-mobilizing oxidants. In the second study, the analytical solution to describe the one-dimensional displacement of the center of mass of a tracer during a push-pull test was expanded to account for displacement during the injection phase. The expanded solution improved the theoretical description of the displacement of a tracer during a push-pull test and the in situ application demonstrated an improvement for the estimation of effective porosity. In the third study, an analytical model was developed to describe the breakthrough of a potentially reactive solute due to non-reactive mixing and was applied to an in situ data set. The analytical model accurately predicted the breakthrough curve of nonreactive solutes and allowed for quantifying the rate and extent of reactive solute mass transfer and transformation. In the fourth study, the exposure history dependence of microbial mediated ethanol transformation was demonstrated to last up to six weeks in the absence of ethanol injections with no apparent enrichment of a select microbial community. This suggested that the predominant mechanisms of adaptation may exist at the enzymatic- and/or genetic-levels. In conclusion, the single-well push-pull test method was utilized and improved to characterize hydraulic parameters and processes, and microbial mediated transformations of substrates in groundwater

The interplay between transport and reaction rates as controls on nitrate attenuation in permeable, streambed sediments

Anthropogenic nitrogen fixation and subsequent use of this nitrogen as fertilizer has greatly disturbed the global nitrogen cycle. Rivers are recognized hotspots of nitrogen removal in the landscape as interaction between surface water and sediments creates heterogeneous redox environments conducive for nitrogen transformations. Our understanding of riverbed nitrogen dynamics to date comes mainly from shallow sediments or hyporheic exchange flow pathways with comparatively little attention paid to groundwater-fed, gaining reaches. We have used 15N techniques to quantify in situ rates of nitrate removal to 1m depth within a groundwater-fed riverbed where subsurface hydrology ranged from strong upwelling to predominantly horizontal water fluxes. We combine these rates with detailed hydrologic measurements to investigate the interplay between biogeochemical activity and water transport in controlling nitrogen attenuation along upwelling flow pathways. Nitrate attenuation occurred via denitrification rather than dissimilatory nitrate reduction to ammonium or anammox (range = 12 to >17000 nmol 15N L-1 h-1). Overall, nitrate removal within the upwelling groundwater was controlled by water flux rather than reaction rate (i.e. Damköhler numbers 80% of nitrate removal occurs within sediments not exposed to hyporheic exchange flows under baseflow conditions, illustrating the importance of deep sediments as nitrate sinks in upwelling systems

Nitrogen dynamics and retention in the river network of a tropical forest, Luquillo Mountains, Puerto Rico

This dissertation identifies gaps in the scientific understanding of nutrient cycling, particularly nitrogen (N) cycling, in streams and riparian zones of tropical montane forests, and addresses several of those gaps with original field-based research using study watersheds in the Luquillo Mountains of Puerto Rico as the model system. The Luquillo Mountains have features typical of mature montane tropical forests, such as high background N concentrations in streams and groundwater relative to streams in other biomes. As a USDA Forest Service Experimental Forest, the Luquillo Mountains are accessible to researchers and have abundant monitoring and experimental datasets from which to build hypotheses and experimental approaches. Chapter 1 is a review of the state of the literature on biological response to nutrients, particularly N, in streams of the Luquillo Mountains. This chapter also includes a gap analysis of research questions that are of greatest importance to the environmental regulatory and management community, that have not yet been addressed. Chapter 2 looks in-depth at ammonium (NH4+) cycling in headwater streams. Headwater streams in tropical forests are typically light- and organic matter-limited in their demand for nutrients. Ammonium can serve as both a nutrient and an energy source, leading to a hypothesized high demand that can be compared across streams using ambient uptake velocity (vf). This study experimentally enriched headwater streams with transient NH4+ pulses to determine NH4+ demand and mechanisms for uptake. Ambient vf ranged from 0 to 2.9 mm min-1, lower than other tropical streams and streams in other biomes in the literature. Though demand was relatively low, areal uptake rate in the streambed was high due to high background NH4+ concentrations. When compared with streams in other geological regions of the Luquillo Mountains, the streams in this study stand out for their sandy substrate and their low phosphorus concentrations, suggesting that NH4+ removal pathways may be limited by nutrients or habitat for NH4+-oxidizing microorganisms. Chapter 3 focuses on N cycling in riparian zones. Riparian zones are widely understood as nitrogen N cycling hotspots, but significant gaps remain in our understanding of the complex biogeochemistry of NH4+ transformations in riparian groundwater. Tropical forest watersheds in particular have distinctive N biogeochemistry that is still poorly understood. This study was the first to examine in-situ NH4+ cycling in a riparian aquifer, using push-pull tests PPTs to experimentally enrich groundwater with NH4+ and trace NH4+ removal from solution, and the first to apply the Damkohler number to riparian NH4+ dynamics to measure the balance between residence time and reaction rate. The rate constant k for NH4+ retention during the five PPTs ranged from 0.13 to 0.68 hr-1; the residence time ranged from 27 to 512 days; and the Damköhler number ranged from 72 to 11620, indicating that nearly complete removal of added NH4+ would occur over transport from the PPT well to the stream. Low dissolved oxygen availability and lack of net nitrate production indicate that nitrification was not the dominant pathway of NH4+ removal. Iron was abundant in surface riparian soils in the form of HCl-extractable Fe(III), and declined to near zero at the depth of the PPTs accompanied by abundant HCl-extractable Fe(II), suggesting that Fe(II) production and NH4+ oxidation could be coupled. Sorption-desporption reactions and the associated equilibrium potentially explain why high background NH4+ concentrations persisted before and after the PPTs, though we expect that NH4+ pulse experiments like those conducted here saturate abiotic storage within soil and provide a determination of biotic NH4+ removal

Final report for DOE Grant No. DE-FG02-07ER64404 - Field Investigations of Microbially Facilitated Calcite Precipitation for Immobilization of Strontium-90 and Other Trace Metals in the Subsurface

Subsurface radionuclide and metal contaminants throughout the U.S. Department of Energy (DOE) complex pose one of DOEâÃÂÃÂs greatest challenges for long-term stewardship. One promising stabilization mechanism for divalent ions, such as the short-lived radionuclide 90Sr, is co-precipitation in calcite. We have previously found that that nutrient addition can stimulate microbial ureolytic activity that this activity accelerates calcite precipitation and co-precipitation of Sr, and that higher calcite precipitation rates can result in increased Sr partitioning. We have conducted integrated field, laboratory, and computational research to evaluate the relationships between ureolysis and calcite precipitation rates and trace metal partitioning under environmentally relevant conditions, and investigated the coupling between flow/flux manipulations and precipitate distribution. A field experimental campaign conducted at the Integrated Field Research Challenge (IFRC) site located at Rifle, CO was based on a continuous recirculation design; water extracted from a down-gradient well was amended with urea and molasses (a carbon and electron donor) and re-injected into an up-gradient well. The goal of the recirculation design and simultaneous injection of urea and molasses was to uniformly accelerate the hydrolysis of urea and calcite precipitation over the entire inter-wellbore zone. The urea-molasses recirculation phase lasted, with brief interruptions for geophysical surveys, for 12 days followed by long-term monitoring which continued for 13 months. Following the recirculation phase we found persistent increases in urease activity (as determined from 14C labeled laboratory urea hydrolysis rates) in the upper portion of the inter-wellbore zone. We also observed an initial increase (approximately 2 weeks) in urea concentration associated with injection activities followed by decreasing urea concentration and associated increases in ammonium and dissolved inorganic carbon (DIC) following the termination of injection. Based on the loss of urea and the appearance of ammonium, a first order rate constant for urea hydrolysis of 0.18 day-1 rate with an associate Rf for ammonium of 11 were estimated. This rate constant is approximately 6 times higher than estimated for previous field experiments conducted in eastern Idaho. Additionally, DIC carbon isotope ratios were measured for the groundwater. Injected urea had a ÃÂô13C of 40.7ÃÂñ0.4 âÃÂð compared to background groundwater DIC of ÃÂô13C of -16.6ÃÂñ0.2âÃÂð. Observed decreases in groundwater DIC ÃÂô13C of up to -19.8âÃÂð followed temporal trends similar to those observed for ammonium and suggest that both the increase in ammonium and the sift in ÃÂô13C are the result of urea hydrolysis. Although direct observation of calcite precipitation was not possible because of the high pre-existing calcite content in the site sediments, an observed ÃÂô13C decrease for solid carbonates from sediment samples collect following urea injection (compared to pre-injection values) is likely the result of the incorporation of inorganic carbon derived from urea hydrolysis into newly formed solid carbonates

Fluid-rock interaction: A reactive transport approach

Fluid-rock interaction (or water-rock interaction, as it was more commonly known) is a subject that has evolved considerably in its scope over the years. Initially its focus was primarily on interactions between subsurface fluids of various temperatures and mostly crystalline rocks, but the scope has broadened now to include fluid interaction with all forms of subsurface materials, whether they are unconsolidated or crystalline ('fluid-solid interaction' is perhaps less euphonious). Disciplines that previously carried their own distinct names, for example, basin diagenesis, early diagenesis, metamorphic petrology, reactive contaminant transport, chemical weathering, are now considered to fall under the broader rubric of fluid-rock interaction, although certainly some of the key research questions differ depending on the environment considered. Beyond the broadening of the environments considered in the study of fluid-rock interaction, the discipline has evolved in perhaps an even more important way. The study of water-rock interaction began by focusing on geochemical interactions in the absence of transport processes, although a few notable exceptions exist (Thompson 1959; Weare et al. 1976). Moreover, these analyses began by adopting a primarily thermodynamic approach, with the implicit or explicit assumption of equilibrium between the fluid and rock. As a result, these early models were fundamentally static rather than dynamic in nature. This all changed with the seminal papers by Helgeson and his co-workers (Helgeson 1968; Helgeson et al. 1969) wherein the concept of an irreversible reaction path was formally introduced into the geochemical literature. In addition to treating the reaction network as a dynamically evolving system, the Helgeson studies introduced an approach that allowed for the consideration of a multicomponent geochemical system, with multiple minerals and species appearing as both reactants and products, at least one of which could be irreversible. Helgeson's pioneering approach was given a more formal kinetic basis (including the introduction of real time rather than reaction progress as the independent variable) in subsequent studies (Lasaga 1981; Aagaard and Helgeson 1982; Lasaga 1984). The reaction path approach can be used to describe chemical processes in a batch or closed system (e.g., a laboratory beaker), but such systems are of limited interest in the Earth sciences where the driving force for most reactions is transport. Lichtner (1988) clarified the application of the reaction path models to water-rock interaction involving transport by demonstrating that they could be used to describe pure advective transport through porous media. By adopting a reference frame which followed the fluid packet as it moved through the medium, the reaction progress variable could be thought of as travel time instead. Multi-component reactive transport models that could treat any combination of transport and biogeochemical processes date back to the early 1980s. Berner and his students applied continuum reactive transport models to describe processes taking place during the early diagenesis of marine sediments (Berner 1980). Lichtner (1985) outlined much of the basic theory for a continuum model for multicomponent reactive transport. Yeh and Tripathi (1989) also presented the theoretical and numerical basis for the treatment of reactive contaminant transport. Steefel and Lasaga (1994) presented a reactive flow and transport model for nonisothermal, kinetically-controlled water-rock interaction and fracture sealing in hydrothermal systems based on simultaneous numerical solution of both reaction and transport This chapter begins with a review of the important transport processes that affect or even control fluid-rock interaction. This is followed by a general introduction to the governing equations for reactive transport, which are broadly applicable to both qualitative and quantitative interpretations of fluid-rock interactions. This framework is expanded through a discussion of specific topics that are the focus of current research, or are either incompletely understood or not fully appreciated. At this point, the focus shifts to a brief discussion of the three major approaches to modeling multi-scale porous media (1) continuum models, (2) pore scale and pore network models, and (3) hybrid or multi-continuum models. From here, the chapter proceeds to investigate some case studies which illuminate the power of modern numerical reactive transport modeling in deciphering fluid-rock interaction

Earth Observing System. Volume 1, Part 2: Science and Mission Requirements. Working Group Report Appendix

Areas of global hydrologic cycles, global biogeochemical cycles geophysical processes are addressed including biological oceanography, inland aquatic resources, land biology, tropospheric chemistry, oceanic transport, polar glaciology, sea ice and atmospheric chemistry