River Water Intrusion Research Articles

Constructing an Efficient Health Assessment Model for Drainage Network to Evaluate the Drainage Network in Zone A of Zhenjiang City

Aiming at the problem of low pollutant concentration in the sewage treatment plant due to external water intrusion into the sewage collection system, which in turn leads to low pollutant reduction efficiency. A sewage system in Zhenjiang City is taken as an example. Analyze the situation of external water intrusion in the sewage pipe network and determine the external water intrusion proportion based on the water quality and quantity method. First, the dry season flow rate of the sewage pipe is obtained according to the monitoring data of the flowmeter. Then, the key research areas are screened out based on the changes in the concentration of water quality characteristic factors. Furthermore, chemical oxygen demand and electrical conductivity are used as water quality characteristic indicators to characterize shallow groundwater and river water. In addition, the proportions of groundwater and river water intrusion in the sewage pipe network are quantitatively analyzed based on the chemical mass balance equation. At the same time, the dredging detection method is used to assist in the investigation, and finally, the engineering rectification of the problems found in the drainage is carried out. The results show that the water quality and quantity method can effectively identify the types of external water and analyze the proportion of external water intrusion, which is of exemplary significance for the evaluation of sewage collection systems.

Influence of saline intrusion on the wetland ecosystem revealed by isotopic and hydrochemical indicators in the Yellow River Delta, China

Yellow River Delta wetland is a nature reserve with important ecological function and service values. It is of great significance for regional ecological protection and species diversity maintenance to systematically evaluate the impact of saline intrusion on wetlands. Based on the sampling campaigns for precipitation, seawater, river water and wetland water in dry, transition and wet season, this study aims to assess the temporal and spatial characteristics of hydrochemistry and water quality in wetland areas and quantify the ecological risks brought by saline intrusion. Isotopic evidence shows that wetland water is affected by intense evaporation, and its source has significant seasonal variation. Piper diagram suggests that the hydrochemical characteristics of wetland water are controlled by the supply from Yellow River water and saline intrusion. Water quality assessment based on metrics of soluble sodium percentage (Na %), sodium adsorption ratio (SAR) and Kelley's radio (KR) indicates that the sodium and salinity hazards caused by saline intrusion lead to negative effects on plant growth for most wetland sites. The risk from high salinity is more serious than that from sodium hazard for the wetland ecosystem. From dry to wet season, water quality in wetlands shows a pronounced trend of deterioration. End member mixing analysis (EMMA) based on principal component analysis (PCA) is employed to quantify the spatio-temporal characteristics of saline intrusion. The results show that the proportion of seawater in wetlands increases at first and then decreases along the stream direction, reflecting the influence of hydrogeological and hydrodynamic variation on water interaction. The proportion of seawater in wetland water ranges between 0 % and 11.86 %, but it poses a severe threat to water quality and ecosystem safety in wetland area. This study will provide insights for environmental risk assessment and water resources management in coastal wetlands.

Quantifying the dynamics of sub-daily to seasonal hydrological interactions of Ganges river with groundwater in a densely populated city: Implications to vulnerability of drinking water sources

Groundwater resources in South Asian cities are facing immense stress due to over-extraction leading to environmental, social and economic instabilities. The perennial mega rivers of Himalayas form the lifeline for South Asia, underpinning food and water security for a large population both directly and indirectly through exchange with groundwater systems. The present study delineates the spatio-temporal variation in patterns and processes of sub-hourly to annual-scale hydrological exchanges between the Ganges and its adjoining highly exploited aquifer in a urban-peri urban reach. Multivariate statistical analyses established river water-groundwater interaction in this region with ~40% loading of first principal component, i.e river water during monsoon on the shallow aquifer. The part of the aquifer detached from the main confined aquifer show an influence of precipitation (the second principal component) with loading of ~90%. Again the part of the aquifer suffering infiltration of local surface water bodies show effect of precipitation with a second principal loading of ~80%. Fourier transformation is used in the hydrograph to remove influence of heavy urbanization on the hydrographs. This study proves that the phenomenon of infiltrating river water during monsoon plays a primary role in controlling aquifer storage although contaminating the aquifer simultaneously. However, during pre and post-monsoon the flow path reversal helps in maintaining river baseflow. Cross-correlation between the river and piezometric series show increased delay of pressure head propagation of the infiltrating river waterfront, with increasing distance. These observations are also substantiated by stable isotope signatures. The present study provides an understanding of potential groundwater vulnerability resulting from waste water and irrigational contamination through river water intrusion which would eventually lead the government to implement proper water and environmental management policies towards availability of long-term sustainable water resources for the residents.

River water intrusion as a source of inflow into the sanitary sewer system.

Previous studies on the extraneous water problem (or infiltration/inflow) in sanitary sewer systems assumed that the wastewater flow is mainly composed of foul sewage (FS), groundwater infiltration (GWI) and rainfall-derived inflow and infiltration (RDII). Most existing assessment methods are based on this assumption. In 2018, China initiated the 'Protection of the Yangtze River Program', and the two-year research data showed that it was neither the GWI nor the RDII but the direct surface water intrusion (DSWI), which has rarely been reported in literatures, that serves as the main source of the extraneous water in many local sewer systems. The discovery has enriched the understanding of the extraneous water in sewer systems. Meanwhile, it brings new challenges for the assessment of extraneous water. In this study, starting from the analysis of the low influent concentration of chemical oxygen demand (COD) of the wastewater treatment plant in a southeastern city in China, a river water intrusion point was successfully localized and the volume of river water intrusion was quantified by a series of field experiments. The methodology used in this study can also be applied in other areas with DSWI.

Tryptophan-like fluorescence as a fingerprint of dry-weather misconnections into storm drainage system

BackgroundInappropriate dry-weather misconnections into storm drainage system are a demanding environmental problem worldwide, which leads to unexpected dry-weather discharge into surface waters. It often costs a large amount of manpower and resources to identify the source of misconnections and estimate its contributions. In this study, we evaluated the possibility of quantifying proportional source contribution in a storm drainage system with dry-weather misconnections from domestic sewage and river water inflow, using rapid and low-cost fluorescence spectroscopy methods. For this purpose, samples of both misconnection sources and outflows of storm drainage system were collected and analyzed in a downtown catchment of Shanghai, China.ResultsResults showed that fluorescent peak intensity of tryptophan-like T1 in domestic sewage (802 ± 126 a.u.) was significantly higher than that in urban river water (57 ± 12 a.u.), while fluorescent peak intensities of tryptophan-like T2 in urban river water (732 ± 304 a.u.) was much higher than that in domestic sewage (261 ± 64 a.u.) due to increased algal activity in the local river and upstream inflow chemistry. However, only peak T2 passed the conservative behavior test in the incubation experiments, which could be used as a fingerprint for quantitatively identifying the misconnections. We further developed a Bayesian fluorescence mass balance model (FMBM) to infer the percentage of dry-weather misconnections into the storm drainage system as a function of fluorescence intensities of peak T2 in the samples of sources and outflow. It was found that the maximum posteriori probability estimate of the percentage of river water intrusion into the storm drains was up to 20.8% in this site, which was validated by the results of on-site investigation.ConclusionOur findings implied that in situ fluorescent sensors and Bayesian FMBM for the fingerprint fluorescence peak could be applied to fast track inappropriate dry-weather misconnections into storm drainage system qualitatively and quantitatively with low costs.

Kilometer‐Scale Hydrologic Exchange Flows in a Gravel Bed River Corridor and Their Implications to Solute Migration

AbstractA well‐characterized field site along a major, gravel bed river corridor was used to investigate the dynamic pathways and impacts of subsurface hydrogeologic structure on kilometer‐scale hydrologic exchange flows between river water and groundwater. An aqueous uranium (Uaq) plume exists within a hyporheic alluvial aquifer at the site that discharges to the Columbia River. We performed temporally intensive monitoring of specific conductance (SpC) and Uaq concentrations within the plume for a 2‐year period at varying distances from the river shoreline, both within and outside a presumed subsurface pathway of lateral hydrologic exchange. SpC and Uaq were utilized as in situ tracers of hydrologic exchange and associated groundwater‐surface water mixing. Seasonal river stage variations by more than 2 m caused distinct events of river water intrusion and retreat from the nearshore, hyporheic alluvial aquifer, resulting in highly dynamic SpC and Uaq patterns in monitoring wells. Simulations of hydrologic exchange and mixing were performed with PFLOTRAN to understand the observed SpC and Uaq behaviors linked to predominant flow directions and velocities in the river corridor as influenced by river stage dynamics and variable aquitard topography. By coupling robust monitoring with numerical flow and transport modeling, we demonstrate complicated multidirectional flow behaviors at the kilometer scale that strongly influenced plume dynamics. Therefore, hyporheic aquifer must be frequently monitored under different flow conditions if water quality is of concern. The resulting hydrologic understanding enables improved interpretation of hydrogeochemical data from this site and other large gravel bed river corridors in the United States and elsewhere.

Trace metal cycling in karst aquifers subject to periodic river water intrusion

River water that enters critical zone aquifers can deliver metals and alter groundwater redox potential, resulting in precipitation and/or dissolution of metal oxides. These effects on metal concentrations should be particularly dynamic in karst critical zones where surface water and groundwater are linked through preferential flow paths (e.g., caves and conduits). Metals can be either toxins or micronutrients and thus their concentrations are important to water quality. To evaluate trace metal dynamics in karst critical zones, we measured Pb and Sr isotopic ratios and major and trace metal concentrations of Fe,Mn-oxide deposits collected from two water-filled cave networks (Madison Blue and Peacock springs) in the carbonate upper Floridan aquifer (Florida, USA). We also measured dissolved and total metal concentrations in water from conduits of both cave networks during three river water intrusion events and subsequent flood recessions. The Fe,Mn-oxides contain anthropogenic Pb that increases in concentration from interior to exterior of the oxides, indicating they formed and continued to grow over the past several decades. Sr isotope ratios of the Fe,Mn-oxides are intermediate between flood water and groundwater values and indicate they form during intrusion events. This result is corroborated by variations in Fe and Mn concentrations in water during intrusion and recession. The Fe,Mn-oxide interiors are more altered than exteriors as shown by depletion of Mn concentrations and the lack of a positive Ce anomaly in the interior of the Fe,Mn-oxides. Most trace metal concentrations correlate with Mn concentrations suggesting remobilization of trace metals over time periods longer than individual intrusion events. Cycling between accumulation and dissolution of Fe,Mn-oxides presents challenges for evaluation and remediation of trace metal contamination of carbonate critical zones.

Detection of drinking water contamination event with Mahalanobis distance method, using on-line monitoring sensors and manual measurement data

Abstract Concerns about drinking water (DW) quality contamination during water distribution raise a need for real-time monitoring and rapid contamination detection. Early warning systems (EWS) are a potential solution. The EWS consist of multiple conventional sensors that provide the real-time measurements and algorithms that allow the recognizing of contamination events from normal operating conditions. In most cases, these algorithms have been established with artificial data, while data from real and biological contamination events are limited. The goal of the study was the event detection performance of the Mahalanobis distance method in combination with on-line DW quality monitoring sensors and manual measurements of grab samples for potential DW biological contamination scenarios. In this study three contamination scenarios were simulated in a pilot-scale DW distribution system: untreated river water, groundwater and wastewater intrusion, which represent realistic contamination scenarios and imply biological contamination. Temperature, electrical conductivity (EC), total organic carbon (TOC), chlorine ion (Cl-), oxidation–reduction potential (ORP), pH sensors and turbidity measurements were used as on-line sensors and for manual measurements. Novel adenosine-triphosphate and flow cytometric measurements were used for biological water quality evaluation. The results showed contamination detection probability from 56% to 89%, where the best performance was obtained with manual measurements. The probability of false alarm was 5–6% both for on-line and manual measurements. The Mahalanobis distance method with DW quality sensors has a good potential to be applied in EWS. However, the sustainability of the on-line measurement system and/or the detection algorithm should be improved.

Hydrological and sedimentation conditions in a non-tidal lagoon during ice coverage – The example of Vistula Lagoon in the Baltic Sea

The main objective of this study was to compare conditions for sedimentation in a lagoon in the presence and absence of ice coverage. The winter conditions of Vistula Lagoon (hydrology, suspended solids in water, ice, and snow) were studied during the period 2011–2016. Three-dimensional numerical modelling was used to simulate the water level variations and currents under ice coverage. Satellite images were collected to reproduce the ice-coverage cycle.In general, the hydrological and sedimentation conditions in the winter period without ice coverage were highly similar to those in the warm period of the year. The suspended sediment concentration was 22–32 mg/l.Ice cover eliminates wind forcing, and water exchange with the sea and river discharge become the main forcing factors. Strong vertical salinity stratification is established under ice, and river water intrusion spreads much further under ice in the lagoon area than in the absence of ice.The water level variation in the inlet is the main dynamic factor influencing the water motion under ice throughout the lagoon. Water level variations with a period of less than 12–24 h are damped by the ice cover, and oscillations with a period of longer than 1 day easily spread under the ice.The currents in the lagoon are characterised by vertical stratification with a maximum in the middle layer due to friction at the bottom and at the ice layer, currents are sufficient to produce re-suspension only in the vicinity of the inlet. In the absence of ice, the currents have maximums at the surface and in the near-bottom layer.The suspended solid concentrations under the ice cover (5–10 mg/l) are less than those in the summer (20–35 mg/l), and vertical distribution is characterised by two maximums: near the bottom and at the surface. The sedimentation rate under ice was experimentally estimated as 2–4 mm/year.During the ice period, the rivers are the single sediment sources, and most sediments settle in the close vicinity of the river mouths and are redistributed along the lagoon area only after the ice melt.Air-born sediments stored in the ice and snow cover (on the ice) contain particles of anthropogenic origin (ashes and soot), inorganic mineral substances and organic substances. Because it appears that most ice melts within the lagoon, the sedimentation peak is expected to occur in spring and is caused by both the increase of river inflow and melting of ice.

Microbial Reduction of Fe(III) and SO42- and Associated Microbial Communities in the Alluvial Aquifer Groundwater and Sediments.

Agricultural demands continuously increased use of groundwater, causing drawdown of water table and need of artificial recharge using adjacent stream waters. River water intrusion into groundwater can alter the geochemical and microbiological characteristics in the aquifer and subsurface. In an effort to investigate the subsurface biogeochemical activities before operation of artificial recharge at the test site, established at the bank of Nakdong River, Changwon, South Korea, organic carbon transported from river water to groundwater was mimicked and the effect on the indigenous microbial communities was investigated with the microcosm incubations of the groundwater and subsurface sediments. Laboratory incubations indicated microbial reduction of Fe(III) and sulfate. Next-generation Illumina MiSeq sequences of V4 region of 16S rRNA gene provided that the shifts of microbial taxa to Fe(III)-reducing and/or sulfate-reducing microorganisms such as Geobacter, Albidiferax, Desulfocapsa, Desulfuromonas, and Desulfovibrio were in good correlation with the sequential flourishment of microbial reduction of Fe(III) and sulfate as the incubations progressed. This suggests the potential role of dissolved organic carbons migrated with the river water into groundwater in the managed aquifer recharge system on the indigenous microbial community composition and following alterations of subsurface biogeochemistry and microbial metabolic activities.

PFLOTRAN-E4D: A parallel open source PFLOTRAN module for simulating time-lapse electrical resistivity data

Time-lapse electrical resistivity tomography (ERT) is finding increased application for remotely monitoring processes occurring in the near subsurface in three-dimensions (i.e. 4D monitoring). However, there are few codes capable of simulating the evolution of subsurface resistivity and corresponding tomographic measurements arising from a particular process, particularly in parallel and with an open source license. Herein we describe and demonstrate an electrical resistivity tomography module for the PFLOTRAN subsurface flow and reactive transport simulation code, named PFLOTRAN-E4D. The PFLOTRAN-E4D module operates in parallel using a dedicated set of compute cores in a master-slave configuration. At each time step, the master processes receives subsurface states from PFLOTRAN, converts those states to bulk electrical conductivity, and instructs the slave processes to simulate a tomographic data set. The resulting multi-physics simulation capability enables accurate feasibility studies for ERT imaging, the identification of the ERT signatures that are unique to a given process, and facilitates the joint inversion of ERT data with hydrogeological data for subsurface characterization. PFLOTRAN-E4D is demonstrated herein using a field study of stage-driven groundwater/river water interaction ERT monitoring along the Columbia River, Washington, USA. Results demonstrate the complex nature of subsurface electrical conductivity changes, in both the saturated and unsaturated zones, arising from river stage fluctuations and associated river water intrusion into the aquifer. The results also demonstrate the sensitivity of surface based ERT measurements to those changes over time. PFLOTRAN-E4D is available with the PFLOTRAN development version with an open-source license at https://bitbucket.org/pflotran/pflotran-dev.

River stage influences on uranium transport in a hydrologically dynamic groundwater‐surface water transition zone

AbstractA well‐field within a uranium (U) plume in the groundwater‐surface water transition zone was monitored for a 3 year period for water table elevation and dissolved solutes. The plume discharges to the Columbia River, which displays a dramatic spring stage surge resulting from snowmelt. Groundwater exhibits a low hydrologic gradient and chemical differences with river water. River water intrudes the site in spring. Specific aims were to assess the impacts of river intrusion on dissolved uranium (Uaq), specific conductance (SpC), and other solutes, and to discriminate between transport, geochemical, and source term heterogeneity effects. Time series trends for Uaq and SpC were complex and displayed large temporal and well‐to‐well variability as a result of water table elevation fluctuations, river water intrusion, and changes in groundwater flow directions. The wells were clustered into subsets exhibiting common behaviors resulting from the intrusion dynamics of river water and the location of source terms. Hot‐spots in Uaq varied in location with increasing water table elevation through the combined effects of advection and source term location. Heuristic reactive transport modeling with PFLOTRAN demonstrated that mobilized Uaq was transported between wells and source terms in complex trajectories, and was diluted as river water entered and exited the groundwater system. While Uaq time‐series concentration trends varied significantly from year‐to‐year as a result of climate‐caused differences in the spring hydrograph, common and partly predictable response patterns were observed that were driven by water table elevation, and the extent and duration of river water intrusion.

Four‐dimensional electrical conductivity monitoring of stage‐driven river water intrusion: Accounting for water table effects using a transient mesh boundary and conditional inversion constraints

AbstractThis paper describes and demonstrates two methods of providing a priori information to the surface‐based time‐lapse three‐dimensional electrical resistivity tomography (ERT) problem for monitoring stage‐driven or tide‐driven surface water intrusion into aquifers. First, a mesh boundary is implemented that conforms to the known location of the water table through time, thereby enabling the inversion to place a sharp bulk conductivity contrast at that boundary without penalty. Second, a nonlinear inequality constraint is used to allow only positive or negative transient changes in EC to occur within the saturated zone, dependent on the relative contrast in fluid electrical conductivity between surface water and groundwater. A 3‐D field experiment demonstrates that time‐lapse imaging results using traditional smoothness constraints are unable to delineate river water intrusion. The water table and inequality constraints provide the inversion with the additional information necessary to resolve the spatial extent of river water intrusion through time.

Modelagem da intrusão da água de um rio com diferentes concentrações de sais

Para considerações de intrusão de água em meio subterrâneo a capilaridade, também entendida como força de sucção, é uma grandeza física que influencia a dinâmica do processo. Neste trabalho é apresentada uma investigação do comportamento do escoamento subterrâneo e as interações entre água doce e água salinizada na área de descarga de um rio. Objetivou-se realizar uma modelagem preditiva para identificar e representar o processo de intrusão das águas de um rio meio ao lençol freático comparando diferentes concentrações de sais nas águas do rio e águas subterrâneas. Como modelo conceitual foi definido um rio em região de descarga. A modelagem foi realizada no software SUTRA sendo o método de solução por elementos finitos. Observou-se a formação de uma camada de água doce sobre o lençol freático salinizado quando considerada intrusão de água doce. Para as considerações com intrusão de água salinizada foi identificada a formação de fingers. Os resultados do modelo possibilitaram identificar que a intrusão de água doce no lençol freático salinizado é dificultada pela presença de sais. A intrusão é mais difícil quanto maior a concentração de sais nas águas subterrâneas.

Single-cell genomics reveals metabolic strategies for microbial growth and survival in an oligotrophic aquifer.

Bacteria from the genus Pedobacter are a major component of microbial assemblages at Hanford Site (a largely decommissioned nuclear production complex) in eastern Washington state, USA, and have been shown to change significantly in abundance in response to the subsurface intrusion of Columbia River water. Here we employed single-cell genomics techniques to shed light on the physiological niche of these micro-organisms. Analysis of four Pedobacter single amplified genomes (SAGs) from Hanford Site sediments revealed a chemoheterotrophic lifestyle, with the potential to exist under both aerobic and microaerophilic conditions via expression of both aa3-type and cbb3-type cytochrome c oxidases. These SAGs encoded a wide range of both intra- and extracellular carbohydrate-active enzymes, potentially enabling the degradation of recalcitrant substrates such as xylan and chitin, and the utilization of more labile sugars such as mannose and fucose. Coupled to these enzymes, a diversity of transporters and sugar-binding molecules were involved in the uptake of carbon from the extracellular local environment. The SAGs were enriched in TonB-dependent receptors, which play a key role in uptake of substrates resulting from degradation of recalcitrant carbon. Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR)-Cas mechanisms for resisting viral infections were identified in all SAGs. These data demonstrate the potential mechanisms utilized for persistence by heterotrophic micro-organisms in a carbon-limited aquifer, and hint at potential linkages between observed Pedobacter abundance shifts within the 300 Area (in the south-eastern corner of the site) subsurface and biogeochemical shifts associated with Columbia River water intrusion.

Assessment of River Water Intrusion at the Periphery of Bagmati River in Kathmandu Valley

The present study was carried out to find out the possible impact of Bagmati river water on its nearby ground water resource within Kathmandu valley (KV). For this, five sampling stations were selected viz. Gokarna, Tilganga, Sankhamul, Teku and Sundarighat . From each station, one river water sample and six ground water samples from both sides of the river were taken on proportion basis by dividing the strata lying within 50 m, 50-100 m and 100-150 m distance from the river collected. River water analysis showed that it was less polluted during winter season as compared to other seasons and got severely degraded as it entered to the urban core. Regression analysis showed that the mean value of ground water parameters like pH, conductivity, chlorides, free carbon dioxide, alkalinity, hardness, nitrate, ammonia, iron, and ortho-phosphate were decreased from within 50 m to 50-100 m, and 100-150 m distance from the river bank. Microbial analysis of ground water showed that 88% of the samples were contaminated with fecal matter. So the ground water along Bagmati river should could not be used as consumptive purposes without treatment. The finding showed that the ground water was more contaminated along the river banks and towards the urban corridor than the upstream parts which might be due to intrusion of nearby polluted river water. Nepal Journal of Science and Technology Vol. 14, No. 1 (2013) 137-146 DOI: http://dx.doi.org/10.3126/njst.v14i1.8934

Imaging high stage river‐water intrusion into a contaminated aquifer along a major river corridor using 2‐D time‐lapse surface electrical resistivity tomography

Key Points Inland high‐stage river water intrusion is imaged using time‐lapse ERT. Transient water‐table elevation constraints enable accurate imaging. River water intrusion is dominated by preferred flow pathways.

Monitoring groundwater‐surface water interaction using time‐series and time‐frequency analysis of transient three‐dimensional electrical resistivity changes

Time‐lapse resistivity imaging is increasingly used to monitor hydrologic processes. Compared to conventional hydrologic measurements, surface time‐lapse resistivity provides superior spatial coverage in two or three dimensions, potentially high‐resolution information in time, and information in the absence of wells. However, interpretation of time‐lapse electrical tomograms is complicated by the ever‐increasing size and complexity of long‐term, three‐dimensional (3‐D) time series conductivity data sets. Here we use 3‐D surface time‐lapse electrical imaging to monitor subsurface electrical conductivity variations associated with stage‐driven groundwater‐surface water interactions along a stretch of the Columbia River adjacent to the Hanford 300 near Richland, Washington, USA. We reduce the resulting 3‐D conductivity time series using both time‐series and time‐frequency analyses to isolate a paleochannel causing enhanced groundwater‐surface water interactions. Correlation analysis on the time‐lapse imaging results concisely represents enhanced groundwater‐surface water interactions within the paleochannel, and provides information concerning groundwater flow velocities. Time‐frequency analysis using the Stockwell (S) transform provides additional information by identifying the stage periodicities driving groundwater‐surface water interactions due to upstream dam operations, and identifying segments in time‐frequency space when these interactions are most active. These results provide new insight into the distribution and timing of river water intrusion into the Hanford 300 Area, which has a governing influence on the behavior of a uranium plume left over from historical nuclear fuel processing operations.

Spatial and temporal dynamics of the microbial community in the Hanford unconfined aquifer

Pyrosequencing analysis of 16S rRNA genes was used to study temporal dynamics of groundwater bacteria and archaea over 10 months within three well clusters separated by ~30 m and located 250 m from the Columbia River on the Hanford Site, WA. Each cluster contained three wells screened at different depths ranging from 10 to 17 m that differed in hydraulic conductivities. Representative samples were selected for analyses of prokaryotic 16S and eukaryotic 18S rRNA gene copy numbers. Temporal changes in community composition occurred in all nine wells over the 10-month sampling period. However, there were particularly strong effects near the top of the water table when the seasonal rise in the Columbia River caused river water intrusion at the top of the aquifer. The occurrence and disappearance of some microbial assemblages (such as Actinobacteria ACK-M1) were correlated with river water intrusion. This seasonal impact on microbial community structure was greater in the shallow saturated zone than deeper zone in the aquifer. Spatial and temporal patterns for several 16S rRNA gene operational taxonomic units associated with particular physiological functions (for example, methane oxidizers and metal reducers) suggests dynamic changes in fluxes of electron donors and acceptors over an annual cycle. In addition, temporal dynamics in eukaryotic 18S rRNA gene copies and the dominance of protozoa in 18S clone libraries suggest that bacterial community dynamics could be affected not only by the physical and chemical environment but also by top-down biological control.

A field‐scale reactive transport model for U(VI) migration influenced by coupled multirate mass transfer and surface complexation reactions

This study explores field‐scale modeling of U(VI) reactive transport through incorporation of laboratory and field data. A field‐scale reactive transport model was developed on the basis of laboratory‐characterized U(VI) surface complexation reactions (SCRs) and multirate mass transfer processes, as well as field‐measured hydrogeochemical conditions at the U.S. Department of Energy, Hanford 300 Area (300 A), Washington. The model was used to assess the importance of multirate mass transfer processes on U(VI) reactive transport and to evaluate the effect of variable geochemical conditions caused by dynamic river water‐groundwater interactions on U(VI) plume migration. Model simulations revealed complex spatiotemporal relationships between groundwater composition and U(VI) speciation, adsorption, and plume migration. In general, river water intrusion enhances uranium adsorption and lowers aqueous uranium concentration because river water dilution increases pH and decreases aqueous bicarbonate concentration, leading to overall enhanced U(VI) surface complexation. Strong U(VI) retardation was computed for the field‐measured hydrogeochemical conditions, suggesting a slow dissipation of the U(VI) plume, a phenomenon consistent with field observations. The simulations also showed that SCR‐retarded U(VI) migration becomes more dynamic and synchronous with the groundwater flow field when multirate mass transfer processes are involved. Breakthrough curves at selected locations and the temporal changes in the calculated mass during the 20 year simulation period indicated that uranium adsorption/desorption never attained steady state because of the dynamic flow field and groundwater composition variations caused by river water intrusion. Thus, the multirate SCR model appears to be a crucial consideration for future reactive transport simulations of uranium contaminants at the Hanford 300 A site and elsewhere under similar hydrogeochemical conditions.