H21B-1001 0800h
Stream Transient Storage as a Function of Land Use in Jackson Hole, Wyoming.
Stream fluvial geomorphic structure establishes patterns of surface water and subsurface head distributions, often driving hyporheic exchange through steps, riffles, and meanders. We performed conservative tracer experiments and geomorphological assessments (topographic thalweg surveys, channel width, and sediment size analyses) in six streams: two agricultural streams, two urban streams, and two reference streams surrounded by native vegetation, in Jackson Hole WY, to investigate stream water residence time distributions in relation to land use. Urban stream channels were the least complex, with very little variation in streambed elevation and narrow channel widths. Agricultural streams were moderately complex, with the widest channels on average, compared to pristine stream channels which contained more riffles, pools, steps and splits. Consequently, relative to advection ({\it t$_{adv}$}), transient storage processes increased with increasing channel complexity. Urban channels had total relative residence times of 2-5 {\it t$_{adv}$}, agricultural streams 6-7 {\it t$_{adv}$}, and pristine streams had the longest relative residence times, 13-20{\it t$_{adv}$}. We suggest that human land use simplifies fluvial geomorphic structure directly lowering stream transient storage, which may also alter the potential for biogeochemical processing.
H21B-1002 0800h
Water Quality Parameters as Environmental Tracers of Stream Exchanges with Ground Water
Water quality data is collected on a routine basis in both surface water and ground water. This paper evaluated the potential use of water quality parameters as environmental tracers of near stream ground water along a 130 km reach of the Russian River in Northern California. For a water quality parameter to be useful as an environmental tracer it must show temporal variability at a boundary (typically the upper boundary) as well as spatial variability in ground water. Three water quality parameters (temperature, specific conductance, and chloride) were sampled quarterly in conjunction with traditional environmental tracers (deuterium and 18-oxygen isotopes) in the river and nearby ground-water wells. For the Russian River, surface water was characterized by an increase in daily mean temperature, specific conductance, and chloride concentration progressing from the upper reach to the lower reach of the study area, with a strong seasonal variation throughout the entire reach. Deuterium showed a strong seasonal variation, whereas 18-oxygen had only a minor seasonal variation in the surface water. A high degree of hydrologic connectivity between the surface water and ground water was indicated at four sites, where hydrochemical changes indicate surface water was infiltrating into the alluvial aquifer. The chemistry of the ground water at these sites was characterized by seasonal trends and dampened and delayed tracer signals relative to the river. At other locations, hydrologic connectivity was less evident and there was no strong correlation in hydrochemical changes between the surface water and ground water. The preliminary results of the study indicated that water quality parameters used as environmental tracers have clear potential to characterize the spatial and temporal variability of the Russian River.
H21B-1003 0800h
Development of Artificial Lenses of Fresh Groundwater in Desert Conditions
A significant proportion of the world's deserts is covered by soils characterized by low hydraulic conductivity, high runoff coefficient and high levels of salinity. The groundwater is usually also saline in such regions. It has been proposed to use clayey watersheds for collecting runoff water during seasonal precipitation and infiltrating it into the saline water table, thus creating an artificial lens of fresh groundwater (ALFGW). The National Institute of Deserts, Flora and Fauna of Turkmenistan constructed in the Kara-Kum Desert a pilot system for ALFGW formation (infiltration basin with recharging wells). It was found that over 3-4 years (with a mean annual runoff of 10,000-15,000 m3/km), about 20,000-25,000 m3 of surface water could be infiltrated. This created a lens of maximum 7 m thickness that could be used as a fresh water reservoir. To understand the process associated with ALFGW formation and its pumping, we applied a mathematical model of density driven flow and transport in the unsaturated-saturated zones. The FEFLOW code was used for simulations and the model was calibrated with available field data. It was found that there is a relatively sharp interface between the ALFGW bottom and the saline groundwater, while changes in water salinity are minor within the ALFGW. Simulations of the ALFGW indicated that, with time, a vortex flow develops under the lens edges. This leads to an increase in mixing between the fresh and saline water zones, thus increasing the lens areal extent while decreasing its thickness. The process mainly depends on the hydraulic parameters and the water infiltration regime. Lowering the hydraulic conductivity and the infiltration rate, leads to an increase in the ALFGW; however, increasing the infiltration time raises water losses by evaporation. During pumping of the ALFGW, up-coning of saline water occurs, which depends on the pumping rate, the well screen parameters, well location, and ALFGW characteristics. Using two pumping wells, which are screened in both the fresh and saline water zones, considerably decreases the up-coning effect and the risk of salinization of the freshwater zone. It was found that for sustainable pumping, discharges of wells in fresh and saline water zones is proportional to the transmissivities of these zones.
H21B-1004 0800h
Development of an Unsaturated Region Below a Perennial River
Field observations at the Russian River Bank Filtration Facility in Sonoma County, California indicate that an unsaturated region exists below the streambed near two adjacent groundwater pumping wells located along the riverbank. Understanding the conditions that give rise to unsaturated flow below the streambed is critical for improving and optimizing riverbank well pumping operations. To investigate the development of an unsaturated region below a perennial river near pumping wells, a three-dimensional model was developed using the multi-phase subsurface flow model, TOUGH2. The model is based on the region around the two pumping wells in the Russian River Bank Filtration Facility. The pumping wells consist of 9 perforated pipes that are projected horizontally into the aquifer at a depth of approximately 20 m below the land surface. A grid was developed for the TOUGH2 model with finer resolution near the wells to represent individual pipes. The effect of varying the pumping operation and the streambed permeability on the extent of the unsaturated region was investigated with the TOUGH2 model. The formation remained saturated below the streambed when only one of the wells was pumped at a rate of 1600 m$^{3}$/hr, but an unsaturated region developed below the streambed when the two wells each pumped at a rate of 1600 m$^{3}$/hr. This unsaturated region was deeper when the permeability of the streambed was lower than the aquifer material compared to when the streambed and aquifer permeabilities were the same.
H21B-1005 0800h
High Cell Density Production of Europium-Labeled {\it Escherichia coli} for Tracing of Bacteria in Mantled Karst of Northwest Arkansas
The preparation of europium-labeled {\it E. coli} as a bacterial tracer in our study is separated into two major steps: the production of large quantities of cells, and the labeling of the cells at high density. Indigenous {\it E. coli} isolated from a natural spring at the University of Arkansas's Savoy Experimental Watershed (SEW), Savoy, Arkansas was fermented in BIOFLO II (New Brunswick Scientific, Edison, NJ) bioreactor using a fed-batch technique. Either a concentrated glucose solution or an ammonium hydroxide solution was pulsed into the reactor automatically using closed-loop pH control in a reactor feeding strategy designed to optimize cell growth. {\it E. coli} cells were harvested at the stationary phase of the bacterial growth profile, washed and centrifuged prior to the europium labeling step. A concentrated europium chloride solution was prepared by dissolving europium (III) chloride in 1-L of deionized water; the salt solution was chilled at 6$^{o}$C overnight. A batch of 100-g wet weight of the washed {\it E. coli} was suspended in the chilled europium salt solution, and the cells were incubated at 6$^{o}$C for 2 hours with stirring. After the cold incubation, the cells were washed with chilled deionized water and centrifuged repeatedly to remove excess europium. We have successfully prepared 760-g wet weight of labeled {\it E. coli} using the high cell density fermentation and europium labeling technique in a 9-day period. Preparation of large quantities of viable europium-tagged bacteria is critical for use as an environmental tracer. The europium uptake by the {\it E. coli} was found to be 15-mg europium per gram of labeled cell (wet weight). A field injection of multiple tracers along with the europium-tagged {\it E. coli} was successfully performed during the summer of 2004 at SEW to elucidate the transport, storage and viability of fecal contaminants in a karst basin. Prior investigations suggest that, unlike conservative tracers, {\it E. coli} become deposited along the flow path in the aquifer, and are resuspended with subsequent recharge events. Fluorescein and Rhodamine WT were injected along with the europium tagged {\it E. coli}. Both dyes were conservatively transported to the primary spring discharge points in about 10-days. Unlike the dyes, {\it E. coli} moved in distinct pulses associated with recharge events. These findings document that the fate and transport of {\it E. coli} is significantly different than conservative tracers.
H21B-1006 0800h
Characterizing Stream-Aquifer Interaction at an Acid Mine Drainage Site
The acid mine drainage site at Davis Mine in Rowe, Massachusetts, has been studied over several years. The site has been inactive for nearly 100 years but still exhibits pyrite oxidation and microbially mitigated sulfate reduction. A major feature of the site is a stream which passes over and through aging tailings piles. Understanding the hydraulic connection between the stream and the surrounding groundwater is critical to analyzing oxidation and reduction processes. The stream emanates from a spring at a mine shaft opening and travels several hundred meters before joining a larger stream. Data has been collected continuously and episodically from the stream and several wells located nearby. Data includes water levels, streamflow, pH, sulfate and other chemical signatures of the oxidation and reduction reactions. Analysis of the data is presented and approaches to estimating hydraulic properties between the stream and the aquifer are discussed.
H21B-1007 0800h
Response of a shallow sandy unconfined aquifer to tidal loading from a low-relief estuary
A field study of the physical and chemical processes occurring in a sandy, unconfined aquifer (Scotland, UK), connected to a low-relief estuary was performed, with an emphasis on groundwater salinization and water table fluctuations. Data were collected from two adjacent monitoring transects located 20 m apart. These extended from the line of zero flux inland towards the estuary. Drill logs show that geological variations between the two monitoring transects are negligible. One transect consisted of three Multi-Level Sampling (MLS) boreholes, allowing detailed depth-specific pore water collection. Water level measurements were derived from pressure transducers located in fully slotted boreholes in the second transect. Water table elevations, pore water Electrical Conductivity (EC) and salinity data, concentrations of halogen ions Cl and Br as well as exchangeable cations Mg, Ca, K and Na were collected during 2001-4 along the monitoring transects. The aquifer response induced by high-low and neap-spring tidal estuarine oscillations was measured. Water level data indicate groundwater fluctuations are influenced by tidal loading from the estuary. Spectral analyses of ground and estuary water level time-series indicate the dominance of the principal lunar semi-diurnal frequency. For all frequencies, damping increases rapidly inland; phase shifts are also observed. The influence of the site settings (especially beach slope) on the tidal signal propagation is discussed. Pore water chemistry indicates a limited salinization zone at depth in the aquifer, and Br/Cl ratio calculations confirm its marine origin. Variations in EC and Cl concentrations do not follow any tidal pattern even near the shore, but are more likely influenced by seasonally varying groundwater recharge. Exchangeable cation concentrations in the aquifer salinization zone show no significant changes over time , except very near to the estuary. Here, the link between observed groundwater chemistry variations and tidal forcing is established by cross-analysis of physical and chemical data. This saline zone where exchange processes are influenced by tidal loading, has been identified as the landward front of the active estuarine intrusion in the aquifer.
H21B-1008 0800h
Groundwater System Investigation of the Cheonggyecheon Watershed Area
The Cheonggyecheon is an urban stream in the center of the Seoul, Korea. Because of fundamental solution of the deterioration on covering structure and expressway, the Seoul Metropolis decided 'Cheonggyecheon restoration' and began on July 1, 2003. The purpose of the Cheonggyecheon restoration is a plan for the afforestation in Seoul, but the Cheonggyecheon needs more water for the maintenance of flow. The Cheonggyecheon is a disconnected stream because of a urban stream which has many influenced factors such as pumping out, groundwater leakages into subway stations and leaking water from water supply and sewage line. So, the objectives of this study are analyzing influenced factors in the groundwater budget in the Cheonggyecheon watershed and suggesting the amount of input and output water. There are many factors influenced by the infiltration in this study area such as a flow rate of the stream, a thickness of the stream bed, a grain size of the stream bed, and a permeability of a waterproof material after restoration. For investigation of the hydrogeologic parameter in the study area, we perform the hydraulic tests; pumping test, slug test, and infiltration test. Groundwater quality data were collected for an investigation of the origin of groundwater in the study area and the index parameter after restoration. For estimating a level of sensitive factors after restoration construction, we conduct sand tank test and numerical test. In comparison of the results of the sand tank test and the numerical test, we decide the prime control factor of infiltration water in the Cheonggyecheon watershed, and suggest the amount of water budget in this study site.
H21B-1009 0800h
Using Geophysical Methods to Detect Submarine Groundwater Discharge in Sarasota Bay, FL
Submarine groundwater discharge (SGD) can be an important pathway for nutrients entering coastal systems. However SGD flow paths can be difficult to identify and flow volumes difficult to quantify. This study assesses whether geophysical techniques are potentially cost effective methods for detecting the presence or lack of SGD within Sarasota Bay, Florida. In this area, a rapid increase in urbanization has led to increased nitrogen loading into the bay, with some 10% of this loading attributed to SGD. In this study we compare the results of both marine resistivity and marine electromagnetic surveys against geochemical techniques (radon and methane tracers) as well as the conventional method of SGD detection, seepage meters. Marine resistivity surveys offer access to deeper and more open waters, while the marine electromagnetic technique is applied in very shallow areas and those where a long streamer cannot be towed. Preliminary results show that there appears to be a correlation between high seepage as measured by tracer studies and a decrease in the seafloor conductivity measured with geophysical methods. Along with changes in seafloor conductivity, there appear to be small but detectable changes in the surface water temperature gradient in close proximity to SGD discharge points.
H21B-1010 0800h
Seasonal Evolution of Hyporheic Zones in Arctic Tundra Streams, North Slope, Alaska
The depth of the active layer in permafrost dominated areas like Arctic Alaska is greater under streams than in adjacent terrestrial areas. We proposed that as the sub-stream active layer increases throughout the summer the extent of the hyporheic zone within these streams also increases. We did conservative stream tracer experiments to examine the hydrologic characteristics of four permafrost-dominated, tundra streams in Arctic Alaska, during the summer of 2004. The streams differed in morphology (alluvial to peat) and size (2nd to 3rd order). We added Rhodamine WT in pulses to these study reaches throughout the warming season to document the dynamics of the downward thaw and its influence on hyporheic exchange. Our results indicate that the hyporheic zone increases in response to seasonal warming, as expected. Tracer breakthrough curves, normalized to advection time ({\it t$_{adv}$}) collected throughout the early portion of the warming period (May - June) suggest increased transient storage associated with the hyporheic zone, which is in agreement with advancing thaw conditions occurring below the channel. However, by late in the warming period (August), when thaw advancement rate was at a minimum, the conditions at each of these sites shifted, indicating less hyporheic influence on transient storage. We hypothesize that changes in surface and subsurface dynamics, modified by geomorphic form, can be used to explain the confounding observations found late in the thaw season and plan to explore this possibility through additional groundwater modeling.
H21B-1011 0800h
A stream tracer technique employing ionic tracers and specific conductance data
The stream tracer technique and transient storage models (TSMs) have become common tools in stream solute and hyporheic exchange studies. The expense and logistics associated with water sample collection and analysis often results in limited temporal resolution of stream tracer breakthrough curves (BTCs). Samples are often collected without a priori or real-time knowledge of BTC information, which can result in poor sample coverage of the critical shoulder (initial rise) and tail (post-steady state fall) of the BTC. We illustrate the use of specific conductance (SC) measurements as a surrogate for conservative dissolved tracer (Br) samples. The advantages of collecting SC data for use in the TSM are (1) cost, (2) ease of data collection, and (3) well-defined breakthrough curves, which strengthen TSM parameter optimization. This method is based on developing an ion concentration (IC)-SC relationship from limited discrete tracer solute samples. SC data can be collected on a more frequent basis at no additional analysis cost. TSM simulations can then be run for the conservative tracer data derived from SC breakthrough curves and the IC-SC relationship. This technique was tested in a 120 m reach of stream (2-60 m sub-reaches) in the Maimai M15 catchment, New Zealand during baseflow recession. Dissolved LiBr was injected for 12.92 h, with Br- as the conservative ion of interest. Four TSM simulations using the OTIS model are optimized using UCODE to fit (1) Br data derived from the Br-SC relationship (n = 1307 observations at each of two stream sampling sites), (2) all stream Br data collected (n D 58 in upper reach, n = 60 in lower reach), (3) half of the stream Br data collected, and (4) 20 stream Br samples from each site. No two simulations resulted in the same optimal parameter values. Results suggest that the greater the frequency of observations, the greater the confidence in estimated parameter values. Br-SC simulations resulted in the best overall model fits to the data, with the lowest calculated error variance of 6.37, narrowest 95% parameter estimate confidence intervals, and the highest correlation coefficient of 0.99, among the four simulations. This is largely due to the improved representation of the shoulder and tail of the BTC with this method. The IC-SC correlation method is robust in situations in which (1) changes in background SC data can be accounted for, and (2) the data used to define the IC-SC relationship are representative of the range of data collected. This method provides more efficient sample analysis, improved data resolution, and improved model results compared to the alternative stream tracer data gathering methods presented. Additionally, we describe a new parameterization of the cross-sectional area of the stream during flow recession, as a function of discharge, based on a stream hydraulic geometry relationship. This variant of the OTIS model provides a more realistic representation of stream dynamics during unsteady discharge.
H21B-1012 0800h
Model Development Framework for the Groundwater/Surface Water Interface: Approaches, Concerns and Challenges
The need and ability to understand contaminant transport across the groundwater/surface water interface (GWSI) is an important part of overall human health and environmental risk assessment. Model development and enhancement for GWSI may best be approached from a systems-based development of a framework that assists in guiding regulatory agencies and interested parties in identifying existing tools, and needed capabilities for contaminated sites of concern that are close to a surface water body. In this context, modeling approaches for groundwater/surface water interactions in the stream/river, estuarine, and lacustrine environments are presented, systematically classified, and critiqued so as to understand the needs for future model development. Primary issues requiring resolution at present are identified as (i) linking large-scale flow and transport models to process-level models; (ii) defining groundwater/surface-water interactions in a spatial and temporal framework; (iii) defining GWSI; and (iv) techniques of scaling-up for process-level models. The objective of this work is to develop a roadmap to achieve modeling capabilities that will enable a scientifically-defensible and cost-effective evaluation of contaminant fate and transport at and across the groundwater/surface water interface.
H21B-1013 0800h
Radium and Radon as Tracers of Ground Water Flow Into Upper Newport Bay, CA
Subsurface waters are highly enriched in isotopes of radon and radium, relative to surface waters. Thus, the measured isotope distribution and the decay constants of Ra-223 (half-life 11.4d), Ra-224 (half-life 3.6d), and Rn-222 (half-life 3.82 d) can provide constraints on transport within an aqueous system, making them potential tracers of groundwater flow and boundary exchange. Boundary exchange mechanisms within Upper Newport Bay estuary include molecular diffusion through pore waters, macrofaunal irrigation of sediments, tidal pumping, river inputs, tidal exchange with the Lower Bay, radioactive decay, and (for radon) gas exchange with the atmosphere. Groundwater flow is also a potential source of these isotopes, but because it is likely to be localized, it is difficult to measure. By developing budgets for Rn and Ra, and placing constraints on other processes, an estimate for groundwater flow was calculated. Sampling in February, April, and August of 2004 included analysis of water column and pore water samples for radium and radon. Concentrations of both Rn and Ra in the water column were smallest in February (71+/-11 dpm/m3 Ra-223, 595+/- 51 dpm/m 3 Ra-224) but were about twice as large in April (188+/-17 dpm/m3 Ra-223, 1080+/-dpm,m3 Ra-224) and August (128+/-21 dpm/m3 Ra-223, 977+/-48 dpm/m3 Ra-224). This reflects the influence of rapid flushing with storm runoff in February, when salinity was also greatly reduced. Pore water concentrations were 1800 dpm/m3 for Ra-223, 20,000 dpm/m3 for Ra-224, and 300 dpm/l for Rn-222. Both core incubations and benthic chamber deployments were used to directly measure radium and radon fluxes from sediments. Fluxes measured (in atoms/m2-sec) from in situ chambers were 13 (Ra-223), 36 (Ra-224), and 450 (Rn-222) in August. These exceeded diffusive fluxes from pore waters by factors of 6, 3.5, and 3 for the 3 isotopes, respectively. This high in situ flux indicates that irrigation by macrofauna must enhance transport, and the different enhancements for each isotope indicates the effect occurs on a variety of length scales. A mass balance for Upper Newport Bay indicates the sediment flux is responsible for ~20-30% of the isotope input. River runoff supplies 10% of the Ra-223 and 20% of the Ra-224 and the Rn-222 inputs. Approximately 30-40% of the supply is brought in by the flood tide. The remaining input must be supplied by a combination of tidal pumping and groundwater flow. An upper limit for groundwater flow can be calculated if it alone balances the isotope budgets equivalent to a distributed flow of 2-4 cm/day.
H21B-1014 0800h
Surface Water - Groundwater Interaction Research in Chalk Catchments: UK Lowland Catchment Research Programme (LOCAR)
The focus of new European legislation on integrated management and, in particular, on ecological quality, raises major scientific and technical questions. These require improved understanding of catchment systems and hydro-ecological interactions that can only be obtained from integrated and multi-disciplinary experimental research. The main water supply aquifers in the United Kingdom, namely the Cretaceous Chalk and Permo-Triassic Sherwood Sandstone, are situated, for the most part, in lowland England, particularly in the Midlands, South and South East. These aquifers have a major, often dominant influence on the river systems that they underlie. These lowland permeable catchments present a particular set of challenges; management pressures are great, the scientific understanding of the major UK aquifers is poor, and tools for the integrated modelling of surface water-groundwater interactions and associated hydro-ecological processes are limited. In response to these factors, the LOwland CAtchment Research programme (LOCAR) was conceived. The programme also provides intrumented catchments to address some of these scientific issues. This paper describes the programme and early results of research into the influence of lithostratigraphy and karst features on surface water/groundwater interaction in the two Chalk LOCAR catchments.
H21B-1015 0800h
Analysis of a Monsoon Flood Event Effect on Surface and Groundwater Interactions in a Regional Semiarid Watershed
Although sporadic and infrequent, flooding events in ephemeral watersheds are a critical component to the water, sediment and biogeochemical cycles in arid and semiarid regions. In the Southwestern United States, intense thunderstorms during the summer monsoon season interact with landscapes characterized by topographic complexity and soils of low infiltration capacity to produce large magnitude floods and flash floods. In this study, we examine the hydrometeorological conditions and hydrologic response of an extreme monsoon flood event in the Rio Puerco watershed of north-central New Mexico and its downstream effects in the Rio Grande, a major continental-scale river basin. The summer storm in early September 4-11, 2003 generated flash flooding in headwater basins and river flooding extending through the semiarid basin and downstream into the Rio Grande for several tens of kilometers. We characterize the hydrometeorological conditions prior to the flood event using precipitation estimates from rain gauge records, NEXRAD radar data, and synoptic weather conditions over the 18,000 km2 Rio Puerco basin. Then, we present the spatial and temporal variability in hydrologic response based on a set of nested stream gauges in river channels and irrigation canals as well as a network of instrumented well transects installed along the Rio Grande alluvial aquifer. Our analysis illustrates the propagation, dampening, and attenuation of a large monsoonal storm through a semiarid ephemeral tributary into a regional river system from both a surface and groundwater hydrology perspective, including the water exchanges observed between the two systems. By estimating the frequency of the rainfall and flood event in the system relative to the historical record and known shifts in climate regime, we discuss the importance of extreme flood events in semiarid tributary systems and their downstream effects in the surface and groundwater interactions of regional river basins.
H21B-1016 0800h
Can rock electrical properties be used as quantitative tool for fluid reactivity interactions responsible of rock permeability variations?
In crystalline rocks, where the matrix permeability is small, fluid flow occurs essentially through a fracture network. Fluid-rock interactions changing the fracture network geometry by mineral dissolution or precipitation may change the effective permeability of the rock. The objective of this work is to experimentally quantify, the effective permeability variation of the rock during the reaction of mineral dissolution-precipitation due to fluid flow through the fractures. An original experimental device has been built to measure in situ and continuously the different physical parameters (permeability, pH, electrical conductivity of both rock and fluid) during the solution flow through a single open fracture of a cylindrical granite sample. In addition, the out put fluid was regularly sampled and the fluid chemical composition was analyzed. Experiments, where a granite rock reacts with CO2 saturated reactive fluids having an ionic strength of 0.1 mol/kg (similar of natural fluids), shows a strong increases of the rock effective permeability (up to 15%) and a decrease of the electrical formation factor. The increase of pH, calcium, silica and fluorine confirm the dissolution of the main fracture mineral (calcite, fluorite and prehnite ) responsible for the `erosion' of the fracture walls as confirmed by both Scanning Electron Microscopy and surface RAMAN determinations. However in our experimental conditions, fluid chemical composition as well permeability and electrical formation factor quickly reach steady state indicating the neoformation of secondary minerals.
H21B-1017 0800h
Mobility of major and trace elements in a coupled groundwater-surface water system: Merced River, CA
Trace element transport in coupled surface water/groundwater systems is controlled not only by advective flow, but also by redox reactions that affect the partitioning of various elements between mobile and immobile phases. These processes have been examined in the context of a field project conducted by the U.S. Geological Survey (USGS) as part of the National Water-Quality Assessment (NAWQA) program. The Merced River flows out of Yosemite National Park and the Sierra Nevada foothills and into California's Central Valley, where it joins the San Joaquin River. Our field site is approximately twenty river kilometers from the confluence with the San Joaquin River. This deep alluvial plain has minimal topography. Agricultural development characterizes the land surrounding this reach of river; consequently, the hydrology is heavily influenced by irrigation. Riverbed groundwater samples were collected from ten wells aligned in two transects across the river located approximately 100 m apart. The wells were sampled from depths of 0.5 m, 1 m, and 3 m below the sediment-water interface. Groundwater flowpath samples were taken from wells positioned on a path perpendicular to the river and located 100 m, 500 m, and 1000 m from the river. The saturated groundwater system exists from 7 to 40 m below the surface and is confined below by a clay layer. Each well location samples from 3-5 depths in this surface aquifer. Samples were collected in December 2003, March-April, June-July, and October 2004. This served to provide an evenly-spaced sampling frequency over the course of a year, and also to allow observation of trends coinciding with the onset of winter, the spring runoff, and early and late summer irrigation. An initial survey of the elements in the riverbed samples was conducted using Inductively-Coupled Plasma Mass Spectrometry (ICP-MS). Elements for further study were selected based on variability in this survey, either with respect to depth or location, as well as to cover a range of expected geochemical behaviors. Further ICP-MS measurements focused on eight elements: strontium, barium, uranium, molybdenum, manganese, iron, phosphorus, and bromine. Bromine is a conservative tracer. Molybdenum, manganese, and iron will precipitate when oxidized, and uranium will precipitate when reduced. Strontium and barium are not redox-active but may be affected by dissolution-precipitation and sorption reactions. Phosphorus is a nutrient that will cycle actively in areas of biological productivity. Generally, these elements appear to behave as expected based on physical waterflow and assumed redox conditions. The two transects of wells across the river bracket a zone of known denitrification, which implies that sediment conditions favor oxidation upriver and reduction downriver. This trend is borne out both by the redox-sensitive elements at each transect, and by the strontium and barium, which bind to precipitated iron and manganese oxides in oxidizing conditions and are released into the dissolved state in reducing conditions. The flowpath samples appear to be enriched in strontium, phosphorus, and bromine when compared to the riverbed samples, and they are depleted in manganese and iron.
http://water.usgs.gov/nawqa
H21B-1018 0800h
Using a Heat Pulse to Measure the Flux Between Groundwater and Surface Water
EPA estimates that 10 percent of the sediments under the surface waters of the United States are contaminated and approximately 20 percent of the superfund sites include contaminated sediments. The risk associated with these contaminated sediments is directly related to the flux of water passing between the groundwater surface water. Design of potential corrective actions requires knowledge of the fluxes. In an effort to reliably measure the temporal changes in water flux that may change direction as well as amplitude over time, we have developed an instrument capable of measuring flow and direction that is capable of measuring water flux with less than 10 percent error. The instrument measures the movement of a pulse of heat injected into a pipe by watching the change in temperature at four locations, two on each side of the heater and calculates the advective flux based on both the peak arrival time and moment analysis of the thermograph. We present the design of the instrument, illustrate the calibration and data analysis and discuss sources of error in the measurement.
H21B-1019 0800h
Modelling Groundwater-Seawater Interactions in the Aral Sea Region
The Aral Sea, the former world's fourth biggest inland water body in 1960, has experienced dramatic shrinkage due to the failed mega scale hydrological engineering, which resulted in losing nine tenth of its original sea volume. Furthermore, the salinity has increased from about 10 g/L in the 1960's to about 90 g/L presently. Such dramatic changes affect also the coastal aquifers in the region. In particular, the risk of salt water intrusion (SWI) into these aquifers must be considered when using the groundwater as local fresh water supply. While, on the one hand, the potential risk of SWI may have decreased in the region due to the increasing distance between the receding sea and the freshwater aquifers, the considerably increased sea water density may, on the other hand, imply an increased potential risk of SWI. We address the effects of sea level lowering and increased sea water salinity on the salinity transition zone in coastal aquifers, as well as the submarine groundwater discharge (the mixed flow of salt water originating from the sea and fresh groundwater discharge) pattern in the region.
H21B-1020 0800h
Nonlinear Analysis of Tidal Forcing of Water Level Fluctuations in an Unconfined Permeable Coastal Aquifer, Georgia Coastal Ecosystems LTER
The characteristics of tidally-forced water level fluctuations in unconfined permeable aquifers provide insight into physical, chemical, and even biological processes occurring at the margin between open water and marsh-upland systems. It has previously been well-established that permeable aquifers act as low-pass linear filters when upland water table fluctuations are driven by tidal fluctuations in adjacent low-friction estuarine systems. For example, diurnal or semi-diurnal tidal components that dominate in the estuary are attenuated in the upland, leading to water table fluctuations increasingly dominated by the longer period (fortnightly) tidal components with increasing distance from the tidal creek boundary. This analysis, which relies on a linearized solution to the Boussinesq equation, breaks down when the boundary between upland and estuary is nonvertical and/or non-stationary, when there is significant physical heterogeneity (e.g., permeability differences) between upland and adjacent marsh, or when other factors complicate the system. Published studies have adopted various mathematical approximations to deal with these deviations from linearity, but none is fully satisfactory, with some solutions yielding highly non-physical results. This study uses a new, high-quality, 8-week time series of water level fluctuations in shallow ($<$ 6 m) groundwater wells in a permeable upland and narrow, fringing, low permeability marsh and coincident tidal fluctuation data acquired in an adjacent tidal creek (the forcing function) to examine the impact of various nonlinear processes on the propagation of the tidal forcing into the aquifer. The raw time series reveal clear asymmetry to the water level fluctuations, particularly within the permeable upland, indicating that boundary effects or changes in permeability from marsh to upland may affect the character of the water level signals. Although spectral analysis of the data do reveal the expected linear filtering effect of the aquifer, we also observe generation of new harmonics, as predicted by nonlinear tidal forcing theory. To avoid some of the problems with the existing analytical solutions to the nonlinear problem, we develop a nonlinear numerical model that permits us to test the sensitivity of upland water level fluctuations to various processes. The results have implications not only for the physics of groundwater-surface water interaction at the upland-estuary interface, but also for the exchange of chemical species at these interfaces due to tidal forcing.
H21B-1021 0800h
Groundwater Response To Stream Stage In Coastal Aquifer
A two-dimensional semi-analytical solution is presented to analyze the stream-aquifer interaction in a coastal aquifer where groundwater responds to tidal effect. The conceptual model considered in this study is a two-dimensional subsurface system bounded by stream and coastline in right angle. The stream is semi-infinitely long in longitudinal extent bounded by coastline, and the streambed partially penetrates the aquifer. The dimensional and non-dimensional boundary value problems are solved for groundwater level in the aquifer with a successive use of Laplace and Fourier transform technique, and the results are obtained by the numerical inversion of the transformed solution. The solution derived herein is verified by reducing the solutions to known one-dimensional problems. Hypothetical examples are used to examine the characteristics of groundwater level variations due to the stream-aquifer interactions and the tidal fluctuations. As the distance increases from coastline the tide effect damps gradually and the effect of stream stage variation prevails. On the contrary, as the distance increases from stream the stream boundary effect decreases and the tide effect prevails. The area of influence caused by stream boundary has increased during the simulation, while the influence zone of coastal boundary was relatively constant. The groundwater level near the zone where two boundary conditions meet may rise by the action of combined effect of the two boundaries or may not change by cancelling the effect of each boundary. Thereafter, care must be taken when hydraulic parameters are estimated using sinusoidal oscillations of hydraulic head in coastal aquifers. An evaluation of parametric sensitivity of the solutions is also conducted to show the impact of stream bed leakance, stream penetration ratio to the aquifer, and storage coefficient of aquifer. The leakance value does not influence the amplitude of groundwater fluctuation, however, the effect of stream boundary increases with increasing leakance value. The sensitivity analysis also implies that the impact of stream penetration on groundwater level increases with stream width, and groundwater level variation due to stream and tide boundaries is very sensitive to storage coefficient.
H21B-1022 0800h
Transient Flow to a Horizontal Well under a Surface Water Body
A horizontal well is sometime installed in an aquifer beneath a surface water body to get better quality water because the lower permeable layer (aquitard) at the bottom of the water reservoir can act as a filter. The horizontal well can have great length of screens, thus can withdraw a significant amount of groundwater for water supply. In this study, we have investigated the transient flow to a horizontal well under a water reservoir. Previous studies in vertical wells related to this subject treated leakage from the surface water body as a volume source term in the governing equation, assuming the leakage distributed uniformly inside the aquifer. This is a necessary simplification to make the mathematical models solvable. In this study, flows in the aquitard and aquifer are treated as two systems which are linked through the continuity of flux and head at the aquitard-aquifer boundary. In particular, we treat leakage as a boundary at the aquitard-aquifer interface, not as a uniformly distributed volume source. The problem presented here concerns the induced three-dimensional interaction of stream and aquifer under a pumping horizontal well. The seepage rate induced by the pumping horizontal well and its dependence on the horizontal well length, well location, aquifer anisotropy, and hydraulic parameters of the aquitard at the bottom of the water reservoir is investigated. The results will be compared with those presented by Zhan and Park (2003). This paper will answer the questions such as under which circumstance the simplification of treating leakage as a volume source is valid. Keywords: transient flow, seepage rate, interaction, horizontal well Reference: Zhan, H. and Park, E., 2003. Horizontal well hydraulics in leaky aquifers. J. Hydrol. 281, 129-146
H21B-1023 0800h
On the Hydrological Setup of a Complex Basin Through its Hydrochemical Puzzle
Agricultural irrigation can make substantial changes to groundwater systems. This study focuses on an arid area in northern Nevada, where 100 years of extensive irrigation has occurred. In particular, the impacts of long-term agricultural activities on the groundwater fluid balance, geochemistry and potential changes to the water quality of a nearby river were investigated. The study, conducted in the Fernley Basin in western Nevada, utilized two different modeling approaches to construct a conceptual model that honors both the chemical and physical aspects of the basin groundwater flow regime. A multi-variable mixing cell model was constructed to represent the hydrochemical approach, while a MODFLOW model was constructed to represent the hydrogeological model. A geochemical and isotopic analysis of the basin and its margin groundwater was conducted to identify all of the water sources that potentially flow through the basin and all water bodies within the basin. Those water bodies and sources were incorporated into a mixing cell model that allowed delineation of the general flow lines connecting the different water bodies within the basin with their sources. The mixing cell model results were used to calibrate the hydrogeological model until a general agreement between the flow lines of both models was achieved. The calibration of the hydrogeologic model included adjustments of the hydraulic conductivities the use of various interpolation methods in order to calibrate the model flow lines into those found by the mixing cell model. The study showed that parallel application of two different modeling approaches, the mixing cell model along with a hydrogeological model, provides an excellent tool for the validation of the conceptual model. Moreover, it allowed calibration of the hydrogeologic model not only to the water heads, as usually done in hydrogeological models, but also to the general flow lines that honors the basin geochemistry.