The 11-year solar cycle,the sun’s rotation,and the middle stratosphere in winter: Part I: Response of zonal means

The effect of the 11-year solar cycle on the response of the stratospheric geopotential height and temperature fields at 10 and 30 hPa in winter to solar activity oscillations with periods related to the period of the Sun’s rotation (27.2 days) is discussed, applying methods of statistical spectral analysis to daily data for the period from 1965 to 1996. Atmospheric responses for three periodicities — 27.2 days (period of the Sun’s rotation), 25.3 days (periodicity caused by the modulation of the 27.2 days oscillation by annual atmospheric variation), and 54.4 days (doubled period of the solar rotation) — are studied. A significant effect of the 11-year solar cycle on the atmospheric response to the 27.2 days solar periodicity has not been found. We explain it by a frequency shift of the response from the 27.2 days to the 25.3 days periodicity via amplitude modulation. For the 25.3 days oscillation, prominent differences between the maximum and minimum of the 11-year solar cycle have been found in the coherence between the 10.7 cm solar radio flux and the height/temperature fields: the relationships are stronger at solar maximum than at the minimum of the 11-year cycle. The same differences, but to a greater extent, are revealed for the oscillation with a period of 54.4 days. Coherence and amplitude estimates for this doubled solar rotation periodicity exhibit strong differences between extrema of the 11-year solar cycle. Phase estimates also demonstrate a clear difference between high and low solar activity: on the average, the delay of the atmospheric response after the solar signal is smaller at solar maximum than at solar minimum. Thus, we conclude that the mechanism of the influence of the 11-year solar cycle on the winter middle stratosphere can include both a direct effect of the frequency corresponding to the doubled solar rotation periodicity and an indirect effect of modulation of the intensity of the interaction between the solar 27.2 days oscillation and seasonal atmospheric variations.     

Influence of a coarse interlayer on seawater intrusion and contaminant migration in coastal aquifers

Vertical 2D slice laboratory experiments were carried out in homogenous and layered sand tanks to elucidate the effects of a highly permeable (coarse‐grained sand) interlayer on seawater intrusion and transport of contaminants to a coastal sea. Tidal fluctuations produced oscillations in the seawater–freshwater transition zone, fluctuations of the contaminant infiltration rate and a zigzag contaminant plume outline. The seawater wedge became discontinuous at the (vertical) edges of the interlayer because of increased lateral movement of the seawater–freshwater interface within the interlayer. The contaminant plume formed a tail within the interlayer depending on the tidal stage, and similar to the wedge, its movement was accentuated. A simple analytical model that neglected vertical flow reliably predicted steady‐state seawater intrusion into the coastal aquifer. Numerical modeling was used to gain insight into the groundwater hydrodynamics and contaminant migration. The numerical results confirmed the experimental findings, i.e. that a highly permeable interlayer can provide a rapid transit path for contaminants to reach the seaward boundary and that the interlayer amplifies the effects of tidal fluctuations, resulting in wider transition zones for the seawater wedge and contaminant plume. Numerical simulations further showed that, with increasing interlayer hydraulic conductivity, the maximum seawater intrusion distance inside the interlayer increases approximately linearly. For the fixed‐head contaminant injection condition used, the model showed that contaminant infiltration increases approximately logarithmically with increasing interlayer hydraulic conductivity (other factors held fixed). Copyright © 2013 John Wiley & Sons, Ltd.     

Tidal effects on variations of fresh–saltwater interface and groundwater flow in a multilayered coastal aquifer on a volcanic island (Jeju Island, Korea)

We conducted various field studies at the seawater intrusion monitoring wells located in the eastern part of Jeju Island, Korea, to observe the tidal effect on groundwater–seawater flow in the coastal aquifer. Studies included monitoring the fluctuations of groundwater and tide levels, electrical and temperature logging, and 2-D heat-pulse flowmeter tests. According to time-series analysis, tidal effects on groundwater level reached up to 3 km inland from the coastline. Water-level variation was more sensitive to tidal fluctuations near the coast, and more related to rainfall toward inland areas. Temporal and spatial variations in the shape and location of the freshwater–saltwater interface were analyzed using data from nine monitoring wells. The results indicated that the interface toe is located at a distance of 6–8 km from the coastline and its location was related to geological layers present. Long-term seasonal variations revealed no major changes in the interface; minor variations were due to moving boundary conditions induced by tidal fluctuations. Using the two-dimensional heat-pulse flowmeter, groundwater flow directions and velocities at four tidal stages were measured on three monitoring wells drilled into the multilayered aquifers. This direct measurement enabled us to relate the differences of flow velocities and directions with geology and tidal fluctuations. Combining the results of EC logging and flowmeter tests, we found a zone where freshwater and saltwater moved alternately in opposite directions, as influenced by the tidal fluctuations. Integrating various physical logging and flowmeter data with water-level fluctuations improved our understanding of the behavior of fresh and seawater flow in the coastal aquifers.     

Crater Lake Temperature Changes of the 2005 Eruption of Santa Ana Volcano,El Salvador,Central America

A sudden eruption at Santa Ana occurred on 1 October 2005, producing an ash-and-gas plume to a height in excess of 10 km above the volcano. Several days before, thermal infrared images of the crater provided precursory signals of the eruption. A significant increase in the extent and intensity of the fumarolic field inside the crater rim and of the surface temperature of the crater’s lake was observed. Changes in energy input was also estimated to explain the increase in lake temperature based on energy/mass balance calculations.     

A hydrographic time series station in the Wadden Sea (southern North Sea)

In the tidal inlet between the East Frisian islands of Langeoog and Spiekeroog, southern North Sea, a time-series station was set up in autumn 2002 as part of the research programme BioGeoChemistry of Tidal Flats run by the University of Oldenburg. The purpose of the station is to provide continuous data on physical, biological and chemical parameters. In addition to instruments recording basic hydrographic and meteorological parameters, the time-series station is equipped with acoustic Doppler profilers for measuring surface waves and current profiles. Compact optical spectrometers are being used for spectral measurements of seawater transmission and for daylight reflectance. Additional sensors were installed for measuring oxygen, nutrients and methane in the seawater. The data shall help to quantify the flux of dissolved and suspended matter between the backbarrier tidal flat and the open sea and to characterise the material transformation in the tidal flat area by biogeochemical processes over the tidal cycle. Due to its novel design, operation of the station is also possible during winter and under extreme weather conditions (gales, storm surges, and sea ice) when data sampling with conventional platforms such as research vessels, buoys, or smaller poles could not be performed in the past. In this way, time series of data are obtained, which include events that are most relevant to the evolution of this coastal area. The performance of the station and its equipment are presented with data covering 6 years of operation. Time series of air and water temperature as well as seawater salinity demonstrate the multiyear dynamics of these parameters in the East Frisian Wadden Sea. Hydrographic data collected under specific meteorological conditions such as gales and storm surges exemplify the all-weather capabilities of the station and its value for studying hydrographic processes in the Wadden Sea.     

Effect of tidal forcing on a subterranean estuary

Groundwater flow and chemical transport in subterranean estuaries are poorly understood despite their potentially important implications for chemical fluxes from aquifers to coastal waters. Here, a numerical study of the dynamics in a subterranean estuary subject to tidal forcing is presented. Simulations show that salt transport associated with tidally driven seawater recirculation leads to the formation of an upper saline plume in the intertidal region. Computed transit times and flow velocities indicate that this plume represents a more active zone for mixing and reaction than the dispersion zone of the lower, classical salt wedge. Proper conceptualisation of this surficial mixing zone extends our understanding of processes within the subterranean estuary. Numerical tracer simulations reveal that tidal forcing may reduce the threat of a land-derived contaminant discharging to the marine environment by modifying the subsurface transport pathway and local geochemical conditions. Mixing and stratification in the subterranean estuary are strongly affected by both inland and tidal forcing. Based on the estuarine analogy we present a systematic classification of subterranean estuaries.     

Physical and dynamical oceanography of Liverpool Bay

The UK National Oceanography Centre has maintained an observatory in Liverpool Bay since August 2002. Over 8 years of observational measurements are used in conjunction with regional ocean modelling data to describe the physical and dynamical oceanography of Liverpool Bay and to validate the regional model, POLCOMS. Tidal dynamics and plume buoyancy govern the fate of the fresh water as it enters the sea, as well as the fate of its sediment, contaminants and nutrient loads. In this context, an overview and summary of Liverpool Bay tidal dynamics are presented. Freshwater forcing statistics are presented showing that on average the bay receives 233 m³ s^− 1. Though the region is salinity controlled, river input temperature is shown to significantly modulate the plume buoyancy with a seasonal cycle. Stratification strongly influences the region’s dynamics. Data from long-term moored instrumentation are used to analyse the stratification statistics that are representative of the region. It is shown that for 65% of tidal cycles, the region alternates between being vertically mixed and stratified. Plume dynamics are diagnosed from the model and are presented for the region. The spring–neap modulation of the plume’s westward extent, between 3.5 °W and 4°W, is highlighted. The rapid eastward erosion of the plume during spring tides is identified as a potentially important freshwater mixing mechanism. Novel climatological maps of temperature, salinity and density from the CTD surveys are presented and used to validate numerical simulations. The model is found to be sensitive to the freshwater forcing rates, temperature and salinities. The existing CTD survey grid is shown to not extend sufficiently near the coast to capture the near coastal and vertically mixed component the plume. Instead the survey grid captures the westward spreading, shallow and transient, portion of the plume. This transient plume feature is shown in both the long-term averaged model and observational data as a band of stratified fluid stretching between the mouth of the Mersey towards the Isle of Man. Finally the residual circulation is discussed. Long-term moored ADCP data are favourably compared with model data, showing the general northward flow of surface water and southward trajectory of bottom water.     

Temporal variations and scaling of streamflow and baseflow and their nitrate-nitrogen concentrations and loads

The patterns of temporal variations of precipitation (P), streamflow (SF) and baseflow (BF) as well as their nitrate-nitrogen (nitrate) concentrations (C) and loads (L) from a long-term record (28 years) in the Raccoon River, Iowa, were analyzed using variogram and spectral analyses. The daily P is random but scaling may exist in the daily SF and BF with a possible break point in the scaling at about 18 days and 45 days, respectively. The nitrate concentrations and loads are shown to have a half-year cycle while daily P, SF, and BF have a one-year cycle. Furthermore, there may be a low-frequency cycle of 6–8 years in C. The power spectra of C and L in both SF and BF exhibit fractal 1/f scaling with two characteristic frequencies of half-year and one-year, and are fitted well with the spectrum of the gamma distribution. The nitrate input to SF and BF at the Raccoon watershed seems likely to be a white noise process superimposed on another process with a half-year and one-year cycle.     

The structure of the coastal density front at the outflow of Long Island Sound during spring 2002

South of the eastern end of Long Island (Montauk Point) along the Eastern U.S. coast, a coastal density front forms between the buoyant outflow plume of the Long Island Sound (LIS) and the denser shelf waters offshore. During a 2-day cruise in April 2002, measurements of the density and velocity structure of this front were obtained from high-resolution CTD and ADCP data. Transects show the front intersecting the bottom inshore of the 30 m isobath and shoaling offshore. Variability in the location of the front is small offshore of the 40 m isobath, yet tidal excursions of the front along the bottom are significant (5 km) inshore of this depth.The frontal structure of the LIS plume was similar to observations of bottom-trapped coastal density fronts and shelf break fronts. A coastal jet in the along front direction was the main feature of the mean velocity field and was found to be in thermal wind balance with the mean density field. Stronger than expected offshore velocities near the surface, most likely a result of wind forcing, were the only exception to these similarities. In addition, analysis of temperature and salinity gradients along isopycnals gives evidence of secondary cross-frontal circulation and detachment of the bottom boundary layer. Characteristics of the LIS plume are used to evaluate recent analytical models of bottom-trapped coastal density fronts and bottom-advected plume theory, finding good agreement.     

2017年新疆精河MS6.6地震遥感大气温度变化特征分析

在中国地震台网中心2016年底利用热红外遥感技术预测2017年新疆西部地区为潜在M_S 6.6±0.2地震危险区的基础上,分析2017年8月9日精河M_S6.6地震临震时段引潮力变化,并选用18时（UTC）中国大陆近地表50m高度处的遥感大气温度数据,以震前引潮力值最高点时刻（8月1日）为时间背景,获取地震前后（8月2~13日）连续的大气温度日增量分布图像,跟踪分析精河M_S6.6地震短临大气温度变化。结果显示：地震发生在天体引潮力由高峰—低谷连续周期变化的低谷时段,而大气温度变化过程显示,在全国大范围内,仅震中附近大气温度升高明显,其异常演化经历了起始—加强—高峰—衰减—再增强—发震—平静的动态过程。增温过程与潮汐变化具有同步性,这表明引潮力对本次地震具有触诱发的作用,而大气温度变化反映了本次地震地应力的变化过程,也说明在地震预测实践中,从中、短临多时间尺度综合分析遥感大气温度和引潮力变化,将有助于提高地震预测能力。     

International tidal gravity reference values at Wuhan station

The international tidal gravity reference values at Wuhan station are determined accurately based on the comprehensive analysis of the tidal gravity observations obtained from 8 instruments. By comparing these with those in the tidal models given by Dehant (1997) while considering simultaneously (i) the global satellite altimeters tidal data, and (ii) the Schwiderski global tidal data and the local ones along the coast of China, it is found that the average discrepancy of the amplitude factors and of the phase differences for four main waves are given as 5.2% and 3.6% and as 0.16° and 0.08° respectively. They are improved evidently compared to those determined in early stage, indicating the important procedures in improving the Wuhan international tidal gravity reference values when including the long-series observations obtained with a superconducting gravimeter, and when considering the influence of the ocean loading and of the nearly daily free wobble of the Earth’s core.     

中纬度电离层foF2潮汐的变化特性

本文通过对1960年以来全球磁纬度40°N至50°N内测高仪台站的观测数据进行研究,提取了电离层F₂层临界频率（f_oF₂）的潮汐,揭示了其变化特征及可能的形成原因.研究发现,周日和半日的迁移潮汐分量（即DW1和SW2）强度最大,并且显示出明显的年变化和半年变化.周日潮汐的3波分量（即DE3）作为典型的非迁移潮汐分量,相对较弱,显示出微弱的半年变化.在冬季,DW1和SW2与太阳活动指数（F₁₀₇）呈现正相关性,其相关系数分别大于0.88和0.65.相反,在夏季,DW1和SW2与太阳活动指数呈现负相关性,特别是SW2,其相关系数在6月份达到-0.72.在相对于纬向均值的归一化处理之后,上述潮汐强度和太阳活动指数之间的正/负相关性被显著增强/削弱.其中,归一化后的夏季DW1和SW2与太阳活动指数的相关系数达到-0.8.更加深入的讨论显示出上传的大气潮汐波动可能是电离层潮汐除了太阳辐射之外的重要驱动源,并且这种驱动机制在SW2中更加强烈.     

Multi-source water pollution in a highly anthropized wetland system associated with the estuary of Huelva (SW Spain)

Major ions, nutrients, trace elements and pesticides distribution were studied in a coastal wetland heavily impacted by human development in Spain. Past land use has altered the local hydrodynamics leading to the partitioning of the ecosystem into a tideland subject to marine influence, and an artificial freshwater reservoir created by stream impoundment. The tideland stretch is flooded twice a day with a heavy metal plume that emerges from the mine-polluted estuary of Huelva and propagates landward depicting the same dispersal trend of major seawater ions. Additionally, the tidal channel receives acid discharges from industrial point sources that contribute to metal enhancement. The impounded area and stream tributaries are affected by agrochemicals runoff (nitrate, phosphate, pendimethalin, simazine, diuron and therbuthylazine) from surrounding agricultural lands. The tidal regime plays a crucial role in the transport and dispersion of pollutants, except in the artificial reservoir where freshwater exhibits a seasonal mineralization pattern.     

Wind-driven dispersal of volcanic ash plumes and its control on the thermal structure of the plume-top

The dispersal of an ash plume as it propagates downwind with the ash settling under gravity is presented. It is shown that wind shear, the initial plume height and the plume grain size distribution have an important role in the dispersal of the ash, in particular in determining the evolution of the upper surface and the leading edge of the ash cloud. Once the ash has thermally equilibrated with the atmosphere, the temperature of the upper surface of the ash plume is directly related to its altitude. As a result we can use the model to interpret satellite images of the temperature of the upper surface of ash plumes. These calculations are compared with new analyses of satellite data from the 18 May 1980 eruptions of Mount St Helens in which both thermal infra-red and visible GOES satellite data were examined. In accord with the data, the model is able to predict the rate at which the Mount St Helens ash plume propagated downwind through a combination of the wind shear and gravitational settling of different size particles. The model is also able to explain the observed thermal structure of the upper surface of the ash plume, in which the temperature initially decreases, but then increases with downwind distance as the ash falls through the tropopause.     

Characterization of contrasting flow and thermal regimes in two adjacent subarctic alpine headwaters in Northwest Canada

Alpine headwaters in subarctic regions are particularly sensitive to climate change, yet there is little information on stream thermal regimes in these areas and how they might respond to global warming. In this paper, we characterize and compare the hydrological and thermal regimes of two subarctic headwater alpine streams within an empirical framework. The streams investigated are located within two adjacent catchments with similar geology, size, elevation and landscape, Granger Creek (GC) and Buckbrush Creek (BB), which are part of the Wolf Creek Research Basin in the Yukon Territory, Canada. Hydrometeorological and high-resolution stream temperature data were collected throughout summer 2016. Both sites exhibited a flow regime typical of cold alpine headwater catchments influenced by frozen ground and permafrost. Comparatively, GC was characterized by a flashier response with more extreme flows, than BB. In both sites, stream temperature was highly variable and very responsive to short-term changes in climatic conditions. On average, stream temperature in BB was slightly higher than in GC (respectively 5.8 and 5.7°C), but less variable (average difference between 75th and 25th quantiles of 1.6 and 2.0°C). Regression analysis between mean daily air and stream temperature suggested that a greater relative (to stream flow) groundwater contribution in BB could more effectively buffer atmospheric fluctuations. Heat fluxes were derived and utilized to assess their relative contribution to the energy balance. Overall, non-advective fluxes followed a daily pattern highly correlated to short-wave radiation. G1enerally, solar radiation and latent heat were respectively the most important heat source and sink, while air–water interface processes were major factors driving nighttime stream temperature fluctuations.     

Transport of variable-density solute plumes in beach aquifers in response to oceanic forcing

A comprehensive numerical study was undertaken to investigate transport of a variable-density, conservative solute plume in an unconfined coastal aquifer subject to high and low frequency oceanic forcing. The model combined variable-density saturated flow for groundwater and solute transport, and wave hydrodynamics from a 2D Navier–Stokes solver. A sinusoidal tidal signal was specified by implementing time-varying heads at the seaward boundary. The solute plume behavior was investigated under different oceanic forcing conditions: no forcing, waves, tide, and combined waves and tide. For each forcing condition, four different injected solute densities (freshwater, brackish water, seawater, brine) were used to investigate the effects of density on the transport of the injected plume beneath and across the beach face. The plume’s low-order spatial moments were computed, viz., mass, centroid, variance and aspect ratio. The results confirmed that both tide- and wave-forcing produce an upper saline plume beneath the beach face in addition to the classical saltwater wedge. For the no-forcing and tide-only cases (during rising tides), an additional small circulation cell below the beach face was observed. Oceanic forcing affects strongly the solute plume’s flow path, residence time and discharge rate across the beach face, as well as its spreading. For the same oceanic forcing, solute plumes with different densities follow different trajectories from the source to the discharge location (beach face). The residence time and plume spreading increased with plume density. It was concluded that simulations that neglect the effect of waves or tides cannot reproduce accurately solute plume dispersion and also, in the case of coasts with small waves or tides, the solute residence time in the aquifer.     

Base flow‐driven shifts in tropical stream temperature regimes across a mean annual rainfall gradient

Climate change is expected to affect air temperature and watershed hydrology, but the degree to which these concurrent changes affect stream temperature is not well documented in the tropics. How stream temperature varies over time under changing hydrologic conditions is difficult to isolate from seasonal changes in air temperature. Groundwater and bank storage contributions to stream flow (i.e., base flow [BF]) buffer water temperatures against seasonal and daily fluctuations in solar radiation and air temperature, whereas rainfall‐driven runoff produces flooding events that also influence stream temperature. We used a space‐for‐time substitution to examine how shifts in BF and runoff alter thermal regimes in streams by analyzing hydrological and temperature data collected from similar elevations (400–510 m above sea level) across a 3,500‐mm mean annual rainfall gradient on Hawai'i Island. Sub‐daily water temperature and stream flow gathered for 3 years were analyzed for daily, monthly, and seasonal trends and compared with air temperature measured at multiple elevations. Results indicate that decreases in median BF increased mean, maximum, and minimum water temperatures as well as daily temperature range. Monthly and daily trends in stream temperature among watersheds were more pronounced than air temperature, driven by differences in groundwater inputs and runoff. Stream temperature was strongly negatively correlated to BF during the dry season but not during the wet season due to frequent wet season runoff events contributing to total flow. In addition to projected increases in global air temperature, climate driven shifts in rainfall and runoff are likely to affect stream flow and groundwater recharge, with concurrent influences on BF resulting in shifts in water temperature that are likely to affect aquatic ecosystems.     

Effects of solar and geomagnetic activities in variations of power spectra of electrical and meteorological parameters in the near-Earth atmosphere in Kamchatka during October 2003 solar events

We perform spectral analysis of records of meteorological (temperature, humidity, pressure of the atmosphere) and electrical (strength of quasi-static electric field and electric conductivity of air) parameters observed simultaneously at the Paratunka observatory during the solar events of October 21–31, 2003. Also, we use simultaneous records of X-ray fluxes of solar radiation, galactic cosmic rays, and the horizontal component of the geomagnetic field. We show that the power spectra of the meteorological parameters under fine weather conditions involve oscillations with a period of thermal tidal waves (T ～ 12 and 24 h) caused by the influx of thermal radiation of the Sun. During strong solar flares and geomagnetic storm of October 29–31 with a prevailing component of T ～ 24 h, their spectra involve an additional component of T ～ 48 h (the period of planetary-scale waves). With the development of solar and geomagnetic activities, the power spectra of atmospheric electric conductivity and electric field stress involve components of both thermal tidal and planetary-scale waves, which vary highly by intensity. In the power spectra of galactic cosmic rays accompanying the strong solar flares, components with T ～ 48 h were dominant with the appearance of additional (weaker by intensity) components with T ～ 24 h. The simultaneous amplification of components with T ～ 48 h in the power spectra of electric conductivity and electric field strength provides evidence of the fact that the lower troposphere is mainly ionized by galactic cosmic rays during strong solar flares and geomagnetic storms. The specified oscillation period with T ～ 48 h in their spectra, as well as in the spectra of X-ray radiation of the sun, is apparently caused by the dynamics of solar and geomagnetic activities with this time scale.     

Investigation of submarine groundwater discharge to tidal rivers: Evidence for regional and local scale seepage

Fluxes of submarine groundwater discharge (SGD) were investigated into two tidal rivers on the north and south shore of Long Island, NY, during July 2015. Ground‐based handheld thermal infrared (TIR) imagery, combined with direct push‐point piezometer sampling, documented spatially heterogeneous small‐scale intertidal seepage zones. Pore waters were relatively fresh and enriched in nitrogen (N) within these small‐scale seeps. Pore waters sampled just 20 cm away, outside the boundary of the ground‐based TIR‐located seepage zone, were more saline and lower in N. These ground‐based TIR‐identified seeps geochemically represented the terrestrial fresh groundwater endmember, whereas N in pore waters sampled outside of the TIR‐identified seeps was derived from the remineralization of organic matter introduced into the sediment by tidal seawater infiltration. A ²²²Rn (radon‐222) time‐series was used to quantify fresh SGD‐associated N fluxes using the N endmembers sampled from the ground‐based TIR pore water profiles. N fluxes were up‐scaled to groundwater seepage zones identified from high‐resolution airborne TIR imagery using the two‐dimensional size of the airborne TIR surface water anomalies, relative to the N flux from the time‐series sampling location. Results suggest that the N load from the north‐shore tidal river to Long Island Sound is underrepresented by at least 1.6–3.6%, whereas the N load from SGD to a south‐shore tidal river may be up to 9% higher than previous estimates. These results demonstrate the importance of SGD in supplying nutrients to the lower reaches of tidal rivers and suggest that N loads in other tidal river environments may be underestimated if SGD is not accounted for.     

Simulation of Tidal Effects on Contaminant Transport in Porous Media

A one-dimensional numerical model is developed with oscillating velocities and dispersions to simulate the migration process of a contaminant plume within tidally influenced aquifers. Model simulations demonstrate that a major effect the tidal fluctuation has on the migration process of a contaminant plume is the exit concentration discharging to the tidal estuary. Tidal fluctuation causes the exit concentration levels to be significantly diluted by the surface-water body of the estuary. Sensitivity analyses demonstrate that tidal fluctuation hastens the rate of plume migration near the bank of the estuary because of the relatively high advective and dispersive fluxes induced by tides. However, tides affect the migration process only over a short distance from the tidal-water interface (about 40 ft for the parameters used in this study). If the contaminant plume is located far beyond the interface, tidal fluctuations will not affect the rate of plume migration until an existing regional ground-water flow velocity brings the plume to the tidally active zone. With or without tides, the rate of contaminant migration increases with higher regional hydraulic gradient. Furthermore, the effects of tidal fluctuations on the transport process become insignificant with higher regional hydraulic gradients.