Freshwater-saltwater Interface Research Articles

Application of Stable Isotopes of Water to Study Coupled Submarine Groundwater Discharge and Nutrient Delivery

Submarine groundwater discharge (SGD)—including terrestrial freshwater, density-driven flow at the saltwater–freshwater interface, and benthic exchange—can deliver nutrients to coastal areas, generating a negative effect in the quality of marine water bodies. It is recognized that water stable isotopes (18O and 2H) can be helpful tracers to identify different flow paths and origins of water. Here, we show that they can be also applied when assessing sources of nutrients to coastal areas. A field site near a lagoon (Ringkøbing Fjord, Denmark) has been monitored at a metric scale to test if stable isotopes of water can be used to achieve a better understanding of the hydrochemical processes taking place in coastal aquifers, where there is a transition from freshwater to saltwater. Results show that 18O and 2H differentiate the coastal aquifer into three zones: Freshwater, shallow, and deep saline zones, which corresponded well with zones having distinct concentrations of inorganic phosphorous. The explanation is associated with three mechanisms: (1) Differences in sediment composition, (2) chemical reactions triggered by mixing of different type of fluxes, and (3) biochemical and diffusive processes in the lagoon bed. The different behaviors of nutrients in Ringkøbing Fjord need to be considered in water quality management. PO4 underneath the lagoon exceeds the groundwater concentration inland, thus demonstrating an intra-lagoon origin, while NO3, higher inland due to anthropogenic activity, is denitrified in the study area before reaching the lagoon.

Steady-State Approximate Freshwater–Saltwater Interface in a Two-Horizontal-Well Scavenging System

In this study, an approximate mathematical formulation is derived to delineate a steady-state freshwater–saltwater interface by a horizontal scavenger well system in a confined aquifer that consists of two horizontal pumping wells. One horizontal well is above the freshwater–saltwater interface to recover freshwater. The other horizontal well is below the interface to extract saltwater. A set of dimensionless variables are introduced to facilitate solution derivations. Critical pumping rates that correspond to critical upconing and downconing interfaces are then derived for two different interface extents. Finally, the horizontal scavenger well system is applied to a real-world case, the 1,500-foot sand in Baton Rouge, Louisiana, and is compared to the current vertical scavenger well system, where the Field-Critchley analytical solution is adopted. The real-world case study demonstrates that among many advantages, the horizontal scavenger well system can recover more freshwater and extract less saltwater than the vertical scavenger well system. The study also indicates that the current freshwater and saltwater pumping rates are far below the safe zone and that freshwater is pulled into the saltwater well.

Numerical Simulations of Seasonally Oscillated Groundwater Dynamics in Coastal Confined Aquifers.

Studies investigating the effects of inland recharge on coastal groundwater dynamics were carried out typically in unconfined aquifers, with few in confined aquifers. This study focused on the groundwater dynamics in confined aquifers with seasonally sinusoidally fluctuated inland groundwater head and constant sea level by numerical simulations. It is known that the mixing zone (MZ) of saltwater wedge in response to the seasonal oscillations of inland groundwater head swings around the steady-state MZ. However, our simulation results indicate that even the most landward freshwater-saltwater interface over a year is seaward from the steady-state location when the hydraulic conductivity K is ≤10-4 m/s under certain boundary conditions with given parameter values. That is, seasonal oscillations of inland groundwater head may reduce seawater intrusion in confined coastal aquifers when K ≤ 10-4 m/s. Sensitivity analysis indicates that for aquifers of K ≤ 10-4 m/s, the larger the inland head fluctuation amplitude is, the less the seawater intrudes. This is probably due to the reason that the seawater intrusion time decreases with the increase of fluctuation amplitude when K ≤ 10-4 m/s. Numerical simulations demonstrate that seasonal inland groundwater head oscillations promote the annual averaged recirculated seawater discharge across the seaward boundary.

The Dynamics of Sea Tide-Induced Fluctuations of Groundwater Level and Freshwater-Saltwater Interface in Coastal Aquifers: Laboratory Experiments and Numerical Modeling

Laboratory experiments were conducted in a vertical, two-dimensional, rectangular flow tank, simulating the response of a phreatic coastal aquifer to a sea tide. Imposed sinusoidal fluctuations of the saltwater level at one side of the flow tank caused three types of fluctuations: (a) hydraulic head throughout the aquifer, (b) saturation degree within the capillary fringe, and (c) salt concentration surrounding the freshwater-saltwater interface (FSI), all recorded by head, saturation, and salinity sensors, respectively. Significant time lags were observed both in the saturation degree within the unsaturated zone and in the salinity within the FSI. All measured values, recorded by the three types of sensors, were simulated and reproduced using a numerical model. The calibrated model was used for mapping the time lags throughout the aquifer. It was found that the time lag of saturation fluctuations within the unsaturated zone increased upward from the groundwater level as the unsaturated hydraulic conductivity decreased. Similarly, the time lag of salinity fluctuations within the FSI increased downward, with distance from the groundwater level. We interpret the low hydraulic conductivity at the capillary zone as the source of attenuation of both saturation and salinity, because both are controlled by the vertical advection of the whole freshwater body. This advection is significantly slower compared to the dynamics of pressure diffusion. The uniqueness of this study is that it provides quantitative data on the attenuation at the capillary zone and its effect on the salinity time lag in coastal aquifer systems.

Groundwater salinity during 500 years of anthropogenic-driven coastline changes in the Motril-Salobreña aquifer (South East Spain)

The freshwater–saltwater interface position in groundwater at coastal aquifers is determined by the location of the coastline. Anthropogenic-driven changes as modification of the land uses in catchments or the engineering construction in rivers can alter the transport of sediments to the coastal areas affecting to the coastline shape in detrital systems. This is the case of the Motril-Salobrena aquifer where the rapid coastline progradation over the last 500 years generated new land previously covered by the sea at a fast rate, 3 m year−1 during hundreds of years. The effect of these changes in the salinity of the aquifer and the flushing time by freshwater has been examined with a paleo-hydrogeological model simulating the transient evolution of the groundwater salinity for the last 500 years with SEAWAT. The results of the model indicate a differentiated flushing time depending on the hydraulic properties in the region ranging from 50 years in the western sector to up to more than 200 years in the central sector for the shallower parts of the aquifer. In the deeper layers, the time can be highly increased but the uncertainties in the hydraulic properties generate different scenarios both in flushing time and water circulation in the aquifer. The sectors in the aquifer where changes took place at a faster rate are also the most sensitive to the effect of global changes, especially those associated to human activity as they occur at shorter time periods.

Combined influence of weir construction and sea-level rise on freshwater resources of a coastal aquifer in northern Germany

A significant volume of an aquifer along the coastline in the German Bight is salinized by seawater intrusion. The mean sea-level rise (MSLR) is expected to continue in the future due to global climatic change, subsequently degrading the fresh groundwater resources. To impede further salinization in the future, a solution is proposed based on weir construction in an existing canal hydraulically connected to the aquifer. The effect is twofold: (1) the elevated groundwater level can upgrade present fresh groundwater resources by shifting the saltwater–freshwater interface position further seaward, or by inhibiting its landward movement, and (2) the inland water level can be elevated, expanding surface water ponds. A fully coupled three-dimensional numerical surface-subsurface model (a modified HydroGeoSphere code) was used to simulate the effects of variable weir construction heights under different MSLR rates, and to quantify the gain of aquifer freshwater volume and loss of usable land due to surface ponding. Construction of a higher weir increases the desalinized aquifer volume and decreases the newly salinized aquifer volume under future MSLR. A minimum height of a weir was determined under a certain MSLR rate to maintain the present freshwater resource. Both weir construction and MSLR can cause the loss of land usage. Computed loss-gain ratio curves can be utilized to determine the optimal weir height, meeting the economic requirements of coastal land management under future MSLR.

Combined geophysical and analytical methods to estimate offshore freshwater extent

Offshore fresh groundwater is increasingly suggested as a potential water resource for onshore human demands. In many cases, onshore pumping already draws significant fresh groundwater from offshore. However, offshore aquifers and the extent of offshore freshwater are usually poorly characterised due to data scarcity. This study combines geophysical data, hydraulic information and a first-order mathematical analysis to investigate offshore freshwater extent in the Gambier Embayment (Australia). A large seismic data set, combined with onshore and offshore bore-log geological profiles, are used to explore the regional offshore hydro-stratigraphy. Aquifer hydraulic parameters and onshore heads are obtained from onshore investigations. A novel application of Archie’s law, geophysical data and onshore hydrochemical data provide useful insights into the salinity profiles within four offshore wells. These are compared to steady-state, sharp-interface estimates of the freshwater extent obtained from a recently developed analytical solution, albeit using simplified conceptual models. Salinities derived from resistivity measurements indicate that in the south of the study area, pore water with total dissolved solids (TDS) of 2.2 g L−1 is found up to 13.2 km offshore. Offshore pore-water salinities are more saline in the northern areas, most likely due to thinning of the offshore confining unit. The analytical solution produced freshwater-saltwater interface locations that were approximately consistent with the freshwater-saltwater stratification in two of the offshore wells, although analytical uncertainty is high. This investigation provides a leading example of offshore freshwater evaluation applying multiple techniques, demonstrating both the benefit and uncertainty of geophysical interpretation and analytical solutions of freshwater extent.

Numerical modelling study of seawater intrusion in Indus River Estuary, Pakistan

Seawater intrusion is one of the major problems in the Indus River Estuary (IRE), Pakistan. A FVCOM-based numerical model was constructed to study the seasonal variability of currents and salinity in the IRE. The model was well calibrated with observed water level, current velocity and salinity data. Model results indicate that current velocity in dry seasons is smaller than that in wet seasons. Salt-water intrudes far upstream for about 65 km. In flood seasons, freshwater discharge is the main component controlling currents and saltwater intrusion in the IRE. Currents are all seaward during both the flood and ebb phases. Saltwater-freshwater interface is pushed seaward to the mouth of the estuary. Both field data and numerical results reveal that stratification (mixing) occur at the mouth of the IRE during wet (dry) seasons, dominated by estuarine circulation and tidal straining.

Baseline evaluation of freshwater\u2013saltwater interface in coastal aquifers of Badagry, south-western Nigeria

This study focused on the evaluation of the baseline condition of saline water–groundwater interface in the coastal aquifers along Badagry, south-western Nigeria. Geologically, Badagry lies within the coastal sands and recent alluvium of the Dahomey Basin. Two-dimensional electrical resistivity tomography (2D-ERT) data along 15 survey lines were acquired in the study area, adopting the Wenner electrode arrays system with minimum spacing of 10 m. Ninety-one water samples were also collected, and their physical parameters were measured using hand-held devices. The (2D-ERT) data were processed and interpreted with the aid of DiProWin software. Sandy topmost layer, freshwater sand, brackish water sand and saline water sand were delineated from the study. Brackish water sand and freshwater sand layers were dominant in areas with high proximity to the Atlantic Ocean located in the south, which were not observed in the northern part of the mapped area. The study established the freshwater–saltwater interface at a depth of 10 m and above, in areas around the coast, while the interface occurs at depth greater than 50 m in areas 3–5 km from the coastal area. Observations from the pH and the TDS show that 87.3% of the samples are slightly acid, while less than 12.7% of them are slightly alkaline, while the TDS vary from 8 to 520 mg/l. The EC of the samples varies from 13 to 1140 µS/cm. All water samples are fresh groundwater, which confirms the occurrence of freshwater aquifer even in areas closer to the Atlantic Ocean.

Freshwater-salt water interface dynamics during pumping tests

Freshwater-salt water interface (FSI) location is very important information for decision maker in managing coastal aquifer system, however, its temporal change have been hard to get using conventional method such as EC monitoring at one or several fixed depths, geophysical logging or remote sensing techniques. A FSI tracking device, which has density between freshwater and salt water and hence can moves up and down as the freshwater-salt water transition zone moves, is used to get a temporal change data for the interface during several different types of pumping tests, which were performed at coastal monitoring wells in Seocheon, middle west of Korean Peninsula. Four short period pumping tests, three long-period pumping tests, one step-drawdown test, one reverse step-drawdown test were performed at different pumping rate ranging 19.86 to 48.71 m3/d for different pumping period of 60 minutes to 2851 minutes. Time series data shows that the Interface-Egg rises up from -86.0 to -77.6 m amsl after 24-hours pumping and to -40.8 m amsl after 2-days pumping and freshwater lens thickness is getting thinner from 88.1 m to 78.4 m after 24-hours pumping and then 42.3 m after 2-days pumping. These salt water up-coning phenomena are supported by EC profiles which were logged before and after the whole pumping periods. Time series data tell us that salt water upconing in the pumping well happens quickly and recovers at a very slow rate which is about 1.5 cm/d at 3 months after stopping pumping. The time series data of groundwater head and the interface-Egg’s location also shows that there is a tidal influence between pumping periods. The FSI tracker is expected to be practically applied to coastal aquifer management preventing from salt water intrusion, especially at dynamically pumping area for agricultural and/or domestic water supply.

A conceptual model for groundwater discharge to a coastal brackish lagoon based on seepage measurements (Ringkøbing Fjord, Denmark)

AbstractGroundwater discharge to a brackish lagoon (Ringkøbing Fjord) was quantified with seepage meters along four transects perpendicular to the shore during four seasons in 1 year. The objectives were to develop a conceptual model of the spatiotemporal variability of seepage and how landscape characteristics and saltwater intrusion affect seepage fluxes. The electrical conductivity of the groundwater was measured in vertical sediment profiles up to 3 m below the lagoon bed to assess the effects of the freshwater–saltwater interface location on flux distribution. The main differences compared with existing theoretical models are a lower discharge near the shore line (0–5 m) and two discharge peaks more offshore (15–20 and >25 m), which allowed the development of a conceptual model that is different from the classical concepts introduced for marine or lake environments and which can be representative of other similar areas. These differences are explained by the vegetation and organic material deposition in the near‐shore environment reducing discharge, the presence of the saline wedge leading to upward flow, and the increase of recirculation of brackish water due to wave pumping in areas offshore most exposed to the wind. The seasonal variability in discharge is smaller than the spatial variability between and within transects along the shoreline. Based on the conceptual model, fresh water discharge in a 20‐m‐wide fringe was calculated to be between 66 and 388 L·day−1·m−1 shore line, corresponding to a difference of a factor of six due to the variations in coast morphology and local characteristics of the area. The seasonal changes were responsible for changes of only 8% to 75% depending on the location.

Transience of seawater intrusion and retreat in response to incremental water‐level variations

AbstractThis paper provides for the first time an experimental study where the impact of sea‐level fluctuations and inland boundary head‐level variations on freshwater–saltwater interface toe motion and transition zone dynamics was quantitatively analysed under transient conditions. The experiments were conducted in a laboratory flow tank where various (inland and coastal) head changes were imposed to the system and the response of the key seawater intrusion parameters was analysed with high spatial and temporal resolution. Two homogeneous aquifer systems of different grain size were tested. The numerical code SEAWAT was used for the validation. The results show that in cases of sea‐level variations, the intruding wedge required up to twice longer time to reach a new steady‐state condition than the receding wedge, which thereby extend the theory of timescale asymmetry between saltwater intrusion and retreat processes in scenarios involving sea‐level fluctuations. The intruding and receding rates of the saltwater wedge were respectively similar in the scenario involving sea‐level and the freshwater‐level changes, despite change in transmissivity. The results show that, during the intrusion phase, the transition zone remains relatively insensitive, regardless of where the boundary head change occurs (i.e., freshwater drop or sea‐level rise) or its magnitude. By contrast, a substantial widening of the transition zone was observed during the receding phase, with almost similar amplitude in the scenario involving a rise of the freshwater level compared with that caused by a drop of the saltwater level, provided that an equivalent absolute head change magnitude was used. This transition zone widening (occurring during saltwater retreat) was greater and extended over longer period in the low hydraulic conductivity aquifer, for both freshwater‐level rise and sea‐level drop scenarios. The concentration maps revealed that the widening mechanism was also enhanced by the presence of some freshwater sliding and into the wedge during saltwater retreat, which was thereafter sucked upward towards the interface because of density difference effects.

Seasonal variability of turbidity, salinity, temperature and suspended chlorophyll in a strongly tidal sub-estuary: The Lynher Marine Conservation Zone

The Lynher Estuary in Southwest England is a small, strongly tidal sub-estuary of the Tamar Estuary. It is a Site of Special Scientific Interest (SSSI), a Marine Conservation Zone (MCZ), a part of the Plymouth Sound and Estuaries Special Area of Conservation (SAC) and a Special Protection Area (SPA). Management of the Lynher SSSI and MCZ stipulates that good water quality and sediment quality should be maintained; as such, a good understanding of its responses to influences such as climate change and changes in agricultural practices within its catchment area is required. Observations of salinity, temperature, suspended particulate matter (SPM) concentrations, estuarine turbidity maximum (ETM) behaviour, and chlorophyll-a are presented for the Lynher over a 1-y period. The dataset provides important baseline information with which to identify future changes and guide management of the SSSI and MCZ as well as adding to our knowledge of estuarine systems. Salt intrusion is largely controlled by tides and runoff. A persistent ETM occurs that is closely associated with the freshwater-saltwater interface at high water (HW) and with a minimum in dissolved oxygen concentrations. HW depth-averaged ETM magnitudes are relatively low, less than 60 mg l−1 and typically 30 mg l−1 over the observation period. Larger tides and stronger flood-tide wind speeds lead to a stronger ETM. Tidal river HW SPM concentrations are intrinsically small (8 ± 8 mg l−1 during the observation period). Surface chlorophyll-a concentrations are low during winter (when they often peak near the ETM) and are much higher during spring and summer.

Contribution of geophysics to geometric characterization of freshwater\u2013saltwater interface in the Ma\xe2moura region (NE Tunisia)

The Maâmoura aquifer is located on the eastern coast of the Cap Bon peninsula. During recent decades, the agricultural exploitation of water has been strongly reinforced, causing an over-pumping of the groundwater resources accompanied by an intrusion of seawater. The geophysical approach based on electrical resistivity tomography has been adopted to obtain high-resolution electrical sections imaging, which allows delineatation of the areas characterized by a high salinization and reconstructs the geometry of the saline wedge front. In fact, the presence of seawater in the aquifers reduces subsurface resistivity values. In this study, five electrical profiles were performed using the Terrameter SAS4000 Lund Imaging System with an array of 64 electrodes. The Wenner configuration was used with a unit electrodes spacing of 5 m. Finally, we use hydrogeological data, and electrical resistivity tomography data superimposed with the location of wells to provide a conceptual framework for the understanding of the freshwater–saltwater interface in the Maâmoura region.

Annotated translation of “Nota in verband met de voorgenomen putboring nabij Amsterdam [Note concerning the intended well drilling near Amsterdam]” by J. Drabbe and W. Badon Ghijben (1889)

The famous report by engineers Drabbe and Badon Ghijben (1889), on an intended well drilling near Amsterdam (the Netherlands), was one of the key documents that contributed to the Ghijben-Herzberg formula, which links water-table elevation to the depth of the freshwater–saltwater interface in coastal aquifers. The report has been often cited but no English translation has appeared in the literature to date. The aim of this annotated translation of the report is to provide the international scientific community with easier access than was hitherto the case, plus electronic access to the original in Dutch. A brief introduction to the report is provided, followed by a translation that follows the original text as closely as possible.

What is the Ghijben-Herzberg principle and who formulated it?

It has been suggested in a number of historical notes that it was neither Willem Badon Ghijben nor Alexander Herzberg who formulated the famous principle now carrying their name, which relates the water-table elevation to the depth of the freshwater–saltwater interface in coastal aquifers. In this paper, a systematic review of the literature pre-dating the publication of their work is presented. The aim is to establish to what extent these previous works captured the essence of the Ghijben-Herzberg principle, that is, the combination of a correct conceptual model of the hydrogeological conditions with a quantitative relationship. It was found that references to coastal fresh groundwater reserves can be traced back to Roman times, while the earliest detailed descriptions of a freshwater lens that could be found dates from the eighteenth century. The correct understanding of the hydrostatic equilibrium between fresh and salt groundwater is evident in works from the early nineteenth century. However, it was Badon Ghijben and Herzberg who combined this with the correct understanding of the groundwater conditions of a freshwater lens. It was further found that Herzberg had already recorded his findings in 1888 in a hand-written report, confirming speculation that such a report might exist.

Assessing seasonal flow dynamics at a lagoon saltwater–freshwater interface using a dual tracer approach

Study regionEastern shore of Ringkøbing Fjord, a coastal lagoon at the west coast of Denmark Study focusA dual tracer approach based on salinity and δ18O is used to assess seasonal dynamics at the saltwater-freshwater interface of a coastal lagoon. At the site, salinity is prone to vary on a sub-seasonal or daily frequency due to riverine freshwater inputs to the lagoon. In contrast, δ18O compositions of end-members only vary seasonally. New hydrological insightsThe dual tracer approach shows to be valuable in coastal settings where end-member concentrations vary substantially over the seasons and hence, an unambiguous end-member definition does not exist. Calculated mixing fractions using only salinity, deviated from the dual tracer approach on average by 18%, but were as high as 97%. Although, these differences decrease to 6% on average when using only δ18O, our study strongly suggests their simultaneous application.Moreover, we found that seawater intrusion occurs during the summer when salinity in the lagoon is high and fresh submarine groundwater discharge (SGD) is low. This process reverses during the winter (wet season) when SGD increases by a factor of 2–3, due to the recession of the saltwater wedge from land. Our findings show that in absence of waves and tides, density-driven dynamics, and particularly the terrestrial freshwater fluxes, create a major impact on saltwater wedge dynamics.

A multi-isotopic approach to investigate the influence of land use on nitrate removal in a highly saline lake-aquifer system

Endorheic or closed drainage basins in arid and semi-arid regions are vulnerable to pollution. Nonetheless, in the freshwater-saltwater interface of endorheic saline lakes, oxidation-reduction (redox) reactions can attenuate pollutants such as nitrate (NO3−). This study traces the ways of nitrogen (N) removal in the Pétrola lake-aquifer system (central Spain), an endorheic basin contaminated with NO3− (up to 99.2mg/L in groundwater). This basin was declared vulnerable to NO3− pollution in 1998 due to the high anthropogenic pressures (mainly agriculture and wastewaters). Hydrochemical, multi-isotopic (δ18ONO3, δ15NNO3, δ13CDIC, δ18OH2O, and δ2HH2O) and geophysical techniques (electrical resistivity tomography) were applied to identify the main redox processes at the freshwater-saltwater interface. The results showed that the geometry of this interface is influenced by land use, causing spatial variability of nitrogen biogeochemical processes over the basin. In the underlying aquifer, NO3− showed an average concentration of 38.5mg/L (n=73) and was mainly derived from agricultural inputs. Natural attenuation of NO3− was observed in dryland farming areas (up to 72%) and in irrigation areas (up to 66%). In the Pétrola Lake, mineralization and organic matter degradation in lake sediment play an important role in NO3− reduction. Our findings are a major step forward in understanding freshwater-saltwater interfaces as reactive zones for NO3− attenuation. We further emphasize the importance of including a land use perspective when studying water quality-environmental relationships in hydrogeological systems dominated by density-driven circulation.

Simulations of freshwater lens recharge and salt/freshwater interfaces using the HYDRUS and SWI2 packages for MODFLOW

Abstract The paper presents an evaluation of the combined use of the HYDRUS and SWI2 packages for MODFLOW as a potential tool for modeling recharge in coastal aquifers subject to saltwater intrusion. The HYDRUS package for MODFLOW solves numerically the one-dimensional form of the Richards equation describing water flow in variablysaturated media. The code computes groundwater recharge to or capillary rise from the groundwater table while considering weather, vegetation, and soil hydraulic property data. The SWI2 package represents in a simplified way variable-density flow associated with saltwater intrusion in coastal aquifers. Combining these two packages within the MODFLOW framework provides a more accurate description of vadose zone processes in subsurface systems with shallow aquifers, which strongly depend upon infiltration. The two packages were applied to a two-dimensional problem of recharge of a freshwater lens in a sandy peninsula, which is a typical geomorphologic form along the Baltic and the North Sea coasts, among other places. Results highlighted the sensitivity of calculated recharge rates to the temporal resolution of weather data. Using daily values of precipitation and potential evapotranspiration produced average recharge rates more than 20% larger than those obtained with weekly or monthly averaged weather data, leading to different trends in the evolution of freshwater-saltwater interfaces. Root water uptake significantly influenced both the recharge rate and the position of the freshwater-saltwater interface. The results were less sensitive to changes in soil hydraulic parameters, which in our study were found to affect average yearly recharge rates by up to 13%.

Estuarine and sediment dynamics in a microtidal tropical estuary of high fluvial discharge: Magdalena River (Colombia, South America)

Relation among estuarine processes, mixing mechanisms and turbidity maximum zone (TMZ) formation was evaluated in a microtidal tropical estuary of high fluvial discharge (Magdalena River, northwestern South America). Particularly, assessing the effects of vertical stratification over mixing, estuarine circulation and suspended sediment transport processes. Measurements of salinity, temperature, density, currents and suspended sediment concentration (SSC) were performed at the Magdalena river mouth during high and low streamflows to distinguish among two contrasting seasonal conditions (flow conditions, wind patterns, wave energy). Salinity, temperature, density and location of the freshwater-saltwater interface (FSI) exhibited a sharp seasonal variability. The river mouth and delta front were stratified regardless of the hydrologic season, whereas the riverine section was stratified during the low streamflow season, reflecting the balance between fluvial advection and vertical stratification. Reduction of the vertical mixing led to a more clearly defined and extended stratification. Bottom friction is the main mixing mechanism, but as stratification is strengthened along the river mouth, the vertical mixing weakens, promoting the developing of TMZ (SSC >4500 mg l−1). Our results indicate that TMZ developing is caused by the convergence of stratified layers near the nodal point and the weakening of bottom turbulence by stratification. Although the TMZ exhibited a strong seasonal variation in its magnitude and extent, it usually formed in the convergence front demarcated by saltwater intrusion. Streamflow is the major influence on the TMZ formation since it allows the development of stratified conditions, which weaken turbulent mixing, and thus promotes particulate matter settling. The Magdalena River estuary can be classified as turbid (4000 mg l−1 < SSCmax < 10,000 mg l−1) and extremely turbid during low streamflow seasons (SSCmax > 10,000 mg l−1). These values are of the same order as the SSC reported for the Yangtze and Yellow estuaries, classified as having the largest SSC values of the world.