Management and sediment dynamics of the St. Lucia Estuary mouth,Zululand, South Africa

St. Lucia Estuary is on the subtropical, predominantly microtidal Zululand coast of South Africa. Lake St. Lucia's surface area fluctuates between 420 and 215 km² and has a mean depth of less than 1 m. The 21-km-long narrows connects Lake St. Lucia with the Indian Ocean. Tidal effects penetrate 14 km up the narrows. The St. Lucia system has changed substantially since the 1930s due to bad farming techniques within its catchment. Large amounts of sediment were deposited in the estuary mouth, resulting in relocation of the Mfolozi River mouth to the south at Mapelane. The St. Lucia catchment was subjected to two devastating floods in the last ten years: Cyclone Domoina during February 1984 and the September 1987 cutoff low flood. After floods scoured out the estuary, marine sand advanced up the estuary at a rate of 1200 m/y as a series of flood-tidal deltas. Over 600,000 m³ of sediment accumulated in the St. Lucia Estuary mouth from February 1988 to November 1989. Of this amount, 466,000 m³ of sediment was removed by dredging, although this has not stopped the shoaling. During high rainfall years, the estuary mouth is able to maintain an open outlet to the sea, but as lake levels drop, shoaling causes the mouth to constrict and eventually close. Without the dredging program the mouth would ultimately close during low rainfall years, causing management problems.     

A modelling study of the effects of land management and climatic variations on groundwater inflow to Lake St Lucia, South Africa

Over the past few years groundwater has been recognized as an important contributor of freshwater to Lake St Lucia, South Africa during periods of prolonged drought. This has led to a management strategy aiming at increasing the groundwater recharge and minimizing groundwater use through active manipulation of the vegetation. For the Eastern Shores on the edge of Lake St Lucia, the replacement of vast areas of pine (Pinus elliottii) plantations with grassland over the past decade, combined with a strict burning regime, has led to a general rise of the water table, which has increased the groundwater seepage to Lake St Lucia. A numerical groundwater model has been applied to assess the effects of local management strategies on the mass balance of a shallow aquifer and these are compared to the effects of predicted climate and sea-level change for this area. The simulations indicate that local management actions that are being applied to the Eastern Shores have positive effects on the groundwater flux into Lake St Lucia and that they outweigh potential negative effects of future climate and sea-level change predicted for this area.     

Groundwater discharge to the Changjiang River, China, during the drought season of 2006: effects of the extreme drought and the impoundment of the Three Gorges Dam

Groundwater can play an important role in the compensation of runoff reduction due to extreme climate events such as droughts, as well as in response to anthropogenic actions such as the construction of a dam. The increase in ²²⁶Ra specific activity and the runoff from September to December in 2006 is used to estimate the total discharge of groundwater along the mid-lower reaches of the Changjiang River. The total groundwater discharge was found to account for 31% of the increased discharge between Yichang and Datong. The groundwater discharge to lakes (i.e. Dongting Lake) constituted the major contribution of groundwater discharge to the mid-lower reaches of the Changjiang River. More importantly, the second impounding operation of the Three Gorges Dam from 20 September to 27 October 2006 induced a water level decrease in surrounding lakes and rivers, which led to an additional groundwater discharge of 63.3?×?10⁸?m³, accounting for 85% of the total groundwater discharge in the same period. Taken together, these observations indicate that groundwater discharge along the mid-lower reaches plays an important role in maintaining stream flow in the drought season, especially in extreme drought years or in response to human activities.     

Influence of sea level rise on iron diagenesis in an east Florida subterranean estuary

Subterranean estuary occupies the transition zone between hypoxic fresh groundwater and oxic seawater, and between terrestrial and marine sediment deposits. Consequently, we hypothesize, in a subterranean estuary, biogeochemical reactions of Fe respond to submarine groundwater discharge (SGD) and sea level rise. Porewater and sediment samples were collected across a 30-m wide freshwater discharge zone of the Indian River Lagoon (Florida, USA) subterranean estuary, and at a site 250 m offshore. Porewater Fe concentrations range from 0.5 μM at the shoreline and 250 m offshore to about 286 μM at the freshwater-saltwater boundary. Sediment sulfur and porewater sulfide maxima occur in near-surface OC-rich black sediments of marine origin, and dissolved Fe maxima occur in underlying OC-poor orange sediments of terrestrial origin. Freshwater SGD flow rates decrease offshore from around 1 to 0.1 cm/day, while bioirrigation exchange deepens with distance from about 10 cm at the shoreline to about 40 cm at the freshwater-saltwater boundary. DOC concentrations increase from around 75 μM at the shoreline to as much as 700 μM at the freshwater-saltwater boundary as a result of labile marine carbon inputs from marine SGD. This labile DOC reduces Fe-oxides, which in conjunction with slow discharge of SGD at the boundary, allows dissolved Fe to accumulate. Upward advection of fresh SGD carries dissolved Fe from the Fe-oxide reduction zone to the sulfate reduction zone, where dissolved Fe precipitates as Fe-sulfides. Saturation models of Fe-sulfides indicate some fractions of these Fe-sulfides get dissolved near the sediment-water interface, where bioirrigation exchanges oxic surface water. The estimated dissolved Fe flux is approximately 0.84 μM Fe/day per meter of shoreline to lagoon surface waters. Accelerated sea level rise predictions are thus likely to increase the Fe flux to surface waters and local primary productivity, particularly along coastlines where groundwater discharges through sediments.     

Suspended particulate matter and dynamics of the Mfolozi estuary,Kwazulu-Natal: Implications for environmental management

 The Mfolozi Estuary on the KwaZulu-Natal coast of South Africa is the most turbid estuary in Natal due to poor catchment management, leading to large quantities of suspended particulate matter (SPM) entering the estuary from the Mfolozi River. This paper quantities some of the solute and sediment dynamics in the Mfolozi Estuary where the main documented environmental concern is the periodic input of SPM from the Mfolozi Estuary to the St. Lucia system, causing reduction of light penetration and endangering biological productivity in this important nature reserve. Synoptic water level results have allowed reach mean bed shear stresses and velocities to be calculated for an observed neap tidal cycle. Results indicate that ebb velocities dominate the sediment transport processes in the estuary when fluvial input in the Mfolozi River is of the order of 15–20 m³ s^–1. Observed and predicted flood tide velocities are too low (<0.35 m s^–1) to suspend and transport significant amounts of SPM. Observed results indicate that although the SPM load entering the estuary is dominantly from the Mfolozi River, the Msunduzi River flow plays a major role in the composition of the estuary's salinity and velocity fields. It is calculated that the Mfolozi Estuary would fill with sediment in 1.3 years if it was cut off from the sea. The major fluvial flood events help maintain the estuary by periodically pushing sediment seawards (spit progrades seawards 5 m yr^–1) and scouring and maintaining the main flow channel in the estuary. During low fluvial flow conditions, tidal flow velocities will become the dominant control on sediment transport in the estuary. Interchange of SPM between the St. Lucia and Mfolozi estuaries under present conditions is complicated by the strong transverse velocity shear between the two systems at their combined mouth. This is creating a salinity-maintained axial convergence front that suppresses mixing of solutes and SPM between the systems for up to 10 h of the tidal cycle during observed conditions. Received: 22 May 1995 · Accepted: 31 July 1995     

Analytical models for the groundwater tidal prism and associated benthic water flux

The groundwater tidal prism is defined as the volume of water that inundates a porous medium, forced by one tidal oscillation in surface water. The pressure gradient that generates the prism acts on the subterranean estuary. Analytical models for the groundwater tidal prism and associated benthic flux are presented. The prism and flux are shown to be directly proportional to porosity, tidal amplitude, and the length of the groundwater wave; flux is inversely proportional to tidal period. The duration of discharge flux exceeds the duration of recharge flux over one tidal period; and discharge flux continues for some time following low tide. Models compare favorably with laboratory observations and are applied to a South Atlantic Bight study area, where tide generates an 11-m³ groundwater tidal prism per m of shoreline, and drives 81 m³ s?^?1 to the study area, which describes 23% of an observational estimate. In a marine water body, the discharge component of any oscillatory benthic water flux is submarine groundwater discharge. Benthic flux transports constituents between groundwater and surface water, and is a process by which pollutant loading and saltwater intrusion may occur in coastal areas.     

Mapping Shoreline Variability of Two Barrier Island Segments Along the Florida Coast

Recent projections of global climate change necessitate improved methodologies that quantify shoreline variability. Updated analyses of shoreline movement provide important information that can aid and inform likely intervention policies. This paper uses the Analyzing Moving Boundaries Using R (AMBUR) technique to evaluate shoreline change trends over the time period 1856 to 2015. Special emphasis was placed on recent rates of change, during the 1994 to 2015 period of active storm conditions. Small segments, on the order of tens of kilometers, along two sandy barrier island regions on Florida’s Gulf and Atlantic coasts were chosen for this study. The overall average rate of change over the 159-year period along Little St. George Island was ??0.62?±?0.12 m/year, with approximately 65% of shoreline segments eroding and 35% advancing. During periods of storm clustering (1994–2015), retreat rates along portions of this Gulf coast barrier accelerated to ??5.49?±?1.4 m/year. Along the northern portion of Merritt Island on Florida’s Atlantic coast, the overall mean rate of change was 0.22?±?0.08 m/year, indicative of a shoreline in a state of relative dynamic equilibrium. In direct contrast with the Gulf coast shoreline segment, the majority of transects (65%) evaluated along the oceanfront of Merritt Island over the long term displayed a seaward advance. Results indicate that episodes of clustered storm activity with fairly quick return intervals generally produce dramatic morphological alteration of the coast and can delay natural beach recovery. Additionally, the data show that tidal inlet dynamics, shoreline orientation, along with engineering projects, act over a variety of spatial and temporal scales to influence shoreline evolution. Further, the trends of shoreline movement observed in this study indicate that nearshore bathymetry—the presence of shoals—wields some influence on the behavior of local segments of the shoreline.     

Estimation of Groundwater and Nutrient Fluxes to the Neuse River Estuary, North Carolina

A study was conducted between April 2004 and September 2005 to estimate groundwater and nutrient discharge to the Neuse River estuary in North Carolina. The largest groundwater fluxes were observed to occur generally within 20 m of the shoreline. Groundwater flux estimates based on seepage meter measurements ranged from 2.86?×?10⁸ to 4.33?×?10⁸ m³ annually and are comparable to estimates made using radon, a simple water-budget method, and estimates derived by using Darcy’s Law and previously published general aquifer characteristics of the area. The lower groundwater flux estimate (equal to about 9 m³ s^?1), which assumed the narrowest groundwater discharge zone (20 m) of three zone widths selected for an area west of New Bern, North Carolina, most closely agrees with groundwater flux estimates made using radon (3–9 m³ s^?1) and Darcy’s Law (about 9 m³ s^?1). A groundwater flux of 9 m³ s^?1 is about 40% of the surface-water flow to the Neuse River estuary between Streets Ferry and the mouth of the estuary and about 7% of the surface-water inflow from areas upstream. Estimates of annual nitrogen (333 tonnes) and phosphorus (66 tonnes) fluxes from groundwater to the estuary, based on this analysis, are less than 6% of the nitrogen and phosphorus inputs derived from all sources (excluding oceanic inputs), and approximately 8% of the nitrogen and 17% of the phosphorus annual inputs from surface-water inflow to the Neuse River estuary assuming a mean annual precipitation of 1.27 m. We provide quantitative evidence, derived from three methods, that the contribution of water and nutrients from groundwater discharge to the Neuse River estuary is relatively minor, particularly compared with upstream sources of water and nutrients and with bottom sediment sources of nutrients. Locally high groundwater discharges do occur, however, and could help explain the occurrence of localized phytoplankton blooms, submerged aquatic vegetation, or fish kills.     

On the advection of turdidity in the saint lawrence middle estuary

The suspension transport away from the extensive turbidity zone of the St. Lawrence estuary is largely determined by the channel topography. The suspended sediments are advected downstream by a 40 km long turbid plume which takes its source in a turbidity maximum at the head of the estuary and flows downstream partly confined by the South Channel. During the ebbing phase of tidal cycles, the turbid waters of the plume are forced downstream through narrow converging sections of the channel, and slowed down through more opened diverging regions, particularly down-stream of the St. Roch Traverse. These, large fluctuations in stratification modulate the vertical transport of suspended material from the bottom to the surface layer. Midway down the estuary, dispersion of the plume occurs along a frontal zone which seasonally migrates 30 km or more in response to changes in fresh water discharge. The plume is reinforced and the turbidity gradient is intensified by local injections of inshore waters from Ste. Anne Bay, a subtidal platform highly enriched in suspended material by intertidal exchanges with large mudllats. Lateral erosion of the plume and cross-channel transport of suspended matter from the South into the North Channel is made possible by large horizontal shears developing in the central part of the middle estuary during the early flood. These are created by a one-hour tidal phase difference between the North and the South Channel.     

Variations of Hydrodynamics and Submarine Groundwater Discharge in the Yellow River Estuary Under the Influence of the Water-Sediment Regulation Scheme

The “Water-Sediment Regulation Scheme” (WSRS) is critically important to the hydrologic evaluation of the Yellow River estuary since a huge pulse of water and sediment are delivered into the sea during a short period. We used the natural geochemical tracers radium (²²³Ra, ²²⁴Ra, ²²⁶Ra) and radon (²²²Rn) isotopes as well as other hydrological parameters to investigate the mixing variations and submarine groundwater discharge (SGD) in the Yellow River estuary under the influence of the 2013 WSRS. Dramatically elevated radium and radon isotopic activities were observed during this WSRS compared with activities measured during a non-WSRS period. Radium “water ages” indicated that the offshore transport rate nearly tripled when the river discharge increased from 400 to 3400 m³/s. We calculated the SGD flux in the Yellow River estuary based on a radium mass balance model as well as radium and radon time-series models. The SGD flux was estimated at 0.02~0.20 m/day during a non-WSRS period and 0.67~1.22 m/day during the 2013 WSRS period. The results also indicate that large river discharge tends to lead more intense SGD along the river channel direction with a large amount of fresh SGD.     

Seepage of groundwater nitrate from a riparian agroecosystem into the Wye River estuary

Intensive research in Chesapeake Bay has indicated that reductions in nitrogen inputs to the bay will be necessary to restore water quality to levels needed for resurgence of bay living resources. Fall-line water quality monitoring efferts have characterized diffuse-source nitrogen inputs from a large percentage of the bay drainage basin, but relatively little information exists regarding rates of nitrogen delivery to tidal waters from coastal plain regions. Extensive nitrate contamination of shallow groundwater due to agricultural activities, coupled with the dominant role of subsurface flow in discharge from Coastal Plain regions of the drainage basin, creates the potential for high rates of nitrogen delivery to tidal waters via groundwater seepage. This study utilized intensive hydrologic and water chemistry monitoring from April 1992 through September 1994 to determine the spatial characteristics of the groundwater-estuarine interface, as well as the rates of subsurface nitrogen transport from an agricultural field into nearshore waters of the Wye River, a subestuary of Chesapeake Bay. The hydrogeologic characteristics of the study site resulted in groundwater discharge to the Wye River occurring almost exclusively within 15 m of the shoreline. Calculated groundwater discharge rates were found to vary widely in the short term due to tidal fluctuations but in the long term were driven by seasonal changes in groundwater recharge rates. The zone of groundwater discharge contracted shoreward during summer months of low discharge, and expanded to a maximum width of approximately 15 m during high discharge periods in late winter. Average discharge rates were more than five times higher in winter versus summer months. Groundwater nitrate concentrations entering the discharge zone were relatively stable throughout the study period, with little evidence of denitrification or nitrate uptake by riparian vegetation. Consequently, nitrogen discharge patterns reflected the strong seasonality in groundwater discharge. Annual nitrate-N discharge was approximately 1.2 kg m^?1 of shoreline, indicating drainage basin rates of nitrogen delivery to tidal waters of approximately 60 kg ha^?1.     

Sediment salt-load in the St Lucia Estuary during the severe drought of 2002–2006

The purpose of this study was to determine whether there was a sufficiently high residual salt load in the dry sediments of the St Lucia Estuary to cause salinity problems should it later fill up with either freshwater or seawater. The estuary lakes have suffered the effects of a severe drought since 2002 with the result that many areas were dry, and the salinity of the residual water varied between 4 psu and up to five times that of seawater. Measurements of the salts content in the sediments to a depth of 20 cm showed that more than 2 million tonnes of salt was held in this layer of the sediment in 2006. Recent management of the estuary (since 1970) has ensured that the mouth was not artificially opened. This was to prevent the inflow of seawater, with its salts, that would otherwise enter while the drought was in place. The results of the sediment salinity data showed that if the drought had been broken and the lake area filled with rain and river water, the resulting salinity would be about 6 psu. In March 2007, Cyclone Gamede in the Indian Ocean off the east coast of South Africa produced a wave climate at sea that resulted in the mouth breaching; thus introducing an estimated 12 million tonnes of salts. The high salinity in the system resulting from this breach is expected to have an adverse effect on the ecology of the system, whereas the residual salinity in the sediments would not have caused an environmental problem. If the estuary and lake system were to fill completely with seawater, the residual salts together with seawater will raise the salinity to an initial value higher than 40 psu, which will have the effect of suppressing much of the important submerged vegetation that is vital for sustaining juvenile fish in the system. Many of the large fauna will also suffer from a shortage of freshwater.     

Distribution and abundance of ichthyoplankton in the Manicouagan River estuary,a tributary of the lower St. Lawrence estuary

The species composition and relative abundance of ichthyoplankton were investigated during summer 1986 at four stations along the salinity gradient in the Manicouagan River estuary, a tributary of the lower St. Lawrence estuary. Physical characteristics of water masses indicated the presence of a strong saline front (>10‰ per km) delineating the freshwater and marine section of the Manicouagan estuary. The estuary supports a depauperate ichthyoplankton community, including four species of pelagic fish eggs and eight species of fish larvae. Species richness increased with salinity. The ichthyoplankton fauna can be divided into two distinct groups: freshwater and marine. These two groups result initially from spawning preferences exhibited by the different species abundance of freshwater larvae was maximal at the head of the estuary and marine larvae were most abundant at the most saline station. The length frequency distribution suggests that marine larvae are not effectively retained within the estuary. The Manicouagan estuary cannot be considered as a major spawning site nor an important nursery zone for any fish found in this area.     

Groundwater hydrochemistry and origin in the south-eastern part of Wadi El Natrun,Egypt

The demand for water is rapidly increasing in Egypt, because of high population and agriculture production growth rate, which makes research of water resources necessary. The regional multi-aquifer system of the Miocene–Pleistocene age is discharged in Wadi El Natrun area. Intensive aquifer overexploitation and agricultural development in the area are related to groundwater quality deterioration. Hydrochemical and hydrogeological data was evaluated to determine the groundwater origin and quality in the south-eastern part of wadi, which appears to be more significant for water supply owing to lower groundwater salinity. The dominance of the high mineralised Cl groundwater type was found; however, also less mineralised SO₄ and HCO₃ types were identified there. Based on the ion relations, halite and gypsum dissolution and ion exchange are the most important hydrochemical processes forming the groundwater chemical composition. The Cl dominated groundwater matches the discharge part of the regional hydrogeological system. Contrary, the presence of HCO₃ and SO₄ hydrochemical types corresponds to the infiltration and transferring parts of the hydrogeological system indicating the presence of zones conducting low mineralised groundwater. The discharge area of the over-pumped aquifer in Wadi El-Natrun lies 23 m beneath the sea level with the shoreline being at the distance of 100 km, thus there is a real risk of seawater intrusion. Using the hydrochemical facies evolution diagram, four samples in the centre of the discharge area indicate advanced seawater intrusion. The zones of the highest demand for groundwater quality protection were indicated based on a spatial pattern of hydrogeochemical composition.     

Groundwater,Acid and Carbon Dioxide Dynamics Along a Coastal Wetland,Lake and Estuary Continuum

Coastal wetlands are hotspots for biodiversity and biological productivity, yet the hydrology and carbon cycling within these systems remains poorly understood due to their complex nature. By using a novel spatiotemporal approach, this study quantified groundwater discharge and the related inputs of acidity and CO₂ along a continuum of a modified coastal acid sulphate soil (CASS) wetland, a coastal lake and an estuary under highly contrasting hydrological conditions. To increase the resolution of spatiotemporal data and advance upon previous methodologies, we relied on automated observations from four simultaneous time-series stations to develop multiple radon mass balance models to estimate groundwater discharge and related groundwater inputs of acidity and dissolved inorganic carbon (DIC), along with surface water to atmosphere CO₂ fluxes. Spatial surveys indicated distinct acid hotspots with minimum surface water pH of 2.91 (dry conditions) and 2.67 (flood conditions) near a non-remediated (drained) CASS area. Under flood conditions, groundwater discharge accounted for ～14.5 % of surface water entering the lake. During the same period, acid discharge from the acid sulphate soil section of the continuum produced ～4.8 kg H₂SO₄?ha^?1 day^?1, a rate much higher than previous studies in similar systems. During baseflow conditions, the low pH water was rapidly buffered within the estuarine lake, with the pH increasing from 4.22 to 6.07 over a distance of ～250 m. The CO₂ evasion rates within the CASS were extremely high, averaging 2163?±?125 mmol m^?2 day^?1 in the dry period and 4061?±?259 mmol m^?2 day^?1 under flood conditions. Groundwater input of DIC could only account for 0.4 % of this evasion in the dry conditions and ～5 % during the flood conditions. We demonstrated that by utilising a spatiotemporal (multiple time-series stations) approach, the study was able to isolate distinct zones of differing hydrology and biogeochemistry, whilst providing more reasonable groundwater acid input estimates and air–water CO₂ flux estimates than some traditional sampling designs. This study highlights the notion that modified CASS wetlands can release large amounts of CO₂ to the atmosphere because of high groundwater acid inputs and extremely low surface water pH.     

Geologic controls of submarine groundwater discharge: application of remote sensing to north Lebanon

Typical of Mediterranean countries, the Lebanese shoreline is well known for its littoral and offshore groundwater discharges, the so-called submarine springs. The tectonic framework of the terrain explains its interruption by dense geologic structures, i.e., fracturing, faulting, karstic routes, as well as acute dips of rock strata seaward. All of these structures serve as hydrologic agents transporting groundwater to the sea. The study aims to locate these groundwater discharges, and to interpret their geologic controls on land. For this purpose, two major lines of approach were followed. The first is an airborne thermal infrared (TIR) survey using radiometers to identify thermal anomalies, thereby determining the exact location of submarine groundwater discharges (SGDs). The second line of approach is the analysis of satellite images (Landsat 7ETM⁺) to delineate the geologic features that govern the mechanism of water flow, thereby determining their sources on land. Twenty-seven major SGDs were identified, 10 of these being offshore springs, the others littoral springs. The springs show a large variety of discharge configurations and extents, mainly parallel or perpendicular to the shoreline or rounded. Three major structural controls were identified to contribute to the transport of groundwater to the sea. These are karstic galleries, faults, and tilted rock strata, their contributions being 48, 37, and 15%, respectively. The SGDs associated with linear passageways, i.e., karstic galleries and faults, are connected with land aquifer formations several kilometers away from the shoreline. Moreover, the presence of impervious rock formations at many localities along the coastline results in a confined hydrologic system, promoting the flow of SGDs into the sea.     

A modeling study of the Satilla River estuary,Georgia. II: suspended sediment

A three-dimensional (3-D) suspended sediment model was coupled with a 3-D hydrodynamic numerical model and used to examine the spatial and temporal distribution of suspended sediments in the Satilla River estuary of Georgia. The hydrodynamic model was a modified ECOM-si model with inclusion of the flooding-drying cycle over intertidal salt marshes. The suspended sediment model consisted of a simple passive tracer equation with inclusion of sinking, resuspension, and sedimentation processes. The coupled model was driven by tidal forcing at the open boundary over the inner shelf of the South Atlantic Bight and real-time river discharge at the upstream end of the estuary, with a uniform initial distribution of total suspended sediment (TSS). The initial conditions for salinity were specified using observations taken along the estuary. The coupled model provided a reasonable simulation of both the spatial and temporal distributions of observed TSS concentration. Model-predicted TSS concentrations varied over a tidal cycle; they were highest at maximum flood and ebb tidal phases and lowest at slack tides. Model-guided process studies suggest that the spatial distribution of TSS concentration in the Satilla River estuary is controlled by a complex nonlinear physical process associated with the convergence and divergence of residual flow, a non-uniform along-estuary distribution of bottom stress, and the inertial effects of a curved shoreline.     

用氢氧稳定同位素评价闽江河口区地下水输入

通过分析闽江河口区降水、地表水和地下水的氢氧稳定同位素特征,揭示降水的环境同位素效应和地下水的形成演化规律,定量评价河口区多种水体的混合过程及地下水输入量。夏季的降水氢氧同位素组成相对贫化,呈现出降雨量效应。在δ18O与δD关系图上,闽江北岸基岩裂隙水、平原及丘陵区浅层地下水均落在福州降水线上,而南岸平原及丘陵区浅层地下水大部分落在福州降水线右下方,其拟合线与降水线交点与5~9月农灌期降水氢氧同位素加权值接近,表明北岸地下水主要来自降水补给,而南岸地下水同时接受灌溉水和降水补给,并在入渗过程中经历了不同程度的蒸发作用。闽江河口段除接受两岸地下水补给外,局部河段还接受断裂带裂隙水补给。将线性端元混合模型、数字高程模型和地下水文分析法结合起来定量评价地下水的输入和各水体的混合过程,结果显示,在河口段淡水区,地下水混合比率上限为8.8%,其中包括0.4%的断裂带裂隙水;在河口段淡咸水混合区,淡水(河水、地下水)和海水的混合比为53:47,其中地下水的保守混合比率为1.7%;枯水期闽江河口段地下水保守输入量为87.0 m3/s,是闽江径流量的12.8%。     

Impact of Farakka barrage on the hydrology and fishery of Hoogly estuary

Construction of the Farakka barrage on the Ganga River in April 1975 to augment water supply to the Calcutta port has brought about a significant increase in freshwater discharge in its distributary, the Hoogly estuary. This has naturally resulted in major changes in the ecology of this estuary, causing modifications in the structure of its fishery resources, fishing pattern, and fish production. This paper presents observations on salinity, plankton, bottom biota, fishery resource, and fish production of different zones of the Hooghly estuary during the period 1982–1992. Comparison with similar studies made before and immediately after commissioning of the Farakka barrage (1975–1977) has revealed that the increased freshwater discharge has resulted in considerable decrease in salinity throughout the estuary. The freshwater zone now extends toward the mouth of the estuary. The true estuarine zone has moved seaward and the marine zone has been restricted to the area near the mouth of the estuary. This has effected major changes in plankton dynamics, sharp decline in the fishery of marine and neritic species in the upper estuary, caused a significant increase in catch ofTenualosa ilisha and an over twofold increase in the average annual fish landings from the estuary as a whole. New zonations have been proposed based on the presently, observed salinity values, which are the most significant factor in determining the fishery of any estuary. An interdisciplinary study of the ecology of the new zones is needed to establish their correct biological characteristics.     

Impact of climate change on groundwater recharge in a small catchment in the Black Forest, Germany

Temporal and spatial changes of the hydrological cycle are the consequences of climate variations. In addition to changes in surface runoff with possible floods and droughts, climate variations may affect groundwater through alteration of groundwater recharge with consequences for future water management. This study investigates the impact of climate change, according to the Special Report on Emission Scenarios (SRES) A1B, A2 and B1, on groundwater recharge in the catchment area of a fissured aquifer in the Black Forest, Germany, which has sparse groundwater data. The study uses a water-balance model considering a conceptual approach for groundwater-surface water exchange. River discharge data are used for model calibration and validation. The results show temporal and spatial changes in groundwater recharge. Groundwater recharge is progressively reduced for summer during the twenty-first century. The annual sum of groundwater recharge is affected negatively for scenarios A1B and A2. On average, groundwater recharge during the twenty-first century is reduced mainly for the lower parts of the valley and increased for the upper parts of the valley and the crests. The reduced storage of water as snow during winter due to projected higher air temperatures causes an important relative increase in rainfall and, therefore, higher groundwater recharge and river discharge.