Saltwater Zone Research Articles

Variable-density solute transport in unconfined coastal aquifers with a subsurface dam

Recently, the influence of subsurface dams on solute transport in coastal aquifers has become a hot research topic. Although many studies have been conducted, the combined effects of subsurface dam and tides on variable-density solute transport remain unclear, and this study aimed to fill this research gap. In the current study, a numerical model representing a 2-D cross-shore coastal aquifer was established. The model simulated cases with and without a subsurface dam, and sensitivity analysis cases with different height and location of the subsurface dam and solute concentration. The results show that a subsurface dam blocks a portion of the solute plume, which can only be discharged by dilution at the edges, thereby altering its discharge pattern and reducing their discharge rate. The addition of a subsurface dam may either prolong or shorten the residence time of solute,depending on the location rather than the height of the subsurface dam. In particular, a more landward subsurface dam would significantly increase the residence time; sensitivity analysis demonstrates that both the landward shift and the height increase of the subsurface dam contribute to a heightened ratio of dynamic mass distribution for the solute plume within the freshwater and saltwater zones of the aquifer, with maximum changes in mass distribution ratios of 87.22% and 300%, respectively. Also, these factors cause the solute to migrate both seaward and landward, respectively, across the primary outflow regions of the aquifer-ocean interface. Results from this study may provide theoretical guidance for the optimal design and environmental impact assessment of subsurface dams.

Geophysical and hydrochemical analysis of saltwater intrusion in El-Omayed, Egypt: Implications for sustainable groundwater management

This study investigates groundwater resources in the El-Omayed area on the northwestern coast of Egypt. The aim is to assess the extent and severity of saltwater intrusion in the shallow aquifer system, as well as its potential impacts on groundwater resources. A combined geophysical and hydrochemical approach was adopted in this investigation. Across the study area, 32 vertical electrical soundings were conducted, along with two electrical resistivity tomography profiles. Sixteen water samples were collected, and then chemically analyzed to evaluate water quality, seawater intrusion, and source of salinity. The results of geophysical interpretation revealed the existence of a thin, shallow freshwater-bearing layer (42–95 Ω.m), predominantly situated in the central parts of the area, followed by saltwater saturated layers of (0.9–2.1 Ω.m) affected by the Mediterranean Sea invasion. The eastern portion of the aquifer also recorded a relatively high seawater intrusion. As evidenced by the actual data gathered from drilled wells, the study area can be divided into three distinct zones: fresh, brackish, and saltwater zones. The study identified the most suitable sites to establish new groundwater wells, in addition to identifying the source of salinity through the investigation of the geochemical properties of water samples.

Vertical distribution of groundwater age in and around the freshwater–saltwater transition zone determined using noble gases and carbon isotopes

The distribution of groundwater ages in coastal aquifers are complicated because of the heterogeneity of the hydrogeology and the mixing of terrestrial and marine water sources. Therefore, multiple studies verifying the groundwater age distribution via borehole surveys are required to better understand groundwater flow in coastal aquifers. This study examined groundwater age distributions in coastal aquifers adjacent to a high-elevation volcano (Mt. Fuji, Japan) by analyzing geochemical and isotopic tracers retrieved from drilling 350- and 160-m-deep boreholes. Brackish groundwater samples collected from the boreholes had high helium isotope ratios, indicating a large contribution of mantle-derived helium. Groundwater ages determined from carbon and helium isotopes indicate that terrestrial groundwater recharged at high elevations flows from the volcano to the coast, mixing with seawater in a transition zone. The groundwater age at the freshwater–saltwater transition zone and the upper part of the saltwater zone is older than that of the underlying saline water. The distributions of the groundwater age in the study area confirm that the conventional groundwater flow system in coastal aquifers is applicable even in deep aquifers. The complex patterns of groundwater age and mixing derived from geochemical and isotopic tracers can guide sustainable use of groundwater resources and the subsurface.

Assessing Nutrient Limitation in Yeongsan River Estuary Using Bioassay Experiments

The Yeongsan River estuary is experiencing increased concentrations of nutrient and organic matter due to its estuary bank (sea dike). The opening of floodgates at the estuary bank leads to a substantial inflow of freshwater into the saltwater zone, thereby resulting in water quality changes. Our study evaluated spatiotemporal variations in nutrient limitation in the freshwater and saltwater zones in the Yeongsan River estuary, which is expected to fluctuate with the changing seasons. We utilized the N:P ratio to evaluate the potential nutrient limitation and conducted bioassay experiments to directly assess actual nutrient limitation. The N:P ratio showed that P was the limiting nutrient in both the long-term (2004–2008) and during our field investigation. However, the bioassay experiment revealed that in the freshwater zone, P was limited in spring and winter (p < 0.05), while no nutrient was limited in summer and fall. In the saltwater zone, we observed P limitation in spring and winter (p < 0.05) and N limitation in fall (p < 0.05), whereas no nutrient limitation was observed in the summer. These results demonstrate that actual nutrient limitation, which directly influences phytoplankton growth, varies spatiotemporally in response to freshwater discharge in the Yeongsan River estuary.

Studies on the dynamic boundary of the fresh-salt water interaction zone of estuary wetland in the Yellow River Delta

Under the interaction between freshwater and seawater, estuary wetland has obvious periodic flooding characteristics and salt gradient change, thus showing diversified ecological service functions. Based on the measured data of salinity in the field, monitoring data of river hydrology, tidal process and the elevation data of estuary geomorphology, the spatial pattern of salt distribution and the dynamic boundary and its area range of the fresh-salt water interaction zone (FSIZ) of the estuary wetland in the Yellow River Delta were studied. The results showed that the salinity of the estuary wetlands in the Yellow River Delta ranged from 0.40 g L−1 to 26.67 g L−1. The bandwidth variation range of the FSIZ in the river was 0.53–1.73 km, and boundary range in saltwater side of the FSIZ outside the river was 0.28–4.27 km. The area of saltwater zone in the Yellow River Delta estuary wetland accounted for >50% of the total area of the study area. The onshore area of estuary wetland was dominated by the FSIZ (38.06%–74.07%), while the area of tidal-freshwater zone only accounted for 3.87%–6.51% of the total onshore area. The dynamic boundary range of the FSIZ in the study area was approximately 140.80 km2 (about 36.01% of the onshore area), of which >92% was located on the saltwater side of the FSIZ. The boundary of saltwater zone, tidal-freshwater zone and FSIZ of the Yellow River Delta estuary wetland were scientifically divided and the dynamic boundary range of the FSIZ was clarified for the first time in the study. Our results can provide a scientific basis for well-understanding the ecological functions of estuary wetlands and the wetland classification restoration and management.

Isotopic compositions (δD, δ18O) and end-member mixing for the control interface in a complex tidal region

Identifying the mixing processes of waters and currents in tidal reach is an important aspect of environmental management to protect freshwater resources and prevent water pollution. In this study, three field investigations conducted in a typical tidal reach in August, November and the following April focused on two isotopes (δD and δ18O) and salinity. A salinity-isotope conservative mixing model was established to differentiate water flows of the important control interface (CI) from freshwater, transition zone and saltwater end-members. Results suggested that the average δD and δ18O values during the ebb and flood tides depleted from August to November, then enriched significantly in the following April and were even higher than those in August. The δD and δ18O values in the saltwater zone enriched markedly compared with those in freshwater zone and transition zone due to the stronger evaporation occurring in the saltwater zone. Based on the revised model, the average contributions of freshwater end-member, transition zone end-member and saltwater end-member in three months were, respectively, 51.50 %, 36.93 % and 11.57 %. However, the contributions of freshwater and transition zones in April end-member were equivalent (47.45 % vs 44.31 %). Meanwhile the largest contribution of saltwater end-member was 20.56 % and occurred in August. The proportions of three end-members that contributed to CI changed with different evaporation scenarios and moisture sources of precipitation. Our research provides important information that furthers our understanding of the isotopes and their applications to environmental management in estuarine regions.

Potential land for agriculture development in three coastal districts of Ben Tre province in the Mekong delta, Vietnam

The research objective was to determine the suitability of land regarding physical and economic conditions as a basis to support managers in planning effective agricultural development strategies that are suitable and sustainable for land potential in a future affected by climate change. In this research, physical and socio-economic data were collected by field surveys, household interviews (330 households), and agricultural manager discussions. Additionally, themethodology of land suitability evaluation was based on Food and Agriculture Organization (FAO)(1976 & 2007) and Geographic Information System (GIS) tools, which were used to determine suitability classifications for land-use types (under different physical and socio-economic conditions) in three coastal districts within the Ben Tre province. The results showed that agricultural production status in the research area was divided into three agro-ecological zones including the freshwater zone, brackish water zone, and saltwater zone of which the highest rate of agricultural land area was aquaculture. The next major proportion of agricultural area belonged to land for planting perennial crops and land for planting annual crops. Besides, 7 and 8 land suitability zones were determined, respectively, depending on physical and economic conditions, for 11 main land-use types. This research contributes toward basic knowledge that aims to support local decision makers in their transformation of agricultural land and adaptation of salinization in the future.

Coupling effects of tide and salting-out on perfluorooctane sulfonate (PFOS) transport and adsorption in a coastal aquifer

The adsorption of perfluorooctane sulfonate (PFOS) on sediments is thought to be enhanced by the ‘salting-out effect’ (SOE), which reduces contaminant solubility as water salinity increases. The process of salting-out has been observed for PFOS in surface estuaries and likely operates in ‘subterranean estuaries’, where tide-induced groundwater circulation creates a dynamic saltwater-freshwater mixing zone in the aquifer adjacent to the ocean. The presence of this dynamic interface between dense seawater and fresh groundwater modifies the flow path of land-derived organic contaminants. However, it is unclear how SOE combined with tidal forcing of salt water, affects the transport and fate of these contaminants in a subterranean estuary. Numerical studies of groundwater dynamics in a coastal aquifer are carried out using a modified version of SUTRA to investigate the behavior of PFOS, as it moves through an aquifer-ocean interface. Three adsorption scenarios are investigated: no adsorption and linear adsorption with a constant coefficient (Kd) and a salinity-dependent Kd (due to SOE). Without adsorption, point-injected PFOS moves and spreads as a plume around the margin of the tide-induced upper saline plume (USP). This is in agreement with previous studies showing that a contaminant plume follows the freshwater discharge path and exits the aquifer around the USP in the intertidal zone. The increase of tidal amplitude enlarges the USP, forcing the discharge points of freshwater and PFOS seaward. In the case of a constant Kd, PFOS concentration decreases in the groundwater as expected with an increase of PFOS residence time in the aquifer. The spatial distribution of the contaminant plume is similar to that of the ‘no adsorption’ case. In the case with the SOE, which enhances the solute adsorption in the saltwater zone, PFOS tends to be intercepted by the USP, concentrated at its center, and subsequently discharged with the tide-induced saltwater circulation. SOE reduces both peak PFOS discharge rate and plume dispersion. In addition, the PFOS flow path moves upward and the PFOS discharges above the USP. The findings highlight the potential importance of investigating the SOE of organic contaminants for benthic organisms when considering their transport and fate in tidally influenced coastal aquifers.

Ultra-High Temperature Drilling Fluid Technology for Drilling Well Jiantan-1

The well Jiantan-1 is a key exploratory well of PetroChina located in the Jianshan structure of the central rhinoplasty belt in the Chaidamu Basin. The main purpose of this 6343 m well is to explore the gas reserves in the base rocks of the Jianshan structure. The highest bottom hole temperature measured is 235℃, one of the highest bottom hole temperatures encountered in onshore drilling. The formations drilled in this area have fractures that are fully developed, long sections of gypsum and mudstones, high pressure saltwater zones, high CO2 concentrations and high geothermal gradient. Borehole wall collapse, mud losses, overflow, creeping and tight hole in the gypsum/mudstone formations are downhole troubles encountered and sometime intertwined with each other during drilling. To deal with these problems, laboratory study was conducted on several aspects of the drilling fluid used, which are high temperature stability, rheology and high temperature filtration control. An organic salt drilling fluid was formulated with additives specially selected, such as ultra-high-temperature filter loss reducers, anti-collapse additives with plugging capacity, lubricants and thermal stabilizers etc. This drilling fluid is able to resist high temperatures up to 240℃. After aging for 72 hours, the HTHP filtration rate was maintained with 10 mL, the coefficient of friction was less than 0.1, and the invasion depth on a sand-bed less than 12 cm. In addition, the fourth and fifth intervals of the well penetrated formations with micro-fissures and a basal rock weathered crust, and mud losses occurred many times into the fissures. A lost circulation material slurry for the well Jiantan-1 was then developed with choice rigid, flexible and gel-like plugging additives which are suitable for use at elevated temperatures. Using this mud loss control slurry, the pressure bearing capacity of the formations into which mud losses have occurred was finally enhanced. The organic salt drilling fluid and the mud loss control slurry played a major role in preventing and controlling borehole wall collapse and mud losses, ensuring the success of the drilling of the first ultra-high temperature Jiantan-1 well in Chaidamu Basin.

Time-lapse electrical resistivity investigation of seawater intrusion in coastal aquifer of Ibeno, Southeastern Nigeria

Saltwater intrusion, erosion, and periodic flooding during severe storms are some of the critical problems bedeviling the precarious coastal environment of Nigeria. A time-lapse study aimed at monitoring the dynamics of saltwater intrusion within the coastland surrounding Ibeno, southeastern Nigeria is presented. Three (3) vertical electrical soundings (VES) and four time-lapse electrical resistivity tomography (ERT) profiles with minimum and maximum electrode spacing of 5 m and 40 m and profile lengths of 120 m and 150 m respectively were acquired in 2016 and 2017 using the Wenner and Schlumberger arrays. Data acquired from the field surveys were modeled with WINRESIST and RES2DINV software to generate 1D and 2D resistivity images of the subsurface. The VES results with a maximum survey length of AB = 300 m revealed a four-layer KH curve. The 2D geo-electrical sections delineated contrast between the high resistivity values obtained for the saturated freshwater zones and the low resistivity values obtained for the saturated saltwater zone. Expectedly, the intrusion was dominant nearer the coastline than inland. The salinity changes are believed to have been caused by both incremental tides passing through highly porous materials in the active-salinity change area and over-pumping of groundwater. The time-lapse resistivity measurements showed significant, active lateral salinity changes rapidly intruding the freshwater aquifers. The study demonstrates the application of time-lapse 2D ERT and vertical electrical sounding in identifying the spatial and temporal changes of saltwater intrusion in the coastal aquifer.

Laboratory modeling for contaminant migration monitoring using electrical resistivity method

The rate at which contaminants migrate in the soil, such as hydrocarbons and saltwater, has long been studied. The contaminants might be as a result of spilled oil from the failed pipeline, sabotaged pipeline in Niger Delta, and saltwater intrusion. A laboratory modeling 2D electrical resistivity experiment was carried out to model the migration pattern of soil contaminants in a sand formation. The electrical resistivity methods are often being used to detect the contaminants’ plume in the subsurface using mini electrodes electrical resistivity imaging. This geophysical method was adopted before (baseline) and after the injection of the contaminant. The model tank was filled with sand, and the contaminants [petroleum and sodium chloride (NaCl)] were injected in four points into the model tank to a depth of 5 cm. A total of three (3) traverses for each segment were established within the tank. The dipole–dipole array was used for the investigation with electrode spacing (a) of 5 cm (0.05 m) and expansion factor (n) varying from 1 to 5. The traverse interval was 11.1 cm apart. A constant station separation of 5 cm (0.05 m) was adopted. The injected contaminants are organic (hydrocarbon) and inorganic (saltwater intrusion), and they are injected at station 9 and station 30 in between traverse 1 and 2 and traverse 2 and 3. Subsequently, the injection of the contaminant into the model tank was undertaken at 24 h. Experimental results obtained from this study show that 24 h after the injection of the organic substance, high resistivity value ranging from 350 to 800 Ωm buildup in some stations, which is directly related to organic (hydrocarbon) accumulation and low resistivity value, is because the porosity in these regions has been affected. 24 h after the injection of the NaCl, the three (3) traverses depict very low resistivity value ranging from 1.2 to 45 Ωm which is related to the presence of saltwater but in traverse 3 station 23 to 25 and traverse 2 station 17 to 18 depict high resistivity value ranging between 105 and 215 Ωm which indicates that the porosity in this region has been affected by the injected fluid. In general, the injected contaminant affected the porosity of the host material, which leads to the high resistivity value in the saltwater zone and low resistivity in the hydrocarbon zone.

A Borehole-Based Approach for Seawater Intrusion in Heterogeneous Coastal Aquifers, Eastern Part of Jeju Island, Korea

Understanding the basaltic aquifer system and seawater intrusion on the volcanic island of Jeju, Korea, has received significant attention over the years, and various methodologies have been suggested in the contributions. Nevertheless, it is still difficult to effectively characterize groundwater systems due to the long period of volcanic activity and the lithological variability of basalt. In this study, geophysical well logging in seawater intrusion monitoring boreholes detected a sudden decrease of electrical conductivity within the saltwater zone in the eastern part of Jeju Island. This anomalous condition cannot be explained by the Ghyben-Herzberg model, which has historically been considered as the basic groundwater model of Jeju Island. This paper focuses on fine-scale temporal and spatial variability of groundwater flow using electrical conductivity and temperature logs and borehole temperature monitoring by a thermal line sensor. On the basis of the results, we evaluate an alternative model to replace the traditional conceptual model in the eastern part of Jeju Island. It is revealed that the area consists of heterogeneous aquifer systems, and the behavior of freshwater and saltwater is understood by temperature monitoring over the entire depth of boreholes. Coastal aquifers flow through two or more independent channels with weak vertical connections. In addition, seawater intrusion does not occur continuously in the vertical direction from the bottom depth, but instead occurs through these multilayered aquifers. In particular, the multilayered aquifers that are responsible for flow pathway, as well as the freshwater–saltwater interface form mainly at lithological boundaries. Our preliminary conceptual model is expected to be improved and revised by various measurements of hydrodynamic parameters such as flowmeter or packer test.

Mapping Shallow Aquifers Intruded by Saltwater in the Coastal Area of Benin River in Western Niger Delta fromDar-Zarrouk Parameters

Shallow aquifers are the major source of groundwater to the inhabitants of the coastal area of the Niger Delta. Groundwater is harnessed by hand-dug wells because they are easier and inexpensive to construct compared to boreholes. The closeness of groundwater water table to the surface encourages the construction of dug wells. The coastal area under investigation is faced with acute groundwater quality and assessment problems arising from saltwater intrusion. Dar-Zarrouk parameter comprising of longitudinal unit conductance(S) and transverse unit resistance (T) were coupled with resistivity to discriminate freshwater from saltwater zones. S across the area ranged from 0.03 to 2.96 Siemens and T from 1.4 to 790 Ω m2. S values greater and lesser than 0.3Siemens were considered as saltwater and freshwater zones respectively, while T values that ranged from 1.32 to 161.33Ω m2 were regarded as saltwater zones. Resistivity high of 185Ωm to 1511.58Ωm was assigned freshwater zones. The resistivity data considered showed evidence of freshwater at depth of 27.6m and saltwater at 34.3m. The lateral interface between saltwater and freshwater is situated at 565m from the sea. The intrusion revealed by the study does not suggest over-withdrawal of groundwater because of the small population of inhabitants but to natural hydrodynamic forcing connected with aquifer adjacent to the sea. However, the tendency of upconing of saltwater into the freshwater aquifer is probably at the depth of 3.9 to 12.3m by over-withdrawing of groundwater may not be ruled out in the future. Induced polarization is suggested as a follow-up investigation to unravel ambiguity that may be associated with resolution of resistivity data.Key words: Benin River, Niger Delta, Saltwater intrusion, Dar-Zarrouk Parameters, longitudinal unit conductance, transverse unit resistance.

Seawater intrusion and retreat in tidally-affected unconfined aquifers: Laboratory experiments and numerical simulations

Based on combined laboratory experiments and numerical simulations, this paper examined seawater intrusion (SWI) and seawater retreat (SWR) processes caused by abrupt inland watertable changes in a laboratory-scale unconfined aquifer (length of 7.7 m and thickness of 1.0 m) subjected to a synthetic sinusoidal tide. The results showed that the salinity distribution was relatively stable and that SWI and SWR processes were almost temporally symmetrical given relatively large horizontal hydraulic gradients (0.0269, 0.0209 and 0.0149) between the inland watertable and the mean sea level. However, the salt distribution changed significantly in response to the inland watertable variations when the horizontal hydraulic gradient was relatively small (0.0030). The speed of the SWI and SWR response to the inland watertable variations was temporally asymmetric, e.g., SWR was quicker than SWI by a factor of 9 with respect to the observed saltwater wedge toe locations. As a relatively thick mixing zone (transition between freshwater and saltwater zones) was induced by the tide, simulated saltwater wedge toe locations, as indicated by the 5%, 50% and 95% isohalines, changed inconsistently. Different hysteresis behaviors were found in the relationship between the SW toe locations and total salt mass stored in the aquifer. Sensitivity analyses demonstrated that the response of both SWI and SWR to the inland watertable variations could be prolonged by a decreased tidal amplitude or decreased tidal period.

Tidal Influences on Biotic and Abiotic Factors in the Seomjin River Estuary and Gwangyang Bay, Korea

To assess changes in abiotic and biotic factors between flood and ebb tides, we investigated the seasonal phytoplankton dynamics and environmental conditions along a salinity gradient at 14 stations in the Seomjin River estuary (SRE), Korea, and conducted bioassays to investigate the effect of nutrient addition (+N, +P, and +NP) on phytoplankton growth. Saltwater intrusion upstream was greatly dependent on the amount of freshwater discharge resulting from seasonal rainfall. There was a strong negative correlation between salinity and the nitrate+nitrite concentration (p 0.01). This indicates that the N and Si loading increased as a result of freshwater input. The algal bioassays showed that high phytoplankton growth rates were usually recorded in response to the +NP treatment, but in the saltwater zone, the phytoplankton community also responded rapidly to the +N treatment, and to the +P treatment in the freshwater zone. The range of nutrient limitation depended on freshwater discharge. The seasonal and horizontal distribution of phytoplankton communities changed along the salinity gradient. The significant differences in abiotic factors between flood and ebb tides play important roles in controlling the biotic factors, including the occurrence of aquatic organisms including microalgae.

Evaluation of Salt-water Intrusion in the Coastal Area of Igbokoda, Southwestern Nigeria

Saltwater intrusion into the coastal aquifer has long been recognized as a major threat to groundwater quality around the world. Groundwater evaluation of salt water intrusions in Igbokoda coastal area, southwestern Nigeria was carried out employing combined Horizontal Profiling and Vertical electrical sounding. Two traverses each with two sounding points were occupied. The result from the survey revealed 4 to 5 major layers comprising the unconsolidated silty sand and sandy clay (overburden), clayey zone, consolidated sand zone, partly intruded salt water intruded sandy clay zone and salt water intruded clay zone. The curves were the complex types KQH, KHA, QH and HKH curves. The overburden has resistivity that ranged from 253 to 1316.7Ω-m, thickness that ranged from 0.2 m to 7m. The clayey zone had resistivity of 846.0 Ω-m and thickness of 4m. The consolidated sand zone had resistivity that ranged from 2848.7 to 2865.7Ω-m and thickness that ranged between 4 and 21m. The partly intruded salt water zone is characterized by resistivity that varies between 18.4Ω-m and 93.0Ω-m and thickness of about 7-25m. The salt water intruded zone is characterized by resistivity that ranges between 4.1Ω-m and 9.7Ω-m and thickness of 4-48m. The partly-salt water intruded zones and salt water intruded zone were characterized with low resistivity while the high resistivity zones of consolidated sand layer constitute fresh water bearing zone that could serve as boreholes in the study area.

Using helicopter TEM to delineate fresh water and salt water zones in the aquifer beneath the Okavango Delta, Botswana

The Okavango Delta is a vast wetland wilderness in the middle of the Kalahari Desert of Botswana. It is a largely closed hydrological system with most water leaving the delta by evapotranspiration. In spite of this, the channels and swamps of the delta remain surprisingly low in salinity. To help understand the hydrological processes at work, we reanalyzed a previous inversion of data collected from a helicopter transient electromagnetic (HTEM) survey of the entire delta and performed an inversion of a high resolution dataset recorded during the same survey. Our results show widespread infiltration of fresh water to as much as ∼200 m depth into the regional saline aquifer. Beneath the western delta, freshwater infiltration extends to only about 80 m depth. Hydrological modeling with SEAWAT confirms that this may be due to rebound of the regional saltwater–freshwater interface following the cessation of surface flooding over this part of the delta in the 1880s. Our resistivity models also provide evidence for active and inactive saltwater fingers to as much as ∼100 m beneath islands. These results demonstrate the great extent of freshwater infiltration across the delta and also show that all vegetated areas along the delta's channels and swamps are potential locations for transferring solutes from surface water to an aquifer at depth.

Assessment of Ecosystem Health during the Freshwater Discharge in the Youngsan River Estuary.

The Youngsan River estuary was physically changed by the construction of a sea embankment at near the mouth of estuary. Weirs were also constructed recently in the freshwater zone and it was reported that algal blooms occur more frequently. The freshwater introduced into saltwater zone from sluice gates of the embankment affects water quality but it has not been addressed that how the freshwater inputs influence the health of marine ecosystem. In this study, we used the data of water properties and phytoplankton communities collected at three stations for 4 days including before the freshwater discharge, during the discharge and after 1 and 2 days of discharge events. WQI(water quality index), TRIX (trophic status index) and P-IBI (phytoplankton index of biotic integrity) were used to evaluate the ecosystem health and long-term data were also utilized to determine the criteria for P-IBI. The results showed that grades of the ecosystem health assessed by the indices were low at the station near the gates and increased as downstream. However, the temporal pattern of grades was different depending on methods. Grades of WQI and TRIX decreased during the discharge and restored after the discharge whereas the grades of P-IBI decreased slightly even after the discharge. This suggests that P-IBI is more applicable to estuarine systems where experience extreme change of water properties than WQI and TRIX since P-IBI includes phytoplankton that can respond quickly to the change.

Drilling Engineering Design and Construction of Well Dong 11

The planned depth of well Dong 11 is 5,780 m, many complicated conditions may exist in the drilling process of this well, such as loose cementation and high risk of hole collapse in the upper formation, leakage of Cretaceous strata, caving mudstone in Jurassic, possible high-pressured saltwater zone and hydrocarbon zone in Sangonghe formation. In addition, this block is located in fault ruptured zone. Based on the above problems, casing program design, BHA design, drill bit selection, drilling fluid design and the cementing design was optimized and the drilling engineering design and site construction of the well was introduced. No complicated condition occurred during the whole well construction. PDC bit was used in the upper well section and PDC bit combined with screw drilling technology was used in lower well section, the average rate of penetration is 4.77 m/h, compared with the offset wells, the drilling speed increased obviously and the good hole diameter validates the serviceability of a strong inhibitive heat-resisting plugging polymer anti-collapse drilling fluid system in this area.

Estimation of the position of the sharp interface in littoral aquifers with hydraulic head differences within the fresh-water zone

In littoral zones where fresh groundwater discharges into the sea, a vertical difference in hydraulic head always exists within the fresh-water zone, i.e., the Dupuit assumption is not satisfied. The hydraulic head difference may affect the sharp interface and needs to be taken into account when position of the interface is determined. Examination of the equilibrium status of a point on the interface reveals that the location of the interface in a homogeneous and isotropic coastal unconfined aquifer can be determined based on the fresh-water head and the salt-water head at one point on the sharp interface. Considering a linear increase or decrease in the hydraulic head within the fresh-water zone, a method for estimating the position of the sharp interface is described on the basis of the fresh hydraulic heads at two arbitrary points in the fresh-water zone and the salt hydraulic head at one arbitrary point in the salt-water zone in the same vertical straight line in the littoral zone. The method using corresponding pressures at the points is also given. The Ghyben-Herzberg model is a specific case of the method. This method requires three nearest observation wells, two revealing the fresh-water zone and one tapping the deeper salt-water zone, or three transducers installed in one well, two in the fresh-water zone and one in the deeper salt-water zone. The Observations of piezometric heads in the northwest Beihai coast in China are used to illustrate the application of the method to determine the position of the sharp interface, which ranges from 32.56 to 49.15 m under the mean sea level during the specific observation period.