滨海盆地变密度地下水流与溶质运移三维耦合数值模型研究

针对滨海含水层的复杂性,以北部湾经济区合浦盆地地下水资源应急潜力评价为例,通过概化出合浦盆地水文地质概念模型的基础上,采用SEAWAT模块建立了合浦盆地变密度地下水流与溶质运移三维耦合数值模型。在对模型进行识别、验证的基础上,假设2025年10月出现极端干旱,在保证不发生海水入侵的条件下,获得了度过整个枯水期各水源地地下水资源应急潜力。结果表明:合浦盆地地下水资源应急潜力为83.13万m~3/d,集中开采区中心地下水水头下降3～8 m, 2 a后水位基本恢复;集中开采区降落漏斗远离海岸线,不受到海水入侵的影响。该方法将滨海水源地地下水应急供水预测和盆地的水文地质结构及当地发展规划紧密结合起来,为合浦盆地地下水资源的合理开发利用和应急能力建设提供了可靠依据。     

Field Investigations for Sustainable Groundwater Utilization in the Konan Basin

The Konan groundwater basin of Kochi Prefecture, Japan, is experiencing freshwater shortages for the past few years, particularly during the winter season. The present study aimed at analyzing the problem and exploring increased groundwater withdrawals to meet burgeoning freshwater demands in various sectors. The trend analysis of waterconsumption indicates that the groundwater demandwould increase by 43 and 52% by the years 2010 and2025, respectively, compared to 32 100 m³ d^-1 at present.The streamflow of the Monobe River reduces to a minimumin winter and it shows a decreasing trend in recentyears. The geologic investigation results indicatethat unconfined (phreatic) aquifers comprisingalluvial sand and gravel and/or diluvial silty sandand gravel are dominant, and that the northwestern andwestern portions of the basin have greater potentialfor groundwater. Groundwater flows essentially fromnorth to south into the ocean. Groundwater levelfluctuations over the basin vary appreciably in spaceand time, indicating a wide variation of naturalrecharge. Field observations confirm the hypothesisthat stream-aquifer interaction also significantlycontributes to groundwater recharge, besides therainfall infiltration. Further, the pumping testsresults demonstrate that though the aquifers arelow-yielding, enhanced groundwater withdrawals couldbe possible. The aquifer hydraulic conductivity ischaracterized as high and ranges from 65 to 804 m d^-1(mean = 229 m d^-1), thereby suggesting great aquiferheterogeneity. Finally, it is concluded that thelong-term sustainability of the scarce groundwaterresource of the Konan basin is doubtful, and that acomprehensive analysis of the groundwater system isessential prior to formulating plans for the futuregroundwater development and management.     

Simulation Modeling for Efficient Groundwater Management in Balasore Coastal Basin,India

The Balasore coastal groundwater basin in Orissa, India is under a serious threat of overdraft and seawater intrusion. The overexploitation resulted in abandoning many shallow tubewells in the basin. The main intent of this study is the development of a 2-D groundwater flow and transport model of the basin using the Visual MODFLOW package for analyzing the aquifer response to various pumping strategies. The simulation model was calibrated and validated satisfactorily. Using the validated model, the groundwater response to five pumping scenarios under existing cropping conditions was simulated. The results of the sensitivity analysis indicated that the Balasore aquifer system is more susceptible to the river seepage, recharge from rainfall and interflow than the horizontal and vertical hydraulic conductivities and specific storage. Finally, based on the modeling results, salient management strategies are suggested for the long-term sustainability of vital groundwater resources of the Balasore groundwater basin. The most promising management strategy for the Balasore basin could be: a reduction in the pumpage from the second aquifer by 50% in the downstream region and an increase in the pumpage to 150% from the first and second aquifer at potential locations.     

Modeling Groundwater Flow and Seawater Intrusion in the Coastal Aquifer of Wadi Ham,UAE

Groundwater pumping from Kalbha and Fujairah coastal aquifer of the United Arab Emirates (UAE) has increased significantly during the last two decades to meet the agriculture water demands. Due to the lack of natural replenishment from rainfall and the excessive pumping, groundwater levels have declined significantly causing an intrusion of seawater in the coastal aquifer of Wadi Ham. As a result, many pumping wells in the coastal zone have been terminated and a number of farms have been abandoned. In this paper, MODFLOW was used to simulate the groundwater flow and assess the seawater intrusion in the coastal aquifer of Wadi Ham. The model was calibrated against a five-year dataset of historical groundwater levels and validated against another eleven-year dataset. The effects of pumping on groundwater levels and seawater intrusion were investigated. Results showed that reducing the pumping from Khalbha well field will help to reduce the seawater intrusion into the southeastern part of the aquifer. Under the current groundwater pumping rates, the seawater will continue to migrate inland.     

潮汐对沿海潜水水位的影响研究

沿海潜水地下水水位受到潮汐海平面的振动影响而上下波动,而地下水水位的位置与沙滩的稳定性和海水入侵有密切的联系。通过潮汐对沿海地下水水位影响的方程形式的概述,综述了在解析解与数值解方面的研究进展,并在此基础上提出了潮汐影响下地下水水位变化的3个特征:振动不对称性、振幅有衰减、相位滞后。     

Regional Water Balance and Economic Assessment as Tools for Water Management in Coastal Lebanon

The Lebanese coast is highly subject to seawater intrusion and groundwater deterioration. The study is carried out in Byblos district (Jbeil Caza) 35 km north of Beirut. It aims to investigate the seawater intrusion, to determine the regional water balance of the region and finally to estimate the economic value of that water for agricultural use. The monitoring of the aquifer was achieved through samples from different wells chosen randomly. As for the regional water balance, it was determined with use of a GIS model. The economic evaluation was carried out, using the contingent valuation method to estimate the willingness to pay of farmers to contribute to the improvement of groundwater quality; two alternative scenarios were proposed and compared with the current situation. The annual regional water balance is positive, which means that the region is rich in water. The monitoring results show that the coastal part of the region is slightly contaminated by seawater intrusion due to the excess of pumping from the aquifer. The economic evaluation estimated that farmers would contribute by 102 US$ yr^− 1 for the first proposal and 166.67 US$ yr^− 1 for the second.     

快速评价海水入侵区地层渗透性实验研究

为快速评价海水入侵区不同埋深地层渗透性,减少实施常规水文地质试验对存在污染含水层的扰动,提出利用振荡试验技术确定滨海地层渗透系数。室内海水入侵砂槽物理模型中实施了注水式振荡试验,在人工潜水含水层和承压含水层中同步监测了试验主井和相邻观测井中水位响应;龙口海水入侵区地下水分层监测井中实施了提水式和注水式振荡试验;利用配线法计算了室内实验和现场实验中目标地层渗透系数。结果表明：海水入侵砂槽物理模型中实施振荡试验为滨海含水层现场利用振荡试验确定不同埋深含水层渗透系数提供了重要经验;相比于提水式振荡试验,注水式振荡试验更易于实施,激发方式（提水和注水）的不同并没有对试验结果造成明显的影响;利用振荡试验在非常短时间内（单井水位恢复时长大部分少于5 min）确定了龙口海水入侵区重要监测断面上不同埋深含水层渗透系数,实验结果准确可靠。     

Groundwater Vulnerability Modeling to Assess Seawater Intrusion: a Methodological Comparison with Geospatial Interpolation

Seawater intrusion has become a growing threat in coastal urban cities due to overexploitation of groundwater. This study examines the accuracy of the commonly used geospatial quality assessment models (GQA) and groundwater vulnerability assessment models (GVA) in determining the extent of seawater intrusion in urban coastal aquifers. For that purpose, interpolation methods (kriging, IDW and co-kriging) and vulnerability assessment models (DRASTIC, EPIK) were compared using groundwater salinity criteria (TDS, Cl^?) collected at three pilot areas along the eastern Mediterranean (Beirut, Tripoli, Jal el Dib). The results showed that while the GIS-based interpolation methods and the vulnerability assessment models captured elements of the groundwater quality deterioration, both had a limited ability to accurately delineate saltwater intrusion. This emphasizes that while interpolation methods and conventional vulnerability models may give general information about groundwater quality, they fail to capture the status of the aquifer at a finer spatial resolution.

     

ANALYTICAL SOLUTION OF GROUNDWATER FLUCTUATIONS IN ESTUARINE AQUIFER

1.INTRODUCTIONTide-induced head fluctuations are a naturalphenomenon in a coastal aquifer[1-3].For an un-confined aquifer,groundwater level changes corre-sponding to the tidal waves.As the tidal propagateinland,their amplitude is attenuated and phase-shifts occur.A typical damping distance for tidalwater table fluctuation in an unconfined aquifer isseveral hundred meters whereas the tidal influenceon a confined aquifer can extended landward byseveral thousand meters[2,4].According to thef…     

Groundwater Level Forecasting in a Shallow Aquifer Using Artificial Neural Network Approach

Forecasting the ground water level fluctuations is an important requirement for planning conjunctive use in any basin. This paper reports a research study that investigates the potential of artificial neural network technique in forecasting the groundwater level fluctuations in an unconfined coastal aquifer in India. The most appropriate set of input variables to the model are selected through a combination of domain knowledge and statistical analysis of the available data series. Several ANN models are developed that forecasts the water level of two observation wells. The results suggest that the model predictions are reasonably accurate as evaluated by various statistical indices. An input sensitivity analysis suggested that exclusion of antecedent values of the water level time series may not help the model to capture the recharge time for the aquifer and may result in poorer performance of the models. In general, the results suggest that the ANN models are able to forecast the water levels up to 4 months in advance reasonably well. Such forecasts may be useful in conjunctive use planning of groundwater and surface water in the coastal areas that help maintain the natural water table gradient to protect seawater intrusion or water logging condition.     

Uncertainty analysis of seawater intrusion forecasting

In order to describe the importance of uncertainty analysis in seawater intrusion forecasting and identify the main factors that might cause great differences in prediction results, we analyzed the influence of sea level rise, tidal effect, the seasonal variance of influx, and the annual variance of the pumping rate, as well as combinations of different parameters. The results show that the most important factors that might cause great differences in seawater intrusion distance are the variance of pumping rate and combinations of different parameters. The influence of sea level rise can be neglected in a short-time simulation （ten years, for instance）. Retardation of seawater intrusion caused by tidal effects is obviously important in aquifers near the coastline, but the influence decreases with distance away from the coastline and depth away from the seabed. The intrusion distance can reach a dynamic equilibrium with the application of the sine function for seasonal effects of influx. As a conclusion, we suggest that uncertainty analysis should be considered in seawater intrusion forecasting, if possible.     

Hydraulic Parameters of Coastal Aquifer Systems by Direct Methods and an Extended Tide–Aquifer Interaction Technique

Adequate and reliable parameters are key to the sustainable management of vital groundwater resources. Present study focuses on the evaluation of direct methods (tidal efficiency and time lag methods) and an extended tide–aquifer interaction technique for determining the hydraulic parameter of coastal unconfined and confined aquifer systems. The hydraulic diffusivities of unconfined and confined aquifer systems were estimated using the tidal efficiency and time lag methods as well as they were optimized using the tide–aquifer interaction model and the Levenberg–Marquardt optimization technique. The hydraulic diffusivities were optimized by the Levenberg–Marquardt technique following two approaches: lumped tidal component approach and multi-tidal component approach. The effect of spring and neap tidal data on parameter estimates was also analyzed. The tide–aquifer interaction data for two unconfined sites and three confined sites were used in this study. For all the five sites under study, the aquifer hydraulic diffusivities based on the time lag method were found to be much larger (2 to 14 fold for the unconfined sites and 5 to 8 fold for the confined sites) than those based on the tidal efficiency method. The analysis of the optimization results indicated that the hydraulic diffusivities following “multi-tidal component approach” are more reliable and accurate for both unconfined and confined aquifers than those obtained following “lumped tidal component approach”. Consequently, the use of “multi-tidal component approach” is strongly recommended for the determination of aquifer parameters by the tide–aquifer interaction technique. Furthermore, the tide–interaction data corresponding to spring and neap tidal events were found to significantly affect the aquifer diffusivities yielded by the tide–aquifer interaction technique. It is concluded that a judicious use of tide–aquifer interaction technique is indispensable for the reliable estimates of hydraulic parameters of coastal aquifer systems.     

TIDAL EFFECTS ON GROUNDWATER DYNAMICS IN COASTAL AQUI-FER UNDER DIFFERENT BEACH SLOPES

The tide induced groundwater fluctuation and the seawater intrusion have important effects on hydrogeology and ecology of coastal aquifers.Among previous studies,there were few quantitative evaluations of the joint effects of the beach slope and the tide fluctuation on the groundwater dynamics.In this article,a numerical model is built by using the software FEFLOW with consideration of seawater intrusion,tide effects,density dependent flow and beach sloping effects.The simulation results are validated by laboratory experimental data in literature.More numerical scenarios are designed in a practical scale with different beach slopes.Results show that the groundwater fluctuation decays exponentially with the distance to the beach,i.e.,,and our simulation further shows that the beach slope influence can be expressed in the form of a logarithm function.While for the same location,the amplitude increases logarithmically with the beach angle in the form,where and are related with the horizontal distance()in the form of a logarithm function.The beach slope has no influence on the phase lag,although the latter increases regularly with the distance from the sea.The beach slope effect on the seawater intrusion is investigated through the quantitative relationship among the relative intrusion length(),the relative enhancement of the tide induced seawater intrusion()and the beach angle().It is shown that the tide effects on a milder beach is much greater than on a vertical one,and both λ and κ can be expressed in logarithm functions of.The tidal effect on the flow field in the transition zone for a particular mild beach is also studied,with results showing that the tide induced fluctuation of is similar to the groundwater table fluctuation while shows a distinct variation along both directions.     

Quantifying a Sustainable Management Space for Human Use of Coastal Groundwater under Multiple Change Pressures

In the densely populated coastal regions of the world, loss of groundwater due to seawater intrusion, driven by changes of climate, sea level, land use and water use, may critically impact many people. We analytically investigate and quantify the limits constraining a coastal aquifer’s sustainable management space, in order to avoid critical loss of the coastal groundwater resource by seawater intrusion. Limiting conditions occur when the intrusion toe reaches the pumping wells, well intrusion, or the marine-side groundwater divide, complete intrusion; in both cases the limits are functions of the seaward groundwater flow remaining after the human groundwater extractions. The study presents a screening-level approach to the quantification of the key natural and human-determined controls and sustainability limits for the human use of coastal groundwater. The physical and geometrical characteristics of the coastal aquifer along with the natural conditions for recharge and replenishment of the coastal groundwater are the key natural controls of the sustainable management space for the latter. The groundwater pumping rates and locations are the key human-determined controls of this space. The present approach to combining and accounting for both of these types of controls is simple, yet general. The approach is applicable across different scales and regions, and for historic, current and projected future conditions of changing hydro-climate, sea level, and human freshwater use. The use of this approach is also concretely demonstrated for the natural and human-determined controls and limits of the sustainable management space for two specific Mediterranean aquifers.     

Evaluation of Analytical Methods to Study Aquifer Properties with Pumping Tests in Coastal Aquifers with Numerical Modelling (Motril-Salobreña Aquifer)

Two pumping tests were performed in the unconfined Motril-Salobreña detrital aquifer in a 250 m-deep well 300 m from the coastline containing both freshwater and saltwater. It is an artesian well as it is in the discharge zone of this coastal aquifer. The two observation wells where the drawdowns are measured record the influence of tidal fluctuations, and the well lithological columns reveal high vertical heterogeneity in the aquifer. The Theis and Cooper-Jacob approaches give average transmissivity (T) and storage coefficient (S) values of 1460 m²/d and 0.027, respectively. Other analytical solutions, modified to be more accurate in the boundary conditions found in coastal aquifers, provide similar T values to those found with the Theis and Cooper-Jacob methods, but give very different S values or could not estimate them. Numerical modelling in a synthetic model was applied to analyse the sensitivity of the Theis and Cooper-Jacob approaches to the usual boundary conditions in coastal aquifers. The T and S values calculated from the numerical modelling drawdowns indicate that the regional flow, variable pumping flows, and tidal effect produce an error of under 10 % compared to results obtained with classic methods. Fluids of different density (freshwater and saltwater) cause an error of 20 % in estimating T and of over 100 % in calculating S. The factor most affecting T and S results in the pumping test interpretation is vertical heterogeneity in sediments, which can produce errors of over 100 % in both parameters.     

潮汐引起滨海承压含水层地下水位变化的数值模拟

海潮波动可以引起海岸带地下水位发生波动。建立基于有限差分法的滨海地区一维承压含水层地下水运动数值模型。通过将潮汐波动概化为正弦波,模拟滨海地区地下水位随潮汐波动的变化。结果表明,受海潮影响的滨海含水层地下水位与海潮有相似的波动特征,但变幅减小,受海潮的影响程度与离海岸的距离有关,随着离海岸距离的增加地下水位的变幅及潮汐效率呈负指函数衰减,地下水位对海潮的滞后时间随距离呈线性增加。例如,当承压含水层导水系数为31．25m^2／h、储水系数为4．5×10^-4及海潮振幅为2．5m、周期为24．7h时,在距离海岸500m处,振幅为1．35268m,潮汐效率为0．54107,滞后时间为1．825h。在距海岸达4389m以远时可以认为地下水位不受海潮波动的影响。     

A Cost-Effective Method to Control Seawater Intrusion in Coastal Aquifers

Intrusion of seawater into coastal aquifers is considered one of the most important processes that degrade water-quality by raising the salinity to levels exceeding acceptable drinking standards. Therefore saltwater intrusion should be prevented or at least controlled to protect groundwater resources. This paper presents a cost-effective method to control seawater intrusion in coastal aquifers. This methodology ADR (Abstraction, Desalination and Recharge) includes; abstraction of saline water and recharge to the aquifer after desalination. A coupled transient density-dependent finite element model is developed for simulation of fluid flow and solute transport and used to simulate seawater intrusion. The simulation model has been integrated with an optimization model to examine three scenarios to control seawater intrusion including; abstraction, recharge and a combination system, ADR. The main objectives of the models are to determine the optimal depths, locations and abstraction/recharge rates for the wells to minimize the total costs for construction and operation as well as salt concentrations in the aquifer. A comparison between the combined system (ADR) and the individual abstraction or recharge system is made in terms of total cost and total salt concentration in the aquifer and the amount of repulsion of seawater achieved. The results show that the proposed ADR system performs significantly better than using abstraction or recharge wells alone as it gives the least cost and least salt concentration in the aquifer. ADR is considered an effective tool to control seawater intrusion and can be applied in areas where there is a risk of seawater intrusion.     

SEASONAL PATTERNS OF RIVER CONNECTIVITY AND SALTWATER INTRUSION IN TIDAL FRESHWATER FORESTED WETLANDS

Hydrology can be difficult to interpret in tidal freshwater forested wetlands because of several influencing factors including river discharge, tidal stage, local precipitation, evapotranspiration, groundwater and prevailing winds. River discharge and tidal stage are considered the primary factors affecting these wetlands and both vary on a seasonal basis and have potential implications for wetland saltwater intrusion. We examined the hydrologic patterns of tidal freshwater swamps along the lower reach (20 km) of the Apalachicola River system in northwest Florida, USA. Water level recorders were installed in five wetlands along this reach and recorded ground and surface water levels at 15‐min intervals from December 2006 to April 2009 (non‐continuously). Seasonal trends related to flooding frequency and duration were compared to long term (2000–2008) records of creek and river salinities measured in the tidal freshwater forested zone. Tidal freshwater swamps tended to maintain a narrow water level range (often ±20 cm of the ground surface) except during high river flows (>680 cms or 24 000 cfs) when levels increased 1 to 2 m above ground and dampened tidal effects. All tidal forested wetlands had the highest mean water levels and most days tidally pulsed during the Jul–Oct season which coincided with peak river/creek salinities and low river‐flows associated with drought conditions. We explored the implications for the timing of salt water intrusion in these swamps and potential changes related to sea level rise and water management in the Apalachicola River drainage basin. Copyright © 2011 John Wiley & Sons, Ltd.     

Estimating Groundwater Withdrawal in Poorly Gauged Agricultural Basins

A method is presented for estimating the annual groundwater withdrawal based on water balance resulting from surface and groundwater hydrological considerations. The unknown groundwater losses of the aquifer are estimated from the groundwater level fluctuations, the specific yield and the groundwater withdrawal prior to the installation of the irrigation network. The meteorological and hydrometric data are used in the Sacramento hydrological conceptual model for the estimation of the stored groundwater volume, via minimization of the difference between the simulated and measured stream discharge. Following the installation of the network due to poorly kept field records, an initial estimation of the groundwater withdrawal is made based on the fluctuations of the groundwater level, the specific yield, and the annual precipitation. The monthly stored groundwater volume is verified against the volume obtained during the recharge of the basin (November–April). The difference between the groundwater volume and the measured discharge of the basin (May–October) is in agreement with the initial estimation of the groundwater withdrawal. This method is applied successfully in an agricultural basin on the island of Crete, Greece and its novelty lies in the fact that it can be used in basins where groundwater withdrawal is not known or data is incomplete.     

潮汐波动对秦山三期核电工程场地裂隙含水层的影响

杭州湾潮汐的波动在临近的含水层中产生的压力波，由于压力波向内地传播，使地下水位和水力坡度连续不断地发生波动，并直接影响场地周围含水层中压力表的读数。为了掌握潮汐对场地地下水的影响，通过比较地下水和地表水平均水位标高计算地下水的平均水力坡度。压力波按正弦波传播，地下水相对于潮水有一时间滞后，本文中计算了不同观测地点的滞后时间，影响因子和波长，为核电站的建设提供了重要的依据。