A FINITE ELEMENT MODEL OF CONTAMINANT MOVEMENT IN GROUNDWATER1

ABSTRACT. Transient, two-dimensional solutions are developed which describe the movement and distribution of a conservative substance in a stream-aquifer system. The solutions are obtained by solving sequentially the groundwater flow and mass transport equations. A variational approach in conjunction with the finite element method is used to solve the groundwater flow equation. Galerkin's approach coupled with the finite element method is used to solve the mass transport equation. Linear approximated triangular elements and a centered scheme of numerical integration are employed to calculate the hydraulic head distribution and the concentration of solute in the flow region. The linear approximation used to define the concentration function within each element is not appropriate for cases involving steep concentration gradients. For such cases, higher order approximations are necessary to assure the continuity of gradients across interelemental boundaries. Numerical examples that illustrate the applicability of the model are presented.     

DECLINING POTENTIOMETRIC LEVELS IN FORT WALTON BEACH AREA,FLORIDA1

ABSTRACT: The Fort Walton Beach area is presently faced with an excessive drawdown of the potentiometric level in the upper Floridan aquifer. Based on available data, the potentiometric level in the Floridan aquifer has dropped 162 feet since 1936. This declining potentiometric level can lead to problems and possible loss of the natural resource on a long-term basis. However, if corrective measures or programs for proper management of groundwater resources are undertaken at this time, the potential problems may be averted.     

广西典型岩溶水系统环境中222Rn的分布及指示意义

放射性同位素氡222是来源于地层中铀衰变生成的惰性元素.由于其半衰期只有3.82d，且其活度在地下水和地表水中差异显著，近年来氡在水文学中的应用日渐兴起.但²²²Rn在岩溶环境中的分布和行为特征少有研究.以广西三个典型岩溶水系统为例，研究²²²Rn在包气带、非饱和带和饱水带中的浓度分布和指示意义.发现²²²Rn在上部包气带中的活度不足500Bq/m³，但在非饱和带中有局部异常高区，与局部小地质构造的分布有关.在饱水带中，地下水的滞留时间、不同地下水组分的混合和氡运移的距离等是导致²²²Rn活度差异的原因.管道介质的²²²Rn活度比裂隙介质的高；含水介质土壤覆盖层大的比土壤浅薄的普遍高.在大型岩溶水系统的排泄区，地下水补给河流，河流也可能短暂反补地下水，由于地下水和河流²²²Rn的差异显著，²²²Rn可能成为地下水-地表水相互作用研究的理想示踪剂.     

WATER SUPPLY IMPACTS OF NUCLEAR FALL1

ABSTRACT: “Nuclear winter,” more properly called “nuclear fall,” could be caused by injection of large amounts of dust into the atmosphere. Besides causing a decrease in temperature, it could be accompanied by “nuclear drought,” a catastrophic decrease in precipitation. Dry land agriculture would then be impossible, and municipal, industrial, and irrigation water supplies would be diminished. It has been argued that nuclear winter/fall poses a much greater threat to human survival than do fall out or the direct impacts of a conflict. However, this does not appear to be true, at least for the U.S. Even under the unprecedented drought that could result from nuclear fall, water supplies would be available for many essential activities. For the most part, ground water supplies would be relatively invulnerable to nuclear drought, and adequate surface supplies would be available for potable uses. This assumes that conveyance facilities and power supplies survive a conflict largely intact or can be repaired.     

贵州织金蒙家大坡地下水水化学特征分析

通过对贵州织金蒙家大坡地下水水化学指标进行分析。结果表明,研究区地下水Ca2+受环境影响最小、空间变异性最小,Na+受环境影响最大、空间变异性最大;各阴阳离子含量与TDS含量呈显著正相关关系,其相关性依次为:Ca2+SO2-4Mg2+Cl-HCO-3K+Na+;TDS含量的增加主要受SO2-4离子的影响,总硬度的变化主要受Ca2+的影响;地下水成分主要来自于碳酸盐矿物的溶解,其次是硫酸盐、含硫矿物、硅酸盐矿物以及盐岩的溶解,地下水类型主要向SO4·HCO3-Ca型和HCO3·SO4-Ca型转化。     

秦皇岛洋河-戴河流域浅层地下水咸化程度评价

为了解秦皇岛洋戴河流域浅层地下水咸化程度,开展了野外调查和水样采集,现场测试EC、TDS指标,并对水样进行了室内水化学分析.应用层次分析法结合Arcgis对选取的6个指标(Cl离子、TDS、Li离子、钠吸附比、硝酸盐、潜在盐度)进行权重计算、归一化处理和栅格计算,得出地下水咸化等级分区(未咸化区、轻微咸化区、中等咸化区、严重咸化区).咸化最严重区域位于沿海平原,包括枣园村、王各庄村.中度咸化区部分位于沿海平原,包括都寨村、西陆庄村、蒋营村、樊各庄,主要是由海水入侵导致的;而樊各庄咸化的原因是海水入侵和工业废水的污染;其他中度咸化区位于低山丘陵区,包括大湾子村、兴隆寨村,则是由水岩作用、农业污水、生活污水造成的.轻微咸化区主要分布在中等咸化区周围,未咸化区集中在山前平原区.     

Influence of Riparian Seepage Zones on Nitrate Variability in Two Agricultural Headwater Streams

Riparian seeps have been recognized for their contributions to stream flow in headwater catchments, but there is limited data on how seeps affect stream water quality. The objective of this study was to examine the effect of seeps on the variability of stream NO₃‐N concentrations in FD36 and RS, two agricultural catchments in Pennsylvania. Stream samples were collected at 10‐m intervals over reaches of 550 (FD36) and 490 m (RS) on 21 occasions between April 2009 and January 2012. Semi‐variogram analysis was used to quantify longitudinal patterns in stream NO₃‐N concentration. Seep water was collected at 14 sites in FD36 and 7 in RS, but the number of flowing seeps depended on antecedent conditions. Seep NO₃‐N concentrations were variable (0.1‐29.5 mg/l) and were often greater downslope of cropped fields compared to other land uses. During base flow, longitudinal variability in stream NO₃‐N concentrations increased as the number of flowing seeps increased. The influence of seeps on the variability of stream NO₃‐N concentrations was less during storm flow compared to the variability of base flow NO₃‐N concentrations. However, 24 h after a storm in FD36, an increase in the number of flowing seeps and decreasing streamflow resulted in the greatest longitudinal variability in stream NO₃‐N concentrations recorded. Results indicate seeps are important areas of NO₃‐N delivery to streams where targeted adoption of mitigation measures may substantially improve stream water quality.     

Calibrating a Basin‐Scale Groundwater Model to Remotely Sensed Estimates of Groundwater Evapotranspiration

Remotely sensed vegetation indices correspond to canopy vigor and cover and have been successfully used to estimate groundwater evapotranspiration (ET_g) over large spatial and temporal scales. However, these data do not provide information on depth to groundwater (dtgw) necessary for groundwater models (GWM) to calculate ET_g. An iterative approach is provided that calibrates GWM to ET_g derived from Landsat estimates of the Enhanced Vegetation Index (EVI). The approach is applied to different vegetation groups in Mason Valley, Nevada over an 11‐year time span. An uncertainty analysis is done to estimate the resulting mean and 90% confidence intervals in ET_g to dtgw relationships to quantify errors associated with plant physiologic complexity, species variability, and parameter smoothing to the 100 m GWM‐grid, temporal variability in soil moisture and nonuniqueness in the solution. Additionally, a first‐order second moment analysis shows ET_g to dtgw relationships are almost exclusively sensitive to estimated land surface, or maximum, ET_g despite relatively large uncertainty in extinction depths and hydraulic conductivity. The EVI method of estimating ET_g appears to bias ET_g during years with exceptionally wet spring/summer conditions. Excluding these years improves model performance significantly but highlights the need to develop a methodology that accounts not only on quantity but timing of annual precipitation on phreatophyte greenness.     

亚热带红壤丘陵区浅层地下水氮淋失特征研究

土壤养分累积引起的氮素(N)淋失是导致农区地下水污染的重要原因,也是农业面源污染的重要形式.本文以湖南省长沙县典型亚热带红壤丘陵流域为研究对象,通过连续定位观测,研究了林地、稻田、菜地和茶园4种代表性土地利用类型浅层(130~150 cm)地下水中N浓度的逐月动态变化特征.连续3年(2010—2013年)的观测结果表明:4种土地利用类型下浅层地下水总氮(TN)平均浓度差异显著(p0.05),其中,林地最低(0.85 mg·L-1),茶园最高(7.64 mg·L-1);从N的形态构成来看,林地、菜地和茶园浅层地下水中N形态以硝态氮(NO-3-N)为主,分别占TN的46.7%、70.2%和72.8%,而稻田浅层地下水N形态则以铵态氮(NH+4-N)为主,占TN 43.5%,表明土壤淹水条件是影响地下水N淋失形态的关键因子.地下水各形态N浓度的动态变化在不同土地利用下也迥然不同:林地地下水各形态N的含量低、变幅小,而稻田、菜地和茶园地下水N浓度变幅较大;采用单因子方法对不同土地利用下地下水的水质进行评价,结果表明:研究区浅层地下水中TN和NO-3-N无显著污染,NH+4-N污染较为严重,而综合指数法(F值法)进一步表明研究区浅层地下水污染主要出现在稻田和茶园,因此,控制稻田和茶园N肥的施用量是预防亚热带红壤丘陵区地下水N污染的关键.     

江苏洋口港地区地下水的环境同位素组成及其形成演化研究

江苏洋口港地区是江苏沿海开发重点建设的深水港区,水质优良的地下水是港区经济社会发展的重要基础.本研究利用近期获得的钻孔剖面地层对比数据资料,沿水文径流剖面分层采集水样,通过识别地表水、各含水层地下水的环境同位素(δD、δ~(18)O、~3H、~(14)C)组成指纹特征,以揭示研究区地下水的形成演化规律.结果表明,研究区浅层地下水主要来源于现代大气降水的入渗补给,大气降水在补给地下水过程中经历了明显的蒸发过程;研究区深层地下水的放射性~(14)C年龄主要为15000~26000 a,其稳定同位素δ~(18)O、δD值比现代降水低,是晚更新世末次冰期(大理冰期)的盛冰期降水入渗补给.地下水含水层的氘、氧、碳同位素分布具有明显的呈层性,随着地下水埋藏深度增加,地下水中的δD、δ~(18)O值呈下降趋势.浅层地下水与地表水水力联系紧密,可更新能力较强;深层地下水径流缓慢,总体上处于封闭-半封闭状态,可更新能力弱.江苏沿海平原天然地下水流动是自晚更新世末期以来,伴随着冰退、海平面上升而调整到目前的模式.末次盛冰期以来的自然地理及地质发展史,决定着研究区第四系地下水流系统的演变格局,现代人类活动加强了浅层与深层地下水之间的水力联系.研究在高强度开发地下水条件下的区域水文循环特征,可为沿海地区地下水演变机理和调控研究提供技术支持.