Ground-Water Quality at a Creosote Waste Site

An abandoned creosote facility in Conroe, Texas, has become a field site for the National Center for Ground Water Research (NCGWR) at Rice University. Ground-water contamination in the shallow aquifer beneath the site was characterized by sampling soils and water quality at 14 monitoring wells and 35 boreholes. Results from six sampling trips over two years for inorganic and organic chemical concentrations in the ground water show wells around the site were contaminated to levels above 800 μg/l for naphthalene, 400 μg/1 for methyl naphthalene, and 150 μg/1 for dibenzofuran. Conservative constituents, traced by chloride concentrations up to 75 mg/l, have migrated 300 ft (90 m) downgradient of the site. Organic contaminants have been adsorbed and microbially degraded in their migration from the waste source as evidenced by their attenuated concentrations. Detailed field pump tests have been performed to evaluate hydraulic conductivity at several of the shallow wells. The U.S. Geological Survey (USGS) Solute Transport Model (Konikow and Bredehoeft, 1978) has been used to predict chloride plume patterns and evaluate parameters which govern transport processes at the Conroe waste site.     

Application of VLEACH to Vadose Zone Transport of VOCs at an Arizona Superfund Site

Cleanup standards for volatile organic compounds in thick vadose zones can be based on indirect risk (transport to ground water) when contamination is below depths of significant direct risk. At one Arizona Superfund site, a one-dimensional vadose zone transport model (VLE-ACH) was used to estimate the continued transport of VOCs from the vadose zone to ground water. VLEACH is a relatively simple and readily available model that proved useful for estimating indirect risk from VOCs in the vadose zone at this site. The estimates of total soil concentrations used as initial conditions for VLF.ACH incorporated a variety of data from the site. Soil gas concentrations were found to be more useful than soil matrix data for estimating total soil concentrations at this arid-zone site. A simple mixing cell model was used with the VLEACH-derived mass loading estimates from the vadose zone over time to estimate the resulting changes in ground water concentrations. For this site, the results of the linked VLEACH/mixing cell simulations indicate it is likely that the federal MCI. for TCE will be exceeded in underlying ground water if remedial action on I he vadose zone is not pursued.     

Synergistic Application of Four Remedial Techniques at an Industrial Site

The soil and ground water at a General Motors plant site were contaminated with petroleum products from leaking underground storage tanks. Based on the initial assessment, the site was complex from the standpoint of geology (clay layers), hydrology (a recharge zone with a perched water table), and contaminant (approximately 4800 gallons of mixed gasoline and oil). After a thorough study of remedial alternatives, a synergistic remedial approach was adopted including pump and treat, product removal, vapor extraction, and bioventing. The system was designed and implemented at the site through 22 dual-extraction wells. Over a 21-month period, 4400 gallons of gasoline and oil were removed from the system, including 59 percent by vapor extraction, 28 percent by bioventing, and 13 percent by pump and treat. Synergism between the various remedial methods was demonstrated clearly. Ground water pump and treat lowered the water table, allowing air to flow for vapor extraction. The vacuum applied for vapor extraction increased the ground water removal rate and the efficiency of pump and treat. The vapor extraction system also added oxygen to the soil to stimulate aerobic biodegradation.     

Monitoring Hydrogeologlcal Conditions in Fractured Rock at the Site of Canada's Underground Research Laboratory

Atomic Energy of Canada Limited is constructing an Underground Research Laboratory (URL) at a depth of 250m in a plutonic rock body near Lac du Bonnet, Manitoba. The facility is being constructed to carry out a variety of in situ geotechnical experiments as part of the Canadian Nuclear Fuel Waste Management Program. A unique feature of the URL, in comparison to other similar facilities such as the Stripa Mine in Sweden, is that it is to be constructed below the ground water table in a previously undisturbed plutonic rock body. One of the main research objectives of the project is to develop and validate comprehensive three-dimensional models of the hydrogeology of the rock mass encompassing the URL site. These models will be used, before excavation of the URL shaft begins, to predict the hydrogeological perturbation that will be created by the excavation of the shaft and the horizontal working levels below the ground water table. As a model-validation exercise, these drawdown predictions will be compared with actual hydrogeological perturbations that will be monitored at the study area over the next several years by an extensive network of instrumented boreholes. Measurements made in an array of boreholes extending to depths of 1,000m on the 4.8 km² study area have established that the permeability distribution in three major extensive subhorizontal fracture zones controls the movement of ground water within the rock mass. Several types of multiple-interval completion systems have been installed in the boreholes to monitor the three-dimensional, physico-chemical hydrogeological conditions within the fractured rock mass. These include conventional piezometer nests and water-table wells that have been installed in shallow holes (less than 30m deep), and multiple-packer/ multiple-standpipe piezometers and multiple-interval casing systems installed in deeper holes (30 to 1,000m deep). An automated, electronic, piezometric pressure-monitoring system has been designed to collect continuous measurements from 75 isolated hydrogeological monitoring positions within the rock mass. Another 200 positions are being monitored frequently using a variety of techniques. Piezometric data have been collected from this monitoring network to establish baseline conditions prior to any excavation into the rock mass. These data have also been used to determine the steady-state, three-dimensional ground water flow regimes that exist at the URL site under natural conditions.     

Importance of unsaturated zone flow for simulating recharge in a humid climate

Transient recharge to the water table is often not well understood or quantified. Two approaches for simulating transient recharge in a ground water flow model were investigated using the Trout Lake watershed in north-central Wisconsin: (1) a traditional approach of adding recharge directly to the water table and (2) routing the same volume of water through an unsaturated zone column to the water table. Areas with thin (less than 1 m) unsaturated zones showed little difference in timing of recharge between the two approaches; when water was routed through the unsaturated zone, however, less recharge was delivered to the water table and more discharge occurred to the surface because recharge direction and magnitude changed when the water table rose to the land surface. Areas with a thick (15 to 26 m) unsaturated zone were characterized by multimonth lags between infiltration and recharge, and, in some cases, wetting fronts from precipitation events during the fall overtook and mixed with infiltration from the previous spring snowmelt. Thus, in thicker unsaturated zones, the volume of water infiltrated was properly simulated using the traditional approach, but the timing was different from simulations that included unsaturated zone flow. Routing of rejected recharge and ground water discharge at land surface to surface water features also provided a better simulation of the observed flow regime in a stream at the basin outlet. These results demonstrate that consideration of flow through the unsaturated zone may be important when simulating transient ground water flow in humid climates with shallow water tables.     

A multiscale investigation of ground water flow at Clear Lake, Iowa

Ground water flow was investigated at Clear Lake, a 1468-ha glacial lake in north-central Iowa, as part of a comprehensive water quality study. A multiscale approach, consisting of seepage meters (and a potentiomanometer), Darcy's law, and an analytic element (AE) model, was used to estimate ground water inflow to and outflow from the lake. Estimates from the three methods disagreed. Seepage meters recorded a median-specific discharge of 0.25 mum/s, which produced a lake inflow rate between 90,750 and 138,200 m3/d, but no detectable outflow. A wave-induced Bernoulli effect probably compromised both inflow and outflow measurements. Darcy's law was applied to 11 zones around the lake, producing inflow and outflow values of 10,500 and 5000 m3/d, respectively. The AE model, GFLOW, coupled with the parameter estimation model, UCODE, simulated ground water flow in a 700-km2 region using 31 hydraulic head and base flow measurements as calibration targets. The model produced ground water inflow and outflow rates of 14,300 and 9200 m3/d, respectively. Although not a substitute for field data, the model's ability to simulate ground water flow to the lake and the region, estimate uncertainty for model parameters, and calculate a lake stage and associated lake water balance makes it a powerful tool for water quality management and an attractive alternative to the traditional methods of ground water/lake investigation.     

Subsurface transfer of chloride after a lake retreat in the central Andes

The area under study covers 3500 km2 in the upstream part of the closed catchment basin of the salt crust of Uyuni. This crust is a remnant of the saline Lake Tauca, which covered the area about 15,000 years ago. In the downstream part of the aquifer, the Cl concentration of ground water and Cl content in the unsaturated zone exceed 20 meq/L and 18 kg/m2, respectively. With the present hydrological conditions under semiarid conditions, the ground water residence time in the study area exceeds 3000 years. Transient simulations over 11,000 years were made using initial conditions as the retreat of Lake Tauca and taking into account a low recharge during the arid mid-Holocene period. The modeling simulates ground water flow, Cl transport, and ground water residence time. It includes the evaporation from the aquifer that leads to the accumulation of chloride in the unsaturated zone. Results of the modeling are consistent with the observations if it is assumed that the Cl previously accumulated in the unsaturated zone was flushed back into the aquifer around 2000 years B.P., contemporaneously with the end of the arid period.     

Demonstration of a Microbiologically Enhanced Vertical Ground Water Circulation Well Technology at a Superfund Site

A full-scale ground water circulation well (GCW) system was installed and operated to demonstrate in situ remediation of soil and ground water impacted with a mixture of chlorinated and nonchlorinated organic compounds at a Superfund site in upstate New York. System performance and applicability under site-specific conditions were evaluated based on the system's ability to meet the New York State Department of Environmental Conservation (NYSDEC) cleanup goals for target compounds in ground water and soil. Contaminants from the unsaturated zone were mobilized (volatilized) by one-way vacuum extraction, and treated via enhanced biodegradation (bioventing). In the saturated zone, contaminants were mobilized by soil flushing (solubilized) and treated by a combination of air stripping and biodegradation. An in situ aqueous phase bioreactor, and an ex situ gas phase bioreactor, were integrated into the system to enhance treatment via bioremediation. After 15 months of operation, the mass of target contaminants in soil and ground water combined had been reduced by 75%. Removal by biological mechanisms ranged from 35% to 56% of the total observed mass reduction. The in situ and the ex situ bioreactors mineralized 79% and 76%, respectively, of their target biodegradable contaminant loads. Results indicate that some mass reduction in target contaminants may have been from aerobic and aerobic processes within the circulation cell. Nonchlorinated compounds were relatively easy to mobilize (volatilize, solubilize, and/or transport) and treat when compared to chlorinated compounds. The data collected during the 15-month study indicate that remediation could be accomplished at the Sweden-3 Chapman site using the technology tested.     

确定太湖流域湖库总磷参照浓度的改进MEI模型

太湖流域湖泊污染严重,非常有必要建立相应的水质基准以便于湖泊水体的保护与修复.根据太湖流域12个受人类影响较小的湖库及太湖早期的总磷、总碱度、平均水深等数据建立了MEI(morphoedaphic index)模型,通过对模型中总磷与总碱度、平均水深因子的相关关系进行分析,并结合太湖流域湖库水深较浅的特征,提出了确定太湖流域湖库水体中总磷参照浓度的改进MEI模型.将该模型应用于太湖,得到太湖总磷参照浓度为0.025 mg/L.研究结果旨在丰富我国水体营养物基准的确定方法,并为太湖流域水体富营养化的控制提供理论依据,同时为长江中下游类似湖库水质基准的建立提供技术支撑.     

Techniques and Equipment Used in Contaminant Detection at Hoe Creek Underground Coal Gasification Experimental Site

Data from an existing network of ground water monitoring wells at the U.S. Department of Energy (DOE) Hoe Creek Underground Coal Gasification (UCG) Experimental Site indicated that organic contaminants, particularly phenols produced during gasification experiments, were threatening neighboring ground water resources. The existing monitoring well network was sparse and further definition of the extent and direction of contaminant migration was needed. Additionally, water level data, important in determining flow directions, was incomplete. A field program was designed and implemented to locate and define the organic contamination and expand the existing ground water monitoring program. The program utilized field analysis of phenol for contaminant detection and well location, followed by completion using gas-drive ground water samplers/piezometers. Geophysical logging was used to permit optimum placement of the samplers. The geologic aspects of the site posed some interesting problems to the installation of the samplers. The contaminant plume edge was defined in the east, west and south directions during the field program. Further work is needed in the north direction.     

Site effects at Euroseistest—II. Results from 2D numerical modeling and comparison with observations

This paper presents results of numerical modeling of site response for Euroseistest. Ground motion across a very detailed model of the subsoil of this valley has been simulated for vertically incident SH waves. The predominance of locally generated surface waves is very clear in the synthetic seismograms. These results are then compared with published studies of observed site effects at this basin and with a detailed analysis of two events in the time domain. It is discussed in which sense it is possible to obtain a good fit between observations and 1D models, even though the real behavior involves locally generated Love waves. For this reason, it can be misleading to rely on an incomplete observation such as empirical transfer functions. Finally, it is stressed that in order to predict ground motion in alluvial valleys the information contained in the phase cannot be neglected.     

Application of harmonic analysis of water levels to determine vertical hydraulic conductivities in clay-rich aquitards

A harmonic analysis method was used to determine vertical hydraulic conductivities (Kv) in geologic media between vertically separated piezometers using water level measurements. In this method, each water level time series was filtered and then decomposed using harmonic analysis into a sum of trigonometric components. The phase and amplitude of each harmonic function were calculated. These data were used to estimate Kv values between vertically separated data sets assuming one-dimensional transient flow. The method was applied to water level data collected from nested piezometers at two thick clay-rich till aquitards in Saskatchewan, Canada. At one site, routine water levels were measured in 12 piezometers (installed between 1 and 29 m below ground surface) since installation (1995). At the other site, water levels were measured in seven piezometers (installed between 4 and 53 m below ground surface) since installation (1998-1999). The Kv calculated using harmonic analysis decreased with depth below the water table at both sites, approaching matrix estimates of hydraulic conductivity between 10 and 11 m and between 21 and 43 m below ground surface. These depths reflected the depth of extensive vertical fracturing at the sites and showed that the depth of fracturing may be site specific.     

The effect of terrace geology on ground‐water movement and on the interaction of ground water and surface water on a mountainside near Mirror Lake,New Hampshire,USA

The west watershed of Mirror Lake in the White Mountains of New Hampshire contains several terraces that are at different altitudes and have different geologic compositions. The lowest terrace (FSE) has 5 m of sand overlying 9 m of till. The two next successively higher terraces (FS2 and FS1) consist entirely of sand and have maximum thicknesses of about 7 m. A fourth, and highest, terrace (FS3) lies in the north‐west watershed directly adjacent to the west watershed. This highest terrace has 2 m of sand overlying 8 m of till. All terraces overlie fractured crystalline bedrock. Numerical models of hypothetical settings simulating ground‐water flow in a mountainside indicated that the presence of a terrace can cause local ground‐water flow cells to develop, and that the flow patterns differ based on the geologic composition of the terrace. For example, more ground water moves from the bedrock to the glacial deposits beneath terraces consisting completely of sand than beneath terraces that have sand underlain by till. Field data from Mirror Lake watersheds corroborate the numerical experiments. The geology of the terraces also affects how the stream draining the west watershed interacts with ground water. The stream turns part way down the mountainside and passes between the two sand terraces, essentially transecting the movement of ground water down the valley side. Transects of water‐table wells were installed across the stream's riparian zone above, between, and below the sand terraces. Head data from these wells indicated that the stream gains ground water on both sides above and below the sand terraces. However, where it flows between the sand terraces the stream gains ground water on its uphill side and loses water on its downhill side. Biogeochemical processes in the riparian zone of the flow‐through reach have resulted in anoxic ground water beneath the lower sand terrace. Results of this study indicate that it is useful to understand patterns of ground‐water flow in order to fully understand the flow and chemical characteristics of both ground water and surface water in mountainous terrain. Copyright © 2007 John Wiley & Sons, Ltd.     

Directional dependence of site effects observed near a basin edge at Aegion,Greece

We study site effects using 520 weak motion earthquake records from a vertical array in Aegion, Greece. The array is inside a basin, has four stations in soil, and one in bedrock (178 m depth). The site is marked by high seismicity and complex surface geology. We first use the records to establish the downhole accelerometer orientations and their evolution with time. Then we estimate site effects using empirical spectral ratios with and without a reference site (standard and horizontal-to-vertical spectral ratio). We find significant site amplification which cannot be accounted for by 1D model predictions, along with a significant difference in the amplification level between the two horizontal components. These are indications of 2D effects, namely surface waves generated at the basin edge. The difference in amplification between the horizontal components is maximised when these are rotated with respect to the orientation of the basin edge. The strongest amplification takes place in the direction parallel to the basin edge (SH, or out-of-plane motion), and is up to 2 times higher than in the perpendicular direction (SV, or in-plane motion). This directional effect on the amplification is corroborated by numerical 2D modelling using incident SH and SV waves, with the former possibly generating strong Love waves. In the records, the directionality is clear for windows containing the largest amplitudes of the records (S waves and strong surface waves), while it tends to vanish for coda-wave windows. This directionality is also observed when using response spectral ratios rather than Fourier ratios. We compute soil-to-rock amplification factors for peak ground acceleration (PGA) and find it is significantly higher than what is predicted by current design codes. We attribute this difference to the basin edge amplification, linear soil behaviour, and to the inability of simple scalar values like PGA to describe complex amplification effects. Finally, we analyse the earthquake records at a surface station near the slope crest and do not observe significant topographic amplification.     

Site effects in Mexico City: Constraints from surface wave inversion of shallow refraction data

In order to understand and simulate site effects on strong ground motion records of recent earthquakes in Mexico City, it is fundamental to determine the in situ elastic and anelastic properties of the shallow stratigraphy of the basin. The main properties of interest are the shear wave velocities and Q-quality factors and their correlation with similar parameters in zones of the city. Despite population density and paved surfaces, it is feasible to gather shallow refraction data to obtain laterally homogeneous subsoil structures at some locations. We focused our analysis in the Texcoco Lake region of the northeastern Mexico City basin. This area consists of unconsolidated clay sediments, similar to those of the lake bed zone in Mexico City, where ground motion amplification and long duration disturbances are commonly observed. We recorded Rayleigh and Love waves using explosive and sledgehammer sources and 4.5 Hz vertical and horizontal geophones, respectively. Additionally, for the explosive source, we recorded three-component seismograms using 1 Hz seismometers. We obtained phase velocity dispersion curves from ray parameter-frequency domain analyses and inverted them for vertical distribution of S wave velocity. The initial model was obtained from a standard first-break refraction analysis. We also obtained an estimation of the Q_S shear wave quality factor for the uppermost stratigraphy. Results compare well with tilt and cone penetrometer resistance measurements at the same test site, emphasizing the importance of these studies for engineering purposes.     

抚仙湖历史水位反演与未来30年水位变化预测

在极端气候事件频发和人类活动加剧的背景下,抚仙湖水位波动显著,尤其是2009—2012年极端干旱事件的发生,使抚仙湖平均水位（1721.31 m）低于法定最低水位（1721.65 m）,给生态环境安全带来严重威胁.因此,找到合适有效的湖泊水位模拟方法,对气候变化影响下的未来水位进行预测,并做好相应的应对准备,对湖泊生态系统的保护至关重要.本文运用DYRESM水动力模型对抚仙湖1959—2050年水位进行了模拟.因抚仙湖流域尚无长时间序列的历史水文观测数据,故利用模型和水量补偿法对抚仙湖入湖水量进行反推,构建了降水量-入湖水量的回归方程,并通过有效的实测入湖水量和水位数据,对回归方程的精度进行了检验.利用全球气候模式BCC-CSM2-MR中SSP245和SSP585两种情景提供的未来气候预估数据,运用DYRESM预测了抚仙湖2021—2050年水位变化趋势.结果表明：（1）构建的DYRESM水动力模型和降水-入湖水量回归方程精度较高,模型结果能很好地反映抚仙湖水位的年际和年内变化趋势,且能有效捕捉到抚仙湖的水位峰值.（2）在SSP245和SSP585两种情景下,抚仙湖2021—2050年多年平均水位分别为1722.98和1723.93 m,较1959—2017年平均水位1721.77 m分别升高1.21和2.16 m.两种情景下抚仙湖未来水位均有部分时段超过法定最高蓄水位（1723.35 m）,但均高于法定最低水位.因此,未来气候变化对抚仙湖水量的影响有限,并不会导致水位过低,当水位超过法定最高蓄水位时,可通过控制出流闸门将水位调节在合理范围内.     

柴达木盆地东部地震地面运动放大效应

柴达木盆地是青藏高原东北部大型断陷山间盆地,该地区的流动观测记录了2008年11月10日发生于大柴旦附近的M_W6.3地震。和附近的基岩上的记录相比,盆地内部的记录显示出非常显著的地面运动放大效应,表现为峰值速度的增大、持续时间的延长,其呈现出长持续时间的后续震相。傅里叶频谱分析表明盆地内部显著的后续震相的频率和直达波相比较低,地面质点运动轨迹图显示后续震相为面波运动特征。为了解释地面运动的差异,构建二维模型,通过交错网格高阶有限差分方法计算了地震波在盆地内部的传播过程,结果显示盆地内部低速层的存在造成直达波的放大以及多次反射与转换,盆地边缘结构造成的波的相干叠加产生了强烈的次生面波,其低频、大振幅、长持续时间的特征是盆地内部地面运动放大的主要原因。     

Biodegradation of Petroleum Hydrocarbons in Fractured, Unsaturated Dolomite at a Field Site

Gasoline constituents were detected in unsaturated soil and rock during abandonment of a leaky underground storage tank (UST). The unsaturated sequence beneath the former UST consists of 90 feet of silty till, fractured dolomite, and friable sand-stone. Pore gas probes were installed in each of the unsaturated units, both in the source area and in a background on-site location. Pore gas samples were collected to evaluate the nature, extent, and fate of residual hydrocarbons in the vadose zone. Pore gas from the till and dolomite in the source area was enriched in petroleum hydrocarbons and carbon dioxide, and was depleted in oxygen, relative to pore gas from the background area. During two years of ground water monitoring at the site, methyl tertiary butyl ether was periodically detected in the ground water beneath the source area as pulses of recharge passed through the unsaturated zone, but no other gasoline constituents were detected. Apparently, the most degradable fraction of the gasoline (aromatic hydrocarbons) is being attenuated in the vadose zone before the water table is reached.     

Radiocarbon and optically stimulated luminescence dating of sediments from Lake Karakul,Tajikistan

Lake Karakul in the eastern Pamirs is a large and closed-basin lake in a partly glaciated catchment. Two parallel sediment cores were collected from 12 m water depth. The cores were correlated using XRF analysis and dated using radiocarbon and OSL techniques. The age results of the two dating methods are generally in agreement. The correlated composite core of 12.26 m length represents continuous accumulation of sediments in the lake basin since 31 ka. The lake reservoir effect (LRE) remained relatively constant over this period. High sediment accumulation rates (SedARs) were recorded before 23 ka and after 6.5 ka. The relatively close position of the coring location near the eastern shore of the lake implies that high SedARs resulted from low lake levels. Thus, high SedARs and lower lake levels before 23 ka probably reflect cold and dry climate conditions that inhibited the arrival of moist air at high elevation in the eastern Pamirs. Low lake levels after 6.5 ka were probably caused by declining temperatures after the warmer early Holocene, which had caused a reduction in water resources stored as snow, ice and frozen ground in the catchment. Low SedARs during 23–6.5 ka suggest increased lake levels in Lake Karakul. A short-lived increase of SedARs at 15 ka probably corresponds to the rapid melting of glaciers in the Karakul catchment during the Greenland Interstadial 1e, shortly after glaciers in the catchment had reached their maximum extents. The sediment cores from Lake Karakul represent an important climate archive with robust chronology for the last glacial–interglacial cycle from Central Asia.     

太湖流域1999年特大洪水和对防洪规划的思考

１９９９年太湖流域梅雨期自６月７日入梅,历时４３ｄ,流域面平均梅雨总量６７０ｍｍ,是常年的３倍,致使流域发生了本世纪以来的特大洪水。项平均连续最大７ｄ,１５ｄ,３０ｄ,４５ｄ、６０ｄ,９０ｄ雨量均超过历史暴雨实测最大值,接近或超过了百年一遇。流域降雨空间分布南部大于北部,浙西区、湖区、杭嘉湖区和浦东、浦西区明显大于湖西区和武澄锡区。太湖最高水位达到５．０８ｍ,超达１９９１年最高历史水位０．２９ｍ。