Estimates of Horizontal Groundwater Flow Velocities in Boreholes

Currently, monitoring tools can be deployed in observation boreholes to better assess groundwater flow, flux of dissolved contaminants and their mass discharge in an aquifer. The relationship between horizontal water velocity in observation boreholes and Darcy fluxes in the surrounding aquifer has been studied for natural flow conditions (i.e., no pumping). Interpretation of measurements taken with dilution tests, the colloidal borescope, the Heat Pulse Flowmeter, and other techniques require the conversion of observed borehole velocity u to aquifer Darcy flux q_∞ . This conversion is typically done through a proportionality factor α = u/q_∞ . In experimental studies as well as in theoretical developments, reported values of α vary almost three orders of magnitude (from 0.5 to 10). This large variability in reported values of α could be explained by: (1) unclear distinction between Darcy flux and water seepage velocity, (2) unclear definition of water velocity in the borehole, (3) effects of well screen and the presence of the measurement device itself on the observable velocities, and (4) hydraulic conditions in the borehole annulus. We address (1), (2) from a conceptual/theoretical perspective, and (3) by means of numerical simulations. We show that issue (1) in low porosity aquifers can yield to order-of-magnitude discrepancies in estimates of q_∞ ; (2) may result in discrepancies of up to 50%, and (3) can cause differences up to 20% of water velocity in the borehole void space compared to the theoretical case of an open borehole.     

金沙江水网各观测井温度梯度的精细测量结果及其分析

Fine measurements have been conducted to temperatures and their gradients of six wells of the Jinsha River Groundwater Observational Network.The results show that the influence depths of sun radiation heat are 50m to 125m,average temperature gradients in the wells range from 0.11 to 2.81℃/hm and most are 1～2℃/hm,and the temperature gradients on varied depth sections of one well are highly changeable.Lithology of strata and their integrity,particularly high-angle crashed fault zones,have imposed major effects on the influence depths of sun radiation heat and temperature gradients of the wells.The micro dynamic characteristics of water temperature,such as coseismic effects,tidal effects and anomalies of the wells prior to earthquakes,probably depend,to a large degree,on the temperature gradients of the depths at which the water temperature sensors are settled.     

Using Hydrochemical Fades to Delineate Ground Water Flowpaths in Fractured Shale

Large differences in chemistry between sampling points separated In short vertical intervals are often observed in contaminant plumes in both granular and fractured aquifers. However, most regional models assume that such differences will be reduced by dispersive mixing during transport. At a field site located in a discharge area on the Oak Ridge Reservation, Tennessee, ground water flows along discrete flowpaths, as evidenced by the presence of four distinct water types—Ca-HCO₃, Ca-Na-HCO₃, and Na-Ca-HCO₃, and Na-Ca-HCO₃-S0₄—in samples collected from shallow (< 3D in) multilevel wells. The preservation of distinct chemical signatures suggests that ground water must he contained in discrete flow zones during much of its transport time. The chemical composition of the water types can be explained primarily by strata-bound flow over varying flowpath lengths and secondarily by mixing of waters during cross-formational flow in a discharge zone. The hydrochemical facies identified by correlation of water types between the boreholes indicate the general orientation of ground water How paths. These inferred flowpaths are oblique to the orientation of the measured hydraulic gradient and are more closely aligned with bedding and the calculated flow direction. Results of this study indicate that discrete multilevel sampling for analysis of major ions, in addition to information gathered from tracer tests, borehole flow tests. and visual core observations, can provide valuable information on flow directions and preferential flowpaths for contaminant transport.     

Stream Temperature Modeling and Fiber Optic Temperature Sensing to Characterize Groundwater Discharge

The Ngongotaha Stream was used as a case study to assess the applicability of fiber optic distributed temperature sensing (FODTS) to identify the location of springs and quantify their discharge. Thirteen springs were identified, mostly located within a 115 m reach, five discharged from the right bank and eight from the left bank. To quantify groundwater discharge, a new approach was developed in which the one-dimensional transient heat transport model was fitted to the FODTS measurements, where the main calibration parameters of interest were the unknown spring discharges. The spatial disposition of the groundwater discharge estimation problem was constrained by two sources of information; first, the stream gains ∼500 L/s as determined by streamflow gauging. Second, the temperature profiles of the left and right banks provide the spatial disposition of springs and their relative discharges. FODTS was used to measure stream temperature near the left and right banks, which created two temperature datasets. A weighted average of the two datasets was then calculated, where the weights reflected the degree of mixing between the right and left banks downstream of a spring. The new approach in this study marks a departure from previous studies, in which the general approach was to use the steady-state thermal mixing model (Selker et al. 2006a; Westhoff et al. 2007; Briggs et al. 2012) to infer groundwater discharge, which is then used as an input into a transient model of the general form of equation to simulate stream temperature (Westhoff et al. 2007).     

Derivation Of Refractive Index And Temperature Gradients From Optical Scintillometry To Correct Atmospherically Induced Errors For Highly Precise Geodetic Measurements

Refraction effects of optical beams are generally caused by an inhomogeneous propagation medium and are a major source of systematic errors in the precise optical determination of angles and distances in the atmospheric surface layer. In this contribution a method for deriving vertical temperature and refractive index gradients from optical scintillation is presented. Knowledge of these gradients is required for the compensation of atmospherically induced errors for highly precise terrestrial geodetic measurements, like direct transfer and levelling. The advantage of the present optical method is, that temperature and refractive index gradients can be derived as line-averaged values over the propagation path, which is not possible by meteorological point measurements. Field observations have been carried out with a displaced-beam scintillometer over flat terrain and under different atmospheric conditions in order to verify this method. The experiments show, that this method allows to derive accurate correction values for precise terrestrial geodetic measurements.     

Distributed Temperature Sensing to Measure Infiltration Rates Across a Groundwater Recharge Basin

Managed aquifer recharge is used to augment groundwater resources and provide resiliency to water supplies threatened by prolonged droughts. It is important that recharge facilities operate at their maximum efficiency to increase the volume of water stored for future use. In this study, we evaluate the use of distributed temperature sensing (DTS) technology as a tool to measure high-resolution infiltration rates at a large-scale recharge facility. Fiber optic cable was laid out inside a spreading basin in a spiral pattern, at two different depths. The cables measured the propagation of diurnal surface water temperature oscillations into the basin depth. The rate of heat propagation is proportional to the velocity of the water, making it possible to estimate the infiltration rate from the temperature measurements. Our results showed that the infiltration rate calculated from DTS, averaged over the entire basin, was within 5% of the infiltration rate calculated using a conventional metering method. The high-resolution data obtained from DTS, both spatially and temporally, revealed heterogeneous infiltration rates throughout the basin; furthermore, tracking the evolution of infiltration rates over time revealed regions with consistently high infiltration rates, regions with consistently low infiltration rates, and regions that evolved from high to low rates, which suggested clogging within that region. Water utilities can take advantage of the high-resolution information obtained from DTS to better manage recharge basins and make decisions about cleaning schedule, frequency, and extent, leading to improved basin management strategies, reduced O&M costs, and increased groundwater recharge.     

Tidally Induced Water Temperature Change in Artesian Wells

Water temperature observation in wells,as a precursor observation project in China,aims to capture stress-strain information during the preparation of earthquakes. The study of the water temperature tidal effect has important significance,for water temperature tidal effect is both a reflection of stress-strain in crust and an interference factor of precursor observations. With a view of thermal conditions in artesian wells,the paper holds that there are two mechanisms for interpreting tidally induced water ...     

GC/MS Nontarget Analysis to Examine an Organic Groundwater Contamination. Part II: Graphical and Multivariate Methods for Searching Key Substances

GC/MS Nontarget Analysis to Examine an Organic Groundwater Contamination. Part II: Graphical and Multivariate Methods for Searching Key Substances In the nontarget analysis, a maximum of organic substances was extracted by a sequence which separates high polar, medium polar, and nonpolar compounds. This leads to the detection of 477 different organic substances in the example of the groundwater contamination investigated. To reduce the high expense for an exact identification of 477 compounds as a first step the individual compound is defined as a data set of retention time and the mass spectrum belonging to this retention time. The table of data contains now 477 individual compounds in groundwater samples collected at 10 different locations. The application of mathematical filters helps to reduce the size of the data set. Graphical methods enable large amounts of data to be visualized in a clear manner and enable to detect patterns in a data set. These patterns are the key to select typical compounds as indicator substances for the contamination source as well as the geogenic background. Similarities between the groundwater samples should not be changed by selection of the indicator compounds. Therefore, cluster analysis was applied as a controlling instrument for the final selection of the indicator compounds. The combination of graphical and multivariate data analysis is a useful tool to deduce indicator compounds for monitoring and control of complex environmental pollution states.     

汶川地震前井水温度异常信息的识别及其特征

通过对我国地震地热台网273口观测井的水温数据进行梳理,发现汶川地震前有10口井记录到客观的异常现象;而这些井的异常信息的持续时间、幅度、空间分布极其复杂,决定了利用这些异常信息进行汶川地震三要素的准确预测还不太现实,但也发现一些有一定科学价值的认识:空间上井震距1000km范围内集中了大部分异常点;异常幅度集中在千分之几到百分之几度;时间上异常具有短临性质,呈现出井震距越大持续时间越短、井震距越小持续时间越长的规律;自流井记录到的异常概率远高于静水位井,不过干井也可以记录到异常信息。通过数据梳理与分析,提出了水温异常取决于井-含水层结构以及观测井所处的水系单元位置这一认识。     

Effects of Changing Meteoric Precipitation Patterns on Groundwater Temperature in Karst Environments

Climate predictions indicate that precipitation patterns will change and average air temperatures will increase across much of the planet. These changes will alter surface water and groundwater temperatures which can significantly affect the local and regional environment. Here, we examine the role of precipitation timing in changes to groundwater temperature in carbonate‐karst aquifers using measured groundwater level and temperature data from the Konza Prairie Long‐Term Ecological Research Site, Kansas. We demonstrate that shifts to increased cool‐season precipitation may mitigate the increases in groundwater temperature produced by increases in average annual air temperature. In karst, the solution‐enlarged conduits allow faster and focused recharge, and the recharge‐event temperature can strongly influence the groundwater temperature in the aquifer. Our field data and analysis show that predictions of future groundwater conditions in karst aquifers need to consider changes in precipitation patterns, in addition to changes to average annual air temperature.     

江苏井网水温水位对几次大地震的同震响应特征及机理分析

本文以2008一2013 年期间发生的几次重大地震为例,对江苏流体井网同震响应的特征进行了分析并对其机理进行了探讨。结果表明,水位的同震变化以振荡型为主,水温以阶跃或趋势变化为主;且同一口井对不同地震其响应形态相同,不受地震方位和震源机制的影响。进一步分析表明,水位和水温的变化幅度随着震级的增大而增大,随着震中距的增大而减小。最后分别对静水位井和动水位井的水位、水温同震响应机理进行了分析。     

Groundwater Deterioration of Shallow Groundwater Aquifers Due to Overexploitation in Northeast Jordan

Groundwater is the major water resource in Jordan and most of the groundwater basins are already exploited beyond their estimated safe yield. Azraq basin is one of the most important groundwater basins in Jordan, which supplies Amman with drinking water. However, due to overpumping from the shallow groundwater aquifers, the water level dropped dramatically and signs of salinization and depletion are starting to occur. The severe drawdown in the Azraq well‐field caused a reverse in the hydraulic gradient and consequently, the saltwater in the center of the basin (Qa‐Azraq) started to move in the direction of the well‐field. The salinization in the shallow aquifer (basalt/B5/B4) is believed to result from one of the following scenarios: (i) a reverse flow from Sabkha to the AWSA well field, (ii) an upward leakage from the middle aquifer system (B2/A7) and the combined B3 Aquitard‐B2/A7 aquifer, (iii) a dissolution process between the water and rock matrix due to lowering of the dynamic water levels during pumping which reached the mineralized formations underlying the Basalt. The salinization trend of some AWSA wells represented by the gradual increase of major ions is associated with rather constant stable isotopic contents. This indicates that these constituents originate from the main minerals existing in the matrix of the aquifers and thus this scenario is the most likely to occur.     

Impact of Temperature on Groundwater Source Attenuation Rates at Hydrocarbon Sites

The temperature sensitivity of microbial populations is reflected in measured source attenuation rates at hydrocarbon‐impacted sites. The objective of this study was to evaluate the correlation between temperature and source attenuation rates (concentration vs. time attenuation rate over many years) of benzene and toluene by analyzing groundwater monitoring data from >2000 hydrocarbon sites. Historical monitoring records were obtained from three databases, processed to yield long‐term multiyear source attenuation rates, and then compared with representative temperatures at each site. Statistically significant and positive relationships between temperature and source attenuation rates were established for benzene and toluene, indicating that temperature does impact hydrocarbon degradation, but is one of many factors that contribute to source attenuation. There was an observed 1.1 to 1.6 times increase in attenuation rates per 10 °C increase in temperature, which is less than the rate increases predicted by the Arrhenius equation. The temperature dependence on attenuation rate is consistent with several lines of evidence that methanogenesis plays a key role in the rate of hydrocarbon source zone attenuation rather than being controlled strictly by the availability of electron acceptors. First, methanogenesis is known to be strongly influenced by temperature, with significantly higher rates up to about 35 °C. Second, the temperature‐degradation rate relationship was stronger at sites with deeper water tables (>30 ft) that are less susceptible to oxygen influx than sites with shallow water tables (<15 ft). Third, dissolved methane concentrations were higher at sites with warmer temperatures. Overall, these results provide indirect support for a conceptual model where methanogenesis is a key degradation process at hydrocarbon sites, and that attenuation of these source zones is temperature‐sensitive.     

Co-Seismic Groundwater Level Changes Induced by the May 12, 2008 Wenchuan Earthquake in the Near Field

The large scales of co-seismic water level changes in mainland China were observed in response to the tragic 2008 Ms 8.0 Wenchuan earthquake. To better understand the mechanism of these hydrogeological phenomena, groundwater-level data at 17 confined wells, with an epicentral distance of <500 km, were collected. We compare the static strain predicted by dislocation theory with the volumetric strain calculated by the tide effect of the groundwater based on poroelastic theory. The results show that the sign of the co-seismic groundwater level change is consistent with the sign predicted by dislocation theory. Additionally, the magnitude of the strain calculated by the two methods is also concordant in half of the wells. In the rest of the wells, the strains inversed from the groundwater level are one or two orders of magnitude larger than the fault dislocation model. These wells mostly have an epicenter distance larger than 300 km; therefore, the dynamic stress induced by the seismic wave may be responsible for the co-seismic water level changes in these wells. According to these results, we roughly estimate that the effect range of the static stress is approximately 300 km for the Wenchuan earthquake, and the dynamic stresses dominate beyond this epicenter distance. In addition, geological and hydrogeological conditions and other mechanisms may be responsible for these changes.