Casing Leakage in Monitoring Wells: Detection, Confirmation, and Prevention

A small but significant proportion of all existing monitoring wells may be affected by leakage through the casing, usually at joints. Casing leakage can render data obtained from a monitoring well unreliable. Anomalous water level, water quality, or isotope data from a particular well are an indication of possible leakage. The occurrence of a casing leak can be confirmed by means of a pressure test using water. The magnitude of the leakage flow can be estimated from the pressure test or from the observed head anomaly. Casing leaks can be largely prevented with adequate care during monitoring well installation, but the possibility that data may be affected by casing leaks should always be taken into account during hydrogeological investigations.     

Study on Active Faults in Quaternary Unconsolidated Sediments by Microstructural Method

Active faults in Quaternary unconsolidated sediments are important indicators to determine paleoearthquake events,They can be studied by macroscopic geological survey,but some problems have been encountered,such as invisible active faults.The authors found an approach to solve these problems by microstructural observation.Firstly,oriented original-state samples of Quaternary Unconsolidated sediments in active fault zones are collected.Secondly,the samples are consolidated in laboratory.Thirdly,microstructureal slides are gound in three-dimension coordinate system.Lastly,microstructures are observed in the microscopic field.By this method,the movement properties of active faults can be determined in lack of the support of macroscopic data.The invisible faults in Quaternary unconsolidated sediments can be found and described.The mechanisms of die-out faults can be also studied.oreover,the boundaries between different unconsolidated sediments in engineering sites can be studied to judge whether they are active faults or not.     

第四纪松散沉积物中活断层滑动面的显微构造研究方法

第四纪松散深积物中的活断层滑动面是重要的古地震遗迹，也是鉴定古地震事件和确定古地震时代的重要标志。以往，以第四纪松散沉积物中活断层滑动面主要借助宏观观察进行研究，但遇到了一些难以解决的问题，如隐性断层问题。作者利用显微构造分析，找到了研究这些问题的途径，其方法是对第四纪松散沉积物进行定向原状样品采集，通过室内样品固结，显微薄片制作和显微构造现象观察等，从显微域里研究各种变形现象。用此方法，可以在没有宏观资料支持的情况下确定断面的运动性质；发现并描述第四纪松散沉积物中的隐性断层，研究断层的尖来机制，还可以对工程建设场地发现的不同松散沉积物相接触的现象进行活断层鉴别。     

Procedures for Sampling Deep Subsurface Microbial Communities in Unconsolidated Sediments

A corehole sampling project utilizing a wireline coring system provided sediment samples for microbiological characterization from deep unconsolidated sediments. Sampling tools were developed or modified to minimize contamination during sample acquisition and to facilitate stringent decontamination requirements. Quality assurance procedures, including the use of tracers, were implemented to minimize and quantify contamination from drilling hardware, drilling fluids and sample processing. Tracers included microspheres, potassium bromide, rhodamine dye, and perfluorocarbons, which enabled the detection and measurement of 1mg of drilling fluid per kg of sediment. In addition, sample processing was performed on-site in an anaerobic chamber to prevent exposure of the subsurface materials to atmospheric oxygen concentrations. Sediment samples were then disbursed to investigators at National Laboratories and universities funded through the Department of Energy Subsurface Science Microbiology Program for microbiological characterization. Results of these efforts demonstrated that representative subsurface samples were collected and disbursed.     

A Multilevel Ground Water Monitoring System for Casing Advance and Bedrock Drilling Methods

A single-hole multilevel sampling piezometer system (MLSPS) has been designed by the Geological Survey of Canada (GSC) to be installed using drilling systems that continuously core (e.g., Rotosonic) or continuously sample (e.g., hollow-stem auger, Becker hammer) overburden and that have the flexibility of allowing additional coring (diamond drilling) or sampling (hammer drilling) of bedrock. The GSC-MLSPS (under license to Solinst Canada Ltd.) uses a patented GSC dry injection system for accurate emplacement of filter packs and seals. This system permits (a) the use of variable screen lengths; (b) the complete evacuation of piezometers before introduction of new ground water (no bailing); (c) the use of a number of types of hydraulic tests (e.g., slug, withdrawal/recovery, vacuum, pressure-pulse); (d) ground water sampling under a nitrogen atmosphere; (e) dissolved gas sampling; (f) a great deal of flexibility in the use of design materials; and (g) the elimination of bridging and collapse of filter packs and seals.     

The Rehabilitation of Monitoring Wells Clogged by Calcite Precipitation and Drilling Mud

Based on aquifer performance tests, 13 out of 15 wells situated at the Mixed Waste Disposal (MWD) area located at the Savannah River site. South Carolina, exhibited high skin factors and low well efficiencies indicative of severely damaged wells. The use of damaged wells in aquifer testing can lead to inaccurate determinations of aquifer properties, and such wells are unusable in future remediation programs. Moreover, damaged wells can go dry during purging, thus compromising sample collection. Pump tests, chemical analyses, and biological investigations revealed that the poor well performance at MWD was attributable to calcite precipitation on the well screen and drilling mud in the filter pack. The calcite problem resulted from improper well installation, and the drilling mud in the filter pack was due to inadequate well development.
Experimental rehabilitation procedures employed on two wells, MWD 5A and 1A, included acidification, swabbing, introduction of surfactants, and surging. Treatment of the wells substantially improved well yields, skin factors, and well efficiencies. Moreover, well rehabilitation was determined to be a reasonable alternative to drilling new wells at the MWD wellfield.     

关于地震地下水观测井建设的若干问题

根据有关的国家与地震行业技术标准,结合建井的实践经验,本文论述了地震地下水观测井的定位、观测井结构设计与钻井施工的技术要求。     

An Inexpensive, Multi-Use, Dedicated Pump for Ground Water Monitoring Wells

A simple, inexpensive sampling pump has lately come into use in ground water monitoring. The pump is referred to as an inertial pump; its only downhole components are a foot valve connected to a length of tubing or pipe. The operating principle of the pump is based on the inertia of a column of water within the riser tubing. Ground water is drawn through the foot valve and up the riser tubing by rapid up and down movements of the tubing. This pumping method is not new, but has only recently been applied to monitoring wells. Foot valves are available in a variety of materials and sizes and can be used in monitoring wells as small as 19mm (3/4 inch) I.D. Flexible polyethylene or Teflon® tubing, and in some cases stainless steel tubing or rigid PVC pipe, is used as the riser. The inertial pump satisfies most of the criteria normally cited for an "ideal" sampling device. The pump is easy to operate, reliable, durable, portable, and virtually maintenance-free. It can be operated manually from as deep as 40m or from as deep as 60m using a motor drive. The pump is inexpensive, and therefore suitable for use as a dedicated sampling pump. Recent tests have shown the pump to be suitable for sampling volatile organics. The inertial pump has a high flow capacity and performs well in silty/sandy environments, which makes it useful for developing and purging monitoring wells. It may also be used to perform field hydraulic conductivity tests.     

The Effect of Construction, Installation, and Development on the Turbidity of Water in Monitoring Wells in Fine-Grained Glacial Till

Twenty monitoring wells were installed in fine-grained glacial till at two sites in southeastern Wisconsin to study the effects of monitoring well construction, installation, and development on the amount of fine-grained suspended material in the well. The types of well screens used were continuous slot, factory slot, factory slot with a filter wrap, and porous piezometer tips. Some of the wells were installed before the open borehole began to fill with water; others were installed after the water levels in the lower section of the borehole had begun to rise. About half of the wells were developed by surging while the others were simply bailed without surging. Installation of the wells in the initially dry holes resulted in wells that yielded samples with very low turbidity compared to wells installed in wet holes. Water samples from wells that were surged were more turbid than those that were not. The type of construction materials investigated had no effect on the turbidity of samples from the wells.     

Types and Usages of Drilling Fluids Utilized to Install Monitoring Wells Associated with Metals and Radionuclide Ground Water Studies

Evaluation for metal and radionuclide contamination associated with selected inactive uranium mill tailings repositories required the installation of monitoring wells in both shallow unconfined and deep artesian aquifer systems. To prevent ionic exchange between the dissolved cationic metals and the cations in bentonitic drilling fluids, organic-based fluids were used during the drilling and installation of monitoring wells. Experience gained at three western U.S. sites involving inactive uranium mill tailings indicates organic drilling fluids can be used to advance drill holes in soil materials ranging from saturated silts and fine sands to unsaturated gravels. However, it has been determined that certain types of these organic fluids can clog very narrow screen slots and remain present in well discharge, even after several hours of well cleaning. Certain types of organic drilling fluids appear to be preferable for these types of water chemistry studies.     

In-Well Degassing of Monitoring Wells Completed in Gas-Charged Aquifers

Total dissolved gas pressure (P_TDG) measurements are useful to measure accurate in situ dissolved gas concentrations in groundwater, but challenged by in-well degassing. Although in-well degassing has been widely observed, its cause(s) are not clear. We investigated the mechanism(s) by which gas-charged groundwater in a recently pumped well becomes degassed. Vertical P_TDG and dissolved gas concentration profiles were monitored in the standing water column (SWC) of a groundwater well screened in a gas-charged aquifer for 7 days before and 15 days after pumping. Prior to pumping, P_TDG values remained relatively constant and below calculated bubbling pressure (P_BUB) at all depths. In contrast, significant increases in P_TDG were observed at all depths after pumping was initiated, as fresh groundwater with elevated in situ P_TDG values was pumped through the well screen. After pumping ceased, P_TDG values decreased to below P_BUB at all depths over the 15-day post-pumping period, indicating well degassing was active over this time frame. Vertical profiles of estimated dissolved gas concentrations before and after pumping provided insight into the mechanism(s) by which in-well degassing occurred in the SWC. During both monitoring periods, downward mixing of dominant atmospheric and/or tracer gases, and upwards mixing of dominant groundwater gases were observed in the SWC. The key mechanisms responsible for in-well degassing were (i) bubble exsolution when P_TDG exceeded P_BUB as gas-charged well water moves upwards in the SWC during recovery (i.e., hydraulic gradient driven convection), (ii) microadvection caused by the upward migration of bubbles under buoyancy, and (iii) long-term, thermally driven vertical convection.     

The Utility of Multiple-Completion Monitoring Wells for Describing a Solvent Plume

At a study site in the midwestern United States, multiple-completion wells demonstrated that a vertical hydraulic gradient was responsible for the contamination pattern exhibited by chlorinated solvent plumes. The typical pattern consisted of little or no contamination in the upper portion of the aquifer with concentrations increasing with depth. When ground water contamination was discovered in an unexpected portion of the site, water level elevations and contaminant distribution data obtained from multiple-completion wells resulted in identification of the source location. The well eventually determined to be located in the source area displayed contaminant levels much higher in the upper zone of the aquifer — the opposite contamination pattern of other on-site wells. Such results indicated that the spill had occurred near this location and that solvent residing along the capillary fringe was continuing to contaminate the aquifer.     

Design for a Packer/Vacuum Slug Test System for Estimating Hydraulic Conductivity in Wells with LNAPLs, Casing Leaks, or Water Tables Intersecting the Screens

"Results indicate that the packer/vacuum system is an adequate method for obtaining values of hydraulic conductivity."     

A Comparison of Sampling Mechanisms Available for Small-Diameter Ground Water Monitoring Wells

The objective of most ground water quality monitoring programs is to obtain samples that are "representative" or that retain the physical and chemical properties of the ground water in an aquifer. Many factors can influence whether or not a particular sample is representative, but perhaps the most critical factor is the method or type of sampling device used to retrieve the sample.
The sampling equipment available today ranges from simple to highly sophisticated, and includes bailers, syringe devices, suction-lift pumps, gas-drive devices, bladder (Middelburg-type) pumps, gear-drive and helical rotor electric submersible pumps and gas-driven piston pumps. New devices are continually being developed for use in small-diameter wells in order to meet the needs of professionals engaged in implementing elaborate ground water monitoring programs.
In selecting a sampling device for a monitoring program, the professional must consider a number of details. Among the considerations are: the outside diameter of the device, the overall impact of the device on ground water sample integrity (including the materials from which the sampling device and associated equipment are made and the method by which the device delivers the sample), the capability of the device to purge the well of stagnant water, the rate and the ability to control the rate at which the sample is delivered, the depth limitations of the device, the ease of operating, cleaning and maintaining the device, the portability of the device and required accessory equipment, the reliability and durability of the device, and the initial and operational cost of the device and accessory equipment. Based on these considerations, each of the devices available for sampling ground water from small-diameter wells has its own unique set of advantages and disadvantages that make it suitable for sampling under specific sets of conditions. No one sampling device is applicable to all sampling situations.