An analysis of the chemical and microbiological quality of ground water from boreholes and shallow wells in Zimbabwe

Groundwater from boreholes and shallow wells is a major source of drinking water in most rural areas of Zimbabwe. The quality of groundwater has been taken for granted and the status and the potential threats to groundwater quality have not been investigated on a large scale in Zimbabwe. A borehole and shallow well water quality survey was undertaken between January, 2009 and February, 2010 to determine the chemical and microbial aspects of drinking water in three catchment areas. Groundwater quality physico-chemical indicators used in this study were nitrates, chloride, water hardness, conductivity, alkalinity, total dissolved solids, iron, magnesium, manganese, potassium, calcium, fluoride, sulphates, sodium and pH. The microbiological indicators were total coliforms, faecal coliforms and heterotrophs. Principal component analysis (PCA) showed that most of the variation in ground water quality in all catchment areas is accounted for by Total Dissolved Solids (TDS), electrical conductivity (EC), sodium, bicarbonate and magnesium. The principal dissolved constituents in ground water are in the form of electrically charged ions. Nitrate is a significant problem as the World Health Organization recommended levels were exceeded in 36%, 37% and 22% of the boreholes in the Manyame, Mazowe and Gwayi catchment areas respectively. The nitrate levels were particularly high in commercial farming areas. Iron and manganese also exceeded the recommended levels. The probable source of high iron levels is the underlying geology of the area which is dominated by dolerites. Dolerites weather to give soils rich in iron and other mafic minerals. The high level of manganese is probably due to the lithology of the rock as well as mining activity in some areas. Water hardness is a problem in all catchment areas, particularly in the Gwayi catchment area where a value of 2550 mg/l was recorded in one borehole. The problems with hard water use are discussed. Chloride levels exceeded the recommended levels in a few areas under irrigation. Most of the chloride is probably from agricultural activity particularly the application of potassium chloride. Fluoride levels were particularly elevated in the Gwayi catchment area and this is because of the geology of the area. There was no evidence of microbial contamination in all the boreholes sampled as the total coliform, faecal coliforms, heterotrophs count was nil. However, severe microbial contamination was found in the wells especially those in clay areas.     

Pestizide in Oberflächenwässern — Befunde in Talsperren und in Uferfiltraten Pesticides in Surface Water — Analytical Results for Drinking Water Reservoirs and Bank Filtrate Waters

The results of a series of investigations are presented, performed by the governmental chemical laboratory Karlsruhe (CLUA) in order to determine pesticides in drinking water for purposes of food monitoring. 231 samples of 8 municipal water supplies (groundwater wells/no bank filtrate) and 34 private water supplies close to the bank of the river Rhine (groundwater wells/partly bank filtrate) were analysed. The sampling sites were located between Mannheim and Greffern, Germany. In 1986 high amounts of atrazine and simazine were determined in the drinking water wells of several private water supplies, especially at the sampling sites in the south. The likewise high chloride content of the samples was taken as a proof of bank filtrate contributing to the water. The decrease of atrazine load in the river water of the Rhine since 1987 has its parallel in the analytical results for the concerned drinking water wells. Obviously the pesticide contamination of water from bank filtrate pumping wells is mainly depending on the pesticide load of the river water. The application of fertilizers and pesticides in the fields seems not to be a significant source of the pesticide contamination of dringing waters derived from bank filtrate water. For reasons of public health a monitoring of pesticide content in bank filtrate pumping wells of private water supplies is recommended.     

Ground and surface water quality along a dambo transect in Chihota smallholder farming area,Marondera district,Zimbabwe

In many smallholder farms in sub-Saharan Africa dambos are used for grazing and crop production especially horticultural crops. Increased use of dambos especially for crop production can result in ground and surface water pollution. Ground and surface water quality along a dambo transect in Chihota, Zimbabwe, was investigated between October 2013 and February 2014. The transect was divided into; upland (control), dambo gardens (mid-slope) and the river (valley bottom). Water samples for quality assessment were collected in October 2013 (peak of dry season) and February 2014 (peak of rainy season). The collected water samples were analysed for pH, faecal coliforms, total nitrogen, electrical conductivity, total dissolved solids (TDS), and some selected nutrients (P, K, Ca, Mg, Na, Zn, and Cu). Water pH was 7.0, 6.4 and 6.1 for river water, garden and upland wells respectively. During the wet season total nitrogen (TN) concentrations were 233 mg/L for uplands, 242 mg/L for gardens and 141 mg/L for the river. During the dry season, TN concentrations were all below 20 mg/L, and were not significantly different among sampling stations along the dambo transect. Dry season faecal coliform units (fcu) were significantly different and were 37.2, 30.0 and 5.0 for upland wells, garden wells and river respectively. Wet season faecal coliforms were also significantly different and were 428.5, 258.0 and 479.4 fcu for upland wells, garden wells and river respectively. The other measured physico-chemical parameters also varied with sampling position along the transect. It was concluded that TN and fcu in sampled water varied with season and that wet season concentrations were significantly higher than dry season concentrations. High concentrations of faecal coliforms and total N during the wet season was attributed to increased water movement. Water from upland wells, garden wells and river was not suitable for human consumption according to WHO standards during both the dry and wet seasons.     

Long-Term Confidence in Ground Water Monitoring Systems

State-of-the-art analytical techniques are capable of detecting contamination In the part per billion (ppb) range or lower. At these levels, a truly representative ground water sample Is essential to precisely evaluate ground water quality. The design specifications of a ground water monitoring system are critical in ensuring the collection of representative samples, particularly throughout the long-term monitoring period.
The potential interfaces from commonly used synthetic well casings require a thorough assessment of site, hydrogeology and the geochemical properties of ground water. Once designed, the monitoring system must be installed following guidelines that ensure adequate seals to prevent contaminant migration during the installation process or at some time in the future. Additionally, maintaining the system so the wells are in hydraulic connection with the monitored zone as well as periodically Inspecting the physical integrity of the system can prolong the usefulness of the wells for ground water quality. When ground water quality data become suspect due to potential interferences from existing monitoring wells, an appropriate abandonment technique must be employed to adequately remove or destroy the well while completely sealing the borehole.
The results of an inspection of a monitoring system comprised of six 4-inch diameter PVC monitoring wells at a hazardous well facility Indicated that the wells were improperly installed and in some cases provided a pathway for contamination. Subsequent down hole television inspections confirmed inaccuracies between construction logs and the existing system as well as identified defects in casing materials. An abandonment program was designed which destroyed the well casings in place while simultaneously providing a competent seal of the re-drilled borehole.     

Utilization of Shallow Geophysical Sensing at Two Abandoned Municipal/Industrial Waste Landfills on the Missouri River Floodplain

In the past 30 to 40 years, floodplain areas of large rivers, such as the Missouri River, have been extensively used for large industrial and municipal landfills. Many of these sites are now causing varying degrees of ground water contamination. Rapid geophysical characterization techniques have proven useful for delineation of anomalous areas indicative of potential contaminant plumes. These methods have also resulted in a cost effective approach to the location and number of monitoring wells.
An effective technique to initially characterize ground water contamination at such landfills along the Missouri River in northwestern Missouri involved a combination of electrical resistivity and electromagnetic conductivity methods. Resistivity was used to obtain soundings of the alluvium by using a modified Wenner array and to corroborate shallow electromagnetic conductivity measurements by using short Wenner array electrode spacings.
Upon confirmation of similar measurements of the upper soils for the two methods, numerous electromagnetic conductivity traverses were made at each landfill site. The data generated from these surveys were graphed and contoured to delineate anomalous areas. Based on the geophysical study, a ground water monitoring well network was then designed for each landfill.
As a result, a minimal number of wells were required to initially characterize the ground water quality at these two sites. In general, analysis of water samples from these wells displayed good correlation with the geophysical results.     

Organic Halogen Group Parameters as Indicators of Ground Water Contamination

A regional survey of Danish ground water demonstrated the presence of adsorbable organic halogens (AOX) in almost all of 142 wells (99 percent). Generally, the presence of AOX was not related to point or non-point source contamination with halogenated organics. However, the AOX concentrations varied with the geology of the aquifers. Extractable organic halogens (EOX) and volatile organic halogens (VOX) were far less prevalent (detected in 4 percent of sampled wells) and the detection could, in most cases, be explained by contamination or chlorination of the wells. The VOX concentrations corresponded to the concentrations of identified, volatile contaminants. The study demonstrates the presence of a natural background level of AOX in the investigated aquifers. This must be considered in the interpretation of AOX results as an indicator of ground water contamination with haloorganics. Similar background levels of EOX or VOX were not delected.     

Contamination from Sand-Bentonite Seal in Monitoring Wells Installed in Aquitards

A six year field experiment has shown that a sand-bentonite mixture used to seal monitoring wells in aquitards contributes solutes to the ground water sampled from these wells. Monitoring wells were installed at field sites with hydraulic conductivity (K) ranging from 5 × 10 ^-9 m/s to 3 × 10¹¹ m/s. In most cases the boreholes remained dry during installation which allowed the placement of a dry powdered bentonite/sand mixture tagged with potassium bromide (KBr) to seal and separate sampling points. Over six years, wells were sampled periodically and ground-water samples were analyzed for Br and Cl and other major ions. Typical Br results ranged from 10 mg/1 to 35 mg/1 in the first 700 days, as compared to an estimated initial concentration in the seal material of about 75 mg/1. After six years the bromide concentrations had decreased to between 3 mg/1 and 5 mg/1. The total mass of Br removed in six years is less than 50% of that placed; therefore the contamination effects, although considerably diminished, persist. The trends of Br, Cl, Na, and SO₄ indicate that varying degrees of contamination occur. These data show that the materials used to seal monitoring wells in aquitards can have a significant and long-lasting impact on the chemistry of the water in the wells.     

Free-Phase Gas Detection in Groundwater Wells via Water Pressure and Continuous Field Parameters

Detection of free-phase gas (FPG) in groundwater wells is critical for accurate assessment of dissolved gas concentrations and the occurrence of FPG in the subsurface, with consequent implications for understanding groundwater contamination and greenhouse gas emissions. However, identifying FPG is challenging during routine groundwater monitoring and there is poor agreement on the best approach to detect the occurrence of FPG in groundwater. In this study, laboratory experiments in a water column were designed to mimic nonflowing and flowing conditions in a groundwater well to evaluate how the presence of FPG affects water pressure and commonly used continuous field parameters. The laboratory results were extrapolated to interpret field data at an abandoned exploration well with episodic release of free-gas CO₂. The FPG effect on water pressure varied between flowing and nonflowing wells, and depending on whether the FPG was above or below the sensor. Electrical conductivity values were decreased and/or behaved erratically when FPG was present in the water column. Findings from this study have shown that the combined measurement of water pressure, electrical conductivity, and total dissolved gas pressure can provide information about the occurrence of FPG in groundwater wells. Measurement of these parameters at different depths can also provide information about relative depths and amounts of FPG within the well water column. This approach can be used for long-term monitoring of groundwater gases, managing gas-locking in production wells with gassy groundwater, and measuring fugitive greenhouse gas emissions from groundwater wells.     

Vulnerability of a public supply well in a karstic aquifer to contamination

To assess the vulnerability of ground water to contamination in the karstic Upper Floridan aquifer (UFA), age-dating tracers and selected anthropogenic and naturally occurring compounds were analyzed in multiple water samples from a public supply well (PSW) near Tampa, Florida. Samples also were collected from 28 monitoring wells in the UFA and the overlying surficial aquifer system (SAS) and intermediate confining unit located within the contributing recharge area to the PSW. Age tracer and geochemical data from the earlier stage of the study (2003 through 2005) were combined with new data (2006) on concentrations of sulfur hexafluoride (SF₆), tritium (³H), and helium-3, which were consistent with binary mixtures of water for the PSW dominated by young water (less than 7 years). Water samples from the SAS also indicated mostly young water (less than 7 years); however, most water samples from monitoring wells in the UFA had lower SF₆ and ³H concentrations than the PSW and SAS, indicating mixtures containing high proportions of older water (more than 60 years). Vulnerability of the PSW to contamination was indicated by predominantly young water and elevated nitrate-N and volatile organic compound concentrations that were similar to those in the SAS. Elevated arsenic (As) concentrations (3 to 19 μg/L) and higher As(V)/As(III) ratios in the PSW than in water from UFA monitoring wells indicate that oxic water from the SAS likely mobilizes As from pyrite in the UFA matrix. Young water found in the PSW also was present in UFA monitoring wells that tap a highly transmissive zone (43- to 53-m depth) in the UFA.     

Low-Cost Apparatus for On-Site Monitoring of Methane in Ground Water

An upsurge in oil- and gas-well drilling in northwestern Pennsylvania and western New York has been accompanied by several incidents of contamination of ground water by methane. Determining which well is causing the contamination is extremely difficult if more than one gas or oil well is present in the area.
The fact that the solubility of methane decreases as the pressure on ground water decreases provides a quantitative basis for monitoring changes in the amount of methane in the ground water. Quantitative measurements of the volume of methane given off by ground water pumped from a well as the water enters atmospheric pressure permit detection of temporal changes in the gas content which are too subtle to be detected visually. These gas volume changes may, in some cases, be correlated with variations in the pressure of methane in the annulus of nearby individual gas/oil wells and thus may provide a means of pinpointing the gas/oil well that is causing the methane contamination.
The basic principle of the gas-volume monitoring apparatus (GVMA) described in this paper is that as a measured amount of ground water enters atmospheric pressure the gas which comes out of solution is trapped and measured. The GVMA can be constructed of materials costing less than $100 and requires no special skills to assemble or operate. In a recent study conducted in a western New York village, four homeowners were able to collect quantitative gas-volume data from their household water wells daily in about one-half hour. Unlike laboratory analyses for dissolved methane, there is no cost involved in monitoring with the GVMA beyond the initial instrument cost and operator time. Another advantage is that the data are available immediately.     

Vertical Contaminant Profiling of Volatile Organic* in a Deep Fractured Basalt Aquifer

Volatile organic compounds delected in ground water from wells at Test Area North (TAN) at the Idaho National Engineering Laboratory (INEL) prompted RCRA facility investigations in 1989 and 1990 and a CERCLA-driven RI/FS in 1992. In order to address ground water treatment feasibility, one of the main objectives, of the 1992 remedial investigation was to determine the vertical extent of ground water contamination, where the principle contaminant, of concern is trichloroethylene (TCE). It was hypothesized that a sedimentary interbed at depth in the fractured basalt aquifer could be inhibiting vertical migration of contaminants to lower aquifers. Due to the high cost of drilling and installation of ground water monitoring wells at this facility (greater than $100,000 per well), a real time method was proposed for obtaining and analyzing ground water samples during drilling to allow accurate placement of well screens in zones of predicted VOC contamination. This method utilized an inflatable pump packer pressure transducer system interfaced with a datalogger and PC at land surface. This arrangement allowed for real lime monitoring of hydraulic head above and below the packer to detect leakage around the packer during pumping and enabled collection of head data during pumping for estimating hydrologic properties. Analytical results were obtained in about an hour from an on-site mobile laboratory equipped with a gas chromalograplvmass spectrometer (GC/MS). With the hydrologic and analytical results in hand, a decision was made to either complete the well or continue drilling to the next test zone. In almost every case, analytical results of ground water samples taken from the newly installed wells closely replicated the water quality of ground water samples obtained through the pump packer system.     

Vertical Delineation of Contamination in Fractured Bedrock Aquifer

The New Jersey Department of Environmental Protection's Technical Regulations require the horizontal and vertical delineation of contamination. Monitor wells screened at increasingly deeper intervals are used to delineate vertical contamination. In New Jersey, the open interval in a bedrock well cannot exceed 7.6 m. Since contamination has been found at depths as great as 91.4 m in a production well in the study area, it would be prohibitively expensive to install monitor wells with 7.6 m open holes at ever-increasing depths until no contamination was found. Isolation of discrete zones in boreholes using pneumatic packers was implemented at a site in north central New Jersey. Ground water samples were collected from selected 6.1 m sections of boreholes drilled into fractured bedrock at three locations on the property and one offsite location. The ground water samples were analyzed in a field laboratory. The analytical results were used to determine the vertical extent of gasoline-related compounds dissolved in the ground water on the property and offsite. These compounds include benzene, ethylbenzene, methyl tertiary butyl ether, toluene, and xylenes. The four boreholes were converted into bedrock monitor wells. The intake interval for each of the wells was selected through evaluation of the vertical distribution of contaminants as determined from analytical results obtained from a field laboratory located onsite. Three wells are used for the recovery of contaminated ground water. The recovered water will be treated at the onsite air-stripping unit. The fourth well is used to chemically and hydraulically monitor the progress of the ground water recovery program.     

Control of Ground Water Contamination at an Active Uranium Mill

Seepage from tailings ponds associated with an active uranium mill in Utah has resulted in contamination of ground water contained in the Dakota-Burro Canyon Formation. This aquifer is used in the area as a supply for domestic and industrial wells.
Results of very low-frequency electromagnetic surveys and ground water quality investigations at the site indicated that the flow of ground water and contaminants is primarily fracture-controlled. Pumping tests were conducted to determine the hydraulic characteristics of the fractured system. The extent of contaminant migration was then determined using an analytical model of transport in fractured aquifers.
Based on these investigations, a plan was designed to control future and remediate past ground water contamination. This plan consists of pumping from a single well intersecting the main fracture that transports contaminants off the site. The effectiveness of the plan was analytically modeled, taking account of the anisotropy of the ground water system. Subsequent monitoring of water levels in the area indicates that the plan has been effective since its inception in November 1983.     

Study of Current Underground Injection Control Regulations and Practices in Illinois

In 1984, the Illinois Department of Energy and Natural Resources was required to assess the regulations and practices of the Illinois Underground Injection Control (UIC) program as it relates to Class I hazardous waste disposal wells. Nine injection wells, including two standbys (one inactive), are currently in operation at seven sites in the state. These wells range in depth from 1540 to 5524 feet (470 to 1683m; most inject wastes into porous carbonate formations (two wells inject into a thick sandstone). In 1984, approximately 300 million gallons (1.1 billion liters) of industrial wastes were disposed of in these wells. Acids were the most common waste disposed of, although water made up 70 to 95 percent of the wastes by volume. Illinois has been granted primacy in operating this program.
The geologic environment, consisting of the unit accepting the waste and confining units lying above and below, has the capacity to accept the waste, to retain it, and to protect all underground sources of drinking water (USDW) from contamination by its injection. The geology of Illinois is relatively simple and includes disposal zones and associated confining units suitable for deep-well injection across the central two-thirds of the state.
The regulatory structure for Class I injection wells is generally adequate in concept and scope to ensure containment of injected wastes and to safeguard underground sources of drinking water in Illinois. There is a need to update and strengthen selected portions of the regulatory practices in the areas of waste sampling protocol, chemical analysis of collected waste samples, and evaluation of injection well testing and monitoring data.
A number of technologies exist that can treat and dispose of most hazardous and non-hazardous waste streams. Each of these technologies has associated with it economic, environmental and societal impacts.     

Screened Auger Sampling: The Technique and Two Case Studies

The screened auger is a laser-slotted, hollow-stem auger through which a representative sample of ground water is pumped from an aquifer and tested for water-quality parameters by appropriate field-screening methods. Screened auger sampling can be applied to ground water quality remedial investigations, providing:(1) a mechanism for determining a monitoring well's optimal screen placement in a contaminant plume; and (2) data to define the three-dimensional configuration of the contaminant plume.
Screened auger sampling has provided an efficient method for investigating hexavalent chromium and volatile organic compound contamination in two sandy aquifers in Cadillac, Michigan. The aquifers approach 200 feet in thickness and more than 1 square mile in area. A series of screened auger borings and monitoring wells was installed, and ground water was collected at 10-foot intervals as the boreholes were advanced to define the horizontal and vertical distribution of the contaminant plumes. The ability of the screened auger to obtain representative ground water samples was supported by the statistical comparison of field screening results and subsequent laboratory analysis of ground water from installed monitoring wells.     

Factors Requiring Resolution in Installing Vadose Zone Monitoring Systems

Increasingly, regulations by federal, state and local agencies are being developed that require the installation of vadose zone monitoring systems for hazardous chemical facilities in addition to, or in lieu of, conventional ground water monitoring wells. Compared to a ground water monitoring approach, vadose zone monitoring systems may permit earlier detection of chemical leakage and less costly cleanup of contamination. The effective use of vadose zone monitoring systems in detecting contamination depends on many factors. Without proper consideration of these factors, a vadose zone monitoring system may not give as high a level of reliability as a ground water monitoring system.
Major factors to consider in installing a vadose zone monitoring system are: type of instrument to use, number of instruments, depth and location of instruments, and frequency of monitoring. Means to evaluate these factors in a comprehensive fashion have been lacking. Based on recent experience in installing and operating vadose zone monitoring systems, criteria and methods useful in resolving the preceding factors have been developed. Types of instruments can be classified as either direct (lysimeter, vapor probe) or indirect (tensiometer, conductivity probe). A combination of the two is needed for reliability. The depth, location and number of instruments depend on the geometry of the facility, the number and size of likely contaminant leakage points in engineered barriers, properties of the material being monitored, the effective radius of monitoring for each instrument, vadose zone properties, and types of remedial actions that are available. The freqency of monitoring largely depends on the rate of movement of the contaminant. Evaluating the preceding factors requires some level of modeling and preliminary field testing.     

Ground Water Monitoring in the U.S.S.R.

Ground water monitoring is considered to be a significant component of the geological service in the U.S.S.R. Currently, the total number of ground water monitoring wells exceeds 30,000. They are divided into two categories, which are the first class, or basic observation wells, and the second class, or auxiliary observation wells. The main objectives of both monitoring networks are briefly described. The general scheme of ground water monitoring organization is also presented.     

Microbial and geochemical quality of shallow well water in high-density areas in Mzuzu City in Malawi

In Malawi, shallow wells constitute the most important water sources for domestic purposes. However, increasing human population coupled with poor sanitation and infrastructure is undermining the quality of shallow well water. An assessment of microbial and geochemical quality of shallow well water in high-density areas of Zolozolo, Ching’ambo and Chiputula in Mzuzu City, Northern Malawi, has been carried out. The study aimed at characterising domestic water sources, identifying possible sources of water contamination and determining levels of microbial and chemical contamination. Arc-view GIS was used to map the water sources. A questionnaire survey was carried out to elicit information on characteristics of drinking water sources. Water samples were collected from quasi-randomly selected shallow wells and analysed for microbial and chemical parameters using standard methods. HCA, performed using R-programme, was used to group sampled sites according to their bio-physicochemical characteristics. Compliance of the water with MBS/WHO water quality guidelines was determined. The WQI was computed to turn multifaceted data obtained from laboratory analyses into simple information that is comprehensible and useable by the public to assess overall quality of water at a specific water points. The GW-chart was used to show hydrogeochemical water types from each sampled site. Microbial analysis revealed that water from 96.3% of shallow wells recorded faecal coliforms ranging from 129 to 920 cfu per 100 ml which were significantly higher than the Malawi Standards and WHO thresholds. In general, shallow well water is of low mineralisation (EC range 80–500 μS cm⁻¹), with hydrogeochemical facies dominated by Ca–HCO₃, which evolves to Ca–Cl water type. The shallow well water registered a WQI range of 50.16–66.04%, with a medium WQ rating. This suggested that the water obtained from the shallow wells is unsuitable for direct human consumption. It was observed that 100% of the shallow wells were at risk of pollution from onsite sanitation because of their proximity to sanitary facilities. It was strongly recommended that onsite treatment interventions have to be mobilised and initiated to protect the households from further possible consequences of using the water.     

Monitoring well interception with fractures in clayey till

When using monitoring wells for investigation of contaminant sources in clayey till, there is a high risk that fractures may cause mobile contaminants to bypass the monitoring wells. This paper indicates that the probability of interception between monitoring wells and hydraulic conductive fractures is often significantly less than 50%. Based on a field experiment and application of a calibrated discrete fracture matrix diffusion numerical model (FRAC3Dvs), the paper also evaluates pesticide-monitoring results for different positions of monitoring well screen relative to fractures. For well screens situated 0.25 and 2 m from a conductive fracture, the first concentrations of the pesticide metabolite (2,6 dichlorobenzamide, "BAM") would be measured two years and 18 years, respectively, after the contaminant had been transported into an underlying aquifer. In this way, underlying aquifers may be subjected to contamination by downward moving contamination without being observed in monitoring wells in the till.     

三峡井网地下流体动态与映震能力的初步分析

系统分析了三峡井网 8口井水位、4口井水温和 4个井台土氡 2 0 0 1年的观测数据 ,建立了各个测项的正常动态 ,并检验了其各自的映震能力。结果表明 ,8口井水位均有固体潮显示 ,其对地壳体应变的响应灵敏度为 10 - 9～ 10 - 10 体应变 /mm。井水位大多有气压效应 ,并受一定程度的降雨干扰。 4口井的水温动态相对平稳 ,日变幅一般为千分之几度。 4个井台的土氡动态起伏较大 ,变化范围在几到几十Bq/L之间。在 2 0 0 1年 10月 16日秭归县梅家河ML3 6地震前 ,有 6口井水位、1口井水温和 1个井台的土氡有异常显示 ,表明三峡井网具有较强的映震能力