Flowpath Independent Monitoring of Reductive Dechlorination Potential in a Fractured Rock Aquifer

The flowpath dependent approaches that are typically employed to assess biodegradation of chloroethene contaminants in unconsolidated aquifers are problematic in fractured rock settings, due to difficulties defining discrete groundwater flowpaths in such systems. In this study, the variation in the potential for chloroethene biodegradation with depth was evaluated in a fractured rock aquifer using two flowpath independent lines of field evidence: (1) the presence of the three biochemical prerequisites [electron donor(s), chloroethene electron acceptor(s), and chlororespiring microorganism(s)] for efficient chloroethene chlororespiration and (2) the in situ accumulation of chloroethene reductive dechlorination daughter products. The validity of this approach was assessed by comparing field results with the results of [1, 2-¹⁴C] cis -DCE microcosm experiments. Microcosms were prepared with depth-specific core material, which was crushed and emplaced in discrete packer intervals for 1 year to allow colonization by the indigenous microbial community. Packer intervals characterized by significant electron donor concentrations, elevated numbers of chlororespiring microorganisms, and high reductive dechlorination product to parent contaminant ratios correlated well with the production of ¹⁴C-labeled reductive dechlorination products in the microcosm experiments. These results indicate that, in the absence of information on discrete groundwater flowpaths, a modified approach emphasizing flowpath independent lines of evidence can provide insight into the temporal and spatial variability of contaminant biodegradation in fractured rock systems.     

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.     

Identifying Aquifer Type in Fractured Rock Aquifers using Harmonic Analysis

Determining aquifer type, unconfined, semi‐confined, or confined, by drilling or performing pumping tests has inherent problems (i.e., cost and complex field issues) while sometimes yielding inconclusive results. An improved method to cost‐effectively determine aquifer type would be beneficial for hydraulic mapping of complex aquifer systems like fractured rock aquifers. Earth tides are known to influence water levels in wells penetrating confined aquifers or unconfined thick, low‐porosity aquifers. Water‐level fluctuations in wells tapping confined and unconfined aquifers are also influenced by changes in barometric pressure. Harmonic analyses of water‐level fluctuations of a thick (～1000 m) carbonate aquifer located in south‐central Oklahoma (Arbuckle‐Simpson aquifer) were utilized in nine wells to identify aquifer type by evaluating the influence of earth tides and barometric‐pressure variations using signal identification. On the basis of the results, portions of the aquifer responded hydraulically as each type of aquifer even though there was no significant variation in lithostratigraphy. The aquifer type was depth dependent with confined conditions becoming more prevalent with depth. The results demonstrate that harmonic analysis is an accurate and low‐cost method to determine aquifer type.     

On Inflow to a Tunnel in a Fractured Double-Porosity Aquifer

Inflow to a tunnel is a great public concern and is closely related to groundwater hydrology, geotechnical engineering, and mining engineering, among other disciplines. Rapid computation of inflow to a tunnel provides a timely means for quickly assessing the inflow discharge, thus is critical for safe operation of tunnels. Dewatering of tunnels is another engineering practice that should be planned. In this study, an analytical solution of the inflow to a tunnel in a fractured unconfined aquifer is obtained. The solution takes into account either the spherical or slab-shaped matrix block and the unsteady state interporosity flow. The instantaneous drainage water table and anisotropic hydraulic conductivities of the fractures network are also considered. Both uniform flux and uniform head boundary condition are considered to simulate the constant head boundary condition in the tunnel. The effects of the hydraulic parameters of the fractured aquifer on the inflow variation of the tunnel are explored. The application of the presented solution to obtain the optimum location and discharge of the well to minimize the inflow to a tunnel is illustrated.     

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.     

Comparison of Methods for Sampling Dissolved Nitrogen in a Fractured Carbonate-Rock Aquifer

As part of an agricultural non-point-source study in the Conestoga River head waters area in Pennsylvania, different methods for collecting ground water samples from a fractured carbonate-rock aquifer were compared. Samples were collected from seven wells that had been cased to bedrock and drilled as open holes to the first significant water-bearing zone. All samples were analyzed for specific conductance, dissolved oxygen, and dissolved-nitrogen species. Water samples collected by a point sampler without pumping the well were compared to samples collected by a submersible pump and by a point sampler after pumping the well. Samples collected by using a point sampler, adjacent to major water-bearing zones in an open borehole without pumping the well, were not statistically different from samples collected from the pump discharge or from point samples collected adjacent to major water-bearing zones after pumping the well. Samples collected by using a point sampler without pumping the well at depths other than those adjacent to the water-bearing zones did not give the same results as the other methods, especially when the water samples were collected from within the well casings. It was concluded that, for the wells at this site, sampling adjacent to major water-bearing zones by using a point sampler without pumping the well provides samples that are as representative of aquifer conditions as samples collected from the pump discharge after reaching constant temperature and specific conductance, and by using a point sampler after pumping the well.     

Some Biases in Sampling Multilevel Piezometers for Volatile Organics

Multilevel piezometers are cost-effective monitoring devices for determining the three-dimensional distribution of solutes in ground water. Construction includes flexible tubing (plastic or Teflon®). Their sampling is subject to a number of'potential biases, particularly: (1) losses of volatile organic solutes via volatilization, (2) sorption onto the flexible tubing of the piezometers, (3) leaching of organics from this tubing, and (4) collection of unrepresentative samples due to inadequate piezometer flushing. It is shown that these biases are minimal or are easily controlled in most situations.
Another source of bias has been recognized. Organic solutes present in ground water above the screened level can penetrate the flexible plastic or Teflon tubing and contaminate the sampled water being drawn through this tubing. Laboratory tests and field results indicate this transmission causes low organic contaminant concentrations to be erroneously attributed to ground water which is free of such contaminants. The transmitted organics apparently desorb from the plastic tubing during flushing of even 40 piezometer volumes.
Recognition of this transmission problem provides for a better interpretation of existing organic contaminant distribution data. Caution is advised when considering the use of these monitoring devices in organic solute contaminant studies.     

Heat Tracing in a Fractured Aquifer with Injection of Hot and Cold Water

Heat as a tracer in fractured porous aquifers is more sensitive to fracture-matrix processes than a solute tracer. Temperature evolution as a function of time can be used to differentiate fracture and matrix characteristics. Experimental hot (50 °C) and cold (10 °C) water injections were performed in a weathered and fractured granite aquifer where the natural background temperature is 30 °C. The tailing of the hot and cold breakthrough curves, observed under different hydraulic conditions, was characterized in a log–log plot of time vs. normalized temperature difference, also converted to a residence time distribution (normalized). Dimensionless tail slopes close to 1.5 were observed for hot and cold breakthrough curves, compared to solute tracer tests showing slopes between 2 and 3. This stronger thermal diffusive behavior is explained by heat conduction. Using a process-based numerical model, the impact of heat conduction toward and from the porous rock matrix on groundwater heat transport was explored. Fracture aperture was adjusted depending on the actual hydraulic conditions. Water density and viscosity were considered temperature dependent. The model simulated the increase or reduction of the energy level in the fracture-matrix system and satisfactorily reproduced breakthrough curves tail slopes. This study shows the feasibility and utility of cold water tracer tests in hot fractured aquifers to boost and characterize the thermal matrix diffusion from the matrix toward the flowing groundwater in the fractures. This can be used as complementary information to solute tracer tests that are largely influenced by strong advection in the fractures.