Investigating Seepage Paths at Golfaraj Earthen Dam, Northwest Iran.

An investigation of seepage was conducted at Golfaraj Reservoir Dam with a particular emphasis on determining the seepage areas based on regional and site-specific hydrogeological studies. The primary goal of the investigation was to develop strategies intended to minimize dam and reservoir seepage. Leakage from the reservoir is a serious problem and of considerable concern to the local populace. Substantial reduction of seepage from Golfaraj Reservoir Dam is the ultimate goal of the investigations conducted. Golfaraj Reservoir Dam, located in East Azerbaijan province, northwest Iran, was built to provide water for agricultural and industrial needs in Golfaraj plain and neighboring lands. The Golfaraj Reservoir was constructed through the Miocene Upper Red Formation, which consists of sequences of sandstone, mudstone, conglomerate, and gypsiferous marl. Following reservoir filling, seepage of water into adjacent formations was found to occur at an estimated rate of 70 L/s. After reservoir impoundment groundwater levels in Shahmar village, 2 km downstream and just north of the dam axis, rose and land surfaces became abnormally wet. Lugeon values in some boreholes drilled around Golfaraj Dam before and after dam construction were high enough to indicate that the dam base has sufficient permeability to allow water to escape by underflow. Twenty-four Casagrande piezometers installed around the dam axis at four sectors provided additional information on seepage pathways through the dam body and underneath or through the cutoff wall. Water-level variations in the Casagrande piezometers confirmed the seepage routes. Study results showed that reservoir water likely seeps through the reservoir bottom and beneath and through the cutoff wall. The west side of the dam and near the reservoir reflected water-level rises in accordance with the rise in reservoir-water level. Seepage in this area is probably due to its proximity to Golfaraj Reservoir. Hydrogeochemical analyses further suggest that the water source of Shahmar Drain, ~ 1800 m north of Golfaraj Dam cannot be from the east or west embankments of the dam because the electrical conductivity in Shahmar Drain water approximates the electrical conductivity of Golfaraj Reservoir water and is lower than the electrical conductivity of groundwater in some of boreholes. Potential future seepage mitigation measures will focus on methods to seal the reservoir floor and cutoff wall sections I2-I2 and I3-I3, although some efforts may be directed at the west side of the dam. Such measures could take the form of installation of a geomembrane barrier over the west side of the dam, concrete cutoff walls downstream of the dam, and pumping wells to intercept seepage.

RAYLEIGH WAVE AND WELL HEAD RESPONSE TO CALCULATE POROSITY IN THE EDWARDS AQUIFER OF SOUTH-CENTRAL TEXAS, USA.

We use the magnitude and centroid period of Rayleigh wave along with the amplitude of fluctuations of water level in a well to calculate effective porosity of a karst aquifer at the site scale. The radial and vertical displacements of Rayleigh wave are first related to the confining pressure of rock, which is then related to fluid pressure via the Gassmann equation. Three seismograms recorded at station 633A of the USARRAY and the induced responses of Well J-17 in the Edwards Aquifer (Texas) allow the calculation of an effective porosity between 17.0 and 24.4 percent, the average of which is close to the total porosity of core samples determined by geophysical well logs. This paper provides an innovative method to measure effective porosity in aquifers. Because of the long wavelengths of Rayleigh wave, the interdisciplinary approach is advantageous in that the resulting effective porosity is at the site scale which includes large conduits or voids.

Groundwater sampling in karst terranes: passive sampling in comparison to event-driven sampling strategy.

Karst aquifers are very easily contaminated because of the surficial features that commonly exist in karst terranes. Pollutant releases into sinkholes, sinking streams, and/or losing streams commonly result in concentrated solutes rapidly infiltrating and migrating through the subsurface to eventually discharge at downgradient springs unless intercepted by production wells, but slow percolation through soils also may result in serious contamination of karst aquifers. The unique features of karst terranes tend to cause significant problems in the interpretation of results obtained from water-quality grab samples of karst groundwater. To obtain more representative samples, event-driven sampling was proposed some decades ago, but event-driven sampling can be difficult and expensive to implement. In this paper, application of passive-sampling strategies is advocated as a means for effectively obtaining representative water-quality samples from karst aquifers. A passive-sampling methodology may be particularly useful for karst aquifers that may be found in complexly folded and faulted terranes. For example, a groundwater tracing investigation of a contaminated site in a karst terrane confirmed that several offsite springs and wells are connected to the contaminated site. Tracer recoveries suggested transport rates that were relatively slow for flow in a karstic aquifer (~0.02 m/s). Breakthrough curves were erratic and spiky. To obtain representative groundwater samples, a passive-sampling methodology is recommended.

On Tracer Breakthrough Curve Dataset Size, Shape, and Statistical Distribution.

A tracer breakthrough curve (BTC) for each sampling station is the ultimate goal of every quantitative hydrologic tracing study, and dataset size can critically affect the BTC. Groundwater-tracing data obtained using in situ automatic sampling or detection devices may result in very high-density data sets. Data-dense tracer BTCs obtained using in situ devices and stored in dataloggers can result in visually cluttered overlapping data points. The relatively large amounts of data detected by high-frequency settings available on in situ devices and stored in dataloggers ensure that important tracer BTC features, such as data peaks, are not missed. Alternatively, such dense datasets can also be difficult to interpret. Even more difficult, is the application of such dense data sets in solute-transport models that may not be able to adequately reproduce tracer BTC shapes due to the overwhelming mass of data. One solution to the difficulties associated with analyzing, interpreting, and modeling dense data sets is the selective removal of blocks of the data from the total dataset. Although it is possible to arrange to skip blocks of tracer BTC data in a periodic sense (data decimation) so as to lessen the size and density of the dataset, skipping or deleting blocks of data also may result in missing the important features that the high-frequency detection setting efforts were intended to detect. Rather than removing, reducing, or reformulating data overlap, signal filtering and smoothing may be utilized but smoothing errors (e.g., averaging errors, outliers, and potential time shifts) need to be considered. Appropriate probability distributions to tracer BTCs may be used to describe typical tracer BTC shapes, which usually include long tails. Recognizing appropriate probability distributions applicable to tracer BTCs can help in understanding some aspects of the tracer migration.

Modeling solute reactivity in a phreatic solution conduit penetrating a karst aquifer.

A two-dimensional model for solute migration, transformation, and deposition in a phreatic solution conduit penetrating a karst aquifer is presented in which the solute is anthropogenic to the natural system. Transformation of a reacting solute in a solution conduit has generally been accepted as likely occurring but actual physical measurements and mathematical analyses of the suspected process have been generally minimally investigated, primarily because of the logistical difficulties and complexities associated with solute transport through solution conduits. The model demonstrates how a reacting solute might decay or be transformed to a product solute some of which then migrates via radial dispersion to the conduit wall where it may become adsorbed. Model effects vary for laminar flow and turbulent flow in the axial direction. Dispersion in the radial direction also exhibits marked differences for both laminar flow and turbulent flow. Reaction zones may enhance subsequent reactions due to some overlap resulting from the longitudinal dispersion caused by flow in the axial direction. Simulations showed that varying the reaction rate coefficient strongly affects solute reactions, but that varying deposition coefficients had only minimal impacts. The model was applied to a well-known tracer test that used the tracer dye, Rhodamine WT, which readily converts to deaminoalkylated Rhodamine WT after release, to illustrate how the model may be used to suggest one possible cause, in addition to other possible causes, for less than 100 tracer-mass recovery. In terms of pollutants in a karst aquifer the model also suggests one possible mechanism for pollutant transformation in a solution conduit.

Hydrogeological and Geochemical Evidence for the Origin of Brackish Groundwater in the Shabestar Plain Aquifer, Northwest Iran.

Shabestar plain aquifer is located in the northeast of the hypersaline Urmia Lake, northwest Iran. There are two types of the aquifer in the plain; an unconfined aquifer that covers the plain and a confined aquifer that is just in the vicinity of the lake. In recent years some of the agricultural wells have become salinized by saline water due to unrestricted groundwater pumping. Groundwater in the confined aquifer in comparison with the above unconfined aquifer is of good quality. The salty Urmia Lake is considered the most probable source of groundwater salinization. Other potential sources of groundwater salinization could include halite dissolution, halite is exposed at the southern end of Shabestar plain, and evaporation from the shallow water table. The water samples, based on their total dissolved solid and chloride contents, are classified in the brackish group. The hydrogeological setting and boreholes log interpretation suggest that the saltwater is the result of Urmia Lake water that is entrapped within the fine-grained matrix from when the lake reached its greatest extent. The ratios of Na/Cl, Br/Cl, (Ca + Mg)/SO4, Mg/Cl, (2Ca + Na)/Cl and Rittenhouse diagram preclude halite dissolution as a salinity source and confirms that the lake water with the composition of sea water is the main cause of groundwater salinization. In addition, Li/Cl ratios indicate that the original briny-water was somewhat affected by evaporation. However, the effect of evaporation was found to be, at most, a minor influence only.

Solute Migration from the Aquifer Matrix into a Solution Conduit and the Reverse.

A solution conduit has a permeable wall allowing for water exchange and solute transfer between the conduit and its surrounding aquifer matrix. In this paper, we use Laplace Transform to solve a one-dimensional equation constructed using the Euler approach to describe advective transport of solute in a conduit, a production-value problem. Both nonuniform cross-section of the conduit and nonuniform seepage at the conduit wall are considered in the solution. Physical analysis using the Lagrangian approach and a lumping method is performed to verify the solution. Two-way transfer between conduit water and matrix water is also investigated by using the solution for the production-value problem as a first-order approximation. The approximate solution agrees well with the exact solution if dimensionless travel time in the conduit is an order of magnitude smaller than unity. Our analytical solution is based on the assumption that the spatial and/or temporal heterogeneity in the wall solute flux is the dominant factor in the spreading of spring-breakthrough curves, and conduit dispersion is only a secondary mechanism. Such an approach can lead to the better understanding of water exchange and solute transfer between conduits and aquifer matrix. HIGHLIGHTS: Euler and Lagrangian approaches are used to solve transport in conduit. Two-way transfer between conduit and matrix is investigated. The solution is applicable to transport in conduit of persisting solute from matrix.

Combined physical and chemical nonequilibrium transport model for solution conduits.

Solute transport in karst aquifers is primarily constrained to relatively complex and inaccessible solution conduits where transport is often rapid, turbulent, and at times constrictive. Breakthrough curves generated from tracer tests in solution conduits are typically positively-skewed with long tails evident. Physical nonequilibrium models to fit breakthrough curves for tracer tests in solution conduits are now routinely employed. Chemical nonequilibrium processes are likely important interactions, however. In addition to partitioning between different flow domains, there may also be equilibrium and nonequilibrium partitioning between the aqueous and solid phases. A combined physical and chemical nonequilibrium (PCNE) model was developed for an instantaneous release similar to that developed by Leij and Bradford (2009) for a pulse release. The PCNE model allows for partitioning open space in solution conduits into mobile and immobile flow regions with first-order mass transfer between the two regions to represent physical nonequilibrium in the conduit. Partitioning between the aqueous and solid phases proceeds either as an equilibrium process or as a first-order process and represents chemical nonequilibrium for both the mobile and immobile regions. Application of the model to three example breakthrough curves demonstrates the applicability of the combined physical and chemical nonequilibrium model to tracer tests conducted in karst aquifers, with exceptionally good model fits to the data. The three models, each from a different state in the United States, exhibit very different velocities, dispersions, and other transport properties with most of the transport occurring via the fraction of mobile water. Fitting the model suggests the potentially important interaction of physical and chemical nonequilibrium processes.

Application of robust statistical methods to background tracer data characterized by outliers and left-censored data.

Accurate analysis of tracer-breakthrough curves is dependent on the removal of measured background concentrations from the measured tracer recovery data. Background concentrations are commonly converted to a single mean background concentration that is subtracted from tracer recovery data. To obtain an improved estimate for the mean background concentration, a statically-robust procedure addressing left-censored data and possible outliers in background concentration data is presented. A maximum likelihood estimate and other robust methods coupled with outlier removal are applied. Application of statically-robust procedures to background concentrations results not only in better estimates for mean background concentration but also results in more accurate quantitative analyses of tracer-breakthrough curves when the mean background concentration is subtracted.

Calculation of elapsed decimal time for tracing studies.

Calculation of time of travel from tracing studies in hydrologic systems is critical to establishing pollutant arrival times from points of inflow to points outflow, calculating subsurface flow velocities, and determining other important transport parameters such as longitudinal dispersion. In addition, breakthrough curve modeling demands accurate time of travel calculations if model results are to have any realistic meaning. However, accurate time of travel calculations are very difficult for long tracer tests in which sampling schedules are not consistent, or when there are major disruptions such as may occur when adverse weather conditions cause automatic sampling equipment to fail. Long and inconsistent sampling times may be accurately converted to decimal times of travel by converting the conventionally recorded Coordinated Universal Time for sampling date and time event to a baseline time standard. By converting to a baseline time standard, all recorded dates and times are linked to the established baseline standard so that each succeeding sampling date and time are correctly determined relative to the previous sampling date and time and to the injection date and time.