Variable Groundwater Research Articles

Modeling the Effects of Variable Groundwater Chemistry on Adsorption of Molybdate

Laboratory experiments were used to identify and quantify processes having a significant effect on molybdate (MoO42−) adsorption in a shallow alluvial aquifer on Cape Cod, assachusetts. Aqueous chemistry in the aquifer changes as a result of treated sewage effluent mixing with groundwater. Molybdate adsorption decreased as pH, ionic strength, and the concentration of competing anions increased. A diffuse‐layer surface complexation model was used to simulate adsorption of MoO42−, phosphate (PO43−), and sulfate (SO42−) on aquifer sediment. Equilibrium constants for the model were calculated by calibration to data from batch experiments. The model was then used in a one‐dimensional solute transport program to successfully simulate initial breakthrough of MoO42− from column experiments. A shortcoming of the solute transport program was the inability to account for kinetics of physical and chemical processes. This resulted in a failure of the model to predict the slow rate of desorption of MoO42− from the columns. The mobility of MoO42− ncreased with ionic strength and with the formation of aqueous complexes with calcium, magnesium, and sodium. Failure to account for MoO42− speciation and ionic strength in the model resulted in overpredicting MoO42− adsorption. Qualitatively, the laboratory data predicted the observed behavior of MoO42− in the aquifer, where retardation of MoO42− was greatest in uncontaminated roundwater having low pH, low ionic strength, and low concentrations of PO43− and SO42−.

The form of the dispersion equation under recharge and variable velocity, and its analytical solution

In this article an attempt is made to describe field scale solute transport parameters in terms of regional hydrologic and aquifer hydraulic properties, such as recharge rate, transmissivity, hydraulic gradient, aquifer thickness and soil porosity. Aquifers subject to natural recharge from rainfall exhibit groundwater velocities which vary with distance and with the recharge intensity. This in turn generates an evolving transport dispersion coefficient which increases with distance even in a homogeneous aquifer with constant dispersivity. The dispersion equation in an aquifer subject to recharge and variable groundwater velocity is one with coefficients given as variable functions of distance. A new stable analytical solution of this equation is presented along with numerical comparisons with the classical convection dispersion equation and sensitivity tests on the effect of hydrologic‐hydraulic variables on the contaminant evolution. It was found that the recharge rate substantially affects the contaminant distribution and may partially explain the scale dependence of dispersion parameters. Transmissivity and hydraulic gradient values also determine the velocity distribution and therefore the rate of migration. It would appear that constant, laboratory scale, dispersivities may be sufficient for the modeling of field scale concentrations if an equation which accounts for the effects of hydrologie and hydraulic variables is used.

Winter Short-Pulse Radar Studies on the Tanana River, Alaska

Subsurface profiles were obtained during airborne and surface short-pulse radar surveys along a winter roadway over the Tanana River near Fairbanks, Alaska. The roadway crossed ice-covered channels and intervening frozen channel bars. The airborne profiles were intended for ice thickness profiling but also revealed sporadic reflections from a deeper horizon beneath the bars. Later profiling from the surface recorded these deeper reflecting horizons in detail, and they were found to correspond with the base of seasonal frost, measured in drill holes. The sediments immediately beneath the frozen material were saturated and represented the top of a seasonally variable groundwater table confined and controlled by frost penetration. The profiles made from the surface also revealed reflections from the bottom of the ice and the channel bottom. However, no significant reflections were observed beneath the channel bottom; reflections from sloping horizons above and below the base of the frost in the bar may indicate alluvial bedding patterns in these deposits. Eleven holes were drilled along the roadway to determine ice thickness, water depth, frost depth and the depth to the river ice-alluvium contact. Wide-angle reflection and refraction soundings were also made to determine electrical properties of materials and to verify our depth interpretations from echo times. These observations indicate that the airborne technique provides an effective method of locating unfrozen channels and measuring the depth of frost penetration beneath bars. The surface surveys revealed additional data on sedimentary structure.

Treatment of spatially variable groundwater levels in one-dimensional stochastic unsaturated water-flow modelling

A stochastic lower-boundary condition for an unsaturated water-flow domain is presented, which can be used in the numerical solution of the unsaturated water-flow equation for soils with a shallow groundwater-table. The resulting stochastic model can be useful in studying the influence of spatially variable soil hydraulic properties and groundwater levels on soil water regime and plant transpiration. The lower-boundary condition employed assumes an exponential relation between the discharge of a watershed and the groundwater level at a specific location. Since it was found that groundwater levels were influenced by the presence of low-conductive clay in the soil profile, tubes from which groundwater levels were measured are divided into two classes. These classes were based on the presence of clay within the first 1.2 m of the soil profile. Scaling was then used to obtain a reference discharge-groundwater-level relationship for each class and for two years. This relationship in combination with its frequency distribution of the set of scale-factor values for each class and year can be used as the stochastic lower-boundary condition for an unsaturated water-flow model to simulate water transport in a soil with a shallow groundwater-table. An application of such a variable lower-boundary condition is given in two simulation examples.

Time‐dependent gravity in southern California, May 1974 to April 1979

The Southern California gravity monitoring project, begun in May 1974, is intended to coordinate gravity measurements with the long‐baseline three‐dimensional geodetic measurements of the Astronomical Radio Interferometric Earth Surveying (Aries) project, which uses radio interferometry with extragalactic radio sources. Gravity data from 28 of the stations, monitored on an approximately 1‐ to 2‐month basis, have a single‐reading standard deviation of 11 μGal that gives a relative single determination between stations a standard deviation of 16 μGal. The averaging of data reduces the uncertainty, and if gravity does not change during the averaging time, it appears that gravity at a station relative to the base can be determined with a standard error of 2 to 3 μGal. Where stations could not be placed on lowporosity bedrock, the effects of variable groundwater levels must be considered. The largest gravity variation observed, 80 μGal, correlates with nearby water well variations and with smoothed rainfall. Smoothed rainfall data appear to be a good indicator of the qualitative response of gravity to changing groundwater levels at other suprasediment stations, but frequent measurement of gravity at a station is essential until the quantitative calibration of the station's response to groundwater variations is accomplished. The largest earthquake to occur during the survey time near the gravity network was the August 13, 1978, Santa Barbara Channel event (ML = 5.1–5.7, MS = 5.6). The closest gravity station to this earthquake, 67 km east of the epicenter, also exhibits the network's largest gravity change that cannot be related to factors other than tectonic distortion. This change is a 50‐μGal low occurring from mid‐1975 to mid‐1977. It is not known if the occurrence of the gravity anomaly is coincidental or related to the process of preparation for the earthquake.

Management model of a groundwater system with a transient pollutant source

A management model is presented for determining the maximum concentration of a transient pollutant source such that space‐dependent groundwater quality requirements are met. The method is illustrated using a one‐dimensional groundwater system with a single pollutant source at an adjacent stream. The chloride concentration of the source is to be managed to meet variable groundwater quality constraints. Management of the pollutant source is demonstrated for a single‐period as well as for repeated‐period pollutant discharges. The Crank‐Nicolson numerical approximation of the convective‐dispersive equation is used in the management model. The pollutant source concentration is treated as a parameter in the resulting system of linear equations. Taking advantage of the block structure of the matrix, the concentrations throughout the system are defined and manipulated as functions of this parameter for each time step. The parameter is maximized by comparison of groundwater quality limits with the groundwater solute concentrations at the corresponding nodes. The minimum value of the parameter over the entire time frame is the maximum concentration allowable in the source water over the management period or periods.