Groundwater Age Distributions Research Articles

The Ribe Formation in western Denmark — Holocene and Pleistocene groundwaters in a coastal Miocene sand aquifer

Abstract The Ribe Formation is a regionally extensive Miocene sand aquifer that is present in western Denmark at depths ranging from 100 to 300 m below ground surface. Groundwater chemistry and isotope data collected from more than 40 wells show that the Ribe Formation mainly contains high quality Cabi-carbonate type groundwater of Holocene age (100–10 000 a bp ). Pleistocene age groundwaters, identified on the basis of stable isotopes, noble gases and corrected 14 C values, are present below the island of Rømø, in discharge areas near the coast, and in hydraulically isolated inland areas. The groundwater age distribution in the Ribe Formation was successfully simulated with a numerical groundwater flow model and particle tracking only when the 14 C content in groundwater was corrected for both geochemical reactions and diffusion. The results indicate that geochemical and physical processes significantly influence the 14 C content of groundwater and that the correction factors required for the two processes are of the same magnitude. Flow modelling results indicate that Pleistocene groundwaters were emplaced at depth within the Ribe Fromation under low base-level conditions that prevailed throughout the late Pleistocene — near the coast these waters are essentially isolated from the present flow system, and Pleistocene freshwater may be present offshore. Seismic surveys show that conditions offshore are favourable for the presence of Pleistocene freshwater within the Ribe Formation and other aquifers.

On the distribution of multicomponent mixtures over generalized exposure time in subsurface flow and reactive transport: Foundations, and formulations for groundwater age, chemical heterogeneity, and biodegradation

The fate of materials undergoing transport and reactions in natural porous media sometimes depends on the time of exposure of the conveyed material to other materials present in the system. The distribution of groundwater age, the effects of mineral chemical heterogeneity on reactive solute transport, and the occurrence of lag in reaction systems are some areas of hydrogeology that involve exposure time in an important way. A general balance equation for accounting for such effects is provided through an extended transport operator that incorporates generalized exposure time as an additional independent coordinate. Evolution of material distributions over exposure time appears within this transport operator as a convective process that represents space‐ and time‐dependent generalized exposure of material constituents undergoing physical transport and nonequilibrium chemical and microbiological mass transformations. The general equation is derived from basic mass balance arguments by treating the constituents as a mixture of overlapping continua and developing evolution equations for the mixture material densities in the new dimensions of space, time, and exposure time. Example applications of the approach to each of the three examples above are described.

Simulation of groundwater age distributions

The objective of our work is to examine how to simulate the age of groundwater in such a way that it can be compared to actual measurements. We start by showing that computation of kinematic age, the one obtained by tracking water along streamlines, is ill posed in heterogeneous aquifers. This, together with its inability to account for mixing processes, makes it inadequate for comparison with age measurements, which are the result of some averaging of the age distribution in the water sample (the type of averaging depends on the measurement procedure). Therefore we go on to write the equations for the cumulative distribution function of residence time under transient flow conditions. This allows us to derive transient equations for the mean age, as well as for the higher‐order moments of its distribution, which generalize previous results by others. These moments can be used for approximating age measurements, which need not be equal to the mean age of the water sample. Using both a synthetic and a real example, we show that mean age is an acceptable estimate of radiometric age measurements in many cases. Including a second‐order correction (variance of residence time distribution) always improves results. On the other hand, higher‐order approximations converge slowly for old (compared to half‐life) waters, to the point that third‐order approximations often worsen the results.

Direct Simulation of Ground Water Age in the Rabis Creek Aquifer, Denmark

AbstractThe steady‐state ground water age distribution in the Rabis Creek Aquifer in Denmark is simulated directly and compared to the age estimated from observed tritium concentrations at three wells. The simulation code is based on an existing solute transport code, modified to include a zero‐order rate term of unity to account for the aging of ground water by one day per one day elapsed. The directly simulated ages of the observed tritium peaks agree to within less than four years with the observations. The tritium peaks would have been located to within 2 m vertically if the directly simulated ages had been used to select sampling depths in the three wells. An analytical solution that assumes that the geology is uniform and rectangular is shown to be a useful tool as a firsthand estimate of the age distribution. However, ages from a numerical direct age simulation that account for nonuniform aquifer properties agree better with the observed ages.

Patterns and Age Distribution of Ground‐Water Flow to Streams

AbstractSimulations of ground‐water flow in a generic aquifer system were made to characterize the topology of ground‐water flow in the stream subsystem and to evaluate its relation to deeper ground‐water flow. The flow models are patterned after hydraulic characteristics of aquifers of the Atlantic Coastal Plain and are based on numerical solutions to three‐dimensional, steady‐state, unconfined flow. The models were used to evaluate the effects of aquifer horizontal‐to‐vertical hydraulic conductivity ratios, aquifer thickness, and areal recharge rates on flow in the stream subsystem. A particle tracker was used to determine flow paths in a stream subsystem, to establish the relation between ground‐water seepage to points along a simulated stream and its source area of flow, and to determine ground‐water residence time in stream subsystems. In a geometrically simple aquifer system with accretion, the source area of flow to streams resembles an elongated ellipse that tapers in the down gradient direction. Increased recharge causes an expansion of the stream subsystem. The source area of flow to the stream expands predominantly toward the stream headwaters. Base flow gain is also increased along the reach of the stream. A thin aquifer restricts ground‐water flow and causes the source area of flow to expand near stream headwaters and also shifts the start‐of‐flow to the drainage basin divide. Increased aquifer anisotropy causes a lateral expansion of the source area of flow to streams.Ground‐water seepage to the stream channel originates both from near‐ and far‐recharge locations. The range in the lengths of flow paths that terminate at a point on a stream increase in the downstream direction. Consequently, the age distribution of ground water that seeps into the stream is skewed progressively older with distance downstream. Base flow ia an integration of ground water with varying age and potentially different water quality, depending on the source within the drainage basin. The quantitative results presented indicate that this integration can have a wide and complex residence time range and source distribution.

Hydrology of meteoric diagenesis: Residence time of meteoric ground water in island fresh-water lenses with application to aragonite-calcite stabilization rate in Bermuda

The average age of ground water that discharges at the shoreline from an island fresh-water lens is equal to the volume of ground water in the lens divided by the total recharge to the island. This island average residence time, τ 0 is easily calculated by Dupuit-Ghyben-Herzberg analysis (DGH). From estimates of the controlling variables (recharge, hydraulic conductivity, and porosity), it is estimated that τ 0 is usually on the order of 1 to a few tens of years in fresh-water lenses of small (width on the order of 100 to a few thousand meters), strip islands where calcarenite is undergoing early diagenesis. Lateral variation in interstitial velocity and distribution of ground-water age within the lens can be calculated from an approximate theory using DGH potentials and an assumption that discharge is uniformly distributed with depth. Results are within a few percent of those from rigorous flow-net construction. Velocities range laterally through two orders of magnitude. Contours of ground-water age are nearly horizontal over most of the lens; at a depth of about 40% of the depth to the interface, the ground-water age is τ 0 /2, and it equals τ 0 at about 60% of the depth to the interface. Flow-net construction shows that velocities are greatest at the water table and decrease rapidly downward to the value given by DGH. Representative velocities halfway between the ground-water flow divide and the shoreline are 10 to 100 m/yr in these islands. Application of these calculation procedures to Bermuda leads to a revised estimate of ground-water age for water samples that have been used to estimate the rate of aragonite-to-calcite transformation in the fresh-water lenses of that island. This rate is an order of magnitude less than that in lenses in Holocene oolitic cays of the Bahamas. The ratio of stabilization rate to amount of aragonite appears to be about the same in the two settings. The value of the ratio implies a half-life of 6,000-7,000 yr for aragonite-to-calcite transformation in these lenses.

Ground‐Water Age Distribution in Madrid Basin, Spain

ABSTRACTThe alluvial deposits that occupy the Madrid Basin in central Spain form an aquifer system covering an area of 5,000 km2 (2,000 mi2) and with thicknesses of 2,000 m (6,600 ft) or more. Average annual precipitation is 500 mm (15 in.) and average annual temperature is 15°C (59°F). The precipitation is sufficient to provide a net surplus to ground‐water recharge which, in turn, supports dry weather flow of major streams in the basin. A distribution of surface recharge and hydraulic conductivities were obtained from a previous study utilizing a two‐dimensional finite‐difference model of the same vertical cross section as this study. In this study a flow net and a discrete‐state compartment (or “mixing‐cell”) model were employed to calculate the age distribution of ground water circulating through the aquifer. Carbon‐14 decay ages were determined for nine ground‐water samples taken from eight locations. The ages obtained with the flow net and with the mixing‐cell models are mutually consistent and generally agree with the carbon‐14 decay ages. The calculated ages range from zero at the recharge boundaries to over 100,000 years at discharge boundaries in stream channels. The results obtained are to be regarded as preliminary. Their principal value will be to guide future C‐14 field sampling programs.