Daniel Ronen

MICROSCALE CHEMICAL HETEROGENEITY IN GROUNDWATER

Ronen, D., Magaritz, M., Gvirtzman, H. and Garner, W., 1987. Microscale chemical hetereogeneity in groundwater. J. Hydrol., 92: 173-178. Large variations in the concentration of Cl-, NO a and SO42 in the upper water layers of a polluted aquifer were found between samples that were vertically separated by as little as 3 cm. This was disclosed when a multilayer dialysis cell device was used to obtain undisturbed, real-time samples from test wells. This first time presentation of chemical fine structure provides new insights into mechanisms of contamination of an aquifer by surface inputs and raises the question of the validity of groundwater quality data based on single samples, however obtained.

Anion exclusion during transport through the unsaturated zone

A 20-year chronological record of the flow of water and anions, along 27 m, in the unsaturated zone of a phreatic aquifer was reconstructed. Water was traced according to its tritium content, using the difference between the environmental tritium content of rain and irrigation water. Anions were traced using data about their sequential input to the overlying cultivated field and their concentration in the profile. Evidences of anion exlcusion were found along a 10-m thick clay loam layer. The vertical velocities of water molecules and anions (SO2−4 and Cl−) were calculated to be 0.7 ± 0.05 and 1.35 ± 0.05m yr−, respectively. The average thickness of the water films and the equivalent distance of exclusion, at a 15% gravimetric water content, were 25 and 12A, respectively. These field data fit and support the theoretical relationship between the water film thickness and the relative exclusion concentration proposed by Bresler (1973).

Will the Dead Sea die

The level of the Dead Sea (the lowest surface on Earth) is currently declining at a rate of 0.8 m/yr, and has dropped about 20 m since the beginning of the twentieth century; it reached −410 m in 1997. We address the question of whether the level of the Dead Sea will continue to decline. A numerical model, developed in this study to determine the water balance, accounts for the increase in salinity and the concomitant decrease in the rate of evaporation that accompanies reduction in the activity of the water. Simulations based on ranges of water withdrawal scenarios suggest that the Dead Sea will not “die”; rather, a new equilibrium is likely to be reached in about 400 yr after a water-level decrease of 100 to 150 m.

The development and influence of gas bubbles in phreatic aquifers under natural flow conditions

In a phreatic aquifer, bubbles may result from the entrapment of air during groundwater recharge and/or bacterial metabolism. The calculated critical depth of about 1 m at which bubbles are most likely to be found in a granular aquifer, coincides with the depth of 0.60 m of an almost stagnant water layer (specific discharge 1 × 10-6 cm sec-1) found at the water table region under natural flow conditions. Bubbles clog pores and therefore reduce the hydraulic conductivity without significantly reducing the volumetric water content. Stagnation at the water table region results since prevailing pressures (in the order of 10-1 atmospheres) are not sufficiently large to move bubbles through porous media in a water environment.

Aquifer Characteristics Derived From the Interaction Between Water Levels of a Terminal Lake (Dead Sea) and an Adjacent Aquifer

The water level of the Dead Sea, a terminal hypersaline lake (total dissolved solids, approximately 340 g/L) has decreased at an average rate of 0.5 m/yr since 1960 and by 0.8 m/yr between 1981 and 1989. The dramatic long-term water level variation of the Dead Sea and the seasonal short-term fluctuations are accompanied by parallel variations of groundwater levels in an adjacent aquifer. A general methodology based on a simplified yet reliable one-dimensional flow model, together with continuous measurements of groundwater levels in observation wells, enables analysis of aquifer structural and hydraulic properties. Furthermore, this analysis enables prediction of future groundwater levels in unconfined and confined aquifers due to future changes in lake levels.

A multi-layer sampler for the study of detailed hydrochemical profiles in groundwater

Abstract A modular multi-layer sampler was developed and utilized for sampling undisturbed groundwater chemical profiles. Sampling at 3 cm depth intervals is based on the dialysis-cell method and has no depth limitations. The sampler may be used for water quality monitoring and research, both in existing or specially drilled screened wells

Production of CO2 in the capillary fringe of a deep phreatic aquifer

CO2 profiles obtained along a 30-m-thick unsaturated zone under land irrigated with sewage effluents show two production regions: a seasonal one in the root zone and another, at steady state, near the water table (29 m). On an annual basis the CO2 flux from the deep source toward the atmosphere (6.3 g C m−2 yr−1) is balanced by a similar influx of soluble organic carbon (SOC) from sewage effluents. The δ13C values of soil CO2 indicate that CO2 is produced from plant material in the root zone and from biodegradation of total sedimentary organic carbon in the capillary fringe. High CO2 concentration in the capillary fringe (up to 2%) is likely to reflect a decrease in diffusivity relative to the unsaturated zone due to increase in both water content and tortuosity induced by the capillary fringe structure. The long residence time of SOC and CO2 in the unsaturated zone (29 years at the study site) suggests that the unsaturated zone of deep aquifers may have a significant storage capacity for carbon and may act as a temporary carbon sink.

Anthropogenically induced salinization of groundwater: A case study from the Coastal Plain aquifer of Israel☆

Abstract One of the most acute problems facing water resources in semi-arid regions is the ongoing salinization of groundwater. The example discussed in the paper is from the Coastal Plain aquifer of Israel where most attention has been given until recently to salinization due to seawater encroachment. The present study revealed that large amounts of salt have been added to this aquifer along its eastern boundary which is in close proximity to a syncline filled by Tertiary chalks and marl. The saline water is characterized by high tritium levels and a stable isotope composition which differs from that of local rains. It is suggested that intensive irrigation with water imported from the Sea of Galilee induced flow in old fractures and in solution channels developed in the underlying chalks. These channels are filled with soluble salts which accumulated during the Holocene in the Tertiary aquitard and were mobilized as a result of increased irrigation of overlying soils. The soluble salts reached groundwater and ultimately flowed into the adjacent sandy Coastal Plain aquifer. Overpumping of the Coastal Plain aquifer caused continuous lowering of the groundwater table enhancing flow and considerable transport of solutes from the adjacent aquitard.

Field observations of a capillary fringe before and after a rainy season

Abstract Field measurements of profiles of the water content of a 7-m deep, sandy, phreatic aquifer have been conducted at the same site before and after the rainy season. The field site is an uncultivated region located in a citrus orchard, north of the city of Netanya, on the Coastal Plain aquifer of Israel. In two single-day campaigns, 13 boreholes were drilled in relatively homogeneous sandy sediments in an area subjected to infiltration of rain only. Twelve of the thirteen boreholes were located within a radius of 5 m. Water content, as a function of depth, was obtained from continuous cores. We found that the distribution of water within the capillary fringe (CF) was compact — denoting that there was an abrupt change in water content with increasing height above the water table. The height of the CF was about 1.4 m at each point above the water table, and over a horizontal distance of about 4 m, this height varied by up to 33% and 50% before and after the rainy season, respectively. Saturated conditions were detected in some regions of the CF and unsaturated conditions were found up to 1.5 m below the water table. The distribution of water in the capillary fringe remained compact while being displaced vertically as the water table rose 35 cm after the winter rains. These observations are consistent with pore level models of the water distribution within the capillary fringe in a porous media.

Characterization of suspended particles collected in groundwater under natural gradient flow conditions

Microscale Eulerian variations in the flux, mineralogical composition and size of suspended particles have been found in a contaminated sandy aquifer under natural gradient flow conditions ( ) during an 8-month study period. Particle variability has been detected along a 16-m saturated section of the aquifer at a scale of centimeters and meters in the vertical and horizontal dimensions, respectively. The average concentration of particles in groundwater varied between 1 and 40 mg/L, but high concentrations of up to 5000 mg/L were determined in specific 3-cm vertical intervals of the aquifer. The particles were primarily composed of CaCO3 (11–57%), quartz (7–39%) and clays (8–43%). Most of the particles were within the 140–3000 nm size range with size modes varying from 310 to 660 nm. The large amounts of suspended particles are considered to be related to high inputs of dissolved organic carbon into groundwater, from sewage effluents which have been used for agricultural irrigation since the early 1960s. As a result of organic matter biodegradation in the saturated zone, anoxic conditions developed and the pCO2 content of groundwater increased dramatically (pCaCO2 = 10−1.8 to 10−1.3 atm). It is postulated that part of the carbonate cement of the rocks dissolved and detrital CaCO3, quartz and clay were released as colloidal particles. Part of the clay particles could have also been transported through the unsaturated zone into groundwater after mobilization from surface layers as a result of the intermittent input of water of different chemical quality. In the prevailing anoxic conditions of groundwater at the study site (dissolved oxygen concentrations of <1 mg/L) colloidal stability is enhanced by organic matter coating of particles.