Salinization Processes in Arid Environments Posters
Presiding: M I Dragila, Oregon State University; N Weisbrod, Ben Gurion University of the Negev
H21B-01 0830h
Is There Enhanced Evaporation In Surface-Exposed Fractures Due To Convection Of Water Vapor?
Diffusive fluxes limit the potential amount of evaporation from surface-exposed fractures and other discontinuities crossing the land surface. However, if density differences between the air above a fracture and the moist air within the fracture voids exist, convective flux might be developed. If so, evaporation could increase by up to several orders of magnitude, dependent on temperature differences and the size of the aperture. Theoretical calculations suggest that under typical high deserts conditions convection is likely to occur during the night while diffusion is likely to control evaporation from fractures during the day. Field experiment carried out in the Negev desert of Israel showed that the amount of salts accumulated within a fracture in six months is much higher than what could be explained by diffusion alone. This further support the existence of additional mechanism that enhances evaporation. The major objective of this work is to experimentally explore the existence of convective condition in natural fractures in the field. A surface exposed fracture in the Negev desert of Israel was instrumented in a way that the temperature and relative humidity within 120 cm deep fracture are constantly monitored. These parameters are also collected 20 cm above land surface, just above the fracture surface. To explore the potential of convective flux of moist air in larger discontinuities (e.g., karsts systems), a large diameter uncased borehole, 55 m depth, was also instrumented with thermocouples and relative humidity probes. Preliminary results indicate that: (1) convective conditions exist at least in the upper part of a fracture from early evening to late morning (the existence of convection cell deeper in the fracture is still unclear); and (2) convective conditions exist almost 24 hours a day (excluding at noon for a short period) in the large borehole. These field measurements suggest that large amounts of salt could accumulate within surface-exposed fractures due to enhanced evaporation controlled by convection. Subsequently, these salts could find their way to the groundwater, bypassing the thick vadose zone.
H21B-02 0830h
Modeling Salt Redistribution in Fractured Porous Media Caused by Convection Driven Evaporation Within the Fracture
A set of numerical experiments demonstrate a dramatic increase in salt precipitation on fracture surfaces caused by convection driven pore water evaporation within the fractures. A model of soil evaporation was created using the EWASG module of TOUGH2, a three dimensional, multiphase, multicomponent, finite difference porous media simulator. This model was tested against laboratory and historic field data and used to investigate the potential effect of fracture-air convection on evaporation and matrix salt redistribution. System evaporation was found to increase 21% over a 100 day dry spell due to the presence of a large soil fracture. This increase was lower than predicted from historic research, which found a nearly 100% increase in evaporation (Ritchie and Adams, 1974). Salt redistribution due to evaporation driven pore solution flux was found to increase near-fracture- salt content by over 350%. A majority of the increase in near-fracture salt, 73%, appeared as solid phase precipitated salt. Field and laboratory evidence indicate that concentrated salt can occur as precipitated salt along the fracture wall. In low permeability porous media, fractures are the major source of bypass flow to the subsurface. Evaporation and salt precipitation along fracture surfaces is proposed to be a potentially major source of salt flux to the water table, due to the high permeability of the fractures, the low permeability of the matrix, and the potential for preferential flushing of the fracture during rain events. While net evaporation was not significantly affected by the presence of the fracture, the simulated effect on salt transport can have significant application to the fields of waste management, agriculture and environmental remediation.
H21B-03 0830h
Quantifying Evaporation and Salt Accumulation in Fractured Rocks
Evaporation was traditionally considered as the loss of water vapor from land surface and water reservoirs to the atmosphere. Direct loss of water vapor from cracks, fractures and other discontinuities was usually neglected. A few papers published in the early 70's, recent models and field measurements suggest that under typical arid conditions significant amount of water vapor could be transported directly from surface exposed fractures to the atmosphere. Subsequently, salt accumulation along the fracture surfaces is likely to occur as the pore water solution in the upper vadose zone is typically saline. The rare but intensive rain events, often occurring in deserts, could dissolve this accumulated salt and flush it to the underlying aquifers. This process could be of great importance for salinization, especially in low permeability rocks, where without this mechanism salts are likely to accumulate in the upper vadose zone and never reach groundwater. The main objectives of this work is to experimentally quantify the amount of water vapor loss from fractures under controlled conditions and measure the amount and distribution of salts precipitate on the fracture walls. A customized Climate Control Room (CCR) was especially designed and constructed to mimic extreme night-time and day-time temperature conditions, typical for deserts. Within the CCR, two fractured blocks of chalk were installed. The rocks and fractures were instrumented so the temperature at the bottom of the rock is constant. Humidity and temperature within the fracture aperture and within the rock are constantly monitored. A feeding container is attached on each side of the block to supply the rock with pore water solution under constant tension. The inflow of water from the feeding containers into the rocks are constantly monitored and recorded as well as the overall changes of the water content within the block. Preliminary results indicate a measurable water vapor loss from the fracture surfaces to the atmosphere and subsequent salt precipitation on the fracture walls.
H21B-04 0830h
Soil Salinity Dynamics in Arid Non-flooded Riparian Areas
Soil salinity is a common problem in arid riparian areas of the arid Southwest, but the dynamics of soil salinity in these areas are not well understood. The main causes of soil salinity in non-flooded riparian areas are generally known as low precipitation, high evapotranspiration, and capillary flux from saline shallow ground water. However, some riparian areas maintain a relatively low soil salinity for a long period of time with thriving salt-sensitive vegetation such as Cottonwoods while other areas are completely salinized and covered by salt-tolerant vegetation such as Saltcedars. Is this difference in soil salinity caused by a small amount of deep infiltration sufficient to leach salts back to the ground water or by ground water dynamics that 'wash' the soil profile from below? The results of this study, using the modeling program HYDRUS-1D, indicate that differences in soil salinity levels among different riparian areas are not caused by a small amount of deep infiltration but by ground water fluctuations that 'wash' the soil profile from below.
H21B-05 0830h
Salinization of Surface Water and Groundwater, Hueco Bolson and Mesilla Valley, New Mexico, Texas and Chihuahua: Sources of Sulfate as Indicated by Stable O, H and S Isotopes.
Salinity and Δ34S values in Rio Grande surface water increase downstream within the Hueco Bolson, with sharp increases detected in El Paso and near Fabens at times of low flow. Fluctuations at El Paso reflect variations in relative input of high-Δ34S salty groundwater discharging from the Mesilla Valley upstream. Near Fabens, further addition of water with Δ34S > 9 ‰, [SO4] > 300 mg/L and Cl/SO4 near 1 is required. Such water can be derived from Hueco Bolson groundwater north of the river or from salty sediment beneath the flood plain, but not from urban effluent or agricultural drainage. Sulfate Δ34S and Δ18O values are consistent with Hueco Bolson groundwater as the source of high-Δ34S sulfate. The origin of flood-plain groundwater is identified as pre-dam (pre-1916) or post-dam river water according to degree of evaporation as expressed in O and H isotope content. Sulfate geochemistry and salinization mechanisms differ in the two types of groundwater. Pre-dam groundwater in Hueco Bolson and Mesilla Valley has increased in salinity relative to contemporaneous river water as a result of addition of salty basin water with Δ34S > 9 ‰; and Cl/SO4 > 1 in the Hueco Bolson where interaction with salty sediment below the flood plain is likely. Post -dam groundwater salinity in the Hueco Bolson has increased by addition of sulfate with Δ34S commonly lower than values in present-day river water, and can be explained by oxidation of sulfide in alluvium, but not by addition of urban wastewater, or of gypsum and sulfuric acid as agricultural amendments. Post-dam groundwater salinity in the Mesilla Valley most commonly results from addition of salty basin water with Δ34S > 8 ‰, [SO4] > 200 mg/L and Cl/SO4 >2.
H21B-06 0830h
Geochemical Classification of Groundwater Salinization in the Northern Hueco Bolson Aquifer
The Hueco Bolson aquifer is the primary water resource for the cities of El Paso and Juarez. Identification of the primary groundwater recharge zones and the dynamics of salinization are needed in order to predict the impact of current and future development of water resources. Excessive withdrawal has resulted in a persistent increase in salinity, forcing the El Paso Water Utility (EPWU) to abandon several once highly productive wells. The mechanisms for this pumping induced salinization are not clear, but may include upward movement of deep saline groundwaters, lateral migration of saline waters from elsewhere in the aquifer and pumping induced leakage of saline waters from mud and clay layers. Isotopic and chemical analysis of groundwaters from a recently constructed well field with multiple discrete vertical zone samples has provided a unique opportunity to characterize these deep salinity sources and identify the primary mechanisms of pumping induced salinization. Analysis of O and H isotopes was used to delineate two primary recharge zones in the northern portion of the aquifer. The western section along the Franklin Mountains indicates a linear trend of oxygen/hydrogen isotope ratios indicative of mountain front recharge with highly evaporated waters at depth. The eastern portion closer to the center of the basin shows a more complex picture of shallow and deep evaporated waters, with a mixing zone of a less evaporated source at intermediate depth. Incorporation of anion ratios and S isotope data with that of these recharge zones indicates multiple sources of salinization. High anion ratios and S isotope values characteristic of deep saline waters appear at depths greater than 300 meters, with S isotope values higher than 9‰ and Cl/Br ratios greater than 10,000. Mixing trends of these highly saline deep waters appear at the lowest intervals of some wells above 300 meters, indicating upward movement of deep saline groundwaters as a possible source of increased salinity. Additionally, data from shallow well intervals and less evaporated mountain front waters show a range of anion and sulfur isotope signatures distinct from those of the deep saline waters (e.g. S isotopes 4 to 9‰ and Cl/SO4 ratios of 200-8000). This range of values and their distributions with depth suggest multiple salinity sources in addition to the deep saline waters. Ultimately, this understanding of recharge and salinity sources will be incorporated into the EPWU groundwater model, which will aid in determining strategies that reduce pumping-induced salinization of the aquifer.
H21B-07 0830h
Integrated Modeling of Regional-Scale Flow and Reactive Salt Transport in the Western San Joaquin Valley
An integrated model of regional-scale subsurface flow and reactive salt transport and its application to the western San Joaquin Valley study area is presented. The integrated model uses the MOD-HMS model to simulate three-dimensional subsurface flow and transport, and couples it to the major ion chemistry modules of the UNSATCHEM model. Unique features of the integrated model include the full coupling of the vadose zone and groundwater systems, and the accounting for major ion chemistry effects on soil and groundwater salinization. The model was applied to the western San Joaquin Valley study area to simulate changes in regional-scale soil and groundwater salinization that occurred over the last 50 years. Model simulation results were compared to historical observations of water table depths, groundwater pumping, subsurface drainage, and soil and groundwater salinity. Based on the general correspondence between observed and simulated relationships it was concluded that these processes are adequately represented in the model.