Martha Conklin

The effect of water content on solute transport in unsaturated porous media

The effect of water content on NaCl transport in unsaturated porous media was investigated under steady state flow conditions for water contents ranging between full saturation and 15% by volume. The experiments were conducted in a 25 cm column packed with homogeneous sand. Results of the experiments indicate that solute transport in unsaturated porous media is subject to greater velocity variations and slower solute mixing than one in saturated media. As a result, NaCl breakthrough curves (BTCs) show earlier initial arrival and greater tailing and variance as the average water content decreases. These results suggest that transport processes in our experiments have not fully developed to the Fickian regime at lower water contents. Because the classical convection- dispersion equation does not adequately describe the movement of solutes under the pre- Fickian regime, a mobile-immobile model was employed to reproduce the BTCs obtained under unsaturated conditions. In general, the results indicate that at lower water contents the medium has a greater fraction of immobile water, higher dispersion, and slower mass transfer between the mobile and immobile regions. A power law relationship between dispersion and water content-normalized velocities was found to exist for our experiments and other experiments reported in the literature using different porous media. Thus we suggest dispersivity is not only a function of properties of the media but also of water content.

Predicting changes in hydrologic retention in an evolving semi-arid alluvial stream

Abstract Hydrologic retention of solutes in hyporheic zones or other slowly moving waters of natural channels is thought to be a significant control on biogeochemical cycling and ecology of streams. To learn more about factors affecting hydrologic retention, we repeated stream-tracer injections for 5 years in a semi-arid alluvial stream (Pinal Creek, Ariz.) during a period when streamflow was decreasing, channel width increasing, and coverage of aquatic macrophytes expanding. Average stream velocity at Pinal Creek decreased from 0.8 to 0.2 m/s, average stream depth decreased from 0.09 to 0.04 m, and average channel width expanded from 3 to 13 m. Modeling of tracer experiments indicated that the hydrologic retention factor (Rh), a measure of the average time that solute spends in storage per unit length of downstream transport, increased from 0.02 to 8 s/m. At the same time the ratio of cross-sectional area of storage zones to main channel cross-sectional area (As/A) increased from 0.2 to 0.8 m2/m2, and average water residence time in storage zones (ts) increased from 5 to 24 min. Compared with published data from four other streams in the US, Pinal Creek experienced the greatest change in hydrologic retention for a given change in streamflow. The other streams differed from Pinal Creek in that they experienced a change in streamflow between tracer experiments without substantial geomorphic or vegetative adjustments. As a result, a regression of hydrologic retention on streamflow developed for the other streams underpredicted the measured increases in hydrologic retention at Pinal Creek. The increase in hydrologic retention at Pinal Creek was more accurately predicted when measurements of the Darcy–Weisbach friction factor were used (either alone or in addition to streamflow) as a predictor variable. We conclude that relatively simple measurements of channel friction are useful for predicting the response of hydrologic retention in streams to major adjustments in channel morphology as well as changes in streamflow.

Metal ion-sulfur(IV) chemistry. 3. Thermodynamics and kinetics of transient iron(III)-sulfur(IV) complexes

Stability constants for the formation of Fe(III)-S(IV) complexes at µ = 0.4 M were determined spectroscopically. Values of K_1 = 10^(6.6) M^(-1) for Fe^(3+) + SO_3^(2-) ⇌ FeSO_3^+ and K_4 = 10^(7.3) M^(-1) for FeOH^(2+) + SO_3^(2-) ⇌ HOFeSO_3 were obtained. Raman measurements indicate that sulfite binds to the metal through oxygen. The kinetics of electron transfer between Fe(III) and S(IV) have been studied, and a mechanism is proposed. Kinetic and mechanistic aspects of the Fe(III)-catalyzed autoxidation of S(IV) are discussed in light of these results.

Atmosphere-snow transfer function for H2O2: Microphysical considerations

H2O2 analyses of polar ice cores show an increase in concentration from 200 years to the present. In order to quantitatively relate the observed trend in the ice to atmospheric levels, the atmosphere-snow transfer behavior and postdepositional changes must be known. Atmosphere-snow transfer was studied by investigating uptake and release of H2O2 in a series of laboratory column experiments in the temperature range −3°C to −45°C. Experiments consisted of passing H2O2-containing air through a column packed with 200-μm diameter ice spheres and measuring the change in gas phase H2O2 concentration with time. The uptake of H2O2 was a slow process requiring several hours to reach equilibrium. Uptake involved incorporation of H2O2 into the bulk ice as well as surface accumulation. The amount of H2O2 taken up by the ice was greater at the lower temperatures. The sticking coefficient for H2O2 on ice in the same experiments was estimated to be of the order of 0.02 to 0.5. Release of H2O2 from the ice occurred upon passing H2O2-free air through the packed columns, with the time scale for degassing similar to that for uptake. These results suggest that systematic losses of H2O2 from polar snow could occur under similar conditions, when atmospheric concentrations of H2O2 are low, that is, in the winter.

A moment method for analyzing breakthrough curves of step inputs

This paper presents a new and simple moment method for analyzing breakthrough curves for step inputs. An advantage of moment analyses, in general, is that the underlying physical model is not needed, unlike other techniques such as fitting convective (or advective) dispersion equations. Therefore moment calculations allow multispecies (such as partitioning tracers) to be considered in one, two, or three dimensions. Previous analyses using moments have emphasized the response for a pulse input. Generally, the travel (residence) time is of importance, and previous solution techniques do not perform well for long pulse or step inputs. In this paper the presented method is used to analyze breakthrough curves for step inputs, and the results are compared to fitted analytical solutions for the one-dimensional convection dispersion equation. Only negligible differences between the new moment method and the fitted analytical solutions are found for both simulated and experimental data. Superior performance over previous methods used for evaluating moments for step inputs is demonstrated.

Influence of shifting flow paths on nitrogen concentrations during monsoon floods, San Pedro River, Arizona

Hydrologic flow paths control transport, and therefore are a major constraint on the cycling and availability of nutrients within stream ecosystems. This control is particularly evident in semiarid streams, where hydrologic connectivity between stream, riparian, and upland systems increases greatly during storms in the rainy season, We measured chloride concentrations in base flow, precipitation, soil water, and stream water to quantify the hydrologic connectivity and solute flux between soil water, groundwater, and the stream channel during six summer floods in 2001 (a wet year; 25 cm winter rain) and 2002 (a dry year; 5 cm winter rain) in the San Pedro River, southeastern Arizona. This hydrologic information was used to evaluate observed patterns in nitrate, dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) concentrations in floods. The first floods of each year showed increased stream nitrate concentration that was approximately two orders of magnitude higher than base flow concentration. DOC consistently doubled to tripled during storm events, while DON in 2001 showed no response and showed a marked increase in 2002. A chloride mixing model indicated that soil and groundwater contributions to storm water discharge were related to antecedent conditions and to flood magnitude. Soil and groundwater contributions were the highest early in the 2001 monsoon season following the wet winter, much lower early in 2002 following a dry winter, and lowest during the largest floods of the 2002 monsoon season when flows were derived primarily from precipitation and overland flow. Stream water nitrate-N concentrations during floods were consistently 0.2-0.5 mg/L higher in 2002 than during 2001, suggesting greater over-winter accumulation of soil nitrate during the drier year. This result is consistent with higher mean nitrate-N concentrations in soil water of the riparian zone in 2002 (3.1 mg/L) than in 2001 (0.56 mg/ L). These data highlight the importance of seasonal and interannual variability of hydrology in semiarid regions, and the role of water availability in driving patterns of soil nutrient accumulation and their transport to the stream. Copyright 2007 by the American Geophysical Union.

DERIVED FUNCTIONS OF TIME DOMAIN REFLECTOMETRY FOR SOIL MOISTURE MEASUREMENT

This paper gives a systematic framework for evaluating the time domain reflectometry (TDR) response of soil. TDR measures the soil composite dielectric constant (Ka); thus any factor that has an influence on Ka measurement can affect soil moisture determination. On the basis of known dielectric constants of air, soil, water, and ice, as well as the volumetric fraction of each phase in a bulk soil, functional relationships between volumetric water content (θυ, cm3/cm−3) and Ka are derived in the form of θv=aKa0.5+b. In general, the TDR functions derived from this study are in good agreement with the “universal equation” developed through experiments by Topp et al. [1980] and other available TDR calibration data. This paper demonstrates that (1) soil solid and porosity have little effect on dielectric constant measurement; (2) temperature has a minor influence except for very wet soils; and (3) surface area is an important factor affecting the water content measurement.

Two- and three-parameter calibrations of time domain reflectometry for soil moisture measurement

Time domain reflectometry (TDR) is widely used to measure and monitor soil water. The commonly used calibration curve is the third-degree “universal polynomial” of Topp et al. [1980]. The most common refinement is calibration to a specific soil but still using four parameters (coefficients) from fitting a third-degree polynomial. Here we demonstrate that a three-parameter expression, θυ = aKaα +  )where the three parameters a, b, and α are determined by fitting water content θυ to the dielectric coefficient Ka). This form is consistent with the well-known mixing model. For an isotropic soil with homogeneous water distribution this expression is further simplified to two parameters by taking α=0.5. When α is 0.5, its calibration is equivalent to the linear calibration between θυ and the travel time along the waveguide. In addition, the simple three-parameter expression can be easily inverted without losing accuracy with regard to the original calibration. The TDR calibration expressed in a three-parameter form not only achieves a good fit but also conveys a physical connotation.

Sulfur dioxide reactions on ice surfaces: implications for dry deposition to snow

Abstract Controlled exposure of ice to a reactive gas, SO2, demonstrated the importance of the chemical composition of the ice surface on the accumulation of acidity in snow. In a series of bench-scale continuous-flow column experiments run at four temperatures (−1, −8, −30 and −60°C), SO2 was shown to dissolve and to react with other species in the ice-air interfacial region at temperatures approaching the melting point of ice. Experiments consisted of passing air containing SO2 through glass columns packed with 100-μm ice spheres of varying bulk composition (0–5 μM H2O2, and 0–1 mM NaCl), and analysing SO2 in the air and SO42− in the ice. At all temperatures (−60 to −1°C), increased retention volumes were found for increasing ionic strength and oxidant concentration. At the coldest temperatures and with no NaCl, increased retention volumes for −60 vs −30°C are consistent with SO2 uptake by physical adsorption. At warmer temperatures, −8 and −1°C, the observed tailing in the sorption curves indicated that other processes besides physical adsorption were occurring. The desorption curves showed a rapid decrease for the warmer temperatures, indicating the sorbed SO2 is irreversibly oxidized to SO42−. Results indicate that aqueous-phase reactions can occur below −8°C (i.e. −30 and −60°C). Results for different salt concentrations show that increasing ionic strength facilitates SO2 oxidation at colder temperatures, which is consistent with freezing point depression. One environmental implication is that snowpacks in areas with background SO2, can accumulate acidity during the winter months. As acidity accumulates, the solubility of SO2 will decrease causing a concomitant decrease in the air-to-surface flux of SO2. Modeling dry deposition of gases to snow surfaces should incorporate the changing composition of the ice surface.

Effect of soil type on water quality improvement during soil aquifer treatment

Bench-scale soil column experiments were performed at The University of Arizona to examine the effects of soil type and infiltration rate on the removal of wastewater organics during soil aquifer treatment (SAT). The suitability of such waters for potable uses following a combination of above-ground treatments and SAT polishing was under investigation. SAT was simulated in 1-meter soil columns containing repacked homogenized soils ranging from poorly graded sands to silty sands. Soils were obtained from existing and potential effluent recharge sites in Arizona. All columns received chlorinated/dechlorinated secondary effluent, ponded to a 25-cm depth above the soil surface, under alternating wet/dry conditions. Treatment efficiencies in biologically active and inhibited columns were compared to determine the mechanism(s) of water quality improvements and the sustainability of SAT. Water quality parameters included: (i) non-purgable dissolved organic carbon and (ii) UV absorbance at 254 nm (used as a measure of disinfection-by-product precursors). Differences in through-column removal of non-purgable dissolved organic carbon were significant for columns containing sandy loam (56%), sand (48%) and silty sand (44%). Removal of UV-absorbing organics was not significantly different for columns containing sand and sandy loam (22 and 20%, respectively). There was no significant correlation between infiltration rate and removal efficiency of either organic parameter for both soils.