Journal of Environmental Engineering

Water temperatures mediate the rate of chemical reactions and constrain the habitat space of freshwater organisms in streams. Stream temperatures are therefore of great interest for stream water quality and stream ecology. There is evidence that evaporative cooling imposes an upper bound on stream temperatures. Maximum stream temperatures observed in the United States were explored in this paper. A frequency-based method was employed for the estimation of probable maximum stream temperatures. The method utilizes an enveloping standard deviate, $K_E\text{,}$ and the first two sample moments of a maximum stream temperature series. The parameter $K_E$ was evaluated by analysis of 100 independent, maximum (weekly) stream temperature series from Idaho, Maine, Minnesota, Oklahoma, and Washington and found to be 4.88. Probable maximum weekly stream temperatures were determined for 798 stream temperature records obtained from USGS water quality monitoring stations located throughout the United States. Projections of probable maximum stream temperatures at these stations using $K_E=4.88$ are presented. As to be expected, all probable maximum stream temperatures exceeded observed stream temperatures. Calculated probable maximum stream temperatures are representative of the past (before 1990), but for streams with recorded annual maximum temperatures of about 30°C or higher they may also be indicative for warmer future climate conditions. Effects of latitude and elevation on probable maximum stream temperatures of the past are illustrated in this paper.

Atmospheric reaeration at the free surface of lakes and streams is a relevant process for water quality, thus the amount of oxygen transferred to the water body should be carefully estimated. Recent studies have demonstrated that available equations for estimation of the reaeration rate offer a poor fit with field data different from those for which each equation was originally developed. Thus, none of the available equations is applicable to all stream hydrodynamic conditions; on the contrary, they remain stream-specific, probably since some parameters involved in the process have been neglected in their formulation and their expressions are too simplistic. This paper proposes a comprehensive approach to the mass-transfer process at the air-water interface that is based on dimensional analysis. Careful inspection of equations in the literature shows that the mass-transfer process at the air-water interface has been affected by 14 different parameters. The application of dimensional analysis produces, for a wide rectangular section if wind speed is negligible, a dimensionless equation for the mass-transfer rate, where this rate is a function of the Froude number, channel slope, Reynolds number, Sherwood number, Weber number, and relative roughness. This expression is further developed to address the reaeration process in streams and rivers. As a result, at a fixed temperature, the dimensionless reaeration rate $K_{a\mathrm{ND}}$ (where ND denotes nondimensional) is finally a function of only the Froude number, channel slope, Reynolds number, and relative roughness. Moreover, the application of the Darcy-Wiesbach equation allows this dimensionless rate $K_{a\mathrm{ND}}$ to be considered as a function of only three of the aforementioned parameters. This result provides a comprehensive approach to the reaeration process that can also explain the unreliability of the literature equations available up to now.

Disposal of pyritic mine tailings under shallow water covers is an effective and commonly used method for reducing tailings oxidation, thus minimizing the formation of acid rock drainage. However, wind-wave-induced resuspension of tailings under the water cover could compromise suspended solids discharge criteria and increase the overall sulfide oxidation, since dissolved oxygen concentrations are higher in the water cover than in the pore water. This article reports field measurements of suspended tailings collected approximately 2–10 cm above the tailings surface by means of 16 sediment traps partly buried in the tailings bed. The traps were distributed over an area of about $600\times 300 m\text{,}$ where the water cover depth varied from 0 to 2 m. The traps were sampled on three occasions during a total study period of 93 days distributed over two field seasons. Wind speed and direction were monitored at the site during the study. Hourly average wind speeds ranged from 0 to 10 m/s. Suspended tailings were recovered in every sediment trap on each sampling event. Comparison of particle size distributions for suspended solids and bed tailings suggests that at this site and for the duration of this study, tailings erosion and resuspension occurred primarily in areas where the water cover depth was 1.0 m or less, and that suspended tailings were transported by currents to other parts of the water cover. The amount of tailings resuspension was related to the frequency of strong winds blowing in the direction of maximum fetch. Comparison of suspended solids and bed tailings compositions indicates that suspended tailings oxidized at a larger rate than bed tailings.

In many water quality model applications to real networks, it has been observed that the chlorine concentrations at nodes on the secondary pipes with long residence times do not compare well with the model predictions. Therefore, it is important to better estimate the correct chlorine contents of outer nodes in order to determine the dosing time and the amount of chlorine during secondary chlorination. A model is developed for evaluating chlorine decay in drinking-water distribution networks. The model uses a simplified two-dimensional expression of a chlorine transport and decay equation within a single pipe that includes the bulk-flow reaction, radial diffusion, and pipe-wall reaction of chlorine. The computer program DYNAQ developed in this study, which tracks chlorine transport and decay using a time-driven approach, is also applied to the chlorine measurements taken from a portion of the South Central Connecticut Regional Water Authority’s service area. A comparison of the results of the model with those of a previously developed model is also included in this paper.

A series of three-dimensional numerical modeling experiments was conducted to determine the impacts of reaeration on the bioattenuation of petroleum hydrocarbon contaminants in groundwater systems. The influence of reaeration on plume length and mass loss was affected by four mechanisms including the groundwater mass transfer coefficient $K_{L\text{,}\mathrm{GW}}\text{,}$ the concentration on partial pressure of $O_2$ in the overlying soil gas $P_{O_2}\text{,}$ the aquifer vertical dispersivity ${\alpha }_v\text{,}$ and the reaeration zone Z or region of mixing between contaminated ground water and $O_2$ supplied from the vadose zone. Results from this study showed that reaeration may reduce steady-state hydrocarbon plume length and mass by up to 87% as compared to the case in which reaeration is not considered. Among the four mechanisms, ${\alpha }_v$ exerted the greatest influence on plume length followed by $K_{L\text{,}\mathrm{GW}}\text{,}$ $P_{O_2}\text{,}$ and Z. Comparisons between plume length and the ratio of reaeration transport $K_{L\text{,}\mathrm{GW}}$ to hydraulic conductivity K indicated that appreciable reductions in steady-state plume length were possible when $K_{L\text{,}\mathrm{GW}}/K⩾{10}^{-3}\text{.}$

Biological activity in zones of chemical contamination changes the pore characteristics that control the flow of water and transport of dissolved chemicals in soils. To further the understanding of these processes, column experiments were performed to evaluate the effect of biomass growth on decane or naphthalene dissolved in simulated groundwater on the hydraulic conductivity and dispersivity of sand. The effect of grain size, groundwater flowrate, and nitrogen limitation were investigated. Given the low carbon loading resulting from the solubility of decane and naphthalene, sparse and discontinuous biomass growth reduced the hydraulic conductivity of the sand by 2 to 3 orders of magnitude after 35 to 63 days. This biogrowth initially increased dispersivity of the sand, but after longer periods of growth dispersivity, decreased to stable values near that of the clean sand. The results indicate that biogrowth can have significant effects in natural systems with low carbon loading and nitrogen availability, and should be taken into account when using models to predict contaminant transport in the field.

Anionic surfactant was added during absorption to investigate the solubility of vapor phase naphthalene and ${\mathrm{SO}}_2$ in water. Anionic surfactant employed was sodium dodecyl sulfate. Lower than critical micelle concentration (CMC), the apparent solubility and absorption rates of ${\mathrm{SO}}_2$ or naphthalene with or without ${\mathrm{SO}}_2$ were practically identical to those of pure water. However, higher than CMC, equilibrium ${\mathrm{SO}}_2$ or naphthalene apparent solubility increased linearly in proportion to the surfactant concentrations. The solubilization effect of micelles resulted in the increase. Because the micelle solubilization effect was greater than that of the decrease of the mass transfer coefficient, the gas absorption rate increased. When surfactant concentration was 0.1 M, the enrichment factor (EF) value of naphthalene with ${\mathrm{SO}}_2$ was 4.54, which was only half of its value without ${\mathrm{SO}}_2\text{.}$ When surfactant concentration was 0.2 M, the ${\mathrm{SO}}_2$ EF values increased to 2.24. These empirical findings confirm that to increase the removal efficiency of simultaneous absorption of hydrophobic organic compounds and ${\mathrm{SO}}_2$ via a spray or packed tower, an anionic surfactant can be employed.

Injecting aqueous solutions of hydrogen peroxide $\left(H_2{O}_2\right)$ into hot flue gases can convert nitric oxide (NO) to higher oxidation states $({\mathrm{NO}}_2\text{,}$ ${\mathrm{HNO}}_2\text{,}$ and ${\mathrm{HNO}}_3)\text{,}$ which can then be removed in a wet scrubber. The optimum temperature for such conversion is 500°C (930°F), at which $H_2{O}_2$ is thermally “activated” (split into free radicals). At lower temperatures ultraviolet (UV) light can be used to activate the peroxide molecules. In this pilot plant study at Kennedy Space Center, experiments were done with none, one, or two UV lamps on, with and without ${\mathrm{SO}}_2$ present in the flue gases, at various temperatures, and with various injection rates of peroxide. Temperatures ranged from 117 to 350°C (243 to 660°F), and the molar ratios (peroxide to ${\mathrm{NO}}_x)$ ranged from 0.68 to 5.02. Conversions of NO varied from below 10 to above 70%, with the highest conversions occurring with higher temperatures, higher dosages of hydrogen peroxide, and with both UV lamps turned on. Conversions of ${\mathrm{NO}}_x$ $(\mathrm{NO}+{\mathrm{NO}}_2)$ varied from below 5 to above 40%. The presence of ${\mathrm{SO}}_2$ did not inhibit NO or ${\mathrm{NO}}_x$ conversion.

The feasibility of using sulfur:limestone autotrophic denitrification (SLAD) pond reactors to treat nitrate-contaminated water or wastewater after secondary treatment was investigated with four lab-scale continuously fed SLAD ponds. The start-up period, temperature effects, and effects of different feed solutions were evaluated. With an influent concentration of 30 mg ${\mathrm{NO}}_3^{-}-N/L$ at an HRT of 30 days, the pond reactors had an overall nitrate removal efficiency of 85–100%. Effluent nitrite concentrations were <0.2 mg N/L in all tests. Aerobic conditions could result in a decrease of the SLAD pH of the pond by 2 to 3 units and a large increase in sulfate production $(\sim 1600-1800 \mathrm{mg}-{\mathrm{SO}}_4^{2-}/L)\text{.}$ Under unmixed (anoxic) conditions, the pH and sulfate produced were maintained at approximately 5.5 to 5.6 and $400-600 \mathrm{mg}-{\mathrm{SO}}_4^{2-}/L\text{,}$ respectively, in all the SLAD ponds. Temperature affected the pond reactors adversely. By assuming that a first-order reaction occurred in a SLAD pond reactor, the temperature-activity coefficient, θ was found to be 1.068. Treatment of nitrate-contaminated surface water and wastewater using SLAD pond systems is feasible only if (1) the chemical oxygen demand (COD)/nitrate–N (COD/N) ratio is low (<1.2 with an initial ${\mathrm{NO}}_3^{-}$ concentration of 30 mg-N/L), (2) sulfur:limestone granules are not covered by sediment, and (3) sulfur-utilizing but nondenitrifying bacteria (SUNDB) are greatly inhibited due to the lack of DO in the pond systems. The SLAD ponds are not feasible for the treatment of raw wastewater or surface water if they contain high concentrations of organic matters due to the possible inhibition of sulfur-based autotrophic denitrifiers by heterotrophs (including heterotrophic denitrifiers). In addition, a high sulfate and low DO concentration as well as a low pH in the SLAD effluent of the pond (even when the pond is operated in an unmixed mode) also will limit the application of SLAD pond processes.

Mass transfer effects on the kinetics of 1,2-dibromoethane (EDB) reduction by zero-valent iron (ZVI) in batch reactors, a laboratory scale packed-bed reactor, and a pilot scale packed-bed reactor are described. EDB was debrominated by ZVI to ethylene and bromide. EDB sorption to the cast iron surface was nonlinear and was described by a Langmuir equation. Laboratory scale column studies showed a nonlinear dependence of EDB removal on flow rate and initial EDB concentration. A nonequilibrium model of EDB sorption and reaction dependent on mass transfer was constructed using the laboratory scale data. The model was verified using data from a larger pilot scale packed-bed reactor that was used to remove EDB from contaminated groundwater. The data showed two distinct removal processes, an initial rapid phase dominated by mass transfer followed by a slower phase where surface reactions dominated. The model successfully predicted the transition from mass transfer controlled to surface reaction controlled conditions in the pilot scale data.

The reductive dechlorination of carbon tetrachloride (CT) was examined in the presence of free iron ions $\left({\mathrm{Fe}}^{2+}\right)\text{,}$ sulfide ions $\left(S^{2-}\right)\text{,}$ and freshly precipitated ferrous sulfide (FeS) as reducing agents, and vitamin $B_{12}$ as organic amendment. The reductive dechlorination of CT by the reducing ions ${\mathrm{Fe}}^{2+}$ and $S^{2-}$ in homogeneous phase resulted in the formation of variable amounts of the mono- and di-dechlorination products chloroform (CF) and dichloromethane (DCM). In the ferrous chloride $\left({\mathrm{FeCl}}_2\right)$ solutions (200 mM) where ${\mathrm{Fe}}^{2+}$ was the only electron source, 76% of the original CT was depleted within 1/2 h and about 28% and 8% went to CF and DCM, respectively, at a pH of 3.1 and with no buffer present. These same dechlorination products were observed in unbuffered sodium sulfide $\left({\mathrm{Na}}_{2}S\right)$ solutions (110 and 200 mM) with $S^{2-}$ as the electron source at pH 13.4. Dechlorination products were also observed in heterogeneous systems where FeS (75–200 mM) acted as the bulk reductant and the addition of vitamin $B_{12}$ (0–4 mM) resulted in an enhancement of the dechlorination reaction. When 4 mM of vitamin $B_{12}$ were added to 200 mM FeS, CT was removed continuously and the amount of CF and DCM formed increased significantly over time, yielding a mass recovery of 40% and higher after 1 h and a pseudofirst-order rate constant of 1.91 h⁻¹. The reductive dechlorination of CT in the absence of vitamin $B_{12}\text{,}$ resulted in a slower disappearance of CT and the formation of smaller amounts of CF and DCM accounting for only 1% of the mass loss in the 75 mM FeS suspensions to 15% in the 200 mM FeS suspensions after 2 h.