T.M. Scanlon, J.P. Raffensperger, G.M. Hornberger, R.B. Clapp

Two subsurface pathways are considered to contribute to stream discharge at the 237-ha South Fork of Brokenback Run (SFBR) catchment within Shenandoah National Park, VA: a transient, perched stormflow zone and a permanent, underlying groundwater zone. A modified version of TOPMODEL, simulating the separate processes that generate interflow and baseflow, is coupled with a geochemical model in order to demonstrate the transport of a natural tracer, dissolved silica, to the stream draining the first-order catchment. The presence of a perched stormflow zone, contributing to the rapid rise of the hydrograph during a storm event, is substantiated by data collected from a transect of six crest piezometer nests, with each nest measuring synchronic maximum potentials at depths of 0.5, 1.0 and 1.5 m below the topographic surface. Preliminary batch experiments have shown that water in contact with this shallow subsurface (<2 m) soil attains an average dissolved silica concentration of 9.9 mg/L, while water in contact with lower (>2 m to bedrock) soil and saprolite attains an average concentration of 21.3 mg/L after one day. Mixing of the water discharged to the stream from these two subsurface zones, along with overland flow from a variable source area, is modeled in order to reproduce both the hydrograph and the chemograph for several storms in the 1998 water year.

S.E. Johnson, J.S. Herman, A.L. Mills, and G.M. Hornberger.

The bioavailability and desorption characteristics of nonextractable soil-aged atrazine were investigated to examine the significance of this potentially large contaminant fraction in agricultural soils. Previous work by this research group compared rates and extents of microbially facilitated release and mineralization by a natural microbial consortium with desorption rates in sterile microcosms to examine the relationship between biodegradability and extraction method. The rates of mineralization by the natural soil bacteria, however, were too slow to test the biodegradability of the nonextractable phase. Work is underway to continue this research using a bacterial strain, isolated from a pesticide spill site, that is known to mineralize atrazine within days. Ring- and side-chain-labeled atrazine were applied to soil at a concentration of 5.6 µg/g and aged for three months. At the end of the aging period, the soil samples were extracted by one of three methods--methanol-water (4:1) at 75°C, acetone, and artificial soil water--representing a range of extraction efficiencies. After aging, atrazine extractability with the hot methanol-water method decreased from 96% efficiency for soil aged less than one hour to 66% for soils aged for three months. Acetone and artificial soil water extracted 51% and 29% of the aged atrazine, respectively. Microcosms are constructed with the dried, extracted soil and inoculated with a 10⁶ cells/mL bacterial suspension of the isolate. Mineralization is measured by flushing and trapping ¹⁴CO2 in the headspace gas. Rates and extents of mineralization of the aged and extracted soil treatments will be compared to freshly applied atrazine mineralization and to known rates and extents of desorption of the soil-associated atrazine to explore the relationship between extractability and bioavailability.

L.A. Sprague, J.S. Herman, G,M, Hornberger, A,L, Mills

Insight into colloid mobilization is sought to better assess the potential for colloid-facilitated transport of contaminants. To examine hydrologic controls on colloid mobilization, rainfall simulations were performed on 0.25 m² plots at the field site, an agricultural catchment in Rockingham County, VA. The plots were either near saturation (wet) or near field capacity (dry), and simulated rainfall events of 2.5 (low) and 5.0 cm/hr (high) were performed. Three gravity lysimeters placed at a depth of 25 cm collected infiltrating water. Mass recoveries of water ranged from 25% to 41%, 19% to 50%, 6% to 7% for the low/wet, high/wet, and high/dry plots, respectively; no water was recovered from the low/dry plots. Comparison of mass recovery of water in each of the three plots within treatments indicated a significant amount of physical heterogeneity at the site. Mass recoveries of water among treatments differed significantly due to antecedent moisture conditions but not precipitation rate, indicating that the moisture level of the soil prior to rainfall events controlled the water flux during the event, while the rate of precipitation at the surface had little effect. The rainfall simulations produced an initial high pulse of colloids, with concentrations declining through the course of the experiments. The total mass of colloids collected during the experiments were 150 mg, 1313 mg, and 78 mg for the low/wet, high/wet, and high/dry treatments, respectively. These values differed significantly due to both antecedent moisture conditions and precipitation rate. Strength-of-effects analyses demonstrated that while colloid generation increased with both higher moisture levels and higher precipitation rates, precipitation rate had a stronger effect. Total mass values were normalized to the total volume of water collected at each plot to obtain volume-averaged colloid concentrations of 88 mg/L, 555 mg/L, and 219 mg/L for the low/wet, high/wet, and high/dry treatments, respectively. Volume-averaged concentrations for all the plots were not statistically different, however, suggesting that mobile colloid concentrations depend primarily on water flux.
 
 

J.-W. Choi, J.A. Smith, T.B. Culver, and A.L. Mills
Previous research has shown that nonionic surface-active agents (surfactants) can increase the rate of pollutant desorption from soil to water, even at sub-critical micelle concentrations. The objective of this work is to determine if the surfactant Triton X-100 can increase the rate of TCE desorption and correspondingly increase the bioavailability and biodegradation rate of the solute. A series of batch microcosm studies and computer simulations were performed to determine the impact of the concentration of Triton X-100 on the rates of desorption and biodegradation TCE from a long-term field-contaminated soil under aerobic conditions in the presence and absence of methane. Four equilibrium aqueous concentrations of Triton X-100 (0, 30,150, and 3000 mg/l) were used. Desorption of TCE from the field-contaminated soil was distinctly rate-limited and well described by a desorption model that employed a gamma distribution of soil-water mass-transfer coefficients. A kinetic sorption and biodegradation model was used to analyze and interpret the experimental results. Surfactant concentrations above CMC decreased TCE biodegradation rates as well as methane consumption by native soil microorganisms. However, the inhibitory effect was not apprent at a low concentration (sub-CMC).
 
 
 
 

S.A. Riddle, J.A. Smith, and J.J. Deitsch

A series of kinetic batch sorption experiments were conducted to quantify the effects of solute solubility,
solute concentration (relative to aqueous solubility) and sorbent organic-carbon content on the rate of solute mass-
transfer from water to soil.  The rate of sorption was measured for for 1,2-dichlorobenzene (1,2-DCB),
4-chloroaniline (4-CHLN) and phenanthrene (PHN) on three natural soils at seven different aqueous concentrations.
Equilibrium aqueous concentrations ranged from 0.001 to 0.3 of aqueous solubility.  Organic-carbon contents for the
three soils ranged from 0.53 to 14.6 percent.  Equilibrium isotherms exhibit non-linearity at low aqueous concentrations (relative to solubility) and are approximately linear at high relative concentrations. Isotherms were modeled as the sum of a Freundlich isotherm and a linear isotherm to accurately predict the sorbed concentration over a wide range of aqueous concentrations. The kinetic sorption data were well described by with a distributed rate parameter model using a gamma distribution to approximate a continuous distribution of mass-transfer rate coefficients.  Preliminary results suggest that the mass-transfer coefficient distribution is not affected by relative solute concentration.
 
 
 
 

A. B. Chan Hilton and Teresa B. Culver

Groundwater remediation systems often cost millions of dollars at a single site.  By replacing trial-and-error remediation design with a mathematical optimization algorithm, the most cost-effective policy may be determined, resulting in potentially large cost savings. This work applies genetic algorithms (GAs) to the optimal design of  in situ bioremediation systems.  GAs are search algorithms inspired by the idea of ``survival of the fittest.''  Because no derivative information is required, GAs can handle nonconvex, highly nonlinear, and complex problems.  The optimization model combines the genetic algorithm approach with a two-dimensional flow and transport model for in situ bioremediation.  In this work, a hypothetical in situ bioremediation design problem is developed for a homogeneous, isotropic aquifer. The objective of this design problem is to minimize the total cost of in situ bioremediation remediation (capital and operating costs), given the initial plume, by varying the injection rates at 17 potential wells subject to constraints on the final contaminant concentration at 73 observation wells and the hydraulic heads in the aquifer.  At the end of the one-year remediation period, the contaminant concentration at the observation wells must meet the clean-up standard for the contaminant.  In order to determine the impacts of time-varying designs on the total remediation cost, optimal designs are determined for both steady-state and dynamic cases.  Additionally, the sensitivity of the optimal designs and costs on the water quality goals are evaluated.  Results from both of these issues would allow the designer to choose effective and efficient remediation strategies.
 
 
 
 

L.V. Smith, R.M. Ford, and A.L. Mills

A bioaugmentation scheme for creosote contaminated sediments in the Elizabeth River has been proposed by Environmental Solutions Inc. of Richmond, Virginia.  A bacterial consortium which has been enriched from a marine oil spill was selected for its ability to use polycyclic aromatic hydrocarbons (PAHs) as a carbon source.  The main premise of the strategy is that a porous solid support can be used to deliver the PAH-degrading consortium to the contaminated sediment and to provide nutrients while the bacteria adjust to a new environment.  While the ability of this bacterial consortium to degrade PAHs has been observed, the transport of enough viable cells to the sediment surface remains a critical step in this strategy.

To test the viability of the overall strategy, bench-scale macrocosms  of the river bottom have been set up.  The macrocosms consist of two twenty-nine gallon aquaria layered with contaminated sediment, the bacteria laden solid support, and river water.  The control tank contains bacteria killed by exposure to radiation instead of viable bacteria; consequently, the microbial action of the non-indigenous consortium is the only difference between the two tanks.  Measurements of temperature, pH, and dissolved oxygen levels were taken for comparison of the macrocosm to river conditions.  The progress of the macrocosm bioremediation was monitored by withdrawing cores of sediment and analyzing them for PAHs.  PAH concentration was determined by performing a batch extraction with methylene chloride, sample concentration, and gas chromatography analysis.  The changes in PAH concentration were measured to provide indirect evidence that this consortium was not only surviving but also degrading the contaminants.  Qualitative evidence of the survival of the consortium was gathered using plating techniques.  A comparison of the control and experimental macrocosm results was made to determine the potential of the proposed bioremediation strategy.