Performance of Engineered Streambeds for Inducing Hyporheic Transient Storage and Attenuation of Resazurin.

Several U.S. programs provide financial incentives for stream restoration to improve degraded water quality. These efforts prioritize hyporheic zone (HZ) restoration to enhance contaminant attenuation, but no stream restoration or stormwater best management practice (BMP) explicitly tailors hyporheic residence times to target specific contaminants of concern. Here we present the first physical demonstration of a new BMP called Biohydrochemical Enhancements for Streamwater Treatment (BEST). BEST are subsurface modules that use hydraulic conductivity modifications to drive hyporheic exchange and control residence times, combined with reactive geomedia to increase HZ reaction rates. Experiments were conducted in 15-m long outdoor flumes: one all-sand control, the other with BEST modules. Sodium chloride (conservative tracer) and resazurin (surrogate for a reactive pollutant) injections were conducted, with observations analyzed by stream transient storage models. Results demonstrated that BEST increased the effective HZ size and resazurin transformation both by ∼50% compared to the control. Numerical simulations of extended reach lengths showed that BEST could achieve 1-log removal of resazurin in 111 m, versus 172 m in the control, and 414 m and 683 m in two numerically simulated urban streams. These results emphasize the potential of BEST as a novel HZ BMP to improve streamwater quality.

Effects of chemical oxidants on perfluoroalkyl acid transport in one-dimensional porous media columns.

In situ chemical oxidation (ISCO) is a remediation approach that is often used to remediate soil and groundwater contaminated with fuels and chlorinated solvents. At many aqueous film-forming foam-impacted sites, perfluoroalkyl acids (PFAAs) can also be present at concentrations warranting concern. Laboratory experiments were completed using flow-through one-dimensional columns to improve our understanding of how ISCO (i.e., activated persulfate, permanganate, or catalyzed hydrogen peroxide) could affect the fate and transport of PFAAs in saturated porous media. While the resultant data suggest that standard ISCO is not a viable remediation strategy for PFAA decomposition, substantial changes in PFAA transport were observed upon and following the application of ISCO. In general, activated persulfate decreased PFAA transport, while permanganate and catalyzed hydrogen peroxide increased PFAA transport. PFAA sorption increased in the presence of increased aqueous polyvalent cation concentrations or decreased pH. The changes in contaminant mobility were greater than what would be predicted on the basis of aqueous chemistry considerations alone, suggesting that the application of ISCO results in changes to the porous media matrix (e.g., soil organic matter quality) that also influence transport. The application of ISCO is likely to result in changes in PFAA transport, where the direction (increased or decreased transport) and magnitude are dependent on PFAA characteristics, oxidant characteristics, and site-specific factors.

Metal release from sandstones under experimentally and numerically simulated CO2 leakage conditions.

Leakage of CO2 from a deep storage formation into an overlying potable aquifer may adversely impact water quality and human health. Understanding CO2-water-rock interactions is therefore an important step toward the safe implementation of geologic carbon sequestration. This study targeted the geochemical response of siliclastic rock, specifically three sandstones of the Mesaverde Group in northwestern Colorado. To test the hypothesis that carbonate minerals, even when present in very low levels, would be the primary source of metals released into a CO2-impacted aquifer, two batch experiments were conducted. Samples were reacted for 27 days with water and CO2 at partial pressures of 0.01 and 1 bar, representing natural background levels and levels expected in an aquifer impacted by a small leakage, respectively. Concentrations of major (e.g., Ca, Mg) and trace (e.g., As, Ba, Cd, Fe, Mn, Pb, Sr, U) elements increased rapidly after CO2 was introduced into the system, but did not exceed primary Maximum Contaminant Levels set by the U.S. Environmental Protection Agency. Results of sequential extraction suggest that carbonate minerals, although volumetrically insignificant in the sandstone samples, are the dominant source of mobile metals. This interpretation is supported by a simple geochemical model, which could simulate observed changes in fluid composition through CO2-induced calcite and dolomite dissolution.

Heavy Metal Bioaccumulation in Peruvian Food and Medicinal Products.

To better query regional sources of metal(loid) exposure in an under-communicated region, available scientific literature from 50 national universities (undergraduate and graduate theses and dissertations), peer-reviewed journals, and reports published in Spanish and English were synthesized with a focus on metal(loid) bioaccumulation in Peruvian food and medicinal products utilized locally. The study considered 16 metal(loid)s that are known to exert toxic impacts on humans (Hg, Al, Sb, As, Ba, Be, Cd, Cr, Sn, Ni, Ag, Pb, Se, Tl, Ti, and U). A total of 1907 individual analyses contained within 231 scientific publications largely conducted by Peruvian universities were analyzed. These analyses encompassed 239 reported species classified into five main food/medicinal groups-plants, fish, macroinvertebrates and mollusks, mammals, and "others" category. Our benchmark for comparison was the World Health Organization (Codex Alimentarius) standards. The organisms most frequently investigated included plants such as asparagus, corn, cacao, and rice; fish varieties like trout, tuna, and catfish; macroinvertebrates and mollusks including crab and shrimp; mammals such as alpaca, cow, chicken eggs, and milk; and other categories represented by propolis, honey, lichen, and edible frog. Bioaccumulation-related research increased from 2 to more than 25 publications per year between 2006 and 2022. The results indicate that Peruvian food and natural medicinal products can have dangerous levels of metal(loid)s, which can cause health problems for consumers. Many common and uncommon food/medicinal products and harmful metals identified in this analysis are not regulated on the WHO's advisory lists, suggesting the urgent need for stronger regulations to ensure public safety. In general, Cd and Pb are the metals that violated WHO standards the most, although commonly non-WHO regulated metals such as Hg, Al, As, Cr, and Ni are also a concern. Metal concentrations found in Peru are on many occasions much higher than what has been reported elsewhere. We conclude that determining the safety of food/medicinal products is challenging due to varying metal concentrations that are influenced not only by metal type but also geographical location. Given the scarcity of research findings in many regions of Peru, urgent attention is required to address this critical knowledge gap and implement effective regulatory measures to protect public health.

New Conceptual Model for Soil Treatment Units: Formation of Multiple Hydraulic Zones during Unsaturated Wastewater Infiltration.

Onsite wastewater treatment systems are commonly used in the United States to reclaim domestic wastewater. A distinct biomat forms at the infiltrative surface, causing resistance to flow and decreasing soil moisture below the biomat. To simulate these conditions, previous modeling studies have used a two-layer approach: a thin biomat layer (1-5 cm thick) and the native soil layer below the biomat. However, the effect of wastewater application extends below the biomat layer. We used numerical modeling supported by experimental data to justify a new conceptual model that includes an intermediate zone (IZ) below the biomat. The conceptual model was set up using Hydrus 2D and calibrated against soil moisture and water flux measurements. The estimated hydraulic conductivity value for the IZ was between biomat and the native soil. The IZ has important implications for wastewater treatment. When the IZ was not considered, a loading rate of 5 cm d resulted in an 8.5-cm ponding. With the IZ, the same loading rate resulted in a 9.5-cm ponding. Without the IZ, up to 3.1 cm d of wastewater could be applied without ponding; with the IZ, only up to 2.8 cm d could be applied without ponding. The IZ also plays a significant role in soil moisture distribution. Without the IZ, near-saturation conditions were observed only within the biomat, whereas near-saturation conditions extended below the biomat with the IZ. Accurate prediction of ponding is important to prevent surfacing of wastewater. The degree of water and air saturation influences pollutant treatment efficiency through residence time, volatility, and biochemical reactions.

Estimating historical exposure to perfluoroalkyl acids in Security, Fountain, and Widefield Colorado: use of water-infrastructure blending and toxicokinetic models.

Drinking water can be a major source of poly- and perfluoroalkyl substance (PFAS) exposure for humans. The lack of historic data on PFAS drinking-water concentrations and consumption patterns are a limiting factor for developing estimates of past exposure. Here, in contribution to a community-scale PFAS health effects study near fire training facilities that contaminated a local aquifer with PFASs, we present a novel water-infrastructure, mass-balance mixing model coupled to a non-steady state, single-compartment toxicokinetic model that used Monte Carlo simulations to estimate the start of PFAS exposure in drinking water for individuals within three PFAS-impacted communities in El Paso County, Colorado. Our modeling focused on perfluorohexane sulfonic acid (PFHxS) because median serum PFHxS concentrations in a sample of local residents (n = 213) were twelve times the median observed in the U.S. National Health and Nutrition Examination Survey (2015-2016). Modeling results for study participants were grouped according to their community of residence, revealing a median start of exposure for the town of Fountain of 1998 (25-75% interquartile range [IQR], 1992 to 2010), 2006 (IQR 1995 to 2012) for Security, and 2009 (IQR 1996-2012) for Widefield. Based on the towns' locations relative to an identified hydraulically upgradient PFAS source, the modeled exposure sequencing does not completely align with this conceptual flow model, implying the presence of an additional PFAS source for the groundwater between Widefield and Fountain.

Kinetic metal release from competing processes in aquifers.

Understanding groundwater time scales wherein kinetic metal-desorption and mineral-dissolution are important mechanisms is essential for realistic modeling of metal release. In this study, release rate constants were compiled and the Damköhler number was applied to calculate residence times where kinetic formulations are relevant. Desorption rate constants were compiled for arsenic, barium, cadmium, copper, lead, mercury, nickel, and zinc, and span 6 orders of magnitude, while mineral-dissolution rate constants compiled for calcite, kaolinite, smectite, anorthite, albite, K-feldspar, muscovite, quartz, goethite, and galena ranged over 13 orders of magnitude. This Damköhler analysis demonstrated that metal-desorption kinetics are potentially influential at residence times up to about two years, depending on the metal and groundwater conditions. Kinetic mineral-dissolution should be considered for nearly all residence times relevant to groundwater modeling, provided the rate, solubility, and availability of the mineral generates a non-negligible concentration. Geochemical models of competitive desorption and dissolution for an illustrative metal demonstrate total metal concentrations may be sensitive to dissolution rate variations despite the predominance of release from desorption. Ultimately, this analysis provides constraints on relevant processes for incorporation into transport models.

Comparison of methods to estimate air-water interfacial areas for evaluating PFAS transport in the vadose zone.

When performing calculations or numerical simulations for the fate and transport of PFAS and other surface-active solutes in the vadose zone, accurately representing the relationship between the area of the air-water interfaces (Aaw) as a function of water saturation (Sw), and changes in that relationship resulting from changes in soil texture, are equally important as accurately characterizing interfacial adsorption coefficients and the concentration dependence for PFAS solutes. This is true because the magnitude of the Aaw directly governs the degree of air-water interfacial adsorption, which contributes to the transport retardation of these solutes within unsaturated porous media. Herein, a well-known thermodynamic-based model for predicting the Aaw-Sw relationship is evaluated through comparisons to literature data collected using various measurement techniques for model sands and a limited number of soils using data collected from the current published literature. This predictive model, herein termed the Leverett thermodynamic model (LTM), relies on the characterization of the soil-water retention curve (SWRC) for a given soil, using the van Genuchten (VG) equation for the pressure head-vs-Sw relationship. Therefore, methods to estimate the VG equation parameters are also compared as to the Aaw-Sw relationships predicted. Comparisons suggest that the LTM provides the best estimate of the actual Aaw-Sw relationships for water containing non-surface-active solutes. Because PFAS solutes are also surface-active, Aaw measurement methods utilizing surface-active tracers are considered to provide the most accurate representation of the Aaw-Sw relationship for these solutes. Differences between Aaw-Sw relationships derived from tracer methods and the LTM are described in relation to media surface roughness effects. Based on the available literature data, a practical empirical model is proposed to adjust the LTM prediction to account for the effects of surface roughness on the magnitude of the Aaw for surface-active solutes. Finally, example retention calculations are performed to demonstrate the sensitivity of the predicted Aaw-Sw relationship on the vadose zone transport of of a representative PFAS, perfluorooctane sulfonate.

Simulated leaching of PFAS from land-applied municipal biosolids at agricultural sites.

Biosolids are an important resource for agricultural practice but have recently received increased focus as a potential source of per- and polyfluoroalkyl substances (PFAS) in the environment. Few studies have investigated the transport of PFAS through the unsaturated zone under conditions relevant to biosolids application sites. Herein, the unsaturated flow and transport model HYDRUS is used to evaluate the leaching of per- and polyfluoroalkyl substances (PFAS) from land-applied biosolids used in agricultural practice to determine the impacts of PFAS leaching on underlying groundwater resources. This numerical case study was based on conditions and operations at two test sites in central Illinois where biosolids were applied at agronomic rates and where PFAS contents and desorption characteristics were previously characterized. Each site possessed different vadose zone soil textural heterogeneity. Simulations were performed under actual present-day meteorological conditions and extended 150 years beyond the initial biosolids application. These long-term simulations demonstrate how soil equilibrium sorption/desorption processes within the biosolids-amended surface soils effectively control the transport rate of individual PFAS to groundwater. Air-water interfacial (AWI) adsorption, which is sometimes considered to be a significant source of PFAS retention in vadose zone soils, was observed to have minimal impacts on PFAS leaching rates within the biosolids-amended surface soils at these sites. Additionally, the impact of AWI adsorption was found to be most significant for PFAS transport within the underlying vadose zone soils when these soils were more texturally homogeneous and considerably less significant within the texturally heterogeneous soils represented herein. The results of multiple long-term simulations were used to develop an empirical equation that relates predicted maximum PFAS pore-water concentrations reaching the saturated zone with changes in PFAS concentrations in the biosolids-amended soil for various biosolids re-application events. This approach is shown to be very useful in developing site-specific PFAS soil screening levels and/or maximum leachate levels for PFAS in support of establishing best management practices (BMPs) for land application of biosolids.

Modeling PFAS Fate and Transport in Groundwater, with and Without Precursor Transformation.

Groundwater professionals require tools to evaluate a variety of technical issues related to per- and polyfluoroalkyl substances (PFAS). These include the potential impact of PFAS precursors on groundwater plumes of perfluoroalkyl acids (PFAAs). Numerical modeling results show that, by adjusting the mass loading rate, source zones with or without a precursor can produce similar PFAA plumes. However, if a precursor is present, it can impact PFAA plume concentrations and extend PFAA plume durations by decades. Additional research regarding in situ precursor transformation rates-and improvements in source area characterization-will further advance the predictive value of modeling.

Air-water interfacial adsorption coefficients for PFAS when present as a multi-component mixture.

Surface tension isotherms and calculated air-water interfacial (AWI) adsorption data are presented for solution mixtures of per- and polyfluoroalkyl substances (PFAS), specifically a series of binary and one ternary mixtures of homologous linear perfluorocarboxylic acids (PFCAs) in a simulated groundwater, and two 8-component mixtures containing both PFCAs and linear perfluoroalkane sulfonates (PFSAs). In all cases, non-ideal competitive adsorption was observed that favored the most surface-active component(s) of the solution mixture. The multi-component extended Langmuir (EL) isotherm model was observed to accurately predict the competitive adsorption observed in the binary and ternary PFCA solution mixtures. However, the predictive utility of the EL model was observed to diminish when mixtures contained both PFCAs and PFSAs, which differ in their hydrophile structure, resulting in overpredictions and underpredictions of the AWI adsorption isotherms derived from measured data depending on the specific components present in the solution mixtures. Observations indicate that the individual component adsorptive affinities for the AWI can change in response to competitive preferential adsorption as their solution concentrations increase that is not being captured by the EL model. Our results demonstrate that alternative mathematical models are needed that support concentration dependent affinity coefficients for non-similar mixtures of PFAS, such that the transport of individual target PFAS components within a larger mixture of components can be accurately predicted across a wider range of solution concentration.

Model evaluation of potential impacts of on-site wastewater systems on phosphorus in Turkey creek watershed.

Nutrient loading to surface water systems has traditionally been associated with agricultural sources. Sources such as on-site wastewater systems (OWS) may be of concern especially in rural, nonagricultural watersheds. The impact of various point and nonpoint sources including OWS in Turkey Creek Watershed was evaluated using the Watershed Analysis Risk Management Framework, which was calibrated using 10 yr of observed stream flow and total P concentrations. Doubling the population in the watershed or OWS septic tank effluent P concentration increased mean stream total P concentration by a factor of 1.05. Converting all the OWS to a conventional sewer system with a removal efficiency of 93% at the wastewater treatment plant increased the mean total P concentration at the watershed outlet by a factor of 1.26. Reducing the soil adsorption capacity by 50% increased the mean stream total P concentration by a factor of 3.2. Doubling the initial P concentration increased the mean stream total P concentration by a factor of 1.96. Stream flow and sediment transport also substantially affected stream P concentration. The results suggest that OWS contribution to stream P in this watershed is minimal compared with other factors within the simulated time frame of 10 yr.

Subsurface transport potential of perfluoroalkyl acids (PFAAs): Column experiments and modeling.

Transport of ten perfluoroalkyl acids (PFAAs) was studied with one-dimensional (1-D) saturated column experiments using four soil types with an organic carbon fraction (foc) range of ~0-0.045. Columns were operated under conditions relevant to aqueous film-forming foam (AFFF)-impacted fire protection training areas to determine the ability of equilibrium transport parameters to describe 1-D PFAA transport, if rate-limited sorption influences PFAA transport, and if kinetic parameters can be used to evaluate factors causing rate-limited sorption. Results of initial screening of PFAA breakthrough found that over half of the breakthrough curves deviated from equilibrium transport and merited further investigation. Subsequent analysis showed that, in many cases, these deviations could be accounted for by considering the range of applicable equilibrium Kd values (i.e. based on standard deviation) applicable to the solid phase. Thus, transport of the majority of PFAAs in 3 soils with foc of 0-0.017 was not impacted by rate-limited sorption. Further, low sorption led to transport that was essentially simultaneous for the majority of PFAAs in these porous media. Exceptions were observed for long-chain PFAAs, and also in a fourth soil with foc of 0.045, which indicated the potential for rate-limited sorption to impact transport in some scenarios. Subsequent flow interruption experiments isolating kinetic behavior confirmed rate-limited sorption caused nonequilibrium transport. Linear free energy relationships (LFERs) developed in previous work to predict the inverse relationship between mass transfer coefficients (k) and sorption parameters (i.e., Kd) were used to estimate values of k for PFAAs in this study. Resulting k values were 10-3 to 10-8 h-1, consistent with previously measured kinetic parameters for other polar and anionic compounds. Models incorporating estimated k values resulted in improved predictions of breakthrough observed in nonequilibrium scenarios (R2 0.83-0.98), but k values will require further validation prior to broader application. This work illustrates rate-limited sorption considerations are needed to describe 1-D column saturated transport for some PFAAs and solid phases. At field scales, subsurface heterogeneity and PFAA precursor transformation may be equally or even more important in determining saturated PFAA transport, but kinetic parameters in this study may help to determine relative contributions of rate-limited sorption to overall transport.

Evaluating air-water and NAPL-water interfacial adsorption and retention of Perfluorocarboxylic acids within the Vadose zone.

The release and transport of linear perfluorocarboxylic acids (PFCA) within the vadose-zone beneath per- and polyfluoroalkyl substance (PFAS)- and non-aqueous phase liquid (NAPL)-contaminated source areas is influenced by multi-phase interfacial retention phenomena. Conceptually, interfacial adsorption results in retardation of PFCA velocities in subsurface multiphase systems. However, site hydrochemical factors influencing interfacial adsorption are not yet fully elucidated. Herein, air-water and NAPL-water interfacial tension isotherms were prepared for six homologous PFCAs of environmental significance for deionized water and five synthetic groundwaters of increasing ionic strength. The isotherms were successfully modeled by the Langmuir-Szyskowski equation and parameters used to fit the measured data are provided. Concentration-dependent interfacial adsorption coefficients and retardation factors are also provided for each PFCA and ionic strength condition and are evaluated to assess their significance. Simplifying relationships for predicting interfacial adsorption based on PFCA chain length were found to be less appropriate for natural groundwaters that contain a mixture of dissolved divalent and monovalent ions. Air-water interfacial (AWI) adsorption increased in a threshold manner with ionic strength from 0 to 6 mM, whereafter further adsorption was marginal. PFCA retention within water-unsaturated porous media is shown to depend on a number of inter-related factors and conditions that complicate the use of retardation factors within analytical models typically used for predicting transport rates under field conditions. Numerical simulation is thus necessary to model fundamental fate and transport processes. Mathematical relationships for incorporating interfacial adsorption in future and existing unsaturated flow and transport models are described.

Evaluating emerging organic contaminant removal in an engineered hyporheic zone using high resolution mass spectrometry.

The hyporheic zone (HZ), located at the interface of surface and groundwater, is a natural bioreactor for attenuation of chemical contaminants. Engineered HZs can be incorporated into stream restoration projects to enhance hyporheic exchange, with flowpaths optimized to promote biological habitat, water quantity, and water quality improvements. Designing HZs for in-stream treatment of stormwater, a significant source of flow and contaminant loads to urban creeks, requires assessment of both the hydrology and biogeochemical capacity for water quality improvement. Here, we applied tracer tests and high resolution mass spectrometry (HRMS) to characterize an engineered hyporheic zone unit process, called a hyporheic design element (HDE), in the Thornton Creek Watershed in Seattle, WA. Dye, NaCl, and bromide were used to hydrologically link downwelling and upwelling zones and estimate the hydraulic retention time (HRT) of hyporheic flowpaths. We then compared water quality improvements across hydrologically-linked surface and hyporheic flowpaths (3-5 m length; ∼30 min to >3 h) during baseflow and stormflow conditions. We evaluated fate outcomes for 83 identified contaminants during stormflow, including those correlated with an urban runoff mortality syndrome in coho salmon. Non-target HRMS analysis was used to assess holistic water quality improvements and evaluate attenuation mechanisms. The data indicated substantial water quality improvement in hyporheic flowpaths relative to surface flow and improved contaminant removal with longer hyporheic HRT (for ∼1900 non-target compounds detected during stormflow, <17% were attenuated >50% via surface flow vs. 59% and 78% via short and long hyporheic residence times, respectively), and strong contributions of hydrophobic sorption towards observed contaminant attenuation.

Microbial community composition of a hydrocarbon reservoir 40 years after a CO2 enhanced oil recovery flood.

Injecting CO2 into depleted oil reservoirs to extract additional crude oil is a common enhanced oil recovery (CO2-EOR) technique. However, little is known about how in situ microbial communities may be impacted by CO2 flooding, or if any permanent microbiological changes occur after flooding has ceased. Formation water was collected from an oil field that was flooded for CO2-EOR in the 1980s, including samples from areas affected by or outside of the flood region, to determine the impacts of CO2-EOR on reservoir microbial communities. Archaea, specifically methanogens, were more abundant than bacteria in all samples, while identified bacteria exhibited much greater diversity than the archaea. Microbial communities in CO2-impacted and non-impacted samples did not significantly differ (ANOSIM: Statistic R = -0.2597, significance = 0.769). However, several low abundance bacteria were found to be significantly associated with the CO2-affected group; very few of these species are known to metabolize CO2 or are associated with CO2-rich habitats. Although this study had limitations, on a broad scale, either the CO2 flood did not impact the microbial community composition of the target formation, or microbial communities in affected wells may have reverted back to pre-injection conditions over the ca. 40 years since the CO2-EOR.

Estimating biozone hydraulic conductivity in wastewater soil-infiltration systems using inverse numerical modeling.

During operation of an onsite wastewater treatment system, a low-permeability biozone develops at the infiltrative surface (IS) during application of wastewater to soil. Inverse numerical-model simulations were used to estimate the biozone saturated hydraulic conductivity (K(biozone)) under variably saturated conditions for 29 wastewater infiltration test cells installed in a sandy loam field soil. Test cells employed two loading rates (4 and 8cm/day) and 3 IS designs: open chamber, gravel, and synthetic bundles. The ratio of K(biozone) to the saturated hydraulic conductivity of the natural soil (K(s)) was used to quantify the reductions in the IS hydraulic conductivity. A smaller value of K(biozone)/K(s,) reflects a greater reduction in hydraulic conductivity. The IS hydraulic conductivity was reduced by 1-3 orders of magnitude. The reduction in IS hydraulic conductivity was primarily influenced by wastewater loading rate and IS type and not by the K(s) of the native soil. The higher loading rate yielded greater reductions in IS hydraulic conductivity than the lower loading rate for bundle and gravel cells, but the difference was not statistically significant for chamber cells. Bundle and gravel cells exhibited a greater reduction in IS hydraulic conductivity than chamber cells at the higher loading rates, while the difference between gravel and bundle systems was not statistically significant. At the lower rate, bundle cells exhibited generally lower K(biozone)/K(s) values, but not at a statistically significant level, while gravel and chamber cells were statistically similar. Gravel cells exhibited the greatest variability in measured values, which may complicate design efforts based on K(biozone) evaluations for these systems. These results suggest that chamber systems may provide for a more robust design, particularly for high or variable wastewater infiltration rates.

Modeling potential vadose-zone transport of nitrogen from onsite wastewater systems at the development scale.

Water in the urban front-range corridor of Colorado has become an increasingly critical resource as the state faces both supply issues as well as anthropogenic degradation of water quality in several aquifers used for drinking water. A proposed development (up to 1100 homes over two quarter-quarter sections) at Todd Creek, Colorado, a suburb of Westminster located about 20 miles northeast of Denver, is considering use of onsite wastewater systems (OWS) to treat and remove domestic wastewater. Local health and environmental agencies have concerns for potential impacts to local water quality. Nitrogen treatment in the vadose zone and subsequent transport to ground water at a development scale is the focus of this investigation. The numerical model HYDRUS 1D was used, with input based on site-specific data and several transport parameters estimated from statistical distribution, to simulate nitrate concentrations reaching ground water. The model predictions were highly sensitive to mass-loading of nitrogen from OWS and the denitrification rate coefficient. The mass loading is relatively certain for the large number of proposed OWS. However, reasonable values for the denitrification rate coefficients vary over three orders of magnitude. Using the median value from a cumulative frequency distribution function, based on rates obtained from the literature, resulted in simulated output nitrate concentrations that were less than 1% of regulatory maximum concentrations. Reasonable rates at the lower end of the reported range, corresponding to lower 95% confidence interval estimates, result in simulated nitrate concentrations reaching groundwater above regulatory limits.

Determining conditional stability constants for Pb complexation by carboxymethyl-beta-cyclodextrin (CMCD).

Carboxymethyl-beta-cyclodextrin (CMCD) has been proposed for remediation of metal-contaminated sediments. This research presents stability constants for CMCD-lead complexes, and demonstrates a rigorous methodology for estimating stability constants for metal-complexing agents. The conditional stability constant for the lead-CMCD aqueous complex was determined to be 10(5.18) with the 95% confidence interval ranging from 10(5.14) to 10(5.22). The best fit for experimental data was made by assuming a reaction between divalent CMCD(2-) and Pb(2+) and using the WATEQ activity coefficient formulation. The optimized value was derived from experimental data with the geochemical model PHREEQC coupled to UCODE_2005, a parameter optimization program. Like FITEQL, UCODE has a built-in option to optimize parameter values by minimizing the weighted sum of squared residuals (WSSR). However, our approach not only allows rapid, automatic optimization of the stability constant, but also allows determination of uncertainties in estimated parameter values and statistical analysis to assess the appropriateness of the conceptual model. The automation of the process allows testing of multiple conceptual models and the final values produced are internally consistent with the PHREEQC database. In this case five different conceptual models to describe the metal complexation and protonation reactions of CMCD were considered.

Using PHREEQC to simulate solute transport in fractured bedrock.

The geochemical computer model PHREEQC can simulate solute transport in fractured bedrock aquifers that can be conceptualized as dual-porosity flow systems subject to one-dimensional advective-dispersive transport in the bedrock fractures and diffusive transport in the bedrock matrix. This article demonstrates how the physical characteristics of such flow systems can be parameterized for use in PHREEQC, it provides a method for minimizing numerical dispersion in PHREEQC simulations, and it compares PHREEQC simulations with results of an analytical solution. The simulations assumed a dual-porosity conceptual model involving advective-reactive-dispersive transport in the mobile zone (bedrock fracture) and diffusive-reactive transport in the immobile zone (bedrock matrix). The results from the PHREEQC dual-porosity transport model that uses a finite-difference approach showed excellent agreement compared with an analytical solution.