Vadose Zone Soil Research Articles

Leaching Behavior of Aluminum, Arsenic, and Chromium from Highway Structural Fills Amended with High-Carbon Fly Ash

Coal combustion byproducts such as high-carbon fly ash (HCFA) are commonly used in highway construction as a stabilizing agent or soil amendment. Annually, 131 million tons of fly ash is produced in the United States. Unfortunately, only 40% of this fly ash is recycled, most of which are Class C and Class F types. These types of fly ash are generally used in concrete and cement production. However, HCFA cannot be used in such processes because of its high carbon content. Therefore, large amounts of HCFA are being put into landfills and causing significant economic loss and loss of valuable land space. Reuse of HCFA in geotechnical structures, particularly in road embankment construction, keeps these materials out of landfills. However, it is important to determine the environmental suitability of HCFA as a soil amendment because it poses a great potential for metal leaching. To evaluate environmental suitability, a series of column leach tests were conducted to determine the leaching of metals (aluminum, arsenic, and chromium) from soils amended with fly ash. In addition, numerical computer modeling programs were used to predict the metal concentrations in the soil vadose zone and ground-water. Results from laboratory tests indicated that metal concentrations increased with an increase in fly ash content and did not exceed the maximum concentration limits for drinking water, with few exceptions. Numerical modeling techniques predicted that leached metal concentrations decreased in the soil vadose zone with time and that concentrations of leached metals would be below environmental regulatory limits.

Soil gas carbon dioxide probe: laboratory testing and field evaluation

An automated semi-continuous on-line instrument has been developed to measure CO2 gas concentrations in the vadose zone. The instrument uses semi-permeable polymer tubing (CO2 probe) for diffusion based sampling, coupled to an infra red sensor. The system operated automatically by intermittently purging the CO2 probe, which was installed in the vadose zone, with a non-CO2 gas at a low flow rate. The gas exiting the CO2 probe was monitored at the ground surface using a miniature infra red sensor and the response related to the vadose zone soil gas CO2 concentration. The in situ CO2 probes provided a reliable monitoring technique under long-term (18 months) aggressive and dynamic field conditions, with no interference observed from non-CO2 gases and volatile organic compounds. The probes provided data that were comparable to conventional grab sampling techniques without the labour-intensive sample collection and processing associated with these conventional techniques. Also, disturbance to vadose zone CO2 profiles from repeated grab samples during long-term semi-continuous monitoring could potential be reduced by using the diffusion based sampling technique.

Pilot-scale vadose zone biobarriers removed nitrate leaching from a cattle corral

Agricultural activities that involve animal wastes can result in the contamination of subsurface soils by nitrates (NO₃). In saturated or nearly saturated soils, microbial biobarriers are a common method used to remove contaminants from water. This study evaluated the presence of NO₃ in the soil profile beneath a cattle corral and the use of biobarriers to prevent the movement of NO₃ through these semiarid vadose zone soils. Soils from beneath the pen contained as much as 330 mg kg⁻¹ (330 ppm) nitrate-nitrogen (NO₃-N), and a well located ~50 m (~55 yd) from the pens contained 18 mg L⁻¹ (18 ppm) NO₃-N, indicating that denitrification activity in the surface soils of the corral had not prevented NO₃ from migrating into deeper soils where denitrification activity was limited by electron donor (i.e., carbon [C] substrate) availability. Biobarriers used to intercept the percolating soil water contaminated with NO₃ included a control that contained only sand and treatment biobarriers that contained sand blended with sawdust, soybean (<i>Glycine max</i> L.) oil, or sawdust and soybean oil. A distinctive attribute of the study was that the biobarriers were installed near the surface of a semiarid soil well above the water table. Results show that all of the treatment biobarriers removed NO₃ from percolating soil water. Water collected from beneath the control barriers contained 99 mg L⁻¹ (99 ppm) NO₃-N while NO₃-N levels in water that moved through the sawdust, soybean oil, and sawdust plus soybean oil barriers averaged 4, 17, and 28 mg L⁻¹ (4, 17, and 28 ppm) NO₃-N for removal efficiencies of 96%, 83%, and 72%, respectively. In waters from the sawdust plus soybean oil barriers, 6.8 mg L⁻¹ (6.8 ppm) nitrite-N was detected. With all three substrate combinations, biobarrier life was such that barriers of this type should last for many years before substrates need to be replaced.

Sensitivity of surface flux simulations to hydrologic parameters based on an uncertainty quantification framework applied to the Community Land Model

Uncertainties in hydrologic parameters could have significant impacts on the simulated water and energy fluxes and land surface states, which will in turn affect atmospheric processes and the carbon cycle. Quantifying such uncertainties is an important step toward better understanding and quantification of uncertainty of integrated earth system models. In this paper, we introduce an uncertainty quantification (UQ) framework to analyze sensitivity of simulated surface fluxes to selected hydrologic parameters in the Community Land Model (CLM4) through forward modeling. Thirteen flux tower footprints spanning a wide range of climate and site conditions were selected to perform sensitivity analyses by perturbing the parameters identified. In the UQ framework, prior information about the parameters was used to quantify the input uncertainty using the Minimum‐Relative‐Entropy approach. The quasi‐Monte Carlo approach was applied to generate samples of parameters on the basis of the prior pdfs. Simulations corresponding to sampled parameter sets were used to generate response curves and response surfaces and statistical tests were used to rank the significance of the parameters for output responses including latent (LH) and sensible heat (SH) fluxes. Overall, the CLM4 simulated LH and SH show the largest sensitivity to subsurface runoff generation parameters. However, study sites with deep root vegetation are also affected by surface runoff parameters, while sites with shallow root zones are also sensitive to the vadose zone soil water parameters. Generally, sites with finer soil texture and shallower rooting systems tend to have larger sensitivity of outputs to the parameters. Our results suggest the necessity of and possible ways for parameter inversion/calibration using available measurements of latent/sensible heat fluxes to obtain the optimal parameter set for CLM4. This study also provided guidance on reduction of parameter set dimensionality and parameter calibration framework design for CLM4 and other land surface models under different hydrologic and climatic regimes.

Pedotransfer Functions for Predicting Soil Hydraulic Properties of the Chinese Loess Plateau

Soil hydraulic properties play an important role in modelingwater and solute movement within the vadose zone of soils. Direct measurement of hydraulic parameters at a wide range of scales involves considerable time, labor, and money. Pedotransfer functions (PTF) may provide an alternative way of estimating these parameters indirectly from easy-to-measure soil properties. The Loess Plateau of China lacks large databases of hydraulic parameters and also the PTF that could determine them accurately enough for scientists and policy makers to address many of the region's related problems, such as severe soil erosion. In this study, new PTF for saturated hydraulic conductivity (K-s), field capacity, and saturated soil-water content were developed. Multiple linear regression was used to analyze 252 data sets of the hydraulic and basic soil properties, as well as altitude, to derive the PTF. A further 130 data sets were used for validation. The predictive capabilities of the PTF were the best for saturated soil-water content (R-adj(2) = 0.78) and least for log K-s (R-adj(2) = 0.36). Bulk density, soil organic carbon, and soil particle composition were identified as significant input variables for the PTF. The inclusion of a topographic factor (altitude) significantly improved the predictive capability of the PTF for log K-s. Compared with established PTF, the PTF developed in this study predicted the hydraulic parameters more accurately as indicated by higher R-2 and lower RMSE values when predicted, and measured parameter values were compared, and the greatest improvement was obtained for log K-s. The new PTF are the first set of PTF based on data from the Loess Plateau. Their better performance makes them applicable for a variety of purposes in the Plateau region and possibly in other loess regions around the world.

Uncertainty assessment and implications for data acquisition in support of integrated hydrologic models

The data set used for calibration of regional numerical models which simulate groundwater flow and vadose zone processes is often dominated by head observations. It is to be expected therefore, that parameters describing vadose zone processes are poorly constrained. A number of studies on small spatial scales explored how additional data types used in calibration constrain vadose zone parameters or reduce predictive uncertainty. However, available studies focused on subsets of observation types and did not jointly account for different measurement accuracies or different hydrologic conditions. In this study, parameter identifiability and predictive uncertainty are quantified in simulation of a 1‐D vadose zone soil system driven by infiltration, evaporation and transpiration. The worth of different types of observation data (employed individually, in combination, and with different measurement accuracies) is evaluated by using a linear methodology and a nonlinear Pareto‐based methodology under different hydrological conditions. Our main conclusions are (1) Linear analysis provides valuable information on comparative parameter and predictive uncertainty reduction accrued through acquisition of different data types. Its use can be supplemented by nonlinear methods. (2) Measurements of water table elevation can support future water table predictions, even if such measurements inform the individual parameters of vadose zone models to only a small degree. (3) The benefits of including ET and soil moisture observations in the calibration data set are heavily dependent on depth to groundwater. (4) Measurements of groundwater levels, measurements of vadose ET or soil moisture poorly constrain regional groundwater system forcing functions.

From atmospheric winds to fracture ventilation: Cause and effect

Vadose zone fractures and soil cracks exposed to the atmosphere have an impact on gas exchange processes at the Earth–atmosphere interface. In this study we explored and quantified the role of ground‐surface winds on fracture ventilation. While the governing physical mechanisms that cause ventilation are relatively well understood, this is the first work to quantify these processes in natural fractures and to determine the net effect on gas exchange. In this study field measurements pointed to a correlation between surface wind velocity and the ventilate rate of surface‐exposed fractures. To better explore and quantify this phenomenon, laboratory experiments were carried out using a Hele‐Shaw chamber to simulate a natural fracture and the ventilation of smoke, used as a gas tracer, was explored as a function of controlled surface‐wind and fracture aperture. It was found that ventilation depth is linearly correlated to wind velocity and nonlinearly with fracture aperture. Results were used to formulate an empirical model for Earth‐atmosphere air exchange. This model can be used to estimate by how much the presence of fractures enhances that exchange under windy conditions. Incorporating this venting process into Earth‐atmosphere gas exchange simulations is another step toward improving our ability to better predict and quantify soil aeration, soil temperature variation, water vapor loss and processes related to climate change, such as the fate and transport of greenhouse gases.

Pine Straw Harvesting Effects on Vadose-Zone Water Content of a Leadvale Silt Loam in Western Arkansas

This study addresses concerns that harvesting marketable pine straw from forests may decrease timber productivity by allowing water to evaporate more quickly from the soil surface. Three harvesting schedules and a control treatment (no straw harvest) were replicated six times on 24 plots (0.18 ha each), and compared to determine harvesting effects on water content of the soil vadose zone in an established (16 yr) loblolly (Pinus taeda L.) plantation (3.0 × 1.5 m2 tree spacing). Pine straw harvesting tended to decrease volumetric soil water content (%) at depths below 20 cm, but the effect was significant (p < .05) only at the 50-cm depth in Weeks 3 and 4 (late June) of the study, when water content at this depth averaged 20.9% for soils where straw was harvested annually, and 30.2% for soils where the straw was never harvested (control). In soils where pine straw had been allowed to accumulate for at least a year after the previous harvest, average water content was not significantly different than in the control plots. Therefore, pine straw harvesting can potentially lengthen drought-stress periods for loblolly pine on some soils during the 1st yr after pine straw has been removed.

Vadose Zone Biodegradation of Benzene Vapors in Repacked and Undisturbed Soil Cores

Bench testing of petroleum biodegradation rates in vadose zone soils is typically associated with errors because transport conditions in laboratory systems are different from those found in the vadose zone. This work addressed the effect of soil structure and gas transport properties on hydrocarbon biodegradation in the unsaturated zone and we present a novel method for measuring biodegradation rates in intact and undisturbed soil columns of 100 cm3. To determine whether soil structure and gas diffusivity, defined as the ratio of the gas diffusion coefficient in soil to that in free air (Dp/D0), influence the outcome of aerobic benzene biodegradation experiments, measurements using identical sandy soils were performed on (i) undisturbed 100‐cm3 core samples; (ii) sieved (2‐mm) and repacked 100‐cm3 core samples; and (iii) soil samples (10 g) prepared as slurry microcosms. While slurry reactor experiments changed the first‐order rate constant (kw,1) significantly compared with undisturbed core samples, this was not the case for soil cores that had been sieved and repacked. This suggests that soil structure on a millimeter scale does not affect aerobic biodegradation in relatively unstructured sandy soils. Within differently textured soil cores, the biodegradation rate was found to increase with gas diffusivity when Dp/D0 &lt; 0.02. This establishes gaseous O2 and petroleum vapor diffusion and distribution in soil profiles as a controlling factor for natural biodegradation of petroleum vapors.

Experimental and numerical analysis of metal leaching from fly ash-amended highway bases

A study was conducted to evaluate the leaching potential of unpaved road materials (URM) mixed with lime activated high carbon fly ashes and to evaluate groundwater impacts of barium, boron, copper, and zinc leaching. This objective was met by a combination of batch water leach tests, column leach tests, and computer modeling. The laboratory tests were conducted on soil alone, fly ash alone, and URM–fly ash–lime kiln dust mixtures. The results indicated that an increase in fly ash and lime content has significant effects on leaching behavior of heavy metals from URM–fly ash mixture. An increase in fly ash content and a decrease in lime content promoted leaching of Ba, B and Cu whereas Zn leaching was primarily affected by the fly ash content. Numerically predicted field metal concentrations were significantly lower than the peak metal concentrations obtained in laboratory column leach tests, and field concentrations decreased with time and distance due to dispersion in soil vadose zone.

Review of Microbial Processes in the Near-Surface Environment and Their Implications for the Chemical Fingerprinting of Hydrocarbon Fuels

The chemical composition of middle-distillate fuels released in the subsurface environment is predominantly affected by biochemical processes resulting in selective degradation of hydrocarbons by indigenous microorganisms. The resulting progression in pattern recognition-based fingerprints depends on the composition of the hydrocarbon product, parameters of the product release (e.g., quantity and rate of release) and environmental conditions. The changes for saturated hydrocarbons follow a common sequence of removal of different compound classes: n-alkanes > isoalkanes > alkylcyclohexanes, with a systematic relationship between decreasing relative degradation rates and increasing chain length. However the timescale of these transformations is defined by the subsurface redox condition. Aerobic biodegradation of saturated hydrocarbons can be observed on a relatively short timescale of years to decades, whereas anaerobic degradation is a much slower process. The fact that aerobic biodegradation produces measurable changes in n-alkane concentrations within this timescale makes it a logical option for evaluating time elapsed since a fuel release into the subsurface environment. This paper provides the synopsis of major modes and pathways of biodegradation processes in the near-surface environment and their effect on the temporal changes in the fingerprints of middle-distillate fuels. In particular, a linear decrease in n-alkane concentrations with time allows for the time of fuel release estimates, utilizing the zero-order kinetics model for the Christensen and Larsen method. This model also allows for establishing applicability limits of this approach. The model is relevant only for hydrocarbon contamination in unsaturated (vadose) zone soil where the availability of sufficient concentration of molecular oxygen favors aerobic biodegradation.

Chromium(VI) generation in vadose zone soils and alluvial sediments of the southwestern Sacramento Valley, California: A potential source of geogenic Cr(VI) to groundwater

Concentrations of geogenic Cr(VI) in groundwater that exceed the World Health Organization’s maximum contaminant level for drinking water (50 μg L−1) occur in several locations globally. The major mechanism for mobilization of this Cr(VI) at these sites is the weathering of Cr(III) from ultramafic rocks and its subsequent oxidation on Mn oxides. This process may be occurring in the southern Sacramento Valley of California where Cr(VI) concentrations in groundwater can approach or exceed 50 μg L−1. To characterize Cr geochemistry in the area, samples from several soil auger cores (approximately 4 m deep) and drill cores (approximately 25 m deep) were analyzed for total concentrations of 44 major, minor and trace elements, Cr associated with labile Mn and Fe oxides, and Cr(VI). Total concentrations of Cr in these samples ranged from 140 to 2220 mg per kg soil. Between 9 and 70 mg per kg soil was released by selective extractions that target Fe oxides, but essentially no Cr was associated with the abundant reactive Mn oxides (up to ∼1000 mg hydroxylamine-reducible Mn per kg soil was present). Both borehole magnetic susceptibility surveys performed at some of the drill core sites and relative differences between Cr released in a 4-acid digestion versus total Cr (lithium metaborate fusion digestion) suggest that the majority of total Cr in the samples is present in refractory chromite minerals transported from ultramafic exposures in the Coast Range Mountains. Chromium(VI) in the samples studied ranged from 0 to 42 μg kg−1, representing a minute fraction of total Cr. Chromium(VI) content was typically below detection in surface soils (top 10 cm) where soil organic matter was high, and increased with increasing depth in the soil auger cores as organic matter decreased. Maximum concentrations of Cr(VI) were up to 3 times greater in the deeper drill core samples than the shallow auger cores. Although Cr(VI) in these vadose zone soils and sediments was only a very small fraction of the total solid phase Cr, they are a potentially important source for Cr(VI) to groundwater. Enhanced groundwater recharge through the vadose zone due to irrigation could carry Cr(VI) from the vadose zone to the groundwater and may be the mechanism responsible for the correlation observed between elevated Cr(VI) and NO3- concentrations in previously published data for valley groundwaters. Incubation of a valley subsoil showed a Cr(VI) production rate of 24 μg kg−1 a−1 suggesting that field Cr(VI) concentrations could be regenerated annually. Increased Cr(VI) production rates in H+-amended soil incubations indicate that soil acidification processes such as nitrification of ammonium in fertilizers could potentially increase the occurrence of geogenic Cr(VI) in groundwater. Thus, despite the natural origin of the Cr, Cr(VI) generation in the Sacramento Valley soils and sediments has the potential to be influenced by human activities.

Sea level rise and South Florida coastal forests

Coastal ecosystems lie at the forefront of sea level rise. We posit that before the onset of actual inundation, sea level rise will influence the species composition of coastal hardwood hammocks and buttonwood (Conocarpus erectus L.) forests of the Everglades National Park based on tolerance to drought and salinity. Precipitation is the major water source in coastal hammocks and is stored in the soil vadose zone, but vadose water will diminish with the rising water table as a consequence of sea level rise, thereby subjecting plants to salt water stress. A model is used to demonstrate that the constraining effect of salinity on transpiration limits the distribution of freshwater-dependent communities. Field data collected in hardwood hammocks and coastal buttonwood forests over 11 years show that halophytes have replaced glycophytes. We establish that sea level rise threatens 21 rare coastal species in Everglades National Park and estimate the relative risk to each

CO2CRC Otway project–Soil gas baseline and assurance monitoring 2007–2010

A four-stage soil gas monitoring study was conducted from March 2007 to March 2010 in the western Port Campbell Embayment of the onshore Otway Basin, SE Australia. This study evaluated existing vadose zone soil gases over the CO2CRC Otway Project’s site for CO2 storage, and nearby deep natural CO2 accumulations. Carbon dioxide, methane and helium were the main gases analysed and much of the time CH4 concentrations were too low for accurate analysis. Carbon dioxide concentrations ranged from atmospheric background levels of 0.033% up to about 10% of the total soil gas. The δ13CCO2 values are typical for organic matter decomposition as the main source of the CO2 in the soil gas. Modern radiocarbon ages, determined from 14C analysis, and low helium concentrations support the conclusion that CO2 in the soil gas is predominantly from modern organic sources. There was a relative increase in CO2 concentration measured in locations where the Port Campbell Limestone is near the ground surface, which implies additional CO2 has been produced from the weathering of limestone.

Predicting Chemical Fingerprints of Vadose Zone Soil Gas and Indoor Air from Non-Aqueous Phase Liquid Composition

Complex mixtures of volatile organic chemical (VOC) vapors can exist over subsurface accumulations of non-aqueous phase liquids (NAPLs) and contaminated soils. The ability to predict the relative soil gas chemical composition arising from such NAPLs is relevant to studies of the sources and fate of soil gas, and in assessing the possible intrusion of soil gas chemical constituents to indoor air. However, it is difficult to directly compare the chemical ‘fingerprints’ of NAPL and vapor because the differences are significant in liquid-gas partitioning coefficients among the many volatile hydrocarbons that compose the liquid NAPL. In this article, detailed chemical characterization data from analysis of liquid NAPLs are used to calculate equilibrium vapor phase distributions of 66 diagnostic paraffin, isoparaffin, aromatic, naphthene, and olefin (PIANO) compounds. Measured NAPL and predicted vapor phase chemical composition are presented for a diverse group of 10 hydrocarbon products that include crude oil, solvents, gasoline, distillate fuels, and coal tar. The laboratory investigation and field study described in this article demonstrate that chemical fingerprints of such predicted vapor phase hydrocarbons can be used as compositional benchmarks for reconciling source(s) of soil gas and indoor air-borne hydrocarbons.

In Situ Bioremediation of Perchlorate in Vadose Zone Soil Using Gaseous Electron Donors: Microcosm Treatability Study

A series of microcosm experiments were performed to determine the effectiveness of various gaseous electron donors (including hydrogen, 1-hexene, ethyl acetate, and liquefied petroleum gas [LPG]) for supporting biological perchlorate reduction under different electron donor concentrations and soil moistures. Under high soil moisture (16% w/w) conditions, complete or partial perchlorate degradation was achieved with all of the tested electron donors, except for ethyl acetate. Hydrogen was the most promising of the tested electron donors, achieving complete perchlorate degradation with first-order rate constants ranging from 0.13 to 0.20 day(-1) and reducing concentrations to non-detectable levels within 35 to 42 days. The LPG and 1-hexene each promoted partial perchlorate reduction, with average first-order rate constants of 0.05 and 0.11 day(-1), respectively. Although significant perchlorate reduction was observed with as little as 13% moisture, the moisture content for complete perchlorate degradation in this particular soil was determined to be 17%.

Relationship Between Subsurface Landfill Gas and Arsenic Mobilization into Groundwater

The maximum contaminant level for arsenic was reduced by the U.S. Environmental Protection Agency (U.S. E.P.A.) for Drinking Water Standards from 50 micrograms per liter (µg/L) to 10 µg/L, effective January 23, 2006. The subject site is a double‐lined sanitary landfill facility located in the mid‐Atlantic region of the United States. Arsenic was reported above the maximum contaminant level in a downgradient monitoring well (MW‐18) in July 2005. Since July 2005, arsenic levels in MW‐18 fluctuated above and below 10 µg/L. This research focuses on determining whether reducing conditions in groundwater, enhanced by subsurface landfill gas emissions, were causing naturally‐occurring arsenic to mobilize from the native variably‐saturated vadose zone soils into groundwater.The groundwater data collected from the impacted well (MW‐18) were compared to an upgradient well (MW‐8) to determine whether significant differences existed during the time period of April 2004 to April 2007. Linear regression analysis was also used to determine whether other parameters had a significant relationship with the arsenic concentrations detected in MW‐18. The groundwater located in MW‐18 was consistently more reduced than groundwater located in the upgradient/background well MW‐8, and this was most likely attributed to the presence of subsurface landfill gas in the area. According to the U.S. E.P.A., oxidation‐reduction potential (ORP) values in groundwater less than 50 millivolts (mv) suggest that a reducing environment may be present. The data presented in this study indicate that arsenic can mobilize into groundwater under moderately reducing conditions, with ORP measurements averaging 53 mv.

Development of a treatment solution for reductive dechlorination of hexachloro-1,3-butadiene in vadose zone soil

The biodegradation of chlorinated organics in vadose zone soils is challenging owing to the presence of oxygen, which inhibits reductive dehalogenation reactions and consequently the growth of dehalorespiring microbes. In addition, the hydraulic conductivity of vadose zone soils is typically high, hence attempts to remediate such zones with biostimulation solutions are often unsuccessful due to the short residence times for these solutions to act upon the native bacterial community. In this study we have identified sodium alginate as a hydrogel polymer that can be used to increase the residence time of a nutrient solution in an unsaturated sandy soil. Additionally we have identified neutral red as a redox active compound that can catalyse the reductive dechlorination of the chlorinated organic hexachloro-1,3-butadiene by activated sludge fed with lactate and acetate. Finally we have shown that a nutrient solution amended with neutral red and sodium alginate can lower the redox potential and reduce hexachloro-1,3-butadiene concentrations in a contaminated vadose zone soil.

Fate of trace organic compounds during vadose zone soil treatment in an onsite wastewater system

During onsite wastewater treatment, trace organic compounds are often present in the effluents applied to subsurface soils for advanced treatment during vadose zone percolation and groundwater recharge. The fate of the endocrine-disrupting surfactant metabolites 4-nonylphenol (NP), 4-nonylphenolmonoethoxylate (NP1EO), and 4-nonylphenolmonoethoxycarboxylate (NP1EC), metal-chelating agents ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA), antimicrobial agent triclosan, stimulant caffeine, and antibiotic sulfamethoxazole during transport through an unsaturated sandy loam soil was studied at a field-scale test site. To assess the effects of effluent quality and hydraulic loading rate (HLR) on compound fate in the soil profile, two effluents (septic tank or textile biofilter) were applied at two design HLRs (2 or 8 cm/d). Chemical concentrations were determined in the two effluents and soil pore water at 60, 120, and 240 cm below the soil infiltrative surface. Concentrations of trace organic compounds in septic tank effluent were reduced by more than 90% during transport through 240 cm (often within 60 cm) of soil, likely due to sorption and biotransformation. However, the concentration of NP increased with depth in the shallow soil profile. Additional treatment of anaerobic septic tank effluent with an aerobic textile biofilter reduced effluent concentrations of many compounds, but generally did not affect any changes in pore water concentrations. The soil profile receiving septic tank effluent (vs. textile biofilter effluent) generally had greater percent removal efficiencies. EDTA, NP, NP1EC, and sulfamethoxazole were measured in soil pore water, indicating the ability of some trace organic compounds to reach shallow groundwater. Risk is highly dependent on the degree of further treatment in the saturated zone and the types and proximity of uses for the receiving groundwater environment.

Review of vadose zone soil solution sampling techniques

Understanding the composition and fluxes of vadose zone soil water is extremely important to many environmental studies, and hence the monitoring of soil solution is of basic interest for different scientific and practical fields, including pedologic studies, water-use management, fate and transport of environmentally consequential pollutants, monitoring of disposal from mining and industries, nutrient management of agricultural and forest ecosystems, ecology, and environmental protection. Soil solution sampling techniques for effectively monitoring the quality and quantity of vadose zone soil pore water have been used to assess the persistence and transport of potential groundwater pollutants, assess the ecological and human health impact of such pollutants, and develope appropriate remediation strategies. However, there is still no consensus as to best techniques for soil solution collection at most field or laboratory soil conditions. The purpose of this review is to evaluate different laboratory and in-situ techniques of vadose zone soil solution sampling. This comprehensive review presents and discusses advantages and disadvantages of these techniques, problems and limitations of some of these techniques, proper installation, operation and pretreatment of samplers, interaction of pollutants with sampler materials, and proper selection of samplers under a wide range of potential pollutants measurements to provide a background and guidelines for the evaluation of recent developments.