Vadose Zone Materials Research Articles

Groundwater quality assessment and its vulnerability to pollution: a study of district Nowshera, Khyber Pakhtunkhwa, Pakistan.

Groundwater is the drinking water source for the majority of rural settlements of district Nowshera, Khyber Pakhtunkhwa, Pakistan. The study aimed to analyze the groundwater quality and its vulnerability to pollution and to develop its spatial distribution mapping. For this purpose, forty-eight groundwater samples were collected from dug wells, tube wells, and hand pumps of sixteen villages andanalyzed for physicochemical parameters. The XY coordinates of the sample's sourcesweremarked by Magellan Triton 1500 handheld global positioning system (GPS). The results were compared with WHO and Pak-EPAguidelines. The results of the majority of selected parameters were found within the WHO and Pak-EPA guidelines; however, in certain areas the concentration of total dissolved solids (TDS), chlorides(Cl-), and alkalinity were higher than the guideline limits. Based on cumulative water quality the excellent water quality prevails over an area of 376 km2(21% of district area), good water quality 726 km2(42%), poor 424 km2(24%), very poor 116 km2(6%), and unfit for drinking 84 km2(4%). The water of the Nizampur and Rashaki areas were categorized unfit for drinking. The groundwater quality of nearly one-half of the district varies from poor to very poor, and the soil type and vadose zone sediment/material was found the key reason for groundwater contamination. Based on the infiltration capacity of vadose zone material, the study area was divided into four water pollution vulnerable zones. The low vulnerable zone covers an area of 104 km2, moderate 862 km2, high 667 km2,and very high 93 km2. The most important factor which determines the vulnerability of the groundwater to contamination is the vadose zone material/sediment which in turn determines the soil infiltration capacity. The generated groundwater susceptibility and water quality maps provide critical information for identifying optimal locations for supply wells.

An operational methodology for determining relevant DRASTIC factors and their relative weights in the assessment of aquifer vulnerability to contamination

The DRASTIC index used to assess the vulnerability of aquifers to contamination, has been subject to various adjustments to improve its reliability. These adjustments include adding and/or eliminating certain aquifer factors and modifying the factor weights. Nonetheless, there is no consensus about which factors, or their respective weights, are most important for assessing aquifer vulnerability. In the present study, we propose an operational methodology that: (1) identifies the relevant factors for assessing aquifer vulnerability to contamination; and (2) determines the relative importance of the selected factors. We applied this approach to a large data set of granular aquifers from a region in Canada, which includes information for DRASTIC factors, combined with groundwater quality and land-use data. We found that for our study region, topography (terrain-slope) is an irrelevant factor for assessing the vulnerability of aquifers to contamination. On the other hand, the relevant factors ranked according to their relative importance (from highest to lowest), are (1) water table depth; (2) hydraulic conductivity; (3) characteristics of vadose zone materials; and (4) recharge. Our approach can serve as an initial step for identifying the relevant aquifer factors when assessing aquifer vulnerability and determining the relative importance of the relevant factors to validate weights attributed to these factors. Our methodology can help adapt index-based methods of aquifer vulnerability assessment to a range of study regions.

Sulfonamide antibiotics in groundwater and their migration in the vadose zone: A case in a drinking water resource

Vadose zone is an important channel for sulfonamide contamination into groundwater, and the processes of adsorption and migration in the vadose zone are complicated. In this study, we focused on three sulfonamide antibiotics (sulfamethoxazole, sulfamethazine and sulfamethoxypyridazine) in the groundwater of Limin drinking water resource in northeastern China and assessed their migration potential in aquifers. Analysis was based on adsorption batch tests, a column experiment. Some key influencing factors including pH, iron, manganese and ammonia were considered. The results showed that the concentration of sulfamethoxypyridazine in groundwater is higher, ranging between 7 and 30 ng/L, the concentrations of sulfamethoxazole and sulfamethazine are relatively lower and no more than 1 ng/L. The adsorption capacity of the three antibiotics on vadose zone materials shows the following order: sulfamethoxypyridazine > sulfamethazine > sulfamethoxazole. Furthermore, for the three sulfonamides, the adsorption is higher in the medium of 0–20 cm (finer particle size, higher clay content and lower sand content) than that in 80–100 cm. Coexisting ions (iron, manganese and ammonia) promote the adsorption of sulfonamides, and the adsorption of sulfonamides decreases as the water pH increases. The column experiment also shows that the three sulfonamides have strong migration abilities and pose a considerable risk to groundwater.

In-situ Observation and Nitrate-N Load Assessment in Madhubani District, Bihar, India

Abstract Fertilizers may leach through the vadose zone and eventually reach groundwater in agriculturally intensive areas. Thus, the main focus of this study was to investigate Nitrate-N load and vulnerability of groundwater resources using in-situ observed hydrogeological data. Soil water flow and contaminant transport equation was numerically simulated using HYDRUS 1D for constant head and atmospheric top boundary conditions. Sub-surface materials were distributed based on the lithologs of the target area. Observed water table locations were considered as bottom boundary condition to respective numerical domain. The time taken by Nitrate-N to reach groundwater table was considered to estimate vulnerability index. The results show that Nitrate-N load is higher in constant head boundary conditions than atmospheric boundary conditions. The eastern part of the study area shows high vulnerability than northern part followed by western part. In-situ observed nitrate concentrations were well matched with simulated results. The high vulnerability in eastern and northern part is due to alluvial sandy lithologs and very shallow groundwater table. These findings are in line with the observed low water table depths, less runoff, and higher hydraulic conductivity of the vadose zone material in these area. In western part, forest cover dominated land use causes low pollution vulnerabilities to groundwater resources. This study may help to frame agricultural and soil-water conservation practices with more sustainable remedial techniques.

Vulnerability assessment of an agro-stressed aquifer using a source-pathway-receptor model in GIS

Agro-stresses on greater Punjab aquifer are rising because of unchecked growths of agriculture. Secondary salinity and pollution loads from non-point sources (NPS) are two environmental concerns of these growths. In order to assess the impacts of NPS pollution on underlying aquifer, a source-pathway-receptor model commonly refered to as DRASTIC is applied in a GIS environment. The model is applied by combining data sets that describe the depth to groundwater, recharge rates, aquifer material, soils composition, topography, vadose zone materials, and hydraulic conductivity. Resultant vulnerability values are scaled into three vulnerability zones based on DRASTIC Vulnerability Index as low, moderate and high vulnerability. A major portion of the study area about 1798 km squares (57.6%) fall under low vulnerability zone, while the porportions of the study area under moderate, and high vulnerabilities are 790 km2 (25.3%), and 551 km2 (17.6%), respectively. The vulnerability of Tehcil Kot Addu is related positively to the recharge rates and agricultural landuse. Results of this study can be used for sustainable agriculture in the study area and other regions having silmilar hydrogeological characteristics.

Experimental and modeling of the unsaturated transports of S-metolachlor and its metabolites in glaciofluvial vadose zone solids

The transport of pesticides to groundwater is assumed to be impacted by flow processes and geochemical interactions occurring in the vadose zone. In this study, the transport of S-metolachlor (SMOC) and its two metabolites ESA-metolachlor (MESA) and OXA-metolachlor (MOXA) in vadose zone materials of a glaciofluvial aquifer is studied at laboratory scale. Column experiments are used to study the leaching of a conservative tracer (bromide) and SMOC, MESA and MOXA under unsaturated conditions in two lithofacies, a bimodal gravel (Gcm,b) and a sand (S-x). Tracer experiments showed water fractionation into mobile and immobile compartments more pronounced in bimodal gravel columns. In both lithofacies columns, SMOC outflow is delayed (retardation factor>2) and mass balance reveals depletion (mass balance of 0.59 and 0.77 in bimodal gravel and sand, respectively). However, complete mass elution associated with retardation factors close to unity shows that there is no adsorption of MESA and MOXA in either lithofacies. SMOC transport is characterized by non-equilibrium sorption and sink term in both bimodal gravel and sand columns. Batch experiments carried out using agitation times consistent with column water residence times confirmed a time-dependence of SMOC sorption and high adsorption rates (>80%) of applied concentrations. Desorption experiments confirm the irreversibility of a major part of the SMOC adsorption onto particles, corresponding to the sink term in columns. In the bimodal gravel column, SMOC adsorption occurs mainly on reactive particles in contact with mobile water because of flow regionalization whereas in the sand column, there is pesticide diffusion to the immobile water. Such results clearly show that sorption mechanisms in the vadose zone solids below the soil are both solute and contact-time-dependent and are impacted by hydrodynamic conditions. The more rapid transport of MESA and MOXA to the aquifer would be controlled mainly by water flow through the unsaturated zone whereas SMOC transport is retarded by sorption processes within the vadose zone.

Evaluating groundwater recharge processes using stable isotope signatures—the Nabogo catchment of the White Volta, Ghana

This study evaluates the source and evolution of groundwater in parts of the Nabogo catchment of the White Volta Basin in Ghana. Porewater δ18O and δ2H signatures of vadose zone material were used together with the isotope signatures of rainwater, groundwater, and surface water in the area to suggest an evolutionary model and the mode of fluid fluxes through the unsaturated zone material in the area. This study finds that the local precipitation in the study area presents a relatively isotopically heavier signature compared to the average signature of global meteoric water. This is because the local meteoric water line (LMWL) has shallower slope, intercept, and average deuterium excess values compared to the global meteoric water line (GMWL). On the other hand, the local groundwater in the area presents relatively enriched isotopic signatures. Stable isotope profiles suggest piston flow as the main mechanism of vertical water movement in the vadose zone, indicating that direct groundwater recharge from local precipitation is principally based on this mechanism of transport. There is a progressive decline in the deuterium excess data of porewater vertically down the soil profile. This observation is consistent with an evolutionary pattern caused by fractionation processes attending evaporation of infiltrating water. Mineral-weathering processes have not been noted to have an effect on the isotopic signature of groundwater in the area. However, there appears to be a significant impact of evaporation on the total dissolved solid content of surface water in the area. In the case of groundwater, field-measured electrical conductivity (EC), total dissolved solids (TDSs), and pH are, respectively, in the ranges of 30–2407 μS/cm, 15–1196 mg/L, and 5.35–8.36. The EC, TDS, and pH are, respectively, in the ranges of 30–139 μS/cm, 15–70 mg/L, and 6.52–8.66 in surface water in the area.

Estimation of evapotranspiration losses in the vadose zone using stable isotopes and chloride mass balance method

This study used stable isotope and chloride data of porewater to evaluate water fluxes through the vadose zone and thereby estimate evapotranspiration water losses in parts of the Nabogo catchment of the White Volta Basin in Ghana. The overall objective was to develop a framework so that the recharge regime can be properly conceptualized for numerical hydrological modeling. Unsaturated zone material was sampled at regular intervals of 50 cm to a maximum depth of 300 cm in four different locations in the study area. Rainwater, groundwater and surface water were simultaneously sampled and analyzed for their δ18O and δ2H characteristics. Porewater, extracted from the unsaturated zone material was analyzed for the δ18O and δ2H and chloride content and profiles were created to gauge the isotopic evolution of precipitation and estimate evaporative losses at each interval of the sampling. The chloride mass balance technique was used to estimate the fraction of infiltrating water remaining at each interval of the sampling. Transpiration losses through the entire profile were then estimated. This study finds that the vertical infiltration of water through the vadose zone is dominated by piston flow and a mixture of piston and preferential flows. In the shallow subsurface (0.0–3.0 m), evaporative losses estimated from stable isotope data fall in the range of 29.3–52.4 % (322.3–576.4 mm/year) of the annual precipitation, with an average of 40 % (or 440 mm/year). Estimated vadose zone recharge at the maximum depth of sampling ranges between 11.1 and 185 mm/year with an average of 32.9 mm/year, representing 1.1, 18.5, and 3.29 % of the annual precipitation, respectively. Estimated transpiration losses within this interval range between 29.1 % (290 mm/year) and 69.5 % (695 mm/year), with an average of 54.7 % (547 mm/year) of the annual precipitation. Transpiration losses appear to increase down the profile and apparently account for a significant percentage of water losses in the vadose zone. A significant proportion of the original precipitation is lost within the upper 300 cm (3 m) of the vadose zone. The Water Table Fluctuations method was independently used to estimate saturated zone groundwater recharge and indicates that recharge rates range between 64.65 and 151.2 mm/year with an average of 102.5 mm/year which, respectively represent 5.9, 13.7, and 9.3 % of the average annual precipitation in the area. The apparently higher estimates from the water table fluctuations method may arise from uncertainties in the specific yield values used for the vadose zone material.

Protective Capacity Assessment of Vadose Zone Material by Geo-Electrical Method: A Case Study of Pakistan

Rapid and poorly managed agricultural landuse in Punjab plain is a major contention in groundwater quality deterioration. This study is focused on the determination of groundwater vulnerability potential of vadose material using geo-electrical method. Total of eight electrical resistivity soundings with Schlumberger electrode configuration are taken near Khizarabad Dairy Form, district Sargodha. The instrument ABEM Terrameter SAS 300 is used for data acquisition. The quantitative interpretation is done by 1D modelling of resistivity soundings by using IX1D software. The results characterize the area into three geo-electrical layers as dry alluvium (15.7 - 32.4 Ωm), saturated alluvium with high resistivity (10 - 20 Ωm) and low resistivity (>10 Ωm). Aquifer vulnerability is assessed by calculating Dar Zarrouk parameter (Longitudinal Conductance) and thickness of vadose zone. Resultant vulnerability is divided into three zones as low (>3.8 siemens) moderate (1.6 - 2.5 siemens) and high (<1.6 siemens). It is concluded from this study that Vertical Resistivity Sounding (VES) can be used for vulnerability assessment of an unconfined sandy alluvial aquifer from agricultural contaminants.

Groundwater Vulnerability Assessment to Contamination Using Soil Moisture Flow and Solute Transport Modeling

The soil moisture flow and solute transport regimes of the vadose zone associated with specific hydrogeological conditions play a crucial role in pollution risk assessment of the underlying groundwater resources. An effort has been made to map the vulnerability of shallow groundwater to surface pollutants of Doon Valley, an intermontane watershed in North India situated between the lesser Himalayas and Shiwalik ranges, using soil moisture flow and contaminant transport modeling. The classical advection dispersion equation coupled with the Richard’s equation is numerically simulated at different point locations for assessing the intrinsic vulnerability of the valley. The role of soil type, slope, and land-use cover is considered for estimating the transient flux at the top boundary from daily precipitation and evapotranspiration data of the study area. The time required by solute peak to travel from the surface to the groundwater table at the bottom of the soil profile is considered as an indicator of vulnerability index. Results show a high vulnerability in the southern region, whereas low vulnerability is observed in the northeast and northern parts of the valley. These findings are in line with the observed low water table depths, less runoff, and higher hydraulic conductivity of the vadose zone material in the southern part of the valley. The study may assist in decision making related to planning of industrial locations and the sustainable water resources development of the valley.

Denitrification in vadose zone material amended with dissolved organic matter from topsoil and subsoil

Limited information is available on the potential of subsoil and underlying alluvial gravels (vadose zone) to denitrify leached nitrate, though it is recognized that C availability is an important constraint. In this laboratory study, we used samples (<4 mm fraction) from subsoil (0.4–0.85 m depth) and alluvial gravel layers (1.2–4.8 m) derived from a site in Canterbury, New Zealand, to quantify denitrification potential and examine the denitrification response to inputs of dissolved organic C (DOC). As C sources, we used DOC extracted in either cold (20 °C) or hot (80 °C) water from topsoil and subsoil (assumed to represent C substrates likely to be transported into the subsoil in drainage). Measurements of potential denitrification enzyme activity (DEA, via samples amended with glucose and nitrate-N) showed the alluvial gravel samples had denitrification capability although rates were low (<5 μg N2O–N kg−1 h−1) compared with those reported for topsoils. The effect of the soil-derived DOC, at addition rates 16–215 mg kg−1, on denitrification was determined in a 7-d anaerobic incubation using acetylene to block reduction of N2O to N2. There was a lag of 24–36 h, during which N2O production was low, followed by a period of rapid increase (36–96 h), and a final phase (120–168 h) in which N2O concentrations remained relatively stable. During the rapid phase, the N2O production rate was very high (up to 440–500 μg N kg−1 h−1 at the highest DOC addition rate) and exceeded that measured in the 8-h DEA assay (a measure of indigenous denitrifier enzymes). The post-96 h decline in N2O production rate was attributed to depletion of available C. The temporal pattern of CO2–C production was similar to that of N2O, though the lag period was shorter (12–24 h). The ratio of N2O–N to CO2–C increased with time; the maximum was circa 0.3:1 at the highest addition rate of hot water extractable DOC. Our results suggest that, with input of DOC substrate, alluvial gravel materials could generate enough enzyme during a 1–2 day anaerobic period to denitrify significant quantities of nitrate. However, in situ denitrification in alluvial gravels may be low because of biophysical limitations such as low C inputs of bioavailable C and anaerobicity mostly confined to localized zones or lenses.

Laboratory investigations of the effects of nitrification-induced acidification on Cr cycling in vadose zone material partially derived from ultramafic rocks

Sacramento Valley (California, USA) soils and sediments have high concentrations of Cr(III) because they are partially derived from ultramafic material. Some Cr(III) is oxidized to more toxic and mobile Cr(VI) by soil Mn oxides. Valley soils typically have neutral to alkaline pH at which Cr(III) is highly immobile. Much of the valley is under cultivation and is both fertilized and irrigated. A series of laboratory incubation experiments were conducted to assess how cultivation might impact Cr cycling in shallow vadose zone material from the valley. The first experiments employed low (7.1mmolN per kg soil) and high (35mmolNkg−1) concentrations of applied (NH4)2SO4. Initially, Cr(VI) concentrations were up to 45 and 60% greater than controls in low and high incubations, respectively. After microbially-mediated oxidation of all NH4+, Cr(VI) concentrations dropped below control values. Increased nitrifying bacterial populations (estimated by measurement of phospholipid fatty acids) may have increased the Cr(VI) reduction capacity of the vadose zone material resulting in the observed decreases in Cr(VI). Another series of incubations employed vadose zone material from a different location to which low (45meqkg−1) and high (128meqkg−1) amounts of NH4Cl, KCl, and CaCl2 were applied. All treatments, except high concentration KCl, resulted in mean soil Cr(VI) concentrations that were greater than the control. High concentrations of water-leachable Ba2+ (mean 38μmolkg−1) in this treatment may have limited Cr(VI) solubility. A final set of incubations were amended with low (7.1mmolNkg−1) and high (35mmolNkg−1) concentrations of commercial liquid ammonium polyphosphate (APP) fertilizer which contained high concentrations of Cr(III). Soil Cr(VI) in the low APP incubations increased to a concentration of 1.8μmolkg−1 (5× control) over 109days suggesting that Cr(III) added with the APP fertilizer was more reactive than naturally-occurring soil Cr(III).

Pedotransfer functions for isoproturon sorption on soils and vadose zone materials

Sorption coefficients (the linear K(D) or the non-linear K(F) and N(F)) are critical parameters in models of pesticide transport to groundwater or surface water. In this work, a dataset of isoproturon sorption coefficients and corresponding soil properties (264 K(D) and 55 K(F)) was compiled, and pedotransfer functions were built for predicting isoproturon sorption in soils and vadose zone materials. These were benchmarked against various other prediction methods. The results show that the organic carbon content (OC) and pH are the two main soil properties influencing isoproturon K(D) . The pedotransfer function is K(D) = 1.7822 + 0.0162 OC(1.5) - 0.1958 pH (K(D) in L kg(-1) and OC in g kg(-1)). For low-OC soils (OC < 6.15 g kg(-1)), clay and pH are most influential. The pedotransfer function is then K(D) = 0.9980 + 0.0002 clay - 0.0990 pH (clay in g kg(-1)). Benchmarking K(D) estimations showed that functions calibrated on more specific subsets of the data perform better on these subsets than functions calibrated on larger subsets. Predicting isoproturon sorption in soils in unsampled locations should rely, whenever possible, and by order of preference, on (a) site- or soil-specific pedotransfer functions, (b) pedotransfer functions calibrated on a large dataset, (c) K(OC) values calculated on a large dataset or (d) K(OC) values taken from existing pesticide properties databases.

Soil Vapor Extraction Performance in Layered Vadose Zone Materials

A pilot soil vapor extraction (SVE) system was installed at a small landfill within the Savannah River Site to address trichloroethylene (TCE) contamination present in the vadose zone. The SVE system has been operating since September 1999 and numerous tests have been performed on the system. A model was developed to simulate SVE at this site, incorporating the effects of contaminant behavior in a layered subsurface as well as the effects of contaminant diffusion into and out of soil aggregates. The objectives of this study were to: (i) compare the field data from the site with predictions from this mechanistic model; and (ii) establish the case for closure based on field observations and model predictions. A dense non‐aqueous‐phase liquid TCE source was discovered at the site during the course of operation. Location of this source compares well with the predicted residual source from the application of the diffusion components of the SVE model to soil gas TCE concentration rebound observations. Collectively, the field observations and the model predictions strongly support the observations that a substantial portion of the source contamination at the site has been removed by the SVE system and that the criteria for site closure have been met.

Relationship between dielectric permittivity and water content for vadose zone materials of volcanic origin

Seven materials, encompassing a young volcanic soil, its parent material, two older buried soils (Palaeosols), and an underlying older volcanic deposit were investigated in a laboratory study to determine the relationship between dielectric permittivity (ϵc) and water content (θ). Empirical 3rd-order polynomial models generally fitted the data better than semi-physical 3-phase mixing models and normalised conversion functions. Based on their different dielectric response, which presumably is mainly due to their different bulk density (and thus porosity), the materials could be grouped into Ignimbrite materials and Palaeosol materials. Relative to the Ignimbrites, Palaeosols showed a particularly muted response at the low end of dielectric permittivity measurements, which could be due to a particularly high amount of bound water in these allophane-rich materials and/or to greater microstructural and phase configuration effects. Applying the Topp equation resulted in substantial underestimations of the water contents of both groups. A relationship previously developed on New Zealand sandy volcanic soils fitted our Ignimbrite materials well, but still underestimated the water contents of the Palaeosols substantially. Normalised conversion functions described both material groupings reasonably well, demonstrating that bulk density differences can explain a substantial part of the variation of ϵc–θ relationships between different materials.

Denitrification capacity in the vadose zone at three sites in the Lake Taupo catchment, New Zealand

Land use in the Lake Taupo catchment is under scrutiny, as early signs of deteriorating water quality in Lake Taupo have been observed. Although the fate of contaminants in soil and groundwater are comparatively well studied, the transformations in the lower vadose zone, i.e. the zone between the soil and the groundwater, are less well understood. The capacity for NO3-N removal via biological denitrification, based on utilising the resident C substrate, in the vadose zone of the Lake Taupo catchment is quantified in this work. Complete vadose zone profiles were sampled at 3 sites (Rangiatea, Waihora, and Kinloch), from the soil surface down to the watertable in approximately 0.5-m depth increments. Texture, allophane content, pH, and concentrations of extractable NO3-N, NH4-N, and dissolved organic carbon were determined. Incubations were undertaken to determine the denitrification capacity of the vadose zone materials amended with NO3-15N, but no added carbon substrate, and maintained under anaerobic conditions at 28°C. Gas samples were taken from the headspace after 48 h and analysed for N2 and N2O. In soil depths down to about 1.2 m, the denitrification capacity ranged from 0.03 to 9.18 kg N/ha.day, and below this depth it ranged from &lt;0.01 to 0.09 kg N/ha.day. A palaeosol layer in the Waihora profile had an enhanced denitrification capacity compared with the other samples in deeper zones of the profiles. In the surface sampling, at least 99.9% of the gas recovered from the 15N applied was in the form of N2. In contrast, no N2 gas production could be detected in any sample from below the second sampling depth, with only N2O detected. Denitrification capacities of all vadose zone materials were low when compared with other studies. Thus, careful land management is required to avoid groundwater contamination by nitrate leaching from the root-zone of the pasture.

Sorption−Desorption of Imidacloprid and Its Metabolites in Soil and Vadose Zone Materials

Sorption-desorption is one of the most important processes affecting the leaching of pesticides through soil because it controls the amount of pesticide available for transport. Subsurface soil properties can significantly affect pesticide transport and the potential for groundwater contamination. This research characterized the sorption-desorption of imidacloprid (1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro-2-imidazolidinimine) and three of its metabolites, 1-[(6-chloro-3-pyridinyl)methyl]-2-imidazolidinone (imidacloprid-urea), 1-[(6-chloro-3-pyridinyl)methyl]-4,5-dihydro-1H-imidazol-2-amine (imidacloprid-guanidine), and 1-[(6-chloro-3-pyridinyl)methyl]-1H-imidazol-2-amine (imidacloprid-guanidine-olefin), as a function of changing soil properties with depth in two profiles extending from the surface to a depth of 1.8 or 8 m. Sorption of each compound was highly variable and hysteretic in all cases. Normalizing the sorption coefficients (K(f)) to the organic carbon or the clay content of the soil did not reduce the variability in sorption coefficients for any compound. These results illustrate the importance of evaluation of the sorption data used to predict potential mobility. Understanding the variability of soil properties and processes as a function of depth is necessary for accurate prediction of pesticide dissipation.

Pesticide adsorption in the vadose zone: a case study on Eocene and Quaternary materials in Northern France.

We present a set of adsorption coefficients measured on various Eocene and Quaternary materials sampled from the vadose zone of a catchment in Northern France for three herbicides, atrazine, isoproturon and metamitron. Some vadose zone materials were found to have higher adsorption coefficients than the topsoil. The adsorption coefficients were strongly dependent on the clay content of the material. From 83% to 97% of the variability in the adsorption coefficients could be explained by a linear relationship to the clay content. Adsorption coefficients normalized for clay content, Kclay, ranged between 1.6 and 17.6 litre kg(-1) for atrazine. Neglecting the adsorption properties of the vadose zone and relying exclusively on Koc values to predict mobility may bias regional or local risk assessment of groundwater contamination by pesticides. More information on the adsorption properties of geological materials should be collected to improve our ability to predict pesticide concentrations in groundwaters.

Sorption of Pesticides Atrazine, Isoproturon, and Metamitron in the Vadose Zone

Knowledge of the sorption properties of the vadose zone is essential to provide sound risk assessments of groundwater contamination by pesticides. This study explored the sorption properties of the vadose zone of a 187‐ha agricultural catchment located 120 km northeast of Paris, France. Eocene geologic materials composing the vadose zone include limestones, marlstones, clays, and sands. Distribution coefficients (Kd) of 14C‐labeled atrazine (6‐chloro‐N2‐ethyl‐N4‐isopropyl‐1,3,5‐triazine‐2,4‐diamine), isoproturon [3‐(4‐isopropylphenyl)‐1,1‐dimethylurea], and metamitron [4‐amino‐4,5‐dihydro‐3‐methyl‐6‐phenyl‐1,2,4‐triazin‐5‐one] were measured in batch on 30 vadose zone samples, and their sorption isotherms measured on six representative samples. Kd values for each pesticide were highly variable and ranged from 0.10 to 2.99 L kg−1 for atrazine, 0.06 to 1.27 L kg−1 for isoproturon, and 0.06 to 2.08 L kg−1 for metamitron. Two vadose zone materials, a lignitic clay and a clay with low organic C content, were found to have Kd values higher than those of the topsoil. Most of the sorption isotherms in the vadose zone were found to be linear. Kds could be well predicted by the clay content (particle‐size fraction &lt; 2 μm after organic matter and carbonates removal) of the vadose zone. This result suggests that simple but efficient pedotransfer functions may be derived to predict the sorption properties of the vadose zone at the catchment scale.

An engineering approach to fingered vadose pollutant transport

Results of many independent experimental findings related to fingered preferential flow are combined into a concise, conservative engineering methodology for predicting pollutant transport through fingered flow paths. Preferential flow can occur in all textures of soil, with fingering occurring in less structured coarse soils, giving over to macropore flow in finer-textured soil regimes. A simple example comparing groundwater loading of naphthalene using plug flow and fingered flow assumptions illustrates that fingered flow can increase contaminant loading to groundwater aquifers by over two orders of magnitude at sites with coarse vadose zone materials.