Pesticide Concentrations In Groundwater Research Articles

Методы изучения миграции пестицидов: анализ, сравнение, рекомендации по использованию при оценке риска воздействия на грунтовые воды

Groundwater pollution with pesticides has affected every country on every continent. An important challenge for researchers and regulatorу bodies in all countries is to develop methods and tools for predicting the risks to groundwater that are associated with pesticide use. In this paper, the generally accepted methods for studying pesticides migration are reviewed and analyzed from the point of view of their applicability for assessing the risk of pesticides impact on groundwater. Using examples, it is shown that the available methods may be divided into two categories, direct and indirect ones. The indirect methods (mobility indices and field studies of pesticide migration) make it possible to compare the migration ability of a pesticide molecule and to determine the depth of penetration of the pesticide into the soil. The direct methods (modeling, lysimetric studies, and monitoring) provide for the determination of pesticide concentrations in groundwater or groundwater leachate, which makes such methods useful in assessing the risk of pesticide use. Thus, by comparing the measured pesticide concentrations in water with acceptable threshold values, it is possible to determine the risk level of a pesticide. Examples of calculating the mobility indices indicate that their estimates differ from each other. Disadvantages of migration field experiments are related to analytical problems and to the short duration of a study. Modeling the migration of pesticides is an effective tool that makes it possible to quickly determine pesticides concentration of in water runoff and to identify the pesticides that can pollute groundwater under specific soil and climatic conditions. Calculations show that for 40 pesticides out of 180 licensed for use in the Russian Federation, their predicted concentrations exceed 1 µg/l. The second method that allows to directly determine concentrations in leachate is lysimetric experiments. A long-term study of the migration of 4 pesticides showed that all toxicants migrate beyond the soil profile. A third useful tool is pesticides monitoring in groundwater. Further development and dissemination of this method for controlling pesticides in groundwater is an important task for the regulatory authorities and the scientific community of the Russian Federation in the near future.

Preliminary Evaluation of the Possible Occurrence of Pesticides in Groundwater Contaminated with Nitrates—A Case Study from Southern Poland

This paper addresses groundwater pollution and the potential presence of pesticides within the catchment areas of two reservoirs that are sources of drinking water. The two reservoirs are Goczałkowice and Kozłowa Góra, both in Southern Poland. Agricultural and rural areas dominate both catchments. Archival data showed local groundwater contamination with nitrates. This indicated the possible presence of pesticides in shallow groundwater. In total, 13 groundwater samples from shallow sandy aquifers were collected. All the samples were tested for the presence of 35 organophosphate pesticides and 28 organochlorine pesticides. Additionally, in order to determine the current groundwater conditions, physicochemical parameters were measured in the field, and water samples were subjected to analysis of their chemical composition (incl. the determination of nitrates). The research outcomes showed that pesticides were not detected above the detection limits in any of the samples. Due to variations in the persistence and degradation rates of pesticides, the occurrence of these substances in the groundwater environment and the possibility of their migration to aquifers should not be completely excluded. Natural processes and factors (e.g., sorption, biodegradation, hydrolysis and redox conditions) may gradually reduce the pesticide concentrations in groundwater. The chemical analyses revealed high concentrations of nitrates in the groundwater. This suggests the possible influence of agriculture and fertilizer application on groundwater quality; however, a proportion of NO3- ions may be connected with improper sewage management within the two catchments. The absence of pesticides in groundwater impacted by agriculture may result from processes occurring in the aquifer and the rapid degradation of these compounds due to photolysis and prevailing weather conditions. In the vicinity of dwellings, nitrates also originate from domestic wastewater. Thus, the occurrence of pesticides in groundwater contaminated with NO3 cannot always be expected.

Spatial assessment of pesticide leaching risk to groundwater: sub-national decision making and model output aggregation.

This study compares standard regulatory methodology (fixed scenarios and models) to spatial modelling at a 1km landscape resolution for the evaluation of predicted environmental concentrations of pesticides in groundwater. The use of spatial modelling in the decision-making processes is discussed and three options for the sub-national evaluation and restriction of substances based on spatial environmental fate modelling are examined. Wheat and sugar beet are tested with two modified FOCUS substances (A and D) in the PEARL and GeoPEARL models. The 80th percentile value in time and space, aggregated to three different sub-national divisions of interest to a regulator, is used as a regulatory relevant output. Means and medians of predicted environmental concentrations at the national level are not useful summary statistics in the age of extensive and freely available geospatial data. A better statistic to use is the P80 (or other desired threshold/percentile combination) in time and space of predicted environmental concentration, combined with flexible and adaptable sub-divisions of the country based on the desired protective target. Tier 3b modelling is shown to provide an increase in localism and regulatory nuance over Tier 1 scenarios when combined with soil and aquifer type sub-national units. © 2019 Society of Chemical Industry.

Modelling pesticides leaching in cropping systems: Effect of uncertainties in climate, agricultural practices, soil and pesticide properties

Modelling of pesticide leaching is paramount to managing the environmental risks associated with the chemical protection of crops, but it involves large uncertainties in relation to climate, agricultural practices, soil and pesticide properties. We used Latin Hypercube Sampling to estimate the contribution of these input factors with the STICS-MACRO model in the context of a 400 km2 catchment in France, and two herbicides applied to maize: bentazone and S-metolachlor. For both herbicides, the most influential input factors on modelling of pesticide leaching were the inter-annual variability of climate, the pesticide adsorption coefficient and the soil boundary hydraulic conductivity, followed by the pesticide degradation half-life and the rainfall spatial variability. This work helps to identify the factors requiring greater accuracy to ensure better pesticide risk assessment and to improve environmental management and decision-making processes by quantifying the probability and reliability of prediction of pesticide concentrations in groundwater with STICS-MACRO.

Spatial distribution and health risk assessment for groundwater contamination from intensive pesticide use in arid areas.

Arid and semiarid areas face major challenges in the management of scarce groundwater. This valuable resource is under pressures of population, economic expansion, contamination and over-exploitation. This research investigates groundwater vulnerability to pesticide contamination in the Al-Kharj area of Saudi Arabia. It explores the spatial distribution of pesticide concentrations in groundwater and other relevant factors. Thin permeable soils, permeable aquifers and shallow water tables, which are prevalent in the area, are especially vulnerable to pesticides. Analyses of 40 groundwater samples were performed using a gas chromatograph mass spectrometer coupled with a quadrupole mass spectrometer with a GC column. The analysis was conducted to detect 32 pesticides from different chemical families, and a total of 22 pesticides were detected. All 40 water samples were positive for at least one of the pesticides studied. In total, 21 compounds were above the quantification limit and 10 of them exceeded the legal limit. Total pesticide levels ranged from 0.18 to 2.21μg/L, and 68% of the analyzed samples exceeded the maximum allowable pesticide concentrations established by the European Community. Comparison of the daily intake peak (DIP) and daily intake mean (DIM) relative to the acceptable daily intake (ADI) shows that groundwater contamination with pesticides is a serious problem. Prolonged exposure to pesticides can cause adverse effects to human health and the ecosystem. Spatial distribution maps of groundwater contamination were developed using GIS. These maps will help risk managers identify vulnerable sources and provide a relative assessment of pesticide hazards to human health and the environment.

Spatial Inference of Nitrate Concentrations in Groundwater

We develop a method for multiscale estimation of pollutant concentrations, based on a nonparametric spatial statistical model. We apply this method to estimate nitrate concentrations in groundwater over the mid-Atlantic states, using measurements gathered during a period of 10 years. A map of the fine-scale estimated nitrate concentration is obtained, as well as maps of the estimated county-level average nitrate concentration and similar maps at the level of watersheds and other geographic regions. The fine-scale and coarse-scale estimates arise naturally from a single model, without refitting or ad hoc aggregation. As a result, the uncertainty associated with each estimate is available, without approximations relying on high spatial density of measurements or parametric distributional assumptions. Several risk measures are also obtained, including the probability of the pollutant concentration exceeding a particular threshold. These risk measures can be obtained at the fine scale, or at the level of counties or other regions. The nonparametric Bayesian statistical model allows for this flexibility in estimation while avoiding strong assumptions. This method can be applied directly to estimate ozone concentrations in air, pesticide concentrations in groundwater, or any other quantity that varies over a geographic region, based on approximate measurements at some locations and perhaps of associated covariates. An S-PLUS package with this capability is provided as supplemental material.

Fifth national survey of pesticides in groundwater in New Zealand

A total of 163 wells were sampled in New Zealand as part of the 2006 national survey of pesticides in groundwater. The aims of the survey were to update the national overview of pesticides in New Zealand's groundwater systems, to investigate temporal variation in pesticide concentrations between surveys, and to identify environmental factors associated with pesticide contamination of groundwater. Thirty‐one wells (19%) tested positive for pesticides, with 13 (8%) wells having two or more pesticides detected. Pesticides were measured in one or more wells in 11 out of the 14 regions sampled; they were not detected in wells from the Northland, Hawke's Bay, and Taranaki regions. Nineteen different pesticides and metabolites were detected in groundwater. Herbicides were the most common pesticide group detected (12), followed by insecticides (5) and fungicides (2). There was a total of 50 pesticide detections and, of these, 37 were herbicides and 25 were triazine herbicides. Levels of only two of the 50 pesticide detections exceeded 1 μg litre‐1. Alachlor was detected in one well at a concentration of 34 μg litre‐1 exceeding the maximum acceptable value (MAV) of 20 μg litre‐1. All other pesticides were detected at concentrations less than 6% of the MAV. There were significant differences between shallow and deep wells with shallow wells being associated with pesticide detections. Higher dissolved oxygen and nitrate concentrations (indicating recent water), lower groundwater pH levels, urban and horticultural land use and unconfined aquifers were associated with pesticide detections. Comparisons of total pesticide concentrations in wells included in the previous surveys indicate that total target pesticide concentrations in groundwater are decreasing.

Risk analysis of pesticides applied to rice paddies using RICEWQ 1.6.2v and RIVWQ 2.02

Rice is commonly cultivated in Southern Europe in large river basins where the extensive use of herbicides may impact water quality at basin scale level. Consequently, pesticide risk assessment would be more relevant when considering a rice cultivation area as a whole. Accordingly, two watershed-scale scenarios were developed to represent the specific agronomic, water and pesticide management practices of rice cultivation in Greece. These scenarios were utilized for the parameterization of the rice water quality model (RICEWQ) 1.6.2v and the surface water model river water quality (RIVWQ) 2.02. in order to predict the environmental concentrations of pesticides in groundwater (GW) and surface water (SW) systems within a rice area. The annual average pesticide concentrations at 1 m depth, predicted by RICEWQ were used to assess the risk for GW contamination. The predicted exposure concentrations derived through modelling and eco-toxicological endpoints obtained from the literature, were used to calculate acute and long-term toxicity exposure ratios (TERs) for aquatic organisms. Application of these scenarios with RICEWQ and RIVWQ models enabled us to perform a risk assessment exercise for several herbicides registered in Greece for use in rice. Such a risk assessment approach could be further scaled up to represent a whole rice cultivated basin in order to be used for regulatory purposes.

Effects of Soil Variability and Weather Conditions on Pesticide Leaching— A Farm‐Level Evaluation

In line with European regulations, Dutch law imposes an environmental threshold of 0.1 microg L(-1) on pesticide concentrations in ground water. During registration, the risk of exceeding this threshold is assessed through simulations for one or a few standard scenarios that do not reflect spatial variability under field conditions. The introduction of precision agriculture, where soil variability is actively managed, can increase control over pesticide leaching. This study presents a step-wise evaluation of the effects of soil variability and weather conditions on pesticide leaching. The evaluation was conducted on a 100-ha arable farm and aimed at identifying opportunities for precision management. As a first step, a relative risk assessment identified pesticides presenting a relatively high risk to the environment. Second, the effect of weather conditions was analyzed through 20 years of simulations for three distinct soil profiles. Results were summarized in cumulative probability plots to provide a probabilistic characterization of historical weather data. The year matching 90% probability (1981) served as a reference to simulate pesticide leaching from 612 soil profiles. After interpolation, areas where concentrations exceeded the environmental threshold were identified. Out of a total of 19 pesticides, isoproturon [N-dimethyl-N'-(4-(1-methylethyl)phenyl)urea], metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one], and bentazon [2,1,3-benzothiadiazin-4(3H)-one, 3-isopropyl-, 2,2-dioxide] showed the highest risk for leaching. Leaching was strongly affected by soil variability at the within-field, field, and farm levels. Opportunities for precision management were apparent, but depended on the scale level at which environmental thresholds were implemented. When legislation is formulated in this issue, the presented step-wise evaluation can serve as a basis for identification and precision management of high-risk pesticides.

AGRICHEMICAL TRANSPORT TO GROUNDWATER THROUGH COASTAL PLAIN SOILS

A 1ha field with Pine Flat loamy sand (coarseloamy, siliceous, thermic Typic Paleudult) and Troup loamy sand(loamy, siliceous, thermic Grossarenic Kandiudult) surface soils, located near Plains, Georgia, was studied for four years(1993 to 1996) to evaluate potential agrichemical transport to groundwater. The field was managed to produce summer cornand winter wheat. Commercial fertilizer, the herbicide atrazine, and the insecticide carbofuran were applied to the field in1993, 1994, and 1995. Average annual application rates were 266 kg nitrogen ha1, 2.5 kg atrazine ha1, and 2.4 kgcarbofuran ha1. Monthly soilwater and groundwater samples were collected. The samples were analyzed for nitratenitrogen (NO3N), chloride, atrazine, carbofuran, and deethylatrazine (DEA). Soilwater and groundwater samplesindicated elevated NO3N concentrations (>5 ppm) in the vadose zone at 4.3 m and in the aquifer at 10 m (>4 ppm). Of thestudied pesticides, carbofuran and DEA were observed at the greatest concentrations in groundwater.<br><br>Both NO3N and pesticides were transported during groundwater recharge following periods of excess precipitation.Peak pesticide concentrations in groundwater were observed in late 1994, driven by a large precipitation event in July of 1994when 565 mm of rain fell over a 4day period. Atrazine and carbofuran concentrations in groundwater did not exceed theEPA maximum contaminant levels of 3 ppb and 40 ppb, respectively. Spatially averaged concentrations observed in monthlygroundwater collected directly below the field were well below these standards. Concentrations of NO3N, atrazine, DEA,and carbofuran observed in groundwater from the onfield wells were significantly different from upgradient anddowngradient concentrations (p = 0.05). These data indicate a significant impact to the local groundwater. Nitrate N wastransported downgradient from the field at the largest concentrations. Peak concentrations of atrazine and DEA weresimultaneously observed in the groundwater, indicating similar transport rates for both compounds and rapid transformationfrom atrazine into DEA in the rootzone.

Simulation of picloram, atrazine, and simazine leaching through two New Zealand soils and into groundwater using HYDRUS-2D

Two 15 m×15 m field plots, a Te Awa silt loam and a Twyford fine sandy loam, located in Hawkes Bay, New Zealand, were applied with bromide, picloram, atrazine, and simazine. The Te Awa subsoil was a heterogeneous coarse sand and sandy gravel, and the Twyford subsoil was a more homogenous fine sandy loam. The underlying aquifers were composed of alluvial gravels at both sites with the water tables generally between 4–5 m below ground level. The sites were monitored for 2.2–3.5 years at approximately monthly intervals using suction cups in the unsaturated zone and monitoring wells in groundwater. HYDRUS-2D was used to simulate water movement and solute transport in soil and groundwater in a domain with a depth of 10 m and length of 68 m, including a 4.5-m unsaturated zone. The model simulated well the general trend of field observations for soil water content ( θ) and potential ( ψ s), and the values matched better for the soils with less heterogeneity. For the soils with significant surface cracks, the simulated θ values were overestimated. On the other hand, for the soil layer perching on top of a less permeable layer, the simulated θ values were underestimated. Simulated pesticide concentrations using the “best available literature values” (BALVs) of organic carbon distribution coefficient ( K oc) and half-life ( T 1/2) were generally lower than those observed. At early times in the trails, most simulations using BALVs were still within the same order of magnitude as observed concentrations for the shallow depths. However, at greater depths and later times, there were major differences between observed and simulated concentrations. The model was then calibrated for K oc and T 1/2 values using observed data with an aid of the PEST optimisation package. Despite higher organic contents found in the topsoil, optimised K oc values for pesticides were consistently lower for the topsoil than for the subsoil, and were also lower than the BALVs except for picloram, possibly as a result of preferential flow in the topsoil. Compared to the BALVs, picloram was much more persistent but slightly more sorptive in both soil types, and atrazine and simazine were more persistent in the Te Awa soils but less persistent in the Twyford soils. The optimised K oc values for all three pesticides were generally greater, or close to, the BALVs in the subsoil of both sites. HYDRUS-2D provided a reasonably good link for pesticide transport in both the unsaturated zone and groundwater. Simulated pesticide concentrations in groundwater using optimised values were generally similar to the observed values. Both observed and simulated bromide and pesticide concentrations indicated that solutes leached more quickly through the soils that were coarser and more heterogeneous, but were more diluted in the groundwater system that was more heterogeneous, conductive, and dispersive. Significant levels of picloram were found in groundwater 22 and 53 m down-gradient of the Twyford and Te Awa plot, respectively. Both simulated and observed atrazine and simazine concentrations in groundwater were less than detection limits.

Simulation of pesticide concentrations in groundwater using Agricultural Drainage and Pesticide Transport (ADAPT) model

A water quality model for subirrigation and subsurface drainage, ADAPT (Agricultural Drainage And Pesticide Transport), was tested with field data collected under various water table management practices near Ames, IA. Atrazine and alachlor concentrations at various soil depths for water table depths of 30, 60, and 90 cm were simulated using ADAPT model for corn growing seasons of 1989 through 1991. Daily pesticide concentrations in groundwater predicted by the model were compared with available observed data for the same site. Predicted values of atrazine and alachlor concentrations in groundwater decreased when shallow water table depths were maintained in the lysimeters. Similar trends were noticed with the observed data. Reasonable agreement was obtained between the observed and predicted values of atrazine and alachlor for 1989 to 1991. However, in few cases, results showed a wide variation between observed and predicted values. Because no observed data was available for pesticide concentrations in the unsaturated zone, predicted results could not be compared. Based on our investigation, it appears that ADAPT may be used for predicting subsurface water quality under water table management practices; however, further validation is necessary with more field observed data from similar studies before wider application of this model is made.

GROUNDWATER RESIDUES OF ATRAZINE AND ALACHLOR UNDER WATER-TABLE MANAGEMENT PRACTICES

This study was conducted to investigate the effects of various water-table management (WTM) practices on the concentrations of two surface-applied herbicides, atrazine and alachlor, in a shallow groundwater system. Groundwater samples were collected by installing piezometers and suction tubes at Iowa State University’s research centers near Ames and Ankeny during three corn-growing seasons, 1989-1991. At the Ames site, experiments were conducted by maintaining constant water-table depths (WTD) of 0.3, 0.6, and 0.9 m in nine field-type lysimeters, and groundwater samples were collected from various depths during the corn-growing seasons. At the Ankeny site, a dualpipe subirrigation system was installed on a 0.85 ha field, and variable water-table depths were maintained. Analysis of water samples collected in 1989, 1990, and 1991 clearly indicates that atrazine and alachlor concentrations in groundwater could be substantially reduced by maintaining shallow WTD during the growing season. It was also observed that atrazine concentrations were higher than those of alachlor. Alachlor was not detected in many samples; however, atrazine was detected in all samples, with high concentrations at the Ames site at the 0.9 m WTD, and at the Ankeny site at deeper WTD. Pesticide concentrations in groundwater decreased with soil depth and time. Results of this study suggest a positive influence of WTM practices in reducing pesticide concentrations in groundwater.

An analytical model for the assessment of pesticide exposure levels in soils and groundwater

The movement and degradation of pesticide residues in soils and groundwater are complex processes affected by soil physical, (bio)chemical, and hydrogeological properties, climatic conditions, and agricultural practices. This work presents a physically-based analytical model suitable for long-term predictions of pesticide concentrations in groundwater. The primary interest is to investigate the impact of soil environment, related physical and (bio)chemical processes, especially, volatilization, crop uptake, and agricultural practices on long-term vulnerability of groundwater to contamination by pesticides. The soil is separated into root and intermediate vadose zones, each with uniform properties. Transport in each soil zone is modeled on the basis of complete mixing, by spatial averaging the related point multiphase-transport partial differential equation (i.e., linear-reservoir models). Transport in the aquifer, however, is modeled by a two-dimensional advection-dispersion transport equation, considering adsorption and first-order decay rate. Vaporization in the soil is accounted for by assuming liquid-vapor phase partitioning using Henry's law, and vapor flux (volatilization) from the soil surface is modeled by diffusion through an air boundary layer. Sorption of liquid-phase solutes by crops is described by a linear relationship which is valid for first-order (passive) crop uptake. The model is applied to five pesticides (atrazine, bromacil, chlordane, heptachlor, and lindane), and the potential for pesticide contamination of groundwater is investigated for sandy and clayey soils. Simulation results show that groundwater contamination can be substantially reduced for clayey soil environments, where bio(chemical) degradation and volatilization are most efficient as natural loss pathways for the pesticides. Also, uptake by cross can be a significant mechanism for attenuating exposure levels in ground-water especially in a sandy soil environment, and for relatively persisting pesticides. Further, simulations indicate that changing agricultural practices can have a profound effect on vulnerability of groundwater to mobile and relatively persisting pesticides.

Pesticides and Nutrients In Southern U. S. Shallow Ground Water and Surface Runoff

Nutrient and pesticide concentrations in shallow groundwater (3m) and surface runoff were determined for conventional-till (CT) and no-till (NT) soybean watersheds. Groundwater NO3N concentrations were similar for both tillage systems, averaging about 7-8 ppm, but exceeded 10 ppm for some storms. Annual mean groundwater NO3N concentrations were only 0.34 ppm in a riparian zone downslope from the CT watershed. Higher nutrient concentrations in NT surface runoff reflected surface residue leaching. Runoff from both watersheds, 2 days after a broadcast application of 0-20-20, had high nutrient concentrations that decreased during subsequent storms. Pesticide concentrations in groundwater of the NT watershed were as high as 250 ppb at a depth of 1.5 m within 1 week after application (1st rainfall). Concentrations beneath the CT watershed were &amp;lt; 10 ppb maximum. One month after application, pesticide concentrations in groundwater beneath both watersheds had decreased to about 10% of their respective 1-week values. Similar total pesticide losses in runoff occurred for both tillage systems. NT reduced sediment loss but increased pesticide movement into the soil profile. Results of companion studies with corn at another site indicated similar trends for nutrients and pesticides in shallow groundwater. Within 11 months after application, herbicides were still detectable at very low concentrations (&amp;lt;1 ppb). Soil insecticides, applied at planting, were not found in groundwater. Herbicides and insecticides were detectable in both the water and sediment phases of runoff for 5 months after application. At the 1.5-m depth, the mean NO3N concentration in groundwater for conventional-, reduced-and no-till corn was 4.78 ppm compared with 6.96 ppm at the 3-m depth.

Pesticide Contamination of Groundwater in Virginia: BMP Impact Assessment

The Nomini Creek Watershed/Water Quality monitoring project was initiated in 1985, as part of the Chesapeake Bay Agreement of 1983, to quantify the impacts of agricultural best management practices (BMPs) on improving water quality. The watershed monitoring system was designed to provide a comprehensive assessment of the quality of surface and groundwater as influenced by changes in land use, agronomic, and cultural practices in the watershed over the duration of the project. The primary chemical characteristics monitored include both soluble and sediment-bound nutrients and pesticides in surface and groundwater. Water samples from 8 monitoring wells located in agricultural areas in the watershed were analyzed for 22 pesticides. A total of 20 pesticides have been detected in water samples collected. Atrazine is the most frequently detected pesticide. Detected concentrations of atrazine ranged from 0.03 - 25.56 ppb and occurred in about 26 percent of the samples. Other pesticides were detected at frequencies ranging from 1.6 to 14.2 percent of all samples collected and concentrations between 0.01 and 41.89 ppb. The observed concentrations and spatial distributions of pesticide contamination of groundwater are compared to land use and cropping patterns. Results indicate that BMPs are quite effective in reducing pesticide concentrations in groundwater.

Multivariate Geostatistïcal Analysis of Ground‐Water Contamination: A Case History

AbstractA case history is presented for the application of multivariate geostatistical methods to the problem of estimating pesticide concentrations in ground water from measured concentrations of nitrate and pesticide, when pesticide is under‐sampled. The shallow, poorly confined, sand and gravel aquifer underlying the lower Malheur River basin near Ontario, Oregon is contaminated by nitrate and metabolites of the herbicide Dacthal (dimethyl tetrachloroterephthalate) or DCPA. The results of extensive ground‐water sampling indicate that a significant positive correlation exists between measured nitrate and DCPA concentrations in the aquifer. This suggests that future sampling should include a large number of the less‐expensive nitrate analyses, and these data should be used to support the interpretation of fewer, more expensive DCPA analyses. Sample variograms were computed for nitrate and DCPA concentrations and were fit with isotropic, spherical variogram models with correlation ranges of 4 km. Incorporating measured nitrate concentrations in the DCPA estimates obtained by cokriging reduced estimation variances from 14 to 34%. A simple economic analysis demonstrated that for this aquifer, acquiring additional nitrate samples is a cost‐effective way to reduce estimation variances for DCPA.

Pesticides pollution of groundwater in the humid United States

Since 1980, many studies have begun to look at the occurrence of pesticides in groundwater; these range from controlled field/plot research studies, to simple monitoring of public water supplies to assess the occurrence of pesticides. The compounds most commonly detected are: (1) mobile and/or volatile soil fumigants and nematicides, used on vegetable or specialty crops; (2) commonly used herbicides from the humid corn-belt regions. The fumigants-nematicides have been the most widely looked for compounds. The herbicides commonly detected are also some of the most widely used of all pesticides, but pesticide properties clearly affect how commonly they are detected. In nearly all corn-belt areas, the herbicide atrazine is the most commonly detected compound. A general overview shows that a total of 39 pesticides have been detected in groundwater from 34 states or provinces. This includes occurrences related to commercial point sources (but not manufacturing sites, or detections exclusively referred to commercial sources) as well as nonpoint sources. From states with appropriate studies, there appears, within the context of pesticide properties, some general proportionality between pesticide use and detections in groundwater; this is also confounded by climatic differences. Kansas studies suggest that 8–10% of private, rural drinking-water supply wells, and 10% of susceptible public water-supply wells may exhibit pesticide contamination. Surveys in Minnesota and Iowa, with more intensive pesticide usage suggest that 20–30% of the more susceptible public water-supply wells, and 30–60% of private wells may exhibit pesticide residues. In contrast however, Illinois, a very high use state, has detected few pesticides in public water-supply wells (except where related to a commercial point source); why such differences occur among adjacent states is not clear. Pesticides are leaching through the soil and into groundwater far more commonly than the preconceptions of a decade ago would have predicted. Point sources may be widespread but are not the sole cause; it is also clear that many pesticides are leaching to groundwater from routine, nonpoint source use. Controlled plot studies show the intermittent, often rapid delivery of many pesticides to shallow groundwaters. The preferential flow of water and solutes through soil and rock materials clearly contributes to the delivery of pesticides to groundwater. The occurrence of preferential flow also implies that notions about the ‘relative susceptibility’ of various soils to pesticide leaching may not be accurate. Some studies show pesticide leaching more commonly in finer-textured soils than coarser, sandy textured soils. Generally, the concentrations of pesticides in groundwater are low, in the 0.1–5.0 μg l −1 range. Even at these concentrations there are concerns for long-term, chronic exposure to a large segment of the public through drinking-water supplies. While concerns with groundwater merit attention, the concentrations of pesticides in drinking water derived from surface waters will typically be greater. Current policy discussions often fail to understand the difference between groundwater protection and drinking-water protection programs and may not provide adequate protection for private rural groundwater supplies. Research and policies to resolve the problems of agricultural impacts on the environment will require a new focus on integrated farm-management systems, that enhance efficiency and reduce off-site impacts.