Groundwater Monitoring Studies Research Articles

Impact of hydrogeology factors on geotechnical conditions of the heritage Kyiv-Pechersk Lavra Monastery complex: Lessons from three decades of monitoring

Abstract The architectural ensemble of the Kyiv-Pechersk Lavra monastery located in the city of Kyiv on the Right Bank of the Dnipro River, which represents historical and cultural heritage of world significance, suffers from a wide range of hazardous geological processes. The review and analysis of data of the long-term (since the 1990s) groundwater monitoring studies of the Kyiv-Pechersk Lavra shows that hydrogeological hazards caused by a complex combination of natural and anthropogenic factors (as well as sometimes by engineering miscalculations) have led to a significant number of emergency situations that threaten the heritage architectural monuments. Monitoring data illustrate a wide range of hydrogeological hazards (flooding, subsidence, landslides, suffusion), and emphasize the importance of consideration of hydrogeological aspects for safeguarding architectural monuments in the urbanized context. The paper also summarizes experience in engineering measures aimed at mitigating of hydrogeological hazards and discusses the pending research tasks

Statement of the Scientific Panel on Plant Protection Products and their Residues (PPR Panel) on the design and conduct of groundwater monitoring studies supporting groundwater exposure assessments of pesticides.

Groundwater monitoring is the highest tier in the leaching assessment of plant protection products in the EU. The European Commission requested EFSA for a review by the PPR Panel of the scientific paper of Gimsing etal. (2019) on the design and conduct of groundwater monitoring studies. The Panel concludes that this paper provides many recommendations; however, specific guidance on how to design, conduct and evaluate groundwater monitoring studies for regulatory purposes is missing. The Panel notes that there is no agreed specific protection goal (SPG) at EU level. Also, the SPG has not yet been operationalised in an agreed exposure assessment goal (ExAG). The ExAG describes which groundwater needs to be protected, where and when. Because the design and interpretation of monitoring studies depends on the ExAG, development of harmonised guidance is not yet possible. The development of an agreed ExAG must therefore be given priority. A central question in the design and interpretation of groundwater monitoring studies is that of groundwater vulnerability. Applicants must demonstrate that the selected monitoring sites represent realistic worst-case conditions as specified in the ExAG. Guidance and models are needed to support this step. A prerequisite for the regulatory use of monitoring data is the availability of complete data on the use history of the products containing the respective active substances. Applicants must further demonstrate that monitoring wells are hydrologically connected to the fields where the active substance has been applied. Modelling in combination with (pseudo)tracer experiments would be the preferred option. The Panel concludes that well-conducted monitoring studies provide more realistic exposure assessments and can therefore overrule results from lower tier studies. Groundwater monitoring studies involve a high workload for both regulators and applicants. Standardised procedures and monitoring networks could help to reduce this workload.

Geophysical investigation of dambo groundwater reserves as sustainable irrigation water sources: case of Linthipe sub-basin

The geophysical investigation of dambo groundwater reserves using electrical resistivity methods was conducted in Linthipe 4B sub basin in Central region of Malawi. With the increasing over-utilization of shallow wells in dambos for smallholder irrigation, this study was carried out to investigate whether dambo groundwater reserves could serve as sustainable irrigation water sources and to examine the aquifer characteristics in Linthipe sub basin. Vertical Electrical Sounding (VES) points were established in the basin using the Schlumberger configuration array. Data was analysed using IPI2win and Surfer software applications. Contrary to other commercial software applications which are costly for government departments when analysing geophysical data, partial curve matching and one dimensional (1-D) computer iteration techniques were used to interpret the VES curves and the pseudo-cross section resistivity profiles due to their simplicity and cost-effectiveness. The study has revealed that the aquifer properties in the basin are exceptionally variable in terms of state of weathering, depth, thickness, lithology, aquifer recharge configuration, geologic material and hydraulic gradients. These variations showed that the shallow wells in the basin have significant fractures suggesting high water potential for climate smart irrigation usage in the study area. The potential groundwater zones for climate smart irrigated farming were detected at VES 1, 2, 3, 4, 5 and 6 noted by low aquifer resistivity with high values of aquifer thickness. However, VES points, 7, 8 and 9 were likely to be zones of low water bearing potential because they had high values of aquifer resistivity with low aquifer thickness. The findings also validated the effectiveness, timeliness, and efficiency of using vertical electrical resistivity technique in exploring groundwater for irrigation. The results from this study have highlighted that feasible shallow well depth for a sustainable irrigation system is a function of geologic resistance and aquifer thickness when the geologic material has low resistance and broader thickness. This study noted that knowledge of aquifer recharge rates in dambos is required in order to effectively control abstraction rates for sustainable irrigation in basins. There is a need to promote the usage of geophysical studies, reforestation, river basin management trainings and aquifer recharge technologies in exploring sites for shallow well development for irrigation. The study further recommends the usage of geographical information systems (GIS) and Artificial Intelligence (AI) in improving the results of groundwater monitoring studies in Malawi.

Hydrogeological investigation of soil salinity adjacent to a flood protection infrastructure

Groundwater modelling is a commonly used technique to determine the influence of surficial processes on subsurface aquifers. In this study, a groundwater monitoring study was conducted on an agricultural field adjacent to a large drainage canal to determine the effects of the canal's operation as it relates to soil salinity during periods around flood events. The groundwater monitoring program consisted of twenty standpipes instrumented with groundwater pressure transducers that took four daily measurements. A finite element model was generated using data collected from the 2017 flood year to determine the effect of the flood on the local groundwater regime within one area consisting of four standpipes. The analysis of the model calibration yielded good to excellent results using the Nash–Sutcliffe Model Efficiency technique, with three of the four standpipes. The salt content within the model area was primarily gypsum initially derived from the underlying till and bedrock units. Model analysis indicated that various processes might impact soil salinity, including canal seepage, evapotranspiration and excessive snowmelt/recharge. The largest flood event on record for the canal was used in the calibrated model to determine the maximum extent of influence the canal has on the adjacent lands. The maximum extent of impact was found to be 112 m in the alluvial sediments and 240 m in the surficial sand structure.

Conducting groundwater monitoring studies in Europe for pesticide active substances and their metabolites in the context of Regulation (EC) 1107/2009

Conducting groundwater monitoring studies in Europe for pesticide active substances and their metabolites in the context of Regulation (EC) 1107/2009

Groundwater monitoring for 1,3-dichloropropene in high fumigant use areas of North America and Europe.

BACKGROUND1,3‐Dichloropropene (1,3‐D) is a soil fumigant used for the control of nematodes in high‐value fruit, nut and vegetable crops globally. Extensive water monitoring efforts have been undertaken over the past four decades by public and private institutions, given the widespread agricultural use of 1,3‐D, and environmental fate and metabolism data suggesting the potential for 1,3‐D to leach into groundwater. The aim of this study is to review the results of groundwater monitoring studies for 1,3‐D conducted in North America and the European Union (EU) since 1980.RESULTSAnalysis of > 50 000 water samples by state and federal agencies in the USA resulted in 151 detections of 1,3‐D. An additional 4000 samples analyzed in groundwater studies specifically targeting high 1,3‐D use areas in Europe and the USA resulted in 74 detections of 1,3‐D or its primary metabolites. The combined detection rate of 1,3‐D and its primary metabolites in high‐use areas of the EU and North America was 0.7%.CONCLUSIONSThe availability of extensive groundwater monitoring information developed through decades of study supports the conclusion that soil fumigation with 1,3‐D poses an inconsequential risk for drinking water exposure. The lack of significant detections of 1,3‐D from targeted monitoring studies is due to the high volatility of 1,3‐D, the rapid degradation of 1,3‐D in most agricultural soils, the rapid degradation of 1,3‐D and its metabolites in aerobic aquatic systems, and the rapid hydrolysis of 1,3‐D in water. © 2019 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Assessing biosynthetic potential of agricultural groundwater through metagenomic sequencing: A diverse anammox community dominates nitrate-rich groundwater.

BackgroundClimate change produces extremes in both temperature and precipitation causing increased drought severity and increased reliance on groundwater resources. Agricultural practices, which rely on groundwater, are sensitive to but also sources of contaminants, including nitrate. How agricultural contamination drives groundwater geochemistry through microbial metabolism is poorly understood.MethodsOn an active cow dairy in the Central Valley of California, we sampled groundwater from three wells at depths of 4.3 m (two wells) and 100 m (one well) below ground surface (bgs) as well as an effluent surface water lagoon that fertilizes surrounding corn fields. We analyzed the samples for concentrations of solutes, heavy metals, and USDA pathogenic bacteria of the Escherichia coli and Enterococcus groups as part of a long term groundwater monitoring study. Whole metagenome shotgun sequencing and assembly revealed taxonomic composition and metabolic potential of the community.ResultsElevated nitrate and dissolved organic carbon occurred at 4.3m but not at 100m bgs. Metagenomics confirmed chemical observations and revealed several Planctomycete genomes, including a new Brocadiaceae lineage and a likely Planctomycetes OM190, as well novel diversity and high abundance of nano-prokaryotes from the Candidate Phyla Radiation (CPR), the Diapherotrites, Parvarchaeota, Aenigmarchaeota, Nanoarchaeota, Nanohaloarchaea (DPANN) and the Thaumarchaeota, Aigarchaeota, Crenarchaeota, Korarchaeota (TACK) superphyla. Pathway analysis suggests community interactions based on complimentary primary metabolic pathways and abundant secondary metabolite operons encoding antimicrobials and quorum sensing systems.ConclusionsThe metagenomes show strong resemblance to activated sludge communities from a nitrogen removal reactor at a wastewater treatment plant, suggesting that natural bioremediation occurs through microbial metabolism. Elevated nitrate and rich secondary metabolite biosynthetic capacity suggest incomplete remediation and the potential for novel pharmacologically active compounds.

A Survey Sampling Approach for Pesticide Monitoring of Community Water Systems Using Groundwater as a Drinking Water Source

The ability to infer human exposure to substances from drinking water using monitoring data helps determine and/or refine potential risks associated with drinking water consumption. We describe a survey sampling approach and its application to an atrazine groundwater monitoring study to adequately characterize upper exposure centiles and associated confidence intervals with predetermined precision. Study design and data analysis included sampling frame definition, sample stratification, sample size determination, allocation to strata, analysis weights, and weighted population estimates. Sampling frame encompassed 15 840 groundwater community water systems (CWS) in 21 states throughout the U. S. Median, and 95th percentile atrazine concentrations were 0.0022 and 0.024 ppb, respectively, for all CWS. Statistical estimates agreed with historical monitoring results, suggesting that the study design was adequate and robust. This methodology makes no assumptions regarding the occurrence distribution (e.g., lognormality); thus analyses based on the design-induced distribution provide the most robust basis for making inferences from the sample to target population.

A Comparison of 18Oδ Composition of Water Extracted from Suction Lysimeters, Centrifugation, and Azeotropic Distillation

The representativeness of soil pore water extracted by suction lysimeters in ground-water monitoring studies is a problem that often confounds interpretation of measured data. Current soil water sampling techniques cannot delineate from which soil volume a pore water sample is extracted, neither macroscopic, microscopic, or preferential flowpath. This research was undertaken to compare δ18O and Br− values of extracted suction lysimeters samples from intact soil cores with samples obtained by the direct extraction methods of centrifugation and azeotropic distillation. Also, the study was concerned with determining what portion of soil pore water is sampled by each method and explaining differences in concentrations of the extracted water from each method to allow a determination of the accuracy and viability of the three methods of extraction. Intact soil cores (30 cm diameter by 40 cm height) were extracted from two different sites. Site 1 was rapid infiltration basin number 50, near Altamonte Springs in Seminole County, Florida. Site 2 was the Missouri Management System Evaluation Area (MSEA) near Centralia in Boone County, Missouri. Isotopically (18Oδ) labeled water and bromide concentrations within water samples taken by suction lysimeters was compared with samples obtained by methods of centrifugation and azeotropic distillation. The 18Oδ water was analyzed by mass spectrometry while bromide concentration, applied in the form of KBr was measured using standard IC procedures. Water collected by centrifugation and azeotropic distillation data were about 0.25‰ more negative than that collected by suction lysimeter values from a sandy soil and about 2–7‰ more negative from a well structured soil. Results indicate that the majority of soil water in well-structured soil is strongly bound to soil grain surfaces and is not easily sampled by suction lysimeters. In cases where a sufficient volume of water has passed through the soil profile and displaced previous pore water, suction lysimeters will collect a representative sample of soil pore water from the sampled depth interval. It is suggested that for stable isotope studies monitoring precipitation and soil water, suction lysimeter be installed at shallow depths (10 cm). Samples should also be coordinated with precipitation events. The data also suggest that each extraction method samples a separate component of soil-pore water. Centrifugation can be used with success, particularly for efficient sampling of large areas. Azeotropic distillation is more appropriate when strict qualitative and quantitative data on sorption desorption, and various types of kinetic studies may be needed.

Nitrate Movement in Shallow Ground Water from Swine‐Lagoon‐Effluent Spray Fields Managed under Current Application Regulations

Rapid increases in the swine (Sus scrofa domestica) population in the 1990s and associated potential for nitrate N pollution of surface waters led the state of North Carolina to adopt stringent waste management regulations in 1993. Our objectives were to characterize (i) nitrate N movement from waste application fields (WAFs) in shallow ground water, and (ii) soil, hydrologic, and biological factors influencing the amount of nitrate N in the adjacent stream. A ground water monitoring study was conducted for 36 mo on a swine farm managed under new regulations. Water table contours and lack of vertical gradients indicated horizontal flow over most of the site. Nitrate N concentrations in water from shallow wells in WAFs averaged 30 +/- 19 mg L(-1) and delta15N ratios for nitrate N were between +20 and +25 per mil. Nitrate N concentration decreased from field-edge to streamside wells by 22 to 99%. Measurement of delta18O and delta15N enrichment of nitrate in ground water throughout the WAF-riparian system indicated that denitrification has not caused significant 15N enrichment of nitrate. Over a 24-mo period, delta15N ratios for nitrate N in the stream approached delta15N ratios for nitrate N in ground water beneath WAFs indicating delivery of some waste-derived nitrate N to the stream in shallow ground water. Nitrate N concentrations in the stream were relatively low, averaging 1 mg L(-1). Dilution of high nitrate N water in shallow horizontal flow paths with low nitrate N water from deeper horizontal flow paths at or near the stream, some denitrification as ground water discharges through the stream bottom, and some denitrification in riparian zone contributed to this low nitrate N concentration.

Environmental aspects of geothermal energy utilization

Geothermal energy is a clean and sustainable energy source, but its development still has some impact on the environment. The positive and negative aspects of this environmental impact have to be considered prior to any decision to develop a geothermal field, as well as possible mitigation measures. The main environmental effects of geothermal development are related to surface disturbances, the physical effects of fluid withdrawal, heat effects and discharge of chemicals. All these factors will affect the biological environment as well. As with all industrial activities, there are also some social and economic effects. In Iceland an enforcement program was launched in the early 1990s to study the environmental impact of developing geothermal resources. Work began on tackling the environmental issues relative to the high-temperature geothermal fields under development in Iceland. Research was conducted on microearthquake activity in geothermal areas and a methodology developed for mapping steam caps. The foundations were laid of networks for monitoring land elevation and gravity changes. Baseline values were defined for the concentrations of mercury and sulfur gases. Groundwater monitoring studies were enforced. Atmospheric dispersion and reaction of geothermally-emitted sulfur gases and mercury were studied. Aerial thermographic survey methods were refined and tested and their capacity to detect and map changes in surface manifestations with time was demonstrated. To further the use of geothermal energy worldwide the International Energy Association set up a Geothermal Implement Agreement (GIA) in 1997; its environmental Annex has been actively implemented, with several projects still under way.

Leaching of Pesticides Through Normal‐Tillage and Low‐Tillage Soil—A Lysimeter Study. I. Isoproturon

Isoproturon is a herbicide, which was used in Denmark against grass weeds and broad‐leaved weeds until 1998. Isoproturon has frequently been detected in ground water monitoring studies. Leaching of isoproturon (N,N‐dimethyl‐N′‐(4‐(1‐methylethyl)phenyl)urea) and its metabolites, N′‐(4‐isopropylphenyl)‐N‐methylurea and N′‐(4‐isopropylphenyl)urea was studied in four lysimetres, two of them being replicates from a low‐tillage field (lysimeter 3 and 4), the other two being replicates from a normal tillage field (lysimeter 5 and 6). In both cases the soil was a sandy loam soil with 13–14% clay. The lysimetres had a surface area of 0.5 m2 and a depth of 110 cm. Lysimeter 3 and 4 were sprayed with unlabelled isoproturon while lysimeter 5 and 6 was sprayed with a mixture of 14C‐labelled and unlabelled isoproturon. The total amount of isoproturon sprayed onto each lysimeter was 63 mg, corresponding to 1.25 kg active ingredient per ha. The lysimeters were sprayed with isoproturon on October 26, 1997. The lysimetres were installed in an outdoor system in Research Centre Flakkebjerg and were thus exposed to normal climatic conditions of the area. A mean of 360 l drainage water were collected from lysimeter 3 and 4 and a mean of 375 litres from lysimeter 5 and 6. Only negligible amounts of isoproturon and its primary metabolites were found in the drainage water samples, and thus no significant difference between the two lysimeter sets was shown. In a total of 82 drainage water samples, evenly distributed between the four lysimetres isoproturon was found in detectable amounts in two samples and N′‐(4‐isopropylphenyl)urea was found in detectable amounts in two other samples. The detection limit for all the compounds was 0.02 μg/l. 48% and 54% of the added radioactivity were recovered from the upper 10 cm soil layer in lysimeter 5 and 6, respectively, and 17 and 14% from 10–20 cm's depth. By extraction first with an aquatic CaCl2 solution 0.49% of the added radioactivity was extracted from the upper 10 cm layer in lysimeter 5. In the subsequent extraction with acetonitril, 1.19% of the added radioactivity was ekstracted. In lysimeter 6, upper 10 cm, 0.2% were extracted with water and 0.56% were extracted with acetonitril. Below 10 cm's depth no measurable amounts could be extracted.

Assessing the natural attenuation of organic contaminants in aquifers using plume-scale electron and carbon balances: model development with analysis of uncertainty and parameter sensitivity

A quantitative methodology is described for the field-scale performance assessment of natural attenuation using plume-scale electron and carbon balances. This provides a practical framework for the calculation of global mass balances for contaminant plumes, using mass inputs from the plume source, background groundwater and plume residuals in a simplified box model. Biodegradation processes and reactions included in the analysis are identified from electron acceptors, electron donors and degradation products present in these inputs. Parameter values used in the model are obtained from data acquired during typical site investigation and groundwater monitoring studies for natural attenuation schemes. The approach is evaluated for a UK Permo-Triassic Sandstone aquifer contaminated with a plume of phenolic compounds. Uncertainty in the model predictions and sensitivity to parameter values was assessed by probabilistic modelling using Monte Carlo methods. Sensitivity analyses were compared for different input parameter probability distributions and a base case using fixed parameter values, using an identical conceptual model and data set. Results show that consumption of oxidants by biodegradation is approximately balanced by the production of CH 4 and total dissolved inorganic carbon (TDIC) which is conserved in the plume. Under this condition, either the plume electron or carbon balance can be used to determine contaminant mass loss, which is equivalent to only 4% of the estimated source term. This corresponds to a first order, plume-averaged, half-life of >800 years. The electron balance is particularly sensitive to uncertainty in the source term and dispersive inputs. Reliable historical information on contaminant spillages and detailed site investigation are necessary to accurately characterise the source term. The dispersive influx is sensitive to variability in the plume mixing zone width. Consumption of aqueous oxidants greatly exceeds that of mineral oxidants in the plume, but electron acceptor supply is insufficient to meet the electron donor demand and the plume will grow. The aquifer potential for degradation of these contaminants is limited by high contaminant concentrations and the supply of bioavailable electron acceptors. Natural attenuation will increase only after increased transport and dilution.

Assessing the ground-water contamination potential of agricultural chemicals: a flexible approach to mobility and degradation studies†

Understanding the fate of agrochemicals in surface and sub-surface environments is of vital importance for the protection of water quality and for responsible environmental stewardship of crop-protection products. This paper demonstrates the need for a versatile approach to study experimental designs, aimed at a fuller understanding of the mobility and degradation of particular compounds in surface and sub-surface environments. Where appropriate, the environmental profile of a crop-protection product is built up using a combination of the following four study types: (a) radiolabelled laboratory studies to establish the routes of degradation and key degradates, supported by non-radiolabelled small-scale field studies to quantify key degradates under field conditions; (b) small-plot radiolabelled field studies for tracking the fate of products with low usage rates or those exhibiting rapid and extensive metabolism; (c) small-scale prospective ground-water studies (PGWs) to assess the potential for a compound's sub-surface mobility in vulnerable ground-water settings, and (d) large-scale ground-water monitoring studies to measure actual environmental concentrations of an in-use product. The determination of which studies are required is product-specific. Examples include radiolabelled laboratory and field studies conducted to investigate the rapid dissipation of the post-emergence herbicide tralkoxydim and its subsequent metabolites in surface soil. A PGW approach is illustrated, used to assess the degradation and mobility of the contact herbicide fomesafen under vulnerable ground-water conditions. Finally, large-scale monitoring studies are described which are used to assess the impact of the selective post-emergence herbicide fluazifop-P-butyl and fomesafen in vulnerable ground-water regions of northern Italy and Germany. These examples illustrate how flexibility and diversity of study design are essential to the development of a meaningful database of environmental fate information for crop-protection products. Such a database provides critical data for risk assessments and predictive modelling, and enhances our fundamental understanding of environmental science. © 1998 Society of Chemical Industry

Monitoring Groundwater for Pesticides at Selected Mixing/Loading Sites in Arkansas

Groundwater monitoring studies have been conducted in recent years to survey contamination due to pesticides, yet few have addressed wells where pesticides are mixed, loaded, or rinsed. Beginning in 1990, a monitoring study conducted over a 2-year period included five collections at each of 16 mixer/loader locations to assess any pesticide and nitrate contamination. At sites in 11 counties, samples for pesticide analysis were extracted with solid-phase extraction (SPE) disks. Samples were screened using gas chromatography-electron capture detection (ECD) and high-performance liquid chromatography−UV detection (LCUV) for 17 pesticides commonly used in Arkansas. Detections were confirmed by gas chromatography−mass spectroscopy (MS) or co-chromatography. Fourteen samples revealed atrazine (1 detection), cyanazine (4), parathion-methyl (2), metolachlor (2), norflurazon (1), pendimethalin (1), propanil (2), or trifluralin (1) at eight locations during the 2-year study. Two detections of parathion-methyl and one detection of trifluralin were above the Lifetime Health Advisory Level (LHAL) of 2 μg L-1. Data suggested a high correlation between pesticide used and pesticide detected at sites sampled. Three wells contained NO3-N concentrations of 10 mg L-1 or higher, but these did not correlate with pesticide concentrations. The pesticide's proximity to the wells during mixing, rinsing, or loading was considered to be a greater influence on temporary contamination of groundwater than chemical or site-specific characteristics.

Use of Ground Water Monitoring Data for Pesticide Regulation

Growing public concern for pesticide residues in drinking water has resulted in tighter federal and state regulations to prevent ground-water contamination. There are many uncertainties in estimating human exposure risk, yet standards are being set to protect human health. In the last several years, drinking water health standards have been developed for a number of pesticides, often at levels of a few μg L−1 or less. Ground-water monitoring studies, first required by the U.S. Environmental Protection Agency in 1984, have now become an integral requirement to support registration of pesticides shown to have leaching potential. Regulators are now faced with deciding the extent to which pesticides can still be used while continuing to meet legal requirements to protect the public and preserve ground-water resources. The implications are that pesticide use may be severely limited in regions with ground-water resources considered especially vulnerable to contamination, dramatically affecting the type of agricultural practices and crops grown in these regions.

Small‐Scale Retrospective Ground Water Monitoring Study for Simazine in Different Hydrogeological Settings

AbstractA ground water monitoring study was conducted for the triazine herbicide simazine at 11 sites in the United States. The study used carefully selected, small‐scale sites (average size: about 33 acres) with documented product use and sensitive hydrogeological settings. The sites selected were Tulare County, California (two sites); Fresno County, California; Sussex County, Delaware; Hardee and Palm Beach counties, Florida; Winnebago County, Illinois; Jackson County, Indiana; Van Buren and Berrien counties, Michigan; and Jefferson County, West Virginia. These sites satisfied the following criteria: a history of simazine use, including the year prior to the start of the study; permeable soil and vadose zone; shallow depth to water; no restrictive soil layers above the water table; and gentle slopes not exceeding 2 percent. A variety of crop types, climates, and irrigation practices were included. Monitoring well clusters (shallow and deep) were installed at each site except in California and West Virginia, where only shallow wells were installed. Simazine was monitored at these sites at quarterly intervals for a two‐year period during 1986–1988.The results of the study showed that out of 153 samples analyzed, 45 samples showed simazine detections. A substantial majority of the detections (32 out of 45) occurred in Tulare, Fresno, and Jefferson counties. The detections in these areas were attributed to mechanisms other than leaching, such as drainage wells, karst areas, surface water recharge, or point source problems. An additional 11 detections in Van Buren County were apparently due to an unknown upgradient source. Only one detection (in Palm Beach County, Florida) near the screening level of 0.1 ppb was attributed to possible leaching. The results of this investigation support the hypothesis that simazine does not leach significantly under field use conditions.

A Large‐Scale Retrospective Ground Water Monitoring Study for Metolachlor

AbstractA large‐scale retrospective ground water monitoring study'for metolachlor, 2‐chloro‐N‐(2‐ethyl‐6‐methylphenyl)‐ N‐(2‐methoxy‐l‐methylethyl) acetamide, was conducted. The study consisted of the collection and analysis of ground water samples from 240 existing wells (203 domestic farm wells and 37 monitoring wells) in selected areas of four states: the Dougherty Plains, Georgia; McLean County, Illinois; Floyd and Mitchell counties, Iowa; and the Central Sands area of Wisconsin. These locations were selected on the basis of high metolachlor use and hydrogeological vulnerability.The wells selected were all screened in the uppermost aquifer, were relatively shallow, and were located close to fields where metolachlor has historically been used. Pesticide handling and storage practices on the farms where the wells were located were documented, and wells were selected to avoid obvious point sources. The wells selected for monitoring included drinking water, irrigation and monitoring wells. Monitoring was at quarterly intervals for a one‐year period.Metolachlor was detected in 89 of 920 samples (9.7 percent) from 39 of 240 wells (16.3 percent) with a screening level (quantitation level) of 0.1 ppb. Ten out of 240 wells sampled (4.2 percent) showed detections in all sampling rounds.Of the samples analyzed, 5.1 percent had detections in the range between 0.1 and 0.5 ppb. Thus, non‐detections plus detections below 0.5 ppb accounted for 95.4 percent of all samples, and detections below 0.5 ppb represented more than half of all the detections found in the study (47 out of 89 detections). In the 31 samples where metolachlor was detected above 1.0 ppb, every detection could be related to a known or suspected point source.Based on the frequency and magnitude of metolachlor detections in this study, there appears to be no significant measurable field leaching of metolachlor to ground water under conditions of high product use and high ground water vulnerability. Instead, measurable detections of metolachlor, particularly those exceeding 1.0 ppb, appear to be associated with point sources.

Small‐Scale Retrospective Ground Water Monitoring Studies for Agricultural Chemicals: Study Design and Site Selection

AbstractA small‐scale retrospective ground water monitoring study is designed to determine if, after years of normal usage, an agricultural chemical has leached to ground water. These studies are conducted on actively planted fields in hydrogeologically vulnerable areas. Because the results of such studies may have larger ramifications for the particular agricultural chemical over much broader hydrogeologic and geographic areas, care must be exercised in selecting particular fields to be monitored and in establishing site‐specific ground water monitoring networks.The U.S. Environmental Protection Agency, Office of Pesticide Programs, has established draft guidelines and suggested field protocols for such studies. These guidelines serve as a baseline for such studies, though many other hydrogeologic and agronomic factors play significant roles. Small‐scale retrospective studies are ordinarily broken into a series of tasks that lead to the selection of a suitable test site(s) by determining on a national scale where to monitor; meeting minimum hydrogeologic and agronomic criteria; and establishing a ground water monitoring program.A detailed hydrogeologic vulnerability analysis is conducted in product use areas for the chemical being studied. A series of screening steps results in a subset of moderate‐ to high‐use counties that are underlain by permeable soils and vulnerable to ground water impacts from agricultural chemicals. Once these counties are determined, suitable test fields can be selected.The individual site selection criteria can be exhaustive. However, these criteria in combination with one another are necessary to ensure that the selected sites will yield information as to whether or not the chemical has leached through the soil profile and the unsaturated zone to ground water. The following criteria are considered essential to a particular site: flat topography; “workable” field size; documented history of use of the chemical being monitored; avoidance of regional and farm‐specific point sources; typical irrigation and farm practices for the crop on which the chemical is applied; farmer cooperation; field accessibility; permeable soils and unsaturated zone; and shallow water table. There are also other hydrogeologic and agronomic parameters that need to be assessed prior to site selection.The hydrogeologic and agronomic criteria are not always easily met. For example, the topography and shallow water‐table criteria could eliminate key agricultural areas in the United States where the agricultural chemical to be monitored may be used. However, the data generated during these field‐scale retrospective studies, when properly conducted, can provide information regarding whether or not a particular agricultural chemical has leached to ground water under worst‐case conditions.

Field research studies on the movement and degradation of thiodicarb and its metabolite methomyl

The movement and degradation of thiodicarb and its metabolite methomyl were measured at three locations in the United States. At a site near Clayton, North Carolina, soil samples were used to determine the degradation rate of thiodicarb in surface soils and methomyl in both surface and subsoils. At sites near Palermo, New York, and Oviedo, Florida, the shallow groundwater was monitored following multiple foliar applications of thiodicarb. These groundwater monitoring studies were conducted under conditions favorable to movement of pesticides to groundwater since surface soils were loamy sand or sand, subsoils were sand, water tables ranged between 0.1 and 1.8 m, and rainfall was supplemented by irrigation to ensure that the sum of irrigation and rainfall was at least 1.5 times the normal monthly rainfall. In these studies the half-life of thiodicarb was only a few hours in surface soils, while the half-life methomyl was about two days in surface soils and about 0.5 to 1.6 months in subsoils. Although methomyl residues were detected in shallow groundwater in the Oviedo and Palermo studies (designed to be representative of worst case situations), these residues were present in only one well cluster and one sampling interval, and at concentrations substantially below the established health advisory level.