Agriculture is a major sector of the economy of the State of Alabama and contributed $70.4 billion to the State economy in 2010 (Auburn University Department of Agricultural Economics and Rural Sociology, 2013). Crop agriculture generates approximately $1.2 billion in annual sales and accounts for about 2.8 million acres of land cover in the State (U.S. Department of Agriculture [USDA] National Agricultural Statistics Service, 2019c). Major crops in Alabama include cotton, soybeans, peanuts, and corn. Historically, the use of agricultural pesticides has limited the effects of pests, such as insects, competitive weeds, and fungi, on crops and has enabled increases in production (Tudi and others, 2021). Although there are significant agricultural benefits to the use of pesticides, many have the potential to migrate from intended application areas into other environmental compartments. Leaching through soils and into groundwater supplies is one example of unwanted movement of pesticides and can limit future uses of the affected waters (Arias-Estévez and others, 2008; Tudi and others, 2021).

The Alabama Department of Agriculture and Industries (ADAI) is responsible for permitting the use of agricultural pesticides in the State. Monitoring the environmental fate of agricultural pesticides has been ongoing for many years as part of the agency’s pesticide management plan. Since 2009, the ADAI and U.S. Geological Survey (USGS) have cooperated on a project to investigate pesticide occurrence in shallow groundwater in areas of Alabama with intensive agricultural land use (fig. 1). The project is intended to evaluate the number, types, and magnitude of pesticide compound detections in a network of wells and to identify any apparent patterns of change over time.

Monitoring the occurrence and concentrations of pesticides in shallow groundwater underlying agricultural areas provides valuable data to understand the potential migration and persistence of these compounds in the groundwater system. The USGS places priority on collecting data to support the preservation of water resources for future uses (U.S. Geological Survey, 2021). The datasets produced by this study help to support USGS goals to monitor the quality of the Nation’s freshwater supplies.

The frequency and magnitude of pesticide detections in groundwater are related to the application of pesticides, which will tend to increase with increases in cropland area. Total cropland data reported in the 2017 Census of Agriculture (USDA National Agricultural Statistics Service, 2019a) are summarized in figure 3A–B for the counties with wells documented in this report. Limestone County (in the Tennessee River valley region) had the greatest acreage of cropland (fig. 3A), totaling about 160,000 acres and covering about 40 percent of the total county area (fig. 3B). Baldwin County had the third greatest cropland acreage but had the lowest percentage of county area in cropland. As much as possible, study wells were located near denser concentrations of cropland (fig. 1).

The cropland data layer (CDL) is produced by the USDA from remotely sensed data to identify crop types in the conterminous United States. CDL data allow for a more detailed view of specific crops in the vicinity of the study wells. Major crop types identified within a 1-kilometer radius of study wells in the 2018 CDL (USDA National Agricultural Statistics Service, 2021) were summarized by region (fig. 4) and evaluated as a potential predictor of pesticide compound detections. Cotton, peanuts, soybeans, corn, and grassland/pasture were the primary agricultural crops around study wells. Some regional differences in crop type were evident. On average, cotton accounted for about 15 percent of the buffer areas around the Baldwin County wells, and about 30 percent of the buffer areas around the Wiregrass region and Tennessee River valley region wells. Peanuts represented 10–15 percent of well buffer areas of Baldwin County and the Wiregrass region but were not present in the Tennessee River valley region well buffer areas. Soybeans and corn were much more prevalent in the Tennessee River valley region than in the other two regions. Grassland/pasture crops accounted for approximately 13 percent of land use in buffer areas around Baldwin County wells and more than 7 percent of buffer areas in the Tennessee River valley, but only about 1 percent of buffer areas in the Wiregrass region.

Estimates of agricultural pesticide use have been compiled by the USGS for the conterminous United States (Thelin and Stone, 2013; Baker and Stone, 2015; Wieben, 2019). County-level pesticide use was calculated using methods outlined in Thelin and Stone (2013) and Baker and Stone (2015) and crop-specific acreage and pesticide application rates estimated by the U.S. Department of Agriculture (Wieben, 2019). Pesticide use was estimated for 70 of the pesticides evaluated in this study. Total estimated use for the 70 pesticides was calculated by year and county for 2013–17 for the Alabama counties with study wells (fig. 5; Wieben, 2019). Total pesticide use was greatest in the Tennessee River valley region counties of Limestone and Madison during all years except 2014, when use in Baldwin County exceeded use in Madison County, and 2017, when use in Baldwin County exceeded use in Limestone and Madison Counties. Pesticide use in Limestone, Madison, and Baldwin Counties was consistently greater than use in Colbert County or any of the Wiregrass region counties (specifically, Henry, Houston, and Geneva Counties).

The pesticide compounds with the top five total greatest estimated use during 2013–17 in the study counties were all herbicides: glyphosate, pendimethalin, 2,4-D, atrazine, and acetochlor (table 2). Glyphosate, an herbicide widely used on broadleaf weeds and grasses, had the highest use estimates, almost six times greater than those for the next most commonly used pesticides in Alabama. Glyphosate use has been increasing since the introduction of genetically engineered glyphosate-resistant strains of soybeans, corn, cotton, and other crops in the mid-1990s. Because these crop strains are resistant to its effects, glyphosate can be applied throughout the growing season to control weeds (Benbrook, 2016). In the environment, glyphosate binds tightly to soils and has a half-life of a few days, so it is often not detected in groundwater (Henderson and others, 2010). Pendimethalin is used as a pre-emergent herbicide to control broadleaf weeds and grasses. The U.S. Environmental Protection Agency has noted that pendimethalin strongly binds to soils and has a low potential to contaminate groundwater (U.S. Environmental Protection Agency, 1997). The herbicide 2,4-D kills primarily broadleaf weeds by affecting plant cell division. The persistence of 2,4-D in the environment depends upon the chemical form used, with half-lives ranging from a few days to about half a year (Jervais and others, 2008). Atrazine is another pre-emergent herbicide commonly used on corn. Because of its longer half-life, medium solubility in water, and poor binding to soils, atrazine is often found in groundwater (Hanson and others, 2020). Acetochlor is a chloroacetanilide pre-emergent herbicide often used on corn, cotton, soybeans, and other crops to control annual grasses and some broadleaf weeds. Acetochlor is often used along with another pre-emergent herbicide, commonly atrazine, to achieve control of a broader array of weed types (Meyer and Scribner, 2009; Minnesota Department of Agriculture, 2022). Other pesticides with high total use in the study counties included the herbicides metolachlor and dicamba, insecticides chlorpyrifos and acephate, and fungicide tebuconazole.

Project design included site selection, routine sampling, and recurring data evaluation. The selection of sites was largely unchanged from the previous site selection description (Welch, 2015), but some sites were dropped from the network and replaced with substitute wells for 1 year or on a permanent basis. The years sampled for each well are listed in table 3 and in Gill (2023). Sample collection and field analysis methods followed standard USGS procedures. Laboratory methods were used to determine pesticide concentrations and some nitrate concentrations. Changes and improvements in laboratory analyses that affected sample collection and determination of pesticide concentrations are described in the “Laboratory Methods” section.

Basic field parameters (temperature, specific conductance, pH, turbidity, and dissolved oxygen) were measured during sample collection at all sites. In addition, rapid field chemical tests of nitrate concentrations were performed on raw water samples from the sites. Shallow groundwater collected at 19 shallow groundwater sites in Alabama during 2018 and 2020 was analyzed for nitrate concentration by laboratory methods to confirm nitrate results provided by field chemical tests in previous years.

From 2014 to 2020, a total of 75 environmental samples were collected from 22 groundwater sites. Concentrations of 230 pesticide and pesticide degradate compounds were determined (Gill, 2023). Some compounds were not analyzed in all samples, so the number of samples analyzed is listed along with the number of detections for each compound in table 7. The results of these analyses are summarized by the frequency of pesticide detections, magnitude of pesticide concentrations, and relation of the results to well locations and characteristics.

From the beginning of this project in 2009 through the summer of 2020, a total of 178 environmental samples were collected from 24 shallow groundwater sites in Alabama and evaluated for concentrations of 289 distinct pesticide compounds (Gill, 2023). Only a few of the analyzed pesticide compounds were detected frequently throughout the project timeframe. More than half (178) of the analyzed compounds were not detected in any of the environmental samples, and an additional 38 compounds were only detected once. Five compounds, metolachlor SA, atrazine, CIAT, metolachlor, and OIAT, were detected in more than 50 percent of the samples during the project timeframe (Gill, 2023).

As described in the methods, samples collected before 2014 were analyzed for about 145 pesticide compounds, and samples collected from 2014 through 2020 were analyzed for more than 200 pesticide and pesticide degradate compounds. Detections using the earlier methods were reported as concentrations in micrograms per liter, whereas under the newer method, they were reported as concentrations in nanograms per liter. A USGS comparative study of field methods attempted to link datasets produced by both of the methods and identified problems with censored data resulting from the great difference in method reporting limits (Martin and others, 2017). Despite these concerns, datasets from the entire period of record for this study were converted to common units of nanograms per liter and results of the two methods combined for the 230 pesticide compounds analyzed by both methods (Gill, 2023). The combined datasets were used to make a preliminary assessment of changes in pesticide compound detection frequencies and concentrations through time.

Twenty-seven pesticide or degradate compounds were detected in more than 15 percent of samples during the 2009–20 time period (fig. 18). There were only slight changes in the ranks of the most frequently detected compounds when the entire study period was compared to the 2014–20 period, suggesting that the occurrence of these compounds is consistent over time. Metolachlor, atrazine, and their degradates were among the most frequently detected compounds and were detected across all regions. Norflurazon, fluometuron, and their degradates were also commonly detected, but most of the detections occurred in the Tennessee River valley region, where estimated use was greater. The fungicide degradate 1H-1,2,4-triazole was also commonly detected throughout the project, with detections observed across all regions.

Although most compounds were detected at similar frequencies, a few compounds were detected in more than 15 percent of samples in only 1 of the 2 periods (figs. 13 and 18). Five compounds were detected in more than 15 percent of samples during 2009–13 but not during 2014–20. Analyses of 3 of these 5 compounds (sec-alachlor sulfonic acid, acetochlor ESA, and 3,4-dichloroaniline) were not continued through the later part of the study period. Aldicarb sulfoxide and diuron were analyzed throughout the project but were detected less frequently in samples collected during 2014–20. Two herbicides, tebuthiuron and bentazon, were detected in more than 15 percent of samples collected during 2014–20 but not during 2009–20. Tebuthiuron was detected in wells in all regions, but all detections in Baldwin County were from well BC-3. Bentazon was also detected in all regions, but most detections were in Baldwin County and the Wiregrass region.

Concentration data for compounds having a consistent record of sample detections throughout the project were examined for any apparent temporal patterns in the magnitude of concentrations. For each pesticide compound, the number of years analyzed, number of years with detections, and number of samples with detections were determined overall, by region, and by individual well. Only four compounds were detected during the first and second 5-year periods of the project, in more than 20 percent of sampled years, and in more than 40 percent of analyzed samples. For atrazine, CIAT (deethylatrazine), metolachlor, and metolachlor SA, detections and changes in detections through time were examined in detail. Compound concentrations in each well were plotted (figs. 19–22), and annual concentrations were plotted for TNV-5 (fig. 23), a well with frequent detections of all four compounds. Metolachlor SA was only analyzed during 2012 and 2014–20. Inclusion of the four detections from 2012 did not alter conclusions drawn from the discussion of 2014–20 results; however, metolachlor SA concentrations in individual wells also were plotted for comparison to parent compound concentrations.

Atrazine was frequently detected in all three regions but occurred at lower concentrations in the Baldwin County wells than in the other two regions. The greatest concentrations and variations in concentration over time occurred in wells in the Tennessee River valley region (fig. 19). The greater prevalence of cotton, corn, and soybean crops in the Tennessee River valley region (fig. 4) relative to Baldwin County and the related use of atrazine are likely explanatory factors for the differences between concentrations in Baldwin County and the Tennessee River valley region.

The variability of concentrations and occurrence of CIAT, a degradate of atrazine, was similar to that of the parent compound (fig. 20). Median concentrations of CIAT, like atrazine, were consistently lower in the Baldwin County sites than in the Tennessee River valley region sites. Concentrations in the Tennessee River valley region wells also were generally more variable from year to year than in wells in the other regions. Concentrations of atrazine and CIAT varied widely among the Wiregrass region wells, and median concentrations were greatest in WG-6 and WG-8, two shallow monitoring wells installed for this project.

Metolachlor was one of the most commonly used pesticides in the study area and was commonly detected at the study sites (fig. 21). Metolachlor concentrations were most variable in the Tennessee River valley region sites, but a similar range in concentration was observed in all the regions. One well, BC-2, had a median metolachlor concentration of 2,620 ng/L, two orders of magnitude greater than all other median values. Greater metolachlor concentrations at BC-2 may be related to the well’s shallow depth and the location beside a wetland area that is a local topographic low. Metolachlor concentrations at this site decreased slightly through time but still exceeded most other detections. Median concentrations of metolachlor at the other sites ranged from 4.6 to 69.5 ng/L.

As noted previously, metolachlor SA is a persistent and commonly detected degradate of metolachlor. Metolachlor SA was analyzed in 2012 and 2014–20 and was generally present more frequently and at greater concentrations than the parent compound (fig. 22). Metolachlor SA was the most commonly detected compound during 2014–20. Only a few additional samples were collected in 2012. Median concentrations varied widely within all regions. The greatest median concentration occurred at BC-2, which, as noted above, also had the greatest median concentration of the parent compound metolachlor. Concentrations of metolachlor SA were not strongly related to well depth and did not appear to increase or decrease consistently through time.

Atrazine, CIAT, and metolachlor were frequently detected in samples collected at well TNV-5 during 2009–20 (fig. 23). Detected concentrations of these three compounds were plotted by year. There were relatively small variations between years, but most changes in concentrations were within a range of about 20–30 ng/L. Atrazine was the most variable, having a difference of almost 350 ng/L between spring and summer samples collected in 2009. No distinct pattern of decreasing or increasing concentrations of these compounds was observed during the study period.

The importance of analyzing for pesticide degradates was noted by Scribner and others (2004) in their survey of parent and degradate pesticide occurrence and by Meyer and Scribner (2009) in their review of water-quality techniques and surveys used to evaluate the environmental transport and fate of pesticides. Some transformation products of pesticides can have differing chemical characteristics that make them more persistent or more biologically harmful in the environment than the parent pesticide compound (Anagnostopoulou and others, 2022). Laboratory schedule 2437 included analyses for 119 degradates of the most commonly used pesticides. There were 42 parent pesticides with at least one degradate compound that was also evaluated, and 4 degradate compounds analyzed without an analyzed parent compound. During 2014–20, there were 22 parent pesticides detected in study wells, and degradate compounds were detected for 18 of these parent pesticides. In addition, eight degradates were detected in samples without detections of their parent compound.

Some pesticide degradate compounds, such as metolachlor SA, were detected more frequently and at greater concentrations than the parent compounds in this study. The most frequently detected degradates were metolachlor SA, OIAT, 1H-1,2,4-triazole, and demethyl norflurazon. For these compounds, degradates were detected 120, 102, 4,200, and 135 percent as frequently, respectively, as the parent compounds. Maximum detected concentrations of 14 degradates were greater than maximum concentrations of the parent compounds. Continued monitoring of these commonly detected degradate compounds will be helpful to evaluate the long-term effects of agricultural pesticide use on groundwater resources.

Since 2009, the U.S. Geological Survey and the Alabama Department of Agriculture and Industries have partnered to monitor pesticide occurrence in groundwater near areas of agricultural land use in Alabama. Groundwater samples have been collected from a small network of wells located in three regions of the State with concentrated areas of agricultural land use: Baldwin County, the Wiregrass region, and the Tennessee River valley region. Samples collected during 2014–20 were examined to identify the compounds most frequently detected and those detected in the greatest concentrations. Results of the first 5 years of sampling were summarized in a previous report and were combined with the more recent results in this report to evaluate apparent patterns of pesticide occurrence through time. In each year of sample collection, 15 wells were sampled, but because of changes in sites, 23 wells were sampled in the three regions during the course of the project.

Groundwater samples collected from 22 wells during 2014–20 were analyzed for concentrations of 230 pesticide and pesticide degradate compounds. Ninety-two compounds were detected at least once in environmental samples, but 37 of those compounds were detected in only one sample. Two additional pesticide compounds were not detected in environmental samples but were detected in one replicate sample each. Metolachlor sulfonic acid (SA), a degradate of the herbicide metolachlor, was the most commonly detected compound, occurring in about 70 percent of samples. Metolachlor, atrazine, and 2-chloro-4-isopropylamino-6-amino-s-triazine (CIAT), an atrazine degradate, were also detected in more than 50 percent of samples. Metolachlor and metolachlor degradates were some of the compounds detected frequently at the greatest concentrations. The maximum detected concentration of a pesticide was 62,500 nanograms per liter (ng/L) of metolachlor. Metolachlor SA was detected at the next greatest concentration, 14,800 ng/L, and was also the most frequently detected compound among the 50 greatest concentrations.

There were differences among the study regions in the most frequent detections and greatest concentrations of pesticide compounds, most likely related to regional differences in factors such as prevalent crop types, types and amounts of pesticides used, and well characteristics. In Baldwin County, the most frequently detected compounds were metolachlor, metolachlor degradates, 2-hydroxy-4-isopropylamino-6-amino-s-triazine (a degradate of atrazine), and 1H-1,2,4-triazole, and the greatest concentrations were metolachlor and its degradates. In the Wiregrass region, the most frequently detected compounds were metolachlor SA, atrazine, 1H-1,2,4-triazole, 2-hydroxy-4-isopropylamino-6-amino-s-triazine, and metolachlor, and the single greatest detected concentration was 1H-1,2,4-triazole. In the Tennessee River valley region, the most frequently detected compounds were atrazine, CIAT (an atrazine degradate), demethyl norflurazon (a degradate of norflurazon), metolachlor, and imidacloprid, and the greatest detected concentrations were of metolachlor, metolachlor degradates, norflurazon, fluometuron, and their degradates.

All detected concentrations of pesticide compounds were below their U.S. Environmental Protection Agency maximum contaminant levels and human-health benchmarks, as well as U.S. Geological Survey health-based screening levels (HBSLs), and concentrations do not currently warrant any human health concerns. Detected concentrations of a few compounds were more than 10 percent of applicable benchmarks. The single detection of fentin was 18.3 ng/L, approximately 92 percent of the lower limit of the HBSL cancer range (20 ng/L). The maximum concentration of fluometuron was 645 ng/L, approximately 32 percent of the lower limit of the HBSL cancer range (2,000 ng/L). The maximum concentration of atrazine was 373 ng/L, or 12.4 percent of the maximum contaminant level (3,000 ng/L). The maximum detected concentrations of aldicarb sulfone (947 ng/L) and metolachlor (62,500 ng/L) were more than 10 percent of their noncancer HBSLs, 6,000 and 600,000 ng/L, respectively.

Results produced using multiple laboratory methods were combined to provide a continuous time series of data from the beginning of the project in 2009 through sampling in 2020. More than half of the 289 pesticide compounds analyzed during this period were not detected in any samples. Only four compounds were detected at great enough frequency throughout the 10 sampling years to evaluate patterns of change through time. Metolachlor and its degradate, metolachlor SA, were frequently detected in all regions. Concentrations detected in samples from representative wells from each region suggested that there was not a pattern to the changes in concentration through time. Atrazine and its degradate, CIAT, were detected in wells from all regions, but the variability and magnitude of concentrations were greatest in the Tennessee River valley region. There was no clear pattern of change in concentrations through time in these wells.

Pesticide degradates were commonly detected and many detected degradates were present at greater concentrations than the parent compounds. Many degradate compounds have similar or greater detrimental effects to human and environmental health, and some may be more likely to persist in groundwater systems. Continued monitoring of degradate compounds will be an important part of the assessment of effects of pesticide use on groundwater quality.

Selected water sampling results were evaluated with respect to well depths. The number of detected pesticide compounds and the sum of detected pesticide concentrations were determined for each sample. Field analyses of nitrate concentrations were made to compare water-quality to the U.S. Environmental Protection Agency maximum contaminant level for nitrate. Overall, well depths appeared to be inversely related to detections of pesticide compounds and nitrate concentrations. Increasing well depth may offer some protection from surface application of pesticides and fertilizers.