Introduction

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.

Description of Study Regions

Groundwater wells sampled in this project were located in three regions of intensive row-crop agricultural land use in Alabama: Baldwin County, the Wiregrass Region, and the Tennessee River valley region. These three regions are distributed geographically across the State. Baldwin County lies in the extreme southwestern part of the State, the Wiregrass region is in the southeastern corner of the State, and the Tennessee River valley region extends from east to west across the northern part of the State (fig. 1).

Baldwin County lies between the eastern shore of Mobile Bay and the Florida State line and extends south to the Gulf of Mexico (fig. 1). Soils in the area are ancient marine and fluvial sediments of the coastal plain (Mitchell, 2017). Crops in Baldwin County are varied, with common cultivation of soybeans, peanuts, and cotton but also fruit and vegetable crops (table 1; USDA National Agricultural Statistics Service, 2019a). From 1991 to 2020, mean monthly temperatures in Baldwin County ranged from a low of 51 degrees Fahrenheit (°F) in January to a high of almost 82 °F in July (fig. 2A; National Centers for Environmental Information, 2021a). From 1991 to 2020, mean monthly precipitation in Baldwin County was lowest in the fall and greatest in the summer, ranging from about 3.9 in. for October to more than 8.1 in. for July (fig. 2B; National Centers for Environmental Information, 2021a).

Table 1.    

Major crops, based on percentage of harvested cropland, in study regions in Alabama (U.S. Department of Agriculture [USDA] National Agricultural Statistics Service, 2019a).

Table 1.    Major crops, based on percentage of harvested cropland, in study regions in Alabama (U.S. Department of Agriculture [USDA] National Agricultural Statistics Service, 2019a).

Rank of crop (greatest to least area)	Region
	Baldwin County	Wiregrass		Tennessee River valley
		All counties in region	Counties where project wells are located	All counties in region	Counties where project wells are located
1	Peanuts	Hay	Peanuts	Hay	Soybeans
2	Cotton	Cotton	Cotton	Soybeans	Corn for grain
3	Soybeans	Peanuts	Hay	Corn for grain	Wheat
4	Hay	Corn for grain	Corn for grain	Cotton	Cotton
5	Corn for Grain	Soybeans	Vegetables	Wheat	Hay

The Wiregrass region extends into parts of nine southeastern Alabama counties (fig. 1; Olliff, 2018). The study sites were located in agricultural areas of three counties in the Wiregrass region, namely Henry, Houston, and Geneva Counties near the City of Dothan, Alabama (fig. 1). Soils in the Wiregrass region are loamy or sandy loam and support the cultivation of peanuts, cotton, soybeans, and vegetable crops (table 1; Mitchell, 2017). From 1991 to 2020, mean monthly temperatures are very similar to temperatures in Baldwin County, ranging from 49 °F in January to 82 °F in July (fig. 2A; National Centers for Environmental Information, 2021c). From 1991 to 2020, mean monthly precipitation in the Wiregrass region was lowest in the fall and greatest in the summer, ranging from about 3.2 in. for October to 6.7 in. for July (fig. 2B; National Centers for Environmental Information, 2021c).

The Tennessee River valley region extends across multiple counties in northern Alabama (fig. 1). Fertile soils formed from the weathering of limestone are used to produce cotton, soybeans, corn, and wheat (table 1; Mitchell, 2017; USDA National Agricultural Statistics Service, 2019a). From 1991 to 2020, mean monthly temperatures in the Tennessee River valley region ranged from about 40 °F in January to about 79 °F in July (fig. 2A; National Centers for Environmental Information, 2021b). From 1991 to 2020, mean monthly rainfall in the Tennessee River valley region was lowest in the summer and greatest in the winter, ranging from about 3.4 in. for August to almost 6.0 in. for December (fig. 2B; National Centers for Environmental Information, 2021b).

Agricultural Row Crops and Estimated Pesticide Use in Study Regions

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.

Methods

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.

Site Selection

The project design included collecting samples from 5 wells in each of the regions for a total network size of 15 wells. Because of changes in well accessibility and availability for sampling, wells were added or removed from the network during the project, so from 2009 to 2020, samples were collected from 24 distinct well sites (table 3; figs. 6–8). All project data supporting this report were published as a data release (Gill, 2023). All sampled wells were in the target agricultural regions except NAWQA LUSCR1-9, an existing agricultural monitoring well in Macon County that was substituted for a disabled well in the Wiregrass region during sampling in 2016. No pesticide compounds were detected in the sample collected from NAWQA LUSCR1-9. Results of the single sample from NAWQA LUSCR1-9 are included in the accompanying data release (Gill, 2023) but are not included further in this analysis. Study wells were either installed specifically for monitoring and evaluating water-quality on a recurring basis (monitoring wells) or routinely used to supply water for domestic, agricultural, or other uses located near areas of agricultural land use (production wells). Well types and depths varied across each of the three study regions.

Table 3.    

Groundwater sites sampled as part of pesticide occurrence study near areas of agricultural row cropping in Alabama, 2009–20 (Gill, 2023; U.S. Geological Survey, 2023).

[ID, identifier; cnty, county; AL, Alabama; P, agricultural and (or) domestic water uses; M, monitoring use only]

Table 3.    Groundwater sites sampled as part of pesticide occurrence study near areas of agricultural row cropping in Alabama, 2009–20 (Gill, 2023; U.S. Geological Survey, 2023).

Station ID	Station name	Report well identifier	Well depth (feet)	Use of water	County	Region	Year(s) sampled
302434087462601	Well Ag & Industries 9 Baldwin Cnty AL	BC-1	65	P	Baldwin	Baldwin County	2009–16, 2018
302438087325801	Well Ag & Industries 8 Baldwin Cnty AL	BC-2	45	P	Baldwin	Baldwin County	2009–16, 2018, 2020
302610087463401	VV 22 Baldwin County	BC-6	240	P	Baldwin	Baldwin County	2020
303231087525801	Well Ag & Industries 7 Baldwin Cnty AL	BC-3	130	P	Baldwin	Baldwin County	2009–16, 2018, 2020
303252087500801	Well Ag & Industries 6 Baldwin Cnty AL	BC-4	140	P	Baldwin	Baldwin County	2009–16, 2020
303406087501401	Well Ag & Industries 6 - Replacement Baldwin Cnty	BC-4A	1140	P	Baldwin	Baldwin County	2018
303432087485501	Well Ag & Industries 5 Baldwin Cnty AL	BC-5	110	P	Baldwin	Baldwin County	2009–16, 2018, 2020
310153085225901	Well S 10 310153085225901 Houston Cnty AL	WG-1	95	P	Houston	Wiregrass	2009–16, 2018, 2020
310453085383001	Well Ag & Industries 4 Geneva Cnty AL	WG-2	140	P	Geneva	Wiregrass	2009–16, 2018, 2020
311048085403201	Well Ag & Industries 3 Geneva Cnty AL	WG-3	310	P	Geneva	Wiregrass	2009–16, 2018, 2020
311928085191101	Well AG & Industries 1 Henry Cnty AL	WG-4	231	P	Henry	Wiregrass	2009–15, 2018
311929085191201	Ag & Industries 1 Shallow	WG-5	54.34	M	Henry	Wiregrass	2019, 2020
312113085191501	Well X-5 Henry Cnty AL	WG-6	47.5	M	Henry	Wiregrass	2015
312118085193001	Well X 4 Henry Cnty AL	WG-7	245	P	Henry	Wiregrass	2012–14, 2016, 2018
312119085192901	Well X 6	WG-8	43.68	M	Henry	Wiregrass	2019, 2020
312143085230301	Well Ag & Industries 2 Henry Cnty AL	WG-9	105	P	Henry	Wiregrass	2009–11
343940087290601	Well EHR-LUS37 Colbert Cnty AL	TNV-1	41.5	M	Colbert	Tennessee River valley	2012, 2014–16, 2018, 2020
344124086531401	EHR-LUS18	TNV-2	24	M	Limestone	Tennessee River valley	2009–13, 2015
344131087335201	EHR-LUS33	TNV-3	53	M	Colbert	Tennessee River valley	2009–13
344150086523401	EHR-SUS14	TNV-4	107	P	Limestone	Tennessee River valley	2016, 2018, 2020
344348086493401	EHR-LUS35	TNV-5	53	M	Limestone	Tennessee River valley	2009–16, 2018, 2020
345112086313401	EHR-LUS13	TNV-6	35	M	Madison	Tennessee River valley	2009–16, 2018, 2020
345822086254001	EHR-LUS11	TNV-7	50	M	Madison	Tennessee River valley	2009–16, 2018, 2020
322500085551201	Well NAWQA LUSCR1-9 Macon Cnty AL	Macon	32	M	Macon	Other	2016

¹

Depth estimated from depth of nearby well.

In Baldwin County, samples were collected from domestic or agricultural production wells ranging in depth from 45 to 240 feet (ft) below land surface (table 3), with a median depth of 130 ft below land surface. All Baldwin County wells were completed in the interbedded sand and gravel layers of the Miocene-Pliocene aquifers (Miller, 1990; Robinson and others, 1996).

In the Wiregrass region, samples were collected from 6 production wells and 3 monitoring wells (table 3). Depths in the production wells ranged from 95 to 310 ft below land surface, with a median depth of 186 ft. Depths in the monitoring wells ranged from 44 to 54 ft below land surface. Deeper production wells access water from the Ocala Limestone or Lisbon aquifers, whereas monitoring wells are completed in the overlying sand and gravel alluvium and residuum (Scott and Cobb, 1988; Scott and others, 1967).

In the Tennessee River valley region, samples were collected from 7 monitoring wells and 1 deeper agricultural production well (table 3). Depths in the monitoring wells ranged from 24 to 53 ft below land surface, with a median depth of 43 ft. The monitoring wells were finished in a regolith composed of clay, silt, and variously sized chert that overlies, and was formed by weathering of, Mississippian-age Tuscumbia Limestone and Fort Payne Chert bedrock (Kingsbury, 2003). The depth of the production well was 107 ft below land surface. This well was screened in the Fort Payne Chert and was sampled in 2016, 2018, and 2020 as a substitute location for a monitoring well that was dry.

Nitrate Field and Laboratory Method Comparison

In 2018 and 2020, nitrate concentrations in 30 samples were determined by both laboratory and rapid field test methods (Gill, 2023). Differences between laboratory and field results were less than 1 milligram per liter (mg/L) for all but six sample pairs, and linear regression between the two methods had a coefficient of determination (R²) of 0.71. Deviation in field-determined nitrate concentration could be caused by many factors, such as a presence of particulates in unfiltered well water, extended holding times before field analyses, variation between in situ and analysis temperatures, and water chemistry of the sample, but the higher variance between laboratory and field methods for the six samples could not be attributed to a particular cause. When the six high-variance sample pairs were excluded, laboratory and field nitrate concentrations were in strong agreement (R² = 0.987; fig. 9). The agreement between laboratory and field methods confirmed the suitability of field-determined nitrate concentrations for screening high nitrate occurrence, so all field-determined nitrate concentration data from 2014 to 2020 were incorporated in site-by-site comparisons of nitrate.

Field Tests and Nitrate Results

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.

Field Parameters

Field parameters were measured at regular intervals from the start of pumping to sample collection to determine when adequate purging of water standing in the well had been achieved and sample water would be representative of aquifer conditions near the well. Once parameter values stabilized, a set of measurements was collected at the start of sampling to describe general water quality in the aquifer. Measurements collected at the time of sampling are summarized here.

Field parameter values varied across the study area, but values in individual wells remained similar across multiple years during 2014–20 (Gill, 2023; fig. 10). Water temperatures ranged from 20.6 to 29.1 degrees Celsius (°C) in the Baldwin County wells, from 21.3 to 26.2 °C in the Wiregrass region wells, and from 16.5 to 19.6 °C in the Tennessee River valley region wells (fig. 10A). Cooler temperatures were recorded in the Tennessee River valley region wells that are more northerly and these wells generally were sampled during cooler spring months. Specific conductance ranged from 30 to 162 microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25 °C) in Baldwin County wells, from 49 to 245 μS/cm in Wiregrass region wells, and from 64 to 282 μS/cm in Tennessee River valley region wells (fig. 10B). Median specific conductance in the Baldwin County wells was 125 μS/cm, lower than in the Wiregrass and Tennessee River valley regions, where median specific conductivity was 226 and 215 μS/cm, respectively. pH ranged from 4.2 to 5.3 standard units in Baldwin County wells, from 4.3 to 8.0 standard units in Wiregrass region wells, and from 4.7 to 7.1 standard units in Tennessee River valley region wells (fig. 10C). Low pH values were recorded in the Baldwin County region wells, which were completed in the less-buffered sands of the Miocene-Pliocene aquifers. Median pH values in the Wiregrass and Tennessee River valley regions were 7.5 and 6.5, respectively, reflecting the presence of soils having greater buffering capacity than those in the vicinity of Baldwin County region wells. Dissolved oxygen concentrations ranged from 5.9 to 8.9 mg/L in Baldwin County wells, from 1.3 to 9.5 mg/L in Wiregrass region wells, and from 2.7 to 9.7 mg/L in Tennessee River valley region wells, with most lower values occurring in the deeper wells (fig. 10D). Many of the dissolved oxygen values may have been affected by aeration during well purging. Turbidity for most wells was less than 20 nephelometric turbidity units (ntu), ranging from 0.8 to 22 ntu in Baldwin County wells, from 0 to 60 ntu in Wiregrass region wells, and from 0 to 100 ntu in the Tennessee River valley wells (fig. 10E). Wells BC-4A, TNV-6, and WG-8 had readings above 20 ntu. Alkalinity was not evaluated in many well samples, especially in Baldwin County, because low pH indicated very low alkalinity. Measured field alkalinity values ranged from 0 to 4 mg/L as calcium carbonate (CaCO₃) in Baldwin County wells, from 0 to 114 mg/L as CaCO₃ in Wiregrass region wells, and from 4 to 120 mg/L as CaCO₃ in Tennessee River valley region wells (fig. 10F).

Nitrate Concentrations

Nitrate concentrations greater than 10 mg/L in drinking water are linked to an increased incidence of methemoglobinemia, a serious blood disorder that disrupts the supply of oxygen to cells in infants and young children (U.S. Environmental Protection Agency, 2009). Other studies have indicated that elevated nitrate concentrations in drinking water may cause health problems in adults as well (Ward and others, 2018). During the first several years of sampling, field rapid chemical tests of samples from some study wells indicated that nitrate concentrations approached or exceeded the drinking-water standard of 10 mg/L (Gill, 2023).

In 2018 and 2020, nitrate concentrations in 30 samples collected from 19 wells were determined by laboratory methods as well as by the rapid chemical tests (Gill, 2023). Laboratory results for nitrate as nitrogen ranged from 0.01 to 10.3 mg/L. Field test concentrations for the same samples ranged from 0 to 11 mg/L nitrate as nitrogen.

Measured concentrations of nitrate in a few study wells were close to the drinking-water maximum contaminant level of 10 mg/L and are a cause for concern if well water is used as a drinking-water source (fig. 11). Field nitrate concentrations of 10 mg/L or greater were measured at BC-1, BC-2, BC-5, WG-6, and TNV-6. A field nitrate concentration of 22 mg/L, almost double the drinking-water limit for nitrate, was recorded once each at wells BC-2 and WG-6 (fig. 11, Gill, 2023). Field nitrate concentrations were generally less variable and lower with increasing well depth (fig. 12).

Pesticide Environmental Sampling Results, 2014–20

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.

Frequency of Detections

The occurrence of environmental pesticides is often evaluated by the percentage of samples with detections, or the detection frequency, of pesticide compounds. Pesticide compound detection frequencies were evaluated for the entire study area. The overall evaluation is presented herein and followed by a discussion of regional differences. Detections are summarized by the type of pesticide use and the most commonly detected individual compounds.

Study-Wide Pesticide Detection Frequency

Of the 230 compounds analyzed, 92 were detected in at least one of the environmental samples collected between 2014 and 2020 (table 7). Forty-two parent pesticides (9 fungicides, 24 herbicides, and 9 insecticides) were detected, and 50 pesticide degradates were detected. Many compounds were detected infrequently in the environmental samples, including 37 compounds detected only once.

The most commonly detected compound in samples collected from 2014 to 2020 was the herbicide degradate metolachlor SA, present in about 70 percent of the samples. The parent herbicide metolachlor was detected less often, in almost 58 percent of the samples. Rose and others (2018) noted that metolachlor and its degradates were commonly detected in previous pesticide occurrence studies and that metolachlor SA has a higher water solubility than the parent compound. Another herbicide, atrazine, and 2-hydroxy-4-isopropylamino-6-amino-s-triazine (OIAT), an atrazine degradate, were also detected in more than 50 percent of samples for this time period.

Twenty-four pesticide or degradate compounds were detected in more than 15 percent of samples (fig. 13). Within these 24 most commonly detected compounds, 9 were parent pesticide compounds and 15 were pesticide degradates. Nineteen of the compounds were herbicides or herbicide degradates, 3 were insecticides or insecticide degradates, and 2 were fungicides or fungicide degradates.

The most commonly applied pesticide compounds were not always the most frequently detected. For instance, glyphosate had the greatest estimated application (table 3), but out of 30 samples collected in 2018 and 2020, glyphosate was only detected once, at TNV-1. The glyphosate degradate AMPA was detected twice, also at TNV-1. The other analyzed pesticides with the greatest application rates were evaluated in all 75 samples collected during 2014–20. Neither parent nor degradate compounds were detected for pendimethalin, 2,4-D, or dicamba. Acetochlor and an acetochlor degradate were detected once each. Acephate was detected once, and tebuconazole was detected three times. Chlorpyrifos was not detected, but its degradate was detected once. Parent and degradate compounds of atrazine and metolachlor were frequently detected (table 7; Gill, 2023).

Pesticide Detection Frequency by Region

Mobility and persistence of agricultural pesticides may be affected by the differences in soil types, weather, crop types, and pesticide use among the regions. Detection frequency was examined for the 230 analyzed pesticides and revealed variations between each of the regions.

Baldwin County was the region having the greatest number of unique pesticide compound detections. Fifty-seven pesticide compounds were detected in at least one environmental sample, and 34 of the detected compounds were pesticide degradates. There were 31 herbicides (14 parent herbicides and 17 herbicide degradates), 21 insecticides (5 parent insecticides and 16 insecticide degradates), and 5 fungicides (4 parent fungicides and 1 fungicide degradate) detected in samples collected from Baldwin County wells (Gill, 2023). In order of descending frequency of detection, the five most frequently detected compounds in the Baldwin County region were metolachlor SA (a metolachlor degradate), metolachlor, 1H-1,2,4-Triazole, OIAT (an atrazine degradate), and metolachlor OA (a metolachlor degradate) (table 8). Four of the five most frequently detected compounds were herbicides or herbicide degradates.

Table 8.    

Five most frequently detected compounds within each study region in Alabama (U.S. Geological Survey, 2023).

[NWIS, National Water Information System; H, herbicide; I, insecticide; F, fungicide; %, percent]

Table 8.    Five most frequently detected compounds within each study region in Alabama (U.S. Geological Survey, 2023).

NWIS parameter code	Compound name	Pesticide use group	Rank by compound detection frequency (and detection frequency percentage)
			Baldwin County	Wiregrass region	Tennessee River valley region
Baldwin County
P68651	Metolachlor sulfonic acid (SA)1,2	H	1 (76%)	1 (65%)	10 (71%)
P65090	Metolachlor3	H	2 (48%)	5 (42%)	4 (88%)
P68498	1H-1,2,4-triazole1,2	F	3 (48%)	3 (46%)	12 (46%)
P68659	2-Hydroxy-4-isopropylamino-6-amino-s-triazine (OIAT)1,2	H	4 (40%)	4 (42%)	9 (75%)
P68650	Metolachlor oxanilic acid (OA)2	H	5 (40%)	21 (15%)	17 (33%)
Wiregrass region
P68651	Metolachlor SA1,2	H	1 (76%)	1 (65%)	10 (71%)
P65065	Atrazine1	H	26 (8%)	2 (54%)	1 (100%)
P68498	1H-1,2,4-triazole1,2	F	3 (48%)	3 (46%)	12 (46%)
P68659	OIAT1,2	H	4 (40%)	4 (42%)	9 (75%)
P65090	Metolachlor3	H	2 (48%)	5 (42%)	4 (88%)
Tennessee River valley region
P65065	Atrazine1	H	26 (8%)	2 (54%)	1 (100%)
P68552	CIAT2	H	31 (4%)	10 (27%)	2 (100%)
P68567	Demethyl norflurazon2	H	21 (12%)	15 (23%)	3 (100%)
P65090	Metolachlor3	H	2 (48%)	5 (42%)	4 (88%)
P68426	Imidacloprid	I	62 (0%)	24 (8%)	5 (88%)

¹

Compound was in top five for more than one region.

²

Compound is a pesticide degradate.

³

Compound was in top five for all regions.

In the Wiregrass region wells, 43 pesticide compounds were detected in project groundwater samples. Sixteen parent pesticide compounds, 26 pesticide degradates, and 1 compound that is both a parent pesticide and a degradate of another pesticide were detected. Most of the detected compounds were herbicides (13 parent herbicides and 19 herbicide degradates). There also were 7 insecticide compounds (2 parent insecticides and 5 insecticide degradates) and 4 fungicide compounds (1 parent fungicide, 2 fungicide degradates, and 1 compound that was either a parent or degradate) detected in Wiregrass region samples (Gill, 2023). The five most frequently detected compounds in the Wiregrass region well samples were metolachlor SA, atrazine, 1-H-1,2,4-triazole, OIAT, and metolachlor (table 8).

In the Tennessee River valley region wells, 48 pesticide compounds were detected in project groundwater samples. Twenty-five of the detected compounds were parent pesticides and an almost equal number (23) were pesticide degradates. Thirty-three of the detected compounds were herbicides (14 parent compounds and 19 degradates), 8 were insecticides (5 parent compounds and 3 degradates), and 7 were fungicides (6 parent compounds and 1 degradate) (Gill, 2023). The five most frequently detected pesticide compounds in the Tennessee River valley region were atrazine, 2-chloro-4-isopropylamino-6-amino-s-triazine (CIAT), deethyl norflurazon, metolachlor, and imidacloprid. Imidacloprid is an insecticide and the other four of the top five compounds are herbicides or herbicide degradates (table 8).

The amount of applied pesticides and the crop types grown may explain the high frequencies of detection for the most frequently detected compounds. The herbicides atrazine and metolachlor were commonly used in all three regions and were, respectively, the 2nd and 8th most heavily used pesticides in Baldwin County, the 9th and 12th most heavily used in the Wiregrass region, and the 4th and 7th most heavily used in the Tennessee River valley (Wieben, 2019). The degradate of the fungicide propiconazole, 1H-1,2,4-triazole, was the third most commonly detected pesticide in Baldwin County and the Wiregrass region. Fungicides are used on many crops, but peanut crops, in particular, are heavily treated with fungicides (USDA National Agricultural Statistics Service, 2019b). The common cultivation of peanuts in Baldwin County and the Wiregrass region may explain the frequent detection of this fungicide degradate (fig. 4; USDA National Agricultural Statistics Service, 2021). Norflurazon, an herbicide commonly used on cotton, peanuts, and soybeans (U.S. Environmental Protection Agency, 1996), was applied in similar amounts in the Tennessee River valley and the Wiregrass regions, at about three times the rate of use in Baldwin County (Wieben, 2019). The common detection of the norflurazon degradate demethyl norflurazon in Tennessee River valley wells but not in Wiregrass region wells may be due to the shallower well depths in the Tennessee River valley region (Gill, 2023). Frequent detections of the insecticide imidacloprid also appear to be related to shallow well depths. Imidacloprid is recommended to control aphids, plant bugs, and whiteflies in cotton (Alabama Cooperative Extension System, 2023) and to control an array of soybean pests (Greene, 2017). Cotton is common near Wiregrass region wells, and both crop types are common near Tennessee River valley wells (fig. 4). Although imidacloprid use in the Wiregrass region was approximately twice the use in the Tennessee River valley (Wieben, 2019), detections of imidacloprid occurred almost exclusively in the Tennessee River valley wells, with only two other detections, both in well WG-5, a shallow well installed for this project (Gill, 2023).

Magnitude of Detections

Study-Wide Pesticide Concentrations

Many of the individual detections of pesticide compounds were concentrations of very small magnitude. More than half (409 of 753) of individual detections of pesticide compounds in environmental samples were below the greatest applicable compound reporting limits in effect during the project (Gill, 2023). Detected concentrations above reporting limits were compared to benchmarks and limits determined by U.S. Environmental Protection Agency and USGS for individual compounds (U.S. Environmental Protection Agency, 2009; Norman and others, 2018).

Ten compounds accounted for the 50 greatest detected concentrations in samples collected during 2014–20 (table 9; Gill, 2023). Metolachlor and three metolachlor degradates accounted for 33 of the 50 greatest magnitude detections. Metolachlor was the compound with the second greatest magnitude of use in the study area (Wieben, 2019), and, as noted previously, metolachlor degradates are likely to be detected frequently in groundwater because of their chemical characteristics. The greatest detected concentration of a single pesticide compound in this study was 62,500 ng/L of metolachlor in the June 2020 sample from TNV-1. Metolachlor SA was the compound with the second greatest detected concentration (14,800 ng/L) and the most frequently detected at the greatest concentrations, with 19 detections in the top 50 by magnitude. Most of the high detections of metolachlor SA occurred in Baldwin County wells BC-1, BC-2, and BC-5 (Gill, 2023).

Table 9.    

Frequency of detection for each pesticide compound among the 50 greatest concentrations of individual pesticide compounds in groundwater collected from selected wells in Alabama during 2014–20 (U.S. Geological Survey, 2023).

[ng/L, nanogram per liter]

Table 9.    Frequency of detection for each pesticide compound among the 50 greatest concentrations of individual pesticide compounds in groundwater collected from selected wells in Alabama during 2014–20 (U.S. Geological Survey, 2023).

Pesticide compound	Count of occurrences in the top 50 detected concentrations	Maximum detected concentration (ng/L)	Rank of most commonly detected at the greatest concentrations
Metolachlor sulfonic acid (SA)	19	14,800	1
Demethyl norflurazon	8	4,240	2
Metolachlor oxanilic acid (OA)	7	11,000	3
Metolachlor	6	62,500	4
1H-1,2,4-Triazole	3	2,820	5
Alachlor SA	3	2,400	6
Disulfoton sulfone	1	4,880	7
Disulfoton sulfoxide	1	8,900	8
Hydroxymetolachlor	1	5,130	9
Norflurazon	1	1,330	10

Maximum contaminant levels (MCLs) and human-health benchmarks (HHBPs) set by the U.S. Environmental Protection Agency are limits for contaminant concentrations intended to protect human and environmental health. Health-based screening levels (HBSLs) determined by the USGS (Norman and others, 2018) are non-enforceable thresholds that indicate similar human health risks for compounds that do not have U.S. Environmental Protection Agency MCLs and HHBPs. When guidelines are expressed as a range of concentrations, comparisons in this report have been made to the lower limit of the defined range. Fifty of the pesticides detected during 2014 through 2020 have at least one benchmark. Six compounds have applicable MCLs, 30 have assigned HHBPs (chronic noncancer or carcinogenic), and 20 have HBSLs (noncancer or cancer). All pesticide detections during 2014 through 2020 were at concentrations below assigned MCLs, HHBPs, and HBSLs, and for 40 of the compounds with assigned benchmarks, all pesticide detections were less than 1 percent of the benchmark concentration.

Although no detected concentrations were great enough to warrant human health concerns, a comparison of maximum detected environmental concentrations to the benchmark concentrations was made as a preliminary screening of compounds that might approach problematic levels in the future. Concentrations for 10 detected compounds equaled or exceeded 1 percent of their respective guideline concentrations (table 10). Maximum detected concentrations of alachlor and atrazine were 2 and 12.4 percent, respectively, of their MCLs. Maximum detected concentrations of 1H-1,2,4-triazole and norflurazon were 9.4 and 1.39 percent, respectively, of their chronic noncancer HHBPs. Maximum detected concentrations of five compounds, aldicarb sulfone, aldicarb sulfoxide, fentin, fluometuron, and metolachlor, were between 3 and 16 percent of their respective noncancer HBSL guideline minimums. Diuron, fentin, and fluometuron maximum concentrations were approximately 5.45, 91.5, and 32.2 percent, respectively, of the lower limit of the range of concentrations for cancer HBSL. Fentin, the compound detected at a concentration closest to an applicable benchmark, was only detected once during 2014–20.

Table 10.    

Comparison to human health benchmark concentrations for compounds with maximum detected concentration in groundwater samples from selected wells in Alabama equal to 1 percent or more of benchmark concentration (U.S. Geological Survey, 2023; U.S. Environmental Protection Agency, 2009; Norman and others, 2018).

[USGS, U.S. Geological Survey; ng/L, nanogram per liter; MCL, U.S. Environmental Protection Agency maximum contaminant level; HHBP, U.S. Environmental Protection Agency human-health benchmark for pesticides; HBSL, U.S. Geological Survey health-based screening level; --, no defined benchmark concentration]

Table 10.    Comparison to human health benchmark concentrations for compounds with maximum detected concentration in groundwater samples from selected wells in Alabama equal to 1 percent or more of benchmark concentration (U.S. Geological Survey, 2023; U.S. Environmental Protection Agency, 2009; Norman and others, 2018).

Compound name	USGS para-meter Code	Maximum detected concen-tration (ng/L)	Benchmark concentration						Maximum detected concentration as percentage of benchmark concentration
			MCL (ng/L)	Chronic noncancer HHBP (ng/L)		Carcin-ogenic HHBP (ng/L)	Non-cancer HBSL (ng/L)	Cancer HBSL (ng/L)	MCL	Chronic non- cancer HHBP	Carcin-ogenic HHBP	Non-cancer HBSL	Lower limit of cancer HBSL range
1H-1,2,4-triazole	P68498	2,820	--	30,000	--		--	--	--	19.4	--	--	--
Alachlor	P65064	40	2,000	--	--		--	--	12	--	--	--	--
Aldicarb sulfone	P68529	947	--	--	--		6,000	--	--	--	--	1,215.8	--
Aldicarb sulfoxide	P68530	195	--	--	--		6,000	--	--	--	--	13.25	--
Atrazine	P65065	373	3,000	--	--		--	--	1,212.4	--	--	--	--
Diuron	P66598	109	--	--	--		20,000	2,000–200,000	--	--	--	0.545	15.45
Fentin	P68603	18.3	--	--	--		200	20–2,000	--	--	--	19.15	1,291.5
Fluometuron	P68608	645	--	--	--		40,000	2,000–200,000	--	--	--	11.61	1,232.2
Metolachlor	P65090	62,500	--	--	--		600,000	--	--	--	--	1,210.4	--
Norflurazon	P67685	1,330	--	96,000	--		--	--	--	11.39	--	--	--

¹

Maximum detected concentration is equal to or greater than 1 percent of benchmark concentration.

²

Maximum detected concentration is equal to or greater than 10 percent of benchmark concentration.

Pesticide Detections and Concentrations by Region

The compound most frequently detected in the top 50 greatest concentrations in all regions was metolachlor SA, but there were some differences between the regions for other pesticide compounds detected at maximum concentrations (table 11). Atrazine, fluometuron, and their degradates were among the greatest detected concentrations in the Tennessee River valley but not in Baldwin County or the Wiregrass region. Norflurazon and demethyl norflurazon were detected in the top 50 concentrations in the Wiregrass and Tennessee River valley regions but not in Baldwin County. Concentrations of several disulfoton degradates occurred once in the top 50 concentrations for Baldwin County. AMPA, a degradate of glyphosate, was detected in the top 50 greatest concentrations in the Tennessee River valley region.

Table 11.    

Compounds detected at the top 50 greatest detected pesticide concentrations in groundwater samples from each study region of Alabama during sample years 2014–20 (U.S. Geological Survey, 2023; Gill, 2023).

Table 11.    Compounds detected at the top 50 greatest detected pesticide concentrations in groundwater samples from each study region of Alabama during sample years 2014–20 (U.S. Geological Survey, 2023; Gill, 2023).

Pesticide compound	Count of occurrences in the top 50 detected concentrations	Maximum detected concentration	Rank of most commonly detected at the greatest concentrations
Baldwin County
Metolachlor sulfonic acid (SA)1	14	14,800	1
Metolachlor oxanilic acid (OA)1	9	7,340	2
Metolachlor	6	2,040	3
Alachlor SA1	5	826	4
Dimethenamid	5	842	5
Dimethenamid SA1	4	486	6
1H-1,2,4-triazole1	2	705	7
Aldicarb sulfone1	1	947	8
Disulfoton oxon sulfone1	1	442	9
Disulfoton oxon sulfoxide1	1	505	10
Disulfoton sulfone1	1	4,880	11
Disulfoton sulfoxide1	1	8,900	12
Wiregrass
Metolachlor SA1	13	1,700	1
1H-1,2,4-triazole1	8	2,820	2
Alachlor OA1	7	906	3
Alachlor SA1	6	2,400	4
Norflurazon	5	1,330	5
Demethyl norflurazon1	4	394	6
Metolachlor OA1	3	460	7
4-Hydroxychlorothalonil1	1	140	8
Bentazon	1	119	9
Fipronil amide1	1	114	10
Metolachlor	1	280	11
Tennessee River valley
Metolachlor SA1	13	7,100	1
Demethyl norflurazon1	9	4,240	2
Fluometuron	6	645	3
Norflurazon	6	579	4
Metolachlor OA1	4	11,000	5
Atrazine	3	373	6
Demethyl fluometuron1	3	319	7
2-Chloro-4-isopropylamino-6-amino-s-triazine (CIAT)1	1	207	8
Aminomethylphosphonic acid (AMPA)1	1	540	9
Dechlorometolachlor1	1	638	10
Desamino-diketo metribuzin1	1	248	11
Hydroxymetolachlor1	1	5,130	12
Metolachlor	1	62,500	13

¹

Pesticide degradate compound.

Pesticide Detections and Concentrations by Site

The number and total concentration of detected pesticides were computed for each sample, and the median number of detected pesticides and median total pesticide concentration were determined for each well. The median number of detections and median total concentrations were used to describe differences between sites.

At least one pesticide compound was detected in 70 of 75 samples. Twenty-three pesticides, the maximum number of compounds in an individual sample, were detected in the single sample from BC-4a. No pesticides were detected in five samples from WG-3 or WG-4, the first and fourth deepest wells, respectively. The low numbers of compounds detected in samples from these wells may be related to the distance between the sources of pesticides at the land surface and the aquifer system in which the wells are completed.

The overall median number of detected compounds in a single sample was 10. All wells in the Tennessee River valley region had median numbers of detected compounds greater than the overall median, whereas in Baldwin County and the Wiregrass regions there was a mix of wells with median numbers of detected compounds well below and well above the overall median (fig. 14). There was a significant, moderate, inverse relation (Spearman rho = −0.59, p <0.001) between the number of compounds detected and well depth (fig. 15).

Total detected concentrations have remained similar for wells that have been sampled multiple years over the last 5 years of sampling (fig. 16). BC-2 had the greatest median total concentration. About 92 percent of the total concentrations at BC-2 were from detections of metolachlor and five metolachlor degradates. The most variable total concentrations have occurred at TNV-1, with total detected pesticide concentrations ranging from 381 ng/L in 2015 to about 87,300 ng/L in 2020 (Gill, 2023).

The sum of detected pesticide concentrations in single samples were significantly and moderately inversely correlated to well depth in Baldwin County (Spearman rho = −0.75, p-value <0.01) and the Wiregrass region (Spearman rho = −0.75, p-value <0.01; fig. 17). In these two regions, wells having depths greater than about 125 ft had much lower single sample pesticide concentrations. In the Tennessee River valley region, a significant, strong, positive correlation was found between the single-sample sum of detected pesticide concentrations and well depth (Spearman rho = 0.70, p-value <0.001). Most study wells in the Tennessee River valley region were shallow monitoring wells having very similar depths, with the deepest being 107 ft.

Pesticide Environmental Sampling Results, 2009–20

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.

Pesticide and Degradate Detections

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.

Summary and Conclusions

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.