Environ. Sci. Technol. 2000, 34, 2426-2430
Mecoprop, Isoproturon, and Atrazine in and above a Sandy Aquifer: Vertical Distribution of Mineralization Potential LISE LARSEN, SEBASTIAN R. SØRENSEN, AND JENS AAMAND* Geological Survey of Denmark and Greenland (GEUS), Department of Geochemistry, Thoravej 8, DK-2400 Copenhagen NV, Denmark
The vertical distribution of the potential to mineralize [ring-U-14C]herbicides was investigated in samples from a sandy aquifer. Mecoprop, isoproturon, or atrazine were added to sediments from 0.06 to 7.68 m below the surface, and mineralization at 10 °C was monitored as evolved 14CO . The highest potential for mineralization was found 2 in the plough layer, the upper 0.30 m. Here, approximately 43% of mecoprop, 14% of isoproturon, and 3% of atrazine were recovered as 14CO2 after 85, 267, and 119 days, respectively.. The estimated doubling time, Td, for 14CO2 evolution from the initially added herbicide was approximately 20 days for mecoprop in the plough layer increasing to more than 5 years below 4.73 m. For isoproturon incubated at 10 °C, Td was 700 days in the plough layer increasing to 6 years in the subsurface. At 20 °C the rate of mineralization of isoproturon doubled compared to the rate at 10 °C. Atrazine was mineralized slowly, Td was 5 years in the plough layer, and mineralization of atrazine below 0.70 m was not observed. None of the herbicides was mineralized anaerobically in the presence of nitrate.
Introduction Herbicides leaching through the surface sediment and unsaturated layers pose a risk to the underlying aquifers. Though partial degradation of herbicides or adsorption to soil may prevent or impede contamination of the groundwater, the only way to eliminate xenobiotic compounds from the environment is by complete degradation to CO2. This process is microbially facilitated (1) and is known as mineralization. Mecoprop ((()-2-(4-chloro-2-methylphenoxy)propanoic acid), isoproturon (N,N-dimethyl-N ′-[4-(1-methylethyl)phenyl]urea), and atrazine (6-chloro-N-ethyl-N ′-(1-methylethyl)-1,3,5-triazine-2,4-diamine) have been used as herbicides in large amounts since the mid-1980s. These compounds are the most widely used members of three large herbicide groups, the phenoxy acid, phenylurea, and triazine herbicides. Mecoprop, isoproturon, and atrazine have all been detected in Danish groundwater in concentrations above 0.1 µg L-1 (2, 3), the threshold limit value according to the regulations of the European Union on drinking water (4). Because the herbicides infiltrate the groundwater, knowledge about the fate of the compounds in deeper layers is vital to * Corresponding author phone: +45 3814 2326; fax: +45 3814 2050; e-mail:
[email protected]. 2426
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 12, 2000
an understanding of the potential for natural attenuation in contaminated aquifers. Only few previous studies have treated the mineralization of herbicides in samples from the profile of an aquifer and compared the potential for mineralization in the plough layer, the unsaturated zone, and the saturated zone. To our knowledge, this is the first study of the mineralization of mecoprop in sediments from the saturated zone, and previously the mineralization of isoproturon has been described in one chalk rich aquifer only (5). Atrazine has been studied in depths from 0 to 17.6 m below the surface (mbs) but not in detail in the upper subsurface sediment (6). The scope of this study was to examine and compare the mineralization potential for [ring-U-14C]labeled mecoprop, isoproturon, and atrazine in sediments from the plough layer, the unsaturated zone, and the saturated zone. The site, a sandy aquifer near Fladerne Creek, Denmark, was sampled from 0.06 to 7.68 mbs. In addition, after 230 days of incubation the distribution of 14C from [ring-U-14C]isoproturon was examined in microbially accessible, humic-associated and nonextractable fractions of the sediment.
Materials and Methods Site Description and Sampling. Sediment was sampled from an aquifer in the area of Fladerne Creek, Jutland, Denmark. The area is a moorland plain with meltwater sediments dominated by coarse sand (7). The surface of the field used as sampling site is 41 m above sea level, and the water flow in the aquifer is mainly horizontal (7). The area has been farmland since 1920 and several herbicides including the phenoxy acid MCPA have been applied, but other phenoxy acids or phenylurea and triazine herbicides have never been used. It has previously been shown that sediment sampled from 0, 0.5, and 1 mbs did not contain isoproturon, any of the phenoxy acid herbicides MCPA, mecoprop, 2,4-D, and dichlorprop, or any of the triazine herbicides and metabolites atrazine, simazine, cyanazine, terbutylazine, desisopropylatrazine, and deethylatrazine. However, phenoxy acids have been detected upstream from the site at 2.0-7.5 mbs (7), and presumably they pass the sampling site at similar depth. Sediment samples from the upper 1 m were obtained by digging, while samples from deeper layers were collected as cores (8). Depths stated for samples from cores are the average depths of cores, 22 cm in length. Groundwater was sampled by nitrogen pressure through a Teflon tube lowered into a well (Ø ) 2.0 cm) (9). pH and oxygen content was measured in water passed through a flow cell connected to pH and oxygen electrodes (pH 320 and Oxi 320, WTW, Germany). Nitrate was determined on a Dionex ionchromatograph. Because of a low pH of the sediment, carbonates are solubilized and total organic carbon in the sediment was determined as total carbon by combustion (LECO CS-200). Samples were stored at 4 °C until use. Mineralization Studies. Flasks for aerobic mineralization studies were set up for depths from 0.06 to 7.68 mbs. Sediment corresponding to 10 g (dry weight) was placed in 100 mL airtight glass flasks that had ground joints. The water content of samples from the saturated zone was adjusted with groundwater from the same depth to reach water-holding capacity. Herbicide, either [ring-U-14C]mecoprop (Amersham, Life Science), [ring-U-14C]isoproturon (Amersham, Life Science), or [ring-U-14C]atrazine, (Sigma), was added in 100 µL aliquots of aqueous solutions to reach an activity of 170 Bq in each flask. The resulting herbicide concentrations were 2.2 µg mecoprop kg-1, 3.8 µg isoproturon kg-1, and 5.4 µg atrazine 10.1021/es9911723 CCC: $19.00
2000 American Chemical Society Published on Web 05/17/2000
FIGURE 1. Chemical structures of (A) mecoprop, (B) isoproturon, and (C) atrazine. kg-1 sediment (dry weight) corresponding to 8-71 µg L-1 mecoprop, 14-123 µg L-1 isoproturon, and 20-174 µg L-1 atrazine after the adjustment of the water content. According to the manufacturers, the concentration of radiochemical impurities was
