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Lake Wales Ridge Groundwater


Summary of Reconnaissance Sampling for Selected Triazine Herbicides and Degradates in Ground Water from Lake Wales Ridge Network Wells

To explore simazine degradation in groundwater on Lake Wales Ridge, groundwater samples were collected in July 2003 from five wells in the Lake Wales Ridge (LWR) Monitoring Network. The samples were analyzed by the USGS Organic Geochemistry Research Laboratory (http://ks.water.usgs.gov/researchlab.html) using liquid chromatography / mass spectrometry methods. Well selection focused on sites that previously had yielded the highest simazine concentrations in the network. Based on this selection criterion, these concentrations would be expected to be among the highest in the LWR network wells. (see Lake Study for results of simazine degradates in sampled Ridge lakes.)

Ground water from all five sampled wells yielded detectable concentrations of simazine and all simazine degradates analyzed (figure 1 and table 1), which included: hydroxysimazine (HS), deisopropylatrazine (DIA; CEAT), didealkylatrazine (DDA; CAAT), and deisopropylhydroxyatrazine (DIHA; OEAT). The laboratory reporting limit was 0.025 g/L for all of these compounds.  Concentrations of simazine (0.09 to 5.18 g/L) were typically higher than those of both HS (0.17 to 2.96) and DIA (0.16 to 1.57). HS concentrations generally exceeded DIA concentrations (4 of the 5 wells). At two wells, the concentrations of DDA were of similar magnitude to the HS concentrations.

Simazine concentrations exceeded the drinking-water maximum contaminant level (MCL) of 4 g/L at 1 well; the sum of simazine and the target degradates exceeded 4 g/L at 3 of the 5 wells (table 1 and figure 2). Although Florida human health guidance protocols typically reference the sum of the parent and its degradates to evaluate possible health effects, this presumes a similar mode of action and toxicity of the compounds. The mode of action for toxicity of hydroxysimazine differs from that of simazine, therefore it may be inappropriate to group it with simazine for health guidance evaluations (Dennis Howard, Fl. Dept. of Agriculture and Consumer Services, July 2004, pers. comm.).

Results of a separate USGS study in progress that includes evaluation of pesticides in four Ridge lakes include some detections of simazine and its degradates. Sampling in December 2003 and March 2004 yielded maximum concentrations [simazine (0.06 g/L) and the degradates DIA (0.14 g/L), DDA (0.14 g/L), and HS (0.27 g/L)] (Sherron Kroening, USGS-Orlando, Florida, written commun., July 2004), which were lower than the 1999-2004 regional ground-water maximums based on the Ridge network. 

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