THE GREATER OCCURENCE OF AGRICULTURAL HERBICIDES IN KARST AQUIFERS IN KENTUCKY

Herbicides have a relatively high incidence of occurrence in karst aquifers, suggesting that the fate and transport dynamics of these chemicals in karst settings may be operating differently than in non-karst regions where their occurrence is much less common in groundwater. The Kentucky Division of Water has conducted several groundwater monitoring projects, including: 1) a State-Wide Ambient Groundwater Monitoring Program, 2) focused pesticide monitoring through an MOA with the Division of Pesticide Regulation, and 3) several nonpoint source groundwater assessment projects. To date, approximately 1900 samples have been collected from more than 300 sites throughout the state. Samples are analyzed for a wide range of parameters, including agricultural pesticides and herbicides.

Atrazine is the most commonly detected herbicide, with 509 detections in 1900 samples. Atrazine was detected at 29% (97 of 333) of all monitored sites and 44% (86 of 196) of karst sites (cf. only 8% [11of 137] of non-karst sites). 27% of all samples (509 detections in 1900) indicated the presence of atrazine. 41% of samples (494 of 1199 samples) from karst aquifers contained detections of atrazine, whereas only 2% of samples from non-karst aquifers (15 of 701 samples) contained detections of atrazine. All detections of atrazine above the MCL (0.003 mg/L) were in karst aquifers. Other common agricultural herbicides (metolachlor, simazine, and alachlor) also occur more commonly in karst aquifers.

Questions remain as to whether the greater occurrence of agricultural herbicides in karst aquifers is the result of fate and transport dynamics in karst regions (e.g. macroporic flow and lack of adsorption and degradation), improper application relative to precipitation events and sensitive karst features (e.g. over-spraying to sinkholes), or more row cropping and increased use of herbicides in karst versus non-karst areas. There is clearly a need for study of fate and transport of herbicides and pesticides in karst terrains, as well as the development and implementation of herbicide-specific karst agriculture BMPs.