Summer Cover Crops Reduce Atrazine Leaching to Shallow Groundwater in Southern Florida

Authors: Potter, Thomas; Bosch, David - Dave; JOO, HYUN - UNIV. N.CAROLINA; SCHAFFER, BRUCE - UNIV. OF FLORIDA; MUNOZ-CARPENA, RAFAEL - UNIV. OF FLORIDA

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2007
Publication Date: 7/17/2007
Citation: Potter, T.L., Bosch, D.D., Joo, H., Schaffer, B., Munoz-Carpena, R. 2007. Summer Cover Crops Reduce atrazine Leaching to Shallow Groundwater in Southern Florida. Journal of Environmental Quality. 36:1301-1309.

Interpretive Summary: Intensive crop production in Florida's Miami-Dade County generates an estimated $1 billion annually in combined revenue. A potential limitation to sustained production is that local soil and geologic conditions appear to make the Biscayne aquifer which underlies the entire region vulnerable to pesticide and fertilizer contamination. The aquifer is the potable water source for most of the >3 million people residing in southeastern Florida thus farming impacts on the quality of this water supply must be kept to a minimum. Growers are encouraged to apply best management practices (BMP) which protect water quality. Before making such management decisions they legitimately require reliable data documenting the impact of current practices and the extent to which BMPs can improve water quality while maintaining or improving yields. These were the broad objectives our 3.5-year study. We focused on assessing the potential for residues of the widely used herbicide atrazine to reach groundwater when used for weed control in sweet corn and the extent to which contamination could be reduced by use of a common and low-cost BMP, a cover crop. During the study corn yields were comparable to the most successful growers and atrazine and degradate levels in groundwater beneath atrazine treated plots were low relative to drinking water standards. Cover crops also contributed to 35 to 42 percent reduction in total atrazine residues which reached groundwater. Although the study showed that some atrazine residues may reach groundwater levels detected were generally low. We also showed that residues can be reduced by use of cover crops. Growers and the public will likely benefit by wide-spread adoption of this management practice

Technical Abstract: In many parts of the United States shallow groundwater is vulnerable to agricultural non-point source pollution. This problem was evaluated in Southern Florida where vegetable crops are intensively produced and groundwater quality appears threatened. The study focused on quantifying levels of atrazine (ATZ) and three degradates in groundwater beneath treated fields and whether growing a cover crop during the spring-summer (April-October) fallow season could reduce cumulative groundwater loading with these compounds. Sweet corn (Zea Mays L.) was grown for four consecutive years on six 0.15-ha plots. After each harvest, a cover crop, Sunn Hemp (Crotalaria juncea L.), was planted and maintained on three plots. Other plots were left fallow. The Sunn Hemp was tilled into the soil and ATZ broadcast applied prior to planting subsequent Fall-Winter sweet corn crops. Shallow groundwater beneath and hydraulically up- and downgradient of plots was monitored intensively for ATZ and three degradates. Corn yields were comparable to the region's most successful growers while total ATZ and degradates concentrations were significantly lower in samples collected beneath cover crop plots after the first crop was tilled into the soil. Estimates of cumulative ATZ and degradates loading of groundwater indicated that a '40% reduction was associated with cover crop use. The study also showed that climatic and cropping patterns and induction of enhanced ATZ degradation in soil were linked to the relatively low overall ATZ loading rates observed with and without the cover crop. Groundwater in this region appears less vulnerable to herbicide residue contamination than anticipated.