Pesticides in Lake Water in the Beasley Lake Watershed, 1998-2005

Oxbow lakes are remnants of meandering floodplain rivers, which have been cut off and physically isolated from their respective main river channels. They usually capture only small relic drainages. Beasley Lake Watershed, a Conservation Effects Assessment Project (CEAP) benchmark watershed, is one of three oxbow lake watersheds in the original Mississippi Delta Management Systems Evaluation Area (MDMSEA) project begun in 1994. Best Management Practices (BMPs) originally established in the Beasley Lake Watershed consisted of grade stabilization and water control structures, including slotted-board risers and slotted-inlet pipes, accompanied by numerous grass filter strips along major drainages into the lake. A large forested wetland adjoining Beasley Lake provided additional natural water treatment. Conventional cotton (Gossypium hirsutum L.), corn (Zea mays L.), and soybeans [Glycine max (L.) Merr.], totaling about 660 ha, were the predominant crops from 1995-2000 in the Beasley Lake Watershed. In 2001-2002, reduced-till cotton and soybeans occupied most of the Watershed, and from 2003 to the present time, reduced-till soybeans has been the dominant crop along with 91 ha (north side of the Lake) planted to hardwoods under the Conservation Reserve Program (CRP). BMP modifications over the last several years include: 1) additional slotted-board risers and slotted-inlet pipes (with and without stiff grass hedges and with and without tile drains) instrumented to automatically collect edge-of-field runoff on a flow proportional basis from both cropped and CRP land, 2) conservation tillage, and 3) constructed wetlands. On a monthly basis since 1998, water has been collected from Beasley Lake and analyzed for a suite of 17 current and past-use (residual) pesticides (and/or metabolites). Of the 80 pesticide detections greater than or equal to 0.1ppb in surface water, herbicides accounted for 68, equivalent to 85%. With regard to the 80 detections greater than or equal to 0.1ppb, 44 (55%) occurred in the years 1998-2000, 21 (26%) occurred in the years 2001-2002, and 15 (19%) occurred in the years 2003-2005. As previously mentioned, prior to the year 2001, conventional tillage was employed throughout the Beasley Lake Watershed. In 2001-2002, reduced-till cotton and soybeans occupied most of the watershed. From 2003-present, the north side of Beasley lake was in trees under CRP, with the balance of the watershed in predominantly reduced-till soybeans. Thus, the number of lake water pesticide detections greater than or equal to 0.1ppb was progressively and significantly reduced by moving from conventional tillage to reduced-tillage to reduced-tillage+CRP.