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A surface-water study focusing on pesticides found in the Mermentau Basin

Study Description

This study examines the effects of rice-agricultural land use on water quality in the Mermentau River basin. The Mermentau River basin, historically called coastal or wet prairie, is characterized by a low gradient and a heavy clay soil that is ideal for rice cultivation, although oil and gas production facilities are scattered throughout the region. In some areas, rice is "double-cropped" with crawfish, where crawfish are collected from the flooded fields before the new rice grows.

The study objectives, developed in early 1999, were to determine the occurrence and distribution of agrichemicals such as pesticides and fertilizers in streams of the Mermentau Basin. Part of this study compares the ecological communities of streams with different amounts of rice cultivation in their basins, from very low to almost total basin coverage. In June 1998, reports of widespread mortality of commercially-grown crawfish resulted in adding fipronil to the list of pesticides that we analyze. Fipronil, known by the trade name Icon, is a new insecticide used for control of the rice water weevil. Fipronil may contribute to crawfish mortality in rice fields due to direct exposure to Fipronil-treated rice seed, but it is unknown to what degree or how sensitive crawfish are to this chemical.

Hydrology in this basin is complex: a dense system of canals for rice irrigation modifies historical overland flow, connecting many bayous and canals together such that there may be inter-basin drainage during heavy rainfall events. Both surface and groundwater are extensively utilized for rice cultivation; demand can cause substantial alterations of the natural hydrology. Water withdrawals in early to late spring can cause portions of streams to cease flowing or even flow backwards.

Rice fields are often precision-leveled to distribute water across the fields evenly, but this levelling may increase the erosion of soil when water is released. The use of curved levees following land contours minimizes the amount of leveling to be done. Shortly after seeding, impounded ricefield water (called "tailwater") is released into nearby canals and bayous, and can bring large amounts of sediments and agrichemicals directly into these waterbodies. These short pulses of high sediment load in streams may reduce dissolved oxygen to very low levels for that period.

Sites

Seventeen sites were selected for monthly water sampling from February through June of 2000. A second set of 18 sites were sampled in March, April, and May of 2001. At these sites, invertebrates were collected and aquatic habitats characterized. Site selection was based on sampling representative upstream and downstream locations on all the major tributaries and mainstem of the Mermentau River. Four of the sites (Bayou Lacassine near Lake Arthur, Mermentau River at Mermentau, Bayou des Cannes nr. Eunice, and Bayou des Cannes at Hwy 98 near Iota) are co-located with existing Louisiana Department of Agriculture and Forestry pesticide sampling sites. This surface-water study is also coordinated with our Rice Landuse Ground-water Study.

Methods and Analysis

We collected surface water monthly from February through June of 2000, and again in the spring of 2001, covering most of the growing season. Constituents assayed at these sites include major ions, nutrients, dissolved and organic carbon, chlorophyll, and a variety of soluble and less-soluble organic pesticides and compounds. Methods for collecting and processing surface water are described in the "Field Guide for Collecting and Processing Stream-Water Samples for the National Water-Quality Assessment Program, Open-File Report 94-455". A custom method has been developed for assaying fipronil and its degradates in water and sediment samples.

• Dissolved Major Ions
• Dissolved Nutrients
• Phytoplankton, Chlorophyll a and b
• Dissolved and Suspended Organic Carbon
• Pesticides, including fipronil and degradates

Aquatic invertebrates were collected and habitat characterized for sites sampled in 2001, using the same methods as for the ecological characterization of Fixed Sites. Invertebrates were collected from woody debris since this habitat was available for sampling at all locations. Methods for sampling invertebrate communities are described in "Methods for Collecting Benthic Invertebrate Samples as Part of the National Water-Quality Assessment Program, Open-File Report 93-406". We measured a reduced set of habitat characteristics than for the ecological characterization of Fixed Sites, and at fewer transects. Habitat measurements were made using the same methods as described in "Revised Methods for Characterizing Stream Habitat in the National Water Quality Assessment Program, Water Resources Investigation Report 98-4052".

For questions about our Mermentau Basin Pesticide Study, please contact Dennis Demcheck [ddemchec@usgs.gov].