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The influence of soil drainage characteristics on triazine concentrations in the Contentnea Creek drainage basin of North Carolina, 1993–95

Managing Albemarle-Pamlico Sounds: CCMP Implementation Forum
By Gerard McMahon and D.A. Harned

Abstract

The Contentnea Creek Basin has been given a high priority for management action by the State of North Carolina. The basin contributes as much as 20 percent of the nonpoint-source nutrient loading to the Pamlico estuary, despite having less than 10 percent of the Pamlico Sound drainage area. There has been no systematic assessment of the occurrence and distribution in the basin's surface waters of important agricultural pesticides, such as triazine compounds. Thus, it is uncertain whether nutrient-related management efforts should be extended to include a pesticide-related component; if so, it may be more cost-effective to develop and implement an integrated management approach, rather than implementing sequential management programs at different times. As a first step in investigating potential pesticide-related water-quality concerns, the occurrence and spatial and temporal distribution of triazine herbicides in surface waters was examined at 38 watersheds located in the Contentnea Creek drainage basin during 1993-95. This investigation was conducted as part of the Albemarle-Pamlico drainage basin study of the U.S. Geological Survey's National Water-Quality Assessment Program.

Measurable concentrations of triazine herbicides occur in the surface waters of 38 subbasins of the Contentnea Creek drainage basin. Samples were analyzed by immunoassay, which detects the presence of atrazine and other triazine analogues such as propazine, prometon, and simazine. Some compounds detected by this procedure are used primarily in agriculture, whereas others, such as prometon and simazine, have lion-crop uses. The greatest individual sample concentration, 2.03 ug/L, was measured in the most urbanized subbasin, Hominy Swamp near Wilson. A seasonal pattern exists in measured concentrations at the 38 stations, with the greatest triazine concentrations occurring in May and June, following the use of these herbicides in the basin.

Spatial patterns of triazine herbicides occurrence in surface waters also exist. Concentration measures for this analysis included the median and 75th-percentile concentration of all samples at each location. Triazine concentrations increase as the percentage of the basin with well-drained soils decreases, where well drained soils are defined as soil hydrologic groups A and B.. Greater herbicide concentrations were measured along the main stem of Contentnea Creek than in other subbasins, other than those with predominantly poorly-drained soils or a high proportion of urban land use. Significant negative correlations (p<0.05) occur between the proportion of basin area with well-drained soils and median and 75th-percentile triazine concentrations.

Linear regression was used to better understand factors influencing the variation in instream triazine concentrations. The response variable was the log-transformed value of triazine concentration. Explanatory variables included the percentage of basin area in agriculture, the log of basin area, and the proportion of basin area in well-drained soil, raised to the third power. Two regression relations were examined, one for median concentrations and another for the 75th percentile concentrations. in each regression, the soil-drainage-characteristic variable was the only statistically significant explanatory variable. The sign of the estimated parameter indicates that the greater the percentage of well-drained soil in a basin, the lower the log will be of the triazine concentration. The R-square of the model predicting the 75th-percentile concentrations was 0.76; R-square for the median model was 0.61. This study indicates the importance of understanding and accounting for the influence of natural environmental characteristics, such as soil drainage characteristics of a watershed, when developing watershed management strategies.

McMahon, Gerard, and Harned, D.A., 1997, The influence of soil drainage characteristics on triazine concentrations in the Contentnea Creek drainage basin of North Carolina, 1993-95 [abs]: Managing Albemarle-Pamlico Sounds: CCMP Implementation Forum, New Bern, N.C., June 5-6., 1997.