National Water Quality Assessment Program: Eastern Iowa Basins

Effects Of Soils, Riparian-tree Density, And Hydrology On Water Chemistry And Algal-nutrient Relations In Midwestern Agricultural Streams.

Stephen Porter1, Linda Roberts2, Mitchell Harris3, Kathy Lee4, and Stephen Sorenson5

U.S. Geological Survey, Water Resources Division; 1Denver, CO; 2Iowa City, IA; 3Urbana, IL; 4Mounds View, MN; 5Reston, VA.

Program abstract in Proceedings of the Mississippi River Research Consortium volume 30, annual meeting, April 23-24, 1998, LaCrosse, Wis

Regional water-quality differences in the Midwestern Corn Belt do not appear to be related to large differences in land use or fertilizer and herbicide application. An investigation of 70 streams and rivers in the region was conducted during low-flow conditions in August 1997 to describe water-quality conditions and responses relative to differences in the permeability of basin soils and riparian-zone characteristics. Water-chemistry samples were collected for determinations of nutrients, herbicides and degradates, and other chemical and physical properties. Algal seston and periphyton samples were collected to evaluate algal-nutrient relations. Continuous monitoring of dissolved oxygen and pH was used to estimate stream productivity. Soil drainage in each basin was classified as permeable or impermeable according to Soil Hydrologic Groups developed by the U.S. Soil Conservation Service. Riparian zones were classified by determining the percentage of trees within a 100 m buffer strip on both sides of the stream, using digital raster images from USGS topographic maps. The riparian-tree density was classified as low if the percentage of trees was less than 35%, and high if trees exceeded 35% of the total area of the riparian zone. Nutrients, selected herbicides, and seston chlorophyll a (CHLa) were compared in streams with low riparian-tree density. Average concentrations of dissolved ortho-phosphorus, ammonia nitrogen, dissolved organic carbon, and triazine herbicides were relatively larger in streams that drain permeable soils. In contrast, seston CHLa, nitrate, and chloroacetamide herbicide concentrations were larger in streams that drain impermeable soils. No significant water-chemistry differences were found between soil-classification groups of streams with high riparian-tree density. Average seston CHLa, dissolved oxygen, and triazine herbicide concentrations were significantly higher in streams with low tree density, regardless of soil-permeability characteristics. Estimates of stream productivity and concentrations of particulate organic carbon, nitrogen, and solids were also higher in streams with low riparian-tree density. Because of significant correlations among algal seston, productivity, and total organic nitrogen (N) and carbon (C), we suggest that algal uptake and production influence nutrient transport in agricultural streams. Concentrations of dissolved inorganic N and C were relatively smaller when seston or periphyton CHLa was large, particularly in streams with low riparian-tree density. Stream productivity was positively correlated with seston CHLa, total N and C, and the amount of nitrogen fertilizers applied in the basin. Significant negative correlations were found between algal productivity and dissolved nitrate concentrations, possibly indicating higher rates of nutrient uptake as algal productivity increases. Negative correlations between algal productivity and concentrations of chloroacetamide herbicides may indicate inhibition of algal photosynthesis; however, herbicide relations with soils, riparian tree density, and hydrology may exert a stronger correlative effect. Nutrient and land-use correlations with algal productivity were larger and more significant in streams that drain permeable soils, possibly indicating the importance of ground-water quality. Regional differences in rainfall and stream hydrology prior to the low-flow study may account for water-quality differences independent of soils, tree density, or land use. Average stream flow differed significantly among states in the region. Significantly higher concentrations and yields of nitrate in streams of southern Minnesota may correspond with an extended period of rainfall and resultant bank-full hydrologic conditions for several months prior to the study. The USGS National Water-Quality Assessment (NAWQA) Program provided funding for this study.

Keywords: algae, nutrients, herbicides, Corn Belt

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