Seasonal changes in stream water quality along an agricultural/urban land-use gradient.

ABSTRACT. We investigated downstream changes in dissolved oxygen (DO), pH, nitrate, total nitrogen (TN), total phosphorus (TP), Atrazine[R], E. coli, and total suspended sediments (TSS) levels in two second-order watersheds with various amounts of riparian buffer coverage, and with more than 80% agriculture and 3% residential land-use in the headwaters and 60-65% agriculture and 10% residential land-use lower in the watersheds. DO, pH, nitrate, TP, E. coli and TSS showed little variation in the downstream direction along this land-use gradient or as a function of riparian buffer coverage. However, a decrease in Atrazine and TN concentrations was associated with the increased percentage of land used for housing in the downstream direction from less than 3% to approximately 10% urban land-use. Benchmark analysis indicated overall poor water quality in both watersheds with respect to nitrate, E. coli, TN and TP. This study provides a baseline of water quality data for future studies assessing the impact of changing land-use and riparian zones on water quality at the watershed scale in till landscapes of the midwestern U.S., where rapid population growth leads to the conversion of agricultural lands into residential areas.

Keywords: Water quality, watershed, land-use, benchmark analysis, riparian zones

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Understanding the impact of changing land-use on water quality in glacial till landscapes of the midwestern U.S. is critical for proper management of water resources at the watershed scale. Land-use has been shown to affect nutrients, suspended sediment concentrations and overall stream water quality (Cooke & Prepas 1998; Sharpley et al. 1992; Kuhnle et al. 2000; Vanni et al. 2001). For instance, there is a significant positive correlation between N exports at the watershed scale and the percentage of watershed in crop (Hill 1978; Cooke & Prepas 1998). Urbanization generally results in higher runoff coefficients and higher losses of sediment and associated contaminants (McMahon & Harned 1998; Almendinger 2003).

Coulter et al. (2004) also reported that soluble reactive phosphorus (SRP) losses were greater in agricultural watersheds than in urban watersheds but that total phosphorus (TP) losses were similar in both settings, suggesting that the relative amount of TP and SRP varies as a function of land-use. McKergrow et al. (2003) studied the impact of a 1.7 km stretch of riparian zone on sediment exports in a catchment in Australia and showed that riparian zones led to a 90% reduction in sediment exports at the watershed scale. Data therefore suggest that land-use (agricultural vs. urban) and the occurrence of buffer zones along streams have an effect on water quality. There is nevertheless a lack of data on the impact of subtle land-use changes (

Studies in till landscapes of the midwestern U.S. are especially important since nutrient exports from midwestern states like Indiana, Ohio and Illinois have been linked to pollution in the Gulf of Mexico where an anoxic zone develops every summer (Goolsby 2000; Royer et al. 2006). Land-use is also quickly changing in many watersheds of the midwestern U.S. that are traditionally dominated by agriculture. For instance, Coulter et al. (2004) indicated that in Fayette County, in the Inner Bluegrass region of Kentucky, population grew 15.6% between 1990 and 2000 and that many rural lands that were once managed for agriculture now support urban land-uses. Similarly, in the Tipton Till Plain region of central Indiana, Tedesco et al. (2003) estimated that the population in the Eagle Creek watershed, northwest of Indianapolis, has tripled in the last 40 years. The authors also project that population will continue to grow in this watershed in the years to come leading to the conversion of many agricultural lands into residential areas. Assessing water quality in these changing watersheds is critical in order to comprehend the impact of changing land-use on water quality and gather baseline information on water quality in these watersheds as these changes are taking place.

Nutrient concentration in streams is often reported as an indicator of water quality as excess nutrient in streams has been linked to eutrophication (Cooke & Prepas 1998; Martin et al. 1999). Many studies have also focused on pesticide mobility in soil (Benoit et al. 1998), including Atrazine[R] losses from artificially-drained landscapes of the American midwest as Atrazine is widely used in this area of the country (Kladivko et al. 1999). Many studies also looked at suspended sediment concentration in streams (Rostad et al. 1993; Kronvang et al. 1997) as high levels of suspended sediments have been shown to increase turbidity and limit the amount of light available to aquatic plants (U.S. EPA 2006a). The impact of buffer zones and various land-uses on bacteria concentrations such as total coliform and/or E. coli (Escherichia toll) concentration (as an indicator of fecal contamination) in streams have also received significant attention in the past few decades (Young et al. 1980; Schmitt et al. 1999; Dosskey 2002; Tate et al. 2006). Other indicators of stream health reported in the literature include pH and dissolved oxygen concentration (DO). DO levels less than 4 mg/l have been reported to negatively impact aquatic life productivity and fish health in particular (IAC 2006). High levels of DO (> 125% DO saturation) have also been used as an indicator of poor stream health as they typically result from excessive algal growth in nutrient rich streams (Bright & Cutler 2000).