Near-stream Groundwater Correlations with Streamwater Nutrient Concentrations in an Urbanizing Watershed

We hypothesized that nutrient concentrations in streamwater within different landuse areas may be influenced by near-stream groundwater. We sampled streamwater and near-stream groundwater at 8 urban and 5 non-urban locations monthly from December 2000 through December 200 1 in urbanizing Johnson Creek watershed in Portland, Oregon. We compared soluble reactive phosphorus (SRP), total phosphorus (TP), nitrate + nitrite nitrogen (NOx), and total nitrogen (TN) concentrations in both streamwater and near-stream groundwater. We found significantly higher streamwater and groundwater SRP and TP concentrations within urban areas, while streamwater NOx and TN concentrations were higher in non-urban areas. NOx and TN were correlated significantly between streamwater and groundwater for both urban (adj. R2= 0.873,0.722) and non-urban landuse areas (adj. R2= 0.793,0.918). Over 13 month period, we found a strong correlation between streamwater and groundwater SRP (adj. R2=0.975) in non-urban areas. In contrast, there were no significant correlations between stream water and groundwater SRP and TP in urban areas. These results suggest that streamwater SRP in non-urban areas was tightly linked to near-stream groundwater. In urban areas, however, groundwater influence was not the only control on streamwater SRP, indicating that surface water sources were likely also responsible. (KEY TERMS: landuse; nitrogen, phosphorus, riparian areas, soil, surfacewater) INTRODUCTION Surface water eutrophication resulting from increased inputs of phosphorus (P) and nitrogen (N) by nonpoint sources is the most common water quality problem in the United States (U.S. EPA, 1996). In northwestem Oregon, relatively high nutrient concentrations, particularly for P, in urbanizing watersheds have long been a concern. Studies by Abrams and Jarrell (1995) and Wilson (1997) suggested that high P levels in local soil and deep groundwater may be responsible for the elevated levels of P in the nearby Tualatin River. More recently, however, we demonstrated that streamwater P concentrations were highly correlated with urban landuses while N concentrations were correlated with non-urban, rural landuses in the urbanizing Johnson Creek watershed, a sub basin of the Willamette watershed in Oregon (Sonoda et al. 2001). Our prior work did not find a correlation between deep groundwater and streamwater nutrient concentrations. The objectives of study were: 1) to determine relative concentrations of nutrients (P and N) in near-stream groundwater in both urban and non-urban areas of Johnson Creek, 2) to establish relationships between the streamwater and the near-stream groundwater chemistry within both urban and non-urban landuse areas. METHODS Site Selection and Sampling Thirteen sample sites (8 in urban, 5 in non-urban area) on the main stem of Johnson Creek, Oregon were selected (Figure 1) to collect streamwater and near-stream groundwater samples. Monthly streamwater and near-stream groundwater samples were taken on the second weekend of the month regardless of weather or flow condition from December 2000 to December 2001. All water samples were collected in acid washed 250 ml DHPE NalgeneB sample bottles. Streamwater samples were collected from the middle of the stream, while near-stream groundwater samples were collected using a mini-piezometer (Lee and Cherry 1978) installed in the middle of stream at 60 cm deep using a Cole ParmerB Environmental Sampler. All water samples were stored in an ice chest and transported to the laboratory at Portland State University for nutrient analysis. Respectively, Graduate Research Fellow, Associate Professor, and Research Assistant, Environmental Resources, Portland 67 1 State University, P.O. Box 75 1, Portland, Oregon 97207-075 1 (E-mail/Sonoda: [email protected]). 0 Non-urban Sample Sites Urban Sample Sites RM 8.8 RM 13.2 N A