The role of temporally varying surface water-groundwater (SW-GW) exchange on nitrate removal by streambed denitrification was examined along a reach of Leary Weber Ditch (LWD), Indiana, a small, first-order, low-relief agricultural watershed within the Upper Mississippi River basin, using data collected in 2004 and 2005. Stream stage, GW heads (H), and temperatures (T) were continuously monitored in streambed piezometers and stream bank wells for two transects across LWD accompanied by synoptic measurements of stream stage, H, T, and nitrate (NO 3 ) concentrations along the reach. The H and T data were used to develop and calibrate vertical two-dimensional, models of streambed water flow and heat transport across and along the axis of the stream. Model-estimated SW-GW exchange varied seasonally and in response to high-streamflow events due to dynamic interactions between SW stage and GW H. Comparison of 2004 and 2005 conditions showed that small changes in precipitation amount and intensity, evapotranspiration, and/or nearby GW levels within a low-relief watershed can readily impact SW-GW interactions. The calibrated LWD flow models and observed stream and streambed NO 3 concentrations were used to predict temporal variations in streambed NO 3 removal in response to dynamic SW-GW exchange. NO 3 removal rates underwent slow seasonal changes, but also underwent rapid changes in response to high-flow events. These findings suggest that increased temporal variability of SW-GW exchange in low-order, low-relief watersheds may be a factor contributing their more efficient removal of NO 3 . Key Points:Low-order, low-relief watersheds experience dynamic streambed SW-GW exchange Streambed hyporheic and GW flow vary with season, streamflow, and climate Dynamic exchange may enhance streambed denitrification in headwater streams Supporting Information:Supporting Information S1 Correspondence to: H. I. Essaid, hiessaid@usgs.gov Citation: Rahimi, M., H. I. Essaid, and J. T. Wilson (2015), The role of dynamic surface water-groundwater exchange on streambed denitrification in a first-order, low-relief agricultural watershed, Water Resour. Res., 51, 9514-9538,

One of the most important resources for drinking water is groundwater. Recently, the quality of groundwater has been endangered due to several reasons, including human activities, vast agricultural activities, and the discharge of industrial wastewater containing nitrogen (N) and heavy metals. For this study we investigated the values of NO 3 , NO 2 , and some heavy metals and analyzed the groundwater of the Ardabil Aquifer area in wet and dry seasons. For this purpose we evaluated 76 wells in the study area. In order to analyze the results we used World Health Organization (WHO), U.S. Environmental Protection Agency (EPA), and Institute of Standard and Industrial Research of Iran (ISIRI) standards. The results indicated that, according to WHO and ISIRI, 10.5% of wells and, according to the EPA, 69.7% of wells have NO 3 above the standard level in the wet season, but in the dry season, according to WHO and ISIRI,29% and, according to the EPA, 80.6% of the wells have NO 3 above the permissible level. However, according to all standards, NO 2 values are less than the permissible level for drinking consumption. In terms of evaluation criteria of the simultaneous presence of NO 3 and NO 2 in drinking water (K), 29% of wells in the dry season and 13.15% of wells in the wet season have NO 3 and NO 2 problems. In order to find an appropriate interpolation method and zoning parameter of NO 3 and NO 2 , different interpolation methods were evaluated and Kriging method with the lowest error was chosen for zoning the parameters. According to zoning maps, in the wet season the Namin and south Ardabil areas, which have the most drinking water wells, contain values of NO 3 above the permissible levels, whereas the values of NO 3 are at the mediocre level in the dry season. In addition to NO 3 and NO 2 analysis, the evaluation of heavy metals pollution indices in the aquifer include: heavy metal pollution index (HPI), heavy metal evaluation index (HEI), and contamination degree (C d ). Generally, according to the EPA, in terms of NO 3 and NO 2 pollution, Ardabil aquifer is in an undesirable condition, and according to WHO and ISIRI standards, this area has a mediocre level of undesirable condition. According to HPI and C d indices, most parts are in a high class of heavy metal pollution, as well.