INTERACTIONS BETWEEN GROUNDWATER AND SURFACE WATER AT A MID-UPPER REACH OF THE YADKIN RIVER AND THEIR IMPLICATIONS IN NUTRIENTS LOADING TO THE RIVER FROM RESIDUALS LAND APPLICATION FIELDS
WANG, Shuying, North Carolina Dept of Environment and Natural Resources, 585 Waughtown St, Winston-Salem, NC 27310, MOORE, Angela M., Deparment of Geology, Guilford College, 5800 W. Friendly Ave, Greensboro, NC 27410 and GU, Chuanhui, Department of Geology, Appalachian State University, ASU Box 32067, Boone, NC 28608-2067, shuying.wang@ncdenr.gov
The N.C. Division of Water Quality (DWQ) launched several field investigations in 2011 in order to help determine if the current rules and monitoring requirements for residuals land application permits are adequate to protect the quality of ground and surface waters of North Carolina. This investigation was launched at the subject site in May of 2011. The site is an agricultural field located adjacent to the Yadkin River and two unnamed creeks. The field had received industrial wastewater treatment residuals for 18 years, but in the past five years no residuals were applied onto this field. Forty-nine soil and sediment samples were collected for analyses of metals and nutrients. Thirteen monitoring wells and five piezometers were installed and sampled under different hydraulic conditions for bacteria, dissolved organic carbon, nutrients, major ions, trace metals, etc. Interactions between the groundwater and the Yadkin River have also been studied through water level and temperature monitoring to determine if groundwater discharge is a significant source of nutrient loading to surface water from residuals land application fields.
Elevated nitrate concentrations were detected in groundwater throughout the studied field. Nitrate concentrations decreased horizontally from the field to the Yadkin River: from 37.33 mg/L at a monitoring well just inside of the riparian zone, to 33.75 mg/L at a monitoring well in the middle of the riparian zone, to 7.6 mg/L at a monitoring well in the river bank, to 2 mg/L in the river bed hyporheic zone, and to 1.15 mg/L in the Yadkin River. Denitrification through the riparian and hyporheic zones, in addition to dilution from the river, appears to be responsible for the nitrate reduction. However, the denitrification may not be consistent throughout the studied field. Due to bank storage moving into the alluvial aquifer when the river stage was rising, the hydraulic gradient between the groundwater at a monitoring well more than 50 feet away from the riverbank and the Yadkin River was reversed. This suggests that denitrification may be limited by recharge of oxygenated river water via bank-storage during storm events.
