The 650-acre watershed that drains into the headwaters of North Carolina's Six Runs Creek is a large laboratory for Dan Israel and his research team.
The soil science professor at North Carolina State University and research plant physiologist for USDA's Agricultural Research Service focuses on water quality research in the watershed in Sampson County. Within the watershed, 23 finishing barns produce approximately 43,000 head of hogs annually. All but two of those barns were built after 1993, and must comply with strict state regulations on management of their associated lagoon and sprayfield systems.
Israel is using 90 test wells, drilled in the sprayfields and in forested and riparian areas, to test the impact on water quality. He hopes to bring scientific research into the heated environmental issues of the state's pork industry.
“We've realized we have a unique place to study the impact of intensive animal agriculture,” he says. “We have a laboratory in the field to make measurements to bring rationale to the table.”
All of the farms use traditional lagoon and sprayfield technology. The 60 to 80 acres of sprayfields are cropped with coastal Bermuda grass, overseeded with rye grass and cut for hay or grazed annually. The only other cropping in the area is cotton fields located further away from the creek.
Well Sequence and Testing
Test wells were drilled on one farm on the west side of Six Runs Creek in the spring of 2000. The wells range from 4 to 20 ft. deep, because of variable topography and water table depth.
Based on nutrient management plans and recordkeeping — required for the farm — Israel knows that effluent applications since 1997 supplied about 270 lb. of nitrogen/acre/year.
The wells are arranged in transects from the sprayfields through the forested and riparian area to the stream. Each month, Israel pumps the wells dry twice and then pulls samples from the recharged water. The samples are then tested for nitrates.
As expected, the nitrate levels decline from the fields, through the forested and riparian areas and are lowest in the creek's slowest moving waters.
The wells in the fields have had readings up to 50 ppm (parts per million). The average reading in the shallow groundwater under the fields is from 12 to 14 ppm. These readings are similar to those reported for groundwater beneath row crop fields in the coastal plain of North Carolina, where agronomic rates of nitrogen have been applied.
Samples are also taken from several areas of the stream. In faster moving waters with no wetlands and/or ponding, Israel finds an average of 3 ppm of nitrates. That reading drops to 0.5 to 1 ppm of nitrate in areas where beavers have built dams and the water is much slower moving.
The riparian areas appear to be removing the nitrate, but all are not equal. The areas where the water table is closer to the surface are more effective in removing nitrates. Israel theorizes that the microbes that convert nitrate to nitrogen gas prefer the environment created when the water table is just below the surface. This process allows nitrogen to move from the soil/water system to the atmosphere, which is already 78% nitrogen gas.
“I look at this as an effort to test the idea that careful management of nitrogen inputs to sprayfields, in concert with biological and chemical processes within the natural system, can protect streams like Six Runs Creek from excessive accumulation of nitrate,” he says. “My hope is to develop scientific information that will assist in the development of environmentally sound and economically feasible regulations.”