The Six Runs Creek watershed in Sampson County is like many parts of eastern North Carolina where hog farming caught hold in the 1990s, and swine production grew at an astonishing rate.

In 1993, there were two hog houses within the roughly 650-acre watershed that drains into the headwaters of Six Runs Creek, said Dr. Dan Israel, a soil science professor in the College of Agriculture and Life Sciences. Israel is also a research plant physiologist with the U.S. Department of Agriculture. Now there are 23 hog houses in the watershed, and the area is home to roughly 43,000 hogs.

It’s that kind of animal density that has raised concern about how the North Carolina swine industry manages the waste its animals produce. That high animal density is also why Israel is working in the watershed to better understand the environmental impact of swine waste management in North Carolina.

Virtually all large North Carolina swine farms deal with waste by washing it to nearby lagoons from the houses where the pigs are raised. In the lagoons, microbes work on the waste to decompose it. Decomposition in a lagoon decreases the organic content of manure by approximately 90 percent, Israel said.

But farmers must periodically remove some of the liquid, or lagoons will overflow. They do so by spraying the liquid on nearby fields, where the nutrient-rich liquid fertilizes crops. When the crops are harvested, the nutrients are removed from the area. Yet the nutrients pose an environmental threat if they’re not used by plants. Nitrogen particularly can move through the soil to shallow groundwater, then be carried to surface waters such as streams, rivers, ponds and lakes.

Excess nutrients in surface waters can cause a process called eutrophication. The nutrients fuel the growth of algae in the water. The algae then die, and the decomposition process depletes the water of oxygen. Without enough oxygen in the water, fish and other marine animals begin to die. The result can be fish kills in rivers and streams.

Because of fears that nitrogen in lagoon liquid applied to fields would move into groundwater, North Carolina in 1993 began requiring swine producers to develop waste management plans for the fields on which they spray liquid. These plans stipulate how much liquid may be applied based on the nitrogen content of the liquid and the agronomic needs of the crop being grown in the field (how much nitrogen the crop will use to produce a realistic yield). Plans must also take into account the type of soil in the field.

But how well do these waste management plans work? Do they adequately protect ground and surface water?

To find out, Israel’s research team drilled shallow wells on three Sampson County hog farms. All three farms were established after the 1993 regulations were imposed. The researchers drilled 90 wells in all. On two of the farms, the wells are located in spray fields only, while on the third farm, wells were drilled in the riparian, or forested, area that separates spray fields from Six Runs Creek as well as on the spray fields.

For roughly a year, Israel has been taking samples of groundwater from the wells, then analyzing the samples for nitrate, a form of nitrogen. The wells at the Six Runs Creek site are arranged so that Israel is able to determine whether nitrate is moving from the spray fields to the creek. His team is also sampling and analyzing the water in the creek.

Israel said it is still much too early to make definitive statements about what is happening to the nitrogen applied to the fields. He plans to continue monitoring for at least another year and perhaps considerably longer. He has, however, made some observations.

The spray fields Israel is monitoring receive from 100 to 300 pounds of nitrogen per acre per year in swine waste. Israel said average nitrate concentrations measured in the shallow groundwater beneath the fields have ranged from 4 to 10 parts per million. He added that nitrate concentrations in this range are typical for the groundwater beneath North Carolina fields to which fertilizer has been applied and in which row crops are grown.

The federal Environmental Protection Agency limit for nitrate concentration in drinking water is 10 parts per million; anything over 10 parts per million is considered unsafe. Israel added that the concentrations he has measured decline in the riparian areas between the fields and the creek.

Israel has also found nitrate in the stream in concentrations ranging from 0.4 up to 4 parts per million. While these concentrations are low, nitrate in surface water is a different matter than nitrate in groundwater.

Eutrophication is usually the result of a number of factors, including the presence of nitrogen, Israel said. As a result, there is disagreement among scientists and others over what constitutes an acceptable nitrate concentration in surface water — a level that does not pose a danger of sparking eutrophication. There is general agreement, however, that nitrate concentrations below 10 parts per million can promote eutrophication.

From what he has seen so far, Israel said the width of riparian areas, the depth of the water table beneath a riparian area and the character of a stream all play roles in determining what happens to the nitrate in groundwater moving from the spray fields to the stream.

Riparian areas are known to act as a filter that removes various potential pollutants from groundwater, but Israel’s observations suggest that not all riparian areas are equal in their filtering ability.

Swine waste has been applied to the fields Israel is monitoring for a relatively short time, only four years. As a result, the system — the fields, riparian areas and stream — may not have reached equilibrium, Israel said. Nitrate concentrations in the shallow groundwater may change over time. Continued monitoring will provide an opportunity to observe such changes if they occur.

— Dave Caldwell