Implications of Changing Water and Soil Salinity in North Carolina's Central and Southern Coastal Counties

Over the past 10 years, I have seen the grass steadily disappear on the lawn of a friend’s house on Bogue Sound. It did not occur to me that increasing soil salinity could possibly be the cause until I began learning about the early signs of climate change impacts on North Carolina’s coastal lands.

Academic researchers, the U.S. Geological Survey (USGS), and others are reporting evidence of changing salinity in North Carolina’s Coastal Plain. They attribute this change to forces such as coastal storm flooding (inundation and erosion), drought, and sea level rise. In Hyde County, NC, researchers are monitoring soil salinity movement inland (salinization) on low elevation farmlands as drainage ditches containing surged saline floodwaters overflow onto fields. The salt residue increases soil salinity that current crops (e.g., soybeans) may not be able to tolerate. As a result, productive acreage is lost, and farmers may be influenced to switch to more salt-tolerant crops such as cotton. Forested wetlands along estuaries in northeastern North Carolina are also retreating, leaving “ghost forests” as the salinity gradient advances up the estuary especially in dry seasons and as sea level rises.

Salinity is the concentration of salt dissolved in water or the salt content of soil. Salts are naturally occurring in water and soil, but salt content appears to be moving or increasing in eastern North Carolina soils, surface waters, and groundwaters that are hydrologically connected to the estuaries, sounds, and ocean. While documented in northeastern North Carolina, less is known about the extent of changing water and soil salinity in central and southern North Carolina coastal counties. Traveling north to south, our state’s coastal counties are diverse in topography, ecology, and hydrology (rivers, estuaries, aquifers), and land use also shifts north to south from rural agriculture and forestry to increasingly developed lands for commercial, municipal, and defense usage. However, these differences raise some important questions. For instance, will central and southern North Carolina Coastal Plain counties experience loss of land or land use? What land use is most vulnerable? What is more resilient to increasing salinity? If municipalities are moving from groundwater to surface water supplies, then will they be less impacted by saltwater intrusion, or will the advancing saltwater interface in water supply rivers still pose a drinking water treatment challenge?

A first step to understanding the potential for increasing salinity is to choose an indicator to monitor change. Because seasonal and long-term droughts open the way for saline waters to advance inland, the USGS developed the Coastal Salinity Index (CSI) to align with drought indices. The CSI can provide important information to land managers of coastal natural resources to assess salinity’s role in the loss or change in aquatic and terrestrial habitats and loss of ecosystem services such as carbon storage by trees and peatlands. Onslow County’s New River is a vital resource for Marine Corps Base Camp Lejeune. Preservation of the New River’s ecosystems and their services, which are vulnerable to salinization, is directly linked to achieving the Base’s defense training mission. The recent 10-year Defense Coastal Estuarine Research Program (DCERP) produced data to help assess salinization. DCERP’s data combined with data collected by others can help calculate the CSI to inform Base land managers, as well as the surrounding municipalities, farmers, and foresters.