Land Use / Land Cover and Water Quality
The link between Land Use / Land Cover and water quality is both intuitive and well researched and documented. Development or other activities that take place without proper considerations can lead to significant degradation of streams and ground water, and loss of aquatic life. Even when well planned and well managed, development, agricultural activities, road construction and maintenance, and lawn care all have impacts on water quality. The following case studies illustrate how land use, when mismanaged, can cause severe water quality degradation.
Forests and other areas that have good vegetative cover and little disturbance of their soils allow rainfall and snowmelt to soak into the soil rather than running off the ground, which keep stream flows steady and maintain water quality. In developed areas with a significant percentage of the area covered by pavement and buildings, less rainfall soaks into the soil, causing increased runoff, stream flows with high peaks and low flows in between, and degraded water quality. In the absence of significant point source discharges, land use and practices are recognized to be the most important factor in determining water quality.
Besides the effect on flow, land use directly affects water quality in many other ways.
Sediment is the most frequent cause of water quality impairment in North Carolina. It affects aquatic life, shortens reservoir life, and complicates water treatment. Sources of sediment include cropland and pasture erosion, construction sites, runoff from streets and other impervious areas, and streambank erosion. Streambank erosion in particular is increased by the added runoff due to development. Erosion and sedimentation in the watershed is discussed here.
Pathogens include E. coli (a bacteria used to indicate the presence of fecal waste) and other viruses, bacteria, and protozoa. The source of most water-borne pathogens is fecal material from warm-blooded animals. In rural areas, sources include wildlife, livestock manure, and malfunctioning septic systems. In urban areas the major sources are pet wastes, wildlife that may be present in high numbers (such as birds), and septic systems in unsewered areas.
Nutrients of particular concern in the Lake James Watershed are primarily nitrogen and phosphorus. High concentrations of nitrogen and phosphorus can lead to rapid growth of algae in lakes and reservoirs such as Lake James. Sources of nutrients in developed areas include fertilizer used on lawns, gardens, and golf courses; pet waste runoff; and septic systems. Rural sources include fertilizers used on row crops, pastures and orchards; animal waste; and composed sludge from wastewater treatment plants that is distributed on the land surface.
Pesticides can be a concern in drinking water supplies that use surface water. Sources of pesticides are simpler to identify than sources of pathogens or nutrients. They are limited to pesticide application, either in agricultural or developed areas.
Oxygen-demanding substances consist of organic matter that deplete dissolved oxygen when decomposed by microorganisms. Dissolved oxygen is critical to maintaining water quality and aquatic life. Studies have shown that urban runoff with high concentrations of decaying organic matter (such as leaves, grass clippings, and other organic debris) can severely depress dissolved oxygen levels after storm events.
Metals include lead, copper, cadmium, zinc, mercury, and chromium. They can accumulate in fish tissues and affect sensitive animal and plant species. Nearly all North Carolina’s fresh waters are subject to fish consumption advisories due to mercury, mostly from atmospheric deposition due to coal power plants to our west. Other sources of metals include mining and gravel production operations, automobiles (copper is worn off of brake pads, for example), industrial activities, and illicit sewage connections.
Oil and other petroleum products degrade the appearance of water surfaces, impair fish habitats, and may be toxic to sensitive species. Sources are oil leaks; auto emissions coming off parking lots, roads, and driveways; and improper disposal of waste oil. Concentrations of petroleum-based hydrocarbons can be high enough to cause mortalities in aquatic organisms.
Road salt increases levels of sodium and chlorides in surface and ground water. Snow runoff produces high salt/chlorine concentrations in streams, rivers and lakes.
Trash and other inorganic materials that are washed into the water during rain events present a hazard to human health, as well as create obstacles to boaters and unsightly views. (link to trash section)
Land Use/Cover in the Riparian Zone
All land use and land cover affects water quality, but the land use/cover in riparian areas along the edges of streams and waterways have a particularly significant effect. Pollutants discharged near waterbodies have little opportunity to infiltrate, become attached to soil, or degraded by sunlight or biological activity before impacting the water. If polluted upland water (such as runoff from a road) is dispersed across a wide, well-vegetated riparian area, that “buffer” can serve to diffuse, infiltrate, and actually treat the runoff before it reaches the body of water. For example, bacteria dies as it is exposed to sunlight and the atmosphere. Slow, dispersed flow through a riparian zone can be an effective treatment for septic discharges, animal waste and other bacteria sources. Sediment from upland erosion can also be trapped in a well vegetated riparian zone, preventing it from reaching the stream where it may harm aquatic life.
In addition, the riparian areas immediately adjacent to our streams have a direct impact on the physical environment. For example, trees provide shade that help keep water cool during hot summer days. Removal of the tree canopy over a stream adversely impacts the aquatic life in the stream, particularly sensitive species such as trout. Roots from trees and other vegetation are essential to stabilize stream banks and prevent excess bank erosion. Another important function of the riparian zone is to provide leaf matter, sticks and other materials to streams that provide food and habitat for aquatic life.
Further information about the relationship between land use/cover and water quality can be found at the following links:
The 2018 LJEA analysis of land cover in the Lake James Watershed used the National Land Cover Database for 2001 and 2011. Part of that analysis tabulated the land cover within 100-meter wide riparian zones along the major waterway in each Hydrologic Unit (HU). A map showing those HUs is here. The analysis, documented in the Land Cover changes in the riparian zone for each HU section of this report, helps us understand the differences in water quality observed and allows us to look at how land cover changes may impact current and future water quality.