Water quality and land cover in the Coastal Plain Little River watershed, Georgia, United States

Abstract

Despite efforts to improve water quality in US watersheds, recent assessments of water quality trends in these watersheds indicate concentrations of nitrogen (N) and phosphorus (P) show minimal changes in the majority of studied streams across the nation. Moreover, results of the US Geological Survey (USGS) National Water-Quality Assessment (NAWQA) indicate that nutrient concentrations in streams and groundwater in basins with significant agriculture or urban development are substantially greater than naturally occurring or background levels. Long-term hydrologic and water quality data from watersheds are a key component to building an understanding that will dictate what and where changes in watershed management need to be made to achieve improvements in water quality. Forty-one years of hydrologic and water quality data from the Little River Experimental Watershed (LREW) were assembled to evaluate trends in streamflow quantity and quality from the LREW and relationships to changes in land cover and management. Concentrations and loads of chloride (Cl), ammonium-N (NH₄-N), nitrate plus nitrite-N (NO₃-N), total kjeldahl N (TKN), total P (TP), and dissolved molybdate reactive P (DMRP) from 1974 to 2014 were determined. In general, concentrations of N and P have remained low and stable, with some increases observed from 1980 to 1999. In contrast, Cl concentration appears to be steadily increasing, possibly due to increased fertilization in the LREW. Flow-weighted mean concentrations were 0.21 mg L⁻¹ y⁻¹ for NO₃-N, 0.08 mg L⁻¹ y⁻¹ for NH₄-N, 1.41 mg L⁻¹ y⁻¹ for TKN, 9.30 mg L⁻¹ y⁻¹ for Cl, 0.15 mg L⁻¹ y⁻¹ for TP, and 0.03 mg L⁻¹ y⁻¹ for DMRP. Average annual loads were 0.64 kg ha⁻¹ y⁻¹ for NO₃-N, 0.25 kg ha⁻¹ y⁻¹ for NH₄-N, 4.47 kg ha⁻¹ y⁻¹ for TKN, 29.54 kg ha⁻¹ y⁻¹ for Cl, 0.46 kg ha⁻¹ y⁻¹ for TP, and 0.10 kg ha⁻¹ y⁻¹ for DMRP. The low nutrient loading in the watershed is attributed to the dense riparian buffers within the LREW, which have been shown to be effective for reducing N loading to the stream. Due to (1) small variations in nutrient concentrations and loads, (2) large variability in precipitation, (3) large variability in streamflow, and (4) small variations in land cover, no clear relationships among changes in land cover and management and nutrient loads were found. Loading rates of N and P, although somewhat influenced by changes in concentration, are largely dictated by changes in streamflow volume in the LREW.