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Geologic sources of phosphorus in the Neuse River Basin and implications for management

Albemarle-Pamlico Estuarine Study Comprehensive Conservation and Management Plan Implementation Forum, June 5-6.
By Timothy B. Spruill

Abstract

Excessive phosphorus in freshwater aquatic systems is known to cause accelerated eutrophication. Of four major basins which drain into Albemarle-Pamlico Sounds, the Neuse Basin delivers about 44 percent of the phosphorus loading, even though it comprises only 20 percent of the drainage area. Of the two major basins that drain into the Pamlico Sound, the Neuse and the Tar-Pamlico, Contentnea Creek in the Neuse Basin delivers 20 percent of the non-point-source phosphorus loading, even though it comprises only 8 percent of the land area. Although the Neuse Basin has been identified as a major source of elevated nutrients to the sounds, usual sources of phosphorus to the watershed have, in the past, been identified as point, atmospheric, nitrogen fixation, animal waste, and fertilizer.

Ground- and surface-water data, simultaneously collected as part of the U.S. Geological Survey's National Water-Quality Assessment Program from 26 surface-water sites in the Albemarle-Pamlico drainage study unit during base-flow conditions in August and September 1995, indicate that a major source of phosphorus in the Neuse drainage basin, as well as other Coastal Plain streams of North Carolina, is probably of geologic origin. Concentrations of phosphorus were significantly higher in discharging ground water (median=0.23 milligrams per liter (mg/L)) than in surface water (rnedian=0.07 mg/L). Based on historical data from the Coastal Plain, shallow ground water is typically low in phosphorus (a median of 0.01 mg/L or less), whereas deeper ground water has median concentrations of between 0.2 and 0.3 mg/L. Many of the highest phosphorus concentrations measured (most were greater than 0.5 mg/L) in discharging ground water occurred in the Contentnea Creek Basin although the maximum concentration observed occurred in Ahoskie Creek in the Chowan Basin. Current evidence from shallow wells in agricultural recharge areas of the Coastal Plain indicate that phosphorus in discharging ground water is not of anthropogenic origin. A plot of concentration versus discharge from Contentnea Creek at Hookerton indicates an inverse relationship between dissolved phosphorus and discharge, supporting the interpretation of a baseflow source of phosphorus.

Data from the NAWQA study indicate that ground water can be a significant source of phosphorus loading in Coastal Plain streams of the Albemarle-Pamlico drainage basin. Typically, ground water contributes between 30 to 70 percent of the annual stream discharge, with a median contribution of around 50 percent. Phosphorus concentrations of discharging ground water were significantly (p<0.05) and positively (Spearman rho= 0.64) correlated with the surface-water concentrations in the Albemarle-Pamlico drainage basin. Based on previous information published by the U.S. Geological Survey and new information presented in this paper, ground-water inflows contribute 60 percent or more of the phosphorus load at Contentnea Creek at Hookerton, with the remaining 10 percent from point sources and 30 percent from agriculture and precipitation. Concentrations of phosphorus in the Neuse River at several stations indicate that ground water also contributes significantly to the in-stream phosphorus load, probably on the order of 40 percent or more.

Results from this study reveal a major newly recognized source of phosphorus to Coastal Plain streams. These results reiterate the need for carefully conducted basin-wide studies and monitoring. Water quality problems of the Neuse and other Coastal Plain streams that drain to the Albemarle-Parnlico Estuarine System cannot be solved without a better understanding of the sources of nutrients and processes governing their movement and fate.

New field and analytical techniques will be necessary to provide information requisite for modeling and prediction. There is a need to assess the impact of natural phosphorus inflows to Coastal Plain streams, particularly those of the Neuse River and it's tributary streams. In addition, there is a need to examine how abundant phosphorus loading affects phytoplankton and bacterial populations, organic carbon loads to the sounds and, ultimately, nitrogen cycling; determine whether total maximum daily loads (TMDL's) can be met in reaches of the streams that exhibit elevated phosphorus concentrations; and determine whether best management practices designed to control phosphorus from agriculture or urban areas can be effective in problematic reaches of tributary streams in the Neuse and other Coastal Plain basins.

Spruill, T.B., 1997, Geologic sources of phosphorus in the Neuse River and implications for management: Albemarle-Pamlico Estuarine Study Comprehensive Conservation and Management Plan Implementation Forum, June 5-6.