Nitrogen Sources and Loads in the Lower Susquehanna River Basin

Nitrogen Sources and Measured Nitrogen Loads in Streams in the Lower Susquehanna River Basin, Pennsylvania and Maryland

R. A. Hainly and S. F. Siwiec

The annual input of nitrogen into the 9,300-square-mile Lower Susquehanna River Basin during the mid-1980's is estimated to be 390 million pounds. Nitrogen inputs from five major sources were estimated as part of a nationwide effort by the U.S. Geological Survey's National Water-Quality Assessment Program:

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  SOURCE OF NITROGEN IN THE            ANNUAL                
LOWER SUSQUEHANNA RIVER BASIN      NITROGEN INPUT   PERCENTAGE      
                                     (millions         OF
                                     of pounds)      TOTAL 
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Municipal and Industrial Discharges    12.6            3.2             
Atmospheric Deposition                 98.8           25.3            
Fertilizer Applications                52.4           13.5              
Livestock Manure                      222.0           56.9            
Private Domestic Septic Systems         4.2            1.1              
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TOTAL                                 390.0               
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Municipal and industrial discharges include 72 major sites identified by the Chesapeake Bay Program. Atmospheric nitrogen input includes estimates for wet, dry, and mist deposition processes. Estimated nitrogen inputs from fertilizer, which apply only to agricultural applications, were based on county-level fertilizer sales data. Animal population figures from the Census of Agriculture were used to estimate nitrogen from livestock manure. Estimated nitrogen inputs from private domestic septic systems were based on 1980 census data.

Measured in-stream nitrogen loads in six major tributaries ranged from 20 to 36 percent of the estimated source inputs; measured in-stream nitrogen loads at three mainstem sites ranged from 38 to 47 percent of the estimated source inputs:

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NAME OF MAJOR             ANNUAL NITROGEN   MEAN ANNUAL      PERCENTAGE
TRIBUTARY OR                  INPUT         IN-STREAM LOAD       OF
MAINSTEM SITE              (millions of     (millions of       INPUT
                              pounds)          pounds)  
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Juniata River                  82.9             16.8            20.3       
Sherman Creek                   4.37             1.20           27.5     
Swatara Creek                  17.5              6.34           36.2     
West Conewago Creek            14.2              3.65           25.7     
Codorus Creek                   8.40             2.73           32.5     
Conestoga River                40.8             10.1            24.8    
Susquehanna River at          165.0             77.3            46.8    
  Sunbury, Pennsylvania                                                                  
Susquehanna River above       299.0            141.0            47.2   
  Reservoirs                                                                 
Susquehanna River below       390.0            147.0            37.7   
  Reservoirs                                                                    
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Although this study did not evaluate nitrogen losses other than losses by stream outflow, differences between estimated nitrogen source inputs and measured in-stream loads may be partly attributed to storage of nitrogen in soil and ground water, nitrogen fixation, nitrogen uptake by plants, denitrification, and agricultural best-management practices.

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