Nitrogen concentrations and loads showed less sig- 

 nificant fluctuations below wastewater discharges and 

 incoming tributaries. The EPA criterion for nitrogen was not 

 exceeded at any time in the mainstem upper Clark Fork during 

 the monitoring period. 



Middle Clark Fork 



Nutrient concentrations in the middle Clark Fork are 

 variable as a result of dilution from incoming clean water 

 tributaries and the influences of several major sources of 

 nutrients. Figure 3-28 indicates a significant change in 

 Clark Fork total phosphorus concentrations from station 16 to 

 station 18. These monitoring locations bracket the Missoula 

 municipal wastewater treatment plant discharge, which 

 contributes a significant phosphorus load to the river — about 

 50 tons per year (Figure 3-29) . The wastewater discharge 

 contributes an even more significant nitrogen load to the 

 river, averaging more than 100 tons per year (Figure 3-30) . 

 Exceedences of the EPA nitrogen criterion were not documented 

 during the monitoring period in the middle Clark Fork. The 

 frequency of exceedence of the phosphorus criterion, however, 

 was doubled or tripled from above to below the Missoula 

 wastewater discharge. Frequencies ranged from 8 to 18 

 percent in the Clark Fork above the discharge to 25 to 50 

 percent below for the FY 85-87 monitoring period. 



The Bitterroot River joins the Clark Fork a short 

 distance below the Missoula wastewater discharge. Its inflow 

 is responsible for significant reductions in Clark Fork 

 phosphorus concentrations and in the frequency with which the 

 phosphorus criterion is exceeded. On the other hand, Figure 

 3-30 indicates that the Bitterroot River (bracketed by 

 stations 18 and 20) contributes a significant nitrogen load 

 to the Clark Fork — about 75 to 85 tons per year. Some field 

 research indicates that the lower Bitterroot River receives a 

 considerable volume of nitrogen-rich ground water inflow from 

 the Missoula area. The presumed source of much of this 

 nitrogen is septic drainfield leachate (Kicklighter 1987) . 



The second most significant source of nutrients to the 

 middle Clark Fork is Stone Container Corporation's Frenchtown 

 kraft mill. The facility, which has been in operation since 

 1957, manufactures bleached pulp and unbleached kraft 

 linerboard. The process produces about 16.5 million gallons 

 per day (MGD) of treated wastewater that is stored in ponds 

 and either infiltrated into the shallow ground water or 

 discharged directly to the Clark Fork according to stringent 

 permit limitations. Environmental impact statements were 

 prepared on the facility in 1974 and 1985 (DHES 1974, 1985). 



3-89 



