Vol. 18, No. 3, 1997

Cheap, efficient, and environmentally sound waste disposal will be needed as 
Kentucky's broiler industry expands. The filter strip length needed to 
protect water resources from contaminants in surface runoff is a pressing 
issue in waste management and water quality. In a previous Soil Science News 
and Views (Vol. 15, No. 8) we reported that grass filter strips as 
short as 15 feet can trap over 90% of the fecal bacteria eroding from 
land-applied and incorporated  poultry waste during runoff following 
rainstorms. In this update, we provide some additional information and 
conclusions from that study on filter strip length, based on  
comparisons of filter strips  receiving equal amounts of surface runoff.
Surface Flow, Sediment Runoff, and Fecal Bacteria Trapping

 Grass filter strips promote surface water infiltration. During simulated 
rainstorms, 15 and 30 foot  filter strips, on 9% slopes,  trapped an average 
of 76% and 85% of the surface runoff, respectively. Filter strips effectively 
reduced sediment concentrations in runoff by 79 to 86% (Table 1) with the  
total sediment loss decreasing by at least 95% even in 15 foot filter strips 
(Table 2). 
Fecal coliforms were trapped significantly better than fecal streptococci 
(Table 2)  and increasing the filter strip length from 15 to 30 feet 
increased the mean trapping efficiency of the filters for both fecal 
bacteria. Greater than 90% of the fecal bacteria mass was trapped in some 
cases (Table 2).  However, the average flow-weighted fecal coliform and 
fecal streptococci concentrations always exceeded 100,000 CFU/100 ml  once 
runoff occurred (Table 1). These concentrations are at least 1000 times 
higher than the standard for fecal contamination of primary 
contact water in Kentucky (200 fecal coliforms/100 ml). The flow-weighted 
mean concentration of bacteria leaving filter strips can be higher than 
that entering them because the filter strip abutting the waste-amended 
soil becomes a reservoir for sediment-bound fecal bacteria trapped 
from surface runoff. These bacteria are released by the mechanical action 
of rainfall and lateral surface flow the longer that runoff occurs.


Our data, based on intensity and duration of rainfall greater than that 
likely to occur under natural events, suggested that the benefit of doubling 
the filter strip length to 30 feet was small in this well-drained soil. 
Although fecal bacteria mass was significantly reduced, their concentrations 
exceeded primary water quality standards when runoff occurred. The criterion 
for assessing fecal contamination of water is based on concentration, not 
mass. Grass filter strips longer than 30 feet, greater intervals between 
the time of application and rainfall, or prior treatment of wastes before 
application, would be necessary to prevent  fecal contamination of adjacent 
waters based on a concentration criterion. 

 Table 1. Flow-weighted mean concentrations of sediment and fecal bacteria  
in surface runoff entering and leaving two lengths of grass filter strips 
during rain simulations.

Filter strip Plot Sediment (g/L)    Fecal coliforms    Fecal streptococci 
length (feet)
                  inflow+  outflow   inflow   outflow   inflow   outflow

			              - - -  Millions of CFU/100 ml - - -  
  15          1	   8.3 	   1.7        3.2 	5.6   	4.7   	  10.4  
              2    6.4 	   1.3       15.4  	6.7    53.4       37.1   
  30          3	   4.3 	   0.7       23.8  	8.7    11.0       16.6 
              4	   3.5 	   0.4        0.6 	0.8  	1.4        3.8  

+ Inflow refers to surface runoff entering the filter strips; outflow 
refers to surface runoff leaving the filter strips.
 CFU = Colony Forming Units

Table 2. Trapping efficiency, based on the same loading rates, of grass 
filter strips for sediment, fecal coliforms, and fecal streptococci during 
rain simulations.   

Filter strip                                   Fecal  	       Fecal 
length (feet)   Plot	   Sediment          coliforms	    streptococci

			      - - - - % Trapping efficiency - - - - -    
   15            1	      95 	        55 	           42 
                 2	      98 	        95 	           93
                             ____              ____               ____	

	        Avg.¤       96 ± 2           75 ±  28            68 ± 36 

   30            3            97                92                 67
                 4            99                89                 80
			     ____              ____               ____

		Avg.  	    98 ±  2          91 ±  2             74 ±  9

¤ Average ± one standard deviation