Efficacy of Vegetated Buffer Strips for Retaining Cryptosporidium parvum
Journal of Environmental Quality , 33 , 2004
Tate, K. W., Pereira, M.D.G., Atwill, E. R.
Overland and shallow subsurface hydrologic transport of pathogenic Cryptosporidium parvum oocysts from cattle feces into surface drinking water supplies is a major concern on annual grasslands in California's central and southern Sierra Nevada foothills. Soil boxes (0.5 m wide × 1.1 m long × 0.3 m deep) were used to evaluate the ability of grass vegetated buffer strips to retain 2 × 108 spiked C. parvum oocysts in 200-g fecal deposits during simulated rainfall intensities of 30 to 47.5 mm/h over 2 h. Buffers were comprised of Ahwahnee sandy loam (coarse-loamy, mixed, active, thermic Mollic Haploxeralfs; 78:18:4 sand to silt to clay ratio; dry bulk density = 1.4 g/cm3) set at 5 to 20% land slope, and ≥95% grass cover (grass stubble height = 10 cm; biomass = 900 kg/ha dry weight). Total number of oocysts discharged from each soil box (combined overland and subsurface flow) during the 120-min simulation ranged from 1.5 × 106 to 23.9 × 106 oocysts. Observed overall mean log10 reduction of total C. parvum flux per meter of vegetated buffer was 1.44, 1.19, and 1.18 for buffers at 5, 12, and 20% land slope, respectively. Rainfall application rate (mm/h) was strongly associated with oocyst flux from these vegetated buffers, resulting in a decrease of 2 to 4% in the log10 reduction per meter buffer for every additional mm/h applied to the soil box. These results support the use of strategically placed vegetated buffers as one of several management strategies that can reduce the risk of waterborne C. parvum attributable to extensive cattle grazing on annual grassland watersheds. Cryptosporidium parvum is a ubiquitous protozoal parasite, with specific genotypes able to be transmitted between domestic animals and humans (Peng et al., 1997; Spano et al., 1997; Awad-El-Kariem et al., 1998; Okhuysen et al., 1999; Xiao et al., 2002). Vegetated buffers are widely advocated as a management practice to minimize the likelihood that animal agricultural operations contaminate surface water with enteric microorganisms prevalent in fecal matter, such as generic E. coli (Young et al., 1980; Coyne et al., 1995; Younos et al., 1998; Entry et al., 2000; Rosen et al., 2000). Considerably less work has been conducted on the ability of vegetated buffers to remove pathogenic microorganisms from overland and shallow subsurface flow, such as the protozoa, C. parvum (Mawdsley et al., 1996a; Tate et al., 2000; Atwill et al., 2002; Trask et al., 2004). It can be hypothesized that the efficiency of buffer strip filtration for C. parvum will be considerably greater than the efficiencies observed for enteric bacteria (Gifford and Hawkins, 1978, Gee and Bauder, 1979), given the fact that enteric bacteria such as Salmonella and E. coli are smaller rod-shaped pathogens ranging in length from 2 to 5 μm and 0.5 to 1.5 μm in width, while the infectious stage of C. parvum (oocyst) is spherical and has a diameter of 5 to 6 μm. It has been suggested that the primary mechanism by which vegetated buffers remove waterborne microbial pathogens is via infiltration of overland flow into the soil profile, followed by subsurface filtration and adsorption (Harter et al., 2000; Atwill et al., 2002; Trask et al., 2004). Larger diameter pathogens such as C. parvum are likely to be more susceptible to filtration compared to the smaller enteric bacteria. The situation is less clear for adsorption given the physicochemical processes governing attachment of a biological colloid onto a solid substrate (Walker and Montemagno, 1999) and the ability of C. parvum to desorb from sediments in a time-dependent manner (Harter et al., 2000). When enteric pathogens are excreted by a host in a matrix of fecal material onto the terrestrial component of a watershed, at least three events need to occur in order for C. parvum to reach surface water to become a waterborne hazard. First, C. parvum oocysts need to be released from the fecal matrix; second, the entrained oocysts need to avoid such processes as filtration, adsorption, and settling while being transported in overland and subsurface flow to a receiving body of water (e.g., river or lake); and third, oocysts need to remain infective during the completion of the first two processes. We have determined previously that a meter of vegetated buffer can function to remove 1 to 3 log10 of waterborne C. parvum oocysts entrained in overland flow (Atwill et al., 2002). The objective of this project was to assess the log10 reduction per meter of vegetated buffer when C. parvum oocysts are instead placed into a fecal matrix, as occurs naturally on watersheds with vertebrate hosts, and to determine if land slope (%) influences the ability of a buffer to retain oocysts. For this experiment, we simulated the vegetation, soil, land slope, and rainfall conditions of annual grasslands found in California's central and southern Sierra Nevada foothills. These grasslands occupy over 1.8 million ha in California and are a critical source of the state's surface drinking water supply. Approximately two-thirds of the state's drinking water reservoirs are located within annual grassland landscapes (Forest and Rangeland Resources Assessment Program, 1988).